Underwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water, its contents and its boundaries. The water may be in the ocean, a lake, a river or a tank. Typical frequencies associated with underwater acoustics are between 10 Hz and 1 MHz.  The propagation of sound in the ocean at frequencies lower than 10Hz is usually not possible without penetrating deep into the seabed, whereas frequencies above 1MHz are rarely used because they are absorbed very quickly.  Underwater acoustics is sometimes known as hydroacoustics.
The field of underwater acoustics is closely related to a number of other fields of acoustic study, including sonar, transduction, acoustic signal processing, acoustical oceanography, bioacoustics, and physical acoustics.
History
Underwater sound has probably been used by marine animals for millions of years. The science of underwater acoustics began in 1490, when Leonardo da Vinci wrote the following,[1]
"If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you."
In 1687 Isaac Newton wrote his Mathematical Principles of Natural Philosophy which included the first mathematical treatment of sound. The next major step in the development of underwater acoustics was made by Daniel Colladon, a Swiss physicist, and Charles Sturm, a French mathematician. In 1826, on Lake Geneva, they measured the elapsed time between a flash of light and the sound of a submerged ship's bell heard using an underwater listening horn.[2] They measured a sound speed of 1435 metres per second over a 17 kilometre(Km) distance, providing the first quantitative measurement of sound speed in water.[3] The result they obtained was within about 2% of currently accepted values. In 1877 Lord Rayleigh wrote the Theory of Sound and established modern acoustic theory.
The sinking of Titanic in 1912 and the start of World War I provided the impetus for the next wave of progress in underwater acoustics. Systems for detecting icebergs and U-boats were developed. Between 1912 and 1914, a number of echolocation patents were granted in  Europe and the U.S., culminating in Reginald A. Fessenden's echo-ranger in 1914. Pioneering work was carried out during this time in France by Paul Langevin and in Britain by A B Wood and associates.[4] The development of both active ASDIC and passive sonar (SOund Navigation And Ranging) proceeded apace during the war, driven by the first large scale deployments of submarines. Other advances in underwater acoustics included the development of acoustic mines.
In 1919, the first scientific paper on underwater acoustics was published,[5] theoretically describing the refraction of sound waves produced by temperature and salinity gradients in the ocean. The range predictions of the paper were experimentally validated by propagation loss measurements.
The next two decades saw the development of several applications of underwater acoustics. The fathometer, or depth sounder, was developed commercially during the 1920s. Originally natural materials were used for the transducers, but by the 1930s sonar systems incorporating piezoelectric transducers made from synthetic materials were being used for passive listening systems and for active echo-ranging systems. These systems were used to good effect  during World War II by both submarines and anti-submarine vessels.  Many advances in underwater acoustics were made which were summarised later in the series Physics of Sound in the Sea, published in 1946.
After World War II, the development of sonar systems was driven largely by the Cold War, resulting in advances in the theoretical and practical understanding of underwater acoustics, aided by computer-based techniques.
Theory
Sound waves in water, bottom of sea
A sound wave propagating underwater consists of alternating compressions and rarefactions of the water. These compressions and rarefactions are detected by a receiver, such as the human ear or a hydrophone, as changes in pressure.  These waves may be man-made or naturally generated.
Speed of sound, density and impedance
The speed of sound

c

{\displaystyle c\,}

 (i.e., the longitudinal motion of wavefronts) is related to frequency

f

{\displaystyle f\,}

 and wavelength

λ

{\displaystyle \lambda \,}

 of a wave by

c
=
f
⋅
λ

{\displaystyle c=f\cdot \lambda }

.
This is different from the particle velocity

u

{\displaystyle u\,}

, which refers to the motion of molecules in the medium due to the sound, and relates the plane wave pressure

p

{\displaystyle p\,}

 to the fluid density

ρ

{\displaystyle \rho \,}

 and sound speed

c

{\displaystyle c\,}

 by

p
=
c
⋅
u
⋅
ρ

{\displaystyle p=c\cdot u\cdot \rho }

.
The product of

c

{\displaystyle c}

 and

ρ

{\displaystyle \rho \,}

 from the above formula is known as the characteristic acoustic impedance. The acoustic power (energy per second) crossing unit area is known as the intensity of the wave and for a plane wave the average intensity is given by

I
=

q

2

/

(
ρ
c
)

{\displaystyle I=q^{2}/(\rho c)\,}

, where

q

{\displaystyle q\,}

 is the root mean square acoustic pressure.
At 1kHz, the wavelength in water is about 1.5 m. Sometimes the term "sound velocity" is used but this is incorrect as the quantity is a scalar.
The large impedance contrast between air and water (the ratio is about 3600) and the scale of surface roughness means that the sea surface behaves as an almost perfect reflector of sound at frequencies below 1kHz. Sound speed in water exceeds that in air by a factor of 4.4 and the density ratio is about 820.
Absorption of sound
Absorption of low frequency sound is weak.[6]  (see  for an on-line calculator).  The main cause of sound attenuation in fresh water, and at high frequency in sea water (above 100kHz) is viscosity.  Important additional contributions at lower frequency in seawater are associated with the ionic relaxation of boric acid (up to c. 10kHz)[6] and magnesium sulfate (c. 10kHz-100kHz).[7]
Sound may be absorbed by losses at the fluid boundaries. Near the surface of the sea losses can occur in a bubble layer or in ice, while at the bottom sound can penetrate into the sediment and be absorbed.
Sound reflection and scattering
Boundary interactions
Both the water surface and bottom are reflecting and scattering boundaries.
Surface
For many purposes the sea-air surface can be thought of as a perfect reflector.  The impedance contrast is so great that little energy is able to cross this boundary.  Acoustic pressure waves reflected from the sea surface experience a reversal in phase, often stated as either a “pi phase change” or a “180 deg phase change”.  This is represented mathematically by assigning a reflection coefficient of minus 1 instead of plus one to the sea surface.[8]
At high frequency (above about 1kHz) or when the sea is rough, some of the incident sound is scattered, and this is taken into account by assigning a reflection coefficient whose magnitude is less than one.  For example, close to normal incidence, the reflection coefficient becomes

R
=
−

e

−
2

k

2

h

2

s
i

n

2

A

{\displaystyle R=-e^{-2k^{2}h^{2}sin^{2}A}}

, where h is the rms wave height.[9]
A further complication is the presence of wind generated bubbles or fish close to the sea surface.[10]  The bubbles can also form plumes that absorb some of the incident and scattered sound, and scatter some of the sound themselves.[11]
Seabed
The acoustic impedance mismatch between water and the bottom is generally much less than at the surface and is more complex. It depends on the bottom material types and depth of the layers. Theories have been developed for predicting the sound propagation in the bottom in this case, for example by Biot [12] and by Buckingham.[13]
At target
The reflection of sound at a target whose dimensions are large compared with the acoustic wavelength depends on its size and shape as well as the impedance of the target relative to that of water. Formulae have been developed for the target strength of various simple shapes as a function of angle of sound incidence.  More complex shapes may be approximated by combining these simple ones.[1]
Propagation of sound
Underwater acoustic propagation depends on many factors. The direction of sound propagation is determined by the sound speed gradients in the water.This is an important thing that happens in water, because the speed of sound travel in water with velocity regular. In the sea the vertical gradients are generally much larger than the horizontal ones. Combining this with a tendency towards increasing sound speed at increasing depth, due to the increasing pressure in the deep sea, causes a reversal of the sound speed gradient in the thermocline, creating an efficient waveguide at the depth, corresponding to the minimum sound speed. The sound speed profile may cause regions of low sound intensity called "Shadow Zones", and regions of high intensity called "Caustics".  These may be found by ray tracing methods.
At equator and temperate latitudes in the ocean, the surface temperature is high enough to reverse the pressure effect, such that a sound speed minimum occurs at depth of a few hundred metres.  The presence of this minimum creates a special channel known as Deep Sound Channel, previously known as the SOFAR (sound fixing and ranging) channel, permitting guided propagation of underwater sound for thousands of kilometres without interaction with the sea surface or the seabed. Another phenomenon in the deep sea is the formation of sound focusing areas, known as Convergence Zones. In this case sound is refracted downward from a near-surface source and then back up again. The horizontal distance from the source at which this occurs depends on the positive and negative sound speed gradients. A surface duct can also occur in both deep and moderately shallow water when there is upward refraction, for example due to cold surface temperatures.  Propagation is by repeated sound bounces off the surface.
In general, as sound propagates underwater there is a reduction in the sound intensity over increasing ranges, though in some circumstances a gain can be obtained due to focusing.  Propagation loss (sometimes referred to as transmission loss) is a quantitative measure of the reduction in sound intensity between two points, normally the sound source and a distant receiver.  If

I

s

{\displaystyle I_{s}}

 is  the far field intensity of the source referred to a point 1 m from its acoustic centre and

I

r

{\displaystyle I_{r}}

 is the intensity at the receiver, then the propagation loss is given by[1]

P
L
=
10
log

(

I

s

/

I

r

)

{\displaystyle PL=10\log(I_{s}/I_{r})}

.
In this equation

I

r

{\displaystyle I_{r}}

 is not the true acoustic intensity at the receiver, which is a vector quantity, but a scalar equal to the equivalent plane wave intensity (EPWI) of the sound field.  The EPWI is defined as the magnitude of the intensity of a plane wave of the same RMS pressure as the true acoustic field. At short range the propagation loss is dominated by spreading while at long range it is dominated by absorption and/or scattering losses.
An alternative definition is possible in terms of pressure instead of intensity,[14] giving

P
L
=
20
log

(

p

s

/

p

r

)

{\displaystyle PL=20\log(p_{s}/p_{r})}

, where

p

s

{\displaystyle p_{s}}

  is the RMS acoustic pressure in the far-field of the projector, scaled to a standard distance of 1 m, and

p

r

{\displaystyle p_{r}}

  is the RMS pressure at the receiver position.
These two definitions are not exactly equivalent because the characteristic impedance at the receiver may be different from that at the source.  Because of this, the use of the intensity definition leads to a different sonar equation to the definition based on a pressure ratio.[15]  If the source and receiver are both in water, the difference is small.
Propagation modelling
The propagation of sound through water is described by the wave equation, with appropriate boundary conditions. A number of models have been developed to simplify propagation calculations. These models include ray theory, normal mode solutions, and parabolic equation simplifications of the wave equation.[16] Each set of solutions is generally valid and computationally efficient in a limited frequency and range regime, and may involve other limits as well. Ray theory is more appropriate at short range and high frequency, while the other solutions function better at long range and low frequency.[17]  Various empirical and analytical formulae have also been derived from measurements that are useful approximations.[18]
Reverberation
Transient sounds result in a decaying background that can be of much larger duration than the original transient signal.  The cause of this background, known as reverberation, is partly due to scattering from rough boundaries and partly due to scattering from fish and other biota.  For an acoustic signal to be detected easily, it must exceed the reverberation level as well as the background noise level.
Doppler shift
If an underwater object is moving relative to an underwater receiver, the frequency of the received sound is different from that of the sound radiated (or reflected) by the object.  This change in frequency is known as a Doppler shift.  The shift can be easily observed in active sonar systems, particularly narrow-band ones, because the transmitter frequency is known, and the relative motion between sonar and object can be calculated.  Sometimes the frequency of the radiated noise (a tonal) may also be known, in which case the same calculation can be done for passive sonar.   For active systems the change in frequency is 0.69Hz per knot per kHz and half this for passive systems as propagation is only one way.  The shift corresponds to an increase in frequency for an approaching target.
Intensity fluctuations
Though acoustic propagation modelling generally predicts a constant received sound level, in practice there are both temporal and spatial fluctuations. These may be due to both small and large scale environmental phenomena.  These can include sound speed profile fine structure and frontal zones as well as internal waves.  Because in general there are multiple propagation paths between a source and receiver, small phase changes in the interference pattern between these paths can lead to large fluctuations in sound intensity.
Non-linearity
In water, especially with air bubbles, the change in density due to a change in pressure is not exactly linearly proportional.  As a consequence for a sinusoidal wave input additional harmonic and subharmonic frequencies are generated. When two sinusoidal waves are input, sum and difference frequencies are generated. The conversion process is greater at high source levels than small ones. Because of the non-linearity there is a dependence of sound speed on the pressure amplitude so that large changes travel faster than small ones. Thus a sinusoidal waveform gradually becomes  a sawtooth one with a steep rise and a gradual tail.  Use is made of this phenomenon in parametric sonar and theories have been developed to account for this, e.g. by Westerfield.
Measurements
Sound in water is measured using a hydrophone, which is the underwater equivalent of a microphone.  A hydrophone measures pressure fluctuations, and these are usually converted to sound pressure level (SPL), which is a logarithmic measure of the mean square acoustic pressure.
Measurements are usually reported in one of three forms:-
RMS acoustic pressure in micropascals (or dB re 1 μPa)
RMS acoustic pressure in a specified bandwidth, usually octaves or thirds of octave (dB re 1 μPa)
spectral density (mean square pressure per unit bandwidth) in micropascals-squared per hertz (dB re 1 μPa²/Hz)
The scale for acoustic pressure in water differs from that used for sound in air. In air the reference pressure is 20μPa rather than 1μPa. For the same numerical value of SPL, the intensity of a plane wave (power per unit area, proportional to mean square sound pressure divided by acoustic impedance) in air is about 202×3600=1440000 times higher than in water. Similarly, the intensity is about the same if the SPL is 61.6dB higher in the water.
Sound speed
Approximate values for fresh water and seawater, respectively, at atmospheric pressure are 1450 and 1500m/s for the sound speed, and 1000 and 1030kg/m³ for the density.[19]  The speed of sound in water increases with increasing pressure, temperature and salinity.[20][21]  The maximum speed in pure water under atmospheric pressure is attained at about 74°C; sound travels slower in hotter water after that point; the maximum increases with pressure.[22]
On-line calculators can be found at  and .
Absorption
Many measurements have been made of sound absorption in lakes and the ocean
[6][7]  (see  for an on-line calculator).
Ambient noise
Measurement of acoustic signals are possible if their amplitude exceeds a minimum threshold, determined partly by the signal processing used and partly by the level of background noise.  Ambient noise is that part of the received noise that is independent of the source, receiver and platform characteristics.  This it excludes reverberation and towing noise for example.
The background noise present in the ocean, or ambient noise, has many different sources and varies with location and frequency.[23] At the lowest frequencies, from about 0.1Hz to 10Hz, ocean turbulence and microseisms are the primary contributors to the noise background.[24] Typical noise spectrum levels decrease with increasing frequency from about 140dB re 1 μPa²/Hz at 1Hz to about 30dB re 1 μPa²/Hz at 100kHz. Distant ship traffic is one of the dominant noise sources in most areas for frequencies of around 100Hz, while wind-induced surface noise is the main source between 1kHz and 30kHz. At very high frequencies, above 100kHz, thermal noise of water molecules begins to dominate. The thermal noise spectral level at 100kHz is 25dB re 1 μPa²/Hz.  The spectral density of thermal noise increases by 20dB per decade (approximately 6dB per octave).[25]
Transient sound sources also contribute to ambient noise. These can include intermittent geological activity, such as earthquakes and underwater volcanoes,[26] rainfall on the surface, and biological activity. Biological sources include cetaceans (especially blue, fin and sperm whales),[27][28] certain types of fish, and snapping shrimp.
Rain can produce high levels of ambient noise. However the numerical relationship between rain rate and ambient noise level is difficult to determine because measurement of rain rate is problematic at sea.
Reverberation
Many measurements have been made of sea surface, bottom and volume reverberation.  Empirical models have sometimes been derived from these.  A commonly used expression for the band 0.4 to 6.4kHz is that by Chapman and Harris.[29] It is found that a sinusoidal waveform is spread in frequency due to the surface motion. For bottom reverberation a Lambert's Law is found often to apply approximately, for example see Mackenzie.[30]  Volume reverberation is usually found to occur mainly in layers, which change depth with the time of day, e.g., see Marshall and Chapman.[31]  The under-surface of ice can produce strong reverberation when it is rough, see for example Milne.[32]
Bottom loss
Bottom loss has been measured as a function of grazing angle for many frequencies in various locations, for example those by the US Marine Geophysical Survey.[33]  The loss depends on the sound speed in the bottom (which is affected by gradients and layering) and by roughness.  Graphs have been produced for the loss to be expected in particular circumstances. In shallow water bottom loss often has the dominant impact on long range propagation. At low frequencies sound can propagate through the sediment then back into the water.
Underwater hearing
Comparison with airborne sound levels
As with airborne sound, sound pressure level underwater is usually reported in units of decibels, but there are some important differences that make it difficult (and often inappropriate) to compare SPL in water with SPL in air.  These differences include:[34]
difference in reference pressure: 1 μPa  (one micropascal, or one millionth of a pascal) instead of 20 μPa.[14]
difference in interpretation: there are two schools of thought, one maintaining that pressures should be compared directly, and the other that one should first convert to the intensity of an equivalent plane wave.
difference in hearing sensitivity: any comparison with (A-weighted) sound in air needs to take into account the differences in hearing sensitivity, either of a human diver or other animal.[35]
Human hearing
Hearing sensitivity
The lowest audible SPL for a human diver with normal hearing is about 67dB re 1 μPa, with greatest sensitivity occurring at frequencies around 1kHz.[36] This corresponds to a sound intensity 5.4dB, or 3.5 times, higher than the threshold in air (see Measurements above).
Safety thresholds
High levels of underwater sound create a potential hazard to human divers.[37]  Guidelines for exposure of human divers to underwater sound are reported by the SOLMAR project of the NATO Undersea Research Centre.[38]  Human divers exposed to SPL above 154dB re 1 μPa in the frequency range 0.6 to 2.5kHz are reported to experience changes in their heart rate or breathing frequency. Diver aversion to low frequency sound is dependent upon sound pressure level and center frequency.[39]
Other species
Aquatic mammals
Dolphins and other toothed whales are known for their acute hearing sensitivity, especially in the frequency range 5 to 50kHz.[35][40]  Several species have hearing thresholds between 30 and 50dB re 1 μPa in this frequency range.  For example, the hearing threshold of the killer whale occurs at an RMS acoustic pressure of 0.02 mPa (and frequency 15kHz), corresponding to an SPL threshold of 26dB re 1 μPa.[41]
High levels of underwater sound create a potential hazard to marine and amphibious animals.[35] The effects of exposure to underwater noise are reviewed by Southall et al.[42]
Fish
The hearing sensitivity of fish is reviewed by Ladich and Fay.[43]
The hearing threshold of the soldier fish, is 0.32 mPa (50dB re 1 μPa) at 1.3kHz, whereas the lobster has a hearing threshold of 1.3 Pa at 70Hz (122dB re 1 μPa).[41] The effects of exposure to underwater noise are reviewed by Popper et al.[44]
Applications of underwater acoustics
Sonar

Sonar is the name given to the acoustic equivalent of radar.  Pulses of sound are used to probe the sea, and the echoes are then processed to extract information about the sea, its boundaries and submerged objects.  An alternative use, known as passive sonar, attempts to do the same by listening to the sounds radiated by underwater objects.
Underwater communication

The need for underwater acoustic telemetry exists in applications such as data harvesting for environmental monitoring, communication with and between manned and unmanned underwater vehicles, transmission of diver speech, etc. A related application is underwater remote control, in which acoustic telemetry is used to remotely actuate a switch or trigger an event.  A prominent example of underwater remote control are acoustic releases, devices that are used to return sea floor deployed instrument packages or other payloads to the surface per remote command at the end of a deployment.  Acoustic communications form an active field of research
[45][46] with significant challenges to overcome, especially in horizontal, shallow-water channels. Compared with radio telecommunications, the available bandwidth is reduced by several orders of magnitude. Moreover, the low speed of sound causes multipath propagation to stretch over time delay intervals of tens or hundreds of milliseconds, as well as significant Doppler shifts and spreading. Often acoustic communication systems are not limited by noise, but by reverberation and time variability beyond the capability of receiver algorithms. The fidelity of underwater communication links can be greatly improved by the use of hydrophone arrays, which allow processing techniques such as adaptive beamforming and diversity combining.
Underwater navigation and tracking

Underwater navigation and tracking is a common requirement for exploration and work by divers, ROV, autonomous underwater vehicles (AUV), manned submersibles and submarines alike. Unlike most radio signals which are quickly absorbed, sound propagates far underwater and at a rate that can be precisely measured or estimated.[47] It can thus be used to measure distances between a tracked target and one or multiple reference of baseline stations precisely, and triangulate the position of the target, sometimes with centimeter accuracy.  Starting in the 1960s, this has given rise to underwater acoustic positioning systems which are now widely used.
Seismic exploration

Seismic exploration involves the use of low frequency sound (< 100Hz) to probe deep into the seabed.  Despite the relatively poor resolution due to their long wavelength, low frequency sounds are preferred because high frequencies are heavily attenuated when they travel through the seabed.  Sound sources used include airguns, vibroseis and explosives.
Weather and climate observation
Acoustic sensors can be used to monitor the sound made by wind and precipitation.  For example, an acoustic rain gauge is described by Nystuen.[48] Lightning strikes can also be detected.[49] Acoustic thermometry of ocean climate (ATOC) uses low frequency sound to measure the global ocean temperature.
Oceanography

Large scale ocean features can be detected by acoustic tomography.  Bottom characteristics can be measured by side-scan sonar and sub-bottom profiling.
Marine biology

Due to its excellent propagation properties, underwater sound is used as a tool to aid the study of marine life, from microplankton to the blue whale.  Echo sounders are often used to provide data on marine life abundance, distribution, and behavior information.  Echo sounders, also referred to as hydroacoustics is also used for fish location, quantity, size, and biomass.
Acoustic telemetry is also used for monitoring fish and marine wildlife. An acoustic transmitter is attached to the fish (sometimes internally) while an array of receivers listen to the information conveyed by the sound wave. This enables the researchers to track the movements of individuals in a small-medium scale.[50]
Pistol shrimp create sonoluminescent cavitation bubbles that reach up to 5,000K (4,700°C) [51]
Particle physics
A neutrino is a fundamental particle that interacts very weakly with other matter.  For this reason, it requires detection apparatus on a very large scale, and the ocean is sometimes used for this purpose.  In particular, it is thought that ultra-high energy neutrinos in seawater can be detected acoustically.[52]
See also
Acoustic Tags (Acoustic Telemetry)
Bioacoustics
Hydroacoustics
Ocean Tracking Network
Refraction (sound)
Sonar
Underwater Acoustic Positioning System
SOFAR channel
Underwater acoustic wireless communication system
European Conference on Underwater Acoustics

 at the Woods Hole Oceanographic Institution

How far can sound waves travel through water?


The common cold, also known simply as a cold, is a viral infectious disease of the upper respiratory tract that primarily affects the nose.[7] The throat, sinuses, and larynx may also be affected.[5] Signs and symptoms may appear less than two days after exposure to the virus.[5] These may include coughing, sore throat, runny nose, sneezing, headache, and fever.[2][3] People usually recover in seven to ten days,[2] but some symptoms may last up to three weeks.[6] Occasionally those with other health problems may develop pneumonia.[2]

Well over 200 virus strains are implicated in causing the common cold, with rhinoviruses being the most common.[11] They spread through the air during close contact with infected people or indirectly through contact with objects in the environment, followed by transfer to the mouth or nose.[2] Risk factors include going to child care facilities, not sleeping well, and psychological stress.[5] The symptoms are mostly due to the body's immune response to the infection rather than to tissue destruction by the viruses themselves.[12] In contrast, those affected by influenza can show similar symptoms as people with a cold, but symptoms are usually more severe.[5] Additionally, influenza is less likely to result in a runny nose.[13]

There is no vaccine for the common cold.[2] The primary methods of prevention are hand washing; not touching the eyes, nose or mouth with unwashed hands; and staying away from sick people.[2] Some evidence supports the use of face masks.[14] There is also no cure, but the symptoms can be treated.[2] Zinc may reduce the duration and severity of symptoms if started shortly after the onset of symptoms.[8] Nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen may help with pain.[9] Antibiotics, however, should not be used[15] and there is no good evidence for cough medicines.[5][16]

The common cold is the most frequent infectious disease in humans.[17] The average adult gets two to three colds a year, while the average child may get six to eight.[7][10] Infections occur more commonly during the winter.[2] These infections have existed throughout human history.[18]
Signs and symptoms
The typical symptoms of a cold include a cough, a runny nose, nasal congestion and a sore throat, sometimes accompanied by muscle ache, fatigue, headache, and loss of appetite.[19] A sore throat is present in about 40% of cases and a cough in about 50%,[7] while muscle ache occurs in about half.[3] In adults, a fever is generally not present but it is common in infants and young children.[3] The cough is usually mild compared to that accompanying influenza.[3] While a cough and a fever indicate a higher likelihood of influenza in adults, a great deal of similarity exists between these two conditions.[20] A number of the viruses that cause the common cold may also result in asymptomatic infections.[21][22]
The color of the sputum or nasal secretion may vary from clear to yellow to green and does not indicate the class of agent causing the infection.[23]
Progression
A cold usually begins with fatigue, a feeling of being chilled, sneezing, and a headache, followed in a couple of days by a runny nose and cough.[19] Symptoms may begin within sixteen hours of exposure[24] and typically peak two to four days after onset.[3][25] They usually resolve in seven to ten days, but some can last for up to three weeks.[6] The average duration of cough is eighteen days[26] and in some cases people develop a post-viral cough which can linger after the infection is gone.[27] In children, the cough lasts for more than ten days in 35–40% of cases and continues for more than 25 days in 10%.[28]
Cause
Viruses

The common cold is a viral infection of the upper respiratory tract. The most commonly implicated virus is a rhinovirus (30–80%), a type of picornavirus with 99 known serotypes.[29][30] Other commonly implicated viruses include human coronavirus (≈15%),[31][32]influenza viruses (10–15%),[33] adenoviruses (5%),[33] human respiratory syncytial virus, enteroviruses other than rhinoviruses, human parainfluenza viruses, and metapneumovirus.[34] Frequently more than one virus is present.[35] In total over 200 viral types are associated with colds.[3]
Transmission
The common cold virus is typically transmitted via airborne droplets (aerosols), direct contact with infected nasal secretions, or fomites (contaminated objects).[7][36] Which of these routes is of primary importance has not been determined.[37] The viruses may survive for prolonged periods in the environment (over 18 hours for rhinoviruses) and can be picked up by people's hands and subsequently carried to their eyes or nose where infection occurs.[36] Transmission is common in daycare and at school due to the proximity of many children with little immunity and frequently poor hygiene.[38] These infections are then brought home to other members of the family.[38] There is no evidence that recirculated air during commercial flight is a method of transmission.[36] People sitting in close proximity appear to be at greater risk of infection.[37]
Rhinovirus-caused colds are most infectious during the first three days of symptoms; they are much less infectious afterwards.[39]
Weather
The traditional theory is that a cold can be "caught" by prolonged exposure to cold weather such as rain or winter conditions, which is how the disease got its name.[40] Some of the viruses that cause the common colds are seasonal, occurring more frequently during cold or wet weather.[41] The reason for the seasonality has not been conclusively determined.[42] Possible explanations may include cold temperature-induced changes in the respiratory system,[43] decreased immune response,[44] and low humidity causing an increase in viral transmission rates, perhaps due to dry air allowing small viral droplets to disperse farther and stay in the air longer.[45]
The apparent seasonality may also be due to social factors, such as people spending more time indoors, near infected people,[43] and specifically children at school.[38][42] There is some controversy over the role of low body temperature as a risk factor for the common cold; the majority of the evidence suggests that it may result in greater susceptibility to infection.[44]
Other
Herd immunity, generated from previous exposure to cold viruses, plays an important role in limiting viral spread, as seen with younger populations that have greater rates of respiratory infections.[46] Poor immune function is a risk factor for disease.[46][47] Insufficient sleep and malnutrition have been associated with a greater risk of developing infection following rhinovirus exposure; this is believed to be due to their effects on immune function.[48][49] Breast feeding decreases the risk of acute otitis media and lower respiratory tract infections among other diseases,[50] and it is recommended that breast feeding be continued when an infant has a cold.[51] In the developed world breast feeding may not be protective against the common cold in and of itself.[52]
Pathophysiology

The symptoms of the common cold are believed to be primarily related to the immune response to the virus.[12] The mechanism of this immune response is virus specific. For example, the rhinovirus is typically acquired by direct contact; it binds to human via ICAM-1 receptors and the CDHR3 receptor through unknown mechanisms to trigger the release of inflammatory mediators.[12] These inflammatory mediators then produce the symptoms.[12] It does not generally cause damage to the nasal epithelium.[3] The respiratory syncytial virus (RSV), on the other hand, is contracted by direct contact and airborne droplets. It then replicates in the nose and throat before frequently spreading to the lower respiratory tract.[53] RSV does cause epithelium damage.[53] Human parainfluenza virus typically results in inflammation of the nose, throat, and bronchi.[54] In young children when it affects the trachea it may produce the symptoms of croup due to the small size of their airways.[54]
Diagnosis
The distinction between viral upper respiratory tract infections is loosely based on the location of symptoms with the common cold affecting primarily the nose, pharyngitis (the throat), and bronchitis (the lungs).[7] There can be significant overlap and more than one area can be affected.[7] The common cold is frequently defined as nasal inflammation with varying amount of throat inflammation.[55] Self-diagnosis is frequent.[3] Isolation of the viral agent involved is rarely performed,[55] and it is generally not possible to identify the virus type through symptoms.[3]
Prevention
The only useful ways to reduce the spread of cold viruses are physical measures[14] such as hand washing and face masks; in the healthcare environment, gowns and disposable gloves are also used.[14] Isolation or quarantine is not used as the disease is so widespread and symptoms are non-specific. Vaccination has proved difficult as there are many viruses involved and they mutate rapidly.[14] Creation of a broadly effective vaccine is, thus, highly improbable.[56]
Regular hand washing appears to be effective in reducing the transmission of cold viruses, especially among children.[57] Whether the addition of antivirals or antibacterials to normal hand washing provides greater benefit is unknown.[57] Wearing face masks when around people who are infected may be beneficial; however, there is insufficient evidence for maintaining a greater social distance.[57]
It is unclear if zinc supplements affect the likelihood of contracting a cold.[58] Routine vitamin C supplements do not reduce the risk or severity of the common cold, though they may reduce its duration.[59] Gargling with water was found useful in one small trial.[60]
Management

Treatment for the common cold primarily involves medications and other therapies for symptomatic relief.[10] Getting plenty of rest, drinking fluids to maintain hydration, and gargling with warm salt water are reasonable conservative measures.[34] Much of the benefit from symptomatic treatment is, however, attributed to the placebo effect.[61] No medications or herbal remedies have been conclusively demonstrated to shorten the duration of infection.[62]
Symptomatic
Treatments that may help with symptoms include simple pain medication and medications for fevers such as ibuprofen[9] and acetaminophen (paracetamol).[63] It, however, is not clear if acetaminophen helps with symptoms.[64]  It is not known if over the counter cough medications are effective for treating an acute cough.[65] Cough medicines are not recommended for use in children due to a lack of evidence supporting effectiveness and the potential for harm.[66][67] In 2009, Canada restricted the use of over-the-counter cough and cold medication in children six years and under due to concerns regarding risks and unproven benefits.[66] The misuse of dextromethorphan (an over-the-counter cough medicine) has led to its ban in a number of countries.[68] Intranasal corticosteroids have not been found to be useful.[69]
In adults short term use of nasal decongestants may have a small benefit.[70] Antihistamines may improve symptoms in the first day or two; however, there is no longer-term benefit and they have adverse effects such as drowsiness.[71] Other decongestants such as pseudoephedrine appear effective in adults.[72][70] Ipratropium nasal spray may reduce the symptoms of a runny nose but has little effect on stuffiness.[73] The safety and effectiveness of nasal decongestant use in children is unclear.[70]
Due to lack of studies, it is not known whether increased fluid intake improves symptoms or shortens respiratory illness.[74] As of 2017 it is unclear if heated and humidified air via the RhinoTherm device was of any benefit.[75] One study has found chest vapor rub to provide some relief of nocturnal cough, congestion, and sleep difficulty.[76]
Some advise to avoid physical exercise if there are symptoms such as fever, widespread muscle aches or fatigue.[77][78] It is regarded as safe to perform moderate exercise if the symptoms are confined to the head, including runny nose, nasal congestion, sneezing, or a minor sore throat.[77][78]
Antibiotics and antivirals
Antibiotics have no effect against viral infections or against the viruses that cause the common cold.[79] Due to their side effects, antibiotics cause overall harm but are still frequently prescribed.[79][80] Some of the reasons that antibiotics are so commonly prescribed include people's expectations for them, physicians' desire to help, and the difficulty in excluding complications that may be amenable to antibiotics.[81] There are no effective antiviral drugs for the common cold even though some preliminary research has shown benefits.[10][82]
Alternative medicine
While there are many alternative treatments used for the common cold, there is insufficient scientific evidence to support the use of most.[10] As of 2018 there is insufficient evidence to recommend for or against honey.[83] As of 2015, there is tentative evidence to support nasal irrigation.[84]
Zinc supplements may shorten the duration and reduce the severity of symptoms if supplementation begins within 24hours of the onset of symptoms.[8][58][85][86] Some zinc remedies directly applied to the inside of the nose have led to the loss of the sense of smell.[87] A 2017 expert panel, however, found the evidence to be insufficient to recommend zinc's use.[16]
Vitamin C's effect on the common cold, while extensively researched, is disappointing, except in limited circumstances: specifically, individuals exercising vigorously in cold environments.[59][88] There is no firm evidence that Echinacea products provide any meaningful benefit in treating or preventing colds.[89] It is unknown if garlic is effective.[90] A single trial of vitamin D did not find benefit.[91]
Prognosis
The common cold is generally mild and self-limiting with most symptoms generally improving in a week.[7] Half of cases go away in 10days and 90% in 15days.[92] Severe complications, if they occur, are usually in the very old, the very young, or those who are immunosuppressed.[17] Secondary bacterial infections may occur resulting in sinusitis, pharyngitis, or an ear infection.[93] It is estimated that sinusitis occurs in 8% and ear infection in 30% of cases.[94]
Epidemiology
The common cold is the most common human disease[17] and affects people all over the globe.[38] Adults typically have two to three infections annually,[7] and children may have six to ten colds a year (and up to twelve colds a year for school children).[10] Rates of symptomatic infections increase in the elderly due to declining immunity.[46]
Native Americans and Inuit are more likely to be infected with colds and develop complications such as otitis media than Caucasians.[33] This may be explained by issues such as poverty and overcrowding rather than by ethnicity.[33]
History

While the cause of the common cold has only been identified since the 1950s, the disease has been with humanity throughout history.[18] Its symptoms and treatment are described in the Egyptian Ebers papyrus, the oldest existing medical text, written before the 16th century BCE.[96] The name "cold" came into use in the 16th century, due to the similarity between its symptoms and those of exposure to cold weather.[97]
In the United Kingdom, the Common Cold Unit was set up by the Medical Research Council in 1946 and it was where the rhinovirus was discovered in 1956.[98] In the 1970s, the CCU demonstrated that treatment with interferon during the incubation phase of rhinovirus infection protects somewhat against the disease,[99] but no practical treatment could be developed. The unit was closed in 1989, two years after it completed research of zinc gluconate lozenges in the prophylaxis and treatment of rhinovirus colds, the only successful treatment in the history of the unit.[100]
Society and culture
The economic impact of the common cold is not well understood in much of the world.[94] In the United States, the common cold leads to 75–100 million physician visits annually at a conservative cost estimate of $7.7 billion per year. Americans spend $2.9 billion on over-the-counter drugs and another $400 million on prescription medicines for symptom relief.[101] More than one-third of people who saw a doctor received an antibiotic prescription, which has implications for antibiotic resistance.[101] An estimated 22–189 million school days are missed annually due to a cold. As a result, parents missed 126 million workdays to stay home to care for their children. When added to the 150 million workdays missed by employees suffering from a cold, the total economic impact of cold-related work loss exceeds $20 billion per year.[34][101] This accounts for 40% of time lost from work in the United States.[102]
Research directions
Antivirals have been tested for effectiveness in the common cold; as of 2009, none had been both found effective and licensed for use.[82] There are ongoing trials of the anti-viral drug pleconaril which shows promise against picornaviruses as well as trials of BTA-798.[103] The oral form of pleconaril had  safety issues and an aerosol form is being studied.[103] DRACO, a broad-spectrum antiviral therapy, has shown preliminary effectiveness in treating rhinovirus, as well as other infectious viruses.[104]
The genomes for all known human rhinovirus strains have been sequenced.[105]

Works cited

Wikipedia's health care articles can be viewed offline with the Medical Wikipedia app.
 Media related to Common cold at Wikimedia Commons
 at Curlie


List of ''Billboard'' 200 number-one albums of 2018

The highest-selling albums and EPs in the United States are ranked in the Billboard 200, which is published by Billboard magazine. The data are compiled by Nielsen Soundscan based on each album's weekly physical and digital sales, as well as on-demand streaming and digital sales of its individual tracks. In 2018, a total of 41 albums claimed the top position of the chart. Beginning with rapper Eminem's ninth studio album, Revival, issue dated January 3,[1] another one of which, Taylor Swift's sixth studio album, Reputation returned to the top slot a week later after three weeks from the slot, issue dated December 2, 2017.[2]
Drake's Scorpion was the best selling album of 2018. Rapper Post Malone's Beerbongs & Bentleys is the second best-selling album with 3,251,000 total album units, according to Nielsen Music. Of that sum, 374,000 were in traditional album sales.[3] In the meantime, another hip-hop artist Travis Scott's third studio album, Astroworld, sustained the second biggest first-week sales with 537,000 album-equivalent units, of which 270,000 were pure sales.[4]
Chart history

See also
2018 in American music
List of Billboard Hot 100 number-one singles of 2018

List of Billboard 200 number-one albums of 2018

What was the top-selling rock album in 2018?


Symbols of Ireland are marks, images, or objects that represent Ireland. Because Ireland was not partitioned until 1922, many of the symbols of Ireland predate the division into Southern Ireland (later Irish Free State and then Ireland) and Northern Ireland.
Unlike other countries (such as the United States, with the state symbols), Irish and Northern Irish state symbols are rarely defined by official Acts; they are defined by common usage or by various interest groups.
These symbols are seen in official capacities, such as flags, coats of arms, postage stamps, and currency, and in URLs.
They appear less formally as recurring themes in literature, art and folk art, heraldry, monuments, clothing, personal decoration, and as the names of parks, bridges and streets.
Insignia
Coat of arms

The arms of Ireland are a Celtic harp (cláirseach) on an azure field. Northern Ireland uses a much more complex arms; notable features are the Irish elk and the banner of the House of de Burgh.
Flags

The Union Jack has been the flag of the United Kingdom since 1801. It is derived by combining Saint George's Cross (for England), St Andrew's Cross (for Scotland) and Saint Patrick's Saltire. St Patrick's saltire has been used as a symbol of Ireland since the 1780s.
The flag of Ireland is a tricolour of green, white and orange, first flown in 1848. The colours stand for Irish Catholicism, Irish Protestantism, and peace between the two.
Although it was originally intended as a symbol of peace and ecumenism, the tricolour is today seen by Irish unionists as a symbol of Irish republicanism and the Irish Republican Army. Many other flags are suggested as the cross-border flag for Ireland.
The flag of Northern Ireland (Ulster Banner) was used officially between 1953 and 1973.
Motto

The Republic of Ireland has no national motto. Northern Ireland has used the motto Quis separabit?, Latin from Romans 8:35: Quis nos separabit a caritate Christi..., "Who shall separate us from the love of Christ?"
The motto Hiberni Unanimes pro Deo Rege et Patria ("The Irish United for God, King, and Country") was used by Confederate Ireland.[1]
The Society of United Irishmen (1780s–1798) used Equality: it is new strung and shall be heard as a motto.[2]
Flora and fauna
Bird

The northern lapwing (Vanellus vanellus) was declared the Republic of Ireland's national bird by a committee of the Irish Wildlife Conservancy in 1990.[3]
Northern Ireland does not have an official national bird, but the Eurasian oystercatcher (Haematopus ostralegus) was unofficially selected in 1961.[4][5]
The Irish Examiner has put the rook (Corvus frugilegus) forward as a possible national bird, due to their "wild hardiness, spirit, and resilience, in the face of all difficulties, and their ability to cope with style and a bit of craic, with anything that the world throws at them."[6]
In 2016 Niall Hatch of BirdWatch Ireland listed ten possible national birds: European robin, peregrine falcon, common house martin, Eurasian curlew, roseate tern, barn owl, common swift, Bohemian waxwing, Eurasian blackcap, northern pintail.[7][8]
Dog breed
The Irish wolfhound is generally considered Ireland's national breed, although some, including Michael Collins, have put forward the Kerry Blue Terrier as an alternative.[9][10][11][12]
Fish
The northern pike (Esox lucius) is Northern Ireland's national fish.[13]
"Coarse fish" have been described as the Republic of Ireland's national fish; this category includes the likes of bream, perch and carp.[14]
Domesticated mammal
Sheep and cattle are widely used as symbols of Ireland. Sheep feature commonly in souvenirs sold to tourists,[15][16] while cattle have been important in Irish mythology and folklore for centuries, as sources of milk, cheese, leather and beef, as well as social status. Cattle feature in the Táin Bó Cúailnge and Táin Bó Flidhais, and the tale of Glas Gaibhnenn.[17][18]
Wild mammal
The national land mammal/animal is unclear. The Irish hare (Lepus timidus) has been described as a national animal,[19] as has the red deer (Cervus elaphus).[20]
Plant and flower

The national plant is the shamrock (Trifolium dubium or Trifolium repens).
Fuchsia magellanica 'Riccartonii' (hummingbird fuchsia, hardy fuchsia; in Irish deora Dé, "tears of God")[21] has sometimes been described as the national flower, despite not being a native plant.[22][23]
Flax (Linum usitatissimum) is widely used as an emblem of Northern Ireland, such as in the emblem of the Northern Ireland Assembly, referencing Ulster's linen industry.
Tree

The national tree of the Republic is the sessile oak (Quercus petraea), declared as such in 1990 by Taoiseach Charlie Haughey.[24][25]
Other accounts give the strawberry tree (Arbutus unedo) as the national tree.[26]
Culture
Dance
Irish dance is Ireland's national dance.
Instrument
The Celtic harp (cláirseach), great Irish warpipes (píob mhór) and uilleann pipes have all been described as national musical instruments.
National anthem
"Amhrán na bhFiann" ("The Soldiers' Song") is the national anthem of the Republic of Ireland. Written in English by Peadar Kearney and set to music by Patrick Heeney in 1907, it was translated to Irish by Liam Ó Rinn in 1923; the Irish-language version is considered the official anthem. "God Save Ireland" was used from the 1870s until independence.
"God Save The Queen/King" is commonly used by Northern Ireland, as is the Londonderry Air.
"Ireland's Call" has been used by the Ireland national rugby union team and others since 1995.
Personifications
There are several national personifications, all female:
Hibernia, an attractive, vulnerable girl
Erin
Kathleen Ní Houlihan
The Sean-Bhean bhocht: a poor old woman who needs the help of young Irish men willing to fight and die to free Ireland
Banba, Fódla and Ériu, three goddesses
Róisín Dubh (Dark Little Rose)
Poet
Thomas Moore (1779–1852) and W. B. Yeats (1865–1939) are both considered the national poet.
Other
Drink
Irish breakfast tea is described as the national (non-alcoholic) drink.[27][28]
Among alcoholic drinks, Irish whiskey is the national spirit,  with Jameson Irish Whiskey and Tullamore Dew the main whiskey brands.[29]
Poitín, made from potatoes and with an alcohol content of up to 90% ABV, is the traditional Irish moonshine,[30] brewed illegally since 1661, when a duty was placed on whiskey and other spirits were made illegal. According to one guidebook, "If you see a product labelled "poteen" in an airport or a bar, it's simply a white duty-paid Irish schnapps. By definition, poteen is illegal and can't be sold."[31]
Stout, a dark beer made using roasted oats or barley, is the national beer, with Guinness heavily promoted as a symbol of Ireland.[32]
[33][32][34][33]
Stout was largely unknown in Ireland when Arthur Guinness opened his brewery in 1759; the traditional beer was Irish red ale.[35][36]
Irish coffee, made with coffee, whiskey, sugar and cream and invented in Foynes in 1943, has been described as the national cocktail.[37]
Food

The national dish of Ireland is the Irish stew; other national dishes include the full Irish breakfast/Ulster fry and bacon and cabbage.[38]
Gemstone
The national gemstone is Connemara / Conamara marble.[39]
Patron saint
Saint Patrick, a 5th-century bishop born in Britain, is Ireland's patron saint.
Sport
Gaelic games constitute Ireland's national sports: Gaelic football, ladies' Gaelic football, hurling, camogie, rounders and Gaelic handball.
Irish road bowling (bullets) is another Irish sport.
Weapon
Gerald of Wales saw the battle axe as Ireland's national weapon.[40]

National symbols of Ireland, the Republic of Ireland and Northern Ireland