For anyone new to the world of acoustics, there is a multitude of terms, coefficients and numbers that are thrown around. This flood of information can seem intimidating, especially to beginners. In this series, acoustician Cameron Girard of Acoustics First® hopes to help you distinguish between what’s useful and what’s not.
Part 3: Perception of Volume
Our ears are wonderful and intricate tools that many of us take for granted. It is very important to understand the idiosyncrasies of our hearing when considering the effect a certain acoustic treatment will make. For starters, let’s take a look at how our ears interpret volume.
Volume (Sound Intensity)
Sound levels (i.e. how loud something is) are typically expressed in decibels (dB). Human hearing ranges from 0dB (threshold of hearing) to 130dB (threshold of pain). The following chart displays common sound sources and their typical dB level.
Do two candles really burn twice as bright?
If one trombonist plays at 70dB, how much louder would it be if another trombonist started playing at 70dB? One might assume that the two trombonists combined would play at 140 dB, but this is not the case. Since decibels are logarithmic values, they cannot be combined by normal algebraic addition. When two sources at the same level play, 3dB should be added to the value to find the combined sound level. So in adding another trombonist, you would really only increase the level to 73dB, a much smaller jump than expected.
“Doubling” the amount of players will double the acoustic power, but what do we actually hear? The loudness perception table shown below displays how these decibel changes are actually perceived by the listener.
Loudness Perception Table
|Change of Level||Approx. Perceived Difference||Volume Gain Factor||Acoustic Power Gain Factor|
|+10dB||“twice as loud”||2.000||10.000|
|– 3dB||“noticeably quieter”||0.812||0.500|
|– 6dB||“significantly less loud/noisy”||0.660||0.250|
|– 10dB||“half as loud”||0.500||0.100|
*Chart Courtesy of David Eagan’s Architectural Acoustics (New York: McGraw-Hill, 1988),
As you can see, doubling the acoustic power (a change of 3dB) would be “noticeable” but not “significant”. It would take a jump of 10dB to make something sound twice as loud. Keep this chart in mind when reviewing acoustic predictions, particularly those that that pertain to noise reduction/control and sound isolation.