Microphone Basics: Transducers, Polar Patterns, & Frequency Response
Microphone Basics: Transducers, Polar Patterns, & Frequency Response
There is no such thing as a one size fits all microphone. If such a device were to exist, it would have to tick a lot of boxes; so many, that it would practically have to morph from one form to another. There are just too many variables in application to produce one unit that can deliver in every scenario. Therefore, in the absence of a magical microphone to rule them all, it is important to understand a few basics that will aid in your selection and operation of the right mic for your application. The following article will cover what you need to know.
There are 3 main technical points that are fundamental when it comes to microphone selection: 1. Transducer type, 2. Directionality (or polar pattern), and 3. Frequency response.These 3 things play a huge role in the sound and suitability of your microphone for any given application.
(Pictured above: Shure PGA57 - dynamic mic)
Understanding Microphone Transducers
A microphone is essentially a transducer, which converts acoustic energy into electrical energy. The type of transducer is defined by the operating principle, with the two primary configurations being dynamic and the condenser mic elements.
Dynamic mics come in two distinct varieties: moving-coil and ribbon. Here's how they work:
Moving coil dynamic microphones are made up of a diaphragm, voice coil, and magnet that form a sound driven electrical generator. Essentially, as sound waves hit the diaphragm, the attached voice coil moves within the magnetic field to generate an electrical signal that corresponds to the original sound waves. This design makes for a very simple and robust microphone that can handle high sound pressure levels without distorting.
Ribbon microphones are a variation on the dynamic microphone operating principle that consist of a thin piece of metal – typically corrugated aluminium – suspended between two magnetic pole pieces. As the piece of metal vibrates in response to a sound wave, the magnetic lines of force are broken – generating an electrical voltage. The output of ribbon microphones tends to be quite low and depending on the gain of your mixer or recording device, additional pre-amplification may be necessary.
It's important to note that ribbon microphones are less durable than moving-coil dynamic microphones. The ribbon element is typically no more than a few microns thick and can be deformed by a strong blast of air, or by blowing into the microphone. Also, phantom power applied to a ribbon microphone could be harmful.
Ribbon microphones are highly regarded in studio recording for their "warmth" and wide frequency response. Their advantage lies in the low mass of the ribbon, which enables better response to rapid transients thanks to less inertia. Ribbon microphones, therefore, have a more linear frequency response than moving-coil dynamic mics – making them a popular alternative to condenser microphones when a warmer tone is desirable.
Condenser microphone elements use a conductive diaphragm and an electrically charged backplate to form a sound-sensitive capacitor. As the diaphragm vibrates in response to sound waves, the distance between the mic and backplate fluctuates within the electrical field to create the signal. In order to use this signal, all condensers have active electronic circuitry; often referred to as the 'preamp'. The inclusion of a preamp means that condenser microphones require phantom power or a battery to operate. (For a detailed explanation of phantom power, see our previous post.)
Condenser mic designs allow for smaller mic elements, higher sensitivity and a smooth response across a very wide frequency range. The main limitations of a condenser microphone relate to its electronics. These circuits can handle a specified maximum signal level from the condenser element; therefore, a condenser mic has a maximum sound level before its output starts to distort. Some condensers have switchable pads or attenuators between the element and the electronics to allow them to handle higher sound levels.
Directionality or Polar-Pattern
Directionality refers to the sensitivity relative to the direction or angle of sound arriving at the microphone. Directionality is usually plotted on a graph referred to as a polar pattern. A polar pattern graph shows the variation in sensitivity as you move 360 degrees around the microphone.
There are a number of different directional patterns available. The three most common patterns are omnidirectional, unidirectional, and bidirectional.
Omnidirectional microphones have equal response at all angles. Its coverage or pickup angle is a full 360 degrees, which comes with a number of distinct advantages or disadvantages depending on your application. On the positive side, omnidirectional mics have a very natural and open sound that is perfect for capturing organic sounds, such as an acoustic guitar. On the other hand, Omni mics pick up more room ambience, which can be desirable so long as you have great sounding room acoustics. In any case, the balance of direct and ambient sound can be controlled by varying the distance of the microphone from the instrument.
In a live scenario, omnidirectional microphones will leave you very susceptible to feedback; particularly when using stage monitors. To cut a long story short, if you want greater control of the sound entering your microphone, a unidirectional microphone is more likely to fit the bill.
Unidirectional microphones are most sensitive to sound arriving from one particular direction. The most common type is a cardioid (heart-shaped) response. This polar pattern has full sensitivity at 0 degrees (on-axis) and is least sensitive at 180 degrees (off-axis). Unidirectional microphones are used to isolate the desired on-axis sound from unwanted off-axis sound. To further emphasise this point, the cardioid mic picks up around one-third as much ambient sound as an Omni. By pointing the microphone directly at your desired sound source and away from the undesired room or ambient noise it is possible to reduce bleed significantly.
(Pictured above: KSM44 with switchable polar pattern)
Other variants on the unidirectional polar pattern include supercardioid and hypercardioid options. Both patterns offer narrower front pickup angles than the cardioid – 115 degrees for the supercardioid and 105 degrees for the hypercardioid – alongside greater rejection of ambient sound. Additionally, while the cardioid is least sensitive at the rear (180 degrees off-axis), the supercardioid is least sensitive at 125 degrees and the hypercardioid at 110 degrees. When placed properly they can provide more 'focused' pickup than the cardioid pattern, but they also have less rejection at the rear. If you're using either of these polar patterns on stage with wedge monitors, it's important make sure you avoid placing the wedges directly behind the mic in this instance. Instead, place them either side at the mics least sensitive angle.
Bidirectional microphones have full response at both 0 degrees (front) and at 180 degrees (back). They are least sensitive at the sides. This mic can be used to your benefit when picking up two sound sources such as two vocalists facing each other; however, in most cases, it's typical just to use one side.
Other directional-related microphone characteristics:
Off-axis colouration – A microphone's frequency response may not be uniform at all angles. Typically, high frequencies are most affected, which may result in an unnatural sound for off-axis instruments or room ambience.
Proximity effect – For unidirectional microphone types, bass response increases as the microphone is moved closer to the sound source. When micing close with unidirectional microphones (less than 1 foot), be aware of proximity effect; it may help to roll off the bass until you obtain a more natural sound. You can 1. roll off low frequencies at the mixer, 2. use a microphone with a bass roll-off switch, or 3. use an omnidirectional microphone, which does not exhibit proximity effect.
Last but not least is frequency response. This term refers to the variation in output level or sensitivity of a microphone over its useable range from lowest to the highest frequency.
Virtually all microphone manufacturers will list the frequency response of their microphones as a range, for example, 20 – 20,000Hz. This range is usually illustrated with a graph that indicates relative amplitude at each frequency. Two main categories apply here: 1) flat frequency responses and 2) shaped frequency responses.
A microphone with equal response at all frequencies is said to have a 'flat' frequency response. These microphones typically have a wide frequency range and tend to be used to reproduce sound sources without colouring the original source. These characteristics are desirable when capturing instruments such as acoustic guitars or pianos and for distant micing techniques.
A microphone with peaks or dips at certain frequencies is said to have a 'shaped' response. This response is designed to enhance a frequency range that is specific to a given sound source. For instance, a microphone may have a peak in the 2-10Khz range to enhance the intelligibility or presence of vocals. Another example includes kick drum microphones, which often have an increased bass response, combined with a scooped mid and presence peak. Shaped frequency responses allow us to focus on desirable frequencies for a number of given applications.
(Pictured above: frequency response chart for Beta52 kick drum mic)
Also worth considering, is that although dynamic, condenser, and ribbon microphones may have similar published frequency response specifications their sound qualities can be quite different. A primary aspect of this difference is in their transient response. Essentially, condenser and ribbon microphones will typically sound more natural as the diaphragm or ribbon can respond to sound faster, and this results in higher sensitivity, alongside greater high-frequency detail.
The Bottom Line
So there you have it; your crash course in microphone transducers, polar-patterns, and frequency response is complete. Mastering the basics at this stage in the signal chain is essential to getting a great end result. If you only take one thing away from this article, it should be that inputs are more important than outputs. Failing to get things right at the start of the signal chain will only result in a headache further down the line. Thankfully, armed with the fundamentals, you can now make a more informed choice to ensure your signal – whether it be for live or studio purposes – gets off on the right foot.
About the Author
About the Author
Marc forms part of our Pro Audio team at Shure UK and specialises in Digital Marketing. He also holds a BSc First Class Hons Degree in Music Technology. When not at work he enjoys playing the guitar, producing music, and dabbling in DIY (preferably with a good craft beer or two).