How to choose a Microphone


The microphone is probably the most important tool we use when recording sound. Plugging it into the best pre-amplifier will optimise the quality of the signal but no amount of processing will fix the wrong sound coming from your microphone. The device for turning movements in the air into an electrical signal is a creative instrument and should be regarded as part of the performance.
Before we think about pressing the record button we must consider the characteristics of the sound source and the way it behaves in the acoustic environment of your recording space. It is equally important to also know what you don’t want to record.
We use the microphone as an extension of the ear, placing the mic where we can hear the best sound. Our ear drum moves with the changing pressure of the passing sound wave. The mechanism inside our inner ear triggers nerves that we perceive as sound. The way the brain interprets the signal from the inner ear is where things get wonderfully complicated. In a similar process the vibration of a thin membrane in the mic (the diaphragm) is turned into electrical energy – a constantly changing voltage that is analogous to the sound wave. The voltage from the microphone is not intelligent so we have to carefully control how it receives the sound in the first place.
Next I will try to translate some of the technical terms and help to explain why you should place the right mic in the right place, capturing the sound of the source that will fit perfectly in the final mix.
Frequency Response. We can perceive a range of tones that vibrate with frequencies of appoximately 20Hz – 20KHz (Hz = cycles per second). It is useful to divide this range into different frequency bands: Bass <250Hz, Lo-Mid 250-1KHz, Mid 1-4KHz, Hi-Mid 4-10KHz, Hi 10KHz and higher. A microphone may reproduce all tones within our range of hearing but each one will give a unique balance of these frequencies. The spoken voice will have a fundamental tone in the ‘low-mids’ with the intelligible frequencies in the ‘mids’ and ‘hi-mids’. The most appropriate mic should therefore have an even balance of lo-mid tones and a lift or presence in the hi-mids. A ‘high pass filter’ or ‘lo roll off’ switch is often available to remove unnecessary bass. The key is to sculpt the shape of the sound at the earliest opportunity so we don’t have to fix it later.
Directivity / Polar Response. Most mics will be uni-directional (cardioid) so you can point it toward the source and reject sound coming from behind (180 degrees off axis). If you want to get the sound of the space as well, then use an omni-directional mic that responds equally from all directions. A ‘figure of 8’ will have maximum rejection at the sides (90 degrees off axis) but has an equal sensitivity at the front and the back of the mic. Using the right pattern is all about minimising unwanted sound.
When recording many sources at the same time we may wish to isolate each sound so that we can adjust the balance in the mix. With a single source we usually want to eliminate the reflections from the room so that we can control the sense of space by adding reverb during mixing.
Sensitivity / Self-Noise. When recording a sound that is extremely loud, check how much sound pressure that the mic can handle. A ceiling of max 130 dB SPL would accept a most deafening performance but an even higher rating allows more headroom and guarantees no distortion at extreme levels. However if the source is very quiet then turning your pre-amp up full will create its own noise and distortion. Your specialist microphone may need to generate upwards of 40mV/Pa (volts per force of sound pressure) and produce very little noise of its own. This ‘self-noise’ will be relative to an ambient sound level of less than 20 dB SPL.
So far we’ve talked about specific properties listed in the manual of the microphone. The numbers and graphs in the manual are unfortunately open to interpretation by the manufacturer. These details may be useful in helping us choose the magical solution for the demands of the recording but ultimately we need to listen to and compare from a selection of the best candidates. In [the separate panels] I have listed some of the most commonly used mics for particular applications. These will provide a good reference for testing with other mics offering alternative flavours.
If, like most creative types, we are too impatient to read product manuals then let’s break it down into just two main types and their properties:
Dynamic microphones. The diaphragm is attached to a coiled wire that moves through the magnetic field of a permanent magnet. An electrical current is induced onto the wire as the diaphragm vibrates in sympathy with the sound pressure. The mechanism is heavy so these mics lack sensitivity and have a relatively poor high frequency response. These are ideal for close miking percussion and are more likely to survive the odd strike from a drum stick. They are less prone to malfunction caused by dirt, dust, shock and humidity.
Condenser microphones. The diaphragm is one half of a capacitor. The oscillation of this membrane creates a corresponding change in the capacitance of the circuit and an analogous voltage is generated across it. This voltage is amplified inside the microphone and therefore needs to be powered from an external source. A biasing voltage is most often supplied from the pre-amp of the mixer and is called ‘phantom power’. These mics are more sensitive and have a better high frequency response. The disadvantages are a lower tolerance to shock and humidity and the necessity of the external phantom power.
Every experienced engineer will have his or her favourites. A small collection of good microphones should cover a whole range of different applications with minimal compromises. A trusted retailer will usually have a good selection for hire and be able to recommend something that fits the budget.
As with every aspect of audio technology the same golden rule applies – use your ears! If you listen with due diligence then you can trust your own perception and make the world sound like a better place.
Neumann U87
Neumann U87
Shure SM57
Shure SM57
AKG D112
AKG D112

Tags: iss025 | microphone | kmr audio | audio | frequency response | polar response | dynamic microphone | condenser microphone | shure | neumann | akg | electro voice | N/A
Contributing Author N/A

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