Microphones: Reading the spec sheet
Are you planning to record an interview, lecture, teleseminar, conference, or some other event involving speech? It’s been said that a recording can only be as good as the microphone that’s used – and the statement is absolutely true. Choosing the right microphone is as crucial as choosing the right recording device.
The choice of microphone for a recording depends on several factors such as location (indoors/outdoors), number of speakers, distance of the microphone from the speaker/s, etc. In this post we’ll look at some key properties of a microphone that should be considered before making a purchase.
Every microphone has diaphragm i.e. a thin membrane of plastic, aluminium, or paper located in the head of the microphone that vibrates when hit by sound waves. Vibration of the diaphragm leads to movement in other parts of the microphone that generate electrical signals (or audio signals) that can be recorded. We’ll discuss the ‘other parts’ and their movement in a minute.
There are 3 types of diaphragms – large, medium, and small – that differ in diameter.
Large diaphragms have diameters ranging from 3/4 inch upto to 1 inch and sometimes more. Their large surface area and comparatively flexible surface makes them more sensitive to sound.
Because of their construct, they also have lesser ‘self-noise’ i.e. noise added to the recording by the microphone’s internal sounds. But while the general sound quality is quite decent, they don’t capture higher frequencies very well.
Small diaphragms (with diameters 5/8 inch or less) are lighter and therefore more responsive to higher frequencies. When sound waves strike a diaphragm from the side, they take time to travel across it’s body. If the diaphragm is large, the time taken to travel across will be longer and may result in loss of vibrations leading to reduced quality.
That’s why large diaphragms work best when the sound waves strike from directly in front rather than the sides. With small diaphragms the loss of vibrations is negligible and therefore the sound quality is sharper no matter which direction the sound is coming from.
A transducer is a device that converts one form of energy to another. Technically speaking, microphones are transducers that convert acoustic energy (sound waves) to electrical energy.
Based on the kind of transducer used, there are 4main types of microphones available – Dynamic, Condenser, Ribbon, and Crystal. Dynamic and Condenser are the most widely used.
These simple, rugged microphones work on the principle of electromagnetism. When sound waves hit the the diaphragm of a dynamic microphone, the vibration makes a coil of wire (or a ribbon of metallic foil) move back and forth against a fixed magnet. This creates electric current or audio signals that can be recorded.
Dynamic microphones are simply constructed and sturdy. They don’t require an external battery or other power source to operate and compartively less expensive. They can handle high sound pressure levels without distortion but can’t capture high frequencies very well due to their mechanical construct. Ideal for rough use in live performances and on location recordings.
These sensitive microphones work on the principle of capacitance (the ability of a body to hold electrical charge). Condenser (aka capacitor) microphones consist of two plates formed by a thin conductive diaphragm and a metallic back plate – these together form a capacitor.
A fixed charge is placed across this capacitor through an external power source such as a battery or a phantom power supply. When the diaphragm vibrates under the force of sound waves, there are fluctuations in the capacitance leading to changes in voltage between the plates (voltage being inversely propotional to capacitance and capacitance being inversely proportional to the distance between the plates). These changes in voltage can then be recorded.
Condenser microphones are sensitive to a wider range of frequencies and can pick up softer sounds. They also have a flatter frequency response. The downside is that they are fragile and therefore unsuitable for rough use. They are also quite expensive. Best for in studio or home recordings.
A microphone can pick up sounds from all directions or a specific direction depending on its technical built. This sensitivity to sound from one or more directions is called directionality. Based on this property, there are 3 main types of microphones available in the market –
Directional or Unidirectional Microphones
These microphones pick up sounds from all directions and are generally not recommended for recording speech. That said, they’re a reasonably good choice when recording the voices of a large number of people together – such as a conference where several people are seated around a table.
Because of the simplicity of their design, omnidirectional microphones are considered more rugged and produce a more ‘natural’ sound recording (as compared to bidirectional/unidirectional microphones). They are also less sensitive to wind/pop noises, resulting in clearer recordings.
The drawback is that because they record sounds from all directions, a lot of unwanted sounds such as shifting chairs, doors opening, side conversations, vehicles passing by, etc. can also get recorded.
When using an omnidirectional microphone in an indoors setting, it’s a good idea choose a quiet room and lay down some ground rules for the participants before beginning recording. When outdoors, the microphone should be placed as close to the speaker as possible.
Directional (aka Unidirectional) Microphones
These microphones pick up sounds from a single direction – though single is a relative term. There are 3 major categories of directional (or unidirectional) microphones –
These come in two varieties – the long shotgun and the short shotgun microphone. These microphones are best for outdoor recordings because they have a very narrow ‘angle of acceptance’ or ‘pickup pattern’ or simply put, the area from which they pick up sound.
This means they can pick up the particular sound they’re directly towards and reject all other surrounding noise to produce a very clear recording.
Long shotgun microphones provide the highest level of directionality and pick up sound from only the person/object that they’re pointed at. This leads to a very high quality of recording and makes them ideal for noisy locations.
The downside of using these microphones is that since they’re very long (and often used with a boom), they normally need a second person to operate. Also, the microphone must consistently be pointed directly towards the speaker – any movement away from the speaker will result in an interruption of the flow of sound to the microphone and thereby impact the recording.
Short shotgun microphones are, well, shorter. Their properties are similar to the long shotgun microphones, the only difference lying in the slightly wider ‘angle of acceptance’ of the short shotgun.
They are more portable as well; this makes them the microphones of choice for outdoor recordings that require a degree of mobility – such as interviews conducted on the street.
The high level of directionality of shotgun microphones is especially useful when the subject is not close to the microphone. The more the distance between the speaker and the microphone, the more directional the microphone needs to be.
Cardioid microphones are the ones most commonly used for speech recording. That’s because they are directional like the shotgun microphones yet offer flexibility in terms of the ‘angle of acceptance’ as well as size and portability. They come in 3 varities – cardioid, hypercardioid, and supercardioid.
Cardioid microphones are considered best for recordings that involve a large number of speaker (as long as they’re seated within the pickup range and direction of the microphone).
They pick up sound mainly from the direction that they’re pointed in, some from the far sides of the microphone, and very little from behind the microphone. Since they’re unidirectional, they eliminate a lot of surrounding noise and produce high quality recordings.
Hypercardioid microphones are a step higher than the cardioids in terms of directionality. They have a narrower angle of acceptance and are very effective in eliminating noise from the rear of the microphone.
The Supercardioid microphones on the other hand, eliminate more of the sounds from the sides of the microphone than from the rear.
The hypercardioid and the supercardioid microphones offer a high level of directionality (though lesser than the shotgun microphones) along with portability.
Also, they don’t need to be pointed directly towards the speaker and therefore offer more flexibility of movement both for the microphone and the speaker.
As the name suggests, these microphones pick up sounds from two opposite directions and reject sound from other sides. Also known as figure-of-eight microphones, these microphones are useful for recording two-person interviews where two people are seated on the opposite sides of a table. Bidirectional microphones are not very commonly used because of their limited application.
Some microphones come with multiple options that allow users to switch from let’s say cardioid to omnidirectional mode when required.
This property relates to the accuracy with which a microphone reproduces a range of sound frequencies. For example, a frequency reponse of 50-14Khz +/-3db indicates that the microphone covers frequencies between 50-14,000hz with a variation in volume not exceeding 3 decibles.
A microphone with a poor response to high frequencies will produce dull sounds while one with a poor response to low frequencies will sound tinny.
In an ideal world, microphones would have a flat frequency response, i.e., the sound output will be exactly the same as the original sound input. However, this is never the case because no microphone is perfectly flat. A variation of +/-3db is common.
Every circuit offers some level of resistance to alternating current (or audio signals) know as impedence. The lower the impedence of a microphone the better the quality of the recording. Most microphones are low-impedence, but when reading a spec sheet, here’s the general description to go by –
Less than 600Ω – Low impedence
600Ω – 10,000Ω – Medium impedence
Greater than 10,000Ω – High impedence
High impedence microphones are cheaper but their audio quality over long distance is not so great – leading to loss of high frequencies over long cables. Low impedence microphones are generally considered better.
Okay, so that was about the basic properties of a microphone. While this post is not exhaustive, it should make it easier for you to read a ‘spec sheet’ when you go out to buy a microphone. In the next post, we’ll cover some common types of microphones.
In the meanwhile, happy recording!