Microphones used in recording speech
Table of contents:
What are your goals?
Before you read the rest of this article, you need to try to clearly define your goals. Let me give you a few examples:
- conversational analysis
- acoustic analysis
- analysis and re-synthesis
- broadcast quality recordings
- archival quality recordings
- oral history
- language documentation
- distribution on optical media and/or the Internet
- distribution at a museum kiosk
Each of these goals requires a different approach. The choice of microphone and microphone technique will be crucial in reaching these goals. Below you will find some general principles governing microphone design, and these general principles should give you enough information to try to narrow down your microphone choices. I recommend that you follow the following guidelines:
- Decide what your goals are.
- Research your typical recording environment (e.g., a remote location, a library, a sound-proof booth, etc.).
- Decide whether you need a condenser or dynamic microphone.
- Decide what polar patter you need.
- Match the microphone to the pre-amplifier or recorder (more on this here).
- Decide on the brand (e.g., availability, price, warranty, etc.).
- Go to the store to see the microphone before you buy it (if you can find it in a store, of course).
- Buy it and try it before your return policy expires. If you don't like it, return it.
The dynamic microphone has a diaphragm that consists of Mylar plastic that has a finely wrapped coil of wire (so-called voice coil) attached to its inner face. This coil is suspended within a strong magnetic field. Whenever a sound wave hits the diaphragm, the coil is displaced in proportion to the amplitude of the wave, causing the coil to cut across the lines of magnetic flux supplied by the permanent magnet. Since the mass of the diaphragm and the coil is quite large, compared to the pressure changes in the sound wave, the dynamic microphone may not respond well to sharp transient sounds and it may fail to record minute changes in voice intensity. This does not mean that dynamic microphones should not be used for speech recording. On the contrary, there are several low-cost, rugged microphones, such as Shure SM58 or Shure SM48 (once recommended by Kay Elemetrics), that can be used successfully for many speech recording applications. Dynamic microphones do not require any external power supply, which is an important advantage. However, they typicically require pre-amplifiers (or recorders) with high-gain and low-noise. This limits the number of compatible portable field recorders. See this post for more details.
The condenser microphone works on a different principle. A thin plastic diaphragm coated on one side with
gold or nickel is placed at a close distance from a stationary backplate. Once a polarizing voltage (from a 48 V phantom power supply) is applied to these plates, the two surfaces create capacitance that varies as the diaphragm moves in response to a sound wave. Since the diaphragm is very light, the response of the condenser microphone is very accurate, often producing a recording that is extremely rich in both frequency and dynamic response.
Condenser microphones are usually more fragile than dynamic microphones and require a 48 V (the exact required voltage may vary) phantom power supply. Some of them can use battery packs, but some rely on an external power source. This makes the condensern microphone a bit more cumbersome to use in the field, though several DAT and HDD recorders have an on-board phantom power supply.
Usually, high-quality speech recordings require the sound source to be fairly close to the microphones diaphragm. This may trigger a so-called proximity effect. Proximity effect is the increase in the low-frequency sensitivity of a microphone when the sound source is close to it. This is particularly true of cardioid, directional microphones. To counter that, most high-end directional microphones use a low-frequency roll-off filter to restore the response to its flat, natural balance. Some microphones have a user-selectable switch to control the filter. The proximity effect may be responsible for speech spectra showing emphasis in the low-frequency range, around the first and second harmonics (see below).
If your research requires detailed acoustic analysis of low-frequency sounds (e.g., F0, vowel nasalization, spectral tilt, voice quality, etc.) it is imperative that you use a microphone that does not cause proximity effect. The figure below shows frequency repsponse graphs of Shure Beta 87a (solid and dashed lines) and Earthworks M30 (dotted line). Note the low-end "bump" of the Shure microphone when it is placed close to the sound source. The M30, on the other hand, shows no such bias.
The microphones polar pattern should play a crucial role in choosing a microphone for a specific recording application. The polar pattern is a plot of the sensitivity of a microphone as a function of the angle around that device. There are several common polar pattern types used in microphones today. The omnidirectional microphone records sound equally from all directions. Such microphones are most commonly used as built-in or lavalier types. They seem to be very good for recording interviews, though their 360-degree pick-up range introduces too much noise to the signal for it to be used reliably in acoustic analysis, when the microphone is used in a noisy environment (e.g., respondent's living room).
Cardioid polar patter of a Shure Beta 87a microphone
The cardioid (heart-shaped) pattern is most sensitive to sounds coming from the front. It is 6 dB less sensitive to sounds from 90 degrees to the sides, and, in theory, is completely insensitive to sounds coming from the rear. The most important attribute of a cardioid (directional) microphone is its ability to discriminate between direct sounds (coming from the direction in which it is pointed) and reverberant, unwanted sounds from all other directions. This type of polar pattern usually produces signals that are substantially less noisy than those captured with an omnidirectional microphone
Polar patterns and proximity effect
Omnidirectional microphones are generally free from proximity effect. If you are recording speech in a sound-proof environment, it might be best to use a quality omnidirectional microphone, especially one in a headset configuration, such as the Shure Beta 53 or Beyerdynamic Opus 55 Mk 2. When selecting an omnidirectional microphone, make sure it has a flat response in the low end. It is not uncommon for manufacturers to roll-off (attenuate) low frequencies (below 150 Hz) on many vocal and interview microphones.
Please, refer to this page for specific equipment recommendations.