In terms of popular music, the sound and style of the vocals has always been the most discriminating factor on record or in a concert situation. The human voice with its unsurpassed flexibility of expression and its ability to contribute words to music, has always had an important place in the works of composers. But it was the microphone (for broadcast and, later on, sound reinforcement use) that freed the singer of the necessity of a strong and well-trained voice to stand up against an orchestra and which led the way to today's highly personalised style of singing. Interestingly, more microphones have been designed for vocal application than for any instrument.

Vocal microphones have to fulfill a wide range of requirements: tailored frequency response, proximity effect, wind and pop screen, good directional characteristics for feedback safety, ruggedness and suppression of structure-borne vibrations, as well as hum from stray magnetic fields. These mics are built to perform in a rough stage act, but some are also excellent mics for the home recording studio.

Human Voice

The human voice covers a frequency range from 85 Hz to about 9 kHz (males) and 170 to about 10 kHz (females). Fundamental frequencies are up to 1000 Hz, the rest consists of harmonics.

Normally, consonants deliver sound over a wide range, the 's' sometimes reaching to above 10 kHz.

Bass frequencies do not contribute to the intelligibility of the sound. For optimum transmission of speech, a bass cut from 200 Hz or 300 Hz down is usually provided. Many vocal microphones improve intelligibility by lifting the presence between 3 kHz and 7 kHz. In a musical context, this will also help to push a voice through a mix.

The sound generated by the vocal chords is amplified by resonances in the throat, mouth, and nose, and each vowel has a particular frequency range, within which all partial tones are amplified by resonance. The particular frequency range is determined by the position of our tongue, lips, palate, and cheeks. It can be simulated electronically by amplifying a certain frequency range and filtering out the rest - that's what a wah-wah pedal does, for example.

Vocal Microphones

The diagram we should first look at, in order to get an idea of a microphone's sound characteristics, is the one showing the frequency response. Most vocal microphones are easy to recognise because of their 'presence' lift. Microphones for paging purposes display a somewhat limited frequency response, especially cutting in the bass region. (Figure 1.) Reading a frequency response diagram is fairly easy: the peaks and dips tell you what frequencies will be emphasised or attenuated; the horizontal axis gives the frequency in Hz, the vertical axis the mic sensitivity in dB. (As a guide; 6-10 dB up is perceived as twice as loud). The dB readings as such do not show the microphone's actual sensitivity at a given frequency (measurement equipment is always adjusted to keep the curve nicely in the middle) but they do show sensitivity variations across the frequency spectrum.

For years, engineers who designed microphones believed in a 'flat' frequency response (no peaks, no dips, absolutely linear) as the ultimate goal. But it's a matter of taste. Judged by the ear, microphones with an uneven response curve are often preferable especially in the higher frequency range, where nonlinearity seems to be acceptable for a good sound. Also, microphones with the same frequency response won't sound the same: there's more to the sound of a mic, especially with dynamic types, than its stated frequency response.

Proximity Effect

Everyone who has been singing into a microphone for some time is familiar with it: as you come close, it will sound fuller, more bass-heavy. This over-emphasised pick-up of bass frequencies (below 1 kHz) at close working distances is called proximity effect.

However, not all microphones show a proximity effect: omnidirectional microphones and specific dynamic cardioid designs (two-way microphones) exhibit no proximity effect.

The proximity effect is often used by vocalists to add more bottom end to their voice by coming close to the microphone. However, as the voice becomes fuller, intelligibility may get lost because of the bass rise. Also, for harmony singing with one mic to each singer, voices tend to blend less nicely if the physical distance between the mic and singers is too small. Most vocal microphones feature a bass roll-off (measured at the regular 1 metre distance) in order to compensate for the bass rise at close distances and ensure tonal balance. Some incorporate an additional switchable bass-cut. Figure 2 shows a frequency response chart for different working distances showing the proximity effect.

Holding Microphones

No joke! More musicians than you would care to believe deteriorate the acoustical performance of their microphones by holding them wrongly or (more often) sticking tape onto the microphone body in awkward places. Any unidirectional microphone needs to sense the sound pressure at two points to get the directional information. This calls for an additional sound entry for the transducer, which is often integrated into the front grille, but not always. AKG's two-way microphones and EV's variable 'D' microphones both employ rear sound entry ports, placed at almost the rear end of the mic; AKG's vocal mic D330 has entry ports behind the grille. Whatever you do, don't cover entry ports with your hand or with tape.

Next month more about vocal mics and recording vocals.