Microphones on the slyWhen miniature microphones are hidden in live-soundreinforcement, sound quality and clarity are sacrificed.
Jan 20, 1997 12:00 PM, By Peter MappMapp is a consultant in the United Kingdom.
Microphones often need to be placed in far from ideal positions for recording and live-sound reinforcement. For example, during live performances, miniature radio microphones are hidden in shirts, sleeves, hats and many other less-than-ideal positions for recording and live-sound reinforcement. These "secret" placements often result in loss of sound quality and clarity. Even the standard tie-clip microphone position is far from ideal.
If you knew what the different placements can do to the frequency spectrum, you could correct for it with your equalizer. Yet no published information concerning the effects of miniature microphone positioning and corresponding performance seemsto exist. Here is an attempt to redress the balance.
Body and head-worn microphonesThe basic test setup used for the investigation is shown in Figure 1. A miniature reference microphone measures the speech signal directly in front of the test subject's mouth. A second test microphone simultaneously measures the acoustic speech output signal of the "talker" at various test positions. Each microphone signal is analyzed by a dual-channel FFT analyzer, giving the transfer function. The difference between the magnitude of the two signals is plotted as the desired result. The test microphone was calibrated to the reference mic to within better than 0.5 dB, having been first measured to ensure that it had a flat response within +/-0.5 dB over the range of interest.
Microphones often must be hidden in less-than-ideal positions. (See Figure 2.) One of the most common locations for a miniature radio microphone capsule is a jacket lapel or equivalent position. Figure 3 shows the acoustic effect of this mounting position both in terms of signal level reduction and spectral changes. The level is reduced significantly, equivalent to a loss of 4 dBA. Up to 1.6 kHz or so, the spectrum of the speech picked up is effectively just reduced in level. At frequencies greater than 1.6 kHz, however, the signal frequency response drops off rapidly, leading to a dull sound with lack of clarity and reduced intelligibility.
Microphone performanceFigure 4 shows the effect of the other main mounting position: the tie. This position is similar in character to the lapel position, exhibiting a 4 dBA loss of signal level. However, a peak at around 500 Hz to 800 Hz also occurs. This well-known effect is caused by the talker's chest resonance. The need for corrective equalization can clearly be seen, but for the first time the exact nature of the problem is highlighted. The high-frequency rolloff is essentially the same as the lapel position, albeit slightly steeper.
In musical productions, one of the most popular positions for a microphone is on the forehead or within the hair at the front of the head. Figure 5 shows the effect of this mounting position. Apart from an overall loss of signal level amounting to -3 dBA, the overall response is not too bad. But then peaks are introduced at 160 Hz, 1 kHz and 2.5 kHz. A general loss of high frequencies (>2.5 kHz) is also exhibited.
An alternative position is mounting the microphone on the front of the ear. Figure 6 shows the response graph for this location. An overall level reduction of 5 dBA occurs with a reasonably flat response out to 1.6 kHz, thereafter followed by the familiar high-frequency rolloff.
Eyeglasses also offer a useful and concealed mounting position. (See Figure 7.). This spot gives a much better overall response, with only 1 dBA loss of signal and much-reduced high-frequency losses.
Recently a miniature, flat microphone has been developed for mounting directly on the face. Figure 8 illustrates why this is a good choice acoustically. With this mounting position, the high-frequency rolloff, typical of the other mounting positions, is diminished, and the overall signal loss is reduced to 0 dBA.
Further experiments look at the way human speech is affected by microphone positioning below the plane of the mouth but away from the body. Figure 9 compares the spectra and output levels experienced directly in front with the same data at 4, 8 and 12 inches (100, 200 and 300 mm) below the mouth. The evenness of the responses and the relative signal strength losses are clearly shown. Overall reductions of -3 dBA, -7 dBA and -9 dBA were measured corresponding to the 4, 8 and 12 inch positions. The figure also clearly shows how a dip in the spectrum moves up in frequency with increasing distance. Be sure to note how the high-frequency content is not attenuated in the same manner as the body-worn microphones at similar distances in relative positions.
Figure 10 throws some useful light on the question of sound radiation from a human talker and is taken from the work of Dunn and Farnsworth.
Locally reflecting surfacesThe effect of locally reflecting surfaces further complicates situation with regard to lecterns. Reflections from the lectern surface can strongly interact and interfere with the direct sound from the talker and produce highly audible comb filtering, peaks and dips in the response. (See Figure 11.) The presence of a person at a lectern can also seriously affect the local sound field. New reflections can be created and directed straight into the face of the microphone. Figure 12 shows how strong this acoustic effect can be. The introduction of the talker at the lectern causes narrowband peaks of 12 dB to 14 dB to be created.
Directional microphones can help overcome such problems, but then it becomes easy for the talker to go "off axis," and voice pickup is lost. A common method of overcoming this is using one highly directional microphone one each side of the lectern. (See Figure 13.) Although this method can work, a talker who tends to move about can cause phase shifts to occur between the arriving signals. This causes large interference dips within the resultant reinforcement problem.
Although placing microphones in less than ideal positions is a necessity for recording and live-sound reinforcement, it is helpful to know what these placements will do to sound quality and clarity. As in many other aspects of sound-system design, knowledge is definitely power.