The directivity patterns given in Section 5.2 are only true where ka is small and are thus ideal. In practice at high frequencies, ka will not be small and the actual polar diagram will differ due to diffraction becoming significant.
Figure 5.10(a) shows the result of a high-frequency sound arriving off-axis at a large diaphragm. It will be clear that at different parts of the diaphragm the sound has a different phase and that in an extreme case cancellation will occur, reducing the output significantly.
When the sound is even further off-axis, shading will occur. Consequently at high frequency the polar diagram of a nominally omni microphone may look something like that shown in Figure 5.10(b). The high-frequency polar diagram of an eight may resemble Figure 5.10(c). Note the narrowing of the response such that proper reproduction of high frequencies is only achieved when the source is close to the axis.
The frequency response of a microphone should ideally be flat and this is often tested on-axis in anechoic conditions. However, in practical use the surroundings will often be reverberant and this will change the response at high frequencies because the directivity is not independent of
Figure 5.10 (a) Off-axis response is impaired when ka is not small because the wavefront reaches
different parts of diaphragm at different times causing an aperture effect. (b) Polar diagram of practical omni microphone at high frequency shows narrowing of frontal response due to aperture effect and rear loss due to shading. (c) Practical eight microphone has narrowing response at high frequency.
frequency. Consequently a microphone which is flat on axis but which has a directivity pattern which narrows with frequency will sound dull in practical use. Conversely a microphone which has been equalized flat in reverberant surroundings may appear too bright to an on-axis source.
Pressure microphones being omnidirectional have the most difficulty in this respect because shading makes it almost impossible to maintain the omnidi- rectional response at high frequencies. Clearly an omni based on two opposing cardioids will be better at high frequencies than a single pressure capsule.
It is possible to reduce ka by making the microphone diaphragm smaller but this results in smaller signals making low noise difficult to achieve. However, developments in low-noise circuitry will allow diaphragm size beneficially to be reduced.
In the case of the shotgun microphone, the tube will become acoustically small at low frequencies and will become ineffective causing the polar diagram to widen. If high directivity is required down to low frequencies the assembly must be made extremely long. The parabolic reflector microphone has the same
characteristics. As such microphones are not normally used for full frequency range sources the addition of a high-pass filter is beneficial as it removes low frequencies without affecting the quality of, for example, speech.
The electrical output from a microphone can suffer from distortion with very loud signals or from noise with very quiet signals. In passive microphones distortion will be due to the linear travel of the diaphragm being exceeded whereas in active microphones there is the additional possibility of the circuitry being unable to handle large amplitude signals. Generally a maximum SPL will be quoted at which a microphone produces more than 0.5% THD.
Noise will be due to thermal effects in the transducer itself and in the circuitry. Microphone noise is generally quoted as the SPL which would produce the same level as the noise. The figure is usually derived for the noise after A weighting (see Section 3.4). The difference between the 0.5% distortion SPL and the self-noise SPL is the dynamic range of the microphone. 110 dB is considered good but some units reach an exceptional 130 dB.
In addition to thermal noise, microphones may also pick up unwanted signals and hum fields from video monitors, lighting dimmers and radio trans- missions. Considering the low signal levels involved, microphones have to be well designed to reject this kind of interference. The use of metal bodies and grilles is common to provide good RF screening.
The output voltage for a given SPL is called the sensitivity. The specification of sensitivity is subject to as much variation as the mounting screw thread. Some data sheets quote output voltage for 1 Pa, some for 0.1 Pa. Sometimes the output level is quoted in dBV, sometimes dBu (see Section 2.7). The outcome is that in practice preamplifier manufacturers provide a phenomenal range of gain on microphone inputs and the user simply turns up the gain to get a reasonable level.
It should be noted that in reverberant conditions pressure and pressure gradient microphones can give widely differing results. For example, where standing waves are encountered, a pressure microphone positioned at a pres- sure node would give an increased output whereas a pressure gradient micro- phone in the same place would give a reduced output. The effect plays havoc with the polar diagram of a cardioid microphone.
The proximity effect should always be considered when placing micro- phones. As explained in Section 3.16, proximity effect causes an emphasis of low frequencies when a PG microphone is too close to a source. A true PG microphone such as an eight will suffer the most, whereas a cardioid will have 6 dB less proximity effect because half of the signal comes from an omni response.