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The frequency response of a microphone is often comprised of certain frequency bands that the microphone will accentuate or attenuate. The matching of a sound source with similar frequency characteristics may or may not provide the recordist with the desired sound. The microphone may cause accentuation of certain characteristics of the sound source; perhaps a microphone with somewhat opposite frequency characteristics as the sound source will be a more appropriate choice. Again, this decision is dependent upon the final, desired sound.
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Figure 5-1. Polar Patter Spheres and the Microphone Axis. (click on image for full size)
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Microphones do not respond equally to sounds arriving at different angles to its diaphragm. The directional response of a microphone is its sensitivity to sounds arriving at various angles to the diaphragm. The polar pattern of a microphone depicts the sensitivity of a microphone to sounds at various frequencies in front, in back, and to the sides; the actual pattern is spherical around the microphone. Directional response measures the microphone's sensitivity to sounds arriving from angles, but calculates this sensitivity at only a few frequencies.
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Sounds directly in front of the microphone diaphragm are considered to be on-axis. Sounds deviating from this zero-degree point on the polar curve are considered off-axis, and are plotted in relation to the on-axis reference level. The frequency response of most microphones will vary markedly to sounds at different angles. Even microphones that show no pronounced frequency areas of accentuation or attenuation (flat frequency response) on-axis, will show an altered frequency response at the sides and the back of the polar pattern.
These variations in frequency response at different angles are commonly called off-axis coloration. These changes are more pronounced at or towards the attenuated angles of the microphone patterns. In the intermediate angles between directly on-axis and the dead areas of directional patterns, a slight change in the angle/direction of the microphone can make a substantial difference in the frequency response of the captured sound source. Frequencies about 4 kHz are most dramatically altered by slight angle changes.
The amount of off-axis coloration is an important measure of the microphone's suitability to a variety of situations. This is especially pronounced in stereo microphone array recording techniques. Instruments at the edges of the array's pick-up pattern and the reverberant sound of the hall will arrive at the array mostly from angles that are off-axis; the sound qualities of those instruments and of the reverberant energy may be altered significantly by the off-axis coloration. Off-axis coloration may have a profound impact on the sound qualities of the recording.
Microphones have different response times, before they will begin to accurately track the waveform of the sound source. This transient response time distorts the initial, transient portions of the sound's timbre. Slow transient response is most noticeable when the microphone is applied to a sound source that has a fast initial attack time, in the dynamic envelope, and a large amount of spectral energy during the onset.
Transient response is a microphone performance characteristic that is not included in the manufacturers specifications information, that accompanies promotional literature and owner's manuals, and is not governed by a standard of measurement.
The acute hearing of the recordist will be the primary judge of this microphone characteristic. The recordist must become aware of differences in timbre that are present in the early time field of the sound source, as compared with the live sound source. These differences are derived through the critical listening process, and are vitally important to the recordist, who must be in control of capturing and shaping the sound as desired. Two microphones with identical frequency response curves may have completely different sound characteristics caused by different transient response times.
All microphones will respond differently to the same sound source, at the same distance and angle. The ability of each microphone to capture the detail of a source's timbre will be different. Microphones will have different sensitivities in relation to distance. The distance sensitivity of a microphone is often influenced by the polar response of the microphone and/or its transduction principle (condenser, moving coil, ribbon, etc).
Directional patterns often are able to capture timbral detail of a sound source at a greater distance than an omni-directional microphone. This is primarily the result of the ratio of direct to indirect sound, and a masking of timbre detail, but it may also be attributed to transduction principle, depending on the particular circumstances. Similarly, condenser microphones will have a tendency towards greater distance sensitivity than dynamic microphones, due to its more sensitive transfer of energy. Many times a microphone with a small-sized diaphragm will have greater distance sensitivity than a microphone with a larger diaphragm, all other factors being equal.
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