Testing, Testing, 1, 2, 3…Frequency Response and Polar Data for Microphones
An excerpt from chapter 7 of the soon to be published
Eargle’s Microphone Book, 3e.
This is a new edition of what is considered to be
“…a “must-have” reference source for recording engineers, producers or anybody who desires to better understand and utilize mics.” – Music Connection Magazine
Frequency response data can be presented in many ways, and the cautious user of such data should always consider how the data is presented. Figure 7.1A shows raw microphone measurement data on a scale that is 10 dB vertical. Figure 7.1B shows the same measurement plotted on a 40 dB vertical scale. Figure 7.1C shows the same measurement plotted on a 40 dB vertical scale with 1/3 octave band averaging applied. Figure 7.1D shows the same measurement plotted on a 40 dB vertical scale with full octave band averaging applied. Figure 7.1E shows the same measurement plotted on an 80 dB vertical scale with full octave band averaging applied. Much of the microphone response data supplied by manufacturers is not only presented with a very coarse vertical resolution, but also averaged by the marketing department having an artist produce a nice-looking approximation of the actual measurement. It is sometimes possible to look at the data as presented and determine if what is presented is actual measured data or an artist’s representation of the measured data.
Frequency response data should always state the physical measuring distance so that an assessment of proximity effect in directional microphones can be correctly made. If no reference is made, it can be assumed that measurements are made at 1 meter. Data for professional microphones may be presented with tolerance limits, as shown in Figure 7.2. Here, the data indicate that the microphone’s response falls within a range of _2 dB above about 200 Hz (slightly greater below that frequency); however, there is no indication of the actual response of a sample microphone.
If the data can be presented with clarity, some manufacturers will show proximity effects at distances other than the reference of one meter, as shown in Figure 7.3. This data is especially useful for vocal microphones that are intended for close-in applications.
Many manufacturers show response at two or more bearing angles so that the variation in response for those off-axis angles can be clearly seen, as shown in Figure 7.4. Here, the response for a cardioid is shown on-axis and at the nominal null response angle of 180_. For supercardioid and hypercardioid microphones, the response at the null angles of 110_ and 135_ may also be shown.
Taking advantage of normal microphone symmetry, polar plots may be restricted to hemispherical representation, as shown in Figure 7.5. For microphones that are end-addressed, it is clear that response will be symmetrical about the axis throughout the frequency band.
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Ray A. Rayburn is a Senior Consultant with K2 Audio LLC. He is a member of the AES Standards Working Group on Microphones, and Chair of the Standards sub-committee on Interconnections. He is also a recording engineer with a lifetime interest in microphone use, testing, and design.