Don Davis, Eugene Patronis and Pat Brown
In order to determine the electrical input level to a sound system we need to measure the electrical output generated by the system microphone when it is subjected to a known sound pressure (SP). In making such measurements an LP of 94 dB (1 Pa) is recommended as this value is well above the normally encountered ambient noise levels.
Everyone seriously interested in the field of professional sound should own or have easy access to a precision sound level meter (SLM). Among other uses, a SLM is required to measure ambient noise, to calibrate sources, and on occasion to serve as input for frequency response, reverberation time, signal delay, distortion, and acoustic gain measurements.
Setting up the microphone measurement system shown in Fig. 17-1 requires a pink noise generator, a micro-voltmeter, a high-pass and low-pass filter set such as the one illustrated in Fig. 17-2, a power amplifier, and a well-constructed test loudspeaker in addition to the SLM.
Select a measuring point (about 5 ft to 6 ft) in front of the loudspeaker, and place the SLM there. Adjust the system until the SLM reads an LP of 94 dB (a band of pink noise from 250–5000Hz is excellent for this purpose). Now substitute the microphone to be tested for the SLM. Take the microphone open circuit voltage reading on the micro-voltmeter. The voltage sensitivity of the microphone can then be defined as
SV = 20 dBlog(Eo) (17-1)
where, SV is the voltage sensitivity expressed in decibels referenced to 1 volt for a 1 Pa acoustic input to the microphone,
Eo is the open circuit output of the microphone in volts.
The open circuit voltage output of the microphone when exposed to some other arbitrary acoustic level LP is calculated from
where, Eo is now the open circuit voltage output of the microphone for an arbitrary acoustic input of level LP.
For example suppose a sample microphone is tested by the conditions of Fig. 17-1 with the result that the open circuit voltage is found to be 0.001V. The voltage sensitivity of this microphone as calculated from Eq. 17-1 is then
SV = 20 dBlog(0.001) = –60 dB
This result would be read as −60 dB referenced to 0 dB being 1 volt per pascal (1V/Pa). If this same microphone were exposed to an acoustic input level of 100 dB rather than the test value of 94 dB, then its
open circuit output voltage from Eq. 17-2 would become
Many current microphone preamplifiers have input impedances that are at least an order of magnitude or larger than the output impedances of commonly encountered microphones. In such instances, Eq. 17-2 can be employed to determine the maximum voltage that a given microphone and sound field will supply to the preamplifier input. The voltage sensitivity of Eq. 17-1 is the one currently employed by most microphone manufacturers.
Another useful sensitivity rating for a microphone is that of power sensitivity. In this instance the focus is placed upon the maximum power that the microphone can deliver to a successive device such as a microphone preamplifier when the microphone is exposed to a reference sound field. In this instance the reference power is one milliwatt or 0dBm and the reference sound field pressure is one pascal or 94 dB. This rating is more complicated as it involves the microphone output impedance. All microphones regardless of whether the construction is moving coil, capacitor, ribbon, etc. have intrinsic output impedance that in general is complex and frequency dependent. Strictly speaking, in order for such a device to deliver maximum power, it must work into a load that is matched on a conjugate basis with the reactance of the load being the negative of the reactance of the source and the resistance of the load being equal to the resistance of the source (see Chapter 8 Interfacing Electrical and Acoustic Systems, Section 8.1, Alternating Current Circuits).
Suppose then that the real part of the microphone’s output impedance is Ro. This being the case, the available input power in watts that the microphone can deliver to the input of a successive device, AIP, is given by
If AIP is referenced to one milliwatt and the microphone is exposed to a sound field of one pascal then,
This can be converted to a power level by taking the logarithm to the base ten of Eq. 17-4 and then multiplying by 10 dBm to yield
LAIP = (– 6 + 30 + SV – 10logRo) dBm (17-5)
LAIP expresses the power sensitivity of a microphone in terms of dBm/Pa. If our example microphone has an Ro of 200 Ω along with its voltage sensitivity of −60 then its power sensitivity would be
−6 + 30 − 60 − 23 = −59 dBm/Pa
Another useful way to express the power sensitivity of a microphone would be to reference the available input power to a sound field of 0.00002Pa. This would produce a result 94 dBm lower than that of Eq. 17-5. If we symbolize this rating by GAIP then,
GAIP = (SV – 10logRo – 70) dBm (17-6)
In this rating system the example microphone would produce −153 dBm at the threshold of hearing. The advantage of this system is that the power level supplied by a given talker’s microphone is obtained by simply adding GAIP to the pressure level of the talker’s voice at the microphone’s position. GAIP as defined here is very similar to the EIA rating for microphones. The EIA rating system differs in that rather than employing the actual output resistance of the microphone, a nominal microphone impedance rating is employed instead.
About the Authors
Don Davis and his wife, Carolyn, founded Synergetic Audio Concepts in 1972. Don and Carolyn are both Fellows of the Audio Engineering Society. Don is a senior member of the IEEE and a gold member of the Acoustical Society of America. Don and Carolyn received dual lifetime achievement awards from NSCA, the Adele De Berri Pioneers of Audio award from InfoComm International, and the USITT Harold Burris-Myer Distinguished Career in Sound Design Award. Both Don and Carolyn were recipients of Indiana’s highest citizen award from the Governor when they were made Sagamore of the Wabash for their contributions to education.
Eugene Patronis, Jr., taught in the School of Physics of the Georgia Institute of Technology for over fifty years. Throughout his tenure he participated in academic research and industrial consulting in the fields of experimental nuclear physics, acoustics, and electronics. He is the author of several patents in audio related topics.
Pat Brown is the president of Synergetic Audio Concepts, Inc., and of Electro-acoustic Testing Company, Inc., both of Greenville, Indiana. He teaches audio engineering seminars worldwide, and develops web-based audio training programs. His background as a musician, sound contractor, and electronics technician and consultant has forged his practical approach to solving audio and acoustic problems.