MED ASSO C IAT E S I NC. ANL- 940 - 1 U L TR ASO NI C M I CRO PH ONE AND AMPL I FIE R
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DOC-262 Rev 1.0 Copyright © 2012
MED Associates, Inc.
Appendix A
Quantitative Analysis of Sound Pressure Levels
Unlike many instruments on the market, the ANL -940-1 Ultrasonic Microphone and Amplifier is a
specialized device that is calibrated to produce a defined signal amplitude in response to a standard
sound pressure level. Thus, it is useful not only for recording the frequency profile of rodent
vocalizations, but also it can provide an accurate measure of the amplitude, or Sound Pressure Level, of
the vocalizations. For researchers interested in quantifying the amplitude of vocalizatio ns, special
consideration should be given to the calibration specifications of the ANL -940-1 instrument, as well as
the physical layout of the recording environment, particularly the distance between the microphone and
the test subject.
To report standard Sound Pressure Levels (Lp), the level should be reported at a distance of 1 meter
between the microphone and test subject. However, it is often inconvenient to configure an
arrangement in the lab where the microphone is at a distance of 1 meter from the t est subject. Thus, it
is necessary to convert measurements obtained a distance of less than one meter to the equivalent
measure that would be obtained at a distance of 1 meter.
Calculating the 1 meter equivalent sound pressure level depends on the express ion of the initial
measurement. If the microphone output is recorded as a voltage, the voltage expected at a distance of
one-meter is obtained by multiplying the measured voltage by the measurement distance in meters.
Example: If a microphone placed 0.25 m (or 1/4 m) from the source produces a sound pressure
level of 30 mV (0.03V), the equivalent pressure at 1 m would be 30 mV * 0.25 = 7.5 mV ( or
30mV / 4). Recall that the ANL-940-1 has a calibration scaling of 1 Pa/V. Thus, the equivalent
sound pressure level at 1 m would be 0.0075 V * 1 Pa/V = 0.0075 Pa.
Considering the large variation of sound pressure levels typically encountered and the need to multiply
or divide to obtain sound pressure in Pa at one-meter, it is usually easier to calculate sound pressure
using logarithms. A standard system has been developed (ANSI S1.1 -1994)
1
to yield a logarithmic
measure of the effective sound pressure level of one sound relative to a reference value. Thus, effective
sound pressure levels are measured in decibels (dB) above a standard reference value.
Sound Pressure Level (SPL), or sound level (L
p
), is defined in ANSI S1.1-1994 as
1
Where p
ref
is a sound pressure level reference value, and p
rms
is the root mean squared (rms) value of
the sound pressure level for the measured signal.
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