In Audio Components, Understanding and Measuring Sensitivity

A common specification for speakers  microphones, and buzzers is sensitivity. This parameter relates electrical signals to sound pressure. Understanding the definition and significance of this parameter is important in that it helps the engineer to intelligently select the proper audio component to suit their design. Also, it is important to understand how to compare sensitivity specifications in the event that they have been characterized under differing measurement conditions.

What is Microphone Sensitivity?

The parameter of microphone sensitivity quantifies the efficiency of a microphone in converting sound into an electrical signal. When driven by a specific input sound pressure, a more sensitive microphone will produce a larger output voltage . An electronic amplifier could be used to boost the output signal from a less efficient microphone, but the amplifier will also boost undesired noise associated with the desired signal. The output voltage from the microphone under test is compared to a reference voltage and the resulting ratio is expressed in the units of dB. The equation describing microphone sensitivity is expressed as:

S = 20 log10(V/Vref)

A standard reference used for comparison is 1 Volt of output signal from the microphone with 1 Pascal of sound pressure applied to the microphone. As a brief review of measurements characterized in dB, a microphone producing 1 volt of output signal from 1 Pascal of sound pressure will have a sensitivity of 0 dB (equal to the reference). A microphone producing twice as much output voltage for the same sound pressure will have a sensitivity rating 6 dB greater while one producing 1/10 as much output voltage will have a sensitivity rating 20 dB less. Almost all microphones will have sensitivity specifications of less than 0 dB (a negative number) because the reference voltage is 1 Volt and the voltage produced by the microphone under test is typically less than 1 Volt.

The sensitivity of a microphone characterized with a pressure level other than 1 Pascal can be adjusted to the equivalent of 1 Pascal by using this formula:

Sadj = 20 log10(1/Pchar)

Sadj is the adjustment to make to the characterized sensitivity Pchar is the pressure level at which the characterization was performed Examples:
-70 dB at 0.1 Pa will be -50 dB at 1 Pa (-70 + 20 log10(1/0.1) = -50 dB) -60 dB at 0.5 Pa will be -54 dB at 1 Pa (-60 + 20 log10(1/0.5) = -54 dB)

Drawing of a typical microphone sensitivity test setup
Typical microphone sensitivity test setup

So, what is Speaker Sensitivity?

Speaker sensitivity is a measure of the ability of a speaker to convert electrical power applied to the speaker into sound pressure or volume as heard by the listener. With a microphone, the sensitivity of a speaker can also be thought of in terms of efficiency; a more efficient speaker will produce a louder sound for a given electrical input signal. A less efficient speaker can be driven with a higher input power level, but the associated amplifier is an added cost to the design and the speaker will need to be able to handle the higher input power. The standard configuration for characterizing a speaker is to place the speaker under test in an anechoic chamber, place a microphone 1 meter in front of the speaker and then drive the speaker with a 2.83 Volt signal (1 Watt into 8 Ohms).

The equation for the sensitivity of a speaker is expressed below as:

S = 20 log10(P/Pref)

Quantities P and Pref are the sound pressure produced by the speaker under test and a reference sound pressure level. The minimum sound pressure level of human hearing (20 µPa) is typically used as the reference pressure. Speaker sensitivity characterization can also be performed at conditions other than the standard 1 Watt, 1 meter and 20 µPa and the sensitivity can then be equated back to standard conditions.

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