DocID025704 Rev 1

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AN4426

Acoustic parameters

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2.6

Frequency response

The frequency response of a microphone in terms of magnitude indicates the sensitivity

variation across the audio band. This parameter also describes the deviation of the output

signal from the reference 0 dB. Typically, the reference for this measurement is exactly the

sensitivity of the microphone @ 0 dB = 94 dBSPL @ 1 kHz. The frequency response of a

microphone can vary across the audio frequency band depending on three parameters: the

ventilation hole, the front chamber geometry, and back chamber geometry. The ventilation

hole and the back chamber geometry have an impact on the behavior at low frequencies

while the behavior at high frequencies depends on the geometry of the front chamber only.

Behavior at high frequencies can be a resonance peak caused by the Helmholtz effect. This

resonance is the phenomenon of air resonance in a cavity. As a matter of fact, it depends on

the dimension of the front chamber of the microphone, representing the sound cavity in

which the air resonates. A microphone with a flat frequency response is suitable when

natural sound and high intelligibility of the system is required. The following figure shows the

response of the MP45DT02. It shows a roll-off at low frequencies and a peak around 18 kHz

Figure 16. MP45DT02 frequency response

The frequency response of a microphone in terms of phase indicates the phase distortion

introduced by the microphone. In other words, the delay between the sound wave moving

the microphone membrane and the electrical signal at the microphone output results in that

this parameter includes both the distortion due to the membrane and the ASIC.

b.

A detailed explanation of the Helmholtz resonance principle and its effect on the chambers of the microphone

is given in Section 2: Acoustic parameters.