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Understanding acoustic design for MEMS microphones

17 Apr 2014  | Alessandro Morcelli, John Widder

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Due to their high performance and small size, MEMS microphones are ideal for use in consumer products such as tablets, laptops, and smartphones. However, the sound inlet of the microphones used in these products is usually not in direct contact with the external environment. This makes it necessary to design an acoustic path from the external environment to the microphone. The design of this acoustic path can have a considerable impact on the overall performance of the system.

The picture below shows an example of a typical acoustic path for the microphone in a tablet.

Figure 1: A typical application example.

All of the components between the external environment and the microphone membrane – the product housing, the acoustic gasket, the PCB, and the microphone – act as a wave guide that shapes the overall frequency response of the system. In addition, the acoustic impedance of the materials used in the sound path also affect the frequency response. Predicting exactly how an acoustic design will perform requires modelling the acoustic path and simulating its frequency response using a professional simulation tool such as COMSOL. However, this article provides some basic guidelines for optimising a microphone's acoustic path.

The Helmholtz resonance
A hollow cavity with a narrow sound inlet will resonate acoustically when excited. This is what produces sound when blowing across the top of an empty bottle. This type of structure is known as a Helmholtz resonator and is named for its inventor, Hermann von Helmholtz. Helmholtz used resonators with different resonant frequencies to identify the frequency components present in music and other complex sounds.

The centre frequency of the Helmholtz resonance is given by the following equation:

where c is the speed of sound in air; AH is the cross-sectional area of the sound inlet; LH is the length of the inlet; and VC is the volume of the cavity. This equation assumes a simple structure comprised of a tube with a uniform cross-sectional area connected to a cavity. The equations describing the behaviour of sound waves in the acoustic path of a microphone with varying cross-sectional areas and different materials are far more complex. This complexity makes it necessary to simulate the acoustic behaviour of the entire sound path in order to accurately predict its overall performance.

For this article, we have simulated the frequency response of different acoustic paths while varying the thickness and hole diameter of the gasket, the diameter of the holes in the product housing and PCB, bends in the acoustic path, and the acoustic impedance of the materials used. The results of these simulations allow designers to predict in general terms how changes to these parameters will affect the overall performance of an acoustic path.

The frequency response of the microphone
The response of a MEMS microphone at low frequencies is determined by the dimensions of the ventilation hole between the front and back sides of the sensor membrane and by the volume of its back chamber, while the high frequency response is determined by the Helmholtz resonance created by the front chamber of the microphone and its sound inlet.

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