MEMS microphone handles loud environments

The VM2020 is an omni directional, differential output Piezoelectric MEMS microphone with an ultra-high Acoustic Overload Point (AOP) using a differential architecture (Figure 1).

Figure 1 A functional block diagram of the VM2020 showing the differential design out of the microphone and  into the Buffer, then differential out of the buffer amplifier as well (Image courtesy of Vesper)

The definition of AOP is at 10% distortion. In the VM2020 distortion reaches 1% at an SPL of 150; the 2% distortion is at a SPL of 153. For many of the voice interface systems, 1% distortion is probably a more important metric because that is when the voice interface begins to get worse. For most of the Woofers, they are close to, but not at 150, so that is where Vesper sets the 1% distortion.

According to the company, the MEMS element itself does not overload until well into the 170s and can almost go to the 180  dB Sound Pressure Level (SPL) before it reached 10% distortion. Because it is a cantilever and with the piezoelectric being a very linear effect, the MEMS helps to get high dynamic range with good sound reproduction. Capacitive MEMS have a backplate, so if a sound is loud enough, the MEMS will strike the backplate. The Vesper solution is really ASIC-limited as to what kind of dynamic range can be achieved with the microphone.

The device supports applications needing high dynamic range or high AOP such as high-end speakers, basically a Woofer-monitoring microphone. The microphone would fit in front of a high quality Woofer. These designs exhibit high SPL and the Woofers have a high non-linearity, so echo-cancelling and voice interface is very difficult with a loud sound source right next to the microphone.

Figure 2 A typical application circuit for the VM2020 (Image courtesy of Vesper)

The product is included in a reference design for Media Tek’s new Smart Speaker and Sound Bar. This microphone is good for speakers, soundbars, and television sets since users will not have to trade off a good voice interface and good music quality. With the VM2020, sound systems will be able to use a less expensive woofer because of good distortion cancellation.