MEMS design for better audio

February 03, 2016 //By Andreas Kopetz
MEMS design for better audio
Multiple generations of mobile phone technology have amazed consumers with rapid improvements in the resolution and image capture capabilities of the camera element. This emphasis on imaging technology has made the phone a replacement for hundreds of millions of stand-alone cameras.

In fact, the success of image recording on mobile platforms calls attention to a less visible but equally important element in a consumer's multimedia experience, the audio quality. This is defined by the quality of MEMS Microphones being used.

Throughout the relatively short history of camera-equipped smart phones, audio quality has not improved at the same rate as video quality. One reason for this was the absence of consumer demand. Quite simply, when you are making a voice call, a good microphone primarily benefits the receiver of the call and not the owner of the phone. This makes it difficult to market a good microphone to the buyer of the smartphone.

Microphones in smartphones today do more than capture voice for transmission. They also work as audio sensors in a very low power mode to support voice activation and control. And they provide high quality audio when the phone is used for video recording. Video recording mode especially drives microphone requirements toward higher acoustic performance. It can be very frustrating when an ultra-HD resolution smartphone video is paired with a low-quality audio recording.

Since the smartphone owner directly hears the performance of the microphone when they play back video, microphone quality can be a differentiator. If the audio is poor it may ruin the recording, even if it is in 4K video resolution.

Acoustic overload point (AOP)

The popularity of video recording (and sharing) places a greater priority on a microphone’s acoustic overload point or AOP. Beside the well-known Signal to Noise Ratio (SNR), the AOP is the most important quality indicator for a microphone.

AOP defines the sound pressure level (SPL) at which the microphone reaches a total harmonic distortion (THD) of 10 percent. As a measurable characteristic, AOP is a good start in evaluating how well a microphone performs at high sound pressure levels, including high noise environments such as concerts and night clubs. In some usage scenarios, even wind noise can cause a microphone to reach its AOP.

In the past, phone manufacturers used 120dB SPL as a baseline AOP level for most microphones. The AOP requirement recently went up to 130 dB SPL and higher. The additional 10 dB results in superior acoustic performance for the end user even in loud concert environments. In addition, higher robustness against wind noise is achieved. Audio algorithms on smartphone level can better process the audio signals without the occurrence of artifacts.

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