MEMS and Electret Microphones and Speakers

MEMS and Electret Microphones and Speakers

Less than five years ago it was pretty cutting edge to have devices in the home or office that either spoke or listened to us. Nowadays they’re everywhere from kitchen appliances to elevators and TV remotes. MEMS and Electret microphones and speakers are at the forefront of this development and RDI has many options available to cover critical applications.

In this note we’re going to use an RDI end product as an example to dive into the differences between microphone types and talk about some of the mechanical and environmental requirements for microphones and speakers that have to operate in indoor and outdoor applications.

The structure of MEMS and Electret microphones is very similar. They are both versions of the condenser microphone, but in one case an electret material is used to provide the charge required to polarize the capacitor instead of a discrete supply voltage for the MEMS and condenser mics.

In the electret mic the mechanical structure is vertical, whereas the MEMS device is horizontal and uses electronic wire bonding to connect the chips. The major difference between a MEMS mic and an electret device is the semiconductor ASIC inside the MEMS mic. The simplest way to think of the difference is to replace the JFET in the electret with the ASIC.

Fig. 1: Electret microphone structure and open MEMS mic showing wire bonds from mic to ASIC

The ASIC be an analog or digital device, and in many cases the extra circuit is an analog to digital converter (ADC) which converts analog signals from the capacitor to a digital format known as pulse density modulation PDM.

MEMS digital microphones are especially valuable in circuits where a microprocessor is present because the processor doesn’t need to convert the sound data, freeing up resources for other functions.

Choosing components for RDI’s CasaCam Security Camera

Fig. 2: RDI’s CasaCam Indoor/Outdoor Security Camara product

The CasaCam camera combines a camera with regular lighting, night vision, PIR and of course a microphone and loudspeaker.

The device connects to its control monitor using a 2-way radio so that users can not only see the subject in the camera, they can talk and listen too.

There were several requirements for the microphone used. The major ones being sensitivity of at least 40dB and omnidirectional response. In addition, because the end product was designed to work outside, a moisture barrier had to be included that would eliminate moisture but still allow sound waves to penetrate without distortion. RDI’s REM-4527F-P-NL was chosen and given a Teflon particle shield instead of a simple dust cover to keep moisture out.

Fig. 3: REM-4527F-P-NL and major specs

Choosing a Loudspeaker for the CasaCam Product

The loudspeaker design criteria were fairly simple because the camera is powered from a line socket, meaning the amplifier designed could drive lower impedance voice coils without worrying about battery power. The frequency response had to cover typical human speech and the sound quality had to be comparable with mobile devices such as the iPad, etc.

Anyone who has ever designed a loudspeaker circuit knows the electronics only take the desired sound so far. The major work is in the speaker enclosure. That is true whether the sound system is a high-fidelity setup or a simple alert system like CasaCam.

There are 3 critical pieces to any enclosure design as shown below.

Fig. 4: Typical Loudspeaker Enclosure comprising Protective Grill, the speaker and Rear Enclosure

The protection is typically a grill with enough holes to allow sound to come through unfiltered, and sufficiently small to block particles entering and compromising the speaker. The Loudspeaker itself is sandwiched between the grill and the most important element for sound quality, the rear enclosure.

The aim of the design is obviously to let sound escape to the front via the grill, and block sound escaping backward via the enclosure.

Rigid materials that do not absorb sound should be used to create the enclosure. Typically, an air channel is created at the rear of the enclosure to allow the pressure caused by the moving speaker cone to dissipate.

RDI designed the CasaCam speaker enclosure with rigid plastic and a 4mm diameter hole in the rear to allow pressure changes.

Fig. 5: The reinforced Rear Enclosure on the CasaCam on the left and Protective Grill on the right

The world of audio control and feedback is changing very rapidly. RDI can help design your electronic and mechanical audio needs and has helped many companies bring their audio products to mass markets quickly. We’d be delighted to discuss your MEMS and electret microphone and speaker needs via our Contact program or by calling our OEM Solutions hotline at the top of this page.

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