Microphone mounting

It's a subject that seems so simple, but gets messed up so often and in so many ways. This is a first in what will be a series of posts on dealing with microphone gaskets in a way that both works and is reliable in manufacturing.

Here I'll deal with MEMS microphones, and specifically bottom ported MEMS, but the same principals apply to ECMs and other types of microphones.

The goal of mounting a microphone inside a product is to create an acoustical pathway from the world outside your product to the microphone, without letting the sound resonate or leak to the interior of your product. Seems simple enough, right? Well, we have only ever seen one customer get it right the first time, so in this and following posts I'll cover some of the basics of mounting a microphone inside a product.

A final design example

In the image to the right, we see a bottom-ported microphone in relation to a piece of curved plastic. How do you mount the microphone to this plastic properly? Let's take a look at the final results, then we'll step through the important points.

Port Length

Keep the port length as short as possible. Any acoustical port will have an open-tube resonance. Keep the port length as short as possible to keep the resonance *well* outside the frequency band of interest. You can use a port resonance calculator to *estimate* the resonance. We usually wan at least so-called 'wideband' audio -- and keep good fidelity at least up to 7kHz. To prevent too nasty of a resonance, let's keep the resonant peak at 12kHz, The port length calculator here shows us that a port length of 7mm corresponds to a port resonance of 12kHz. So, make sure the port length is short, but at *most* 7mm.

PCB Location

The whole PCB assembly must be located with precision. This means locating by means of screw alone is not a good idea. You can use either a shoulder on a screw boss to locate the PCB precisely, as in this diagram, or you can use a simple pin/slow arrangement in the PCB. In either case, you're looking for tight tolerances on the absolute position of the PCB relative to the outer plastics.

Ensure PCB is compressed tightly

If using shouldered bosses, be sure that they do not protrude through the PCB so that when the screws are assembled that you get a tight compression between the screw head -> PCB -> plastics.

Gasket Compression

You must ensure that under all circumstances and tolerance variations, the gasket remains compressed. We are fans of Rogers Poron material. It is literally made for making gaskets, and is very good. There's lots to say on gasketing materials, but at the moment, let's just stick with Poron -- a very tight, but still open-cell foam. It doesn't take a compression set and keeps its sealing properties through extreme conditions.In this design, the gasket is in magenta, compressed to about 40%. With all tolerances, it should be fine because of the small mechanical loop from the screw -> pcb -> plastic -> screw.

Avoid severe Helmholtz Resonantors

It's important to keep the volume behind the port but in front of the PCB very small. The reason is because any time there is something that looks like a necked down bottle, you get a resonance, potentially ruining the microphone's frequency response.

Keep your friends close. But keep your screws closer.

It's important to prevent the PCB vibrating relative to the plastics. This necessitates putting screws close to the acoustical port. If you have a large PCB, be sure to have screws or other positive means of compression, near your microphones. Basically, the mechanical loop from the screw head to the PCB, plastic to gasket and back needs to be small.

Conclusion

That gives a basic summary of a few of the important details of how to properly mount microphones. As always, if you need some assistance with microphones and loudspeakers, please give us a call. We're here to help!

Don't Blow A Gasket, or How to Mount a Microphone