Hi everyone, sort of a random question here, but:

I've got two MEMs microphones that are positioned kind of far from the circuit to which I want them connected. Each microphone has a v+, audio out, and GND pin. For simplicity, I'm thinking of using a USB cable to transmit the voltage, ground, and audio out signals from the MEMS microphones to the circuit, since USB has four pins/internal wires. I'm thinking I could use the first pin for the v+ signal for both microphones, the second pin for GND for both microphones, the third pin for one microphone's audio output, and the fourth pin for the other microphone's output. The circuit is only about 6 ft from the microphones, so it wouldn't be a far distance. Any reason I shouldn't do this? E.g., would the audio signals degrade terribly on the USB cable?

 

I think that will work, but personally I’d keep the power pins on opposite sides just like a normal USB. It may never matter but it reduces the chances of bad things happening in the future.

 

I agree with Wayne’s comment. If your using a USB cable, there will be a time when someone will plug it into a standard USB port. The microphones audio out might not like having +5 VDC applied to it.

 

Not all USB cables are shielded, so there might be some extra noise pickup with cheaper cables.

ak

 

Not all USB cables are shielded...

Huh, I didn't know that. Are unshielded ones just two-conductor ones for power? Or maybe were made when USB was back at 1.0?

 

Don't know. I do know that I've cut open some unshielded ones. Rare, but something to keep an eye on.

ak

 

Don't know. I do know that I've cut open some unshielded ones. Rare, but something to keep an eye on.

ak

Discount stores do various desk toys that run off USB power but don't use any data - its not entirely unheard of to find examples with only the 2 supply wires and no screening whatsoever.

 

Thanks for the feedback everyone!

I chose a USB cable just because it has four wires and I have a bunch of spares laying around, but is there another cable you'd recommend instead?

 

The correct twin shielded mic cable may work best. But try the USB and see how it goes. An advantage of the USB cable is that you have it, but do connect it as mentioned above so the power and gnd are correct. It will be plugged into a USB port at some time.

Years ago I got a call to fix a speaker for a movie projector. Just to make it easy, a 3 pin power plug was connected to it, so a standard power extension lead could be pressed into service. Well, a helpful person took it on them self to assist the setup and plugged it into a power point. I wish I was there to see it as an 8 ohm speaker tried to handle 240VAC. I reckon it made quite a noise!
A new speaker and rewiring the plug fixed it. So they could still use power extension lead, I wired active and neutral together as one speaker wire, and the earth as the other. So if it was plugged into a power point, the fuse will blow, not the speaker. Not really ideal.
A better idea would have to buy speaker leads!
So, if you reuse any sort of connector, assume it will at some time be plugged into the "real" socket. Make sure it can survive.

 

Thanks for the feedback everyone!

I chose a USB cable just because it has four wires and I have a bunch of spares laying around, but is there another cable you'd recommend instead?

If you have electret mic capsules - most just have grounded aluminium can and a signal connection. Rl is external and you have some choice where you put it. Putting Rl right next to the mic means separate power & signal lines - move Rl to inside whatever the mic lead plugs into and you can use a single core screened audio lead.

 

Do you think electret mics would be the best for my application? I don't really know anything about audio, but I'm most interested in detecting patterns in lower frequencies, between 20Hz and 200Hz. Small size and power consumption is also critical to my application, since the mic will be in a wearable device. That's why I was attracted to MEMS mics. If the mic is directional, that'd be even better (trying to avoid background noise).

 

Do you think electret mics would be the best for my application? I don't really know anything about audio, but I'm most interested in detecting patterns in lower frequencies, between 20Hz and 200Hz. Small size and power consumption is also critical to my application, since the mic will be in a wearable device. That's why I was attracted to MEMS mics. If the mic is directional, that'd be even better (trying to avoid background noise).

they're one of the easiest types to use, dynamic types have very low output and usually need a fair bit of low noise gain. Ribbon mics need HUGE gain. Carbon granule mics are noisy and require a source of excitation current - unless you use them as the emitter resistor for a common base first stage.

 

Ok so it sounds like my best bet is to stay away from ribbon or carbon granule mics, and stick with either the electret or mems mics.

 

Ok so it sounds like my best bet is to stay away from ribbon or carbon granule mics, and stick with either the electret or mems mics.

There may not be any ribbon mics about these days. Telephones now use almost exclusively electret mics - there were electret conversion kits marketed for older phones with carbon mics. anything still has one could probably be flogged to a movie set.

 

If you post a datasheet for the mems mic you are using we can comment on its applicability.

ak

 

It's the ADMP401 (now INMP401), available here: https://www.sparkfun.com/products/9868. Datasheet attached. Thanks!