"Noise-cancelling" microphones?

[evandude]

I have been looking at a lot of the low-priced computer microphones, and i have one that was pretty cheap (10 bucks or so).

A lot of the boom mics claim to be "noise cancelling". Just to be clear, I am NOT talking about the $70+ noise-cancelling microphones, which are probably nice and complicated and probably work great. I am talking about the $10-20 ones. However, they don't have any external power (unless power is supplied via the 1/8" jack or something, but they go into standard PC microphone jacks, and as far as I know, those aren't powered...)

Therefore, I figure they could only include PASSIVE filtering, not some form of active filtering (detecting noise and adjusting accordingly, whatever) as the name might suggest. However, i've seen the inside of one, and there is no circuitry besides the microphone element and a mute switch.

Do they ever put any sort of filtering circuitry in the mic element itself? If not, then the only way the microphone is any better than a standard one is if they used a higher-quality microphone element to begin with, which is an understandable difference but hardly counts as "noise cancelling"

The biggest thing that gets me is people I know who have tried them have said they are FAR superior to other cheap, but non-noise-cancelling microphones... just orders of magnitude better... So i'm trying to figure out if it's due just to a better quality mic element, or if there is some passive filtering circuitry. If it is just passive filtering, then potentially I could add some simple external filtering circuitry and get equally good results with a non-noise-cancelling mic. I know I could just buy a $20 noise-cancelling mic and get it over with but part of me would feel ripped off if it were just a passive filter difference, not to mention, if i could improve it with just a simple passive filter, I could probably do even better adding an active filter of some sort, possibly adjusting the response to best match the environment I use it in (which is in a car, so road noise is a huge problem)

Any thoughts?

 

[audioguru]

Hi Evandude,
I have a pretty good quality Audio Technica gooseneck mic connected to my computer to measure speaker response. It is electret type and gets its tiny amount of power from my sound card. Its active end is a little bigger than the end of a pencil. Maybe it is what you call a "boom" mic.

A noise-cancelling mic doesn't have filters. If you hear noise, a filter would also filter desireable sounds except if the desireable sound is narrowband like a telephone then filtering out the low and high frequencies might reduce some noise.

Many mics are omni-directional and pickup sound (and noise) all around.
Other mics are uni-directional, also called cardioid and hyper-cardioid and pickup sound very well on axis, a little less to the sides and much less to the rear. They cost a little more and are probably called "noise-cancelling". Because of the long wavelengths of sound, their directionality isn't perfect, being much poorer at low frequencies. They work by using vents mounted on their sides which are ducted to the rear of the diaphram.

Very expensive noise-cancelling mics use 3 uni-directional mics, two at the sides connected out-of-phase with the one at the front. With noise surrounding them, they are very quiet. But when they pickup voices or music from the front, it sounds odd because some desired frequencies are cancelled.

 

[evandude]

Thanks for the info.

This is the "guts" of one of the particular noise-cancelling mics I am referring to.

**broken link removed**

and this is the mic it came from:

**broken link removed**

Doesnt' appear to have any real advanced construction, vents, etc. I am obviously not a microphone expert, but that looks like every other electret mic element i've seen.

I may look into narrow-band filtering. I am only using the mic for voice recognition software, so minimizing noise and passing only enough voice frequency to get my commands heard is of paramount importance, rather than sound definition like you'd want if going for accurate recordings...

 

[audioguru]

Hi Evandude,
Your mic doesn't look like a noise-cancelling type. It looks like it came from a dollar store.

Watch out for filtering voices with a narrow-band filter. Have you ever heard an underwater throat-pickup mic? It picks-up only vocal cord vibrations. The resulting filtering out of very important higher frequencies makes them very difficult to understand speech when heard by a human with a big brain for voice-recognition. Your software wouldn't have a chance. :lol:

 

[zachtheterrible]

hey evandude, just an idea when you are making your noise cancelling mic: If you use an op-amp, you can connect your main mic to one input, and another mic to the other. The other mic can "listen" to the noise that you don't want to hear. The noise that you dont want to hear will then be subtracted from your main mic. I read that somewhere about cancelling noise in a circuit, but im sure it could be applied to this.

 

[audioguru]

Yeah, Zach. They use noise-cancelling systems like that in luxury cars.
But it works only at low frequencies due to their long wavelengths. Only the low frequencies have nearly the same phase. Higher frequencies have changing phase with distance making half of them the wrong phase and therefore the system would amplify them instead of cancelling like at low frequencies.
The same thing applies to adding another mic to cancel ambient noise. You can't place the extra mic near the headset or it would pickup the voice and cancel it too. So it must be located at a distance where higher frequency noise would have changing phase. :lol:

 

[evandude]

that particular mic costs $20 at best buy.

Yeah, I already thought of using a second mic except for the phase issues. oh well. well technically that isn't "my mic" because I haven't bought one yet. I'm glad someone else agrees with my skepticism at those being called "noise cancelling"...

So then I suppose the only reason they are any better than crappier mics is that they are just better quality microphones? i can't think of much else to explain the performance difference...

 

[audioguru]

A cheap mic can be made "noise-cancelling" by using it up close to your mouth (eat the thingy) and turn down the gain. :lol:

 

[evandude]

haha... good point.

well in my car, the mic is attached to the roof, up by the sun visors, to the front right of the driver, and up... I think I am going to buy a new mic sometime, I found one of the "noise cancelling" ones for under $10...

I'm thinking that the only reason they're "noise cancelling" is that they're designed to be somewhat unidirectional. in my case, that works out well, because only the driver really needs to be able to command it.

But, I figure that for only $10 i can hack one of these up, mount it where I want, and still have my current, crappy little lapel mic as a backup in case it doesn't work out.

 

[tansis]

Knew of one guy who used to put a bucket on his head to use the telephone in a machine shop, wierd but it worked.

 

[evandude]

I'm not sure the cops would appreciate me driving down the highway with a bucket on my head :lol:

 

[audioguru]

Most cell phones sound like the user has a bucket over their head! :lol: