sound cancellation

[sunil]

Hello,

I have 2 different recordings. One is my main recording which includes background noises that I wish to eliminate, while the other one only has background noises that I want to remove. How would I subtract the background noises from my main recording?

Additionally there seems to be another issue with these recordings. When I try to align them they do not completely match up, it seems that the background recording somehow seems to have a different bit rate than the main recording. Is there any way to cancel them out if they don't align? If not is there some way I can get them to align and then cancel out the background noises from the main recording?

Any help would be appreciated. Thanks

[steve]

I doubt that this is possible. Usually for sound cancellation to work both audio tracks must be recorded at exactly the same time from (almost) exactly the same point. Perhaps you could say a bit more about what/where/how the two audio tracks were recorded.

[sunil]

So if I get new recordings that are both at the same rates and align them so that timings match, then I will be able to do sound cancellation? How would that work?

[steve]

The most simple form of sound cancellation is to exactly align the two sounds and invert one of them.

Here's a practical demonstration using Audacity 1.3.12:

1) Open Audacity
2) Generate > Tone (default settings) > OK
3) Press Ctrl+D (duplicates the track)
4) double click on one of the tracks so that it is selected
5) Effect > Invert
6) Play.

You will notice that there is no sound - the two tracks are exactly cancelling each other out.

7) Mute one track.

You can now hear the other (non-muted) track.

This only works because the two tracks were identical, which brings us back to the question of how you are going to record two tracks so that the the noise on the two tracks is identical?

If the "noise" sounds are not identical, there are other techniques, but again there are conditions. For example, if the noise is very low bass, or very high pitched, it can be filtered out. Without more details about the type of sounds involved it is not possible to say which technique, if any, is likely to work. For some combinations, such as someone talking in the background with a lot of noise from a television, separation will probably not be possible.

[sunil]

Thanks for the prompt replies.

These recordings are going to be of mosquito swarms for research. However, due to background noises it can be difficult to hear the mosquitoes, therefore one recorder will be placed close to the swarm while the other will be about 6 meters away thus will pick up background noises, but not the mosquitoe noises. Will those recordings be able to do a noise subtraction so that the background can be removed? If not are there any other techniques that may be useful for this?

[steve]

Will those recordings be able to do a noise subtraction so that the background can be removed?

I very much doubt it.

You're going to have lots of fun/problems with this.
Unless you are talking about some exotic giant swarm of mosquitoes armed with vuvuzelas I would guess that the absolute sound level of the mosquitoes will be really low. This in itself will be demanding on the recording hardware and require a very sensitive microphone and very low noise electronics.

I presume that the relevant frequencies will be pretty high, so a lot of low frequency noise can be removed using the high-pass filter effect.

High frequency noise tends to be quite directional, so you may be able to get a better recording if you use a microphone that is quite directional and place it below the swarm pointing up toward the sky. Ambient noise from the sides could be further reduced by shielding it acoustically with something like this: http://www.dv247.com/microphones/se-ele ... oth--31474
A cheaper one: http://www.thomann.de/gb/the_tbone_micscreen_le.htm
Or if the budget does not stretch that far perhaps make something: http://www.wickes.co.uk/General-Purpose ... tapopen=cm
(or even out of old carpet).

[sunil]

ok, I will still try, but thank you for the warning and information

[steve]

Another possibility that may help to some degree is the "vocal isolation" ability of "Extra Boy".
I think the "Lite" version is free. There is also a demo "Pro" version available. http://www.elevayta.com/product13.htm

See here for how to install VST effects in Audacity 1.3 http://wiki.audacityteam.org/wiki/VST_P ... a_versions

[kozikowski]

ElectroVoice once had a technique where they would tie two of their 635A microphones together head to head with heavy packing tape and cross the microphone cable wires.

http://www.electrovoice.com/product.php?id=100

You could use the combination as a noise-canceling microphone by closely talking only into one of the two microphones. The combination would neatly cancel out everything else.

EV was demonstrating their superior manufacturing tolerances. Most microphones aren't made close enough to cancel as these do.

This process also only works if the microphones are right next to each other. Any separation at all --one or two inches -- and the environment starts to distort one noise signal different from the other. No more cancellation.

You have the classic job for an expensive, special purpose microphone. The performers aren't loud enough for a conventional microphone or mixer. Many modestly priced microphones and mixers aren't up to recording a person speaking, and they're certainly not up to recording insect noises.

Koz