Recording and Playing Inaudible frequencies

[Gumby]

I need to do a presentation on Bats, and want to demonstrate to the audience bat calls. Bats make noise above that audible by humans. My aim at the moiment is:

Recording the bat calls to CD and play them back to the audience. They hear silence. Audience put Bat Detectors in ears (I am being serious before you ask), and the play the track again. The bat detectors will convert the previously inaudible sound to audible.

I'd also like to record my own voice if possible, and convert that to a an inaudible frequency that the bat detector could also convert to audible

My questions are:

1. Do CDs record inaudible frequencies?
2. Do PAs reproduce inaudible frequencies?
3. Does the CD/PA produce a whole heap of 'unwanted' inaudible noise that will interfere with the detection of the 'wanted' inaudible frequency
4. What software is recommended for the conversion my audible recording to inaudible?

Thanks in advance for all suggestions and recommendations

Gumby

 

[david strait]

One of my friends who knows way more than I do about gear etc. told me that the mic used while recording would be the weakest link in the chain. He said cd's are no problen and your PA will duplicate what's on the cd, but that some mics are set up to eliminate those frequencies intentionally. This sounds like an interesting project. Good luck with it. Maybe post an MP3 for us here when you get it done. p.s. where can I get those earplugs that convert the frequencies?

 

[AlChuck]

CDs, with a sampling rate of 44.1 kHz, theoretically capture frequencies up to about 22 kHz.

PA speakers typically reproduce up to about this range too, at best.

You can record to a computer at a higher sampling rate -- up to about 192kHz, I believe, which corresponds to a frequency respones of around 96 kHz -- but I don't know if there are any transducers that will accurately reproduce sounds that are up there.

You left out part of the chain. What are you going to use as a transducer to get the bat call into the computer in the first place? Most mics I know of don't have a frequency response much better than 20 Khz, if that.

Methinks you will need real bats for your demonstration...

 

[AlChuck]

...some mics are set up to eliminate those frequencies intentionally.

Huh?

I think it's more that it's rather difficult and expensive to design a device to capture frequencies that humans cannot hear, so for ordinary use, it makes no sense to design them that way.

 

[mshilarious]

same bat-time, same bat-channel

1. Do CDs record inaudible frequencies?

No, they don't. A standard CD is theoretically capable of reproducing 22 kHz. CD recorders use a device called an anti-aliasing filter to eliminate higher frequency sounds, which is necessary as they can mess up the encoding of audible frequencies.

You could theoretically reproduce up to 48 kHz with a DVD-Audio format (96 kHz sample rate), but again they still have anti-alias filters that would cause problems. Generally the filters on a 96 kHz converter are higher and gentler than those on a standard format CD converter, so that might work if you need reproduction up to maybe 25 kHz. I just checked specs on several converters, and only one listed freq. response, which was only 22 kHz even though it was a 96kHz converter.


2. Do PAs reproduce inaudible frequencies?

I think I've seen specs on some horns that reproduce up to 30 kHz, but generally, no, they aren't designed to. How high are these bat calls?

Crap, I just looked on Google! You're talking anywhere from 40 to 90 kHz. 'Fraid you're outta luck.

3. Does the CD/PA produce a whole heap of 'unwanted' inaudible noise that will interfere with the detection of the 'wanted' inaudible frequency

Don't know, see #2.

4. What software is recommended for the conversion my audible recording to inaudible?

You know, I don't know what bat detectors are, but I assume it's some sort of device that lowers the frequency to audible range. Why not simply record the bat calls with such a device? You'd need a microphone that had a frequency response that high (which is a specialty piece of gear, I imagine), patched into an analog octave device, then record the audible sound from there, no more worries.

 

[Massive Master]

A typical bat click's peak energy is at around 50-55kHz.

So, the first thing you need to do is get a microphone that will pick that up. Then, look for a harmonic at around 5-5.5kHz. Record that.

 

[Bulls Hit]

4. What software is recommended for the conversion my audible recording to inaudible?

A volume knob. (Sorry, that's a hardware solution)

 

[jfrog]

Well, the last time I recorded bats....

Just kidding, I alway wanted to say that.

Seriously, though, if you could get a mic capable of recording them, does audio tape (you remember - the pre-Napster way of song sharing) have sufficient frequency response? It might be something to look into.

 

[Freudian Slip]

Sounds like you have access to bats? You could build a bat encloser for much cheaper than the gear it would cost you to do this, If you can get it.

Need I say more

You know it might be easier to find a unit to produce that frequency rather than record it. Either way.. Big bucks.

F.S.

 

[Bulls Hit]

Sounds like you have access to bats?
F.S.

Yeh he has bat access

 

[radiorickm]

good idea

you have a really good idea here, unfortunately it doesn't seem practical.

there are numerous issues about recording outside the normal AUDIO SPECTRUM, and then trying to decode the bats radar. that would be a trick.


i think an avenue to persue might be using an audio frequency spectrum analyzer and demonstrate where we can hear, showing spikes on the graph, and then where the bats emissions are


just a suggestion