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Recording bats - advice from field recordists appreciated! Condenser Microphones
Old 26th August 2011
  #1
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thenewyear's Avatar
Recording bats - advice from field recordists appreciated!

OK I need a little advice on a very odd little recording project.

My friend (who is a ranger in a national park) wants to record bat calls at their naturally occurring frequency and then play them back again at the naturally occurring frequency. The object of this exercise is to play the calls back to a group of children in an educational setting and say "can you hear the bat calls" to which they say "NO!" and he explains that is because they are outwith the range of human hearing and then he hands out bat detectors to them all which they can switch on, dial into the frequency of the calls and magically hear the (transposed) bat calls. The bats will probably be in the main Common Pipistrels whose calls fall in the 43k - 47k range.

Obviously to record these calls I am going to need high sampling rates and a bunch of equipment whose frequency range extends far above 20k before tailing off.

I'm thinking of using:-
  • a regular bat detector with minijack output
  • Sound Devices 722 running at 192kHz
  • Neumann 183 with windshield (although I have a bunch of other mics if anybody thinks this is not that appropriate)

Approach 1: record the bat calls acoustically at a high sampling rate and later edit the recordings and possibly pitch shift them down to around 22k for greater chance of achieving any meaningful playback from domestic playback equipment that will be found in classrooms

Approach 2: record the bat calls via the bat detector and then later pitch them UP to around 22k

Neither approach is ideal as the kids would be dialing into 22k on their bat detectors when the original cries were at 45k so there would be an element of fibbing to the kids about the frequency of the original calls. However both approaches might achieve that educational magic of turning on the recording to have the kids hearing nothing and then have them dial in with their bat detectors to reveal the calls.

Does anybody who has managed to read this far (well done if you have by the way!) have any advice??

Thanks!
Old 26th August 2011
  #2
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John Willett's Avatar
 

Exclamation

The Sennheiser MKH 8020 would be better than the Neumann 183 as the top end of the 8020 extends to 60kHz - the 183 only goes up to 20kHz and is already beginning to roll off, so the 183 would not hear the bat's frequencies at all.

The MKH 8020 is also an RF condenser and will work better in damp and humid conditions (especially if you are recording in a bat cave).

Also, I think you will find it a good idea to talk to the Wildlife Sound Recording Society.

You'll also need to check with Sound Devices if the 722 goes up that high as the only spec. I can see says up to 40kHz. The Nagra LB may also be worth considering at a similar price (again, you will need to check frequency response with the company).

The first picture is the KM 183's frequency response and the second is the MKH 8020 (the frequencies above 20kHz are in the blue section and you can see that it is still picking up at 100kHz).

.
Attached Thumbnails
Recording bats - advice from field recordists appreciated!-183.jpg   Recording bats - advice from field recordists appreciated!-8020.jpg  
Old 26th August 2011
  #3
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thenewyear's Avatar
Thanks for this John,

I looked at the freq. specs for all the kit. The 40kHz listed against the SD722 is for a Fs of 96kHz (which just about ties in for the Nyquist freq for that rate) so I'm hoping for 192 kHz the range is correspondingly extended to somewhere a little south of Nyquist (although that is pure guesswork).

Likewise with the Neumann I've seen the specs but am *hoping* it extends further and they are only quoting the flat/designed portion of the range and the -3dB point is well beyond that (although looking at the curve on the graph you posted makes me doubt that).

I've read about the MKH's extended frequency response before but have yet to have an opportunity to work with any (nor do I have access to any). Likewise the Nagra isn't an option as I am just using what I have to hand & the recording is this evening! I should add this is very much an unpaid favour to a friend and nothing more.

Alternative mics I have around here are:-

414XLS
451B
SM57
SM58

And thanks for the link to the WSRS I'll ask for some advice there as well.
Old 26th August 2011
  #4
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I had one of the first portable bat detectors from Imperial College in the late 70s
We were making a Bat film for the BBC NHU.
It was Lancaster Bomber build and clunky, and worked by super heterodyning signals from an electret mike.
I recorded the output on my Nagra.
We were underground in a flint tunnel searching for a very rare pipistrel with two experts.
The signal was loud but we could not locate him.
I stepped back to allow the cameraman a shot and the bat squeaked intensely,He was on the cave floor unnoticed, and I had stepped on him.
He was dead.

I could easily hear bats well into my 50's, clicks and tones.
Bat detectors are now cheap and portable
Kids can easily hear Bats
Surely better to go Bat hunting with kids than lab stuff ?

Roger
Old 26th August 2011
  #5
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John Willett's Avatar
 

Exclamation

Quote:
Originally Posted by thenewyear View Post
Thanks for this John,

I looked at the freq. specs for all the kit. The 40kHz listed against the SD722 is for a Fs of 96kHz (which just about ties in for the Nyquist freq for that rate) so I'm hoping for 192 kHz the range is correspondingly extended to somewhere a little south of Nyquist (although that is pure guesswork).

Likewise with the Neumann I've seen the specs but am *hoping* it extends further and they are only quoting the flat/designed portion of the range and the -3dB point is well beyond that (although looking at the curve on the graph you posted makes me doubt that).

I've read about the MKH's extended frequency response before but have yet to have an opportunity to work with any (nor do I have access to any). Likewise the Nagra isn't an option as I am just using what I have to hand & the recording is this evening! I should add this is very much an unpaid favour to a friend and nothing more.

Alternative mics I have around here are:-

414XLS
451B
SM57
SM58

And thanks for the link to the WSRS I'll ask for some advice there as well.
Please - don't "hope" - check !

Talk to SD and make sure for certain that the frequency will go up to above 60kHz at 192kHz.

The Neumann KM 183 will, almost certainly not work at the frequencies you want. Also the Shures you mention are also totally unsuitable for you. From what I see of the specs. the AKGs you mention are no good either - they just do not have the op-end frequency response you require.

The best is definitely the MKH 8020 as they quote it up to 60kHz and it is still picking up at a reasonable level at 100kHz.

The more expensive MKH 800 also goes up to about 50kHz.

The above are RF condensers which are safer in the damp.

The only other mic. I know that goes that high is the Earthworks QTC50 which goes up to 50kHz - this is an electret mic..

You really need to thoroughly check the specifications of the equipment you want for this as there could always be something that limits the frequency response, making it unusable for your purposes.

As far as I know, the only microphones that will work at the frequencies you want are the ones I have mentioned above and you will really have to talk to SD to make sure that all sections of the 722 will handle the frequencies you want at 192kHz.

If the recording is this evening then you are stuffed - none of the microphones you mention go that high. Microphones normally deliberately roll off the high frequencies to prevent unwanted high harmonics from disturbing what is happening lower down. You really need to get hold of one of the ones I mentioned.

Shame you are in Scotland as I may have offered to pop over with my MKH 8020s - but it's far to far away from Oxford for me to do in an evening. It sounds interesting.
Old 26th August 2011
  #6
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John Willett's Avatar
 

Exclamation

For the record - Nagra have just measured an LB in the factory at 192kHz sampling: in REC/REPLAY and at 40 kHz it is -0.3 dB and at 60 kHz it is -1 dB.
Old 26th August 2011
  #7
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If you are serious about this IMO you should check out some B&K mic's. There are small capsule omni mic's with 100kHz extension.

I have both the EW QTC50 and MKH8020 and I would use the smaller Earthworks omni since you will find it hard to have the bats lined up on axis. :-) What I mean is a bigger membrane (16mm for MKH8020) will have little pickup off axis compared to the 5-6mm membrane in the EW omni.


/Peter
Old 26th August 2011
  #8
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PoxyMusic's Avatar
 

Keep 5 mg of Valium in your kit as well.

Bats! <shudder>
Old 26th August 2011
  #9
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Quote:
Originally Posted by John Willett View Post
The Neumann KM 183 will, almost certainly not work at the frequencies you want.
How do you know that from a frequency response graph that only goes to 20kHz? Just because it hasn´t been measured beyond 20k doesn´t mean it´s not picking up anything beyond 20k.
Old 26th August 2011
  #10
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Quote:
Originally Posted by John Willett View Post
You'll also need to check with Sound Devices if the 722 goes up that high as the only spec. I can see says up to 40kHz.

.
Just for the record: Specs say 40kHz@96Khz.
Old 26th August 2011
  #11
Gear Nut
 

I have recorded bats with a DPA4061 to a SD702 @192kHz. At least I could detect and record bats at 50-60kHz.
Old 26th August 2011
  #12
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John Willett's Avatar
 

Exclamation

Quote:
Originally Posted by apple-q View Post
How do you know that from a frequency response graph that only goes to 20kHz? Just because it hasn't been measured beyond 20k doesn't mean it´s not picking up anything beyond 20k.
No - but you can see that the frequency response is already starting to droop at 20kHz which implies that it's on the way down.

If it showed no sign of a droop at 20kHz I would say maybe.

But the manufacturers that advertise mics going up to 50 or 60 kHz have put in extensive design work to get them to go that high - I don't think a normal mic. that's only quoted up to 20kHz will go up there with any usable results - if it did, the manufacturer would say so.
Old 26th August 2011
  #13
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John Willett's Avatar
 

Smile

Quote:
Originally Posted by apple-q View Post
Just for the record: Specs say 40kHz@96Khz.
Yes, but it does not say what it is at 192kHz - which is why I said ask SD.

The electronics may be the limiting factor, maybe not, just ask.

Sometimes I don't understand why questions are asked on internet forums when the best answer would be obtained by ringing up the manufacturer and asking.
Old 26th August 2011
  #14
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John Willett's Avatar
 

Exclamation

Quote:
Originally Posted by Audiop View Post
If you are serious about this IMO you should check out some B&K mic's. There are small capsule omni mic's with 100kHz extension.

I have both the EW QTC50 and MKH8020 and I would use the smaller Earthworks omni since you will find it hard to have the bats lined up on axis. :-) What I mean is a bigger membrane (16mm for MKH8020) will have little pickup off axis compared to the 5-6mm membrane in the EW omni.


/Peter
This is the polar-pattern of the MKH 8020 - yes, the QTC50 holds up better off-axis as it's a smaller diaphragm.

But the noise floor will be higher as it's a smaller diaphragm and the frequency response drops off very quickly above 50kHz - and it's an electret.

.
Attached Thumbnails
Recording bats - advice from field recordists appreciated!-mkh8020_clockfacediagram.jpg  
Old 26th August 2011
  #15
Gear Maniac
 

the problem here is not only about recording

what speakers are you going to playback the bat calls?
anyway, no matter how you do it, the outcome is still - can't hear at all! why bother with the equipment?
Old 26th August 2011
  #16
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Quote:
Originally Posted by thenewyear View Post
OK I need a little advice on a very odd little recording project.

My friend (who is a ranger in a national park) wants to record bat calls at their naturally occurring frequency and then play them back again at the naturally occurring frequency. The object of this exercise is to play the calls back to a group of children in an educational setting and say "can you hear the bat calls" to which they say "NO!" and he explains that is because they are outwith the range of human hearing and then he hands out bat detectors to them all which they can switch on, dial into the frequency of the calls and magically hear the (transposed) bat calls. The bats will probably be in the main Common Pipistrels whose calls fall in the 43k - 47k range.

Obviously to record these calls I am going to need high sampling rates and a bunch of equipment whose frequency range extends far above 20k before tailing off.

I'm thinking of using:-
  • a regular bat detector with minijack output
  • Sound Devices 722 running at 192kHz
  • Neumann 183 with windshield (although I have a bunch of other mics if anybody thinks this is not that appropriate)

Approach 1: record the bat calls acoustically at a high sampling rate and later edit the recordings and possibly pitch shift them down to around 22k for greater chance of achieving any meaningful playback from domestic playback equipment that will be found in classrooms

Approach 2: record the bat calls via the bat detector and then later pitch them UP to around 22k

Neither approach is ideal as the kids would be dialing into 22k on their bat detectors when the original cries were at 45k so there would be an element of fibbing to the kids about the frequency of the original calls. However both approaches might achieve that educational magic of turning on the recording to have the kids hearing nothing and then have them dial in with their bat detectors to reveal the calls.

Does anybody who has managed to read this far (well done if you have by the way!) have any advice??

Thanks!
do like the radio designers
you can record at your normal sample rates
use a miek taht covers less than 20kcps of the bat range
mix it with a another frequency picked to cause images
that will fall in the 20-20kcps standard audio range

select the lower images and wallaah

only custom gear you would need is a radio type mixer circuit which should be very easy at audio freqs
local oscillator (one freq!) and then a simple mixing circuit
to multiply the bat mike stuff with the LO.
select the lower image and you are done.
Old 26th August 2011
  #17
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Quote:
Originally Posted by oldeanalogueguy View Post
do like the radio designers
you can record at your normal sample rates
use a miek taht covers less than 20kcps of the bat range
mix it with a another frequency picked to cause images
that will fall in the 20-20kcps standard audio range

select the lower images and wallaah

only custom gear you would need is a radio type mixer circuit which should be very easy at audio freqs
local oscillator (one freq!) and then a simple mixing circuit
to multiply the bat mike stuff with the LO.
select the lower image and you are done.
That's not an option since it does not work that way.


/Peter
Old 26th August 2011
  #18
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Quote:
Originally Posted by John Willett View Post
This is the polar-pattern of the MKH 8020 - yes, the QTC50 holds up better off-axis as it's a smaller diaphragm.

But the noise floor will be higher as it's a smaller diaphragm and the frequency response drops off very quickly above 50kHz - and it's an electret.

.
The SNR is likely better for QTC off axis. I think you really would need to do some measurements to really know for sure. Guess it all depends on if you can have the source on axis or not.

Being an electret I don't see as a problem. Distortion performance is basically equal for QTC and MKH.


/Peter
Old 26th August 2011
  #19
Gear addict
 

safety first

this sounds dangerous for the bats. If you capture the sounds they make, doesn't that leave them blind? I wouldn't want a whole gaggle of bats colliding willy-nilly around my expensive mics, and of course, there is the issue of their safety.
Old 27th August 2011
  #20
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Quote:
Originally Posted by SonicAlchemist View Post
this sounds dangerous for the bats. If you capture the sounds they make, doesn't that leave them blind? I wouldn't want a whole gaggle of bats colliding willy-nilly around my expensive mics, and of course, there is the issue of their safety.
Yes, that's a possibility to consider, you could be creating a partial 'sonic vacuum' by sucking up all their ultrasonics with your mics and leaving them with none left to go insect hunting with.........

I'd be wearing some sort of helmet protection in case a bat crashed into you at high speed, although with their navigational and aerobatic control skills it would be highly unlikely. Then there's always Hendra and Nipah to worry about, in warmer climates with larger fruit bats: Henipavirus - Wikipedia, the free encyclopedia
and perhaps rabies with the smaller species ! Full body armour is probably the way to go.......
Old 27th August 2011
  #21
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abechap024's Avatar
 

Quote:
Originally Posted by SonicAlchemist View Post
this sounds dangerous for the bats. If you capture the sounds they make, doesn't that leave them blind? I wouldn't want a whole gaggle of bats colliding willy-nilly around my expensive mics, and of course, there is the issue of their safety.
i AGREE, I know when i setup mics people have to turn up just to compensate for the "black wholeness" in the "sound field" its seems to be mostly guitar players effected by this phenomena


Just kidding

I highly doubt recording the bats calls will have and adverse effect on their ability to "see" ultrasonically.


Unless it was a joke and i missed it entirely, in which case I apologize.
But it sounds like a very interesting and good venture! We have lots of bats around here and they are fascinating.
Old 27th August 2011
  #22
Lives for gear
My good friend was recording Attenborough for 'Life on Earth' in Borneo
A remote,innacessible cave, full of a million Bats and a pile of Bat guano 100' high, covered, in giant cockroaches.
Sir D was to do a piece to camera atop the pile of poo, It was a huge cavern, and required a lot of light for filming.
A genny was hired and hauled in.
The genny was fired up, it was very noisy and startled a million rectums, the air was full of acrid falling guano and hydrogen sulphide...
They beat a hasty retreat.
Bats can be hazardous to health and dignity.
Old 27th August 2011
  #23
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boojum's Avatar
Where is Col. Bat Guano when we need him??!!
Old 28th August 2011
  #24
Gear addict
 

Quote:
Originally Posted by abechap024 View Post
Unless it was a joke and i missed it entirely, in which case I apologize.
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