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Full spectrum passive recording (1 Viewer)

Had.enough

Registered User
Supporter
Hi,
I had an audiomoth, but am just about on the verge of giving it up.
(I'm finding the configuring interface very unreliable, the "triggering" non-existent, plus the battery compartment is about to break off from the circuit board.)

However, the recordings I got from it were really useful. In that the sonograms on Audacity covered both birds and bats, and therefore it was ideal to leave overnight. (I have a zoom h2 which is good for bird calls, and a Time expansion bat detector which is good for remote bat calls.)

Question is, is there anything I can get to produce the same "full spectrum recording" ?.
I'm thinking of an ultrasonic mic that can be plugged into the Zoom H2 Line in? then set the zoom H2 to trigger when recording at 256 KHz MP3 format, but don't seem to be able to find anything

Any ideas would be welcome
 

Jon.Bryant

Active member
I initially was looking for a recorder that would do bats and birds, but I could not find a recorder to do both simultaneously and the recorders with ultrasonic capabilities were more expensive. I therefore decided to stick with birds - this response may therefore be able to be improved by someone with a better working knowledge of bat recorders.

I am not sure how the Audiomoth is able to record acoustic and ultrasonic audio in the same track. Technical details on the AudioMoth seem very scant online, and there seem to be no information on the recording format, microphone pickup pattern, mic sensitivity etc.

Looking at something like the Wildlife Acoustics SM4 mini bat, you can record 'full spectrum', but 'full' doesn't seem to mean it covers the full acoustic range - more like full above a reasonably high frequency floor. Furthermore if the SM4 ultrasonic mic is anything to go by, the sensitivity of this ultrasonic mic starts to drop off and the signal to noise ratio increase below 15Khz (so presumably impacting audible sound). I suspect that the choice is to have a mic that works well in the acoustic range or the ultrasonic range, but not both.

With the SM4 mini bat you can install a second acoustic mic, but I understand that you cannot record an acoustic and ultrasonic signal at the same time - you could however program the recorder to make alternating recordings of whatever length you desired - says an hour of bats then an hour of birds.

The Zoom H2N (not sure about the H2) can record wave files with a sample rate of 96Khz, so in theory you could record sound to half this frequency (48Khz), but you would need to find a mic that could achieve 0-48Khz. My acoustic mics are all only sensitive to 20Khz, and I suspect that this will be the norm for most acoustic mics on the market. In comparison the SM4 mini bat records up to 500KHz, although the manual states that a sample rate of 256Khz (recording up to 128KHz frequency) is adequate for most North American and European bats. I think that when you state 256Khz MP3, there may be some confusion between Khz and kbps - MP3 is a lossy format, but is good for achieving reduced files size. I therefore suspect that this format will not support recording of ultrasonic sound, which would be wasted memory for audio recordings. From a brief web search I found the following data regarding MP3 frequency range

1411kbps >20kHz
320kbps 19.5kHz
192kbps 18kHz
160kbps 17kHz
128kbps 16kHz
96kbps 15kHz
64kbps 11kHz
32kbps 5kHz

I imagine the MP3 format you are referring to is 256kbps, so would have a max frequency range of less than 19.5KHz and would be no good for bats.

In summary, I think if you want to record both ultrasonic and acoustic at the same time you probably need two separate recorders. If on the other hand the AudioMoth achieves this amazing feat, I would stick with it - the device also seem very cheap so I can't see how you could beat it on value with anything else.

If you have any information on the frequency range the AudioMoth can record across, it would be very interesting to know.
 

Had.enough

Registered User
Supporter
Hi,
All I can find is "AudioMoth can listen at audible frequencies, well into ultrasonic frequencies. It is capable of recording uncompressed audio to microSD card at rates from 8,000 to 384,000 samples per second.

On Audacity software, I could pull up a spectrogram of a file from audiomoth, and see birds ~8khz, plus bats up at 50khz.
( I'd have to select the bat calls, and reduce the speed by 10x to hear them, or identify by the sonogram).

So it sounds like the zoom is for the birds only.

The pettersson u256 or 386 would be a good choice for me I think
These have a range from 1khz up to 125khz or 192khz. And there is a good android app to manage the "triggering", so i have a manageable amount of data coming off it.
 

Had.enough

Registered User
Supporter

There's an example if you enlarge the image from my blog, #90 was a blackbird I think, from memory, #91 a soprano Pipistrelle. Looks like I was having a similar labour of love back in June!

Suspect I had a duff rechargeable battery giving me grief over the weekend. Have re flashed the latest firmware and all seems good.
I find the audiomoth very difficult to use, any number of simple things can leave you bewildered, such as forgetting to put the memory card in, or trying to make it work outside it's recording time etc.
However, I think I'm good now, and have written down my instructions!
 

Borjam

Registered User
Supporter
I have tried to record bats once using a good but non specifically ultrasonic capable recorder and, to my surprise, it worked pretty well at close range. Bats are loud!

In my case I used a Marantz PMD661MK2 with its builtin microphones.

Anyway, a good professional recorder such as a Sound Devices MixPre 3 II or the MixPre 6 or 10 (both original an series II) that can sample at 192 KHz will be able to record 90 KHz of spectrum as long as you connect a proper microphone.

And even good condenser microphones can record ultrasonic frequencies although with some attenuation, like the humble builtin electrets in my Marantz.
 

Jon.Bryant

Active member
I have tried to record bats once using a good but non specifically ultrasonic capable recorder and, to my surprise, it worked pretty well at close range. Bats are loud!

In my case I used a Marantz PMD661MK2 with its builtin microphones.

Anyway, a good professional recorder such as a Sound Devices MixPre 3 II or the MixPre 6 or 10 (both original an series II) that can sample at 192 KHz will be able to record 90 KHz of spectrum as long as you connect a proper microphone.

And even good condenser microphones can record ultrasonic frequencies although with some attenuation, like the humble builtin electrets in my Marantz.
I would be interested to know what were the frequencies you were successful with using the Marantz? I occasionally see a series of ticks/clicks on NocMig recording at approximately 18,500Khz, which I assume is a bat, although it seems a little low in frequency. I am recording at 96KHz and have sonograms displayed across the full range, so up to 48,000KHz, but never see a peep about the mic threshold of 20,000KHz. Perhaps there would be a response if a bat was very close - as you say, I understand that bats are in fact incredibly loud - even a very weak response from my mic may therefore generate some kind of signal.

It is a bit frustrating that most companies are silent on frequency response of bat equipment, so it is unclear if mics work well across full spectrum.

I have found some further date on the Audiomoth, but it shows a fairly flat response for the mic between 1 and 10KHz, without any data beyond that. I am a bit of a layman, but from my interpretation of sensitivity and signal to noise ratio data for the mic, it does not look great for good quality acoustic recordings - but then I presume the AudioMoth's purpose is passive monitoring of soundscapes for conservation studies. The performance conclusion seems to be backed up by on the following link Equipment Comparison February 2020 – nocmig, where there is mention that some NocMig sounds were hard to detect in the Audiomoth recordings and that noise (hiss) in an issue on full sensitivity.

A paper on the web Ultrasound behavior and guidelines of analog MEMS microphone IMP23ABSU - Application note (st.com), has a frequency response for a MEMS mic - both the AudioMoth and Pettersson u256 or 386 use MEMS mics, but I think different models. If this frequency response if typical for all MEMS type mics, then they are much more sensitive between 15,000KHz and 50,000KHz, with a distinct peak in the frequency response. The acoustic range is flat (as per the other data I found on the AudioMoth), so Audio recordings should not be coloured.

I am not sure how you would set recording levels for full spectrum as the frequency response diagram suggests that if you set a level appropriate for birds, a bat recording at 30,000KHz would probably be clipped (or perhaps the device limiters triggered) - the frequency response below indicates the signal would 18dB (or 64 x louder - and as stated bast calls are loud in any case!). I would imagine from the data that a bespoke bat recorder may have a very low gain range. This could mean that bird sounds then are indiscernible or very weak in recordings.

I suppose a possible solution to recording bats and birds at the same time, would be to use a recorder with capability to record 192KHz or better, and then to have separate acoustic and ultrasonic mics, to record birds on the left channel and bats on the right. Gain could then be set for each channel accordingly. There don't seem to be many ultrasonic mics with standard xlr connections, and the one I found made me fall off my perch when I saw the price! So, I still think it is probably more economic to use two separate passive recorders - one for bats and one for birds.

1606893234079.png
 

Had.enough

Registered User
Supporter
Hi Jon, you say you are a layman here, but you obviously have quite a bit of technical knowledge, particularly relative to myself.
My limit is to turn the device on, and listen back the following day! (With one or two shortcuts)

You are correct, quality is sacrificed with a preference for detection.

My only reservation about two devices is that it is twice the analysis involved, although this is probably false, as there will be fewer triggers on each device!

I'm in a better position now in that I have:

1. The audiomoth seemingly reliable again, with my own instructions to prevent mishaps. This will detect birds and bats

2. A zoom H2, which triggers well for bird calls

3. A bat detector with external battery which runs for 12 hours plus.

All are capable of remote triggering, so once I have tweaked the trigger sensitivities, they should all be good to deploy.

Plus, I haven't shelled out a lot of money!
 

Vollmeise

Well-known member
(...) I suppose a possible solution to recording bats and birds at the same time, would be to use a recorder with capability to record 192KHz or better, and then to have separate acoustic and ultrasonic mics, to record birds on the left channel and bats on the right. Gain could then be set for each channel accordingly. There don't seem to be many ultrasonic mics with standard xlr connections, and the one I found made me fall off my perch when I saw the price! So, I still think it is probably more economic to use two separate passive recorders - one for bats and one for birds(...)
Jon, if a sampling rate of 192 kHz would help (enough for recording Sounds of 96 kHz), the Sound Devices Mixpre-3 II should meet your needs.

A you mentioned, place a mic for birds to the left channel and one for bats to the right.

Other than most field recorders, the mixpre II version offers a bit depth of 32 bit floating point. That means, no matter how low or high your gain settings were when recording, no clipping will ever occur as You can readjust the gain in post processing without any loss. It's kind of a RAW format for audio.

The extremly low noise XLR inputs of the Mixpre Recorders can be used for mics with 3,5 mm jacks also by using an XLR to plug-in-power adapter (like the Rode VXLR+, see atached photo).

Cheers, Stefan
 

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Dale Forbes

SWAROVSKI OPTIK Austria
Jon, if a sampling rate of 192 kHz would help (enough for recording Sounds of 96 kHz), the Sound Devices Mixpre-3 II should meet your needs.

A you mentioned, place a mic for birds to the left channel and one for bats to the right.

Other than most field recorders, the mixpre II version offers a bit depth of 32 bit floating point. That means, no matter how low or high your gain settings were when recording, no clipping will ever occur as You can readjust the gain in post processing without any loss. It's kind of a RAW format for audio.

The extremly low noise XLR inputs of the Mixpre Recorders can be used for mics with 3,5 mm jacks also by using an XLR to plug-in-power adapter (like the Rode VXLR+, see atached photo).

Cheers, Stefan
@Stefan, I use a mixpre II with a shotgun for bird sounds. Any suggestions on an ultrasonic mic for bats to get started? I recently found a scarce bat roosting nearby and this has piqued my interest
 

Vollmeise

Well-known member
@Stefan, I use a mixpre II with a shotgun for bird sounds. Any suggestions on an ultrasonic mic for bats to get started? I recently found a scarce bat roosting nearby and this has piqued my interest
Dale,

send a request to Ivano Pelicella, owner of dodotronics in italy. He's a smart and engaged engineer focussing on parabolic and ultrasonic mics. I'm sure he can recommend and offer the right mic for Your needs and even do some customizations if necessary.

Cheers
 

Jon.Bryant

Active member
Dale,

If Ivano provides any advice on ultrasonic mics it would be great to read it. The only mic I found with an XLR connection, which could be plugged into a standard recorder was Euros 2,500! This was one reason that I concluded that a specialist bat recorder was probably the most economic solution, rather than trying to record bats and birds with one device.

The other probably less important reasons, are that
  1. Even with a 192Khz recorder, the maximum frequency response would be 1/2 the sampling rate so 96Khz - good old Wikipedia states that bat call range from 12-160KHz and that most bat recorders cover up to 125Khz, so I am not sure what may be missed at the top end of the spectrum between 96 and 125KHz?
  2. My MixPre chomps through batteries at 192kHz, 32 bit float. It also cannot be secured in location, so would not be good for overnight deployment away from the house (and a power source). Devices like the Wildlife Acoustics SM Mini Bat are designed with longer term deployment in mind and can be fixed in place with a cable lock, so could be left unattended, if you are not the anxious type.
 

Dale Forbes

SWAROVSKI OPTIK Austria
I have yet to contact Ivano, but watched some videos he and others had produced and that got me thinking that a USB mic plugged in to my phone and a bat app would undoubtedly be more useful for me but I've yet to take the plunge
 

Had.enough

Registered User
Supporter
On Android, at least, there is an app called "Bat Recorder" which seems pretty good once you have familiarised yourself with it. i've only used it without a mic (so only recording hearing range frequencies around the house, no bats!), but it definitely has potential.
 

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