Recording ultrasonic frequencies

Question

I'm looking into putting together a rig that can really capture ultrasonic frequencies for the purposes of varispeeding down during sound design, and I'd like a little input from those of you who have experience in such things.

Most mics preamps and A/D converters really don't list specs above 20kHz, and I'm not entirely up to date on the science of the stuff, so any resources you can direct me to would be appreciated.

Ok, Questions;

1. Am I correct in assuming that the biggest hardware obstacle to recording sounds above 20k is generally just the microphone (not the pre or D/A)?
2. how well would the preamp and A/D of say a 744t handle supersonic frequencies (if recording with an appropriate mic)?
3. How about a lower end recorder like a Zoom H4n (again, with appropriate mic)?
4. What pitfalls should be avoided when processing high sample rate files with supersonic frequencies in them? If I take a file and just trim heads and tails in a two track editor like Izotope RX, will I ruin it or will it handle it?

That is pretty much it. Any input on specific mics and recording devices would be appreciated.

I'm considering the earthworks mic, but I'm not married to anything right now.

Answer 1

In order to record anything "ultrasonic", you'll have to start at 96k or higher. 48k only responds up to 24khz. 96k will respond up to 48khz, and so on and so forth.

So most recorders can capture ultrasonic frequencies, but you need a mic that can handle it too. The Sennheiser MKH800 is rated up to 50k, but unless you have one or $3k laying around, that may not happen.

Here's a schematic that won't cost you very much at all to build a mic that is sensitive to up to 60khz. (granted it won't be as nice and flat as the mkh800, but it's a great mic to play with!)

There's also some test recordings on that page (not ultrasonic stuff, but it at least shows you the quality of the mic)

Here's a great article on ultrasonic recording.

Also, beware of the limitations of different formats. For instance, you may be able to capture 30khz in a BWF, but everything above a certain frequency will be chopped off if you convert to any type of compressed format, including red book CDs.

As for software, I don't know the limitations of RX specifically, but theoretically, anything that can handle BWF files should be able to handle supersonic frequencies. HOWEVER, I can't say as much about the plugins (including RX's own plugins as used in it's stand alone version). Plugins operate at a certain sample rate themselves, and unless they are rated at 192k, they may end up down sampling your recording and you may lose frequencies. I would do your research on the specific software you want to use before manipulating. Or, possibly processing a copied version of the file for comparison if you can't find out info on the software.

Answer 2

I'm using the Sennheiser mkh8050 to record at 192 with my Sound Devices 722. This mic is also rated out to 50kHz, but it's a little more affordable than the Sennheiser mkh800 or the Sanken CO-100k (which I'd love to check out someday...that company is really pushing boundaries with their unique mic design...anybody tried their contact mic that supposed to be able to pick up the sound of ant footsteps?). We also have a couple Neumann Solution D mics where I work and I've tried it with them too going direct digital into ProTools.

Recording at 192kHz and pitching down is certainly fun, but a bit overrated to my ears...(unless you're sound designing for dolphins or dogs). It's certainly not the holy grail to making recordings sound huge. You're much better off finding things to record that sound huge in the first place. But what do I know...I still like the sound of pitch shifting via 2-inch tape.

It does work very well on sounds that speak in the high frequencies, like metal, or glass, but not on everything. Pitching this way often requires a ton of corrective EQ in the mastering stage to make a sound spectrally balanced again...as most mics aren't flat above 21kHz. There's also a lot of strange high frequency hash/harmonics that rear their ugly head when you pitch down, especially on machines.

But experiment and push the boundaries of sound...that's the fun in what we do.

Answer 3

Am I correct in assuming that the biggest hardware obstacle to recording sounds above 20k is generally just the microphone (not the pre or D/A)?

No, it's the hardware between the mic and the ADC, or the ADC's internal digital filtering, whichever cuts off first.

I believe most ADCs use digital filters that scale with the sampling frequency (the sigma-delta ADC datasheet I'm looking at does, at least). So recording at 96 kHz will let the ADC go up to ~48 kHz, for instance. But if the hardware before the ADC is rolling off before this, then those frequencies won't get to the ADC to be sampled, and you'd have to modify the analog hardware to raise the cut-off frequency.

The 744t specifies "10 Hz–40 kHz, +0.1, −0.5 dB", so it will let you go to 40 kHz. :)

As another example, I have the Focusrite Scarlett 6i6, which is advertised as "20Hz - 20kHz +/- 0.1dB", but I've measured it and it actually goes up to ~47 kHz.

Answer 4

So after some tests, here's what I've been able to come up with:

First off, just because a mic isn't rated at over 20k doesn't mean that it isn't capable of recording that high. It really just means that it won't be linear up there, but mics are often not perfectly linear even in the audible spectrum, so that's usually not an issue.

My coworker has the earthworks drum mic set (which is only rated up to 20k) and the tests that we ran worked fine. Also, the mics on the Sony PCM-D50 recorded tons of information up there as well.

Second, I found that iZotope RX is almost the perfect 2 track editor and cleanup software I can think of for working with ultrasonic content.

• destructive 2 track editing of original files (no conversions like with Protools)
• spectral repair is amazing
• and of course, the spectrogram display, which automatically adjusts its range to fit the sample rate of the material that's loaded up

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So, to put it all in perspective here:

below is a recording of a metal gate that I made a few months ago with my trusty PCM D50 at 24bit 96k, with slowed down iterations:

[soundcloud]rcoronado/metal-gate-varispeed-versions[/soundcloud]

Metal gate-varispeed versions by Rcoronado

I've made the file downloadable so that you can check it out yourself.

Here's the spectrogram of the gate at 100% and 96k sampling rate. Notice that the ruler on the right hand side goes up over 30k! Also notice that I had material recordable that actually exceeded the 48kHz frequency response that the Sony is capable of recording.

gate spectrogram 100% http://www.flickr.com/photos/52823418@N02/4873561438

this is the second sound in the soundcloud file, varispeeded using Soundminer down to 41% and output at 48k sampling rate. Notice that the ruler on the right now only goes to 24k, since the new sampling rate of the file is 48k. Also notice how full range this recording looks (and sounds) even slowed down to more than halfway.

gate spectrogram 41% http://www.flickr.com/photos/52823418@N02/4873561644

and here is the third. varispeeded from the original the same as the other. Still full freq sounds, though the tops are starting to feel pretty dull by this point. Still, there are the points where the clanks just shot out some insane frequencies that far exceeded the range.

gate spectrogram 21% http://flic.kr/p/8qB9UH

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edit

I think I'm failing at embedding pix, so here's a link to the photoset of this on flickr:

http://www.flickr.com/photos/52823418@N02/sets/72157624682752988/

So, to return to my original questions:

Q. Am I correct in assuming that the biggest hardware obstacle to recording sounds above 20k is generally just the microphone (not the pre or D/A)?

A.That seems to generally be the case. If a $450 device like the PCM D50 can perform like this, then clearly preamps and A/D converters are there at most price points. Haven't tested my old Zoom H4, but I hate that piece of plastic anyway. :)

Q. how well would the preamp and A/D of say a 744t handle ultrsonic frequencies (if recording with an appropriate mic)?

A. Again, looks fine. I didn't test the 744t that we have on hand, but I'd be very surprised if the sony's pre's and A/D outperforms it.

Q.How about a lower end recorder like a Zoom H4n (again, with appropriate mic)?

A.I may have to test our H4n, but the Sony test feels like a very promising experiment.

Q. What pitfalls should be avoided when processing high sample rate files with supersonic frequencies in them? If I take a file and just trim heads and tails in a two track editor like Izotope RX, will I ruin it or will it handle it?

A. Izotope rocks for this application - specifically because it automatically adjusts the spectrogram to accommodate the higher frequencies.

alright, thx all!

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Further update

It turns out that lots of mics that aren't rated above 20k perform very well up there. I used a pair of AT 4050s to record a bunch of coverage on a Canon t1i that a coworker had last week, and those servos put out some crazy supersonic noises that came back down into audible range like a champ.

Also used a schoeps CMC6 XYpair to record coverage on a medium format camera at 96k, and those mics performed excellently above 20k as well. **

This is all very encouraging

Answer 5

The frequency response of the Sanken CO-100K microphone wiggles its way up to 100kHz

_{(source: sanken-mic.com)}

Answer 6

Eager to hear responses from the group on this one. My only advice is to record at the highest sampling rate possible, ie. 192kHz. This will stave off the Nyquist that Shaun was referencing. And 24-bit, too, of course.

Answer 7

We have a couple of Earthworks mics at work, and they are excellent. They will not be a poor investment.

You may or may not be correct in your assumptions regarding mics and pre's, perhaps someone else here will know. Of course, don't forget Nyquist. You're only going to be able to accurately capture frequencies that are 1/2 of your sampling rate.

I'll run a test tomorrow with our 744T and one of our Earthworks mics that (either QTC30 or SR30) and let you know if I get anything interesting.

Update: I was able to do some quick tests today while waiting for the video editors to get me some footage to finish off yet another test. You can find a description of what I did, some audio clips, and spectral analyses on my website.

FURTHER UPDATE (8/11/10): I've done a noise floor test with a bunch of mics, including the QTC30 to appease Jay's curiosity. They're in their own dedicated post on my site. [link in my comment below if you're feeling lazy]

Answer 8

What are you looking to record and are you going to be able to hear those frequencies when you find them? :)

I'd be curious to hear what applications you have in mind, are you trying to record bat sonar or something?

Answer 9

Can you check back in when you finish and maybe post some samples? I was wondering how they sound.

Answer 10

Old thread though,

How about using a Bat detector (freq. transducer, basically tone generator and mixer and filter) to get the upper freq. sound then convert back to original freq. on PC?

If detection and relative SPL is your matter, it may works too in theory.

Sounds quality will be greatly degraded.