I posed similar questions in this thread.

Afterward I did several tests (which are noted in my response later on in the same thread)

what I found was that even mics that aren't rated above 20k are perfectly capable of capturing great audio far above the audible range - and are capable of it using a pretty wide array of A/D converters and preamps.

everything test here is confirmed capable of recording good stuff well above 20k in my own tests:

• Sony PCM D50
• Schoeps CMC6
• Earthworks DK50
• AT 4050
• Sure VP-88
• Sound Devices 744t
• John Hardy M1
• Digi 192 interface

I'll also reaffirm that higher sample rates really add to the flexibility of sounds. The doors that I recorded at 96k for Tim Prebble's doors project have already been pitched down and employed in all kinds of heavy impact situations, and because they retain high freq content they don't sound "pitched" at all - they just sound much bigger.

The way i think it works is that the samples are distributed equally across the20-20khz range to increase the resolution of the wave form for a more accurate "picture", as opposed to using the samples to register higher tan 20khz frequencies. That way the waveform has much more information and so suffers less degradation in processes like pitch shift, time stretch etc, because the signal processor has more to work with.

This doesn't come from any scientific source, it's just what makes sense in my head and would love to confirm or dis-confirm this; plus i just woke up, so i don't know if what i wrote makes any sense at all...

I discovered a LOT when dealing with the Fireworks Library at 192k eg I thought my HD2 PT rig was powerful (eg on films I often run out of voices, and it can playback 192 tracks at one) but at 192k it became underpowered! The track count, plugin use and even the timeline window became restricted! I would love to work even at 96kHz all the time for better plugin performance but even that would reduce my PT performance too much..

The moral of the story - there is definitely an advantage to using higher sample rates, but practically the workflow tends to be that of recording, editing and manipulating material at 96 or 192, but then do the syncing, layering, editing at 48k.... for now at least!

+1 on the increased resolution of the audible range.

There are definitely some interesting sounds above the 20kHz zone, but you don't necessarily need a mic rated up beyond that to capture them. Many mics will capture those frequencies, but they're response may not be as even as those rated to 50 or 100 kHz.

I wonder if a small diaphragm condenser mic would work better in those applications. Less membrane for the acoustic compressions/rarefactions to excite? Anyone have any thoughts on that idea?

There's no point in recording frequencies above 20 kHz unless you're going to do something with them, like put them through an effect that will modulate or stretch them back into the audible spectrum.

The main benefit of higher sampling rates is not to capture ultrasound, but to reduce audible things like aliasing, phase distortion, etc.

These can be a benefit for synthesis, too. If your square wave generator is poorly-written, it will produce an infinite number of harmonics, which are aliased and sound bad. If the sampling frequency is higher, this effect is reduced. Of course it would be better if the generator were written correctly, but you can't always control that.

+1 to a more precise audible waveform

Since we're talking about digital conversion of sound, the more samples you get.. the more acurate it will be.

But then again.. like Filipe here.. that's just my 2 cents!

I dont agree that sampling frequencies higher than 40kHz increase the accuracy of the recorded frequencies below 20KHz...

Sounds are made up of sine waves and you only need 2 samples to be able to perfectly represent a sine wave since it is a mathematically perfect shape. If you store a million samples for one sine wave it will be no more accurately represented than with 2 samples.

But hey, I could be wrong

Here is an abstract you'll want to look at.

http://jn.physiology.org/content/83/6/3548.abstract

Talks about the impact of frequncies we can't heard in perception of sound. So yeah, it matters :)