Can I record a sound frequency I can't hear and play it back so I can hear it?

Question

Can I record a sound frequency I can't hear and play it back so I can hear it?

If yes, How?

Answer 1

This is actually really straightforward given one caveat:

• Your recording hardware and software needs to have a frequency response which includes the range you want (either very low or very high)

Almost all professional recording software will let you frequency shift - either directly, or by speeding up or slowing the playback of the waveform. Even free software like Audacity will do this.

If you are recording ultrasonic frequencies, just slow your waveform playback down; and do the opposite for subsonics.

Answer 2

To expand on Dr Mayhem's great answer:

It is not possible to hear a frequency that you cannot hear.

It is possible can transform a frequency that is too high or low to hear into one that you can, and you do that by shifting the recording's frequency as Dr Mayhem describes.

You won't be hearing the original frequency, but you'd be hearing a sound at the transformed frequency that has the same characteristics as the original, which I believe is what you're looking for.

Answer 3

I would like to expand on the caveat in @Rory Alsop's answer, since to me he answers one part of your question (can I play it back so I can hear it?) while relegating the arguably far less trivial part (can I record an ultra/infrasonic sound?) to a caveat.

So what are the hardware/software requirements to record inaudible frequencies?

I will concentrate on ultrasound (> 20kHz) since that's where most of the issues come up and I'm guessing it's probably what you're most interested in.

• Hardware issue 1, microphone: Most microphones have a frequency response specification that varies at the low end but goes up to 20,000Hz at the high end (the decent quality ones at least). Above 20k simply isn't measured because the assumption is it will be used to record sound, ie. audible frequencies. We can think of its response above 20kHz as 'undefined behaviour', but most likely there will be a roll-off. That is to say, the microphone will likely not be good at picking up ultrasound.
• Hardware issue 2, DAC: The Digital-Analogue Converter, whether in an audio interface or your computer's internal sound card, filters the incoming analogue signal before digital conversion. This is to avoid aliasing - frequencies above the Nyquist frequency of the sampling rate erroneously manifesting as lower-frequency sidebands, creating unwanted artifacts. Professional audio interfaces support recording at 96 or even 192kHz sampling rate, meaning that in theory if you keep it on those settings you will record some ultrasonic frequencies. However, the purpose of higher sampling rates is NOT for recording ultrasound but: 1. to oversample the audible signal so that certain transformations can be applied with less distortion, and 2. to push the cutoff of the anti-aliasing filter up away from the hearing limit so it doesn't risk affecting anything below 20k. However, exactly where the cutoff is placed for a 192kHz recording and how it affects higher frequencies is an open question, and that information will not necessarily be published as part of the equipment's specifications.
• Software issue, sampling rate: This should be the easiest to get right - make sure that the recording software, as well as the hardware, has as high a possible sample rate. There also might be some anti-aliasing stage to any pitch effects you apply to it, though I'm not sure how likely this is.

So, I'm sorry to be naysayer, and I don't mean to say it's impossible, but you will need to think about these things if you want to capture ultrasonic frequencies.