At trivia tonight one of the questions was, “True or false, a cats purr sounds out at 26 Hz?” I assured my teammates that, since that note is approximately the A below the open E string on a bass, it is not bloody likely. The correct answer was “true” .
In disbelief I found lots of references like this, which seems pretty reputable. Not having either a convenient purring cat, nor a bass I implore someone with the requisite instrument and feline to tune to the cat and let me know the results. Surely they are all wrong, you don’t need a subwoofer to play a purr do you?
While I’m sure my cats are capable of hitting that frequency, it’s not the only one they hit by a long shot - one of my cats regularly makes a sound comparable to an off-centre grist mill running through slush.
Remember a cat’s purr is a rumbling sound. Each cycle of the rumble is obviously made up of much higher frequencies but the rumble itself is quite slow and it wouldn’t surprise me if it was in the region of 26 cycles a second. It’s like someone rolling their Rs, they may roll their Rs at a rate of 5 cycles a second but no one would suggest that they are capable of talking or singing that low.
We need to remember that 26 Hz is just outside of the normal human hearing range. Between 20 and 30 Hz or so is the boundary between hearing a series of sound events individually or as a continuous tone.
I did a very simple test. I went over to Freesound where I downloaded three samples of different cats purring:
Just by listening to the samples in quick succession shows that indeed the base frequency of each purr is very close. How close? I opened up the files in a waveform editor and measured the length of each cycle. The results are:
Cat 1: ~39ms or 25.6 Hz
Cat 2: ~39.65ms or 25.2 Hz
Cat 3: ~43ms or 23.25 Hz
(The order probably doesn’t match that of the files above.)
The waveforms look different in part because each cat has its own voice, but miking also plays a big factor. What’s important is the longest repeating pattern: that’s going to be the base frequency.
It’s only a sample of three cats and I only looked at a very short section of each recording but as you can see, they’re all very close to each other.
