Why can I hear 120 BPM, but not 2 Hz?

Question

What is the difference between BPM (Beats Per Minute, like what a DJ counts to mix) and Hz (Hertz, like what tuning forks are measured in), other than a factor of 60?

Why can I hear a 60 BPM bass beat, but not a 1 Hz tone?

At first I presumed it is because the bass beat is made of a bunch of vibrations much faster than 1 hz, but then (by having a frequency), percussion (like a drum beat ) would have a pitch, which it by definition does not, so it must be one single wave.

That said, 60 BPM means 1 of those single waves a second, so 1 Hz, but I can hear a 60 bpm drum but not a 1 Hz tone! Same goes for 120 bpm and 2 Hz etc.

Furthermore, speeding up a bass beat enough (i.e. high enough BPM) turns into a tone, as in the intro to the Swedish House Mafia song "One.":

There has to be a puzzle piece I am missing here- how does this work?

Answer 1

I think you've misunderstood what Frequency is, with respect to audio. Whilst 'Frequency' typically is 'how frequently something occurs', in audio it's how many times a sine-wave oscillates in a second, rather than how many things you hear in a second.

eg. A standard kick-drum track at 60 BPM means you'll hear a kick-drum sound once-per-second. That actual kick-drum sound though will be a constructed from a combination of frequencies, as you actually correctly originally thought, within the audible range of 20Hz-20kHz. An additional note here is that percussion does actually have a pitch; drums etc. are tuned too, they just won't sound quite as 'musical' as other instruments!

A 1Hz sine-wave is a sound wave that oscillates once-per-second; That is a vibration in air (or other materials) that rises to it's peak, down to it's trough, and back again, once per second (picture waves on the sea!). As Marc W mentioned, most speakers cannot produce a reliable 1Hz sound-wave, and your ears wouldn't be able to perceive it anyway.

Every sound you hear, unless it's a pure tone, will be constructed from a combination of sine-waves and cosine-wave of various frequencies, which is called the Fourier Theory. The frequencies that musical notes etc are made of, however, are not connected to how regularly they are being played, which is what BPM describes.

In short:-

Frequency of a Sound-wave: How many times it oscillates from zero-to-peak-to-trough-to-zero in a second.

BPM: How many times a sound/beat is heard in a minute.

Hope this helps!

Answer 2

It's simple really; Your speakers can't reproduce a 1Hz sinewave, and if it could, you wouldn't hear it due to the limits of human hearing(plus, due to the size of the speaker cone, it would have very little energy).

If your speaker could reproduce a 1Hz sinewave, you would see the speaker cone moving in and out with a one second period. If you were to play a sawtooth wave at 1Hz in this way, it would probably sound like a clicking at 60BPM.

Does this answer your question?

Answer 3

120 BPM is not 2 Hz.

Lets take a typical kick-drum as a beat. Such a beat is not a single movement of the air, and 120 beat in a minute is not 120 movements of air, or 2 Hz.

A kick-drum has a combination of frequencies which all have an "attack" at first, and then those frequencies sustain for a while, eventually die out.

A Picture is a thousand words:

So you "hear" all those frequencies, a 120 times a minute. You might also note that some of the frequencies are actually surprisingly high.

So the answer is, you cannot hear 2 Hz, but you can hear 120 kick-drums in a minute.