Hertz, period and pitch
Frequency and period are two views of the same thing. A 440 Hz tone completes 440 cycles every second, so each cycle lasts 2.27 ms. A 20 Hz tone takes 50 ms per cycle, and a 1000 Hz tone takes 1 ms. The unit is named after the German physicist Heinrich Hertz.
For sound, frequency is the main thing that sets pitch, but the two aren't the same. Frequency is what you measure. Pitch is what a listener hears. Hearing follows ratios, not differences: each octave doubles the frequency, so equal steps in pitch are equal ratios in Hz.
Wavelength: how long a tone is in air
A sound wave has a length too. Wavelength is the speed of sound divided by the frequency. In air at room temperature, sound travels at about 1,125 feet (343 meters) per second, and that speed hardly depends on frequency across the range of hearing. Here's how long some tones are:
- 40 Hz: 28 feet (8.6 meters)
- 100 Hz: 11 feet (3.4 meters)
- 1000 Hz: 1.1 feet (0.3 meters)
This is why rooms matter most for bass. Most rooms have their lowest resonances between 20 Hz and 200 Hz. In a 16-foot (5-meter) room, the lowest one sits at 34 Hz. These resonances are called room modes, and they're why bass can sound boomy in one seat and thin in another.
Frequency isn't loudness
Loudness depends mostly on amplitude, and the ear isn't equally sensitive everywhere. It's most sensitive between 2000 and 5000 Hz, and equal loudness contours show that it needs more sound at the low and high ends to hear a tone as equally loud. So a 40 Hz tone and a 3000 Hz tone at the same volume setting don't sound equally loud.
That's worth remembering when you test speakers. A weak bass tone can mean the speaker can't play it, or only that your ear is less sensitive down there.
Hz beyond sound
Hertz counts anything that repeats, and it shows up across the site. A 144 Hz monitor redraws the screen every 6.94 ms. A DDR5-6000 memory module runs a 3000 MHz clock. Mains power cycles at 50 or 60 Hz, depending on where you live.
Prefixes scale the unit by thousands. A kilohertz (kHz) is 1000 Hz, a megahertz (MHz) is a million, and a gigahertz (GHz) is a billion. Audio work stays mostly between 20 Hz and 20 kHz.