Sound & frequency

Everything in audio — microphones, DSP, loudspeakers, acoustic panels — exists to capture, move or shape one thing: small, rapid changes in air pressure. Before any cable or connector, it helps to understand what that thing is and why its frequency changes how it behaves.

Sound is a pressure wave

When someone speaks, their vocal folds push and pull on the air. Each push squeezes the molecules together (compression); each pull spreads them apart (rarefaction). That pattern travels outward. The air itself barely moves — the pressure change is what travels, at roughly 343 metres per second in room-temperature air.

A microphone is a pressure sensor: a thin diaphragm moved by those changes, turned into a matching voltage. A loudspeaker is the same idea run backwards. In between, every piece of audio gear is handling a voltage that is a copy — an analog — of those pressure changes.

Frequency and wavelength

Frequency is how many complete cycles happen each second, measured in hertz (Hz). We hear it as pitch. Wavelength is the physical distance one cycle occupies in the air. The two are tied together by the speed of sound:

That gives a staggering range. A 20 Hz rumble has a wavelength of about 17 metres — longer than most rooms. A 20 kHz tone is under 2 centimetres. Same air, same speed, and yet the two behave like entirely different things.

The range of human hearing

The textbook range is 20 Hz to 20 kHz. In practice the top end falls with age and noise exposure, and many adults hear little above 15 kHz. Hearing is also far from flat: we are most sensitive between about 2 and 5 kHz, and much less sensitive to low frequencies at quiet levels.

Pitch works on octaves — each doubling of frequency sounds like the same step. 100 Hz to 200 Hz is one octave; so is 5 kHz to 10 kHz. That is why audio graphs use a logarithmic frequency axis, and why the hearing range is better described as about ten octaves than as “19,980 Hz wide.”

Where speech lives

Why wavelength matters in a room

Sound interacts with objects that are similar to or larger than its wavelength. This one idea explains a great deal of room acoustics:

• Low frequencies wrap around things. A 100 Hz wave is 3.4 m long; a table or a person barely registers to it. Bass leaks through walls and doors, and it's why you hear the thump from the room next door but not the words.
• High frequencies travel in beams. Short wavelengths are blocked by heads and furniture and soaked up by soft surfaces. Loudspeakers become more directional as frequency rises.
• Absorbers need depth to work low. A common rule of thumb says porous material works well once it is around a quarter-wavelength thick. A 50 mm panel handles speech frequencies nicely but does little to bass — taming low frequencies needs much deeper treatment.