What Sound Is and How You Hear It

Read first: SoundStudio theory section 1, What sound is, then the opening paragraph of section 5, Loudness. Those two cover the physics. This lesson adds the part the tutorial only touches: your ears are a lopsided measuring tool, and every mixing decision you make runs through them.

The short version of the physics

Sound is air pressure going up and down, fast. A guitar string pushes the air on one side, pulls it on the other, and that ripple of high and low pressure travels to your ear at about 343 meters a second. A microphone turns the ripple into voltage. SoundStudio draws that voltage as a waveform: time runs left to right, pressure runs up and down.

Two numbers describe a simple tone. Frequency is how many full cycles happen in one second, counted in hertz. Amplitude is how tall each swing is. Double the frequency and the note goes up an octave. Make the swing taller and it gets louder. Real sounds are piles of many frequencies at once, which is why a song's waveform looks like a hedge and a test tone looks like a smooth ribbon.

What the ear can hear

The textbook range of human hearing is 20 Hz to 20,000 Hz (20 kHz). Treat both ends as generous. Below about 20 Hz you stop hearing pitch and start feeling pressure in your chest. At the top, most adults lose the highest octave over the years. The SoundStudio tutorial puts a typical middle-aged ceiling at 14 to 15 kHz, and that matches what you find if you test a room full of people over forty. Hearing loss with age is common: the National Institute on Deafness and Other Communication Disorders reports that about one in three people in the U.S. aged 65 to 74 has hearing loss.

This matters for your work in a plain way. If you are 55, you may not hear hiss above 12 kHz that your 20-year-old listener hears clearly. Trust the spectrogram up there, because your ears are no longer a reliable witness.

Why bass sounds quieter

Here is the fact that explains half the arguments in a mixing room. Ears do not hear all frequencies equally. Play a 50 Hz tone and a 1,000 Hz tone at exactly the same sound pressure, and the 50 Hz tone sounds far quieter. The midrange around 2 to 5 kHz sounds loudest of all, partly because your ear canal is a small tube that resonates in that region.

In 1933 Harvey Fletcher and Wilden Munson at Bell Labs measured this. They played tones at many frequencies and asked listeners to adjust a 1 kHz reference tone until both seemed equally loud. Plot the results and you get a family of curves, each one connecting the levels that sound the same. People still call them Fletcher-Munson curves. The modern version is an international standard, ISO 226:2023, "Normal equal-loudness-level contours," which replaced the 2003 edition. It is based on pure tones heard by otologically normal listeners aged 18 to 25, with the sound coming from straight ahead.

Look at the two curves on the left side. At low volume, bass needs a huge boost to keep up. At high volume, the curve flattens and bass catches up. So the same mix sounds thin when you play it quietly and full when you crank it. The file is identical both times; your hearing shifted.

This lopsided hearing is also why a meter that only reads peaks can lie to you about loudness. A loudness meter (lesson 3) weights the measurement to match the ear: it rolls off the deep bass and adds a small lift above about 2 kHz before it averages. That is the K-weighting inside ITU-R BS.1770, and it exists because of curves like these.

Pitch, harmonics and timbre

The tutorial covers octaves (220, 440, 880 Hz) and the twelve semitones in each. The piece to hold onto here is harmonics. A plucked string vibrates at its full length, which gives the fundamental, and also in halves, thirds, quarters and so on. Those extra vibrations land at two, three and four times the fundamental. A low E string on a guitar at about 82 Hz is also putting out energy near 165, 247 and 330 Hz and on up.

The mix of those harmonics is timbre. It is why a fiddle and a voice singing the same A at 440 Hz never get confused. It also explains a trick of hearing called the missing fundamental: a small phone speaker cannot reproduce 82 Hz at all, yet you still hear the low E, because your brain rebuilds the fundamental from the pattern of harmonics above it. Producers use this on purpose when a bass line has to survive phone speakers.