Frequency State / Music Production Reference Library · Volume 1
Every recording, synthesizer patch and live performance begins with the same physical event: something vibrates. A loudspeaker moves air, a guitar string oscillates, or a drum skin flexes after being struck. Those movements create changing pressure in the surrounding air, and our hearing system interprets the resulting pattern as sound.
Understanding this process helps explain why microphones need air movement, why bass behaves differently in a room, and why two instruments playing the same note can sound completely different. This guide introduces the physics of sound and connects it to everyday music production.
How is sound produced?
A vibrating object disturbs the material around it. When a loudspeaker cone moves forwards, it pushes nearby air molecules closer together, raising the local pressure. When it moves backwards, the pressure falls. The disturbance passes from one part of the air to the next, spreading away from the source.
The air itself does not travel all the way from the loudspeaker to your ear. Individual molecules mostly move back and forth around their usual positions while energy is transferred through the medium. This distinction matters: sound is a travelling pressure disturbance, not a stream of air blown across the room.
Consider three instruments. A plucked guitar string sets the guitar body and nearby air vibrating. A kick drum briefly moves its drumhead and creates a complex pressure pulse. A synthesizer ultimately produces an electrical signal which drives a loudspeaker or headphones; the resulting diaphragm movement turns that signal into audible pressure changes. Different sources, same fundamental mechanism.
How sound travels: compression and rarefaction
In air, sound usually travels as a longitudinal wave. The air moves locally in roughly the same direction that the wave propagates. Regions of slightly higher pressure are called compressions; regions of slightly lower pressure are rarefactions. Both move through the air as part of the wave.
A common waveform drawing shows a smooth line rising and falling. That graph is useful, but it is not a picture of air moving up and down like a water wave. It normally plots pressure, or an equivalent audio signal level, against time. The line makes changes in pressure visible.
Sound needs a medium such as air, water or a solid material. It cannot propagate through a perfect vacuum because there are no particles to transmit the disturbance. Sound also travels at different speeds through different media. In room-temperature air, a useful approximate figure is 343 metres per second at 20°C; temperature affects the exact value.
Frequency: why some sounds are high and others low
When a vibration repeats, its frequency tells us how many complete cycles occur each second. Frequency is measured in hertz (Hz). A 100 Hz tone repeats its pattern 100 times per second; a 1,000 Hz tone repeats 1,000 times per second.
For a simple pure tone, increasing its frequency raises the perceived pitch. Doubling the frequency produces an octave: A4 at 440 Hz and A5 at 880 Hz illustrate that relationship. Real musical sounds are usually more complex, however, and pitch perception depends on how their frequency components relate to one another.
For a closer look at frequency, the shapes of sound waves and how oscillators produce them, continue to Frequency, Amplitude and Waveforms.
Amplitude: signal level and perceived loudness
Amplitude describes the size of a variation or oscillation. Greater sound-pressure amplitude will generally produce a stronger sensation of loudness under comparable conditions, but loudness is not determined by amplitude alone. Our ears respond differently to different frequencies, and the duration and character of a sound also influence how loud it seems.
In a DAW, the waveform height represents the recorded signal amplitude on the display’s scale. That is not a direct measurement of how loud the sound will be in your room: playback volume, headphones or speakers, distance and the recording’s spectral content all matter.
Audio meters commonly express level using decibels, but different decibel scales describe different quantities. The distinction between physical sound pressure and digital signal level is explained in The Decibel Explained.
Timbre: why two sounds at the same pitch differ
Play middle C on a piano, then on a synthesizer. Both can have a similar perceived fundamental pitch, yet they are easy to tell apart. That difference is partly timbre: the character of a sound produced by its mixture of frequency components and how those components change over time.
A pure sine wave has one frequency component. A sawtooth-like tone contains a fundamental and many harmonics, giving it a brighter, richer character. Acoustic instruments add their own resonances, noise and evolving attacks. The short burst at the beginning of a piano note is an important part of its identity, even when the sustained pitch is unchanged.
Filters, envelopes, distortion and other production tools work by changing these characteristics. Timbre is therefore not simply a label for an instrument; it is something you can actively shape when designing or processing a sound.
What happens when sound reaches your ear?
The outer ear helps collect incoming sound, and pressure variations move the eardrum. The middle-ear bones transfer this vibration to the inner ear, where the fluid-filled cochlea helps convert mechanical movement into nerve signals. The brain interprets those signals as pitch, loudness, position and sound character.
Human hearing is often introduced as extending from roughly 20 Hz to 20 kHz in young listeners, but that is an approximate range, not a universal limit. Sensitivity varies substantially with frequency and between individuals, and the upper limit commonly decreases with age and noise exposure.
This helps explain why a mix that looks evenly balanced on a spectrum analyser does not necessarily sound balanced, and why monitoring level matters during long production sessions.
Sound in a room: reflection, absorption and distance
A sound wave rarely travels directly from a monitor to your ear without interacting with anything else. It reflects from walls, floors, ceilings and furniture. Reflected waves arrive after the direct sound and can reinforce or reduce particular frequencies, depending on their timing and position.
Low frequencies are particularly influenced by the dimensions of a room, which is why bass can appear much stronger in one listening position than another. Soft furnishings and acoustic treatment can absorb some sound energy, while hard surfaces tend to reflect more of it. Real materials behave differently across the frequency spectrum.
For a practical demonstration, play a steady bass note and move around the room. Notice where it becomes louder or quieter without changing the instrument or playback level. You are hearing the combined effects of the room and the sound source.
How a microphone turns sound into a recording
A microphone responds to incoming sound pressure and converts it into an electrical signal. An audio interface can then sample that changing voltage and store a digital representation. A DAW displays the recording as a waveform, giving us a way to edit, arrange and process the captured signal.
On playback, the process runs in the other direction: a digital-to-analogue converter produces an electrical signal, an amplifier drives headphones or loudspeakers, and a moving diaphragm creates new pressure variations in the air. The recorded numbers are not themselves sound waves travelling through the room.
These stages are explored further in Digital vs Analogue Audio and Sample Rate and Bit Depth Explained.
Try it: three short listening experiments
1. Compare pitch without changing the waveform
Open a synthesizer with a sine-wave oscillator. Hold a note and raise it by one octave. Listen to the change in pitch, then return to the original note. The frequency has doubled or halved, while the basic waveform shape remains the same.
2. Compare timbre at the same pitch
Keep the same note and playback level, but change the oscillator from sine to sawtooth or square. Notice how the character changes even though the musical note stays fixed. Be mindful that different waveforms can have different perceived loudness.
3. Explore room reflections
Clap once in a small furnished room and then in a larger, harder-surfaced space. Listen to the reflections and decay following the initial clap. Avoid excessively loud impulses: the aim is to hear the room, not test hearing limits.
What to remember
Sound is a mechanical disturbance transmitted through a medium. Frequency helps describe repetition and pitch, amplitude describes the scale of variations, and timbre depends on the changing mixture of frequencies and transients. Hearing adds another layer of perception, while rooms and playback systems change what reaches the listener.
These ideas underpin microphone technique, synthesis, equalisation, recording and mixing. Understanding the physical process makes the controls inside a DAW much easier to interpret.
Continue reading
Next chapter: Frequency, Amplitude and Waveforms. You can also explore The Decibel Explained or Digital vs Analogue Audio.
Return to the Music Production Reference Library for the full sequence of chapters.