Frequency is the number of complete pressure oscillation cycles that a sound wave completes in one second, measured in Hertz (Hz). A sound at 1000 Hz means the air pressure at a given point rises and falls 1000 times per second. Frequency is what we perceive as pitch — low frequencies sound deep and bass-heavy, high frequencies sound bright and treble-rich.
Understanding frequency is essential for acoustic design because virtually every acoustic property — absorption, transmission loss, reverberation time, diffraction, resonance — varies with frequency. A room that performs well at 1000 Hz may be completely inadequate at 125 Hz, and designing without considering the full frequency spectrum is one of the most common mistakes in practice.
Real-World Analogy
Think of frequency like the speed of a jump rope. Turn the rope slowly and it makes one large, lazy arc per second — that is low frequency. Turn it as fast as you can and it whips through dozens of cycles per second — that is high frequency. The rope's shape at any instant (its wavelength) gets shorter as you turn faster, just as sound wavelengths get shorter at higher frequencies.
Or consider a guitar string. Pluck the thick, heavy bass string and it vibrates slowly — maybe 80 times per second — producing a deep, low-pitched tone. Pluck the thin, taut treble string and it vibrates over 300 times per second, producing a bright, high-pitched tone. The only difference is how many cycles of vibration occur each second.
Technical Definition
Frequency (f) is defined as the reciprocal of the wave period (T):
f = 1 / T
Where T is the time in seconds for one complete cycle. If a wave completes one cycle in 0.001 seconds, its frequency is 1000 Hz (1 kHz).
Frequency, wavelength (lambda), and the speed of sound (c) are related by:
c = f x lambda
At 20 degrees C in air, c = 343 m/s, so:
- 125 Hz has a wavelength of 2.74 metres
- 1000 Hz has a wavelength of 0.343 metres (34.3 cm)
- 4000 Hz has a wavelength of 0.086 metres (8.6 cm)
The Audible Spectrum
The human ear can detect frequencies from approximately 20 Hz to 20,000 Hz (20 kHz), though the upper limit decreases with age. For practical acoustic design, the range from 63 Hz to 8000 Hz covers the vast majority of architecturally relevant sound.
The ear perceives frequency on a logarithmic scale, not a linear one. A jump from 100 Hz to 200 Hz sounds like the same "distance" as a jump from 1000 Hz to 2000 Hz — both are one octave. This logarithmic perception is why acoustic measurements use octave bands rather than equally spaced frequency intervals.
Frequency Ranges in Acoustic Design
| Range | Frequencies | Examples |
|---|---|---|
| Low (bass) | 20 - 250 Hz | Traffic rumble, HVAC hum, bass guitar |
| Mid | 250 - 2000 Hz | Human voice fundamentals, most musical instruments |
| High (treble) | 2000 - 20,000 Hz | Consonant sounds (s, t, f), cymbals, birdsong |
Speech intelligibility depends heavily on the range from 500 Hz to 4000 Hz, where consonant sounds carry the information that distinguishes one word from another. Bass frequencies give speech warmth and presence but carry relatively little linguistic information.
Why It Matters for Design
Frequency dependence is the defining challenge of acoustic design:
Absorption varies with frequency. A 50 mm mineral wool panel might absorb 95% of sound at 4000 Hz but only 15% at 125 Hz. Specifying materials by NRC alone hides this critical variation. Effective design requires examining absorption coefficients at each octave band.
Transmission loss varies with frequency. A lightweight plasterboard partition might block 40 dB at 1000 Hz but only 15 dB at 125 Hz. Bass frequencies pass through walls far more easily than treble, which is why you can hear your neighbour's subwoofer but not their conversation.
Room modes are frequency-specific. Resonant frequencies depend on room dimensions, and the resulting bass build-up occurs at specific frequencies rather than across the spectrum. Treatment must target the problematic frequencies.
Diffraction is frequency-dependent. Low-frequency sound bends around obstacles easily while high-frequency sound is blocked. A noise barrier that reduces 4000 Hz traffic noise by 20 dB may reduce 125 Hz engine rumble by only 5 dB.
This is why professional acoustic analysis always works across the frequency spectrum — typically the six standard octave bands at 125, 250, 500, 1000, 2000, and 4000 Hz.
How AcousPlan Uses This
Every calculation in AcousPlan operates on the six standard octave band centre frequencies: 125, 250, 500, 1000, 2000, and 4000 Hz. When you assign materials to surfaces, AcousPlan uses the frequency-specific absorption coefficient for each band and calculates RT60 independently at each frequency.
The results dashboard displays RT60 as a frequency curve, not a single number, so you can immediately see if your room has balanced acoustics or if one frequency range is problematic. The compliance checker evaluates performance at each frequency against the target standard — because standards like BB93 and ANSI S12.60 specify frequency-dependent requirements.
The auto-solve algorithm optimises material selection across all six bands simultaneously, balancing the competing demands of bass control and mid/high-frequency absorption.
Related Concepts
- What Are Octave Bands? — How the frequency spectrum is divided for acoustic analysis
- What is Sound Wavelength? — The spatial dimension of a frequency
- What is Acoustic Resonance? — Amplification at specific frequencies
- What is a Decibel (dB)? — The logarithmic unit used to measure sound at any frequency
- What is Sound Pressure? — The physical quantity that oscillates at the wave's frequency
Calculate Now
Ready to see how your room performs across the full frequency spectrum? Use the AcousPlan Room Calculator to analyse RT60 at every octave band from 125 Hz to 4000 Hz.