Acoustic Principles: Understanding How Sound Travels


Acoustic principles – understanding how sound travels – can make a big difference in creating great recordings. There are two main measurable elements of a sound wave: frequency and amplitude. Frequency refers to the rate of forward and backward movement of a compression wave and the up-and-down movement of a waveform.

One wavelength is measured from the time that compression (forward or upward movement) begins, to the backward or downward movement and the return to the starting position. The amount of wavelengths or cycles in an acoustic sound that occur per second is the definition of frequency, and frequency defines pitch, such as 100 cycles per second, which is the same as 100 Hz.

Amplitude is the amount of variation between forward and backward movement, up-and-down movement, or compressions and rare factions. Amplitude defines volume and sound pressure levels (SPLs). The higher the acoustic pressure, the louder the sound. These levels are defined by a logarithmic measurement called decibels (dB). Decibels are a relative measurement that, when applied to SPL, ranges from 0 dB SPL, the threshold of hearing; 60 dB SPL, the average level of speech; to 120 dB SPL, the threshold of pain. This measurement of dB SPL is different from dB on faders of a console, but they both have an effect on volume and loudness.

When we look at acoustic theory, sound waves travel in every direction, but commonly encounter into obstacles such as walls or other barriers in a room. When this happens, the sound waves bounce in a different direction. It takes time for this refraction to happen, so every time sound bounces, it gets more and more delayed.

Sound travels at 344 m, or roughly 1100 feet per second. So, if a sound travels 25 feet to a wall and then 25 feet back, by the time he gets back it will be about 50 ms delayed from the original sound. If the sound wave travels much farther than 50 feet, the effect will be like an audible repeat, also known as an echo.

Reverb is different from this delay or echo effect. Reverb is a combination of thousands of different delays repeating at different times, from different levels, and from different directions. Waves will keep bouncing around in a room until the wave’s energy runs out. So, the way to control acoustics in a room with walls and other barriers is to either divert the energy with reflections or diffusion, or stop the energy with trapping and absorption.