The Complexity of Sound

Sounds are generally much more complex than just a single frequency at a constant sound pressure.

When a note is played on a musical instrument, one gets not just a single frequency (the “fundamental”) but additional higher frequencies known as overtones. These overtones can be multiples of the fundamental frequency (“harmonics”) for some instruments but may not be, for others. The perceived position of a note on a musical scale is its pitch. Pitch is typically closely-related to the fundamental frequency of the note. The same note played on different instruments will have the same pitch yet will sound different and distinctive because of different proportions of overtones. The subjective impression of the combined frequencies is the timbre and distinguishes it from the same note on other instruments.

Most sources of noise will have more than a few dominant frequency components. Road traffic noise and factory noise, for example, have a broad band of component frequencies. Analysis techniques can be applied to determine the spectrum of components.

The loudness of a sound is a perceived quantity related to but distinct from its sound pressure level. Increasing the loudness of a sound (without changing its spectral composition) will increase the measured sound pressure level. But its different frequency components do not contribute equally to the overall loudness. For example, a 40 dB tone at 1000 Hz sounds just as loud as a 63 dB tone at 100 Hz. Determining the overall loudness of a noise source (and relating this to hearing loss risk) is a subject in psychological acoustics.

There is also a temporal aspect to sound. The sound pressure level and frequency spectrum of a noise source can vary over time. Some sounds, such as the noise from helicopter blades or from jackhammers, have a regular interval between impulsive bursts. Noise from a rifle shot is a single event over a short period of time.