Pertaining to vibrations and sound waves whose frequency is too low to be heard as sound by the human ear, i.e. below about 20 Hz. The term is also used loosely to describe any low frequency sound.

Compare: Audio, Bass, Pulse, Rumble, Sonics, Subsonic, Ultrasonic.

Infrasonic frequencies are felt as vibrations which, if intense enough, may result in feelings of nausea, vertigo and eventual black-out or internal hemorrhaging. Such sound or vibration is difficult to contain because of diffraction and resonance effects, and the tendency for these vibrations to be transmitted through earth and building materials. The long-term physiological and psychological effects of constant exposure to these sounds are poorly understood.

See also: Sympathetic Vibration, Transmission.

The intensity level of low frequency sounds may be measured by comparing C-scale and A-scale Sound Level Meter readings. The difference between dBC and dBA levels indicates the amount of low frequency sound present (between 20 and 1000 Hz). Outdoor urban environments are characterized by a difference of at least 10 - 15 dB (dBC - dBA); motorized traffic and some buildings show 20 - 30 dB difference, whereas in natural environments, the difference may drop to 0 - 3 dB. Infrasonic sounds may also be analysed with a vibration analyser.

Compare: Equal Loudness Contours, Noise Criterion, Phon, Sound Transmission Class.

Low frequency audio signals are termed subaudio and are often used as control voltages in electronic sound synthesis.

See: Amplitude Modulation, Frequency Modulation, Sound Synthesizer

Third-octave spectrum analyses of sound sources with high intensities of low frequency energy. Although the infrasonic component cannot be analyzed by the same machine and is not shown here, it is quite possible that such energy exists in the region at the left of the graph, given the high levels from 20-100 Hz.

Left: Boiler-room water pumps, Cape Tormentine, N.B.

Right: Diesel train shunting, Vancouver, B.C.

Left: Inside accelerating subway car, Toronto, Ontario.

Right: Pulsating ventilation duct, Burrard Dry Docks, Vancouver, B.C.

Left: Inside Volkswagen van on freeway.

Right: Industrial and city ambience of Vancouver harbour.