Chapter One: An Acoustics Primer

5. What is Frequency | Page 2

Frequency Ranges

The range of human hearing in the young is approximately 20 Hz to 20 kHz—the higher number tends to decrease with age (as do many other things). It may be quite normal for a 60-year-old to hear a maximum of 16,000 Hz. On the low end, as 20 Hz is approached, with sufficient amplitude, one might feel the vibrations, but not be able to resolve them into a pitch, unless they are reinforced with harmonics above.

Frequencies above and below the range of human hearing are also commonly used in computer music studios. We refer to the frequency ranges as:

<20 Hz
20 Hz — 20 kHz
20 kHz>
sub-audio rate*
audio rate

*The term subsonic is ambiguously used as a substitute, however it is primarily understood to refer to travel speeds through a medium below the speed of sound, therefore sub-audio is a preferred term for audio signals, particularly in electronic music. Scientifically, the term infrasonic also refers to signals below 20 Hz.

Sub-audio rate signals are used as controls in synthesis to produce modulation effects like vibrato and tremolo. The lowest 32' organ pipes (of which only a few exist) also produce fundamental frequencies below our ability to hear them, down to approximately 8 Hz, which corresponds with the lowest MIDI Note Number. The C four octaves below middle C is 16.4 Hz which is also the approximate pitch of the lowest tuba note.

Going Low

Film music commonly doubles bass frequencies synthetically below 20 Hz for power and richness, and as mentioned, more for feel from theater's enormous sub-woofers than for frequency-resolved hearing. In fact, a dedicated audio channel called the LFE (for Low Frequency Effects) was originally developed by Dolby Labs for theater use for explosions, earthquakes, and so forth, and is now a staple item of 5.1 or higher home audio systems, being routed directly to a sub-woofer so as not to degrade the higher-frequency audio from smaller speakers. For larger venues, Meyer Sound, for example, produces a Very Low Frequency Control (VLFC) element, a line array system capable of producing high-powered low frequencies in the 13-30 Hz range for theaters, EDM concerts, NASA's severe vibration testing and wherever else chest-pounding sub-audio shaking is required (they currently market as "Thump and Thunder They’ll Feel In Their Bones" and offer testimonials from Metallica FOH engineers).

human ear
Amazing Factoid #3!

It is believed that many whales and dolphins can create and perceive sounds in the 175 kHz range. Most bats use slightly lower frequencies for their echo-location system. Ferrets have a super-wide hearing range from an estimated 16 Hz to 44 kHz.