I use the term ultrasonic to describe the area of the audio spectrum that lies above 20 kHz. As was pointed out to by a reader in a recent comment, perhaps “ultrasonic” is an oxymoron. If ultra means “going beyond” and sonic means “relating to or using wound waves”, then maybe it is. A quick check of Wikipedia provides the following description and figure:
“Ultrasound is an oscillating sound pressure wave with a frequency greater than the upper limit of the human hearing range. Ultrasound is thus not separated from ‘normal’ (audible) sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults. Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz.”
Figure 1 – Illustration of frequency spectrum from infrasound to ultrasound with acoustics in the middle from Wikipedia [Click to enlarge]
Of course, this whole issue hinges on whether there is a line of demarcation between what is acoustic energy and what is ultrasound? Obviously, there isn’t. The generally accepted upper range of human hearing is conveniently stated as 20 kHz in healthy young adults. I have the sense that this is “convenient” because it’s easy to remember when stated as “20 to 20 kHz”. The actual top frequency is likely to range considerably.
Before arguing about whether it’s time to scrap the old definition and revise upwards the “audio band” of human hearing, let’s look at the frequencies that are output from the instrumental groups found in a traditional symphony orchestra. I was referred to a research project conducted at CalTech about 15 years ago. The resulting paper is titled:
There’s Life Above 20 Kilohertz!
A Survey of Musical Instrument Spectra to 102.4 KHz
California Institute of Technology
You can check out the paper by clicking here.
Here’s the abstract:
“At least one member of each instrument family (strings, woodwinds, brass and percussion) produces energy to 40 kHz or above, and the spectra of some instruments reach this work’s measurement limit of 102.4 kHz. Harmonics of muted trumpet extend to 80 kHz; violin and oboe, to above 40 kHz; and a cymbal crash was still strong at 100 kHz. In these particular examples, the proportion of energy above 20 kHz is, for the muted trumpet, 2 percent; violin, 0.04 percent; oboe, 0.01 percent; and cymbals, 40 percent. Instruments surveyed are trumpet with Harmon (“wah-wah”) and straight mutes; French horn muted, unmuted and bell up; violin sul ponticello and double-stopped; oboe; claves; triangle; a drum rimshot; crash cymbals; piano; jangling keys; and sibilant speech. A discussion of the significance of these results describes others’ work on perception of air- and bone-conducted ultrasound; and points out that even if ultrasound be [is] taken as having no effect on perception of live sound, yet its presence may still pose a problem to the audio equipment designer and recording engineer.”
I applaud the author on establishing that musical partials extend to well above the accepted upper limit of human hearing. It’s clear that we’re exposed to a lot of ultrasonic frequencies during a live performance of a symphony orchestra. Of course, the distances involved in this test are not like being in Carnegie Hall but it reveals something that I’ve known for a long time. Music doesn’t stop at 20 kHz…so why should our recording and reproduction systems?