The Bernoulli effect is central to how we speak and it, at least in part, defines many of the characteristics of the speech spectrum input to hearing aids. Daniel Bernoulli was a member of a Bach-like family where several generations participated in the family business.
Of course for the Bernoullis, it was mathematics and physics, and not music that defined their family. And other than the famous Bernoulli distribution in the field of statistics, he discovered and described the phenomenon whereby the speed of sound is inversely related to the (sound) pressure.
The Bernoulli Effect
The Bernoulli effect was used by the Wright brothers when they defined the shape of their airplane wing; a design that forced higher speeds of air over top of the wing than below, thereby creating a net negative pressure above the wing, also known as “lift”.
It also explains how the reeds of many musical instruments such as the saxophone, clarinet, and oboe, vibrate. And it also explains why a transit passenger should not stand too close to the edge of the subway or train platform when a train is approaching- the high speed of the passing train creates a low pressure zone that can suck an unexpecting passenger inwards towards the deadly train.
It also explains the behavior of our vocal chords and how they vibrate to provide the underlying energy for many of our speech sounds. As air rushes upwards from our lungs they create a high speed of air rushing past the edges of our vocal chords and this creates a low pressure zone, sucking our vocal chords together (and then the sub-glottal pressure blows them open a moment later).
The rate that the vocal chords open (sub-glottal pressure) and close (Bernoulli effect) is called our fundamental frequency or pitch. And because of how the vocal chords are held rigidly at both ends, they function like guitar or violin strings and create harmonics at integer multiples of the fundamental frequency.
My fundamental frequency, or pitch, happens to be exactly 125 Hz. But I also have harmonics at 250 Hz, 375 Hz, 500 Hz, 625 Hz, and so on. This array of harmonics define the important formant pattern that we see in speech, primarily below 3500 Hz. Above 3500 Hz, there is mostly frication noise such as that heard from the sounds ‘f, s, sh, th, …’.
In hearing aids, frequency shifting/compression schemes work well for the higher frequency frication sounds but should not be used for the lower frequency sounds where harmonics are the important aspect. Frequency compression in the lower frequencies would alter the important harmonic structure thereby altering the pitch of person’s voice. It would also be deleterious for music since a B flat would no longer be a B flat.
And this is where Paris Hilton, one of the heirs to the Hilton Hotel chain, comes in…
It turns out that the Hilton Hotels were the first to address and minimize the Bernoulli effect. Who hasn’t taken a shower only to have the (cold) shower curtain come in and stick to them; the high speed of the water coming from the shower head created a low-pressure zone which sucked in the damp, cold shower curtain.
Someone at Hilton Hotels (who obviously knows about the Bernoulli effect) had the great idea of installing shower curtain rods that arc outwards away from the bathtub, thereby increasing the distance between the curtain and the hotel patron. No longer are hotel patrons fair game for being attacked by cold and damp pieces of plastic during their morning ablutions. Since then, virtually every other hotel chain I have had the pleasure to stay at, have followed suit.
**A version of this article originally appeared at the Hearing Review on June 21, 2019, and is republished with permission.
MARSHALL CHASIN, AuD
Dr. Marshall Chasin is Director of Audiology at the Musicians’ Clinics of Canada, Adjunct Professor at the University of Toronto (in Linguistics), and Associate Professor in the School of Communication Disorders and Sciences at Western University. He is the author of over 200 articles and 8 books including Musicians and the Prevention of Hearing Loss. He writes a monthly column in Hearing Review called Back to Basics. Dr. Chasin has been the recipient of many awards over the years including the 2012 Queen Elizabeth II Silver Jubilee Award and the 2017 Canada 150 Medal. He has developed a new TTS app called Temporary Hearing Loss Test app. And he is not as boring as this bio makes him sound!