Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.
Sound seems simple enough, but hear me out: There are many fascinating properties about it you might not know about. Below is but a sampling of surprising moments that are a result of acoustical physics. Some enter the land of classical mechanics while others go the mysterious realm of quantum physics. Let’s get started!
The Color of Sound
Ever wonder why we can call background sounds white noise? It refers to the spectrum of sound, something that Newton tried to develop as a parallel to the spectrum of light. To best hear the spectrum, small spaces are used because we can get weird acoustical properties to arise. This is because of “a change in the balance of the sound” with respect to the different frequencies and how they change in the small space. Some get boosted while others will be repressed. Let’s now talk about a few of them (Cox 71-2, Neal).
White noise is a result of frequencies from 20 Hz to 20,000 Hz all going at once but with different, and fluctuating, intensities. Pink noise is more balanced because the octaves all have the same power associated with them (with the energy cut in half each time the frequency doubles). Brown noise seems to be patterned off Brownian particle motion and is usually a deeper bass. Blue noise would be the opposite of this, with the higher ends being concentrated and nearly no bass at all (in fact, it’s also like the opposite of pink noise too, for its energy doubles each time the frequency doubles). Other colors exist but are not universally agreed upon, therefore we will await updates on that front and report them here when possible (Neal).
I could talk about frogs and birds and other assorted wildlife, but why not dig into the less obvious cases? Those that require a bit more analysis than air passing through a throat?
Crickets make their sounds using a technique known as stridulating, where body parts are rubbed together. Normally, one using this technique would use wings or legs as they have a stridulatory fill allowing a sound to be generated much like a tuning fork does. The pitch of the sound is depended on speed of the rubbing, with a usual rate of 2,000 Hz being achieved. But this is by no means the most interesting sound property of crickets. Rather, it’s the relationship between the number of chirps and temperature. Yes, those little crickets are sensitive to temperature changes and a function does exist to estimate the degrees in Fahrenheit. It is approximately (# of chirps)/15 minutes + 40 degrees F. Crazy (Cox 91-3)!
Cicadas are another summer hallmark of natural noises. They happen to use little membranes underneath their wings that vibrate. The clicks we hear are a result of the vacuum being formed so fast by the membrane. As it should be no surprise to anyone who has been in a cicada environment, they can get loud with some groupings reaching up to 90 decibels (93)!
Water boatmen, “the loudest aquatic animal relative to its body length,” uses stridulating also. In their case, however, it is their genitalia which has ridging on it and it is rubbed against their abdomen. They can amplify their sounds using air bubbles near them, with the result getting better as the frequency is matched (94).
And then there are snapping shrimp, which also make use of air bubbles. Many people assume their clicks are a result of their claws coming into contact but it’s actually the water movement as the claws retract at speeds up to 45 miles per hour! This fast movement causes a pressure drop, allowing a small amount of water to boil and thus water vapor forms. It quickly condenses and collapses, creating a shock wave that can stun or even kill prey. Their noise is so powerful that it interfered with submarine detection tech in WWII (94-5).
I was rather surprised to find that some liquids will repeat a single sound made by someone, making the listener think the sound was repeated. This occurs not in typical everyday mediums but in quantum liquids that are Bose-Einstein Condensates, which have little to no internal friction. Traditionally, sounds travel because of moving particles in a medium like air or water. The denser the material, the faster the wave travels. But when we get to super cold materials, quantum properties arise and strange things occur. This is just another in a long list of surprises scientists have found. This second sound is typically slower and with a lesser amplitude, but it doesn’t have to be so. A research team led by Ludwig Mathey (University of Hamburg) looked into Feynman path integrals, which do a great job of modeling quantum paths into a classical description we can better understand. But when quantum fluctuations associated with quantum liquids are introduced, squeezed states appear that result in a sound wave. The second wave is generated because of the flux the first wave introduced into the quantum system (Mathey).
As cool as that was, this is a bit more every day and yet still an intriguing finding. A team led by Duyang Zang (Northwestern Polytechnical University in Xi’an, China) found that ultrasonic frequencies will transform droplets of sodium dodecyl sulfate into bubbles, given the right conditions. It involves acoustical levitation, where sound provides a force sufficient to counter gravity, provided the object being lifted is rather light. The floating droplet then flattens out because of the soundwaves and begins to oscillate. It forms a larger and larger curve in the droplet until the edges meet at the top, forming a bubble! The team found the greater the frequency then the smaller the bubble (for the energy provided would cause larger droplets to simply oscillate apart) (Woo).
What else have you heard that is interesting about acoustics? Let me know below and I will look more into it. Thanks!
Cox, Trevor. The Sound Book. Norton & Company, 2014. New York. Print. 71-2, 91-5.
Mathey, Ludwig. “A new path to understanding second sound in Bose-Einstein condensates.” Innovations-report.com. innovations report, 07 Feb. 2019. Web. 14 Nov. 2019.
Neal, Meghan. “The Many Colors of Sound.” Theatlantic.com. The Atlantic, 16 Feb. 2016. Web. 14 Nov. 2019.
Woo, Marcus. “To Make a Droplet into a Bubble, Use Sound.” Insidescience.org. AIP, 11 Sept. 2018. Web. 14 Nov. 2019.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2020 Leonard Kelley
Leonard Kelley (author) on December 05, 2020:
It was to me, too. Some supposedly have therapeutic qualities to them but it remains unsettled by science.
Umesh Chandra Bhatt from Kharghar, Navi Mumbai, India on December 04, 2020:
Very informative article. Well presented. Colour of sound was a new thing to me.