Sound Misconceptions, Some

Last night I attended my town’s City Council meeting, they’re revising the Noise Ordinance. It was one of the funniest and conspicuously frustrating things I’ve participated in. Yes, there’s more to the story. Anyway, I think most of us know very little about the mechanics of sound. Audiophiles, sound engineers, musicians, architects, producers, builders, city councils… the majority of us have significant misconceptions about sound—waves humans hear—which results in some pretty wooly communication.

See that bird? It's a brown-throated thrush, but in Germany it's called a Pfleegel flügel, and in Chinese they call it a Keewonton, In Japanese a Towhatowharra, and so on. And when you know all the names of that bird in every language, you know nothing, but absolutely nothing, about the bird. […] Knowing the name of something doesn't constitute knowledge.

[…] In order to talk to each other, we have to have words, and that's all right. It's a good idea to try to see the difference, and it's a good idea to know when we are teaching the tools of science, such as words, and when we are teaching science itself.

–Richard Feynman

Lets start with a very familiar word, deebees. dB (decibel). Decibel is a scale, and on its own it is not specifically a unit of sound. Without context, references or definitions all that is said using “dB” is that there’s a logarithmic scale in play. And even when you put a day or two’s effort into understanding units such as dBA/dBC/dBZ, you still do not understand sound. Without understanding sound power and sound intensity I don’t think you can understand sound pressure, even though these aspects correlate and are similarly scaled. And then even then, with all that under your belt, I think more is needed; bandwidth and how spectral content interplays and sums, divergent and source qualities, differentials, wave functions.… How does an isotropic source differ from a cylindrical or plane wave source. I think all of that is necessary if you really want to know what a datapoint like 90dBA is and how it might likely fit within the whole of a sound field. We have to know what it is we know and know what it is we don’t know, to again quote Feynman. I keep my favorite books close. While an outline of the nature of sound doesn’t fit very well on a page or two, you can reference two great books on acoustics below.

And how about the word acoustics. We all use this word pretty loosely, usually ambiguously. Acoustics is a branch of physics that encompasses much more than sound as it relates to human sense of hearing. If you or I wish to talk about acoustics applied to musical instruments the right term is Music Acoustics, or Musical Acoustics, a branch of acoustics that can get pretty hairy once the solids qualities are considered and how they contribute to tone, and what tone is and what is good and bad tone. Yeah, “hairy” is selling it. If we want to talk about the acoustics of a recording studio or playback room that would be Room Acoustics, or more broadly Architectural Acoustics. And there too the solids, materials, construction methods and quality contribute significantly to the “sound” of a room. Continuing, there’s Environmental Acoustics, Underwater Acoustics, Physical Acoustics, Bioacoustics, and so on.

Sound, wide and deep as it is, that one we have a good handle on right? Maybe not, it requires defining, or needs to be used contextually, which it usually is. So then what’s the problem? Well, we use the word all the time, and so long as we don’t talk about the nature of sound we use the word to good effect. But when discussed technically… I would call most of what is spoken on sound by those without a solid grasp of physics a shitshow. We are so familiar with the word and the effect sound has on us that we think we understand it. We don’t, not the whys and hows. Feynman was right on target when he said, knowing the name of something—even combined with near universal, continual sensory experience—does not constitute knowledge. If we are discussing the sound of a Legendary Pink Dots concert we generally know we are talking about vibrations in air within the full musical spectrum and within a general intensity or sound pressure range, the source being loudspeakers and the receiver our ears. Where everything falls apart is when the physics-deficient start speculating, and worse yet when the same try and visualize misguided explanations for the nature of sound. Here’s an example, there’s a medial device company that had me do some work for them, a sound wave therapy device manufacturer. The technical description on how and what the devices was creating specific to sound (never mind the why) wasn’t quite a shitshow, but it was along the lines of ignorant quackery. “Our sound waves device creates special pressure waves…. What is special about pressure waves in air? Nothing! All sounds in air are pressure waves, there is no shear modulus for gasses. Interested to know more about wave types, shapes and sizes? Really, those two books listed below, buy them and study them. And since you are reading this I’m guessing you like thinking about sound, here’s a fun one. How many ways can square-waves be produced in air? [Hint. See figure 1.1-A in Olson’s Music, Physics and Engineering.]

Start with Olson: Olson, Harry F. (1967) Music, physics and engineering (2nd ed.). Dover. (Original 1952, formerly: Musical engineering). Then: Kinsler, Frey, Coppens, and Sanders (2012) Fundamentals of Acoustics (4th ed.). Wiley.

[Outro] Harvey Day paraphrasing: Ska—it’s what a thirteen year-old boy hears when he gets extra mozzarella sticks.

Sean Casey3 Comments