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Saturday, September 25, 2021

Column: One idiot’s guide to how a guitar works

First I want to talk about the air — and not in some mystical and ancient way like finding your qi. I literally want to talk about the gaseous fluid medium that we, human beings (about 7 billion) like to live in and breathe.

It’s nothing unusual or special, but it’s an important thing to consider that the air consists of a bunch of different molecules bouncing around. Like any other collection of matter, these individual particles interact with one another.

One very important interaction related to what we’re talking about is the propagation of waves. In the air, waves are simple enough. Something disturbs some air, knocking some particles around. Those particles then crash into the neighboring ones. In this brief moment, there’s a pocket of heavily packed air particles directly adjacent to a spot where there’s not so many. One could call this a high pressure zone next to a low pressure zone.

Since nobody is hanging on to all of these air molecules and keeping them this way, the pressure diffuses, and the high pressure zone becomes lower while the surrounding low pressure zones get a little more crowded until the two zones even out. This pattern repeats outward from the source of the disturbance, and the whole phenomenon is called a wave. As such, it can be described with wave-related jargon; high pressure zones, or where the wave has peaked, are crests, and low pressure zones, where the wave is at its lowest, are troughs.

I just threw a lot of words at you that might have gotten pretty boring pretty quickly, so we’ll talk about something fun: slinkies. The simplest way to visualize waves away from the beach is through a technique used by introductory physics instructors everywhere. Grab a friend, hand them one end of a slinky, hold on to the other end and move your end side to side. Ta-dah, you’ve just made a wave. Granted, it’s a longitudinal wave and mildly different from the transverse pressure waves I just described, but if you take a finger and flick a coil of the spring you’re holding, the little disturbance that you see bounce back and forth is a transverse wave.

On the note of making springs vibrate, a guitar string is essentially like a slinky! They’re all held under very high tension. Plucking one of these strings disturbs the air and sends a pressure wave through the surrounding air.

Now, the reason guitars have six strings and all those frets is that the different strings have different tensions. Plucking them disturbs the air a different amount and changes the resulting wave that travels through the air. The reason all of the frets are there is that holding down the string to a different length also alters the tension and enables many different wave frequencies to be produced.

The resulting wave either gets bounced around the body of an acoustic guitar or gets picked up by the appropriately named “pickup” on an electric guitar. With an electric guitar, the pickups translate the waves into electrical signals that cause the speaker to make bigger, stronger waves of the same frequency. In an acoustic guitar, waves rattle around the body of the guitar, and positive superposition (fancy word for waves of the same sorts hitting one another just right and becoming bigger) sends waves through the air, which rattles our eardrums.

Once the eardrum starts moving, it causes a set of very small bones to shake and translate the outside sound into pressure changes on the the cochlea, a fluid-filled organ that’s lined with sensory cells. When the stapes (U-shaped bone included in that set mentioned earlier) vibrates, those vibrations are transferred to the cochlea and disturb the sensory cells, which send impulses to the brain and let it know to express the particular sensations of warm fuzzies associated with hearing a guitar.

It’s worth mentioning that these same principles apply to all musical instruments: vocal cords vibrate when air passes over them and make waves, wind and brass instruments taper in current airflow to pass vibrations along a bell and disturb the surrounding air, pianos use percussed strings and drums use percussed membranes.

ALAN LIN can be reached at science@theaggie.org.

3 COMMENTS

  1. […] Column: One idiot's guide to how a guitar works On the note of making springs vibrate, a guitar string is essentially like a slinky! They're all held under very high tension. Plucking one of these strings disturbs the air and sends a pressure wave through the surrounding air. Now, the reason guitars … Read more on The Aggie […]

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