PHYS341/2024/Project27

Sabine Orthwein

PHYS 341

04/13/2024

PHYS 341 Final Project: Educational Video + Demo

Link to Final Video Project: https://youtu.be/NHkq05CvhLQ

Script:

Today, I will explore the physics of how sound is recorded and played back on a vinyl record.

Physics of Sound

Sound, in its simplest description, is movement. As sound waves move through a medium, molecules vibrate back and forth: displacing the surrounding molecules and causing fluctuations in pressure. When it approaches the ear, sound that is traveling through the air will displace the eardrum and ossicles in the same way it displaced the air molecules. The ossicles then translate the displacement to vibrations that are sent to a part of the cochlea, called the basilar membrane. The basilar membrane is a funny looking, spiral shape, but it serves as an essential mediator between sound vibrations and your brain. [insert photo of the basilar membrane] Vibrations that are sent to the membrane find their place along it, as the different regions along the basilar membrane are excited by different frequencies. In this way, the basilar membrane turns the frequency into a spectrum graph. The activation on the membrane will cause tiny hair cells that correspond to the region, to vibrate: sending an electrical signal to the brain to process it into sound that we recognize.

So what does this have to do with music? Well, when a musical instrument is played, whether that be a trumpet or a human voice, it acts as the instigator for this whole process. The vibration and sound wave of a musical note can vary a lot depending on anything from the shape of an instrument, to the mouthpiece, or even air temperature. Each instrument has its own unique sound qualities and, therefore, a unique sound wave. Singing and whistling can both be produced by the same human body, but have two distinct time graphs. [insert the two time graphs recorded on Audacity] These time graphs can show us how all sound leaves a distinct trace.

Vinyls and Record Players

Now that we have taken a close look into sound and how it is translated from instrument to ear, we can start to apply these concepts to vinyl records. Records are, in essence: spinning, three- dimensional time graphs. As great as regular, two-dimensional time graphs are: without a synthesizer, they are just recorded sound with no way of playing them back. Instead (for the recording process of vinyls), as the sound is recorded in real time, the vibrations are etched into a circular mold while the record spins around at a constant rate: creating those minuscule rings around the record. [insert picture of vinyl] Those rings are actually the 3D imprints of the time graph for the sound. [insert upclose picture of vinyl rings] A hardened mold can then be used to stamp as many vinyl records that are needed.

Although rudimentary, this concept can be displayed by creating notches in this cardboard to create a rhythm.

[Rhythm Demo]

Now that we understand the basic recording process, let’s take a look at how the recorded sound can be played back. The process starts with the disc rotating at the same speed it was recorded at (because, as we said, the grooves are like a 3D time graph, meaning the record is a result of displacement across time: so, if played at any other speed, the relationship between displacement and time wouldn’t be proportional). Then the stylus is placed into the grooves of the record, and dragged along the pattern. The distinct shapes of the path cause the stylus to vibrate in the exact same way that the recording needle did; this vibration is transmitted up the arm, to the body of the record player. Early versions, like the gramophone, used a diaphragm and a large amplification tool [insert photo of gramophone horn] (in other words, one of these big horns) to turn these vibrations into recognizable sound. More modern record players translate the vibrations into electrical signals by using metal coils and magnets. These signals are then sent to more modern versions of amplification: speakers. These speakers then turn those electrical signals back into movement: fluctuating the pressure and displacing air molecules.

It might sound like a confusing process, but making a device that plays back recorded sound is actually pretty easy with the right materials. Let’s make one.

[Stylus Demo]

PHYS 341 Final Project: Educational Video + Demo

Link to Final Video Project: https://youtu.be/NHkq05CvhLQ

Script:

Physics of Sound

Vinyls and Record Players

References