CMI/IUPUI Speaker Restoration Project: Speakers

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The heart of the speaker. Stylish.
This week, we’re wrapping up our background coverage of this project by getting into the real nitty gritty of what goes on behind the scenes when designing an audio speaker. What exactly makes up a speaker? How does it produce music? What clever design decisions do the RCA speakers we’re working with exhibit?

The tweeter nestled inside the subwoofer cone.
Dynamic speaker design basically boils down to a few basic components: a permanent magnet, a “voice coil” that is capable of producing a magnetic field, and some kind of vibrating material. It relies on the vibration caused by alternating current passing through the voice coil creating a magnetic field that interacts with the magnetic field of the larger, permanent magnet. This vibrates the air between the two, causing the vibrating material to move back and forth and produce sound. Modern speakers utilize two separate configurations of this vibrating apparatus, the subwoofer for lower frequencies and the tweeter for the higher ones. Tweeters and subwoofers are often combined in order to maximize the frequency range one speaker can put out, with the smaller tweeter either nestled in the center of the larger subwoofer cone or placed next to it, depending on the speaker design.

The heart of the speaker. Stylish.
How, then, does the tweeter and subwoofer receive the information they need to produce music? Physical vibrations from a microphone, digital data from a computer, or any other musical data that is sent to the speaker must be first converted to an electrical signal that the speaker can understand. This electrical data is sent to the speaker’s crossover, a filter that sorts the high and low frequencies and sends them into the proper part of the speaker. Crossovers are often a combination of capacitors and inductors nestled outside the speaker’s cone and are responsible for the performance and safety of each speaker part (particularly the subwoofer, which might overheat if given too much high frequency signal).
Due to the nature of vibration, each forward vibration of the speaker will produce an equal and opposite backward vibration headed towards the speaker wall. Without proper highly absorptive material, these vibrations bounce back from the wall and interfere with the vibrations heading forward. This interference can heavily distort the sound, both due to the phase shift and some diffraction from hitting the corners of the speaker wall. This is why speaker design and placement are key to get optimal sound in a performance space. The best speakers will be designed either to reduce phase shift or have enough absorption to negate the rebounding waves. They also need to be placed in such a way that sound waves that bounce off walls behind them have minimal interference with the sound being heard.

No sharp edges minimizes frequency diffraction
How do these factors play a part in the speakers we’re restoring? Harry F. Olson (mentioned in previous blog posts) did extensive research in speaker cabinet design that minimized distortion, resulting in the rounded design of our speakers. He concluded that the diffraction of frequencies was a result of the waves bouncing off sharp 90-degree angles in the speaker cabinet, so one without these harsh edges could resonate pleasantly with minimal distortion of sound. Our speakers also have a uniquely textured dome tweeter nestled inside the subwoofer cone so that the speaker as a whole looks simple, clean, yet unique with its wooden case.

When setting up a sound system, there are still so many other factors to consider when trying to recreate the perfect listening environment, something audio engineers struggle even to this day to achieve. While we might not see much of this rounded design in the future (due to the sheer logistics on how one could mass produce speakers like this), it’s fascinating to physically see how small details can have great contributions towards sound quality.

If you’re interested in more in-depth explanations on how speakers function, I’ll leave the links to some of my sources (including Olson’s research) down below:


Physics of Sound eBook chapter focusing on Speakers:
https://soundphysics.ius.edu/?page_id=1343

 Basis of Crossover in speakers:
https://www.audioholics.com/loudspeaker-design/crossover

 Research on Speaker Cabinet design:

http://www.dwdrums.com/images/moonmic/olson_direct-radiator-loudspeaker-enclosures.pdf

 I also wanted to share an really interesting video on how a speaker cone is manufactured by How It's Made:
https://www.youtube.com/watch?time_continue=287&v=VN0tmyyC0ak

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