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Make waves.

Did you know that the fastest vibration we can hear is 20,000 vibrations per second?!

Mar 30, 2022

Words: Gippslandia

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You’ve got to get excited to produce sound.

Firstly, we make an object vibrate. This is our sound source. Then like a riotous mosh pit at a punk concert, the vibrations cause nearby air molecules to bump into each other. The state of their excitement lifts. This results in a wave of vibrations. This ‘Mexican wave’ travels through the air (or water, gas or solid but not in a vacuum) until it reaches your eardrum and causes it to vibrate too. Sound travels faster through water than air, as the water is denser.

The sound changes depending on how fast or slow the source vibrates. The slowest vibration human ears can hear is 20 vibrations per second. That would be a very low-pitched sound. The fastest vibration we can hear is 20,000 vibrations per second, which would be a very high pitched sound. Cats can hear even higher pitches than dogs, and porpoises can hear the fastest vibrations of all (up to 150,000 times per second!).

The stronger the vibrations, the louder the sound, and we know that sounds become fainter the further they travel from the source. a riotous mosh pit at a punk concert, the vibrations cause nearby air molecules to bump into each other.

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With that bit of rudimentary acoustics, we reached out to Richard Northover, the owner of Tropic Design, Traralgon, to learn more about the Inverloch and Traralgon sound shells that he was involved with designing.

Richard explains that the shells are designed to “project the sounds created in the shell out to the audience either passively or assisted by amplification. [In] theory it’s like the old gramophone horn or a loud hailer cone.

“The intent was to provide a facility that produced clear sound from a single acoustic, unassisted performer to a rock band or orchestra.

“Its location is critical to its performance, and my particular design performs best when located in a natural amphitheatre.”

The main design factors that Richard accounted for in the designs were:
- The project budget
- Proposed site and suitability
(What’s interesting is that as the location of the Traralgon sound shell faces west, large orchestras have mentioned that if instruments are tuned at soundcheck, the sun can require them to be retuned after sunset.)

- Orientation and floor levels relative to the surrounding land
(Traralgon is on a flood zone, so the building has to allow crossflow of floodwater across the stage and all the electricals must be above the highest flood levels.)

- The exposure of the site to the prevailing wind
(Wind disturbs sound waves when it exceeds 15km/hr.)

- Finally, construction type and materials to reduce vandalism.

Once the above factors are decided, a basic sound wave deflection diagram is created to “get as much sound reverberating out of the shell with minimal artificial assistance.”

“We also measured the projected sound spread with respect to the side walls to maximise the spread for the audience. The ceiling sound panel and the curved rear wall are part of this calculation.

“The shell shape is to passively amplify and equally distribute sound over an audience in outdoor conditions… It’s effectively a frustum of a cone lying on its side. Design for enclosed spaces is hugely different.”

Richard adds that fortunately both “Traralgon and Inverloch work perfectly; Inverloch, in particular, gives the natural amphitheatre shape”.

He finishes by adding that these projects wouldn’t have happened without the “sheer tenacity of the local Lions Club” facilitating and providing funds for these community venues that fill us with more exciting vibrations.

Gippslandia - Issue No. 22

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