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Perfect Sphere vs Room Modes

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Perfect Sphere vs Room Modes

Postby recordplay » Wed Dec 05, 2018 4:21 pm

If a room was a perfect sphere. How would you (or could you) calculate room modes within it?
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Re: Perfect Sphere vs Room Modes

Postby Hugh Robjohns » Wed Dec 05, 2018 4:33 pm

The axial modes would all be at the one frequency (plus its harmonics of course) because all reflections between opposite surfaces in each axis would be of the same length. The same would apply to the tangential and oblique modes.

Not a good environment to perform or listen...

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Re: Perfect Sphere vs Room Modes

Postby MarkOne » Wed Dec 12, 2018 2:27 am

Not a sphere as such but a hemisphere...

A number of years ago, I was having dinner in a place in Paris that had a crypt-like basement, with a central chamber that was circular with a hemispherical roof. It was weird. The roof acted like an acoustic lens and wherever you sat you could hear quite clearly the people sat across the room from you like they were sat next to you. Also, even the natural background noise we all pretty much ignore most of the time was strangely distorted and amplified

I guess the same sort of effects were at play in the Royal Albert Hall before they installed the acoustic cloud in the dome.

I could imagine a full sphere would be the same effect but doubled.
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Re: Perfect Sphere vs Room Modes

Postby BigRedX » Wed Dec 12, 2018 10:58 am

I've played a few times in a church hall that is a near perfect octagon in shape with a symmetrical pointed roof. The stage is along three walls of the octagon and the acoustics are very weird. Bass frequencies in particular are projected out into the centre of the room and significantly attenuated in the stage area, with the result that if I have my bass rig turned up so I can hear myself properly on stage, the bass guitar is almost drowning everything else out where the audience are standing.
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Re: Perfect Sphere vs Room Modes

Postby Hugh Robjohns » Wed Dec 12, 2018 11:02 am

Years back at the dear old BBC's training centre, there was a recreational building on the site called the Phoenix, with a gym, snooker table, and squash courts downstairs, and a bar upstairs. And in the centre of that bar room was a domed fibreglass skylight... and it behaved exactly as you describe: it acted as a lens or prism to deflect and focus conversations from elsewhere in the room, and it really did sound like invisible ghosts were talking right next to you. It was a very spooky effect, and it caused no end of embarrassment as people heard conversations that were intended to be private. Good teaching aid for sound techs though! ;-)
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