Control Room Project - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 07-22-10, 05:04 PM Thread Starter
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Eric

Join Date: Dec 2009
Location: Dora, Ok
Posts: 193
Re: Control Room Project

It's possible that the ceiling is creating a void toward the floor corners; however, from what I've heard, I'm kinda leaning toward the mode being directly in the middle of the room on the floor (on one end), and directly above (on the other end) where the ceiling comes to a point from all sides. It's a tight corner.

My thinking is this: build a cloud with low frequency absorbers on top, facing the ceiling where that nasty corner is. Because spectral analysis shows a build-up there of 31 Hz, I'll build some absorbers with that low freq in mind. On the underside, I have been considering installing 2-D diffusion, as you have suggested, Bryan.

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Old 07-23-10, 09:01 AM
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fractile

Join Date: Mar 2009
Location: San Francisco
Posts: 212
Re: Control Room Project

It's an interesting problem. A rough calculation in my head tells me there is about 30 feet from the peak to the center of the floor, with the peak as a projective sound source resonating/coupling with the floor at that wavelength.

In my imagination I suggest running a simulation of what a bass trap at 1/2 that wavelength will do, since you mention putting it in the middle, above the cloud.
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Old 07-26-10, 10:53 AM Thread Starter
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Eric

Join Date: Dec 2009
Location: Dora, Ok
Posts: 193
Re: Control Room Project

Hey, fractile,

The peak is actually at about 18ft in there. And the ceiling at the outer edges is 10 ft. The octagon is roughly 21 ft in diameter, when measured perpendicular between facing walls. I actually don't have a simulator (I do my computations manually). If you've got one (and some spare time), I'd be interested in seeing what yours comes up with.
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Old 07-28-10, 06:55 AM
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fractile

Join Date: Mar 2009
Location: San Francisco
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Re: Control Room Project

Ok; I looked at it again and got 18.5' as the half-wave length of 31Hz. I'd mistakenly been looking at the full wave-length, when it is the nodes of the wave that resonate in the space and these nodes are at each end and in the middle of the full wave. Getting my laws of physics straight.

I'm not set up for that simulation, either. Maybe someone around here is. This problem is something to help me comb out my technical proficiency. I was having a little difficulty comprehending why the loudest resonance is at the zero-crossing of the nodes, where the wave is at least amplitude. My best explanation is that this is where the soundwave in the air molecules couples with the boundaries of the room, like the bridge and nut of a guitar; without the guitar you'd barely hear the strings.

My room looks pretty symmetrical, but the doors are like a giant matrix, so I don't worry too much about symmetric placement of the sub; we'll see when I get to that stage of tuning.
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Old 07-30-10, 01:15 PM Thread Starter
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Eric

Join Date: Dec 2009
Location: Dora, Ok
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Re: Control Room Project

Quote:
fractile wrote: View Post
I was having a little difficulty comprehending why the loudest resonance is at the zero-crossing of the nodes, where the wave is at least amplitude.
Not sure about that, either. Where's Bryan when you need him?

I do remember... this applies to a bass trap - maybe it can be translated into room acoustics ... that at the back end of a quarter wave bass trap, the air velocity is zero - because the air is trapped and has nowhere to go. At the opening of the trap, the air is free to move, and at the target frequency, the velocity is the highest (pressure is the lowest).

In any room, if you're playing a stereo, you'll hear the bass getting louder toward the corners, where they seem to be gathering. My guess is that the hard walls offer resistance to the wave fronts, resisting the air movement, creating high pressure at these frequencies. .... The same principle as the bass trap physics, except there is no effort in open air to "trap" the energy.
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Old 07-30-10, 07:01 PM
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fractile

Join Date: Mar 2009
Location: San Francisco
Posts: 212
Re: Control Room Project

Well (that's a deep subject ;-) the way it works, i think, as i mensioned, is that the wave couples with the room, like two ends of a string. The dimension of the room couples first at the lowest frequency harmonic with the output of the lowest frequency response of the speaker, if that is in the dimension of the room at 1/2 wavelength, at the nodes . Other harmonics will be less audible by 6dB or whatever.

The low frequencies have the easiest ability to resonate a space, say, because it is fundamental.

On velocity of the wave, if you look at a sine wave, the velocity is maximum increase/decrease around zero; the rate of change. The amplitude is low, yes, but the dynamic transient is greatest.

It helps me to think in physical terms, say, like the air is a 3D string vibrating, and the room has walls that tie the ends of the string and serve as a sounding board to resonate and amplify the sound. This will be at the fundamental resonant dimension frequency of the room and will drop off at 6dB per octave or something. You will hear other echos and reverb at higher frequencies, but these will not resonate as much as that fundamental. That's about what I've gathered from my reading and listening.
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Old 07-30-10, 07:21 PM
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fractile

Join Date: Mar 2009
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Posts: 212
Re: Control Room Project

I should mention that at the corners of a room the dimensional length is greatest and therefore more easily audible as a resonance, due to some law of physics and the fundamental resonant tone of a room (space). The corners do serve to reflect and amplify/regenerate the wave.
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Old 07-30-10, 07:23 PM
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fractile

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Re: Control Room Project

What I mean is that this will also happen on a sphere with no corners, that the sphere will resonate at its fundamental tone.
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Old 08-05-10, 04:30 PM Thread Starter
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Eric

Join Date: Dec 2009
Location: Dora, Ok
Posts: 193
Re: Control Room Project

Yes, a sphere is the absolute (theoretical) worst case scenario. Whatever its fundamental diameter, it will be repeated from any direction at its center.

I think the 3D picture you've come up with is handy and easy to remember.
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