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post #91 of 203 Old 01-18-2014, 06:06 PM
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@Nils, how loud do you need to run your setup in order for you to feel your riser move?
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post #92 of 203 Old 01-18-2014, 11:45 PM
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Very cool, Fogo!
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Originally Posted by zheka View Post

Also, do you think spring based isolation is essential to achieve the effect or this is something that can be done using absorbers like Auralex floor floaters ?
http://www.amazon.com/Auralex-U-Boat-Floor-Floater/dp/B0002IL6ZS

Yes, both are damped isolators, just have to get the frequency low enough.
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Originally Posted by Trepidati0n View Post

I believe the point is to have a mass that can actually move when a force a light force is applied to it. Imagine standing on the ground and I push you "very very softly". You won't move. Might not even notice it. That is why people buy butt-kickers, etc. They need a strong force to physically "shake them". But what happens if you were standing on ice with Teflon shoes. I could push you ever so softly and you would feel in your body as you moved. This is what I feel FoLLgoTT did. He made so movement of air could move his rise" enough to get that tactile feel. Brilliant move. Honestly, f'ing brilliant. But, you do need his kind of system to generate that SPL force to cause the movement. Secondarily, you don't want too many springs or you just land up being stiff again.

Sorry, but most of that is flawed analogy or incorrect.

Noah
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post #93 of 203 Old 01-19-2014, 12:52 AM - Thread Starter
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Originally Posted by bossobass View Post

I wonder if you can tell the resonant frequency of my wood frame floor system in my HT space?

I don't think I'm able to find the resonant frequency of your floor by watching the frequency response of your subwoofers. smile.gif

But there is definitively something at about 5 Hz which is also visible in the close mic measurement. Do you have a long corridor or something like this? When I open the door of my home theater room there appears a strong mode of the corridor < 10 Hz. And even in the closest mic measurements I could always identify the room modes.

You often stated that in your room it is a big difference when playing the single digits. In my room I could not hear/feel a difference at all when activating a high pass at 10 Hz. Maybe it is my lack of experience with the ULF. But is it possible that your resonance at 5 Hz amplifies the perception? Usually resonances have a big impact on perception. Just an idea... smile.gif
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As I've said in other threads on this subject, it doesn't take much movement underneath you to leap to a whole new experience in movie watching. smile.gif

I agree! No earthquake like vibration is necessary to give a noticeable effect. The passive riser is such a difference to the concrete floor that I can barely describe it. smile.gif
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I also was involved in the discussion years back where some claimed that a resonance could cause a cancellation at that frequency, explaining Ricci's FR suckout around 10 Hz. I argued for proof because I'd never heard of that in anything I read on the subject. When a resonance is externally excited, the result is always a magnitude increase, never the opposite.

I think so, too. A very narrow "suck out" is commonly caused by cancellation. Room modes are the best example.
I can only imagine that something resonates with a phase shift and cancels the original signal. But this phase shift has to be 180° and I never found an evidence, either. I fact several types of absorbers work with resonances and none of them "sucks out energy" up to a notch.
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What was the formula you used for calculating the weight to spring isolators?

There are several mathematical principle I used:

- F = k * X (Hooke's law)
- Fg = m * g (force of gravity)
- n springs in parallel behave like one spring with kall = k / n. Or the other way around the force on one spring is F = Fall / n.
- balance point of the riser is at about 75 % of its length (strongly depends on the specific riser and seats).

I made an Excel sheet with these principles and calculated the compression distance for different spring models (with their datasheets). So I could choose the right one for my riser.

The 3 springs in the front are nearly half as stiff as the 3 springs in the back. This is the result of the balance point, because the couch will be placed in the back.

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Originally Posted by WereWolf84 
@Nils, how loud do you need to run your setup in order for to feel your riser move?

I can feel it down to -25 dB relative to reference SPL. I used the dragon crash scene in "How to train your dragon". When I put the chair on the concrete floor I feel nothing even at reference SPL.
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post #94 of 203 Old 01-19-2014, 01:12 AM
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"I also was involved in the discussion years back where some claimed that a resonance could cause a cancellation at that frequency, explaining Ricci's FR suckout around 10 Hz. I argued for proof because I'd never heard of that in anything I read on the subject."

see "limp mass bass absorber".

with a loaded mass (including furniture etc) of about 30 lbs per sq foot over an air gap of 8", cancellation occurs at 11hz.

something similar is used in the wall of bass that i posted not too long ago. by adjusting the mass and the air gap, you can tune the frequency. by adjusting the amount of absorption inside the device, the q can be varied.

dr. g. and dr. toole discuss a similar concept of lossy boundaries to minimize resonances.

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post #95 of 203 Old 01-19-2014, 01:36 AM - Thread Starter
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Quote:
Originally Posted by LTD02 View Post

see "limp mass bass absorber".

with a loaded mass (including furniture etc) of about 30 lbs per sq foot over an air gap of 8", cancellation occurs at 11hz.

I think this is not correct. A limp mass absorber does not cancel anything. Its mass resonates, this part is true. But the porous absorber behind the limp convert the kinetic energy to heat. This is the functional principle of all resonating absorbers (limp, CBR, Helmholtz etc.). They all convert kinetic energy of the moving mass or of the moving air to heat. There is no cancellation involved.

The wall of bass uses some kind of metal pane absorber in combination with porous absorbers. This is known as VPR or CBR. Again there is no cancellation involved.

I made a few experiments with metal pane absorbers, but they didn't absorb my first length mode enough. So I installed porous absorbers only.
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post #96 of 203 Old 01-19-2014, 02:22 AM
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recall that we are talking about frequencies below the modal region. in that frequency region, it is pressurization of the room that creates room gain.

with a resonating wall or floor, energy leaves the room at that frequency, so the room gain disappears. kind of like opening the wall at the frequency.

my theory is that it was not so much a cancellation that ricci was experiencing, but a frequency where pressure vessel gain was not functioning/disappearing. that then appeared to create a massive cancellation in response.

the width of the suckout also seems a little more consistent with a lower q absorber than the true cancellations which occur higher in frequency and are characterized by narrow dips.

who knows?



oh, and ricci mentioned,

"My best guess is that the floor resonated at this frequency and it would absorb the bass output. The floor would vibrate violently with sine tones at 12-13Hz."

so maybe he almost got it. the floor wasn't absorbing the bass per se, just no longer providing pressure vessel gain at that frequency.

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post #97 of 203 Old 01-19-2014, 02:27 AM
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Quote:
Originally Posted by FoLLgoTT View Post

This is the functional principle of all resonating absorbers (limp, CBR, Helmholtz etc.). They all convert kinetic energy of the moving mass or of the moving air to heat. There is no cancellation involved.

There's also a reactive component. They become sound sources themselves too.

Markus

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post #98 of 203 Old 01-19-2014, 02:31 AM
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Quote:
Originally Posted by FoLLgoTT View Post

Do you have a long corridor or something like this? When I open the door of my home theater room there appears a strong mode of the corridor < 10 Hz.

The other day I did observe such resonance. There was a very obvious difference between door closed and open. Don't have the measurement anymore but could repeat it if anybody is interested.

Markus

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post #99 of 203 Old 01-19-2014, 02:46 AM - Thread Starter
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Quote:
Originally Posted by LTD02 View Post

recall that we are talking about frequencies below the modal region. in that frequency region, it is pressurization of the room that creates room gain.

I don't agree. "Pressurization" is just an approximation for the phenomenon, but not the reality. If it would be real the speed of sound must change to infinity below the first room mode. This is just impossible.

I think "boundary gain" is the correct term. There is more and more constructive interference with decreasing in frequency. But I still believe that the wave theory applies in this frequency region. Why shouldn't it? There is no physical explanation for such a behaviour like "pressurization". smile.gif
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with a resonating wall or floor, energy leaves the room at that frequency, so the room gain disappears. kind of like opening the wall at the frequency.

my theory is that it was not so much a cancellation that ricci was experiencing, but a frequency where pressure vessel gain was not functioning/disappearing. that then appeared to create a massive cancellation in response.

This would mean that the wall works like a limp absorber. Maybe this is true. I cannot prove or disprove it.

Or maybe the walls are translucent to a certain degree at this frequencies. This would mean that the next room dimensions must be added to the room. This would lead to lower room modes (room length + next room length). But this is just another theory without proof. wink.gif
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Originally Posted by markus767 
There's also a reactive component. They become sound sources themselves too.

Yes, this is true. Usually phase shift is 90° at the resonant frequency. But to cancel the original signal it has to be 180°.
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post #100 of 203 Old 01-19-2014, 03:45 AM
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Quote:
Originally Posted by FoLLgoTT View Post


I think "boundary gain" is the correct term. There is more and more constructive interference with decreasing in frequency. But I still believe that the wave theory applies in this frequency region. Why shouldn't it? There is no physical explanation for such a behaviour like "pressurization". smile.gif

Only way I think that pressurization can be thought of as valid is if enough displacement is available to significantly alter the volume of the room despite necessary leakage (HVAC). I keep thinking of your riser as the brake booster on a vacuum-assist power braking system....you are turning a small change in the room's pressure into a palpable movement by using the force multiplier of the riser's total area. Very similar to the elegance that is a vacuum press, with huge clamping forces available using the same principle.

JSS
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post #101 of 203 Old 01-19-2014, 10:16 AM
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Quote:
Originally Posted by FoLLgoTT View Post

I don't agree. "Pressurization" is just an approximation for the phenomenon, but not the reality. If it would be real the speed of sound must change to infinity below the first room mode.

Please explain, unless you mean the zeroth mode at 0 Hz, but then there is no going lower than that.
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Originally Posted by FoLLgoTT View Post

There is no physical explanation for such a behaviour like "pressurization". smile.gif

The ideal gas law works nicely.
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Originally Posted by FoLLgoTT View Post

Or maybe the walls are translucent to a certain degree at this frequencies.

Of course, as we see anytime we hear the sound inside a room from outside.

Noah
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post #102 of 203 Old 01-19-2014, 10:39 AM
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Quote:
Originally Posted by noah katz View Post

Quote:
Originally Posted by zheka View Post

do you think spring based isolation is essential to achieve the effect or this is something that can be done using absorbers like Auralex floor floaters ?
http://www.amazon.com/Auralex-U-Boat-Floor-Floater/dp/B0002IL6ZS
Yes, both are damped isolators, just have to get the frequency low enough.
.
but is not the point of the exercise is to get right balance of spring action and damping? my concern is that the u-boat isolator is not compliant enough to be effective in this application.

@FoLLgoTT
what's the highest peak-to-peak amplitude of the riser vibrations you observed?

thank you very much
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post #103 of 203 Old 01-19-2014, 12:02 PM - Thread Starter
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Quote:
Originally Posted by noah katz View Post

Please explain, unless you mean the zeroth mode at 0 Hz, but then there is no going lower than that.

No, I mean the first and lowest room mode > 0.

The sound pressure propagates with a specific speed (speed of sound). Why should that change with low frequencies? Why should it change when the wave length increases over a point where its half doesn't fit in the room?

From my point of view there is no dynamic pressure which overlays the static pressure. This assumes that the dynamic pressure is simultaneous everywhere in the room (-> speed of sound = infinity). Please tell me the physical principle that explains this dramatically changed behaviour. I just don't believe it and I never found any physical explanation.
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The ideal gas law works nicely.

Please explain to me how it works. I'm not so familiar with gas laws.

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Originally Posted by zheka 
what's the highest peak-to-peak amplitude of the riser vibrations you observed?

I don't know. It is not so easy to get, because I have to measure the voltages, gain of pre amplifier and calculate it including the sensitivity of the sensor. I didn't do that.
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post #104 of 203 Old 01-19-2014, 01:26 PM
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"I don't agree. "Pressurization" is just an approximation for the phenomenon, but not the reality. If it would be real the speed of sound must change to infinity below the first room mode. This is just impossible."

that is the crux of the matter that bb and i discussed at length before and ended up agreeing to disagree. once you get below about 1/4 wavelength the fluid dynamics change from the farfield to the near field. in the near field, air behaves more like an incompressible fluid and the speed of sound increases dramatically (by about a 5x as it turns out).

i posted resources and links in several posts, but don't have the inclination to do so again at the moment. so, we too shall have to agree to disagree. just be aware that there is a rather extensive literature on the topic if you choose to revisit it at some point.
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post #105 of 203 Old 01-19-2014, 01:37 PM
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Quote:
Originally Posted by FoLLgoTT View Post

Yes, this is true. Usually phase shift is 90° at the resonant frequency. But to cancel the original signal it has to be 180°.

Correct. Nevertheless 90° will have some effect.

Markus

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post #106 of 203 Old 01-19-2014, 01:40 PM
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Originally Posted by LTD02 View Post

once you get below about 1/4 wavelength the fluid dynamics change from the farfield to the near field. in the near field, air behaves more like an incompressible fluid and the speed of sound increases dramatically (by about a 5x as it turns out).

I'd think air behave like air regardless of far field or near field.

Markus

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post #107 of 203 Old 01-19-2014, 02:51 PM
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Quote:
Originally Posted by FoLLgoTT View Post

No, I mean the first and lowest room mode > 0.

The sound pressure propagates with a specific speed (speed of sound). Why should that change with low frequencies?.

It doesn't.

But when there are room for many wavelengths there are an equal number high and low pressure areas, so the net average pressure is unchanged.

But at low frequencies much less than a single wavelength exists in the room at any instant in time, so the net pressure in the room varies with time.

Technically I guess the same is true in the first case also if you define pressure as the gas force at a boundary at any instant in time.
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Originally Posted by FoLLgoTT View Post

Please explain to me how it works. I'm not so familiar with gas laws.

Maybe you just don't know it by name, but I'm sure you understand it - if the volume of a container of gas is increased/decreased, then the pressure decreases/increases.

Noah
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post #108 of 203 Old 01-19-2014, 02:52 PM
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"I'd think air behave like air regardless of far field or near field."

can't argue with a statement like that, but what you imply is that the compressibility and the speed of sound are constant across the various fields. they are not.

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post #109 of 203 Old 01-19-2014, 04:30 PM
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Neat FoLLgoTT! I started to contemplate a similar riser construction with vibration dampers a couple of weeks ago. I will follow this thread to pick up your findings but first thing first, a DBA to be built.

A link in laymans language on how a very, very small variation of room volume changes the sound pressure below the lowest room mode (and also the reason why ported speakers are useless for room pressurization as they are not changing the room volume outside the speaker enclosure because the port is in direct communication with the room volume, -the gas laws comes into play): http://www.cartchunk.org/audiotopics/SmallEnclBass.pdf
For those used to the metric system, some of the units may be unfamiliar.
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post #110 of 203 Old 01-19-2014, 07:05 PM
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Originally Posted by Adhoc1 View Post

... ported speakers are useless for room pressurization as they are not changing the room volume outside the speaker enclosure because the port is in direct communication with the room volume, -the gas laws comes into play...

That's true below Fb but not in its operating range, or ported speakers wouldn't be able to generate the SPL that they do (hint: the P in SPL is for pressure).

Noah
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post #111 of 203 Old 01-19-2014, 11:42 PM
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Quote:
Originally Posted by LTD02 View Post

"I'd think air behave like air regardless of far field or near field."

can't argue with a statement like that, but what you imply is that the compressibility and the speed of sound are constant across the various fields. they are not.

Could you provide references?

Markus

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post #112 of 203 Old 01-20-2014, 02:04 AM - Thread Starter
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Quote:
Originally Posted by noah katz View Post

But when there are room for many wavelengths there are an equal number high and low pressure areas, so the net average pressure is unchanged.

But at low frequencies much less than a single wavelength exists in the room at any instant in time, so the net pressure in the room varies with time.

Yes, for one instant in time the pressure is nearly constant in the room. But this is just an approximation. We are talking about dynamic pressure changes from one small surface in the room. These dynamic changes have to be propagated. The gas law doesn't say anything about changing the pressure at one point in the room. It assumes that it changes equally everywhere. So there is still wave propagation, reflection and constructive interference as explained in all the discussions before (see Popalock's thread). I don't find any hint that gas law can be applied for such a situation.

If pressure would change everywhere in the room at the same time a close mic measurement would be no different to every other point in the room. But it is very different. The "pressurization" theory is nothing more than an approximation that doesn't stand any experiment.

Quote:
Originally Posted by Adhoc1 
A link in laymans language on how a very, very small variation of room volume changes the sound pressure below the lowest room mode (and also the reason why ported speakers are useless for room pressurization as they are not changing the room volume outside the speaker enclosure because the port is in direct communication with the room volume, -the gas laws comes into play): http://www.cartchunk.org/audiotopics/SmallEnclBass.pdf

I think Dick Pierce makes wrong assumptions. See my explanation above about small surface vs. everywhere.

Beside that I never heard that a bassreflex subwoofer doesn't have room gain under its port frequency. I always saw the opposite in measurements.
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post #113 of 203 Old 01-20-2014, 03:31 AM
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"That's true below Fb but not in its operating range, or ported speakers wouldn't be able to generate the SPL that they do (hint: the P in SPL is for pressure)."

it is also that same principal that allows a room to be pressurized even though it may have windows or doors.

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post #114 of 203 Old 01-20-2014, 04:59 AM
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^
And a sub is capable of doing that? References please.

Markus

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post #115 of 203 Old 01-20-2014, 08:19 AM
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^ what are you asking for? pressure vessel gain in rooms works whether or not they are air tight. works better the more sealed, but still works otherwise.

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post #116 of 203 Old 01-20-2014, 08:25 AM
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Excellent creativity Follo. That "passive shaker" setup is very interesting, especially to guys with rooms on concrete.

Here are some springs I found that might work: http://www.amazon.com/Mason-SLFH-A-125-Unhoused-Vibration-Deflection/dp/B006W0R97Y

I'm trying to wrap my head around how to better tune a setup like this so that it emphasizes the lowest frequencies and not the highest.
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post #117 of 203 Old 01-20-2014, 09:20 AM
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Awesome set up FoLLgoTT with measurements to match! Just curious, in your wiring schematic you show 7000 Watts at 5.4ohm. In the specs 4ohms is listed at 14,000 (not that I think it will deliver all that) so I thought 5.4 would have been higher?
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post #118 of 203 Old 01-20-2014, 09:31 AM
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Quote:
Originally Posted by coctostan View Post

I'm trying to wrap my head around how to better tune a setup like this so that it emphasizes the lowest frequencies and not the highest.

I am out of my depth here, but would not a riser with low resonant frequency and low Q be the answer?



unfortunately the mass is variable in this situation, but it should be possible to optimize for a single scenario, say total viewer weight is 300lbs + weight of the furniture and platform itself.
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post #119 of 203 Old 01-20-2014, 10:45 AM
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Quote:
Originally Posted by FoLLgoTT View Post

The "pressurization" theory is nothing more than an approximation that doesn't stand any experiment.

It would be more accurate to say that our listening rooms are poor approximations of sealed vessel with rigid walls.

But that only lessens the effect for us, does not eliminate it.

I'm not sure I understand your point, as you seem to alternately agree and disagree that there is room pressurization.
Quote:
Originally Posted by coctostan View Post

I'm trying to wrap my head around how to better tune a setup like this so that it emphasizes the lowest frequencies and not the highest.

You can shoot for resonance at the freq of your choice, but I don't know why you'd want to do this here any more than you would with a sub.

The normalized (to the resonant freq) transmissibility curve is instructive:

https://www.google.com/search?q=forced+vibration+response+curve&espv=210&es_sm=93&tbm=isch&source=iu&imgil=iDFBqsjjU2BTGM%253A%253Bhttps%253A%252F%252Fencrypted-tbn1.gstatic.com%252Fimages%253Fq%253Dtbn%253AANd9GcSSNHAY3AMS0abEDbch0SPz9FETLx0LS_A5l1pUMKBJ1rLqcuvvcg%253B448%253B327%253BzkcbC1YZK6Kp9M%253Bhttp%25253A%25252F%25252Fwww.modalshop.com%25252Ftechlibrary%25252F200.asp&sa=X&ei=Qm7dUpePM8u7oQTc84LwBg&ved=0CEkQ9QEwAw&biw=1920&bih=947#facrc=_&imgrc=iDFBqsjjU2BTGM%253A%3BzkcbC1YZK6Kp9M%3Bhttp%253A%252F%252Fwww.modalshop.com%252Ffilelibrary%252FForced_Vibration_Response_amplitude.jpg%3Bhttp%253A%252F%252Fwww.modalshop.com%252Ftechlibrary%252F200.asp%3B448%3B327

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post #120 of 203 Old 01-20-2014, 11:32 AM - Thread Starter
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Quote:
Originally Posted by noah katz View Post

I'm not sure I understand your point, as you seem to alternately agree and disagree that there is room pressurization.

Maybe it is my poor english. In my native language I'm much more precise. smile.gif

I agree that there is a strong gain, but I disagree that it is caused by a "pressurization" which can be explained by gas laws. I think the reasons for the gain are constructive interference and mirror sources on boundaries. That's my point.

Quote:
Originally Posted by gpmbc View Post

Awesome set up FoLLgoTT with measurements to match! Just curious, in your wiring schematic you show 7000 Watts at 5.4ohm. In the specs 4ohms is listed at 14,000 (not that I think it will deliver all that) so I thought 5.4 would have been higher?

The FP 14000 is rated as 2 x 7000 W at 2 Ohm not 4 Ohm. smile.gif
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