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FoLLgoTT's build: 18 x Peerless XXLS12 - Page 6

post #151 of 203
Poorly stated on my part; what I should have said is the conclusion is wrong, likely confused by the spatial distribution of modal effects having been changed, the same as putting an absorber in the same location.

If you disagree, please tell me what's incorrect
Quote:
Originally Posted by bossobass View Post

The measurements aren't flawed.

Here's my room with 2 doors and 2 windows closed vs the same openings opened:


post #152 of 203
Quote:
Originally Posted by noah katz View Post

Poorly stated on my part; what I should have said is the conclusion is wrong, likely confused by the spatial distribution of modal effects having been changed, the same as putting an absorber in the same location.

If you disagree, please tell me what's incorrect


If there is spatial distribution that's proof enough to conclude that the pressure vessel gain theory is an incorrect premise.

In fact, the spacial distribution of pressure waves in a room holds true to the lowest frequencies. This is what I've found to be supported by the evidence.

There is zero evidence that this known phenomenon changes at any frequency, let alone a frequency whose wave length is >2X the rooms longest dimension.

If there is evidence, please post it.
post #153 of 203
Quote:
Originally Posted by bossobass View Post

If there is spatial distribution that's proof enough to conclude that the pressure vessel gain theory is an incorrect premise.

I don't see how that conclusion follows.
Quote:
Originally Posted by bossobass View Post

In fact, the spacial distribution of pressure waves in a room holds true to the lowest frequencies. This is what I've found to be supported by the evidence.

Why wouldn't it?

Again, it's a smoothly varying function with 0 Hz equating to a pressure assuming a perfectly sealed room with rigid boundaries, which of course doesn't exist in the real world and why you've never observed/measured the resulting effects.
Quote:
Originally Posted by bossobass View Post

There is zero evidence that this known phenomenon changes at any frequency, let alone a frequency whose wave length is >2X the rooms longest dimension.

Who said it does?
post #154 of 203
Thread Starter 
Here are a few more measurents for the actual discussion. I have two small windows in my cellar room.



I measured three states:

1. both windows closed
2. one window open
3. both windows open

Frequency responses:


Decay (both closed):


Decay (both open):


Apparently a strong resonance below 10 Hz is introduced when the windows are opened. This resonance changes its frequency with the number of open windows. It is interesting to see that the frequency increases. The frequency doesn't match the distance to the neighbour house. So the reason must be something different.

My theory is that the whole room acts like a ported subwoofer with a very short port (the window). The increase in frequency of the resonance with two open windows supports that. A bassreflex enclosure has the same behaviour.

This means that an open window is definitely not the same as an equivalent-sized absorber in the same place.
post #155 of 203
Very interesting. 13db difference at 6hz that is essentially "free". Have you listened to any ULF movies with the windows open? The f'n Irene scene should sound/feel more visceral I would think.
post #156 of 203
FoLLgoTT

thank you for doing this
post #157 of 203
Quote:
Originally Posted by FoLLgoTT View Post

My theory is that the whole room acts like a ported subwoofer with a very short port (the window). The increase in frequency of the resonance with two open windows supports that. A bassreflex enclosure has the same behaviour.

^^^^^
THIS

Is great stuff. Postulation, Investigation with data to back it up. Thanks FoLLgott. This and the SBA/DBA thread are the threads I follow most closely.

JSS
post #158 of 203
Yeah, I think you are quite right with your theory, -that the room can be seen as a ported speaker where the open windows act as ports. Can you make a quick calculation versus the rooms approximate volume of air and the area + wall thickness at the windows? Probably that tuning corresponds with the peaks at the low frequencies. Good thing the resonance with open windows were not at 40 Hz or so, -your neighbours would not have been happy about that. For the very same reason it is a good idea to tune a HVAC to a frequency a lot below what your spekaers + room can produce ( => large ducts + lengths + low air flow velocity in duct), no sound will escape as the air in the port acts as a "plug" for frequencies substantially above tuning frequency.

A guy I know actually decreased a problem mode inside his HT by making openings through the walls at the corners towards another rooom / closet which acted as a large Helmholtz resonator. (My opinion:With windows closed, you have a sealed speaker with some losses but there is also some room gain + some pressure gain (less than 12 dB for the ideal fully sealed room). Combined these more or less even out the natural roll off of your sealed speakers.Noah is right about the open windows being perfect absorbers, not for a 6 Hz wave length but for all those close to or smaller than the windows dimensions.

(I will start to try out the DBA idea in a month or so with 4 pcs PB18+ and 4 pcs SBP15 from Acoustic Elegance powered by Crown Machro-Tech 5000VZ. This thread started the idea, thanks FoLLgoTT!)
.
post #159 of 203
Quote:
Originally Posted by FoLLgoTT View Post

Here are a few more measurents for the actual discussion. I have two small windows in my cellar room.

Apparently a strong resonance below 10 Hz is introduced when the windows are opened. This resonance changes its frequency with the number of open windows. It is interesting to see that the frequency increases. The frequency doesn't match the distance to the neighbour house. So the reason must be something different.

My theory is that the whole room acts like a ported subwoofer with a very short port (the window). The increase in frequency of the resonance with two open windows supports that. A bassreflex enclosure has the same behaviour.

Although not nearly as dramatic as your results, mine are the same.

Have you noticed a change in behavior of the drivers from windows closed to windows open?

Have you measured outside the window?

How does the inside of a ported enclosure measure vs outside? (I have no ported subs to measure).
post #160 of 203
Thread Starter 
Hi Dave,
you ask the right questions. I hope I can answer them. smile.gif

I simulated a 4th order bandpass in WinISD using the volume of my room, the 18 speakers in 65 l each and the measurend resonance frequencies.

Frequency response (outside the window):

Simulation:


Measurement:



Cone excursion:

Simulation:


Laser measurement:



I think the measurements confirm the theory that the room acts like a 4th order bandpass. The real port resonance is just not es narrow as the simulated ones. But this may be explained by the fact that the windows have a varying diameter (the frame is smaller than the cutout in the wall). smile.gif
post #161 of 203
Thanks Nils. Superb data.

Does this mean that a bandpass sub has the same FR on both sides of the port?
post #162 of 203
Nils, most excellent.

I agree that the windows turn the room into a Helmholz resonator, which a bass trap on the wall does not do.

The former adds a resonance effect with little damping, while the latter is pure damping.

So I modify my statement to say that the window acts like an absorber below the operating range of the port resonance, as seen by the reduction in SPL at the lowest freq.

I'm at a loss to explain why there isn't a reduction in SPL at higher freq.

Did you take measurements at more than one location, and if so what do they look like?
post #163 of 203
Reason for ”no SPL reduction of higher frequencies”, -my guess is that it is the other way around. Close to 6 Hz there is reinforcement, “far” from 6 Hz there is not reduction, rather there is less reinforcement. The room with open windows acts as a poorly damped Helmholtz (FoLLgoTT apparently gobbled up all pizza slices and cheese doodles, no absorbing leftovers laying around). => So a high Q device with a narrow bandwidth and no reinforcing resonance for frequencies “far” from center tuning, on both sides of 6 Hz, when it is played.

(The windows seem to be around 3 x 1 feet each, in my opinion they cannot work as absorbers for 6 Hz and lower, wave lengths are more than 57 m / 188 feet.)
post #164 of 203
"The real port resonance is just not as narrow as the simulated ones. But this may be explained by the fact that the windows have a varying diameter (the frame is smaller than the cutout in the wall)."

the absorber at the back of the room may be serving to lower the "q" of the front chamber. advanced tab, ql.

we discussed this "room as a large ported sub theory" a couple of years ago. there are a few other measurements floating around that confirm the theory.

while opening a window may be impractical, porting the room to another room or to the rest of the house may provide some significant 'free bass'.
post #165 of 203
Quote:
Originally Posted by Adhoc1 View Post

(The windows seem to be around 3 x 1 feet each, in my opinion they cannot work as absorbers for 6 Hz and lower, wave lengths are more than 57 m / 188 feet.)

It's not the L/W dimensions but the depth dimension that matters.

Actually that's for an actual resistive absorber; a window is simply a "black hole" absorber where energy exits but does not return.

Although I guess no simplification is safe at these frequencies/wavelengths, so I guess I better add the caveat that there be no other buildings or landscape features of significant size within a significant distance.
post #166 of 203
Thread Starter 
Quote:
Originally Posted by bossobass View Post

Does this mean that a bandpass sub has the same FR on both sides of the port?

It seems that the peak of the port is the same on both sides. But the frequencies above that are diminished outside.
Quote:
Originally Posted by noah katz View Post

I'm at a loss to explain why there isn't a reduction in SPL at higher freq.

A ported subwoofer has some "high frequency dirt" directly in front of the port, too. Maybe it looks different when the microphone is placed in a greater distance to the window.
Quote:
Originally Posted by LTD02 View Post

the absorber at the back of the room may be serving to lower the "q" of the front chamber. advanced tab, ql.

You are absolutely right! I forgot the big absorber in the equation. The Q factor must be smaller. smile.gif
post #167 of 203
How much space did you leave between the rear wall and the rockwool?
post #168 of 203
Thread Starter 
Quote:
Originally Posted by mikela View Post

How much space did you leave between the rear wall and the rockwool?

10 cm. And there is a plastic film around the rockwool.
post #169 of 203
Hi!

Does the angle on the baffle wall mean that you should delay the outer bass drivers in order to have a planar wave? Or doesn't it matter that much for the lower frequenzies?
Edited by jeno - 2/16/14 at 3:22am
post #170 of 203
post #171 of 203
Hi Follgott,

4th hit for a google.de search for "Peerless XXLS12" is... guess what...

Cheers
rumpeli

P.S. Abo
post #172 of 203
Quote:
Originally Posted by jeno View Post

This is a cool implementation of the SBA.
http://lambda-labs.com/en/news.html?pagenum=1#/references/wallofbass

Looks like The Grateful Dead's wall of speakers!
post #173 of 203
Thread Starter 
Some time has passed and meanwhile my daughter was born. smile.gif
As a father I now have many other things to do. But there is still some progress in my home theater. Here are the updates:

I installed the rear speakers. These are the Behringer B2031A. I wanted to upgrade, but at the moment there are other things I need the money for. And since they work pretty good, I still use them.



Then I built absorbers for the first reflection of the side walls. The absorbers are 20 cm thick and filled with rockwool. They still absorb noticeably down to 100 Hz. smile.gif





Because of the door on the left side I needed a feew tricks. I wanted a movable absorber which is gapless when the door is closed. So one part is coupled with the door and moves a second part on the wall. The second part is mounted on a rail. This works pretty like a charm! cool.gif





The next thing was to glue the skyline diffusers on the ceiling. They are placed to scatter the first reflection of all three speakers for the three front seats.




Here are the ETC measurements. As you can see the absorbers and diffusers works as expected.




RT60 looks pretty good at the low end, but too high at the top end (250 ms is desired). This is expected, because the room is still without furniture and large areas are untreated. This will change when the couch and chairs are installed.
This RT60 is a good base to start from. It is always easier to absorb the highs then the lows. smile.gif



And finally I installed the screen with a Center Stage XD! cool.gif
  • 4 m (158") wide
  • curved
  • acoustically transparent
  • Cinemascope




The size is truly impressive!

Now everything looks pretty raw. Later when the side walls are covered with fabric no speakers and no devices will be visible. The pipes above the screen will be covered by a small curtain. Masking will be done by a motorized curtain. It will move automatically when a movie with 2,35:1 gets started. Since I'm a software developer I do the programming myself. I've written a MovieManager which uses meta data on every movie to control the masking. This worked very well in my old home theater room. smile.gif
Edited by FoLLgoTT - 3/16/14 at 2:42am
post #174 of 203
Wow! That is mighty impressive! Can you elaborate a little more with regards to how you are doing the side walls? Are they going to have floor to ceiling absorbing panels on each wall, them cover the entire wall with fabric? Are those absorbers mounted I between the studs? I am trying to get a visual picture of the finished walls, and how you plan to make them look with the absorbers and fabric.
post #175 of 203
Quote:

Originally Posted by FoLLgoTT View Post




And finally I installed the screen with a Center Stage XD! cool.gif

  • 4 m (158") wide
  • curved
  • acoustically transparent
  • Cinemascope




The size is truly impressive!

Now everything looks pretty raw. Later when the side walls are covered with fabric no speakers and no devices will be visible. The pipes above the screen will be covered by a small curtain. Masking will be done by a motorized curtain. It will move automatically when a movie with 2,35:1 gets started. Since I'm a software developer I do the programming myself. I've written a MovieManager which uses meta data on every movie to control the masking. This worked very well in my old home theater room. smile.gif

 

 

Wow!!! :eek:

 

You darn Germans are so cutting edge!  Great JOB!!!!


Edited by doublewing11 - 3/15/14 at 4:30pm
post #176 of 203
Looks like you are making great progress with the theater, especially considering the new family member...congratulations on both!
post #177 of 203
Quote:
Originally Posted by FoLLgoTT View Post

Here are the ETC measurements. As you can see the absorbers and diffusers works as expected.

Nice but why does the floor reflection move in time?
post #178 of 203
Folgott is doing a great job, but i do not get the point of having to quote a dozen pics and 3 graphs to say so:eek:
post #179 of 203
Quote:
Originally Posted by nethien View Post

Folgott is doing a great job, but i do not get the point of having to quote a dozen pics and 3 graphs to say so:eek:

 

I agree....................I'll go edit...............didn't realize all his photos show up in my comment.  :o

post #180 of 203
Thread Starter 
Thanks folks! smile.gif
Quote:
Originally Posted by Martycool007 View Post

Can you elaborate a little more with regards to how you are doing the side walls? Are they going to have floor to ceiling absorbing panels on each wall, them cover the entire wall with fabric? Are those absorbers mounted I between the studs? I am trying to get a visual picture of the finished walls, and how you plan to make them look with the absorbers and fabric.

I want to build a wooden frame in front of the side walls. You can see the beginning of this frame at the side surround speakers. On this frame I want to tense a black acoustical transparent fabric (maybe with magnets). This fabric is used to cover the front of loudspeakers. I measured it and it does only very slightly absorb the highs which is negligible.
Behind this fabric I want to put the necessary absorbers/diffusers. I don't have an exact plan for this treatment. I'll have to measure and place them iteretatively. This means I place them where they are needed. E.g. this is mostly in height of the ear. There must be left some reflective surfaces, because I don't want to overdamp the room. It shall sound "alive" and "open". RT60 shall not be too low, especially in the highs. Unfurnately many home theaters are overdamped in the highs.

When all this is done, you'll see only a black wall and some lamps on it. smile.gif
Quote:
Originally Posted by markus767 View Post

Nice but why does the floor reflection move in time?

I guess the microphone was not at the exact same position when I took the second measurement. The height was not the same which caused the reflection to shift slightly to the left.
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