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Single Bass Array with massive damping (measurements inside)
Old 21st July 2013
  #1
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Single Bass Array with massive absorption (measurements inside)

I did an experiment for my new home theater in an completely untreated rectangular room. I placed 2 subwoofers on the front wall in SBA configuration. On the rear wall I build up a wall of 15 blocks of Rockwool Sonorock (6 kPa*s/m²). The porous absorber wall has a depth of 55 cm. The low flow resistance is very suitable for this thickness.
Since the driver grid is not very dense the plane wave is not formed well over about 50 Hz and the modes between the side walls become visible. Later I will install 18 drivers so the upper cutoff frequency of the plane wave will be much greater than the used bandwidth (at about 200 Hz). So let's concentrate only on the modes between the front and back wall. They are at 27 Hz and 56 Hz.

Room size: 6 x 4.8 x 2.2 m (LxWxH)

I measured at different distances to the front wall. The distances are 2m, 3m and 4m.
As you can see the first mode at 27 Hz is damped well, but still visible. The second mode at 56 Hz is completely damped.





Frequency responses in untreated room:


Frequency responses with 55 cm Sonorock at the back wall:


2m:


3m:


4m:
Attached Thumbnails
Single Bass Array with massive damping (measurements inside)-aufbau-ruckwand.jpg   Single Bass Array with massive damping (measurements inside)-abklingspektrum-2m.png   Single Bass Array with massive damping (measurements inside)-abklingspektrum-3m.png   Single Bass Array with massive damping (measurements inside)-abklingspektrum-4m.png   Single Bass Array with massive damping (measurements inside)-amplitudengang-sonorock.png  

Single Bass Array with massive damping (measurements inside)-amplitudengang-unbedampft.png   Single Bass Array with massive damping (measurements inside)-_mg_9412_klein.jpg  
Old 21st July 2013
  #2
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This may may turn out interesting. ( Even though I am thinking about trying out the Double Bass Array you had a thread about on AVS Forum: Double Bass Array (DBA) - The modern bass concept! )
Old 22nd July 2013
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Very nice results for the room center location.
Old 22nd July 2013
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Quote:
Originally Posted by Adhoc View Post
This may may turn out interesting. ( Even though I am thinking about trying out the Double Bass Array you had a thread about on AVS Forum: Double Bass Array (DBA) - The modern bass concept! )
A working DBA is still better in terms of linearity and decay. I had to put 1m porous absorber with a flow resistivity of 3 kPa*s/m² to damp the 27 Hz mode. Hemp would be such a material. But it is expensive and "wasting" 4,8m² of space really hurts.

The DBA has the advantage of beeing very slim. Only 2 x 15-20 cm depth are required. The big disadvantage are the high costs. Drivers and amplifiers are doubled only for a linear response and minimal decay. Note that the DBA's gain usually must be about 5 dB higher than the other channels to get the same subjective loudness.

On the other hand the SBA is cheaper and for the same price the array can be twice as dense. With a fixed amount of money the maximum SPL rises by +6 dB. Additionally the remaining of the first mode "amplifies" subjective loudness. For home theater use this is no disadvantage if the first mode is deep enough. Furthermore the main speakers benefit from the absorbing rear wall.
The next experiment may be a wall full of VPR. This should reduce the overall depth of the absorbers. But I don't know if a VPR really works as well at 27 Hz as porous absorbers...

For music only I would go with the DBA. If wasting space is no problem and a high maximum SPL is required, I would build a damped SBA.
Old 23rd July 2013
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Thanks for the feedback! I already have several amps so no extra money needed there. It would be for a music room / HT, 32 m² / 72 m³, treated concrete bunker were lowest length mode is 22 Hz. There is a group buy in the works for 15” woofers with the following parameters (still pending a bit):
Fs 22 Hz
Re 6,4 ohm
Qms 5,83
Qes 0,30
Sd 754 cm²
X max 19 mm
Le 0,76 mH
The still very "loose" plan is 4 pcs 15" ported boxes at front, and 4 pcs at the back as time delayed closed boxes with Linkwitz transform. Where the ported ones start to loose breath around / before lowest mode, the closed boxes will take over for infra besides evening out modes. There should be quite a bit of room gain in the pressure zone below 22 Hz as the room is well sealed. Time will show... Back tp topic, sorry.
Old 28th July 2013
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Right now I wonder if a back wall full of VPR's would perform equally. Looking at the absorption coefficents from Fraunhofer it should, but most measurements of VPR's here at Gearslutz don't look that good under 50 Hz. On the other hand I never saw a SBA with VPR's at the back wall. Maybe I just have to try it out.

Here is another visualization of the porous absorber.

SBA untreated:


SBA with 55cm rockwool:


Double Bass Array (DBA):
Attached Thumbnails
Single Bass Array with massive damping (measurements inside)-spektrogramm-4m-unbedampft.png   Single Bass Array with massive damping (measurements inside)-spektrogramm-4m-sonorock.png   Single Bass Array with massive damping (measurements inside)-spektrogramm-dba.png  
Old 11th September 2013
  #7
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Thank you for these very helpful graphs FoLLgoTT

I have a friend who is thinking about putting together a small dedicated home theater room. I'm trying to help him out with the most affordable way in getting somewhat even frequency responce and decay time. I've gotten interested in a single bass array with damped back wall for a basis for room treatment.

The idea I have currently for the front wall is to put L, C, R and 4 15" subs flush in a sealed brick wall. No boxes for the subs. The subs would be situated 1/4 and 3/4 positions horizontally and vertically. Behringer dcx2496 would be available for EQ with some kind of a AVR.

Room is 2,5m (height), 4,1m (wide) and 5,2m (lenght) after the front wall with the speakers has been built. Listening distance is about 3,1m for the main listening position. No need to worry about other positions responses.

In my small understanding of SBA he should put massive dampening to the back of the room if he wants to get even response for the region under 100Hz. If he mimics FoLLgoTT:s idea of using rockwool, then how much is enough? Will 0,7m, 0,8m or 0,9m work?.

And over all.
What does this plan sound like?
Problems, things to remember?

There would be Treatment for other frequencies as well. I think that they could be made after the building of front wall and back treatment. Wecould then measure the behavior of new treatments and reject treatment that made the plane wave (low frequensies) work more badly.
Old 11th September 2013
  #8
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Quote:
Originally Posted by Aki H. View Post
If he mimics FoLLgoTT:s idea of using rockwool, then how much is enough? Will 0,7m, 0,8m or 0,9m work?.
Running your suggested depths (70cm vs. 90 cm) through a calculator, the benefit from more depth seems to be rather minimal. Porous Absorber Calculator

BTW, if you're from Finland: I'd go with Knauf rolls of insulation from Byggmax, which amazingly is the cheapest option and among the best (if not THE best) performer for thick bass/broadband traps.
Old 11th September 2013
  #9
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Yeah from Finland... Moro vaan!

Is this it? Can he use these in their plastic coverings or should these be taken off?

What should I put to Flow resistivity in the calculator?
Old 11th September 2013
  #10
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Moro! That's the wool. One will need to open the packs, the roll is tightly packed and you want the fibers to extend to their loose state for best possible low frequency absorbtion. (And to profit from the actual volume of the material). With that much depth (70cm), I'd build a wooden frame for the trap. Remember to support the wool, say every 50 cms of height, to prevent it from sagging. The insulation from the roll is some 58 cms wide... I would also consider the option of making the depth of the trap equal to the width of the insulation to minimize the need for cutting.

I think the estimated gas flow resistivity for this type of wool is around 4000.
Old 12th September 2013
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Quote:
Originally Posted by Aki H. View Post
If he mimics FoLLgoTT:s idea of using rockwool, then how much is enough? Will 0,7m, 0,8m or 0,9m work?.
I used only 55 cm for my test. When I moved the absorbers 40 cm away from the back wall the first length mode was completely gone. I think 70 cm would be fine.

Since a SBA has the same frequency response for all seats in a row it can be easily equalized for one complete seat row. I would choose the first row where the host sits.

Quote:
There would be Treatment for other frequencies as well. I think that they could be made after the building of front wall and back treatment.
Yes, I'll do this like that. I'll place absorbers for frequencies >100 Hz on the side walls and diffusers of course. The absorbers have to be no thicker than 20 cm to absorb the frequencies >100 Hz.

Btw. my front will be a complete baffle wall behind a cuved screen. I can post mode pictures when I start building the room.

Attached Thumbnails
Single Bass Array with massive damping (measurements inside)-hk-planung-2.jpg  
Old 12th September 2013
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Quote:
Originally Posted by FoLLgoTT View Post
I used only 55 cm for my test. When I moved the absorbers 40 cm away from the back wall the first length mode was completely gone. I think 70 cm would be fine.
Ok. Then he could propably get away with 10cm air + 60cm treatment if he wants to save a few cents.

Quote:
Since a SBA has the same frequency response for all seats in a row it can be easily equalized for one complete seat row. I would choose the first row where the host sits.
That is a great benefit with these bass arrays.

He will have two rows (3+3) altough the one in the back is a tight fit. And yes the first row is the one being calibrated. The second row will be against the back wall basically so no one could hope to have it sound good.

Quote:
I'll place absorbers for frequencies >100 Hz on the side walls and diffusers of course. The absorbers have to be no thicker than 20 cm to absorb the frequencies >100 Hz.
My ideas are similar.

I´m still unsure about sidewall and ceiling absorbers though. Side walls are no problem and they will be built last. They can be tuned easily if necessary. Maybe they will be 20 cm or maybe less. With 20 cm absorbition they could perhaps effect a bit too low because sound aproaches these surfaces in angles.

And how about a lowered ceiling?
Would for example a 15 cm drop with absorption in the first reflection points leave the bass response unchanged (good)? Would a dropped ceiling with some dampening effect the height of sub placement in the front wall?

Of course these will be measured and fitted accordingly when the front and back walls are up but it would be good to have somekind of a plan about the ceiling before he can start building it.

Btw. I don´t have good room acoustics modeling software so planning is kind of a pot shot at the moment.

Quote:
Btw. my front will be a complete baffle wall behind a cuved screen. I can post mode pictures when I start building the room.
Baffle wall with subs and mains. Check.
Curved screen. Check.
Amount of subs...

Attached Thumbnails
Single Bass Array with massive damping (measurements inside)-sba01.jpg  
Old 12th September 2013
  #13
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Quote:
Originally Posted by Aki H. View Post
Ok. Then he could propably get away with 10cm air + 60cm treatment if he wants to save a few cents.
There is another advantage. I will leave a gap of 10 cm to avoid clampness on the outer wall. The gap will be ventilated electrically to another room.

Quote:
I´m still unsure about sidewall and ceiling absorbers though.
I'm unsure, too. I'm not able to predict the results. So I have to experiment with absorbers when the bass array is finished.

For the ceiling I have a lot of 2D diffusers. But a few absorbers may be needed. It will be a long way with experiments.

Quote:
Baffle wall with subs and mains. Check.
Curved screen. Check.
Amount of subs...
Your plan looks good and very similar to mine. What kind of main speakers are these?

Regarding the number of subwoofers: if the goal is reference SPL (115 dB) at a given frequency (e.g. 20 Hz) there are three parameters to consider:

1. How much gain has your room?
2. Basreflex or closed boxes?
3. How much displacement volume have your drivers (Sd * Xmax)?

The first point is very important, since it can save you costs. Here is the gain I measured in my room with the SBA in the first post. The measurements where taken with closed boxes. One nearfield measurent and three in-room at different distances to the front wall.





As you can see my room (stone walls, conrete ceiling/floor) has a nice gain under the first length mode (27 Hz). At my listening position I'll still get reference SPL at 5Hz with the planned 18 x Peerless XXLS12. For 20 Hz less drivers are enough, of course.

I suggest you to measure the gain of your friend's room. E.g. a typical scandinavian framehouse may not show that much gain.
Attached Thumbnails
Single Bass Array with massive damping (measurements inside)-roomgain-differenz-alle-eng-.png   Single Bass Array with massive damping (measurements inside)-roomgain-eng-.png  
Old 12th September 2013
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Thanks for that room gain diagram + the details about the room. -Looks encouraging for infra sound and subwoofer investment, as I have a concrete room myself but 7,9 m long instead of about 6,4 m.
Old 15th September 2013
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Quote:
What kind of main speakers are these?
He doesn´t have any yet. I´m a big fan of Genelec speakers and when I found Genelec 1038B´s in the SketchUp warehouse I just had to put them to the sketch.

Quote:
Regarding the number of subwoofers: if the goal is reference SPL (115 dB) at a given frequency (e.g. 20 Hz) there are three parameters to consider:

1. How much gain has your room?

The first point is very important, since it can save you costs.
I am still just doing innitial planning at this stage and I´m not even 100% sure that this theater will be built. But the minute he decides to go on with the build I will do some measurements. But no matter what the room gain is, I´m pretty sure that 4 x 15" elements will be the max for him.

Quote:
2. Basreflex or closed boxes?
This is something I´m unsure of. I think it would be the most simple if there were no boxes. I was thinking about a heavy front wall. There would be something like 6000 litres of space behind the elements (actually more would be availeble if he could sacrifice a part of a closet) Could the elements just be placed airtight to the front wall with no boxes like in my sketch?
Attached Thumbnails
Single Bass Array with massive damping (measurements inside)-sba02.jpg  
Old 21st September 2013
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Quote:
Originally Posted by Aki H. View Post
This is something I´m unsure of. I think it would be the most simple if there were no boxes. I was thinking about a heavy front wall. There would be something like 6000 litres of space behind the elements (actually more would be availeble if he could sacrifice a part of a closet) Could the elements just be placed airtight to the front wall with no boxes like in my sketch?
Maybe this will work. But I'm concerned about the sability of this construction. The front should be mounted to the wall on many points to obtain a high stiffness and to avoid resonating.
Old 21st September 2013
  #17
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I did my final experiment and added a parametric equalizer to the SBA + absorber. Since room modes show a minimum phase behaviour an IIR filter should eliminate the decay.

Measurements were done at 4m distance to the front wall.

Frequency response flat signal + PEQ:


Waterfall flat signal:


Waterfall PEQ:


Spectrogram flat signal:


Spectrogram PEQ:



The results are excellent! SBA + 55 cm Absorber + PEQ looks equal to a full DBA.

The response is valid for a complete seat row. The second seat row shows a wide peak of about +7 dB, but there is no long decay either. The peak is no surprise and should be considered when planning such an array. With a high number of seat rows a thicker absorber or a DBA are the better choices.
Attached Thumbnails
Single Bass Array with massive damping (measurements inside)-amplitudengang-4m-ohne-equalizer.png   Single Bass Array with massive damping (measurements inside)-abklingspektrum-4m-ohne.png   Single Bass Array with massive damping (measurements inside)-abklingspektrum-4m-equalizer.png   Single Bass Array with massive damping (measurements inside)-spektrogramm-4m-ohne.png   Single Bass Array with massive damping (measurements inside)-spektrogramm-4m-equalizer.png  

Old 24th September 2013
  #18
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Quote:
Originally Posted by FoLLgoTT View Post
Maybe this will work. But I'm concerned about the sability of this construction. The front should be mounted to the wall on many points to obtain a high stiffness and to avoid resonating.
Very good point. We will have to plan for high stiffness if we decide to go this way. Also the room behind the speakers and sub elements would have to be made rattle free I guess.

Thank you for your additional measurements! SBA + 55 cm Absorber + PEQ really is a great setup on the right room.
Old 3rd February 2014
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My SBA is finished now. Here are the pictures and measurements.

The absorber is 55 cm thick with an air gap of 10 cm behind it. I used the same rockwool chunks that I used in my experiments.

-3 dB is at 3,3 Hz. I measured 130 dB @ 20 Hz and 115 dB @ 5 Hz. This is more than enough for home theater use!

More information about my SBA can be found here (only in german).
Attached Thumbnails
Single Bass Array with massive damping (measurements inside)-_mg_0085_klein.jpg   Single Bass Array with massive damping (measurements inside)-_mg_0093_klein.jpg   Single Bass Array with massive damping (measurements inside)-absorber-fertig-amplitudengang-.png   Single Bass Array with massive damping (measurements inside)-absorber-fertig-abklingspektrum-.png  
Old 4th February 2014
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Inspiring!

Is that the HKL-01 I see in the pics? Interesting speaker. I´ve had similar ideas myself (thinking of the horn loaded TB 3" dome with a Beyma 12” woofer as low mid driver) but never had the opportunity to try it.
Old 4th February 2014
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maybe a stupid question: will that wall not make the screen (canvas???) rattle and shake? i suppose the canvas would be in front of all those bass drivers...

amazing looking machinery. slutty.
Old 4th February 2014
  #22
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Quote:
Originally Posted by Jens Eklund View Post
Is that the HKL-01 I see in the pics? Interesting speaker. I´ve had similar ideas myself (thinking of the horn loaded TB 3" dome with a Beyma 12” woofer as low mid driver) but never had the opportunity to try it.
Yes, it is.

Using a 12" PA woofer for the range of 100 - 2500 Hz is absolutely stunning. With very low power it produces a very loud and clean sound. Even above reference SPL I never heard any distortion with this system.

Quote:
Originally Posted by korejekt View Post
maybe a stupid question: will that wall not make the screen (canvas???) rattle and shake?
The screen will be acoustical transparent (Center Stage XD). I used a similar configuration in my old home and there were no problems regarding rattling screen. In a few weeks I have built my new screen (4m wide curved). Then I can tell you more.
Old 4th February 2014
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Quote:
Originally Posted by FoLLgoTT View Post
Yes, it is.

Using a 12" PA woofer for the range of 100 - 2500 Hz is absolutely stunning. With very low power it produces a very loud and clean sound. Even above reference SPL I never heard any distortion with this system.
Sounds like a winner.

I actually found that paper a couple of months ago while searching for opinions on the Beyma 12G40 that looks like an amazing value for money. I plan on using it with the CP380M and the H290C waveguide, but I´ve previously used the TB 75-1558SE dome (with the Seas 27TDF) in another 3-way near field design with excellent result.
Old 4th February 2014
  #24
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Quote:
Originally Posted by Jens Eklund View Post
I actually found that paper a couple of months ago while searching for opinions on the Beyma 12G40 that looks like an amazing value for money.
Yes. But today I would take a look at the speakers from B&C. They are pretty cheap and extermely well designed. I have Klippel measurements of the 18TBW100. I've never seen such an symmetrical motor force and suspension!

Quote:
I plan on using it with the CP380M and the H290C waveguide, but I´ve previously used the TB 75-1558SE dome (with the Seas 27TDF) in another 3-way near field design with excellent result.
The 75-1558SE has very low non-linear distortion. In fact it has the lowest THD of the three drivers in my speakers. Considering that the mids are by far the most critical frequencies in terms of THD the Tang Band is an excellent choice for a 3-way design.
Old 4th February 2014
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Indeed. That´s why I choose it for my near field monitor: Room correction EQ - mono or stereo?
Old 4th February 2014
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Quote:
Originally Posted by Jens Eklund View Post
Indeed. That´s why I choose it for my near field monitor: Room correction EQ - mono or stereo?
Very nice monitor! Looks like the Klein + Hummel O 300.

I visited Neumann (formerly K+H) a few years ago. They design the loudspeaker near my home town (next to Sennheiser). They have a well-treated demo room with all their speakers. I could directly switch between the smallest O 110 and the biggest O 500 C + subwoofers. I was surprised how similar all their speakers sounded. Very neutral.
But the big O 500 C was on another level to the rest. Even at approximately 3 m distance they sounded excellend and high SPL were absolutely clean. The head developer told me that they needed >300 prototypes for the waveguide and many different drivers to get the final result. The O 500 C is one of the finest piece of engineering I know of. Sadly discontinued...
Old 4th February 2014
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I have a lot of respect for K&H and the O300 is one of my favorite monitors in that price range and class. It´s so typically German; all about science and no mumbo jumbo.

The O300 was definitely a source of inspiration for my monitor but I wanted something that could play louder and deeper but still sealed enclosure and not “gigantic” in size. Funny thing is I found the GS-3A monitor (very similar design to mine but using the ATC dome and a higher price tag n general) shortly after the final design of Greger was settled ...
Old 7th February 2014
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WOW that's a lot of bass.
Any idea of the max linear SPL?
Old 7th February 2014
  #29
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Quote:
Originally Posted by OpusOfTrolls View Post
Any idea of the max linear SPL?
Since I built closed boxes SPLmax is limited by Xmax below 50 Hz. I attached the result of the simulation and room gain measurements.

I measured with CEA bursts, too. I stopped when I thought the excursion is high enough. So actual SPLmax may be a bit higher, but not much. I didn't measure above 20 Hz, because I didn't want to damages my ears.

When watching movies and cranking up the volume it seems to never reach any limit. I watched the THX trailer "Amazing life" at +10 dB above reference SPL. Additionally the subwoofers were +7 dB hot resulting in +17 dB above reference SPL! It is amazing how much the clothes move and my spring mounted riser gets stimulated! And it always sounds absolutely clean without any "booming". But I would never watch a whole movie with this volume. It is just too much.
Attached Thumbnails
Single Bass Array with massive damping (measurements inside)-roomgain-differenz-alle.png   Single Bass Array with massive damping (measurements inside)-splmax.png  
Old 7th February 2014
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Quote:
Originally Posted by OpusOfTrolls View Post
WOW that's a lot of bass.
Any idea of the max linear SPL?
Define “max linear SPL”. How much distortion and compression is acceptable? Or are we talking about the absolute maximum SPL reached just before mechanical failure of drivers, regardless of the amount of distortion?

Also, some argue (and rightfully so), that for a perceived level of THD at low frequencies, due to how our hearing works regarding the Fletcher–Munson curve, you need extremely low measured THD at low frequencies to be perceived at equal level compared to higher frequencies. The question then becomes; what kind of subwoofer system do we really need to reproduce very low frequencies at fairly high SPL without audible distortion? For those how initially think that the work described in this thread is a complete overkill, might soon realize that it´s actually not, even if it looks bizarre (but beautiful!).
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