Martin Poulin
April 2021:
Below is the house I just acquired. It is situated in Varennes, Québec. The move-in date is scheduled for June 2021.
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The main basement room is 37' long by 20' wide with 7.5' ceiling (suspended tile height). I plan on installing an ambitious sound system there, and will probably lay in a cinder block wall, to serve as a mounting point for an infinite baffle bass system, with at least eight 18'' subwoofers.
I won't make the same mistake I did in my previous house, by using an existing closet volume for such an install. In that previous set up, the closet framing wasn't sturdy enough to serve as an enclosure for the four 18" subwoofer I had. It resulted in the entire house vibrating as vibration transmitted very easely.
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Now, with a 37' long listening room, I'll be able to implement and test the double bass array concept :)
I've ordered four more 12" subwoofer to complement the four I already have. I plan on 8x 12'' subwoofers for the cancellation end of the Double Bass Array.
Distance is your friend and the rear wall won't need all the SPL capacity of the front wall. I calculated at least 12db less output required from subwoofers of the rear wall. Hence 8x12" used instead of the 8x 18" in the front.
This should minimise the need for acoustic treatments, thus making the decor more acceptable.
Stay tuned for the build-up process...
Picture of the left isn't my work. I took it from a thread on AVSforums. It still provide a good idea of my intent.
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It shows a infinite baffle subwoofer installed in a Simple Bass Array setup. It also has a massive porous absorption wall in the rear ( 24'' thick with 12" airgap. 36'' effective thickness).
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My system will look like this but will have the active cancellation rear wall from 10 to 60 Hz instead of the absorption wall.
Of course, for music, Atmos speakers are useless and I will omit them.
August 2021
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Above, you can see a rough start of the room, picture was taken from the back of the room and the intented listening position will be close to where the microphone stand is. Such listening position would be around the first tier of the room lenghtwise.
first thing to go will be the suspended ceiling and I will recycle some of the tiles to use against the metal duck of the HVAC to prevent vibration. At least, attenuate the duck vibration.
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Carpet will be teared down and a solid floor will be installed at the very end of the build.
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All of the drywall will be removed and I plan on installing curtains all around the room to use the wall insulation as sound absorber too.
Rear wall (from the listening spot)
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This narrower end will (16.5 foot wide) be my rear wall.
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I plan on using a relatively deep porous absorber. Maybe 12" thick as I only aim to absorb down to about 60 Hz.
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Below 60Hz the double bass array shall do its magic and cancel my room mode.
September 2021
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-Removed most drywall
-removed ceiling tiles and grid system.
-blocked the 3 windows (with the recycled ceiling tiles I removed)
-recycled some ceiling tiles and screwed them directly onto the metallic duck of the HVAC to hopefully prevent vibration.
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-you can see the foamboard insulation that I partly taped with red vapor barrier tape to reduce humidity infiltration in the basement. I can't understand why it had not been done by the original builder, tape is cheap and it is quick to install. I did it to all the walls.
This wall is my left wall as seen from my listening position.
This is the only wall that isn't backed against the concrete foundation of the basement. It measures 25' long and I have to take few extra steps to ensure rigidity.
-(left picture), added recycled pieces of MDF and chipboard against outter drywall to increase mass of the wall.
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-(right picture), top part of the wall (originally left open) has been closed with various recycled wood pieces and remaining cavity around pipes and seems have been air sealed with expanding foam.
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-not shown, outter drywall (room behing the wall) have been double drywalled to further increase mass of the wall.
Started to fill the rear left wall with a single layer of R12 fiberglass
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From my listening position, this wall is on my rear left and measure about 12' long
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this wall is against the interior garage of the house and doesn't have the foam panel insulation installed. I didn't bother to add foamboard as I will heat my garage in the winter.
View from the rear of the room
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-R12 fiberglass had been added on 4/5 of the walls.
-Remaining 1/5 of the wall (barely seen on the left, partly hidden by the shelf) where I used Roxul Safe N Sound.
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-window pit haven been closed and filled with 2 layer of R12 fluffy fiberglass as windows are very detrimental to the sound of a room.
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-more on the window, I live nearby a busy road so blocking traffic noise from outside is a must.
From my listening position, on my left.
I have 3 doors to my left wall: they communicate with a bathroom, a staircase and a utility room
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Well, all of those 3 doors received the same 3.5" thick Roxul Safe N Sound Rockwool onto them.
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Rockwool is slightly supperior to R12 fluffy fiberglass for sound absorbtion and I used it only on this interior wall where thermal insulation quality isn't required.
Of course, those doors will be covered with fabric or curtain later.
Ceiling:
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-R12 fiberglass had been installed on all the gap between trusses.
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-without an effective airgap, those pieces of R12 fifiberglass acoustic absorbtion are limited but some is better than none.
Moreover, I wanted to retain relative ease of access to my pipes and electrical wires.
Ceiling:
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-(above pictures), you can see I've installed some Philip Newell style panel absorber. (he himself developped them with Tom Hidley)​
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-they are 24" deep as I don't have enough headroom clearance to make them deeper or the angle would be too shallow.
-they all point down at an angle of around 23 degrees. (not ideal but such is my situation)
-they are the whole width of the room (20')
-they are covering a little bit over the first third of the room lenghtwise (may add more later but my listening position is covered)
November 2021
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-A peak of what it would look like.
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-laminate flooring installed, look way better already.
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-Only the JBL 306P are connected for now.
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-2 broadband absorber added. On caster, they measure 6'tall by 6' wide by 16'' deep... they are effective down to about 100hz. I have 2 more in construction stage for the rear of the room.
December 2021
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-added more fiberflass in the Philips Newell style ceiling absorber, ceiling is now packed with fiberglass.
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-added a string with a nut suspended at the end:
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A) This serve to properly place the microphone between measurement of the system. I'm focusing on reverberation time and subwoofer frequency at this time but it is nice to have accurate measuring position for proper comparison.
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B) This work great too to know where to place your head while doing preliminary listening, I must admit, the sound is promising.
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-(bottom of the picture). Added three more shelf stuffed with fiberglass. This one measure 48'' wide by 15'' deep and about 74'' tall. The others two are elsewhere in the room.
Rear wall as it stand now.
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3 broadand absorbant panel on caster.
6' tall by 6' wide by 15'' deep. I have now 5 of those DIY absorber.
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More placement of the absorbtion panels and 12'' subwoofers to be tried before choosing their final resting.
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View from the back wall
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You can see I bought several carload worth of fiberglass :)
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More fiberglass to come. Only after I'm happy with the room reverberation time, I will then, and only then, start to add diffusion.
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Preliminary measurement are +-1.5dB from 13-55Hz (subwoofer only measurement).
Reverberation time within this band is still a tad too long for me to call it victory. More fiberglass needed :)
Better view of the front.
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Temporary computer monitor table to be changed very soon!
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You can see I have 2 more 6' x 6' by 15'' broadband absorber to deal with my side reflection, they are on caster and moving them is easy. They are 12'' away from the wall as having an airgap improve their efficiency.
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I always laught when I see people hanging like 12 pieces of 12''x12''x1'' deep foam in their living room and claim it did improve their sound... Absorbtion need to be substansial to provide real benefit.
Idea for a supporting method for the center channel horn.
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I'm still thinking of a way to hang all my 5 midrange horn and eventually my mid-high horn as well.
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This piece of 2'' x 4'' works very well and I think I'll replicate the idea for my two JMLC left and right horn.
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Beauty of it is that it is still easy to move or tilt the horn at will. Since my room isn't finalized yet, adjustment capacity is still very important.
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The little piece of wood above the Krell TAS amplifier only serve as support for the supertweeter and the mid-high horn. The 340Hz tractrix horn doesn't touch it. I may use a supporting mean from the same 2x4'' piece or simply use a slab of granit to replace it, not sure yet.
Built a new desk for my monitor:
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-I wanted to absorb my floor reflection as much as possible to increase focus and clarity.
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-it measure about 65'' wide by 24'' deep and about 24'' tall. Shall have built it 70'' wide but too late now.
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-I did a 10'' deep cavity in the middle for extending my legs.
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-like my Ikea chair, it is on caster for ease of moving.
My rear channels are finally working:
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-only 2 ways ATM as I await more parts for the dedicated 1'' compression driver.
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-I used my old Precision Device 15'' woofer in a sealed cabinet (55Hz - 800Hz)
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-Treble is a Radian 760Neo PB mounted on a 340 Hertz Tractrix horn. (800Hz 20kHz)
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-Stand are a set of old chair I had, I cut the backrest to obtain a solid platform as those 15'' cabinet weight about 110 pounds.
Per above pic, the rear channels are surrounded by fiberglass.
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-I've wanted to have the rearmost fiberglass shelving installed lengthwise but needed them to be sitting at an angle to keep easy access to my bathroom.
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-Listening test proved this angle to not change much at all.
Here come the ugly.
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-My RT60 measurement always showed anomaly above 2kHz mostly due to diffraction of the 3 horns mouth and the bass enclosures.
-To a lesser extend, the horns interacted with the 4 subwoofers towers too.
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-I had to add fiberglass nearby the horns to correct this. It does look really bad but it is a very effective solution.
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-I still have to find a way for the Beyma CP22 supertweeter as the amplifier sitting on top of them reflect energy as well, I angled the amps backward but it only help so much. More thinking to be done for thoses.
My room does start to feel smaller with all of those shelf of fiberglass filling the listening space.
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Benefit:
-I do get a RT60 of about 220mS in my 5000 cubic foot room. (see pic below for EBU/ITU RT60 recommendation and next pic for RT60 variation relative to room size)
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-I get super clean bass extension down to 10Hz. (Diagonal mode room limitation is at a calculated frequency of 13Hz). Below 10Hz, my SPL drop like a rock and it is pointless to try to reproduce it unless I get a bigger room.
-I currently highpass my subwoofers at about 14Hz as pipe organ usually reach 16Hz anyway and I have a big room mode at 13Hz. I prefer to EQ the 13Hz mode out as my fiberblass does very little to tame it.
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-I measured a 20Hz sinewave at my listening positon, at a SPL of 90 Decibels and obtained a reading of 1.07% THD yesterday. This, you can feel it as well as hear it.
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A friend came to visit me the other day and wanted to listen to my room despite the room not being close to be completed. He noticed the feeling of 'weird silence' once he was in the basement. He told me that it made him feel inconfortable. My background noise is about 28-32dBA on weekeends, not a level most are used to live with. The feeling of 'weird silence', I first had that feeling too when I started.
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Philip Newell is my inspiration for an ''as absorptive room as possible''.
Every HIFI enthusiasm shall read this: https://www.routledge.com/Recording-Studio-Design/Newell/p/book/9781138936072
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Before building my room, I paid a session and went to a studio built following the approach I was about to take: https://www.lelabmastering.com/fr/studio
The Lab Mastering studio, built by Tom Hidley follow the same principle that Phillip Newell recommend and use.
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For the first time, I was exposed to the feeling of 'weird silence' such an absorbtive room immerse you in. The studio employee told me that after a few days or week, you feel accustomed to 'the silence' and it does become the new normal.
Like when you climb on a tall ladder, you feel scared at first but after some time, your brain get used to the feeling of height and you can start to act more normally.
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The studio employee told me that after you become accustomed to a silent room, it become impossible to listen to a 'noisy' and reverberant 'normal room'. He was so right, I deeply agree with him, you just have to train your brain.
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The sound is so clean, exempt from the energy radiated from the room. Reverberant room sound on the bright side and almost artificial in comparaison. Opera or choir are an easy way to hear this. Shall I have no rear channel installed, I might consider a bit of diffraction in the rear to create an feeling of surrounding but with 2 high quality rear channels, I get the feeling of immersion recreated quite convincingly.
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I should add that my 2 rear surround channels are not playing as loud as my front channels despite Dolby and EBU recommending to match the volume of all speakers. I think I'm currently at around 8 decibels softer while using calibration pink noise. (subject to change, I'm not settled on the final level yet)
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I also use a gentle 6 dB per octave high frequency rolloff starting at 2kHz. This curve is very similar to the Harman curve except that I don't apply the 10 Decibels low frequency boost that would normally start at about 70Hz. I really dislike a pronounced LF boost like the Harman curve suggest.
For reference, here is the Harman target curve.
More research by Toole:
Some subjective preference for various correction curve and type of listener.
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This suggest that a 2 or 3 Decibels LF boost is prefered by ''trained listener'' and it is about what I prefer as well. 2 Decibels boost being about my limit. ''Untrained listener'' prefer a large LF boost.
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My front channels have a slight donward tilt above 2kHz too but I tilt less than 6dB, I'm about at 3dB if I remember right.
Background noise in my room measured on a weekend (less road traffic):
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52dB 'Z' weighting
42 dB 'C' weighting
32 dB 'A' weighting
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I live nearby a busy road but the weekends are a bliss. Not sure how good or bad it is but this is the noise curve I consistently get.
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More info on noise here
My pictures may not be exactly show how I've placed all my absorbtion.
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Here a rough Sketch view I drew today
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A picture worth a thousand word they say.
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Approximate scale, I've rounded most number, just to get a general idea.
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Notice that except for the 3 big absorbers on my rear wall, none of my absorbers are directly against the walls, I use airgap to increase the absorber efficiency.
Jan 2022,
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Finally, I feel ready to share a few measurements.
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20Hz and 30 Hz sinewave played at 100dB, my microphone is placed at the listening position. All 16 subwoofers are playing in mono, the DBA isn't working as well as intended and I prefer all subs playing in mono for now.
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I obtain the following results:
0.16% THD at 20Hz
0.13% THD at 30Hz
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This is nothing sort of incredible, I have less distortion in the deep bass than what my midranges provide me!
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A big room full of fiberglass as per above schematic does really work. I'm very satisfied with such result, much better than expected. Subjective sound is powerful, yet immensely clean, reverbs added by the record engineer on some albums are so easy to distinguish.
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This room is like a giant microscope to hear deep into an album.
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My frequency response from 20-100 Hertz is +-2dB at the listening position. Below 20 Hertz, I don't EQ much and keep the level slightly lower to prevent distortion, note that I use a steep crossover at 10Hz.
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You can see a bump at 12 Hertz, this is my very strong diagonal room mode.
Waterfall graph is fairly clean if you forget my 12 Hertz diagonal room mode.
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Below 30hertz, my fiberglass isn't very efficient and I would need some diaphragmic absorbers or few Helmholtz resonator to damp the excess energy.
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At such low frequency, I doubt that it is audible but I really can't say as I never tried to treat this low.
Waterfall graph is fairly clean if you forget my 12hertz diagonal room mode.
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My quest for a very low and uniform RT60 time is quite a success.
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Lower than 100Hz, I need some more absorbtion to lower the RT60 some more but according to research, at less than 0.4 second, I'm well below the maximum ITU specification within the audioband.
April 2022
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Finally in true 7.1 with 2 actives 3 ways speaker on each side.
Added 8 more big broadband absorber (at a cost of roughly $700CAD) in the rear of the room.
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I now cut my subwoofer at 14Hertz, going lower isn't beneficial and to tame my 12Hz mode, I would need diaphragmic absorber.
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Enabling the 7.1 over the 5.1 was a significant step forward in sense of immersion. Soundstage is much better.
Side surrounds
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3 ways active.
bass - 12'' Precision device woofer in a sealed cabinet. All double thickness 0.75'' plywood (50-550 Hz)
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Midrange - Radian 760NeoPB mounted on a 18'' wide Ebay horn ( 550 Hz -8 kHz)
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Supertweeter- B&C DE-35 (8 kHz and up)
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With hindsight, I shall have gone with 15'' woofers instead of 12''. I did pick 12'' woofers to keep the box smaller and save weight on my absorber shelf but reinforcing it to carry an extra 20 pounds would have been easy. I may rebuild a new cabinet later if a pair of nice 15'' come up at deep discount. But I must say that the 12'' are fine for the job and are far from running out of steam.
the 2 above pictures are the new way I've mounted my center channel horns.
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It turned out that getting proper soundstage with voice right in the center was impossible without having the horns mounted in a vertical fashion in order to maintain equidistant measurement between each center horn to the Right and Left channels.
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Doing a small bamboo shelf and mounting the midrange and super tweeter horns in front of my bass amplifier was the best solution.
It took me few minutes to recalibrate the required time delays and voila, it worked.
Rear surrounds
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3 ways active.
bass - 15'' Precision device woofer in a sealed cabinet. All double thickness 0.75'' baltic birch plywood (50-500 Hz)
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Midrange - BMS 4596ND on a 350Hz tractrix horn (500-2 kHz)
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high- Celestion CDX1-1425 mounted on the FaitalPRO STH100 horn (2kHz and up)
January 2023
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More fiberglass added as per new approximate sketch:
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-rear wall is more absorptive than ever.
-ceiling got few more layer in the first reflection point.
-some more fiberglass sheets added on the ground here and there. (not seen on sketch)
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Frequency response flatness improved a tad.
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Note that my DSP setting changed somewhat since the last published data I've posted, I now use less correction than before for flattish FR curve.
I need to re-EQ my bass drivers, the dip at 58 Hertz is probably my 15'' drivers that are now not exactly time aligned as I only readjusted the DSP on the subs. I expect this dip to be gone once I spend the time to re-align everything.