Sonolok Acoustic Panels for the Living Room Wall

Sonolok acoustic panels come in boxes of four. 

The TV in my living room hangs on a wall that's about 3.3m (11') high x 6.7m (22') wide. That wall has had TVs and other things hung on it by previous owners of my condo, all of whom did less than professional repairs to the drywall. It looks awful. It also happens that the opposite side of that wall is my neighbor's bedroom, and it happens that my stereo speakers are positioned along that wall on either side of the TV with their rear radiation bouncing off and going through the wall.

 

This is what I started with. Is it really that hard to patch a hole in the drywall? And whose idea was it to just paint over it?

I've have been thinking about what to do about it for a couple years. I wanted something that would look nice and not seem out of place in the room/building. I considered just repairing the wall and painting it, or hanging wall paper or a wall paper mural. 

On my recent semiannual trip to Costco I saw something called Sonolok acoustic panels on sale for $30 for a box of four panels (they go for $90 per box on the manufacturer's web site and $40 per box at Wayfair). I looked them up and watched some YouTube videos about installing them and decided this was the way to go. The panels will theoretically reduce sound transmission through the wall, though the manufacturer provides no specs to that effect, and will provide some absorption and diffusion of the rear radiation from my speakers, all while looking pretty good and fitting with the look of the room/building. The panels get good reviews of their acoustic properties on amateur audio forums.

Each Sonolok panel consists of a dense synthetic felt pad 600 mm square x 10 mm thick with 27 mm wide x 13mm thick walnut veneered (really?) MDF strips stapled to it. The strips are spaced 13 mm apart. The wood strips are offset on the felt so that two edges of each panel will neatly overlap adjacent panels. Each box of four panels comes with two right side and two left side end-pieces to neatly finish the look of the wall. There are also black drywall screws and a bunch of round, black, felt stickers to hide the heads of the screws. 

My building was converted to condos about 20 years ago and uses steel studs, spaced 24" apart, covered with 5/8" drywall on both sides. I checked building codes to see what the allowable lateral loading on the studs would be (you can't just start putting heavy stuff on the walls without regard to the loading capacity!). This wall, like all the walls in my condo, are non load bearing (the rafters, posts, and beams carry all the load). I found that acceptable loading on non load bearing light gauge steel studs used in my walls was 10lbs/sqft. The wall is approximately 12' high x 22' wide, so 264 sqft. The 5/8" dry wall on both sides of the studs loads it to 4.4 lbs/sqft. The Sonolok panels add about 1.65 lbs/sqft, so the load on the studs will be a bit over 6 lbs/sqft, well within the allowable 10 lbs/sqft limit.  

Front side of one of a Sonolok panel. The felt backing is 600mm x 600mm (NOT 2'x2' as at least one Youtuber would have you believe). The wood slats are 587mm long x 27mm wide x 12mm thick, and are spaced 13mm apart. Don't worry, the dark area is the shadow of my head when I took the picture. The color of the wood slats is perfectly uniform.

The back of a Sonolok panel. The felt pad is 600 x 600 x about 10mm thick. The wood slats are stapled to the felt. Note: at least one YouTube reviewers says the panels are 2' square. NO they are NOT! 600mm is not 2'.

One of the panels with the right and left end pieces. 

When you put the end pieces in place, their slats are spaced the same as the others for a very neat look.

When the panels are installed, there will be neat, uniform horizontal gaps between the wood slats. This eliminates unsightly mismatches in the slat positions if the panels/slats aren't placed perfectly. One YouTube installer actually cut the felt at the top of the panels off so the slats would contact each other. Then he suggested hiding the resulting mismatches by using wood putty and sanding. That's insane!

The recommended mounting method is to put construction adhesive on the back sides of the panels then screw them to the wall to hold them while the glue sets up. I had a different idea. I decided to mount the panels on furring strips that would hold them away from the wall, allowing things like network, power, and speaker cables to be hidden behind the panels.

This is the wall that will be covered with the Sonolok panels. The plan is to start the panels at the top of the baseboard and go up from there. There are a couple electrical outlets and a TV mount that will have to be accommodated. Note: the light spots are reflections of the sun from the disco mushroom sitting in the window.

At the top of the wall there are unevenly positioned rafters. The top edge of the Sonolok panels  get close but won't require any trimming around the rafters.

As usual, I started the project by making a CAD model of the wall and the Sonolok panels so I could figure out the layout before starting the installation. 

This is what the wall will look like when it's done. The green thing is the wood structural post that stands forward of the wall by about 200mm. This image doesn't show the TV wall mount that I'll have to make cutouts for. There are 5 vertical x 11 horizontal Sonolok panels here, with right and left end pieces. The wall is a a few mm wider than the panels and end-pieces so I'll have to finish the right side with a custom end pieces to hide the furring strips.

Tools

 

Besides the usual electric drill/screwdriver and a tape measure, I used a couple special tools for this job:

Laser "tape" measure. IRIC it was about $30 a few years ago when I bought it. I have checked it against a couple different tape measures and found it to be very accurate.

3 axis laser level I bought from Temu for about $40. It projects horizontal and vertical lines so it can be used to check the squareness of walls, the level of floors, and to follow studs in the wall once they have been located using a magnet. I verified it's accuracy by marking a level line (as reported the this level) on the wall then rotating the laser level 180 degrees. The line remained in the same position, so I could trust the accuracy of this unit.

A razor knife with a fresh blade cuts the felt very neatly and I used a pull-saw to cut furring strips and most of the Sonolok wood slats. 

I added additional staples to the Sonolok slats to ensure that every piece would have at least two staples holding it to the felt backing before I sawed through the slats. 

Special considerations

I was planning to use the baseboard as the guide for placing the panels, but the laser level revealed that the baseboard that follows the floor is neither flat nor level. 

The green laser line is aligned to the baseboard at about the middle of the wall...

Left end of the wall at floor level. 

Level at the right end of the wall. The floor at the left side of the wall is about 30mm higher than the floor at the right side of the wall.

The left end-piece and panel had to be cut to fit around the bricks in the upper left corner of the wall.

There are three outlet boxes that had to be allowed for and a wall-mount for the TV. 

Decisions, decisions...

The original plan was to leave the baseboard in place and follow its top edge with the bottom of the first row of Sonolok panels. That wouldn't work because, like the floor, the baseboard's top surface is neither flat nor level. This install was over a wood floor that occasionally gets vacuumed and wet-mopped, so I couldn't just remove the baseboard and run the panels down to the floor because the finish on the MDF slats would not hold up against the vacuum cleaner nozzle or a wet mop. The baseboard hides gaps between the drywall and the floor, and protects the soft drywall from things like vacuum cleaner nozzles and wet mops, so it needed to be left in place.

I settled on mounting the Sonolok panels on furring strips. The furring strips are thicker than the baseboard, so the Sonolok panels overlap the baseboard. I just installed the bottom edge of the Sonolok panels along a level line overlapping the baseboard a little. That allows the panels to be high enough to protect them from the vacuum cleaner and mop. I didn't have to trim the bottom edge of the panels to fit the unflat floor or the baseboard.

This is how the Sonolok panels, mounted on the furring strips, overhang the baseboard. The gap between the Sonolok panel and the baseboard, and a few horizontal gaps between the furring strips allows me to bring speaker cables out from behind the panels. 

Preparations

I put masking tape on the floor at the baseboard and the right corner of the wall and painted the baseboard and right edge of the wall black. I also painted the left edge of the wall black right up to the bricks. Then the power outlet plates and TV mount were removed and the areas around them painted black, as well as the left and right edges of the wall.

Black paint on baseboard, surrounding power outlet, and on the right edge of the wall.

Black paint around power and CATV outlet boxes, TV mount, and baseboard. Note the vertical lines marking locations of studs. I replaced the original white outlets and cover plates with black ones.

I used a magnet to locate the studs (it sticks where there are screw heads holding the drywall to the studs) and drew vertical lines on the wall over the studs, using the laser level as a guide. I added horizontal lines on the wall to indicate where to mount the furring strips. The laser level makes all this very easy to do.

Installation

The next step was to mount the furring strips. I used 1x2" (actual measure 0.75 x 1.5"- why do they do that?) and 1 x 4" (actual measure 0.75 x 3.5") strips that I screwed to the studs using drywall screws intended for use with thin gauge steel studs. I left some gaps between the ends of the furring strips to allow for vertical cable runs in strategic places (close to the speaker locations and power outlets). Blue painter's tape on the baseboard marks the locations of the gaps in the furring strips so speaker cables can be brought out from the wall.

Wherever the ends of the furring strips were going to be visible I painted them black.

This is the layout of the furring strips with some of the Sonolok panels shown so you can see how they attach. The Sonolok panels are screwed to the furring strips without any glue so they can be removed to run cables as needed. The left end-pieces are glued to the furring strips and the ones on the right are screwed down.

I determined where the left edge of the panels would be, and placed the left edge piece and 11 of the panels and a right side end piece along the wall to precisely locate the right edge of the installation about 30mm from the edge of the wall on the right side. 

With all the prep and painting, it took two days work to get the first two rows of panels installed. They were the worst part of the job because they were closest to the floor and required a lot of bending over and sitting on the floor to work. They also involved making cut-outs for the power outlets and TV mount and that took extra time.

Installing the panels. I left horizontal gaps between the furring strips to allow power and CATV cables to go to the TV behind the panels. I considered lowering the TV enough to cover the outlets but decided against it as I need access to the power outlet for vacuuming, etc.

The first two rows almost complete. The left end-pieces are glued and the right end-pieces are screwed to the furring strips.

I used the green 09laser level to ensure the panels were aligned properly before screwing them down on the furring strips. 

I installed the panels using seven screws each- 3 at the top and bottom edges and one in the center. 

Cutouts

The positions of the cutouts were determined as I installed the panels. I started on the bottom row, placing the panels from the left edge and working toward the right edge. When I encountered a power outlet, I laid out its location on the panels using blue painter's tape, then used a framer's square to draw the cutting lines on the tape (sorry, I didn't take any pictures). In places where cutting the slat would result in a piece of the slat being held by a single staple, I added staples to stabilize the positions of those slats (example- the short slats above the TV cutout) before cutting the slats.

At some cutout locations, and at the far right edge of the installation, when viewed from the side, the ends of the furring strips were visible. I painted those black before installing the panels.

Problems

When I bought the furring strips I selected straight, untwisted boards. By the time I got to putting up the third row of panels, the 1x2s that were sitting on the floor in my living room for two days warped so badly I couldn't use them. A couple of the 1x4s also warped to unusable condition. I was able to cut them into shorter pieces, leaving out the twisted parts, and got them to work OK, but next time (hah!) I'll invest in better quality wood. For this application, it would actually be better to use strips of MDF or even plastic because it won't warp. 

The top left side panel had to be trimmed to fit around the bricks up near the ceiling. The slats are held on the felt with staples, and are made of MDF, so it's impractical to try to trim the panel at the shallow angle that would be required to follow the outline of the bricks exactly. I painted 50mm or so of the wall black, up to the bricks, and cut the left end-piece and the panel squarely to fit. Then I marked and cut the two left-most slats and felt on the panel to lengths that would fit under the overhanging bricks. I think it came out pretty good. 

The upper left corner panel was cut to fit around the bricks. First I measured and cut the left end piece, the I measured from the wood on the lower panel up to the bricks following the edge of the wood on the lower panel and trimmed the two left-most slats and felt on the upper panel to fit.

I wasn't sure how I was going to close the edge of the panels at the right side of the wall. Then I looked at the many extra left end-pieces I had and found that the wood strips are just the right width to close off the opening created by the furring strips. I removed the wood slats from the felt and glued them to the right side end-pieces and it looks very professionally finished. If I ever need to run cables through the right side I can remove one of the right end-piece plus slat, cut it open enough to run cables and then put it back. Or just make a new one with some of the many extra end-pieces that were left over from this installation.

Detail of one of the right edge finishing pieces. I removed the slat from a left end-piece and hot melt glued it to the felt on a right end-piece using a simple jig to keep them aligned. The right end-piece with the left end-piece slat attached is just held in place with screws so I can remove it easily to run more cables if I ever need to. Note: I painted the wall and the end of the furring strip black, but didn't really need to.

Jig used to hot-melt glue the left end-piece slat to the right end-piece felt. I used spring clamps to hold the right end-piece against a board, taped off the slat, then applied glue to the felt. Masking tape prevented the glue from going to the wrong places. I set the slat (removed from its felt) from a left end-piece down on the glue.

Installing cables

I prepped a set of 12 gauge speaker cables (I bought a 250 ft spool of 12 gauge speaker cable via Craig's List for $15!) for this installation. I hung the cables using some thin plastic held in place with thumb tacks. The hangers are only there to keep the cables from getting in the way when installing the acoustic panels. I also ran a piece of black network cable to the left side of the wall where the WiFi router will be placed.

Cable hangers made from thin plastic strips tacked to the wall.

I bought a power-strip/surge protector with a flat, rotating plug so that I could route the power cable up the wall under the Sonolok panels. The end with the sockets will be installed on the wall behind the TV. I will use it to power the TV, Shield Pro, and Bluetooth transmitter.

Power outlet under the TV with flat plug that rotates so the cable can go straight up under the Sonolok panels. The other cables are HDMI and optical fiber that go from the TV to the amplifier.

 

Finally

 

It took about 4 days to hang all the furring strips, run the cables, and trim and mount the Sonolok panels. I used 14 boxes of the panels at $30 each, and spent $40 on the laser level, and another $50 or so on furring strips and screws, and maybe $15 for replacement power outlets and face plates, for a total cost of about $500 to cover this 3m x 6.6m (~10'x22') wall.

I don't yet know if the Sonolok panels reduce the sound transmission through the wall a meaningful amount. They will certainly help at least a little. I'll have to ask my neighbor if he notices any difference.

The newly paneled wall is on the west side of the room. The east side wall is mostly very large windows, with heavy velvet curtains that can open and close, and is very reflective for sound with the curtains open and very dead when the curtains are closed. With the curtains open, the stereo image from my speakers seems to be a bit more sharply focused than before the wall was paneled, but that may be because I'm paying more attention to it. With the curtains closed the sound seems less satisfactory, as if the room is just too "dead". 

Living room with Sonolok paneled wall. Windows on the left are the east side of the room, Sonolok wall is on the west side.

This is what it looks like with the curtains open during the day.

 

 

An improved diaphragm stretcher design for Quad ESL-63 speakers

I use a diaphragm tensioning jig to set the resonance of the drivers to match the factory value of about 86 Hz. Even though the drivers play below their resonance in the speakers, there's no hump in the frequency response at resonance. So why does the resonance matter? Setting the resonance to match the factory value does two things. First, it ensures that there is sufficient tension on the diaphragm so that when the HV bias is applied, the diaphragm won't pull to one side and stick to one of the stators. Second, it ensures the sensitivity of the drivers will be uniform if the resonance of the drivers is uniform.

There are companies that sell kits to replace diaphragms on ESL-63s. They recommend tensioning techniques that are far from ideal. Most recommend taping the film down on a flat surface, pulling it tight as you go around the film. Some even provide a spring scale and recommend pulling the film to a specific value on the scale before taping it down. That's the sort of thing hobbyists were doing in the 80s, and isn't likely to result in matching driver resonances.

I rebuilt my ESL-63s using a diaphragm stretcher made from MDF and wood, that allowed the resonance to be set to a specific value. It worked well, but after multiple uses, adhesive from the tape used to secure the film built up on the wood and the tape started letting go before I could glue the diaphragm to the stator grid. I found it very difficult to clean the adhesive off the wood. Also, the opening in the center didn't match the size of the driver, so the resonance measured on the stretcher was different from the resonance measured once the diaphragm was glued to the driver. I'm rebuilding more drivers (going to turn my 63s in 989s) so I decided to try to make a better stretcher, one that would, I hope, provide the same resonance on the stretcher and driver, and wouldn't have the same problem with tape adhesive.

I found some promising aluminum extrusion in my materials pile collection. I tried sticking some of the double sided tape used to hold the film on the stretcher to the aluminum and pulling it off, multiple times and found it didn't leave residue behind like it did on the wood stretcher. The next thing to do was model the aluminum in CAD.

The aluminum I used has this profile. It's 56.8 mm high x 42.5 mm wide. Those wide, flat areas on the top, bottom, and sides are very useful for this application. I have no idea who makes this particular stuff, and neither does Google Lens. Square or rectangular aluminum tubing would work as well and would be it would be easier to design and print corner pieces.

I measured the drivers and found the opening to be exactly 583 x 175 mm, so I cut the aluminum a few mm longer than needed with a saw, then milled it square to exact, matching lengths. The axial holes in the aluminum are sized for a 1/4" tap, so I tapped them with 1/4-20 threads.

The next step was to design and 3D print corner pieces to hold the aluminum rails. I split each corner piece into two identical parts so I could print the part that inserts into the aluminum without using any support material. The screw holes were printed 4mm in diameter and drilled out with a 1/4" drill after printing.

This is what the corner pieces look like. Support material is used inside the screw insertion slot. The slot is about 12mm high to accommodate the button head cap screws that mount the corner pieces on the aluminum rails.

One of the corners holding two of the rails together. It's a tight fit!

The whole frame. The 1/4" hole in the green rail is for the tire tube valve stem. I had to get a 20 x 1 3/8 tube with a 60mm long stem and Presta valve.

Here's a driver sitting on the frame. The opening in the frame matches the driver (583 x 175 mm) within a fraction of a mm. The plastic corners fit so tightly I had to tap them in with a rubber mallet. 

The next part of the design was to make a base that would allow the stretcher to be positioned vertically for resonance testing. In the previous design, when the stretcher tilted up, one edge hit the hinge support and stopped it from tilting further. That meant that during resonance testing, the tensioned diaphragm was in contact with the hinge support. I didn't like that. It doesn't take much to puncture and tear the tensioned film, so it's best to avoid physical contact with the film.

In the new design, I added "pins" to use for the hinge and an additional one for the support/tilt stop. It will allow the stretcher to tilt up to vertical, and I'll add a cord that will prevent it tilting so much that the edge hits the hinge support.

I installed three bolts for the hinge and support/stop.

The stretcher mounted on the stand. Yup, that's all there is to it. All that's left is to add a cord to the support/stop to prevent it from tilting too far, and some neoprene foam to the top surface of the stretcher frame.

First test with 6 um film. Worked well, but pointed out some minor issues. The pump hose has a pressure indicator that makes it hard to attach and detach the hose without accidentally closing or opening the valve. The film ultimately split, I think due to something sharp along the bottom edge of the frame.

After the film split during the first test I used a Scotchbrite pad and ran it over all the edges of the stretcher to smooth out anything sharp that might have caused the film to split. I had no further problems with film splitting.

The pump/hose I was using on the wood stretcher didn't work so well on this one. The pressure indicator on the hose took up too much space. I ordered another small tire pump that came with a hose and no pressure indicator. That solved the pump/valve problem.

There's one more problem I've been struggling with since I bought the roll of film back in the 80s. When the film is pulled from the roll, it generates a static charge that causes it to try to stick to anything and everything nearby (and pulls dust, cat hair, etc., from the air). When I put the film on the stretcher, it immediately curls under and tries to stick to the tape. Then I have to try to pull it free so I can position it where I want.

I decided to try to make a proper dispenser for the film that would kill the static charge generated when the film comes off the roll. I designed a couple conical end pieces with F608 skate wheel bearings and added two grounded steel tubes for the film to pass over as it comes off the roll. 8mm bolts go through the bearings and extend through the printed mounting brackets that are screwed to a piece of wood.

One end of the film dispenser. The other end is a mirror of this one. There are two conical plugs that fit the ends of the film roll, each of which has two F608 skate wheel bearings and an 8mm bolt. The ends of the bolts pass through the end supports that are screwed to a piece of wood. The film comes off the top of the roll, goes down under the bottom steel tube, wraps between the tubes, then pulls over the top of the upper steel tube so both sides of the film contact the steel. 

 

The bad news is this method doesn't actually take the charge off the film. It simply provides a new means of charging the film by sliding the plastic over the steel tubes. The good news is that it doesn't matter. I placed the film dispenser on the work table just behind the stretcher and I worked out a technique for attaching the film to the stretcher that works fine even with the charge on the film. Getting rid of the static charge on the film would require some sort of ion generator that would spray the film with ions and neutralize the charge as it comes off the roll.

Now I pull the film off the roll right over, and attach it to the two front corners of the stretcher. Then I cut the film free of the roll and attach it to the stretcher at the back corners, pulling wrinkles out of the film as much as possible as I do it. Then I attach the film to the short sides of the stretcher and finally to the long sides. The new stretcher allows me to see the film as it attaches to the tape.

Film coming off the roll and getting attached to the stretcher. I've added neoprene foam (black) around the edge of the opening in the top of the stretcher.

Once the film is attached, I put some air in the tube and tilt the stretcher up to check the resonance and adjust the air pressure as needed to get to the target value (about 80-85 Hz). 

Next I wipe the film and driver grid with IPA, then apply the 4693H contact cement to both, wait 15-20 minutes, check and adjust resonance again, and stick them together. I have found that with this stretcher, the final diaphragm resonance will be about 6 Hz higher on the driver grid than it is on the stretcher, so I set the resonance on the stretcher to about 80 Hz.

Resonance testing. I have added a couple marks on the stretcher frame (not visible here) indicating where the center of the diaphragm is to make positioning the microphone as accurate as possible. You can see the thin, black cord that is used to limit the tilt-up.

I pile some weight on top of the driver grid sitting on the stretcher and wait a few hours before letting the air out of the tube and cutting the driver free. Then I use a soldering iron set to about 260C to make the holes in the diaphragm around the center posts. When making the holes, I wear 5x loupes so I can see clearly, and keep the tip of the iron in contact with the post as I circle around it. Sometimes this creates fine plastic hairs that I remove using the soldering iron.

After letting the drivers sit for a couple days, I clean the excess glue off all the edges, tape off all the long edges, spray and wipe the diaphragms with IPA, then position the 3D printed masks in the center holes and on the ends, and spray with Licron Crystal. I make 4 passes, alternating L to R and R to L, then turn the driver around and do it again. I set the driver aside to dry while I spray the next one. I get reliable 10^8-10^9 Ohms/square resistivity using this technique.

Four drivers with newly recemented stators and new diaphragms, with long sides taped for spraying with conductive coating - Licron Crystal.

3D printed masks to block the conductive coating in selected areas of the driver.

I test the resonance after spraying on the Licron Crystal coating. I've been able to get pretty consistent results- here are resonance plots of 4 drivers I recently tested. Note- the lowest frequency peak is the one I use to set the resonance. The the drivers have multiple resonances and the mic picks up other sounds in the room (the AC running, garbage trucks outside, neighbors vacuuming, etc.), so it's safe to ignore the other stuff.

Final testing is done using electronics from an ESL-63 speaker- I have alligator clips on all the wires that go to the driver, and simply connect an amplifier and apply a signal. What I'm mostly looking for here is any hissing or whining noises coming from the driver when the 5.2 kV bias is applied, and any odd sound that a damaged driver might produce.

The two white boxes contain all the electronics from an ESL-63 speaker. I just connect the driver using the alligator clips, power it up and give it an audio signal from an amplifier.

Final testing is done with bias and audio signal applied:

The two halves of the early model drivers I have are held together by the three center screws and the four corner screws that hold the driver in the frame in the speaker. Newer ESL-63 drivers came with small metal clips to hold the two long sides of the drivers together. I think those clips are a good idea, so I ordered some 1/2" binder clips because they should fit in the 15 mm wide spaces in the driver grid. The 1/2" clips are actually 15mm wide (only in 'murica folks!) so I had to grind them all down to fit. 

When I am ready to add more bass panels to the ESL-63s I'll write another blog post.