DIY Sound-absorber panels

A while ago I built new crossovers for my floorstanders and while performing the measurements I noticed some nasty behaviour between 100-200Hz.

Frequency response with the old crossovers, 1/12 smoothing

I was suggested that it was probably because of something called SBIR (Speaker Boundary Interference Response). In short that means that the lower frequencies start wrapping around the speaker hitting the rear wall and then being reflected with a 180 degree phase shift and then interfering with the signals coming directly from the driver. The linked article from GIK Acoustics mentions treating the wall behind the speaker to try to absorb the sound that wraps around the speaker so you get less reflected sound coming back to interfere with the orginal signals from the speaker.

I had a look at buying absorber panels from different companies but they were all so expensive so I figured I could probably do something myself for much cheaper (and it's fun to know you did it yourself). I looked into what is important when chosing the right material and what thickness I would need. Of course, the thicker the better, but there's only so much space between the speakers and the wall and since this is not a dedicated listening room but a living room I do need to think about aesthectics too. For low frequency absorption and thin panels (less than ~100mm) a dense material with high flow resistivity is prefered. I had decided to go with a 70mm panel since it would still look OK and it's easy to find panels and wood for the frames in 70mm thickness.

Flow resistivity is not something that a lot of manufacturers list in the spec sheet for their products. I had a look at different brands of rockwool insulation and couldn't find anything helpful. After some further googling I found this thread with some helpful data on flow resistivity for insulation products here in Scandinavia. Regular fluffy rockwool insulation seems to have values of around 10 kPa*s/m2 while the denser ground insulation slabs seemed to have values of around 40 kPa*s/m2 and up.

There's a great porous absorber calculator here at Acousticmodelling which turned out to be super helpful. I compared two absorbers, both 70mm thick and with 20mm to the back wall. 20mm is the thickness of the baseboard in my living room.

Model of two different absorber panels with different flow resistivity.

The green line is with the denser material and the blue is with the fluffier material. If my goal was to treat reflections anywhere in front of the loudspeakers I would have gone with the fluffier material since it is way better from around 250Hz and up. However, now I'm trying to treat reflections behind the speakers and for that I need good absorption in lower frequencies because those are the ones that wrap around the speaker and my main problem area seems to be between 100-200hz and in that range the green line (dense material) wins.

I ended up buying a pack of three Rockwool 70mm thick 600x1200mm ground insulation slabs. Then for the frames I bought two 4.2m long 28x70mm bits of wood and cut four 1.2m pieces and four 0.656m pieces. I hadn't decided on what fabric to wrap them in yet and since I wasn't too confident about my skills in wrapping frames with fabric I bought the cheapest stuff I could find at Ikea. It was a non-bleached cotton fabric and I started with buying just enough for wrapping one frame once.

One frame being wrapped with fabric.

Result of first attempt at wrapping a frame. Not pretty but the best I could do.

Wrapping the frame wasn't as difficult as I thought it would be. No problems at all getted the fabric stretched evenly. However, the corners had me confused and I also realised that I should probably get the back side stapled BEFORE wrapping the sides over. I ended up going with this triangular deal for the corners. I had a feeling that there is some other nicer way to do it but at the time I couldn't figure it out. Turns out that when I did the very last corner on the second round on the second absorber I finally figured it out. But then it was too late so I did the triangle anyways. Learn by doing! By this stage I had realised that the dirt cheap fabric from Ikea matches the colour of the wallpaper in my living room really well so I went back to the store to buy another three pieces of fabric so I could wrap the first panel with one more layer and then do two layers on the other panel as well.

Left panel with one layer of fabric, right panel with two layers.

Both panels finished.

Both panels finished.

The panels blend in nicely with the wall and don't attract too much attention. I had first planned to go with a coloured or more textured fabric but I'm glad I did it like this. As I mentioned previously my intention was to place these all the way against the back wall with only a 20mm gap for the baseboard down by the floor. Turns out this doesn't work because behind the right speaker there's a power outlet that stick out quite a bit. I ended up having to move the TV-bench and the speakers forward a bit to make enough room. Now the panels ended up 70mm from the wall and this changes the efficiency of them a bit.

Blue line is with 70mm air gap, green with 20mm air gap.

Not a big difference but it does give slightly better absorption below 200Hz. Good for me, because that's exactly what I needed. However, if we compare this new result with the fluffier material from before we get a different picture.

Blue line is the dense panel with 70mm air gap, green is fluffy with 70mm air gap.

Now my dense panel gets beaten by the fluffy panel from around 150Hz and the fluffy panel is way better from 150Hz up to 2-3kHz. This shows that you really need to measure and model BEFORE buying your material. Depending on your situation you might need something different than what you first thought.

Also, something that I didn't consider beforehand but I realised later is that a fluffy panel may sag a little from gravity over time. I'm not sure if it will, but the denser ground insulation panels are incredibly rigid and shouldn't deform over time. That could be something worth thinking about too. Weight is also something to consider if you want to mount the panel on the wall. Ground insulation slabs are HEAVY! My pack of three slabs weighed in at over 20kg. A fluffier material would way much less and be much more wall-friendly. I will end this post with some measurements with/without absorber panels and in the very end I will make a parts list and write the total cost for everything.

Left channel measured at ~1m on-axis with and without absorber panel. Very small difference in frequency response. 1/12 smoothing

Left channel RT60 with and without absorber panel. The biggest difference is between 150-450Hz, as
can be expected since the sound starts to wrap around the speaker at ~500Hz according to the GIK Acoustics article. 

Frequency response at sweetspot with both channels. Red is with absorber, blue is without.

RT60 at sweetspot with both channels. Not as big of a difference here but there is at least a slight difference.

Parts list:

• 1 pack of 3 70mm 600x1200mm Rockwool ground insulation slabs: 519SEK (~€50/$58)
• 2 pieces of 4.2m 28x70mm wood: 66SEK (~€6.50/$7.40)
• 4 pieces of 1.5m "Bomull" fabric from Ikea: 115SEK (~€11.30/$13)
• 1 pack of staples: 59SEK (~€5.80/$6.60)
• 16 wood screws: 0SEK (had those already)

Total cost: 756SEK (~€74/$85) and then I have one slab left so I could build a third absorber for only ~90SEK (~€8.80/$10). So price per unit turned out to be 378SEK (~€37/$42.50) when building two or 282SEK (~€28/$31.50) if building a third one.

Tools required:

• A saw to make accurate 90-degree cuts (or 45-degree cuts if you feel ambitious).
• A drill to pre-drill the holes for the screws. Makes assembly a lot easier and with less risk of cracking the wood.
• Screwdriver
• Staple gun
• Scissors for cutting the fabric.
• Earplugs (recommended because staple guns can be quite loud).

Time required: It depends on how good tools you have, how good you are at using these tools, and how skilled you are in general. I had a terrible saw and did all the drilling and assembly in the small kitchen in my apartment and wrapped the frames on the largest open space available in my apartment. I probably spent 2-3hrs on each panel. If you are more skilled than me (which isn't very unlikely) and have a better workspace you could probably finish a panel in 30-45 minutes. 

CSS EL70eN frame dampening

I know, it's been a while since my last post, but life has kept me busy and away from the audio world.

This summer I brought my dMar-Kel70T to the local hifi-store to try them with better amplifiers but also to let the store owner have a listen to them.

My dMar-Kel70T next to some Dali and B&W speakers at the local hifi-store

The store owner, who has probably been in the audio business since before I was born, thought they had a "nasal" tone and I do agree with him. Without some sort of EQ they can be a bit too nasal. At home I use a MiniDSP to control that and it makes a massive difference, bringing them from sounding like a pair of $500 speakers to >$1000. One suggestion he gave me was to dampen the driver frame and bezel. Since they are a lightweight plastic they do ring quite a bit at certain frequencies, which surely has a negative impact on the sound.

I did some googling and found this thread at the diyaudio.com forums. None of the materials suggested there were easily available (or cheap enough) for me. I did some research and found that blu tack would probably suit my needs. It sticks well, is easy to shape, has a high density (around 1.8g/cm^3), sticks to things, and isn't crazy expensive for the amounts that I would need. I bought two packets for 29SEK each (~$3.50/ea). Each pack has six strips each weighing 10g.

The drivers before my mod.
Note that these have already been partly modified by Dave at Planet_10 hifi

I had no idea how much blu tack I would be able to fit in there. On the first go on the first driver I managed to get around 2.5 strips in there. Then I figured I could use a stick to press out the blu tack better into the spaces, so then I could fit even more in there. In total I used around 40-45g per driver.

Side view of inishedd driver

I also put some on the "legs" of the frame just to add some extra mass to it.

Top view of finished driver

So now to the important question: do I hear any difference? I can't say for sure, but there is a noticeable difference in vibrations in the frame. I started out doing just one driver, put it back in the enclosure and played some music on both speakers. The untreated driver frame had a lot more vibrations in the mid-range frequencies. The untreated driver made a higher pitched (and louder) sound when tapping on the bezel compared to the treated one.

Was this mod worth my time and money? I don't know, but why not? At least I can now have peace of mind knowing that I have done what I can to address the problem.

Update on the new Porta Pro cable

A while ago I changed the cable on my Koss Porta Pro to a new one bought off ebay. You can read about this in my previous blog post. It seemed like a reasonable cable although not the sturdiest. However, considering the original cable lasted me around two-three years I was hoping to get at least a year of use out of this new one. That turned out to be wrong. Today I was going to go for a walk and as the great music fan I am I wanted some music in my ears while walking. To my disappointment I heard only music from the right speaker. The left side was completely dead. I tried replugging the cable to my phone, no difference. I thought maybe there is something wrong with the phone so I tried connecting them to my O2+ODAC combo, but the problem remained. I examined the cable for any visible damage and I couldn't see any. I tried jiggling around the cable along its whole length to see if I can get a connection somehow. No success. I took of the speaker from the frame and opened it up to see if the solder joints were still intact and they were. Apparently the cable must have broken somewhere and I can't find it.

Right now I don't know what to do. A new cable would cost me around 40SEK and a month of waiting and may only last another month or two. A new pair of headphones would cost me 300SEK and would hopefully last at least a year or two. I could try with another, sturdier, cable, such as a normal 3.5mm to 2xRCA but that would probably cost ~80SEK and would require some modifications to the headphones which might not turn out very well. I really like the Porta Pro headphones because in my opinion they are the simply the most portable non-in-ear headphones and they do actually have a decent sound for its price level.

Koss Porta Pro cable replacement guide

My second pair of headphones, the Koss Porta Pro, has served me well the last couple of years and I am surprised at how well they have withstood all the abuse they have been put through. I used them when I was sanding the Oy cabinets. I had them with me on my trip around the world, carelessly packed in my backpack or pockets. They have been with me on over a hundred workout in the gym and then afterwards just tucked in my gym bag along with other items. The frame is still perfectly straight. The foam earpads are looking pretty ok, just a few holes on the edges. The cable has been the part that has handled the abuse the worst. About half a year ago it turned really stiff so it wasn't a very good idea to fold the headphones and then wrap the cable around them. I did that anyways though and eventually the outer insulation snapped in half and it was only being held together by the really thin wires. I solved this temporarily by using some electrical tape. It didn't last very well though and I had to change the tape every two weeks or so. I ordered a 3.5mm -> 2xRCA cable that I was planning to use as a replacement. However, that cable got put in use elsewhere (between the O2+ODAC and my stereo amplifier) so I kept on using the electrical tape. Last week when I was about to go for a walk the sound just died in one side. One of the wires had broken. I managed to fix this with some soldering and lots of electrical tape, a really ugly short term solution. Meanwhile I had ordered a replacement cable from Ebay.

Short term electrical tape solution.

The replacement cable arrived today and I just couldn't wait with replacing the old cable. I found this guide over at iFixit which shows how to disassemble the headphones but also how to do if you have a non-standard replacement cable, similar to what I was planning to do in the beginning.

I decided to write my own guide for you, based on if you have the same cable as I have. I bought this cable from tuttoit on ebay. It is cheap, shipping included and it looks ok. I have no idea about the durability yet but I will come back later to comment on that. As usual it took quite a while for it to arrive but that can be expected when ordering things from Asia with free shipping.

As the iFixit guide mentioned above says you need to start by removing the speakers from the frame. You do this by gently pulling them apart. With a bit of force they will snap off from the frame.

Then you should remove the black foam. It just hangs onto small plastic pins running around the edge on the backside. Gently lift it off the pins and remove it.

Now is also a good time to make sure there is no dirt or hair behind the black cover. That could potentially degrade the sound. If you have had the headphones for a while there will probably be some stuff there. Gently remove it and be careful not to touch the speaker drivers.

The next step is to remove the plastic cover covering the wire. For this you need something pointy, like a flat head screwdriver, knife or similar. Bend gently and it will start coming off.

Take note of how the cables are soldered. Take a picture with your phone or write it down on a piece of paper.

Begin with desoldering the wires to one speaker. Don't do both at the same time. I would recommend trying to be as quick as possible when heating up the joint and removing the cable. The plastic can deform or even melt if you keep the heat on for too long.

New cable soldered to the connector.

As you can see in the picture above I kept the heat for a little too long so the plastic deformed a bit around the hole where the pin on the cover goes. This turned out to be a little problematic since the cover didn't want to come back on again. I had to scrape some plastic out from the hole to make it a little bit bigger.

The last step is to wrap the cable around the pins. This way it is protected in case you would pull on the cable. Put the cover back on and then put the foam pads back on. By this time you have probably forgotten which is right and which is left, but don't worry. It doesn't really matter, just run some stereo channel test track and you can easily figure it out. I used this track on YouTube. Then just push the speakers back in place on the frame you are done.

Result.

Hopefully this will allow me to use these fantastic headphones for another year or two. Sure, they are no hifi headphones but considering how cheap and portable they are amazing. Perfect for when you are out taking a walk, running, or training in the gym. Heck, I actually think they sound better than most other headphones that I have listened to it the same price range.

Estimated time: 10 minutes
Estimated cost: ~40SEK / ~$4.5 / ~€4 / ~£3
Difficulty: Basic
Tools needed: Soldering iron, flat head screwdriver or other pointy object

Experimenting with a quieter power source for the MiniDSP

As exams are approaching I find myself spending more and more time by my desk with faint background music on as I study. As I lean forward over my books my head comes within 40-50cm from the speaker drivers so any noise will be very audible.

I have been running the MiniDSP hooked up to my system even though I am not using any subwoofers at the moment. This is because the mid bass becomes a bit overpowering in my room as soon as I turn up the volume a bit. I have been using a small wall wart as a power supply for my MiniDSP 2x4 for years now but the noise haven't bothered me at all since I haven't been listening near-field. If I sit a couple of meters away from the speakers I can't hear any noise at all.

I did some googling on the problem and found a thread where some guy suggested powering it with a 9v battery to see if it was the wall wart that was introducing the noise, because apparently batteries are supposed to be a quiet (but not very convenient) power source.

By chance, I happen to have some 9v batteries lying around from my previous O2+ODAC builds.

The sacrifical cable to the left, a 2.5mm male to 2x2.5mm female cable.

I also had a spare cable which I ordered just a couple of weeks ago. The plan was to use it as a replacement cable for my Koss Porta Pro headphones, but I realised I should get something sturdier (which I have, just waiting for my heat shrink to arrive from China).

I started by cutting off the useless 2.5mm female connectors. I mean, have you ever used one of those? Well, I haven't.

Shielding/ground and signal wire.

Apparently the cable had a shielding/ground wire and a signal wire, which makes sense after all. I decided to use the outer ground wire since it was thicker and I didn't have to peel an extra piece of insulation.

The next problem was how to connect the wire to the battery, without holding it in place with my hands. One of the connectors on the battery allows for winding the cable around it but the other one does not. But as it happens to be, batteries are not the only things I get left after building an O2+ODAC combo. I get spare battery connectors as well. Might as well sacrifice two of them for this experiment.

Wire soldered to a battery connector.

This way I can attatch the cables to the battery without holding them in place. However, another problem appeared. These connectors are meant to be soldered in place on a circuit board, fixing them in place so they can't rotate and touch eachother which creates a short circuit. My solution to this was electrical insulation tape.

Tape-wrapped and insulated battery connectors.

I used my trusty soldering iron to melt the tape in a few places to fuse it together so it wouldn't fall apart as easily.

Finished "product".

After finishing the battery connectors I simply connected the other ends of the cables to the MiniDSP. No fire, no smoke, no weird sounds. All good. Next step - connecting it to the amplifier. No fire, no smoke, no weird sounds. All good so far. Turn on the amplifier and wait for the "click" that turns on the sound. Dead silent, no background noise. Wow, did this really work? I turn on some music but the speakers are still dead silent. I unplug the battery and plug in the USB cable. Background noise appears again along with the music.

My first thought was regarding the age of the battery. I think the battery I tried was from my own O2+ODAC build several years ago. I take it these batteries are not high-end and maybe they lose charge by time? I fetched my DMM to check the voltage.

3.91 volts, the MiniDSP 2x4 requires at least 4.5 VDC.

Turns out I was right, the battery had lost too much charge. But as it happens to be, I had seen some other batteries in my box full of newer O2+ODAC supplies while I was getting all my tools which where in the same box. I fetch the newer batteries and measure them. The first one measured around 7.5 VDC and the other measured 8.83 VDC as you can see in the picture below.

8.83 VDC, the MiniDSP 2x4 can handle up to 24 VDC.

I connect the new battery to the MiniDSP, I connect the DSP to the amplifier again and as I turn on the amplifier and hear the "click" I can instantly hear the background noise again. I put on some music to see if it actually works with a battery and it did work, however not any better than the wall wart.

I am a bit disappointed with the results, I was really hoping to get rid of the noise with a quieter power supply. I guess it must be the MiniDSP itself making the noise and I will have to delve deeper to try to find a solution to this, because otherwise I wont be able to use the MiniDSP in my current setup.