A comparison of the effectivity of various speaker damping materials

[Rick Sykora]

Here is one of the more interesting charts I found comparing various materials…

Seems unfortunate that many material suppliers do not quote an NRC rating. Sonex is a major proponent of melamine foam and quotes ranges of about .6 to 1. As shown above, that is good but is also a big range. As with many other situations, conditions matter and often getting agreement on a common set is difficult. The Sonex site could be more helpful but touts the fireproof aspect of melamine rather than explain the NRC variability. Like polyurethane foams is not a particularly eco-friendly solution either.

This chart is from a good discussion of sound proofing:

Soundproofing a room | Science of noise reduction

A simple introduction to the science behind soundproofing.

www.explainthatstuff.com

 

[sigbergaudio]

I will immediately start the development of a refrigerated loudspeaker that uses snow as insulation!

 

[Rick Sykora]

After pondering the Dacron sheet result, when back and reviewed my application of Acousta-stuf and increased the quantity I applied. The original quantity was too loosely based on the designer's post (half fill the cabinet) and now I went with the manufacturer's recommendation. This more than doubled the quantity and puts the result more in line with the Dacron result. I updated the original post to include both test cases.

This is sort of a sneak peak at a planned test to measure the effect of various levels of damping material. Loose polyfill damping is more difficult for the novice to determine the correct amount to apply. So, will show how this can affect the speaker performance.

 

[Rick Sykora]

If you want some F13 Felt, 1"

I looked around for my F13 felt and don't seem to find any. I can check again tomorrow but I just built a speaker in August so I may really be out. I do have some 1" Sonic Barrier.


Open cell foam like egg-crate foam or mattress topper will likely measure the worst. In other people's tests that I have seen, melamine foam is one of the best (no idea why). The other thing that people really love is Merino wool (for stuffing, not lining). But you are right, the F13 felt and merino wool are NOT cheap.

If you want some 1" Sonic Barrier and some melamine foam squares PM me the measurements, you need and your address. (I know the measurements of the C-Notes, but not sure exactly what sides, top, back etc. you are lining.) If I happen to find the felt, I can send it too.

Thanks!

The melamine is intriguing but if I decide to do, will order a sheet of it. After further dialog, your 3/8" wool may be closest to the Jantzen but mine adds up to the same thickness. If you find you have enough felt to line the C-Note sides, top and bottom, let me know as will be less work than my 2-inch-wide strips.

 

[Rick Sykora]

Here is 80g of cotton blanket loosely lining most of the cabinet...

So this is a bonus material as it came via my work with a UK gentlemen. He was hoping to resell the Purifi SPK5 design and one got damaged shipping to Amir for testing. I ended up fixing and he left me with a pair for my effort. The SPK5 calls for 300g of cotton damping material which barely fits in the cabinet. When I decided to redo the design and mount the crossover internally and retune the port, it no longer fit. One leftover blanket was 80g and fit inside the C-Note reasonably with some fluffing.

As you can see by the blue trace, the cotton does some nice damping from 300 Hz up and it matches the thicker Sonic Barrier in tamping down the ugliest resonance. Above 1000 Hz, it is not quite as effective but could also be due to not covering the panels closer to the driver as well as the sheet-based materials. Maybe be a case for combining it with some other more effective material but frequency is high enough to be less of a concern in a rear-ported speaker.

 

[Rick Sykora]

Finally, here is some 1/2 inch open cell foam (from Madisound)...

The foam sheet was applied to all of the internal panel except the front and a small portion of the top to keep it from interfering with the port. It shows good overall efficacy with 2 exceptions: the major port resonance and another peak right around 1600 Hz.

As posted earlier, plan to also test some felt but this will require some added effort to tack the felt into place.

 

[ZolaIII]

I whose thinking about baking paper as a low weight cellulose which you can stiff up and shape easily and apply different multi levels\chamber\materials design (back from my light glider modeling days). Eventually with large enough cabinets and multi chamber design targeting specific refractions. Do try silicone mesh if you can find some. Idea is low weight high efficiency multilayer with at least 3 different materials. Handy stiffener would be a graphite spray in a couple of layers for it's EMI blocking properties but quality ones are far from cheap. Good old sheep wool is also a good one (better than cotton), best would be densely woven sheet.

 

[AnalogSteph]

As had posted in the C-Note review thread, here is a look at how changing the port length (to 4 inches) affects the major resonance from Amir's review...

So, this variation keeps the Acousta-Stuf but shortens the port to 4 inches. The red is the original undamped baseline and the yellow is the shorter port. The shorter port does lessen the low bass by about 10 Hz but also has higher bass output above 50 Hz. So, is a tradeoff but sounded fine to me. More notably, the ugly port leakage is reduced by about 30 dB! You may also note a high Q resonance at 1300 Hz. With a rear-facing port, doubt it is audible. If needed, could be reduced using other damping material rather than Acousta-stuf.

This is facinating. So basically the BR tube has a lengthwise resonance of sorts that amplifies a narrow frequency range including all the internal modes going on inside. Which should not be too surprising to anyone who has ever stuck a toilet paper or kitchen towel roll to their ear, mind you, but still. Intuition says it should have pressure minima at the ends and a pressure minimum in the middle, so that's like ƛ/2? That would suggest an effective tube length of about 7.5 inches then, or about 5" when shortened to 4".

Thankfully the BR tube is already placed away from the woofer, giving acoustic stuff a good chance at reducing the problems.

Has anyone ever placed a BR tube externally sticking out from a corner? It wouldn't exactly be easy to do (not to mention look silly) but that's where you would expect a velocity minimum for internal modes....

 

[Salt]

Well done so far

And OT some side effects of this topic (maybe should post in snake oil threat too):
https://www.mundorf.com/audio/en/shop/Damping/ misses the specification if it is the real, one and only audiophile holy unicorn's pubic hair ...

 

[TLEDDY]

Please forgive my abject ignorance! As I have been scanning the posts, I saw the suggestion of using Melamine for sound absorption! Because of my aforementioned lack of knowledge, in my life having used Melamine only as plates, cups, saucers etc., I struggled with visualizing the interior of the speakers stacked like a dishwasher with colorful tableware!, or crushed shards and powder of the same!

I know - how utterly silly is that! So, thank Heavens for the internet and search engine, I found the description and a photo of the material in the appropriate form.

I ended up laughing out loud at myself and had to share with the group…

Tillman

 

[cavedriver]

SB1.2 is the black trace and they both compare well between 300 and 1200 hz. The notch in the green trace at 660 Hz is partly due to the top piece as it actually touches the port tube. Above 1200 Hz, the performance is unexpectedly more mixed as the thinner SB struggles at around 1600 Hz but recovers and matches the thicker version above 2000 Hz.

I was intrigued when you said there was a notch because the damping material touched the port tube. We use round port tubes and generally people say the material used doesn't matter much- cardboard, plastic, etc. work about the same, but I've always been surprised that the tubes themselves don't resonate in some deleterious way. So are you actually seeing that having material touch the tube in a way that might keep it from vibrating reduces portions of the externally measurable noise? And if so, could a test be done to measure packing the port more closely versus no packing? Finally, would this improvement only be over a relatively narrow frequency range that might correspond to vibration of the tube itself, or disruption of standing waves in the tube, or a wider range that might accomplish useful noise reduction with a minimal amount of material (or different, intentional placement of ports)?

 

[Rick Sykora]

I was intrigued when you said there was a notch because the damping material touched the port tube. We use round port tubes and generally people say the material used doesn't matter much- cardboard, plastic, etc. work about the same, but I've always been surprised that the tubes themselves don't resonate in some deleterious way. So are you actually seeing that having material touch the tube in a way that might keep it from vibrating reduces portions of the externally measurable noise? And if so, could a test be done to measure packing the port more closely versus no packing? Finally, would this improvement only be over a relatively narrow frequency range that might correspond to vibration of the tube itself, or disruption of standing waves in the tube, or a wider range that might accomplish useful noise reduction with a minimal amount of material (or different, intentional placement of ports)?

I tested my hypothesis about the notch subsequently. The short answer is a tweener. When I removed the upper damping, the notch was reduced by about half, but still was present. That said, the damping across the back still contacted the port tube but much less so. I did some testing in another thread of how damping the tube affects the system impedance, I was using a cardboard tube and only tested it internally. As strong as the major C-Note port resonance is, seems likely that a thin plastic tube itself could be resonating. Have seen major manufacturers wrap the port tube externally so seems worth a try. Since I have to glue the felt in place, can try a quick port wrap to see before I test the felt.

P.S. just remembered wrapping may be difficult to do with the C-Note port as it so close to the top panel. Will either have to use thin material or try later and move the port. Perhaps outside the box?

 

[Rick Sykora]

With the port mounted externally and no internal damping, the deep notches returned. I tried 4 different damping applications and they made almost no difference. The first emulated having the internal case with sb.75 and balance a piece on top of the port. This made me wonder if the damping was coupling the port to the top so I tried supporting the port with a pipe clamp. Next I wound a thick rope of butyl rubber around the last half of the port. None of these things made and appreciable difference. Finally, I took and formed a jacket of sb.75 and secured it to almost the entire port. Still no significant change. Here is a chart...

So, in this case, appears damping the port externally has no major impact. May be that when the port is inside, the major change is the airflow around the port. It is located closer to the top than is recommended. Later will test with some internal damping applied. Stay tuned!

 

[cavedriver]

Thank you- I think you've confirmed that port tubes themselves resonating don't generally contribute to the noise heard from them. Looking forward to what you learn about the airflow inside the box around the tube. The other instruction that is commonly repeated, specifically for transmission line speakers particularly, is to not install damping in the area around the port tube. How much that applies to bass reflex designs I am unsure of. I also often wonder what is it about the design of the DBR-62's slot port, which is essentially integrated into the floor of the cabinet, that produces such nice bass. When I've asked Andrew Jones about how he is able to make a slot port work well despite common wisdom against them, his answer was, "it depends". Not revealing any secrets, is he... I haven't looked inside a DBR-62 to see where the damping was placed.

 

[Rick Sykora]

Finally in this round, here is F13 felt done in 2 passes…

First (black trace) is 1/4 felt lining much of the cabinet walls except for right above the port. The reason for leaving that space open was to thread another layer for the second pass. This raised the total to half inch. Shown here as the purple trace. A this stage, becomes obvious that you can get comparable improvement with most of the materials if you use enough. This varies based on the material, so will take some of the less expensive options and determine what is enough (and what too much does).

This will mean tearing out the felt and doing some repairs on mounting holes. Throughout the testing the holes have become worn and have done some quick repairs and switched to larger screws. I stripped one hole and repaired it before testing the felt last pass.

 

[Rick Sykora]

Thank you- I think you've confirmed that port tubes themselves resonating don't generally contribute to the noise heard from them. Looking forward to what you learn about the airflow inside the box around the tube. The other instruction that is commonly repeated, specifically for transmission line speakers particularly, is to not install damping in the area around the port tube.

Have not built a transmission line but thought the general rule for any port was to avoid damping around it.

How much that applies to bass reflex designs I am unsure of. I also often wonder what is it about the design of the DBR-62's slot port, which is essentially integrated into the floor of the cabinet, that produces such nice bass. When I've asked Andrew Jones about how he is able to make a slot port work well despite common wisdom against them, his answer was, "it depends". Not revealing any secrets, is he... I haven't looked inside a DBR-62 to see where the damping was placed.

The DBR62 may not rely on damping as much as some fabrication work. Amir did not do nearfields and but notes “some resonances” around 600 Hz. So may not be desirable and may be why Andrew was coy. Best not to draw attention to marginal design compromises. A cutaway is shown on the Crutchfield site and the opening to the slot is circular. They also flared the port opening more than many hobbyists would do for a slot. I like the idea but not until am done with final tuning and that is just so much easier with a tube. Also easier to tune stereo speakers to match using a tube.

 

[Rick Sykora]

Before I repurpose the original speaker, thought it might be good to look at some potential problems with overcompensating damping. In this case, I added about 90g of Acousta-stuf to the NoRez test speaker...

So, as problems go, the additional damping (black trace) has made the original NoRez less effective at dealing with the major problem resonance. It also appears to have slightly lowered both the intended frequency and output of the port.

The takeaway here is that more is not better and slightly detrimental. At the very least, would be a poor use of money spent for added damping.

 

[ryanosaur]

@Rick Sykora
Is the felt you guys are discussing similar to the recycled denim insulation (like Ultratouch) that some use? I've seen some branded as audio filler, but more commonly sold for household insulation.

 

[cavedriver]

@Rick Sykora
Is the felt you guys are discussing similar to the recycled denim insulation (like Ultratouch) that some use? I've seen some branded as audio filler, but more commonly sold for household insulation.

I had assumed they were testing this material, but I admit I don't see where it was mentioned:

Sonic Barrier Acousta-Blue Speaker Cabinet Sound Damping Denim 50mm x 40" x 48"

Sonic Barrier Acousta-Blue Speaker Cabinet Sound Damping Denim 50mm x 40" x 48"Sonic Barrier's Acousta-Blue vs. Other Speaker Cabinet Damping MaterialFirstly, Acousta-Blue's ability to isolate and trap sound waves sets it apart. By preventing unwanted standing wave frequencies inside speaker...

www.parts-express.com

 

[Rick Sykora]

@Rick Sykora
Is the felt you guys are discussing similar to the recycled denim insulation (like Ultratouch) that some use? I've seen some branded as audio filler, but more commonly sold for household insulation.

There is a F13 felt pic on post #40. It is 75% wool as opposed to denim which is usually cotton. Do not have the denim-based Sonic Barrier but expect it is comparable to the cotton blanket I tested.