Yes, it's another Bass Traps thread...

[sploo]

I have just set up a basic home cinema system in a recently redecorated room (5 speakers + projector). The room now has a wood floor and little wall ‘furniture’ (i.e. flat, hard, painted walls).

I’ve noticed that the sound is quite ‘muddy’ and the clarity and crispness of bass that I used to enjoy with my speakers in bigger rooms is missing.

Having spent several days crawling round the floor (installing cabling/skirting board etc.) I noticed just how ‘boomy’ the bass was in the corners – and a little research led me to looking into Bass Traps.

I’ve now spent quite a bit of time reading numerous articles by Ethan Winer, Thomas Andry and many others… which, of course, leads to lots of questions.

Quite a few of my questions are directed at Ethan Winer, but I’ve mailed here in the hope that any responses would be useful to others (and to get other people’s thoughts too).

1) I’m presuming that Bass Traps are what I require to improve the sound – someone please stop me now if I’ve got the wrong end of the stick? (I'll go away and buy some more expensive cables to improve the sound )

2) I’ve downloaded the REALTRAPS Test Tone CD. Whilst I haven’t run through the whole disk yet, I noticed that the bass drivers in my front speakers move violently (large amplitude) at very low frequencies (10-19Hz). As the drivers are rated down to 25Hz (and I don’t use a subwoofer) am I right to assume that; a) It’s not doing the drivers any good, and b) I don’t need to test lower than 25Hz as that’s as low as the drivers go (i.e. there aren’t any effects whereby lower frequencies will be generated in the room)?

3) Looking at the designs on Ethan Winer’s site (http://www.ethanwiner.com/acoustics.html), figure 3a at shows a simple bass trap design. Would there be any reason for not cutting the edges of the panel at 45 degrees and thus run them parallel to the edge walls (it would make the depth of the panel a little less obvious, and hopefully less visually intrusive)? Oh, and I take it that this design isn’t sealed?

Figure 7 shows a sealed design. If the wall (in the diagram) were replaced with another board, creating a portable sealed box, hung on the wall, would this be as effective?

The text also notes that the panel traps are better placed flat against the wall, but could you create a sealed trap, like that in figure 7, but with a corner back, and the fibreglass + plywood front at 45 degrees (i.e. the plan view would be a right angle triangle) and would fit into a corner. This is obviously much more work, but would it be better than the simple version from figure 3a?

On the RealTraps site I’ve found pictures showing the construction of some of the traps, and they look like they’re made from two panels, with the depth being something (plastic?) with many vents. How does this differ from the above designs?

4) My room is roughly 25.5’ long, 9’ wide and 8’ high. The front three speakers are along one of the shorter walls, and the surround speakers are halfway along the long walls. I could get traps into most of the vertical corners (there’s a fireplace sticking out too), but I could also put a thinner trap horizontally along the floor behind the front three speakers. Would this help too?

5) I take it that because the traps absorb sound there's no way they'd increase bass transmission through the walls (I have neighbours)?

6) Finally, does anyone know of suppliers of suitable rigid fibreglass in the UK? Many people have listed suppliers, but all are in the US. If no-one knows of a specific product, what characteristics in board should I be looking for?

Apologies for the long-winded mail, but there’s obviously lots to learn!

 

[Ethan Winer]

Sploo,

> I’m presuming that Bass Traps are what I require to improve the sound – someone please stop me now if I’ve got the wrong end of the stick? (I'll go away and buy some more expensive cables to improve the sound ) <

Yes, forget cables, you need bass traps. You also need absorption at the first reflection points. And probably some general mid/high frequency absorption around the room too.

> am I right to assume that; a) It’s not doing the drivers any good, and b) I don’t need to test lower than 25Hz <

Yes and yes.

> figure 3a at shows a simple bass trap design. Would there be any reason for not cutting the edges of the panel at 45 degrees <

Only because it's a nuisance to do the extra work, and difficult to cut rigid fiberglass at an angle evenly so it looks good.

> I take it that this design isn’t sealed? <

There's nothing to seal. Rigid fiberglass is porous.

> Figure 7 shows a sealed design ... would this be as effective? <

Yes, but most small rooms do well with rigid fiberglass only. Wood panel traps make sense in larger rooms.

> could you create a sealed trap, like that in figure 7, but with a corner back, and the fibreglass + plywood front at 45 degrees <

No, wood panel traps need to be flat against the wall.

> On the RealTraps site I’ve found pictures showing the construction of some of the traps ... How does this differ from the above designs? <

Our traps are proprietary, but basically we bond a limp mass membrane which increases absorption quite a bit at low frequencies compared to plain rigid fiberglass.

> I could also put a thinner trap horizontally along the floor behind the front three speakers. Would this help too? <

All corners benefit from trapping.

> I take it that because the traps absorb sound there's no way they'd increase bass transmission through the walls (I have neighbours)? <

There's no way adding traps will make that worse, and they might even help a little.

> does anyone know of suppliers of suitable rigid fibreglass in the UK? <

My Acoustics FAQ lists a number of international manufacturers, and also the alternate product names - rigid rock wool and rigid mineral wool.

--Ethan

 

[sploo]

Ethan,

First, many thanks for the response. Of course, many useful answers generates yet more questions .

> Yes, forget cables, you need bass traps. You also need absorption at the first reflection points. And probably some general mid/high frequency absorption around the room too.

OK, so I should probably look at something like the 705-FRK for the corner traps, and make a few flat panels for absorbing reflections from 703? I've been reading some of the info at realtraps.com about placement of traps and HF absorbers.

>> am I right to assume that; a) It’s not doing the drivers any good, and b) I don’t need to test lower than 25Hz <

>Yes and yes.

Having nearly blown out my eardrums, I realised that the SPL meter I’ve borrowed is “A” weighted, and thus reads 'under' at low frequencies . I can get hold of the RadioShack meter over here, and that does “C” weighting, but only records down to 50dB. Will this meter be suitable for running the RealTraps Test Tone CD?

Strangely, I can play music at around 75dBA (though I don't often go that loud). However, playing a few tracks on the test CD was quite unpleasant - the speakers weren't distorting, but I wouldn't want to spent too much time in there without ear defence! Maybe my room is *really* boomy under 300Hz?

>> figure 3a at shows a simple bass trap design. Would there be any reason for not cutting the edges of the panel at 45 degrees <

>Only because it's a nuisance to do the extra work, and difficult to cut rigid fiberglass at an angle evenly so it looks good.

Bandsaw with tilting table, dust extractor, and a personal respirator .

> Our traps are proprietary, but basically we bond a limp mass membrane which increases absorption quite a bit at low frequencies compared to plain rigid fiberglass.

Interesting. So it's a double layer of fiberglass with an air gap (with your membrane). I guess your membrane is your 'differentiator', so I won't push you on details

However, copying the physical layout of your traps, I could build a framed ‘sandwich’ of fibreglass/vented sides/fibreglass. How would this configuration - two 1” boards with, say, a 2” air gap, compare to a single 2” board (assuming both designs were placed into a corner)?

I could also make the above with mitred sides, so they fit into the corners (with small side gaps for the ‘vents’), as this may have a slightly higher WAF . Obviously this would reduce the width of the rear panel compared to the front... hmmm, am I going too far for the results I’d get.

> My Acoustics FAQ lists a number of international manufacturers, and also the alternate product names - rigid rock wool and rigid mineral wool.

Ah, yes - Owens-Corning, Knauf, Armstrong, Delta, Johns-Manville, CertainTeed, Roxul, Ottawa Fibre, and Fibrex! I'll look into those.

Many thanks!

 

[Ethan Winer]

Sploo,

> so I should probably look at something like the 705-FRK for the corner traps, and make a few flat panels for absorbing reflections from 703? <

Sure.

> the RadioShack meter over here, and that does “C” weighting, but only records down to 50dB. Will this meter be suitable for running the RealTraps Test Tone CD? <

Sure.

> playing a few tracks on the test CD was quite unpleasant <

So measure 10 dB softer. As long as you're well above the noise floor of the room the readings will be accurate.

> Bandsaw with tilting table, dust extractor, and a personal respirator . <

There you go!

> How would this configuration - two 1” boards with, say, a 2” air gap, compare to a single 2” board (assuming both designs were placed into a corner)? <

I never tested that so I can't say for sure. But I'm sure that an internal air space is not as good as having it solid all the way through.

--Ethan

 

[sploo]

>So measure 10 dB softer. As long as you're well above the noise floor of the room the readings will be accurate.

Noise floor – about 24dB if I remember correctly (tested with the ‘A’ weighted meter). I’ve ordered the RadioShack meter, so will re-test when it arrives. My only concern is that if I’m measuring at, say 60dB, and the RS meter only goes down to 50dB it won’t fully record any big dips.

> I never tested that so I can't say for sure. But I'm sure that an internal air space is not as good as having it solid all the way through.

Right… well, Owens Corning have contacts for Europe, so I’ve emailed them about UK suppliers (cross fingers and hope).

My plan is to make absorbers for the first reflection points with 1” unfaced 703, probably in the same way as Thomas Andry (wooden frame surround, then cover the whole thing in batting and suitable cloth).

When hung on a flat wall like a picture, should they be mounted right on the wall, or with a batten behind to create a gap?

For the bass traps, 2” 705-FRK (foil side facing the room), mitred sides and screwed into vertical wooden battens (see attached image).

I presume there’s no problem hanging them off hooks in the corners, and again wrapping the whole thing in batting and an ‘open’ fabric?

 

[WmAx]

Noise floor – about 24dB if I remember correctly (tested with the ‘A’ weighted meter). I’ve ordered the RadioShack meter, so will re-test when it arrives. My only concern is that if I’m measuring at, say 60dB, and the RS meter only goes down to 50dB it won’t fully record any big dips.

24 db noisefloor? What did you measure this with? Only very expensive, instrumentation quality SPL meters can measure noiselevels this low. That is extraordinarily quiet. If you really have such a noisefloor, count yourself very lucky. Such an ambient noise level normally requires a specially constructed, air tight and physically decoupled suspension room. Alternatively, living away from a road or civilization, with no noises caused by things in the building.

-Chris

 

[Tom Andry]

When hung on a flat wall like a picture, should they be mounted right on the wall, or with a batten behind to create a gap?

If you built the frame with 1x2" boards, it will have a built-in gap so you can hang them flush (cleaner look). Regardless, gap is better (more effective) than no gap.

 

[sploo]

24 db noisefloor? What did you measure this with?

Yes, excuse me while I engage my brain (it's been a long few days). The meter (if I remember correctly) was displaying around 24dB. Checking its specs shows it to be claimed capable down to 35dB +/- about 2dB across its range.

I do live in a fairly quiet area though, so I'd guess the noise floor is reasonable by general domestic standards.

BTW WmAx, you have a pretty 'anti-snake oil' attitude to audio, what's your position on room treatment? For? Undecided? Sceptical?

 

[sploo]

If you built the frame with 1x2" boards, it will have a built-in gap so you can hang them flush (cleaner look). Regardless, gap is better (more effective) than no gap.

Yep, that sounds simple enough, thanks.

I don't think something 2" deep should cause major 'aesthetic' issues as far as WAF goes

 

[Ethan Winer]

Sploo,

> My only concern is that if I’m measuring at, say 60dB, and the RS meter only goes down to 50dB it won’t fully record any big dips. <

Good point. But I have to tell you, measuring your room with an SPL meter and test tones is mostly a waste of time anyway. Without even seeing your room I can tell you that it rings at all the mode frequencies, and the low end response varies by at least 20 dB and probably much more. What else would you like to know about your room?

> When hung on a flat wall like a picture, should they be mounted right on the wall, or with a batten behind to create a gap? <

A gap is always good, as was already explained. It's a free way of getting the equivalent of thicker fiberglass.

--Ethan

 

[Ethan Winer]

Sploo,

> WmAx, you have a pretty 'anti-snake oil' attitude to audio, what's your position on room treatment? For? Undecided? Sceptical? <

I know you're not asking me, but as the world's biggest audio skeptic I can't ignore that question. I've written on this exact topic for pro audio magazines, and I also have an article out now in the current issue of Skeptic magazine. Nobody is more opposed to magical tweaks than I am.

I assure you that room treatment is the polar opposite of audiophile snake oil! It is the one "tweak" that actually makes a huge difference. It is 1,000 times more useful than getting new cables, no matter how crappy your cables are now. The difference between a room that's treated properly and one that's not treated at all is more significant than the difference between any two loudspeakers.

When you're done with your project please come back and report here, to tell me I was right.

--Ethan

 

[WmAx]

Yes, excuse me while I engage my brain (it's been a long few days). The meter (if I remember correctly) was displaying around 24dB. Checking its specs shows it to be claimed capable down to 35dB +/- about 2dB across its range.

I do live in a fairly quiet area though, so I'd guess the noise floor is reasonable by general domestic standards.

35db is the normal low point of very quiet areas. 25db is possible, but it's just a very rare thing. However, as your db meter speced 35db as the lower limit of accuracy, the actual SPL would need to concluded with a calibrated device with a lower level of 25dB.

BTW WmAx, you have a pretty 'anti-snake oil' attitude to audio, what's your position on room treatment? For? Undecided? Sceptical?

Room treatments are true upgrades. The specific treatments that are optimal for you depend on your room, speaker polar response characteristics, preferences and budget(alternatively how much work you will do to DIY).

-Chris

 

[sploo]

Good point. But I have to tell you, measuring your room with an SPL meter and test tones is mostly a waste of time anyway. Without even seeing your room I can tell you that it rings at all the mode frequencies, and the low end response varies by at least 20 dB and probably much more. What else would you like to know about your room?

The exact tonnage of TNT required to rectify the problem

Seriously though, wood floor, walls of plastered then painted brick, and seven internal corners (odd shaped room and a fireplace) I'm pretty screwed really!

A gap is always good, as was already explained. It's a free way of getting the equivalent of thicker fiberglass.

But 2" of 703 against the wall would still be better than 1" with a 1" air gap right?

 

[sploo]

I assure you that room treatment is the polar opposite of audiophile snake oil! It is the one "tweak" that actually makes a huge difference.

Room treatments are true upgrades

Fairly conclusive argument. Chris (WmAx) has previously given me some very useful replies to my other threads in my quest to regain what I loved about my speakers.

Basically, in every other room I've used them (all of which have been much larger, and not in my house!) they put out a really nice clean sound (that I love). In my current room the result is very 'muddy' and unclear.

When you're done with your project please come back and report here, to tell me I was right.

Having built this: http://spikyfish.com/froggypirates/event.htm, throwing a few panels of board on a wall should be a doddle. If that gets the sound anywhere near to what it is in bigger rooms I'll be writing 'Ethan's right' in very large flashing characters

 

[Ethan Winer]

Sploo,

> But 2" of 703 against the wall would still be better than 1" with a 1" air gap right? <

Not by very much - really!

> Having built this: http://spikyfish.com/froggypirates/event.htm <

Wow. Very cool.

--Ethan

 

[sploo]

Not by very much - really!

Well, that would save a little cash I guess. I take it the lack of difference is due to the lower velocity near the wall? (see, I have been reading your docs )

Currently looking into panels from Knauf... assuming Knauf UK sells the same stuff as Knauf US. Owens Corning's European contact doesn't seem to want to send me any email .

Wow. Very cool.

What's even more amazing is that the deranged fool, sorry, friend, that sat in it when it went over a 12m (40ft) tall ramp (max 45 degree slope) still lives, and doesn't require feeding through a straw .

 

[sploo]

Right, an update, some (hopefully useful) information, and more questions.

Information (for UK based 'bass trappers')
Owens-Corning no longer sell in the UK, and will point you to Knauf (http://www.knaufinsulation.co.uk/). They don't sell glass mineral boards, but have a rock mineral slab at various densities, up to 100kg/m^3 (roughly 6.25lbs/ft^3).

It's called "Rocksilk Universal" and is available in 1, 2, 3 and 4" thicknesses, but is sold unfaced.

Knauf put me in contact with a local dealer (CH Insulation, Witney, Oxfordshire) who sell the high density product (RS100), and will also add a foil facing at a reasonable cost.

I was given the following absorption data, which doesn't look hugely different from other products, though why the 2" board is better for higher frequencies than the 3" beats me. They haven't tested the 4" product, so there's no figures for that thickness.

Thickness 125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz
1” 0.05 0.30 0.70 0.95 1.05 1.00
2” 0.35 0.85 1.10 1.10 1.15 1.10
3” 0.44 1.00 1.00 1.00 1.00 1.00

I should point out that I've not actually seen the board yet (I only got the information today).


Questions
The distributor assures me that the foil facing they use is a standard material, and should have the same acoustic affect on the boards as the Owens-Corning FRK boards.

I can understand why adding the facing reduces high frequency absorption, but why does it increase low frequency absoption?

I'm considering using 2" unfaced boards (plus an air gap) on the ceiling for first reflection points, as depth of material here is less intrusive. I take it that ceiling panels are at least as important as wall panels?

I now have ETF (www.acoustisoft.com) and RadioShack SPL meter. I plan test my room in the same way as here: http://www.ethanwiner.com/density/density.html. I'd be grateful for any tips on measurement or interpretation of the results if you think there's anything worth mentioning (though the page is pretty clear).

Cheers,

sploo.

 

[sploo]

Well, I've had a play with ETF, and these are the first results.

The first image (no panels) is the room, which has a wood floor, painted walls (brick and plaster) and is very sparse (a couple of chairs and a sofa).

The ringing is huge - to such a level I'm not 100% sure I've done the measurement correctly, though ETF reported the levels as being good. Peaks match some of the modal frequencies calculated for the room. And, yes, music sounds pretty rough.

I then put some thin compressed paper boards around the room (stuff I've been using when laying the floor), chucked some cushions around, put a duvet on the floor and closed the curtains. Nothing really good enough to absorb deep bass, but it was worth a try.

Different results... similar modal peaks... not much more to say I guess.

Time to get some fiberboards and do a decent before/after test.

 

[Ethan Winer]

Sploo,

> The ringing is huge - to such a level I'm not 100% sure I've done the measurement correctly <

Yeah, that does seem awfully long. The 25+ foot length might be a factor, but it still seems excessive. Did you connect ETF as shown in the image below? This is from the ETF Help file, but I added the Y splitter portion because it wasn't clear enough in the Help file.

--Ethan

 

[bpape]

That does seem awfully long but normally, if the cabling isn't right, you'll get a huge rise at the extreme bottom end (like continuing to rise off the scale at 20Hz). I don't see that here. It' might really be that long.