Room Treatment

[Tenson]

Have you considered building some deeper panel absorbers on the walls directly behind each speaker? Would you be able to pull the cabinet forward 6-7" to fit them in? That would be a brilliant place to have them, acoustically.

 

[ShinOBIWAN]

Shin,

Do you know the resonant freq. of your 4mm ply on 3" cavity membrane absorbers. I suspect they could be well above 100Hz (probably 120-140Hz) and if you're not careful, you could have a nasty "suck out" of energy in the lower midrange with very little effective low-frequency control. Membrane absorbers can be savagely efficient.

Just to be specific, its 3.5" cavity with 3" RW5 and 0.5" air gap. There's 4 panels in total. Largest is tuned to 85hz. And others cover up to ~220hz where the 3" RW5 takes over since its effective from that point.

Key to good performance and ensuring the resonance of the panel isn't raised significantly by the fixing method is to use something will elastic and sealing properties. I used a bead of seal string run around the perimeter then push the panel on to that. The string is quite sticky and I could have risked leaving the ply attached without fixings but I did *gently* put in 4 screws just to hold against movement.

These are tips from Rainer BTW and not from any investigations I've done.

I bought the seal string from a local plumbers merchants but basically the same stuff as this except these folks are charging a fortune for it:

http://www.crazypc.com/products/9270.html

Can you generate some test tones? Place you hand over the membrane absorber and vary the frequency until you feel the panel really get going, that's your resonance.

Thanks I'll give that a try.

 

[Tenson]

Just to be specific, its 3.5" cavity with 3" RW5 and 0.5" air gap. There's 4 panels in total. Largest is tuned to 85hz. And others cover up to ~220hz where the 3" RW5 takes over since its effective from that point.

Key to good performance and ensuring the resonance of the panel isn't raised significantly by the fixing method is to use something will elastic and sealing properties. I used a bead of seal string run around the perimeter then push the panel on to that. The string is quite sticky and I could have risked leaving the ply attached without fixings but I did *gently* put in 4 screws just to hold against movement.

These are tips from Rainer BTW and not from any investigations I've done.

I bought the seal string from a local plumbers merchants but basically the same stuff as this except these folks are charging a fortune for it:

http://www.crazypc.com/products/9270.html



Thanks I'll give that a try.

Nice tip on the seal string. I always used caulk and waited for it to set a little bit before fixing the panel.

From experience, I think the 3.5" cavity with 4mm ply will be tuned higher than 85Hz. More like 100Hz-120Hz. The 4mm ply is too light to resonate that low. 85Hz is probably the lower cut off point. I will be interested to hear what happens when you try some test tones and see where it goes wild.

As you have DRC its not really such an issue anyway. Below 100Hz DRC can be extremely effective, as you know.

I have actually found that large Helmholtz resonators are one of the most effective ways to treat sub 100Hz resonances. They need to be tuned with a mic at their mouth though, as they are very fussy about exact build and theory doesn't get you right there.

 

[ShinOBIWAN]

From experience, I think the 3.5" cavity with 4mm ply will be tuned higher than 85Hz. More like 100Hz-120Hz. The 4mm ply is too light to resonate that low. 85Hz is probably the lower cut off point. I will be interested to hear what happens when you try some test tones and see where it goes wild.

You might be right Simon. All work has been based on theory and I'm slightly ashamed to say I've done zero testing or measurements throughout the build. I knew the advice was coming from a very knowledgeable source and trusted in that. However we know from loudspeaker building that model and reality don't always match up.

If you look at the FR measurement above I have a nasty axial mode at 98hz and a tangential at 77hz. DRC used in these measurements makes the situation look better than it really is. The aim of this panel trap at 85hz was to help deal with these two problems. However even if its 100hz I'll be happy since that catches the nasty axial one at 98hz. As you say, I use DRC so that will work where the physical treatments don't.

I have actually found that large Helmholtz resonators are one of the most effective ways to treat sub 100Hz resonances. They need to be tuned with a mic at their mouth though, as they are very fussy about exact build and theory doesn't get you right there.

Interesting but probably impractical for such a small listening space as mine. I think what I have regarding bass treatment is about effective as I can go without starting to eat up the already limited space within the room. This was one of the hardest things to balance with Reiner. In the end, and as always with audio, it was a compromise between performance and size. I think we hit the sweet spot with what we had to work with.

BTW Simon, not too long back I built some new amplifiers based on aussieamps NXLSP400se modules. These are excellent performers and I think one of the best amps I've had pass through my system. Certainly keepers. They're balanced input and the output is DC protected too. If your looking for an amp project in future than I'd highly recommend these.

 

[andyoz]

Thanks I'll give that a try.

It's harder to feel when the membrane absorbers are tuned higher in freq. Place your hand across the membrane very, very softly of curse so as to not damp it.

 

[Tenson]

Interesting but probably impractical for such a small listening space as mine. I think what I have regarding bass treatment is about effective as I can go without starting to eat up the already limited space within the room. This was one of the hardest things to balance with Reiner. In the end, and as always with audio, it was a compromise between performance and size. I think we hit the sweet spot with what we had to work with.

In one of the acoustics books I have I remember the author saying that this is the biggest problem in acoustic design. Owners want to see the floor space they paid for! However, you have to decide if you want a medium size room that sounds bad, or a small room that sounds big. My room is also pretty small, only 10ft x 12ft. In fact that is the bigger secondary listening room. My main listening room is also my office and thats about 11ft x 9ft. Both sound good though

You don't have to build super huge Helmholtz resonators. I have a couple that are about 30cm x 60cm x 15cm. They are tuned to 65Hz. Build them like a slot ported speaker cabinet. You can even use WinISD to calculate the frequency. A box with a volume of about 15L could be a very effective absorber right down to about 40Hz. Adjusting the internal damping and port length to get the right frequency and Q is the tricky bit, but its not impossible if you can open it up again to change damping.

I'm still using my regulated SKA amps and happy with them.

 

[RobHolt]

Christ, this man doesn't do things by halves!

Looks superb - well done sir.

 

[ShinOBIWAN]

In one of the acoustics books I have I remember the author saying that this is the biggest problem in acoustic design. Owners want to see the floor space they paid for! However, you have to decide if you want a medium size room that sounds bad, or a small room that sounds big. My room is also pretty small, only 10ft x 12ft. In fact that is the bigger secondary listening room. My main listening room is also my office and thats about 11ft x 9ft. Both sound good though

You don't have to build super huge Helmholtz resonators. I have a couple that are about 30cm x 60cm x 15cm. They are tuned to 65Hz. Build them like a slot ported speaker cabinet. You can even use WinISD to calculate the frequency. A box with a volume of about 15L could be a very effective absorber right down to about 40Hz. Adjusting the internal damping and port length to get the right frequency and Q is the tricky bit, but its not impossible if you can open it up again to change damping.

Sounds like a future project once I get the bug to improve things further. Thanks for the advice.

I'm still using my regulated SKA amps and happy with them.

Ah yes, I'd forgot you had built those. Another very nice amp, I wouldn't rush to swap either.

Here's one of the pair of new ones:

 

[ShinOBIWAN]

Christ, this man doesn't do things by halves!

Looks superb - well done sir.

Thanks Rob.

 

[scott_01]

Shinobiwan

Well done, I'll bet it'll sound fantastic. Makes my efforts look positively agricultural, but then again, they are.

When I eventually own a place I'll be trying the same thing. Probably with some more modest speakers though.

P.S. Where did you get the case for the Aussie Amp?

 

[ShinOBIWAN]

Thanks Scott. Enjoy the tube traps.

The amp case is a home theater PC case that was easy to convert. I bought mine from here:

http://www.kustompcs.co.uk/acatalog/mCubed_HFX_Passive_HTPC.html

 

[ShinOBIWAN]

Simon/Andy

I just took my first listen to a single speaker in the room and its not good below 100hz. Awful actually. Seems worse than before.

I've been running sines.

There's something weird going on at around 50-90hz and its at its worst around 70hz. The bass at the listening position is very strong and strangely appears not to come from the speaker but from the rear of the room as opposed to the front right where the speaker is located. Also just turning your head from looking straight forward to looking 90degrees right makes the bass completely dissappear! But move your head straight or to the left and it comes back.

I played music and it sounds crap. Loads of boom and that droning quality which saps any long term listening enjoyment.

DRC does nothing to shift this as it doesn't even appear to be a hump in the response.

I've but a fairly steep low pass filter in at 150hz and listened. Despite the lack of bass it sounds pretty amazing for just a mono playback but the bass issue kills it.

Any thoughts? Rainer is away on holiday with family over Christmas so I can't contact him unfortunately, I'm hoping you guys can help.

BTW the lowest tuned membrane panel seems to resonate most at 105-110hz. Which I guess means it has zero effect on this problem.

 

[Tenson]

Hi Ant,

Have you had the speakers in that position and room before? I assume it was all like this before, only without the panels and sounded okay?

One thought is that the air-tight fabric and closed cell foam on the upper panels is resonating at this frequency and then releasing the energy again. Does the front of those absorbers resonate at all when you place your hand gently on it?

I also wonder if it is an effect of only using one speaker. I always EQ bass as a stereo pair, because both speakers interact with each other. It may sound entirely differnt with both speakers.

You could tune the bass panels lower by swapping the 4mm ply for 6 or 9mm. I'd go with a combination, but no 4mm.

A short decay time in other ranges can act to highlight other areas where the decay is longer. It might be that the bass and mid panels are absorbing a lot above 100Hz and making the rest stand out.

Some to think about there I hope. Don't worry though, I'm sure you can track the issue and sort it out

 

[ShinOBIWAN]

Thanks Simon, your right that the measurements for before and after were taken with the same approximate positioning.

I took measurement to see what was going on:

WFT? Its worse everywhere. Its like I went the other way and downgraded the room acoustics.

I'll check the fabric for vibration as you suggest, perhaps the whole lot is going crazy.

I'm now wondering if putting the speakers back into the alcove formed by that cabinet has done this. I stick my head in there and there doesn't seem to be much going on but perhaps its projecting out into the room or something. This is what I mean about the alcoves:

 

[Tenson]

So the speakers were in the same place before when it sounded okay, but that cabinet wasn't there? Seems probable that could cause a problem.

Unfortunately you won't know unless you take it out the room!

I do find it odd that the RT time has gone up in the mid-range as well. I notice you have the start time of the impulse response gate set earlier on the second measurement. That could account for some of the increase in decay time on the graph through out the midrange? As for the 100Hz'ish bass it does look like a fabric resonance to me, or that the bass panel... panels are not damped well enough but it looked fine from your construction pictures. Was the frame of the bass panels made airtight to the wall so the whole thing forms an air tight cavity?

 

[andyoz]

I haven't had time to read you last few posts fullt but you've discovered one of the problems with membrane absorbers...they can be a bitch to get to work well and when they go wrong, it can cause mayhem with a rooms response.

Your membranes sound like they are tuned too high for my liking.

I only ever ettempt to use membranes down low (<80Hz) and rely on "slotted" aborbers for the 100-200Hz region IMO.

 

[ShinOBIWAN]

So the speakers were in the same place before when it sounded okay, but that cabinet wasn't there? Seems probable that could cause a problem.

That's correct. Its likely contributing something but I feel there's also something more at work here. I walk around the room and the resonances at 50-100hz are very consistent, amazingly so actually - there's virtually no shifting or cancellation at all around the center of the room and only when you move close to the walls do you hear things change. My money is on the panels and/or fabric that's causing this since they're everywhere.

Unfortunately you won't know unless you take it out the room!

I'm might try getting a hand to lay it down on the floor and see if that makes any difference. Thing is bloody heavy and awkward though.

I do find it odd that the RT time has gone up in the mid-range as well. I notice you have the start time of the impulse response gate set earlier on the second measurement. That could account for some of the increase in decay time on the graph through out the midrange? As for the 100Hz'ish bass it does look like a fabric resonance to me, or that the bass panel... panels are not damped well enough but it looked fine from your construction pictures. Was the frame of the bass panels made airtight to the wall so the whole thing forms an air tight cavity?

Yes I cannot figure this out either.

Let me say I'm 100% convinced that above say 200hz its a substantial improvement in detail and overall smooth clean sound than before room treatment but the measurements say otherwise. I don't understand how they can show bad bass and my hearing agrees with that but yet at the same time, everything else sounds greatly improved but looks worse!

I do have one idea and you may be thinking the same as you read this - Software and/or measurements errors.

I will email Uli who wrote STransform(used for the waterfalls) as well as Acourate(the crossover+DRC software I use). STransform uses stockwell tranforms and before now, I have never attempted to use this type of transform to display waterfalls and instead relied on the usual fourier transform. Maybe I'm chopping up the impulse wrong or it needs to approached differently.

What I definitely do know is that it sounds better in the mids and highs but worse in the bass after treatment and I don't need measurements to tell me that. So I think I've messed up with this new software somewhere or maybe even the measurement itself. I will look more closely into this because reliable data is a must otherwise we find ourselves chasing ghosts.

For now I will revert back to software I'm familiar with such as ARTA. I'll post measurements again simon for you to take a look at.

BTW thanks for the help so far, its much appreciated!

 

[ShinOBIWAN]

I haven't had time to read you last few posts fullt but you've discovered one of the problems with membrane absorbers...they can be a bitch to get to work well and when they go wrong, it can cause mayhem with a rooms response.

Your membranes sound like they are tuned too high for my liking.

I only ever ettempt to use membranes down low (<80Hz) and rely on "slotted" aborbers for the 100-200Hz region IMO.

Andy the thought of having to rip half the treatments apart is not appealing.

However if I find it is them then I'll be looking at alternatives regardless of the effort.

 

[ShinOBIWAN]

OK I mailed Uli and got a swift reply. I had messed the plots up. The program STransform works by taking the length of an impulse to transform and then has a window to be passed over this. I was only setting the length and not the window as well. Simple to fix luckily.

I had also made the mistake of mixing sample rates. My 'before' measurements were taken at 44.1Khz but the after were taken at 96Khz. This may seem harmless but you need to double the area of the window in 96Khz to effectively see the same resolution as at 44.1Khz. Look in the impulse slider on the pics below to get an idea of what I'm talking about.

Anyway the upshot is my hearing isn't faulty and yes there is now showing a nice improvement from 100hz and up. However the I was also right about <100hz and its a fair bit worse than before treatment.

Here's a couple of plots at shorter and longer window times:

PS. Y axis = frequency but the numbers are chopped off. They are from top to bottom, 100hz, 1Khz, 10Khz.

Before treatment:

After treatment:

Before treatment:

After treatment:

Argh... I have to leave for work in a couple of minutes but does anyone have further idea's based on this? It seems the panels are working where they should ie. 100hz-200hz and the fibreglass is doing a nice job above that. Can the panels still work at reducing resonances where they are intended to work but also boost at even lower frequencies? This doesn't seem possible to me.

Maybe the theory about the fabric itself flapping is the cause or maybe the cabinet? Or perhaps your right Simon about the rest of the frequency range making these low resonances stick out even more.

Any idea's on a fix. It looks like I'm going to need some big guns to fix this problem.

 

[Tenson]

Nice to see the low frequencies come in earlier than the high frequencies. You don't get that on a normal speaker! Its clear you are using FIR filters.

The bass issues are either from the cabinet, the fabric or the bass panels. I can't think of any short cut to find out which - take them out one by one.