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

[sploo]

OK, well, since my last posting, I've realised I'd bolloxed* up my amp setting: I'd forgotten to set my receiver to 'direct' mode, hence it was adding positional delays that resulted in ETF reporting weird mic to speaker distances.

I have the following connection differences to the diagram Ethan posted:

1) I don't have a mic pre amp + mic, as I'm using the RadioShack SPL meter. It looks like you were using the same thing here: http://www.ethanwiner.com/density/density.html?

2) I've not used a splitter at the amp, so I'm just driving one speaker. I figured that this wouldn't be a problem, and in fact may make it easier to take certain measurements (like recording reflections). Should I really be using both front speakers?

The cables I'm using are the finest 'got nothing suitable, bodge some connectors together to make stuff(TM)'. I could probably borrow some better ones from work tomorrow, but I'd be surprised if my cables were that bad.

If I only show 300ms, I get 'nice' sharp peaks like the graphs of the untreated room on Ethan's site, but there's still lots of noise well past that (and continuing to 1000ms). Very strange...

I must just say a big thankyou to Doug Plumb (the author of ETF) who's been taking the time to answer the many questions I've been sending him over the past couple of days.

* can I get away with using rude British slang on a US forum?

 

[Tom Andry]

* can I get away with using rude British slang on a US forum?

Hey, your screen name is Sploo - I figure that gives you the right to use whatever slang you want

 

[sploo]

Hey, your screen name is Sploo - I figure that gives you the right to use whatever slang you want

It's what happens when you spend a few years at uni - drink all day, then go and try using the uni computer system. I didn't realise until the next day (looking at the logs) why I was unable to spool the doc I wanted. The name's stuck ever since

 

[Ethan Winer]

Sploo,

> I don't have a mic pre amp + mic <

Of course - an RS meter is both mike and preamp.

> I've not used a splitter at the amp, so I'm just driving one speaker. <

In 99.9 percent of music bass instruments and other low frequencies are panned to the middle. So sending ETF to both speakers more closely emulates the response you'll get when playing recorded music. Most pro acousticians suggest measuring L and R separately too. But for me L+R is the most useful.

> The cables I'm using are the finest 'got nothing suitable <

Oh geez...

Get a Y splitter and a couple of $5 cables from Radio Shack. I promise you they are more than up to the task.

--Ethan

 

[sploo]

To be honest, I don't think the cables I was using were that bad, but obviously, no splitter.

I've grabbed some gear from work and will try it when I get home... soon... it's well after 6pm here...

It's occurred to me that I'm probably running the test too low (considering the insensitivity of the RS meter). I'll annoy the neighbours shortly.

BTW Scanning your site for every bit of info sometimes results in unexpected links. Not normally a cello man, but I quite enjoyed your "Cello Concerto in A Minor". Quite partial to a bit of Junko Urayama's piano playing personally.

 

[sploo]

Yep, the sound level was too low. Because of the 'boomy' nature of the room's sound, and the fact I have neighbours, I tend to be a bit cautious with the volume control. Too cautious in this case.

I've also got both speakers running now, and the results are attached.

Surprisingly, apart from the slow decay at 43Hz, most of the decays are no worse than the graphs Ethan posted on his density page.

I must admit I wasn't expecting this as, whilst 'quiet' music (acoustic guitar etc.) is OK, playing something like Rage Against The Machine's 'Bombtrack' is extremely unpleasant - boomy, one-note bass and very unclear.

I've now obtained some material samples for covering traps, and will get the fiberglass soon.

mrnomas, what weight batting did you use? I've got a sample of 2oz, but there are many others. BTW For UK buyers, some places refer to batting as 'wadding'.

My only concerns now are that my room is an ackward shape, and not all corners are going to be easy to get a trap into (curtain poles, window sills etc.), but I'll work round that.

The biggest problem is that there's a large window at the end of each room (behind/in front of the speakers), so it's going to be pretty much impossible to put anything on those walls.

 

[Ethan Winer]

Sploo,

> I quite enjoyed your "Cello Concerto in A Minor" <

Thanks!

> Surprisingly, apart from the slow decay at 43Hz, most of the decays are no worse than the graphs Ethan posted on his density page. <

Yes, your decay times are not bad, but the peaks are very narrow. And that's something else traps help.

--Ethan

 

[sploo]

Yes, your decay times are not bad, but the peaks are very narrow. And that's something else traps help.

I certainly hope it'll help. I'm just a little concerned about that huge 43Hz spike, as I suspect it's too low for the traps to tame it much.

Would adding another layer of board help (4" of foil faced with 4" of plain behind)?

Just a final sanity check - for the side/ceiling first reflection absorbers, is there any reason/preference for using a lower density board? I ask because the data I have for the plain faced high density (6.25lbs/ft^3) 1" or 2" thick boards show them to be good for higher frequencies.

A graph of the room's full requency response shows, what I understand to be, comb filtering, so presumably that'll be helped a lot with side and ceiling absorbers.

Many thanks.

 

[Ethan Winer]

Sploo,

> I'm just a little concerned about that huge 43Hz spike <

That's not as big a problem as you might think because there's very little music down that low.

> Would adding another layer of board help (4" of foil faced with 4" of plain behind)? <

You bet. The thicker the better.

> for the side/ceiling first reflection absorbers, is there any reason/preference for using a lower density board? <

Yes, lower density is better for mid/high frequencies.

--Ethan

 

[sploo]

That's not as big a problem as you might think because there's very little music down that low.

Fair point. I suppose if it still ends up being a problem (movies, some very 'bassy' electronic music) then one of those equalisers that Chris (WmAx) has mentioned might help. One step at a time though...

You bet. The thicker the better.

OK. I'll have to work out a way of holding more board, but I don't think it should be a huge problem.

Yes, lower density is better for mid/high frequencies.

Most interesting. I'll make sure I use the lower density boards.

I've just done a 'mirror test' and found I'll need quite a few boards (6'x4' on each wall and 8'x4' on the ceiling) to cover all first reflection points for the front three speakers, for the row of seats in my listening area. I don't see any particular problems building them as single large frames with multiple 1" unfaced boards. Just checking that doesn't ring any alarm bells with you?

Thinking more generally, in your many articles, do you have anything that explains why lower density boards are better for mid/high absorption, and why adding a foil facing to a board increases bass absorption (or could point me to some material to read)?

As you rightly point out - far too few people consider the acoustics of their room, and whilst I'm still a complete novice at this, it's a fascinating subject and I'd like to learn more.

Cheers!

 

[Tom Andry]

mrnomas, what weight batting did you use?

Without the benefit of the receipt (my wife's filing system is a little....random) I'm pretty sure this is what I used. Foil faced and 2" thick. Hope this helps.

 

[sploo]

Hope this helps.

It does, thanks!

I've just bought some sheets of a similar material, in 1" and 2" thicknesses, un-faced, for high frequency absorption. I'm currently waiting for the 4" high density, foil faced, stuff for the bass traps.

A couple of questions...

I noticed that you made the corner traps with 1' wide boards, so the depth behind them isn't so large. Did you find this still helped smooth out the low end response?

I plan to try and stay near the full 2' board width, but I'm going to have to compromise in a few corners, so it'd be nice to know that a smaller width board is still worthwhile.

Secondly, what were the specifics of the material you used to 'seal' the boards? I've found batting/wadding in weights of 2, 4 and 8oz. The 2oz stuff is thin, but looks like it would be perfectly suitable, but I was just wondering if you used something a little thicker.

Cheers!

 

[Tom Andry]

I noticed that you made the corner traps with 1' wide boards, so the depth behind them isn't so large. Did you find this still helped smooth out the low end response?

Two things - I would have preferred to use the full 2' width but space constraints wouldn't allow it. Second, I am considering building two more corner traps for the front at the full 2' width and using 4" of fiberglass in them. I will probably also bolster the original traps with 2 more inches of fiberglass.

As for as smoothing out response - it definitely helped - but I need more. With a nearly square room it took the response from unbearable to alright. I still have problems but things are much better than before.

 

[Ethan Winer]

Sploo,

You should know that every time I print a document and the Windows Print Spooler icon appears in my System Tray, I now think of you.

> do you have anything that explains why lower density boards are better for mid/high absorption <

Sure, the surface is simply less reflective because it's softer and more like fluffy fiberglass.

> why adding a foil facing to a board increases bass absorption (or could point me to some material to read)? <

The foil coated paper serves as a damped membrane. I'm not into physics deeply enough to explain it in more detail.

--Ethan

 

[sploo]

As for as smoothing out response - it definitely helped - but I need more. With a nearly square room it took the response from unbearable to alright. I still have problems but things are much better than before.

Good news.

I think I'll be able to do a couple of corners near 2' width, (and 8' height). Most are probably going to be around 18" wide.

I reckon I should be able to trap five of the seven (yes, seven) wall-wall corners of my room, and I'm going to try to get one along the rear wall-floor corner behind the speakers.

 

[sploo]

You should know that every time I print a document and the Windows Print Spooler icon appears in my System Tray, I now think of you.

Please accept my condolences. I'm afraid I can't afford to pay for the counselling you're going to need.

Sure, the surface is simply less reflective because it's softer and more like fluffy fiberglass.

That sounds logical. Well, I'll take the boards home tonight, and hopefully a simple ETF sweep test will show some improvement in comb filtering (with the boards just resting on the wall and floor).

The foil coated paper serves as a damped membrane. I'm not into physics deeply enough to explain it in more detail.

Hmmm. Most interesting. Time to speak to a mate who's a physics buff. Either that or just buy some rolls of vinyl flooring!

Thanks again.

 

[sploo]

Certainly a good start...

I rested the six 1" boards against the side walls, over the first reflection points I'd marked. The four 2" boards went on the floor, directly under reflection points I'd marked on the ceiling.

Obviously there's no gaps behind, so I wasn't expecting miracles.

Well, the sound has certainly improved. Of course, it does nothing for the boomy bass, but music sounds a little more focused, or, to put it in a non-audiophile BS way, you can now sit and listen to a RATM track without feeling ill.

A long way to go, but here's the graphs from last night; quite a bit of difference in the 8-12kHz range. An ETC graph also showed me that the energy in the room is dropping much faster.