Basstrap - Jon Risch

[scott_01]

Hi all

Intro I have been planning to sort my room out for some time now and have finally got around to it this weekend. I've been reading anything (free) I could get my hands on and asked a fair bit at the HiFi Show (Thanks HiFi Addict). I'll be posting full details with measurements soon but for now here's the bass traps part

Bass Trap Types Looking at the commercial designs and reading the various eulogies on their websites I decided that the ASC tube traps would meet my requirements. However, they are commercial products and based in the US.

I found a design based on the ASC on this website:

http://www.geocities.com/jonrisch/a.htm

Starting yesterday I began to make a pair. As per Mr Risch's recommendations I decided on 14" diameter traps:

I used the following kit:

Breathing Mask +Gloves x 1
Router (could use saw / Jigsaw) x 1
Workbench x 1
G-clamps x 2
MDF 1m x 1.5m sheets x 3
Wickes Unfaced Insulation x 1 roll (170mm thick)
Polyester batting / Floor covering x 1 roll
Wire mesh rolls x 2
Tacks x 1 pack
Hammer x 1
Wire snips x 1
Zip Ties x 20
Hoover x 1

Step 1

First I drilled some centering holes and cut out all the pieces for 2 traps.

This is what the parts for 1 trap looked like:

Step 2

The wire is bent around the smaller sections of MDF to form a tube with 2 end pieces. These are 2" smaller than the larger ones which will be used as end caps and guides for the insulation. The smaller sections help add rigidity to the wire. The holes cut in the central piece are to ensure that there is equal pressure throughout the enclosure.

The larger pieces are fitted with glue, or you can just nail them down with the splayed out ends of the wire tube like I did.

Step 3

A section of wire is wrapped around the skeleton matching the diameter of the large end caps. This is then laid on the ground and the insulation laid on top.

This is by far the trickiest part. I then placed the skeleton on top and rolled the insulation round, tacking the wire onto the end caps as I went.

The best way of ensuring the wire joins tightly is to use zipties / tiewraps / cable ties:

Step 5

Wrap the bass trap in the polyester batting to contain the insulation. Note I haven't done this on the photo yet. Then start again for the next trap...

Cost

Insulation: 33.85 (Same price for 2 rolls but I only needed one)
Wood: 22.00 (B&Q rip off this one, Travis Perkins or Mica DIy would have been cheaper but I wanted it then and there)
Wire: 25.00
Zip Ties: 2.95
Tacks 1.99

Total 85.79 GBP - Bargain? I think so.

I haven't included tool costs because I already had them or ( in the case of workmate) will be using for lots of other things too over the next few years.

Sound

Placed in the corners of my room behind each speaker the effect is not subtle. There seems to be a vast improvement in bass definition and tone. The mid range has cleared too, the upper mid and treble needs some more attention and details of that will come later.....

 

[I-S]

Great stuff.... Look forward to seeing the measurements.

Can you not get some speaker grille cloth to cover them with to make them a little more domestically acceptable?

 

[Tenson]

You can cover them with any cloth since its mostly low frequencies they absorb.

How many did you make? You can't really have too many

 

[SteveC]

Nice work! I remember reading about this some time back. IIRC the trap needs to be stuffed or packed to the right density - did you manage to do the translation from American-sourced fibre to the Wickes stuff you got OK? What does one need to know in advance about the stuff one buys locally?

 

[Dr Rock]

Glad you noticed a difference! Room treatments really do work....

 

[scott_01]

Great stuff.... Look forward to seeing the measurements.

Can you not get some speaker grille cloth to cover them with to make them a little more domestically acceptable?

Yes, I'll be doing that soon, probably with a little help from my sister. They are a bit rough at the moment.

How many did you make?

Just the two for now, although I think another two (one for each remaining corner) will do the trick eventually.

IIRC the trap needs to be stuffed or packed to the right density - did you manage to do the translation from American-sourced fibre to the Wickes stuff you got OK?

Risch says to use 6" rock wool compressed to 2". I can't seem to get rock wool over here and this insulation was pretty close in thickness at 170mm. Not very scientific I know, but I just wanted to have a go. Besides there were only so many strange looks from DIY salesmen I could take. "What grade is that? You see, I'm building a Bass Trap"

Great stuff.... Look forward to seeing the measurements

That will take me a bit longer as my Presonus Firewire box has decided to crash my machine every time I activate it. I need to investigate alternate Mic pre-amps, and spend some time on the ETF forum too. It's quite complex once you drill beneath the surface.

www.acoustisoft.com

 

[SteveC]

Thanks for the clarification about the stuffing density (it was me)

 

[Tenson]

You might find that rather than having one in each corner, you get better results from having two stacked in each corner nearest the speakers. I certainly found that in my room. Closer to the speakers, more effect.

With ETF you will want to do a full range 5 second MLS measurement and look at the 3D low frequency waterfall plot. That will show the changes most easily.

Rockwool comes in different densities, so 6" or 2" doesn't really say a lot. 3lbs per cubic ft (46 kg meter cubed) is about optimal but you'd probably have a hard time rolling it up.

 

[DavidF]

You might find that rather than having one in each corner, you get better results from having two stacked in each corner nearest the speakers. I certainly found that in my room. Closer to the speakers, more effect.

With ETF you will want to do a full range 5 second MLS measurement and look at the 3D low frequency waterfall plot. That will show the changes most easily.

Rockwool comes in different densities, so 6" or 2" doesn't really say a lot. 3lbs per cubic ft (46 kg meter cubed) is about optimal but you'd probably have a hard time rolling it up.

Is rock wool the stuff to use then Simon?

 

[andyoz]

With ETF you will want to do a full range 5 second MLS measurement and look at the 3D low frequency waterfall plot. That will show the changes most easily.

It can be a real eye opener to see the room modes clearly displayed on a 3D plot.

Post them here when you get a chance.

 

[Tenson]

Is rock wool the stuff to use then Simon?

Depends what the design calls for! As I say, in this case I suspect it may not be feasible to roll it up, in which case it simply won't work in the design. It is an excellent sound absorber though, yes.

 

[scott_01]

Room Mesurement Graphics

Okay, finally I have sorted out

1. A laptop that doesn't sound like a motocross bike (my PC Does) so I can measure the sound field and get a higher than 15db S/N ratio.

2. My pre-sonus soundcard, what a pain in the arse that was.

3. ETF. It is pretty intuitive but there are a couple of small bugs and a knowledge of acoustics helps a great deal. I don't have that so I went on the ETF support forum and did some reading. The most useful bit for me was an explanation of excitation times and the lengths required for adequate bass measurements.

3. Most of my room.

The Story So Far

Okay, for a quick re-cap, 1st thing I did was build those bass traps shown above and put them in the corner next to each speaker. There was a real difference straight away.

What I did next

I went back to Wickes and bought 4 packs of Heavy density insulation, some wood, screws and no more nails. I also bought some of that see through matting you put on plants to stop them getting chilly in the winter.

I made some panels to put at 1st reflection points and across the back wall to prevent comb filtering.

I'll detail construction later with some pictures.

Results

1. With two 90mm thick panels at each 1st reflection point, another 4 on the corners opposite the speakers and two on the wall behind my listening pos. I was disappointed. The sound was truly 'dead' and the bass wasn't great either, considering the effort.

2. I removed the 1st reflector panels and placed them to one side. I put only 1 panel behind me, of 60mm thickness. This improved the mid / treble. It seems that the panels themselves provide a reasonable amount of absorption at mid / high freqs without trying to hammer the 1st reflection points.

The Big One

4. Bearing in mind what Tenson says a couple of posts above I set to constructing some braces for my panels to allow me to stack them on top of each other. I thought I'd load up on the area behind each speaker. I also did some ETF reading and adjusted my measurement technique accordingly.

So here goes:

All measurements at the same SPL (obviously)

6 sec excitation time
1.3 sec pre-excitation time
Gate time 259ms
All figures are gain normalized (reduces the level so you can see it all on the graph).

1. An empty room with no Bass traps.

2. Bass traps spread amongst all corners (panels placed diagonally across the corners)

The Big One

4. Bearing in mind what Tenson says a couple of posts above I set to constructing some braces for my panels to allow me to stack them on top of each other. I also did some ETF reading and adjusted my measurement technique accordingly. Once I'd done that I placed a big 4 panel stack parallel to the biggest corner behind my speakers. I placed the other two John Risch style traps behind the left speaker. They are low enough to get under my TT Shelf.

The sound started to come together here, this was the biggest improvement since actually putting the traps in.

Next step

I moved the traps about 8 inchs from the rear and side walls and also placed a panel horizontally behind my equipment rack. There's a stone fire place which is corners galore. My limited knowledge tells me that corners mean bass peaks. This seemed to improve the measured side a great deal.

Just to clarify. Once I'd done all this I took them down and re-measured the whole thing using the same measurement location and measurement set up. The one external influence that I think did influence things was the huge Que of buses outside my window due to roadworks. I can hear them though the protective glazing.

Sound

A huge improvement. Bass notes on some previously troublesome records really come out. Every record sounds better. I can hear the resonance of the bass much more clearly. 'Holy Holy' on Aretha Franklin's 'Amazing Grace' now has a real bass guitar playing on it, that sounds different as the player alters his style from teh intro to the chorus. 'Concrete Jungle' has real impact and is missing all the low end smear that merged the notes into one. My original copy of 'What's Going On' has just got it's bass back too.

It is also true that improving the bass makes the rest of the music better. The large area of new absorptive material obviously also plays a part and soaks up some of the high freq energy in the room. Sound stage and imaging is much better. Indeed, I was prompted by the improvements to move my speakers about 8 inches. I couldn't hear much of a difference before but I can now.

This post is too long. I will follow with some pictures, I think there may be room for some more improvements.

 

[scott_01]

Freq Response Plot Pictures

All the shots below show the frequency response that corresponds to the waterfall (decay) plots above.

AN UNTREATED ROOM

BASS TRAPS IN ALL CORNERS

BASS TRAPS CONCENTRATED BEHIND THE SPEAKERS

BASS TRAPS BEHIND THE SPEAKERS AND OVER FIRE PLACE*

This room is most probably NOT waf friendly. Not something I have to worry about though.


* I obviously don't use the fire.

 

[bemcsa]

Interesting post as I have been trying to do pretty much the same this weekend (the weather has been crap here too!). I bought a pack of the Wickes high density fibre board (about 200kg/m3) and one of the medium density (45kg/m3) board.

I put two sheets of the medium density board together with one sheet of the high density per panel. I placed the high density board at the back hoping it would help trap the lower frequencies and to give the panel some rigidity. This made 6 panels of 1200x600x120mm. I stacked 2 across the corners behind the speakers that went from floor to ceiling. The other two went in the first reflection points. At the moment they only have some old sheets covering them so I don't have fibres floating around the room.

Unfortunately they made little difference to the lower frequencies (below 200Hz), so I would be interested to know what design you found most successful. Do the tube traps or the panels placed across the corners have the most effect?

I then put my Behringer deq2496 back between the transport and dac; and this together with a Velodyne DD10 provided a reasonably flat curve down to 20Hz. However digital EQ only addresses some of the room issues, so I will be interested in trying any successful solutions that you find.

 

[scott_01]

Hi there.

I think that there are two things that have helped me treat sub 200Hz.

1. The high density board, both the location and the quantity of it. As Tenson mentions further up here the density does have a large impact on what you can absorb. The thicker the high density material you have, the better.

2. The air gap between the panels and the wall. It is now about 8 inches over the whole range of the panels.

Try having a read of this:

http://www.ethanwiner.com/acoustics.html

scroll down a bit and you will see a table about material density and another about spacing and how they affect the absorptive properties of your panels.

I was getting pretty disillusioned with the whole business until this week. Persistence and a bit of reading paid off. One point worth noting is that my Revel M22 are only rated from 45Hz - 16 Khz, hence the huge dip just before the 45Hz point.

I've thought about the Beringer before, does it have a low pass filter and can it be put through the tape loop of an integrated?

On the treble front I'm thinking about some diffusion. This apparently works better than absorption in small rooms. I can see the logic in this having experienced an 'overdamped' room. I need something that isn't too trying to make and that is light enough to hang on the wall.

 

[ShinOBIWAN]

Excellent work Scott. Those look great.

 

[Mr_Sukebe]

I have to say that this to me suggests a genuine business opportunity.
Dealers might well struggle to sell kit when it can be bought from the internet, but offering a service to measure and suggest solutions to resolve room issues is something that just doesn't really exist right now, and could most certainly be done by someone with some expertise.

Simon, do you offer this type of service, if so what's your charges?

 

[Tenson]

Hi Mike,

What you are talking about is simply hiring an acoustician to sort out your room. I don't offer it 'officially' but if someone asks and, most importantly I can get to them by public transport (I don't drive due to illness) then I would charge about £150-200 + travel for what can be done in a day of measurement and set-up. This is usually fine for setting up room EQ or active crossovers.

Given a set of pictures of the room and talking with the owner it would be possible to suggest a shopping list of treatments without even visiting. The time of the visit could then be spent setting up and optimising the use of said treatments. What ends up not being used (if any) can be sent back if the supplier is okay with that.

 

[andyoz]

It is a good business opportunity.

The rates professional acousticians charge can be very scary for residential clients (£80-100/hour ). The industry needs a service that lies somewhere in between the snake oil salesman and the professional acousticians (who have the overhead of high Prof Indemnity Insurance, £10,000 calibrated sound level meters, etc.). You should seriously pursue it further Tenson as the business will all grow on referrals.

 

[Tenson]

The biggest problem is that most people, audiophile or not are not willing to have acoustic treatments in their room in the quantity and position most needed. Either that or they blame the other half

Also for myself at least, as I don't drive and its not really safe for me to do so, it is not very easy to visit a lot of people. This is why I decide if I am willing to do it on a person by person basis.