Bass Traps

Tenson

Tenson

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So, I got a new amp today - Cayin A-88T :D

Lovely sounding amp, however compared to my Cyrus 3 it is a tad slower in the bass and 1.5 db louder at 54Hz.

So now I am thinking I would like to build a bass trap to cut bass at say 55Hz by about 2 or 3 db.

Question one - What sort of a frequency range do bass traps operate? I want it to be most effective at 55Hz but still do something at 10Hz either side. (look at the picture)

FrqResponce.jpg


Question two - Has anyone done such a thing and have any tips before I start reading websites and going mad with working out what would be best?

I have also put a picture of my room. The photo is quite old but the layout is the same. I was thinking it would be best to put the bass trap either on the ceiling above my listening position, or one on each wall either side of me, making two.

The ceiling in my room is half at an angle, because I am at the top of the house. I guess this might be a bad thing if I want to mount a bass trap on it 'coz it would make half of it point a different way other than down at me.

Two on either side seems best. I think they would both be half the size of the one on the ceiling if I did it that way, so I get the same attenuation. Both my walls are pretty bare.

Any help very much appreciated!

Room-blind.jpg
 
Don't bother with a bass trap it will be physically too big if it is to do any good at 55Hz.

First, you should try moving your speakers..

If you don't get better resuults and as you have a peak, you should use parametric EQ.

What mic did you use for your plot? If it isn't a well calibrated mic, you could easily be being misled.
 
Not much chance on moving the speakers I'm afraid! I used to have my system pointing the other way in the room and the bass was VERY boomy.
I have tried using a Behringer digital EQ. It did fix the problem and I could even make the response much flatter all over. BUT, it made the quality worse. Because it is another piece of equipment in the signal line. It would work okay if I had a Transport/DAC setup as it can work in pure digital mode. However I don't! And at £240 I didn't feel it was worth the money.
How big would the bass trap have to be? I can fit maybe about 2m by 2m on one wall and 1.4 by 1.4 on the other. Seems pretty big to me!
 
Oh yeah, I used a Audio-Technica AT3050 microphone. It has a flat responce. Very nice mic I must say ;)

mic-pattern.jpg
 
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Originally posted by Tenson
Oh yeah, I used a Audio-Technica AT3050 microphone. It has a flat responce. Very nice mic I must say ;)
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Bad Tenson :)

You can't use a mic with that kind of polar response for room measurements! You need an omni directional mic. like http://www.behringer.com/02_products/prodindex.cfm?id=ECM8000&lang=eng which is for the Behringer DEQ.

As for a bass trap, it has to be around a 1/4 wavelength across to be effective (especially 3dB!). You'd be looking at something about 5ft in diameter :) Even "anachoic" chambers with 4ft foam wedges are only flat to about 60Hz!
 
Well I did wonder if it needed to be omni, however I wrote to Behringer and asked if I needed to pick up a ECM8000 to go with the DEQ when I had it. They said no, the omni doesn't matter as long as it has a flat response. I mean your ears are not omni directional. Okay so they are, but they work mostly at the front ;)

A 5ft bass trap is quite plausible. How do you change the amount it attenuates?

Sorry I haven't done my homwork for myself yet, I must get reading.
 
Originally posted by Tenson
Well I did wonder if it needed to be omni, however I wrote to Behringer and asked if I needed to pick up a ECM8000 to go with the DEQ when I had it. They said no, the omni doesn't matter as long as it has a flat response. I mean your ears are not omni directional. Okay so they are, but they work mostly at the front ;)
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Nope, it doesn't work like that. LF comes at you from all directions, so anything but omni-directional means that the power is attenuated with the polar response of the mic.

It sounds like Behringers Tech SUpport is upto the usual standard :)

A 5ft bass trap is quite plausible. How do you change the amount it attenuates?
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Internal volume. Think of a box which is 5ft deep filled with glass-fibre with an open top.

The trouble with such devices is that the performance can be bloody unpredictable and is therefore a rather hit and miss affair. It's best to *measure* a selection of devices and pick the one that work (buying based on the spec can end in tears).

For DIY, it's a time consuming to build several incarnations to hit the sweet spot.
 
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Membrane bass traps are most likely to be what you're after, reasonably easy to make and tunable. Basically they're a sealed box filled with glass fibre/rockwool and placed on the wall. The depth of the box, filling and density of the panel facing the room set the absorbant frequency. RPG Modex and Real traps are the names that come to mind at the moment for commercial versions. The BBC engineering site is well worth a visit too.
 
Originally posted by avanzato
Membrane bass traps are most likely to be what you're after, reasonably easy to make and tunable.
Click to expand...
But they have to be big and heavy and you need several of them to do any good.

I'm currently looking at the use of Helmholtz Resonators to absorb room modes. Finding out how to design them is not too hard. Working out their likely efficiency is. I don't know how many I will have to make. Fortunately the required materials are cheap. Don't hold your breaths though - I won't be making any until next year.
 
Well for now I have decided to move my room around so that my setup is on the left wall. I used to have it on the right but it was very boomy. It was better in front of the window. I didn't try it on the left wall because I figured it would be the same as the right, but as someone pointed out the roof is different. So I tried it and it has a much flatter response. The bass hump is still there however, but it moves up to about 65hz-70hz which makes it easier to absorb too :)

I'll get back to you all on the bass traps and helmholtz's after a few days I expect!

Cheers,
 
Here is a link to a reasonable technical description of Helmholtz resonance.

Here is a bit more on design of a Helmholtz bass absorber.

Here is a handy calculator for designing Helmholtz absorbers.

A Helmholtz resonator is a simple harmonic oscillator where the mass is provided by the air in a narrow neck while the spring is provided by a volume of trapped air.

The natural frequency of a Helmholtz Resonator is given by the formula:

f = [v/(2pi)]sqrt[A/(V L)]

v: velocity of sound in air (340 m/s)
A: area of neck
V: volume of Bottle
L: length of neck
pi: 3.1415926536

It is easy to create a spreadsheet to show a range of designs. I propose to use a 150mm cardboard tube or plastic sewer pipe for the cavity and a 75mm cardboard tube inserted from one end as the neck. For a 1 metre high bass trap this would require a neck length of about 30 cm to absorb at 50 Hz or 40 cm to absorb at 45 Hz. I plan to start with a 40 cm neck and shorten it in stages until the correct frequency is achieved.
The experience of others suggests that I will need as many as 8 of these. They could easily be designed to stand one on top of another though, perhaps with one opening to the floor and the other to the ceiling. The pairs could then be stood in corners and would be quite inconspicuous if suitably decorated.
 
If I can be so bold..

Why not try a rug behind the speakers (the most important place IMHO) and another behind your sitting position?

You can put absorbant material behind the rugs to improve their effectiveness. If on a budget a couple of layers of carpet underlay seems to work.

This has the added advantage of looking nice!!!!!
 
Originally posted by bottleneck
If I can be so bold..

Why not try a rug behind the speakers (the most important place IMHO) and another behind your sitting position?

You can put absorbant material behind the rugs to improve their effectiveness. If on a budget a couple of layers of carpet underlay seems to work.
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Down to a frequency of about 250 Hz. Below that, the carpet will have no effect whatsoever.
 
Chris,

you've obviously had a more ''measured'' play with this than I have - I purely tacked things around to see what they sounded like.

Do you know what frequency ''room boom'' occurs at - noticeable room reflections? I have one I want to knock on the head and havent made my DIY effort yet.

It will be a rug with a backing for aethetic reasons, but the backing material is still at the ''dunno'' stage yet.

Chris
 
Thank-you technobear, I have finished moving my room around and got some proper speaker stands now.

The sound is great, better than before except the bass hump is more noticeable as I predicted. It is centred at 68hz as you can see from the pic at the bottom. (yes I'm still using my AT3035 mic :P )

I have done a bit more research on bass traps and for my room I think the helmholtz resonators seem best. I think I will try your suggestion of 1m L x 15cm W bodies with a 7.5cm W neck and slowly shave the length down till the frequency I want is achieved and absorbed.

I recon I have room for 4 of these in my room now. Does it matter if they are not placed symmetrically?? Because I can put one in the corner behind each speaker and one either beside the speaker or just in front and beside. I hope that makes sence. But if I want more I can put two in the rear corner HOWEVER I only have one rear corner to my room so this is where it might become uneven.

I also had the idea of covering the outside with small triangular strips of wood so that they not only absorb the LF but diffuse the HF!

Sounds like a good idea? Any more suggestions before I go and build them??

Cheers,

Oh, what does this person mean on arielaudio when he says 'Properly designed bass traps do not weaken bass in the room. They actually increase the bass power delivered to the listener.'

That seems very strange! I was under the impression that most bass traps worked by either turning the sound energy into kinetic movement energy when moving the plywood or whatever on the bass trap, or in the case of a helmholtz they turn the sound into heat from pressurising the air inside it. Is this all wrong, just too complicated to explain or is that guy chattin' crap?! :)
freq-new.jpg
 
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Christ almighty. The lengths some of these people go to in order to transform their living room beggars belief.... drilling and attaching fibreglass panels to their walls!! they obviously dont plan on selling their house in the future!!

Rugs and foam may not absorb DEEP bass, but at least they dont turn your house into a monstrosity..
 
The bass is increased when you introduce a bass trap which absorbs at the frequency of a dip. The dip is caused by reflected sound cancelling out the sound from the speakers. If you absorb the sound so that less is reflected then you get less cancellation and so you hear more of what is coming from the speakers at that frequency. Broadband bass traps therefore reduce (i.e. fill in) dips as well as reducing peaks. There is nothing to stop you from using a variety of Helmholtz absorbers to deal with different peaks and dips in the room.

As far as finding space is concerned, there is no reason one cannot stand on top of another if they are only a metre high and designed appropriately.
 
Originally posted by bottleneck
Christ almighty. The lengths some of these people go to in order to transform their living room beggars belief.... drilling and attaching fibreglass panels to their walls!! they obviously dont plan on selling their house in the future!!

Rugs and foam may not absorb DEEP bass, but at least they dont turn your house into a monstrosity..
Click to expand...

I really don't care how it looks as long as it sounds good! Besides, it can look good! And you can always put some pollyfiller in holes!
 
Originally posted by technobear
But they have to be big and heavy and you need several of them to do any good.
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Having just unpacked my six Modex Corners I can't argue on the size, and the weight varies from 3.9kg to 7.1 kilos each depending on the tuned frequency.

I've already partially dismantled one and I can't imagine what the materials cost is as there really isn't much to them other than air.

Originally posted by Tenson
I recon I have room for 4 of these in my room now. Does it matter if they are not placed symmetrically?? Because I can put one in the corner behind each speaker and one either beside the speaker or just in front and beside. I hope that makes sence. But if I want more I can put two in the rear corner HOWEVER I only have one rear corner to my room so this is where it might become uneven.
Click to expand...

I'm not up on Helmholz reasonators but if they work well in places with biggest air pressure variation the corners would be the best place to start. From there try putting some in the other 'high pressure' areas of the mode you are trying to tame. These can be calculated. If you have Excel try the 'waves' and 'modes' speadsheet from Harman

I also had the idea of covering the outside with small triangular strips of wood so that they not only absorb the LF but diffuse the HF!

Sounds like a good idea? Any more suggestions before I go and build them??
Click to expand...

I wouldn't bother as the curved surface will give some diffusion anyway and it just makes the construction more fiddly. Hope you'll let us know how well they work when you've done them.
 

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