Bass Trap Project

This page originally from: http://www.cloneensemble.com/basstraps.htm circa 2006/05/20.

Copied with permission.

An incomplete documentation of my DIY bass trap project.

Every home studio owner knows that the acoustics of small rooms are terrible, particularly if they have brick or cinderblock walls and concrete floors.
However, there is a widespread opinion that if you close-mic, these problems don't matter so much when recording. So you just have to "take things into account" when mixing and mastering.
This is true for many instruments, but it is NOT true for instruments that produce a lot of bass, particularly if the sound-generating surface is low-mass. The reason is that standing bass waves don't just interfere with what you hear, they also interfere with the sound-generating surface itelf. So some bass tones will be unnaturally boomy, and others completely dead. No amount of close-micing will avoid this. I noticed this with my bass drum and 16" floor tom.
The solution is to add bass traps to the room. A modest amount of online research clearly indicates that foam wedges are a con. You need high-density fibreglass or rockwool, mounted a distance out from the walls, and the most effective placement is in the corners.
In the US, www.realtraps.com have made affordable professional-grade bass trapping available to home studio and home theatre owners. However, by the time these products make their way to Australia, after shipping, currency exchange, taxes, distributor markup... that affordability evaporates.
Here's how I went about fixing the bass problems in my studio:


The Tasmanian oak slats as delivered by a local joinery firm to my specification. Some further shaping and sanding was required.	Applying stain to the slats. I used a couple of different kinds and applications. (Not shown, several coats of satin polyurathane.)

I used slotted angle iron from my local hardware supplier, here I am cutting it to size and cutting a bevelled profile at the top.	Finishing off the cut edges with various rust-proofing products (to be followed by enamel).

My initial selection of fasteners. Getting black-coloured washers was a complication. I eventually found a plater who could do it.	I realised that the washers would bruise the polyurathane finish on the slats, so I punched some circles of adhesive rubber foam to apply to the inside of the washers.

The "rigid" rockwool (Bradford Fibretex 450, 50mm boards), some sofa-lining fabric (a manufactured tissue) and black car carpet.	Cutting the carpet to size.

Cutting the fabric to size.	Cutting the rockwool.

Attaching the slats to the angle iron.

The carpet face down on the inside of the frame.	The first layer of rockwool. There is a sheet of the sofa fabric between the carpet and the rockwool.

The second layer of rockwool. Note the crudely carved bevel.	The fabric is folded down over the top, and roughly tucked in. I was considering glueing it in place, but this proved unnecessary.

I lashed everything down with thin rope.	This held things together so well I was confident that no fibres would escape even though I had done no sewing or glueing.


The four bass traps installed.	They are held in place with bungy cords and heavy-duty hooks in the wall.	This was pretty straightforward for a brick wall...	...but if I were doing it on a plaster wall I would make sure those hooks were screwed into corner studs.


Bass reverberations of my untreated studio. I am not 100% confident in the accuracy of this analysis, every time I run the test I get different results. However the comparison does give some indication of the benefit.	Bass reverberation with my new bass traps installed. Subjectively, bass sounds a lot more even, and does not vary nearly as much as I move around the room.

Notes on my design:

The cost was approximately Aus$1300, which is about 60% of the cost of equivalent treatment from Shamrock consulting (probably the most affordable commercial option in Australia), and about 30% of the cost of the equivalent amount of RealTraps (when bought in Australia).
Although there are slots between the slats, this is not a slot resonator bass trap. There is no seal at the sides, so they don't form a cavity. And though I haven't done the calculations, I'm pretty sure the resonant frequency (if there was an effective cavity) wouldn't be in the desired range. Those slats are purely there for appearance and structural integrity.
I chose rockwool over fibreglass as it is easier to handle (the fibres are less irritating) and it came in sizes that were more compatible with the sizes of my design. Fibreglass has slightly better acoustic properties, but with the large surface area of my design rockwool was adequate.
They look spectacular, like they belong in a much more expensive studio.
My bass traps are extendable should I move to a new house with a different ceiling height. A length of angle iron extension can easily be bolted on, and I have some spare slats in storage just in case.
The traps are robust, so that if someone bumps into them with a shoulder, they'll easily take it. (I have young kids, so this was important).
If I do move house, I will probably have to disassemble them, they are not terribly transportable...