Bass Traps and Acoustic Absorbers Pt. 1 - Concept Design

Now, I am by no means an acoustics expert so I think that these articles can stand as a good discussion forum. Make sure you leave a comment if you have any suggestions for the design and we can help anyone that is keen to build their own.

Background

When designing these acoustic panels I had a few limitations:

• I am renting so any panels need to be attached with minimal damage and easily removed when I move out.
• Panels need to be suited to different rooms as I don’t know what my next studio will be like.
• I am only working part time, so all costs need to be minimised ie. little or no wastage.
• The two corners behind my listening position are difficult to access as one has a window and the other has a large piece of immovable furniture

My room is square, 4.6m x 4.6m x 2.25m with two brick walls and two plasterboard walls. As you can imagine standing waves and flutter echoes are serious problems in my room. I’ve done a few quick calculations using a room mode calculator and have found that axial room modes occur at around 37Hz, 74Hz, 158Hz, etc. as well as other complex derivatives. I could aim to tackle these frequencies individually using tuned bass traps but I have opted for more versatile broadband bass traps and acoustic panel designs. This involves using insulation to absorb troublesome frequencies.

As standing waves peak at the walls and in the corners of the room, this is where I want to place my traps. Try it. Stick your head into the corner of your room and hear how the frequency response changes. I have considered the use of Superchunk bass traps but have opted to build self standing panels that don’t require being built into the room. It will also make it easier to move to my next studio.

Concept

After much deliberation I decided to build two floor to ceiling 600mm wide x 150mm thick bass traps and six 1200mm wide x 600mm high x 100mm thick broadband absorbers. One of the main reasons for this arrangement was due to the size and availability of acoustic insulation. The insulation that I have chosen to use is Tontine Acoustisorb 3. It is comparable to Owens Corning 703 (one of the most used studio insulation materials used in the USA) and comes in 1.2mx2.4mx50mm sheets.

I briefly looked at normal house insulation products but couldn’t find a material that had the density that I required. Acoustisorb 3 is 48kg/m3 and does not require personal protective equipment to handle or cut, which is a bonus. It also means that I don’t have to worry too much about covering. Acoustisorb 3 comes in a pack of three sheets so I will use one pack to construct the bass traps and a second trap to construct the broadband absorbers.

Design

Many DIY bass traps use 100mm or 150mm wide pine as a simple and strong way of framing the insulation. However, any sound waves contacting these frames is going to be reflected so some people then cut holes and patterns out of the wood to increase the effective surface area of the traps. As Acoustisorb 3 is fairly rigid I can get away with having a fairly simple frame. Rather than framing the insulation, I am going to provide a skeleton for it to be mounted to. I can then use this frame to attach it to the wall.

Down at my local Bunnings there are dozens of different wood cross sections to choose from. I finally decided on 18mm x 38mm x 2.4m lengths of pine as they are rigid enough in both axes as well as being reasonably priced at a convenient length (about $4.80 /length).

You could then construct the frames using pre made L-shaped brackets but I have opted for the simplest and cheapest option. Each corner requires two 50mm wood screws (8 per frame) which minimises drilling and provides enough rigidity to not need cross bracing even on the floor to ceiling frames.

Further Reading

http://www.ethanwiner.com/basstrap.html - Great DIY bass trap article.
http://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/ - forum community dedicated to acoustics and production.
http://www.stereo.net.au/forums/forumdisplay.php?f=17 - An Australian based forum dealing with acoustics.
http://www.radford.edu/shelm/acoustics/bass-traps.html - Fantastic article on building similar acoustic panels.
http://www.spec-net.com.au/company/tontine/acoustic/acous6.htm - Specs for the Tontine Acoustisorb 3.

Next Week

So next week I'll go through the construction process and outline a few tips to help you build the panels a bit quicker and easier. I'll have some pics then too so you can see what they look like. Got any suggestions / questions? Just leave a comment. Do it!

Click here to go to the next installment.