Here is the second in a series on improving your listening and recording environment with Bryan Pape (see the first part here).
Bryan is lead acoustical designer for GIK Acoustics, a respected sound treatment manufacturer and dealer, as well as owner of Sensible Sound Solutions, through which Bryan sells sound treatment services and materials. Bryan also lends his advice to thousands of audio enthusiasts and recording engineers in several online forums, including the popular Gearslutz’ studio construction and acoustics forum.
Q: Most people who record or listen to music need something to absorb some of that sound that bounces around a typical room. They can hire a consultant or purchase pre-made sound treatment, of course, from companies like yours or GIK. But if they want to try to do it themselves, what materials can they use?
A: This depends somewhat on the type of treatment you’re trying to make and how thick it’s going to be. Thicker panels, intended more for bass duties, can use less dense materials once you get to approximately 8″ of thickness. Standard wall insulation will work OK in this application.
For more standard 2″ type panels for reflections, 3lb/cu ft density rigid fiberglass board is my first choice. Many know this as OC 703. OC 703 is actually a model number for an Owens Corning product. There are other equivalent products such as JM814 (Johns Manville).
Acoustic foam is very well known. Most of it isn’t terribly good. The few that are are relatively expensive and don’t perform nearly as well
Similar density mineral wool can also be used. It’s slightly less expensive in some places but is not nearly as easy to work with if you have to cut it. Lastly, there is also 3lb density acoustical cotton. This is LEED certified green material and performs slightly better than the 703, albeit at a higher price. The advantage is that it comes in a variety of colors and does not have sharp fibers like fiberglass and mineral wool so it can be hung uncovered if desired.
Q: Where do you purchase those materials?
A: LOL. Good luck. It’s very difficult to find as a consumer. Most places that have it will only sell to you if you’re a business. There are always exceptions. I do offer 703 and the cotton for sale.
Q: Where can people find directions on how to build such a sound-absorbing panel or bass trap?
A: Again, it really depends on what you’re looking to do. For simple reflection panels, you really don’t need a plan. Build a frame that has an inner set of measurements that are maybe 1/16th” smaller than the 2×4 sheet of material you’re using. Make the frame from 1×3. Friction fit the fiberglass up at the front, flush with the front edge. Wrap with cloth and staple it to the back. Pretty simple.
Q: How thick should absorbers be? What is the difference between a bass trap and a broadband absorbing panel?
A: For standard absorber duties, a 2″ thick panel is fine. For broadband absorbers that will reach lower, use at least 4″ or 6″ of thickness. If you want a broadband bass-ONLY absorber, then you have to bond a facing to the fiberglass to provide a damped membrane and reflect the upper mids and highs. There are also tuned absorbers that only function over a couple of octaves or some even less. These are either rigid undamped membranes made from wood usually, or Helmholz resonators which work similarly to a speaker port or blowing across a bottle. The difference is that these are damped at the tuning frequency vs reinforcing it.
Q: How can people determine where they should install sound treatment like bass traps and broadband absorbers?
A: Every room is different. Corners are generally a good place to use bass absorbers or broadband absorbers. They’re efficient since they are at the end of 2 room dimensions. Remember that you have 12 corners in a room, not just 4. The wall/ceiling and wall/floor are also corners.
After that, it really depends on what particular issues you’re having in the room. Other generally good places for bass absorbers are centered on the rear wall behind the listening position, over your head to help with height related issues, behind your main speakers if they’re close to a wall to deal with boundary reinforcement, etc.
For more standard reflection absorbers, we are trying to deal with any point in the room which reflects directly back at you, arrives within a specific time period, and is a reflection that comes from far off axis where most speakers mid and high frequency response is VERY different than the direct radiated sound. The most common of these are the side wall reflection points and the ceiling between you and the speakers. These reflections, if left untreated, allow a 2nd impulse to reach you at a different time (smearing imaging, dynamics, etc.) and also are of a different frequency distribution which will change the overall tonal balance of what you hear.
Thanks Bryan! And remember, if you have any other acoustic questions Bryan might be able to answer, post them here.