Acoustic Tweaks

What we hear be it in a concert hall or our listening rooms is a complex interaction between the source (our speakers or the intruments) and the various boundaries of the room. Controlling the way that sound is reflected within the room produces a great improvement in the performance of an audio system.

I am firmly convinced that many audiophiles suffer needless "upgrades" to their equipment in a vain attempt to correct problems which are actually caused by their ROOM. I've spent considerably more time learning to properly tune my room than auditioning new equipment. What I learned along the way is that the more "right" i got my room, the better my equipment sounded. You've truely not heard your system until you've heard it in an acoustically tuned room

Before we attempt to treat the room, however, we must attempt to place the speakers properly in the room. No amount of acoustical treatment can ameliorate the effects of poor speaker placment. There's an excellent guide on The Cardas Web Site Another quick and dirty way to try speaker placement is as follows

1. Measure the distance from the floor to the center of your woofer.
2. Multiply that value by 1.63. Move your speakers so that the center of the woofer is this distance from each wall.
3. Multiply the floor-woofer distance by 2.63. Place your speakers so that the FRONT of the cabinet is this distance from the rear wall

Pictured to the right is one of Jon Risch's DIY acoustic panels. In order to be effective, we would like to absorb frequencies from about 125 hz upward. An absorptive panel can only absorb those frequencies whose wavelength is at least 1/4 of the panel's thickness. That is, a six inch panel will begin to lose absorption at about 500 Hz. The panels shown are 8 inches thick and are made of a combination of 6 1/2 inches of kraft-faced spun fibergalass (R-19) building insulation with 2 inches of unfaced Johns Mansville Spin-Glas Fiberglass Board behind that. They provide effective absorption to below 250 Hz.

These panels must be spaced away from the walls by several inches since this increases their absorptive capacity, often by a factor of twofold. Think about it in this simplistic way. The sound waves enter the front of the panel and get absorbed as they pass through the layers. They exit the rear of the panel, bounce off the wall and then get absorbed even more as they pass back through the layers of material.

Here is a drawing of how my acoustic panels are constructed. Click on the Drawing to Enlarge It In order to facilitate spacing them out from the walls, I mounted the panels on a base. The legs of the base are 24-inch sections of 2-inch PVC pipe with a piece of 3/8-inch all-thread rod passing through them. At the frame end, the all-thread runs into a t-nut and is then locked in place with a jam nut. On the base end, the threaded rod is secured with a jam nut and washer.

My frame is 5 1/2 inches deep to accomodate both the 6 1/2_inch fiberglass and the 2-inch rockwool. I did this by stapling the kraft paper backing of the fiberglass batts around the INSIDE of the frame to create a 1-inch cavity into which the rockwool fits. When mounted like this, the rockwool projects an inch outside the back of the frame. It is then concealed by stapling polyester batting across the back of the frame, making a nice clean look.

If you can't find the rockwool, a pretty good absorptive panel can be made using just the 6 1/2-inch fiberglass. Simply reduce the thickness of the frame to 4 1/2 inches. Everybody always asks what the function of the paper backing on the fiberglass insulation it. It's purpose is to facilitate stapling the fiberglass bats to the wooden frame. Its presence doesn't affect the operation of the panel becasue the high frequency sound waves simply pass through the paper. Hold one layer of a brown paper bag over your ear. You'll see that you can still hear!

After completion I attached a layer of polyester batting to the front and then covered the front with decorator burlap which wraps around the frame and gets stapled the the rear as well. Note that the front coverings will compress the fiberglass a little at the edges (see the picture of the "naked" panel above). This is okay, just don't stretch it so tightly that the center is compressed.

How To Cut Straight Edges in Burlap A woman at my local fabric store taught me a really neat trick for cutting strait edges in burlap fabric. Having a square piece of fabric makes covering the panel easier and neater. Here's how:

1. At the measured point, cut a V-shaped notch in the edge of the fabric.

2. Pull the burlap thread at the base of the notch out of the fabric.

3. Slide the fabric down the thread and completely remove the thread from the fabric. You now have a straight line upon which to cut.