"I recently built a heavily insulated isolation booth in my studio, which I mostly use for loud guitar cabinets. It works well, but it there seems to be a huge build up of sound at around 400 Hz. I can go into the booth and easily trigger it with my voice. It sounds almost resonant. I tried to put up acoustic foam, but it didn't help. What else can I do without having to start over?"

Acoustic foam didn't help because most treatment like this does very little to frequencies that low. In fact, if you put up too much you end up killing all the mid and high frequencies, which can make a space sound too bass heavy.

Your best bet is to employ some type of Helmholz Resonator in the booth. These can perform miracles if done properly. Sweetwater carries products from a couple of different companies that can fit the bill, however, for best results you should have one of them make you a custom panel that addresses your specific problem. Your Sweetwater Sales Engineer can help get you going on this.

It is possible to build these yourself. In fact, the construction can be fairly simple. You just build a box that you can attach to one wall of your room. It should be a few feet tall by a few feet wide and about four to eight inches deep. The front surface can be very thin fiberboard or something similar. Fill it half way with dense fiberglass insulation. Then you need to drill holes of the proper diameter with the proper spacing into the front. These form the acoustic interface between the resonant body of air inside the space and the room. If done properly it will effectively kill your problem frequency. The problem is figuring out the hole diameter and spacing. There are math and formulas involved, and it's extremely difficult to get the information across without the benefit of drawings and diagrams. We'll give you the basic formulas here if you want to try to run with them, but we recommend getting a good book on acoustic treatment if you're going to try this. This is also something your Sweetwater Sales Engineer can help you with.

There are two basic formulas you need to calculate the resonant frequency of a perforated Helmholz Resonator (perforated as opposed to slate type, which is harder to build).

F = 200 x (square root of P/Dt), where F is frequency, P is perforation percentage (see below), D is the depth of the air space in inches, and t is the effective hole length in inches + the hole diameter. Hole depth is basically the thickness of your front panel you will be drilling the holes into. A practical hole diameter is somewhere around 3/16 of an inch, or .1875 inches.

In order to figure out P, the perforation percentage, you need the following formula. Understand that the holes are normally drilled in a square pattern, where four holes mark the boundaries of a square space on the panel. Perforation percentage is thus the amount of hole space in that square area as a ratio to the amount of solid space (or non hole area). You just need the percentage of one of the many sets of four holes to plug in the correct value. The formula:

P = ((ϖ(d/2)squared)/((D1)(D2)) x 100, where P = Perforation Percentage, d = hole diameter, D1 = distance of one side of the square made up by the four holes (measured from the center of the holes), and D2 is the distance of the other side. Obviously in a true square D1 and D2 are the same, but this allows for other shapes.

It is worth noting that it's fairly easy to treat more than one problem frequency with this design. All you need to do is have sections of the panel deal with each frequency, which is easy to accomplish by changing the hole spacing (D1 and D2 in the equation) from one part to another. We don't have the space to work through examples of all the math here. Maybe we can come back to it another time if enough people request it.