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Surface Applied Acoustic Treatments
by John Storyk
Recording Studio Advice
Articles for Home & Studio Users

In the last article we left off with a discussion of mid and high frequency control in small listening rooms: basically the control of reflections. The one we discussed at length was console controls. Now we're going to talk about the effect the room's surfaces have on the sound -- and what you can do about it.

It would not be unusual for most home engineer/producers to:
1) move into a room with untreated floors, walls and ceilings
2) set up a modest production or songwriting "rig;"
3) start working and then
4) realize that something is happening in the room that is "coloring" the sound.

The problem often rears it's ugly head when content leaves your room and is played in different environments, particularly more professional ones. You'll notice that your take is "bass heavy" or "bass light," "lacking clarity in the vocal range" or "fuzzy around 800Hz." Unfortunately, your mixing environment is quite likely affected by harsh reflections and room modes.

Nevertheless, your studio is what it is! Therefore, most of you will want to make an attempt at corrective acoustics by way of "surface applied acoustic treatments." The good news is that there has been great progress in cost-effective treatments during the past ten years. We shall review some of them for you.

Prefabricated acoustic treatments that can be applied to your room's walls, ceilings, and even floors come in a number of styles and flavors. One way to categorize them would be by the way they alter acoustic qualities in your room:

- Absorption
Reflections stay the same, they are simply lowered in amplitude.

- Diffusions
Reflections are scattered throughout the room, but total energy stays the same.

- Reflection
Energy is re-directed, but without scattering.

In general, the types of surface treatments we are describing -- relatively light and relatively thin (say from 6" to 8") -- usually work the best at mid and high frequencies. However, most recently there are prefabricated (relatively thin at 8") membrane absorbers that can be very effective in controlling low frequency standing waves in a room.

Analyzing the required frequency "flavors" is a bit tricky, but once accomplished (possibly with minor help from a consultant) the application of these membrane absorbers is relatively easy.

Here is a (partial) summary list of the types of thin surface applied acoustic treatments that are relatively accessible in the marketplace today. This is not a total list, but will get you started.

Absorption -- Mid to High Frequency

1. Fabric-wrapped medium density fiberglass (1", 2", etc.). Standard stuff, been around for a long time, relatively inexpensive, available in various shapes, sizes, manufacturers, etc. (See Figure 1.)

2. Acoustic foam (1",2",4", etc.). Also been around for a long time, and is even cheaper than fabric panels. But not always the look one wants. (See Figure 2.) (NOTE: Excessive use of mid to high frequency absorption without balanced testaments in other frequencies will cause what I call the "Sonex -- Alpha Foam Effect." This means that the room is too dead at high frequencies, yet still booms at low frequencies. You would be better off using half as much in treatments but place the treatments in targeted locations. This requires more thought! Acoustic foam is a great product, but there is a time and place for it.)

3. Slotted, perforated wood absorbers. Yes, this is possible. There is a relatively new product in this country called "TOPAKUSTIK" (manufactured by RPG). The good news is that these panels can offer a relatively high amount of absorption, but still provide reflection. Also, they are not made of foam or fabric, but wood, so they will most likely maintain better over time. The bad news is that they are expensive. (See Figure 3.)
Figure 4 shows the energy/time curve (you may need a little patience looking at this graph) of a small Iso booth (live room) that has been OVER-treated with acoustic foam. The two curves show you the energy vs. time relationship at 1k vs. 125Hz. This room is really behaving like two rooms: rather "dead" at high frequencies, but "boomy" at low frequencies. Recording drums in this room is tough. Even low male voices struggle. There is simply too much high frequency absorption, not enough low frequency absorption, or a combination of both!

Absorption -- Low Frequency
1. As we had previously discussed, the use of prefabricated membrane absorbers now allows you to correct for room modes that you might be "stuck" with due to the size of the room you have moved into. The most reliable and possibly the thinnest on the market currently are "Modex" units from RPG. Several other companies will be coming out with wood prefabricated Helmholtz resonators that will also work, as well as add high frequency diffusion to the room. (See Figure 5.)

There are a number of diffusion products on the market. This subject has been discussed quite frequently during the past 15 years, with RPG being the leader in this field in the USA. (Possibly the area of diffusion is a subject for an entire TAXI Acoustics article!)

Relatively new on the market is a product line that is probably quite useful for home recording enthusiasts. These are lightweight, prefabricated mid/high frequency diffusors. The "skyline" modular (2' x 2') diffusor from RPG works very well in many situations where you would not want to absorb sound, but need to correct for a harsh reflection. Installation on a low flat reflective ceiling (does this sound familiar?) would be ideal in many instances.

Systems Development Group (SDG) also makes a similar product. These lightweight and relatively inexpensive surface mounted diffusors are easy to install, can be taken with you if you move to another room, and allow you to "break up" harsh reflective surfaces without installing excessive amounts of absorption (thus making your room too dead at only high frequencies). (See figure 6.)

In summary, even in a relatively small rectangular room (the type of room you might be working in), there is no real reason why one cannot make some form of modest improvement to room acoustics with the use of thin, light-weight, and moveable surface treatments. A typical 200 to 300 square-foot room can easily be improved for $3-4,000 -- tops! Add $500 for some professional advice on where to place the treatments (assuming you might not be able to deal with this type of analysis) and this becomes a very modest investment, completely in line with the equipment that you are purchasing.

This is a bit more complicated, and will be discussed in a separate article on room layout.

Good luck, and enjoy the music.

Illustrations for this article will be posted our Web site. Go to www.wsdg.com. On the site, go to "RESOURCES," navigate to TAXI, and park in the correct spot.

Figure 1: Typical 1 - 2" fabric wrapped acoustic panel
Figure 2: Acoustic foam
Figure 3: Slotted / perforated wood absorbers. (New product: "TOPAKUSTIK")
Figure 4: ETC curves for ill-treated small Iso booth
Figure 5: Low Frequency membrane absorber, "Modex," by RPG
Figure 6: Modular high frequency diffusors on ceiling (SHAQ Orlando Project Studio)

The information above came from "Studio Buddy -- The Home Recording Helper." It's a self-contained, easy to use database of recording tips designed specifically for people with home studios. If you find this article helpful, you should download the FREE program at:


This series of recording studio and sound engineering articles are being made available courtesy of our friends at Taxi.  The articles aim to help you make better sounding recordings in your home or project studio and will be available here or online at Taxi, complete with pictures using the link at the end of the article or better still - sign up to receive Taxi's Newsletters FREE and get them delivered straight to your inbox!