Placing Your Acoustic Treatments

How and where to position your acoustical absorption for maximum effectiveness

By Phil O'Keefe

You've bought your acoustic foam and / or fiberglass absorbers, and made panels for mounting them as needed, but now where do you put them within the room? In this article, we'll go over some suggestions to help you make good use of your absorbers.

Acoustic foam and fiberglass absorption products are popular with home and project studio owners, and are often used to help correct various acoustic issues that are commonly found in the small rooms that are typically used for such studios. The most common problems you're likely to encounter in small rooms are early reflections from nearby room surfaces, flutter echo from parallel walls, and low-midrange and bass buildup.

2" thick foam or fiberglass panels placed flush against the wall can be effective at dealing with flutter echoes from first reflection points, but since they absorb primarily at midrange and high frequencies, it is easy to over-absorb the mids and highs while leaving the bass frequencies untamed, resulting in a badly unbalanced-sounding room. Whenever possible, I recommend using as much bass trapping and broadband absorption (thick absorbers that work well across a wide range of low, midrange and high frequencies) as possible, and using only as much of the thinner absorptive products as needed to tame flutter echo and early reflections from the side walls. In fact, if you can use thicker absorptive material for your first reflection points and space the material a few inches from the walls to increase its effectiveness at lower frequencies, that's highly recommended.


Studios and Control Rooms - They're Not The Same

Right up front we should discuss the purpose of the room you'll be treating, since control room and studio acoustics often have two different goals. In a control room, monitoring accuracy and controlled RT60 (reverb decay) times at all frequencies is the ultimate objective, while the main purpose of acoustic treatment in a studio or tracking room is more about making it sound good - with "good", as always, being variable and subjective. In this article, we'll focus primarily on control rooms.

Many people are using a single room for recording and mixing, and that's okay too - for single-room studios, I would recommend taking a control room approach to the room so that you have a good place to monitor and mix in; after all, with today's mobile recording options, it's always possible to move to different rooms and acoustical environments, both inside your home and at other locations when tracking.

A small control room being set up, with absorptive panels suspended over the mix position (above the wood diffusers), mounted to the side wall first reflection points, and mounted diagonally in the front room corners.

Control Room Orientation and Symmetry

Your control room setup should be symmetrical, with the speakers set up at equal distances from the centerline of the room so that there is the same amount of distance from each speaker to the nearest side wall. I also generally recommend that the speakers be set up to fire down the long axis of the room, with the desk set so that when you're seated at the mix position, you're located about 1/3 of the room's length away from the front wall. Assuming you're using this type of configuration and room layout, the places I recommend positioning your absorptive panels are:

• Side wall first reflection points

Ideally, you want the sound from your speakers to be the only thing you're hearing - when it's combined with very short / early reflections from nearby walls, it has a detrimental effect on the stereo imaging, transients, and overall sound quality and accuracy of your monitoring system. Sound that radiates from your speakers, strikes a room boundary surface and reflects directly back to where you're seated at the listening position should be absorbed if at all possible. How do you determine where these first reflection points are? The most common approach is to use the mirror technique. While seated at the mixing position, have an assistant hold a mirror flat against the side walls at various locations in the room. Whenever you're able to see the speaker's reflection in the mirror, that's a first reflection point, and a very good location to place some absorption. Since your room should be symmetrical, the treatment should normally be in similar spots for the left and right side walls. 

• Room corners

Bass builds up in your room corners, making them another excellent place to put absorption. Typically, you'll want to put at least one 2' x 4' absorber diagonally across each 90 degree room corner where two walls meet. Thicker panels are recommended for all corner absorption; stick with 4" thick as a minimum, and 6" thick or more is even better. If you have eight foot tall ceilings (or higher) and enough room to stack two 2' x 4' panels vertically so that you have panels diagonally across the entire corner from floor to ceiling, so much the better.

If bass buildup is a big issue in your room - and it is for many small rooms - adding even more absorption to the wall / ceiling corners can often help. As with the front and rear corners of the room, the absorptive panels should be as thick as possible, and mounted diagonally so that it creates a triangular space behind the panel. This will increase low frequency absorption compared to mounting the panel flat to the wall or ceiling.

• Front and Rear Walls

There are more than a few theories about the best room acoustic treatment approaches. One very popular configuration is the Live End / Dead End approach. While there's more to a true LEDE certified room than this, the basics are an absorptive front part of the room, and a more diffusive back end. I really like the sound of a diffusive rear wall since diffusers scatter sound in a variety of directions which helps keep some controlled ambience in the room, but you need a bit of space behind you in order for diffusers to work properly. If you have less than 6-8 feet of distance from you to the rear wall when seated in the listening position, it may be better to make the rear wall highly absorptive instead. If you go this route, thick absorbers on the rear wall are a good approach. If you do have sufficient space and room, diffusers in front of thick mineral wool or fiberglass bass traps can provide some liveliness in the mids and highs while providing additional and beneficial low frequency absorption.

The front of the room can also be a good place for absorption - especially the space between the monitor speakers. In the room pictured above, a large bay window exists behind the curtains, leaving considerable space. This space has been filled with large, thick chunks of absorptive material two feet thick, which provides additional bass trapping for the room.

• Ceiling:

Sound can reflect off the ceiling just as easily as it can off of the side walls, and there are first reflection points on the ceiling that you should address if possible. Companies such as Auralex, Real Traps, and Primacoustic offer absorptive panels that can be suspended from the ceiling that will help tame first reflections from the ceiling. These should be hung so that they are between the two speakers, and directly above and in front of the listening position. If you're not certain of the location to put the panels, the mirror trick can be helpful for the ceiling, just as it is for the side walls.


Keeping It Simple

While these are generalized tips and are in no way a substitute for in-depth room analysis or consultation with a good acoustician, for those who don't have the money or expertise to hire someone or do extensive room analysis themselves, adding thick bass traps to the room corners, as well as absorption to the first reflection points on the front and side walls and ceiling above the mix position, and using either absorption or diffusion on the rear wall can make a significant improvement to the sound within the room, and is well worth the cost, time and effort required to purchase and install these treatments.

 

Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Senior Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.