RealTraps - Front Wall Absorption Must I really treat the whole wall?

By Ethan Winer

Many audiophiles and home theater owners treat the entire front portion of their room with absorption. Their thinking is that loudspeakers radiate a significant amount of sound not only forward, but also from the sides and rear of the enclosure. Since loudspeakers are often close to the front wall, echoes from that wall will arrive soon after the direct sound - the dreaded "early" reflections - and thus harm imaging. So a common treatment is to install 703 rigid fiberglass one inch thick on the entire front wall. Often the side walls in the front portion of the room are also treated, either from floor to ceiling or sometimes to ear height only.

In theory this makes sense - reflections that arrive within about 20 milliseconds of the direct sound from the loudspeakers do indeed harm clarity and imaging. And at low frequencies these reflections skew the frequency response quite badly. But how much sound actually radiates from the rear and sides of a typical loudspeaker? And can relatively thin absorption on the front wall really help?

For the purpose of this article I'll exclude bi-polar speakers that radiate equally from the front and rear by design, and likewise I won't consider wall-mounted surround speakers that are designed specifically to increase ambience by radiating in two opposite directions. Rather, I will focus on the main left and right speakers only, as used in both stereo and multi-channel surround systems.

LOUDSPEAKER DIRECTIVITY

It's important to understand that loudspeaker directivity is highly dependant on frequency. Every loudspeaker radiates omnidirectionally at low frequencies, even when all of its speaker drivers are facing forward. At higher frequencies the sound begins to radiate more as you would expect, in the direction the drivers are facing.

Loudspeaker directivity is displayed using a polar plot. This is a graph that shows loudspeaker (and microphone) radiation patterns on a circle, using different lines for the various frequencies. Unfortunately, it's very difficult to find polar data for consumer loudspeakers - even very expensive models - probably because speaker manufacturers don't want you to know how directional their speakers really are at higher frequencies. They want you to believe you'll get lush, full sound with great imaging no matter where you are in the room. However, loudspeakers sold to the professional audio market often do include such data, so this is what will be presented below.

As you can see in the graphs that follow, all loudspeakers radiate omnidirectionally at the lowest frequencies, regardless of their design. Depending on the cabinet construction, driver type, and other factors, they start to become directional around the low midrange, and become more directional still at higher frequencies. Therefore, the issue is at what frequencies do speakers radiate from the sides and rear, versus the range of frequencies typical front wall absorbers are effective.

ABSORBER EFFECTIVENESS

Most thin absorbers made of acoustic foam or rigid fiberglass are ineffective below about 500 Hz, yet this is precisely where absorption is needed most if the goal is to reduce or avoid reflections off the front wall. Treating the entire front wall won't solve this problem at low frequencies, nor is it needed or even useful at mid and high frequencies.

As you can see in the polar plots below, once you get up to 500 Hz all of the loudspeakers but the Bogen A2 favor front versus rear radiation by at least 10 dB - most differ by 15 to 18 dB. Loudspeakers simply do not radiate much mid and high frequency energy toward that wall. Further, covering an entire wall with absorption risks making the room too dead sounding.

A better treatment solution, in our opinion, is to have as much bass trapping as possible in the corners to solve the skewed low frequency response present in all small rooms. Then you can treat the first reflection points on the side walls and ceiling as described in the article Creating a Reflection-Free Zone. If the room is still too live sounding, or there are obvious echoes between parallel walls or between the floor and ceiling, more absorption can be added at specific locations to avoid those echoes and achieve the ideal balance.

Ethan Winer's living room home theater. The 65-inch TV is slightly behind the three front loudspeakers, and thus is out of the direct line of fire from those speakers. The surround speakers are not visible in this photo - they face the opposing side walls, so little sound gets to the TV's front glass from those speakers either.

ADDED OCTOBER 30, 2006 Since writing this article, several of my friends who are industry professionals suggested I at least try adding absorption in front of my large glass TV as a test, to see if I hear an improvement in imaging. Even though the ETF software shows no significant reflections, I tried it anyway. A friend and I took turns, with one of us listening while the other first held and then removed absorbing panels in front of the TV. The room is shown in the photo at left. The only change either of us noticed was a very sight lowering of the overall room ambience. This makes sense given that the room was already free of all major reflections. One situation where absorption on the front wall might be useful is when the surround speakers face that wall. In my case the surrounds face each other and fire across the width of the room. But if they were facing toward the front of the room, and the total round trip from surround speaker to TV to the listener was ten feet or less, absorption in the front of the room could help in that case.

LOUDSPEAKER POLAR PLOTS

The loudspeaker data below was taken from the respective manufacturer's literature. All of the plots show horizontal axis radiation, and a few also show vertical directivity. Brands and models are presented alphabetically.

Apogee AE-8

Apogee FH-4

Bogen A2

Bogen A8

Bose Panaray LT 9402-III

Carvin TCS210

EAW MK5164

Electro-Voice Sx100+

Electro-Voice XI 1152

TOA F-500WP