Over the past couple of decades movie and music buffs have had to deal with the problem of low-frequency sound distribution in their home theater spaces.   

Lately this topic has also been a popular point of discussion on ElectronicHouse.com.

Looking at the evolution of the home theater, it used to be that audiophiles would move their speakers around to get the best possible sound. Unknowingly, yesteryear’s audiophiles were not only improving the imaging and soundstage of their two-channel systems, they were also adapting their speakers to the peaks and nulls that are created when low-frequency sounds are reflected off of walls, floor and ceiling. 

By the 1980s as the home theater hobby began to unfold the use of dedicated subwoofers gained momentum. In the late 1990s when the DVD format was introduced, subwoofers were used to reproduce the low-frequency effects (LFE) channel found on the audio mix of these discs. 

Consequently, the more subwoofers were used in the home, the more everyone noticed how uneven their multichannel systems’ performance was.

What’s the Problem?
Explaining why a home theater can sound “boomy” or “muddy” when reproducing low frequencies, Joe Finn, Velodyne Acoustics training manager, says that much of the problem lies in the small dimensions of the typical home theater room. 

“Many consumers appreciate the idea that a large area makes twin subs a wise move,” notes Finn. 

“The part that consumers may miss is that all rooms—small, large or in between—suffer from standing wave issues. When excited by deep bass, the room itself resonates, like a water glass struck by a fork.”

Going into more detail, Finn points out that the lower the frequency the larger the sound wave is, and when these large sound waves are reproduced in a small environment like a home theater room, the sound waves reflect off of their surrounding surfaces and they can cancel each out, which causes standing waves.

“Bass wavelengths are long [about 50 to 55 feet for the lowest frequencies] and most home listening rooms are comparatively smaller.  So the deepest bass notes hit the back wall, ceiling and floor, and collide with notes that the sub is producing,” says Finn. 

“This sets up areas in the room where bass energy is too prominent, ‘boomy’ if you sit there, and other areas where bass energy seems lacking. That bass energy doesn’t decay quickly, but stays constant. So the room ‘rings’ for a while, resulting in uneven bass and unhappy consumers.”

Solving Bass Issues
There are ways to deal with standing waves, however. One cure is to have a professional measure and analyze the room. In this scenario the professional installer or acoustician will solve these problems through the use of acoustic treatments to absorb or diffuse the sound. 

The second method to deal with standing waves is to use an equalizer. Today there are a number of equalization products, that vary in price, that are available to installers and consumers. There’s also a growing trend within the electronics market to incorporate built-in equalization into subwoofers and A/V receivers. 

A third method is walk around a room while trying to find the “peaks” and “nulls” created by the reflected waves and positioning the room’s seating at points where the bass is even sounding. 

The final method is to use multiple subwoofers, which was found by experts such as Dr. Floyd Toole, Told Welti and Allan Devantier to have advantages if they are set up correctly. 

Explaining the rationale behind the use of multiple subwoofers, Finn says that using more than one can naturally eliminate some of the inherent problems of reproducing bass in a small room. 

“Twin subs, strategically placed, can help cancel these room resonances,” notes Finn.

“Twin subs are far more capable than one at filling in the valleys and smoothing out peak energy areas. The whole family is more likely to get clear, undistorted bass.  While much research suggests that four subs is really the optimum number, twin-sub systems are nearly as effective—they are much better than a single sub—and are more practical, more cost-effective and easier to position in the room than four [subwoofers].”

Finn sums up that beyond the natural differences between large-venue acoustics and small-room acoustical environments, there’s a lot of attention paid to ensuring those famous halls do sound good during a concert. 

“Many consumers just wouldn’t think about this part. Places like Carnegie Hall have tons of money and engineering talent poured into making them sound good,” he notes.

“But homes like yours and mine don’t benefit from sophisticated acoustical engineering designs. Twin subs can really help. Plus, each sub doesn’t work as hard to fill the space, which results in less distortion and awesome-sounding videos.”

Additional Resources:
Harman International, http://www.harman.com/EN-US/OurCompany/Technologyleadership/Pages/WhitePapers.aspx?CategoryID=White%20papers

http://www.harman.com/EN-US/OurCompany/Technologyleadership/Documents/Scientific%20Publications/13680.pdf

Home Acoustics Alliance, www.homeacoustics.net

THX, www.thx.com

Velodyne Acoustics, www.velodyne.com

At its recent CEDIA Expo trade show demonstration Triad Speakers showed how a sophisticated array of multiple subwoofers in a well-engineered room can perform with multichannel content:

Robert Archer - Senior Editor, CE Pro

Bob is a dedicated audiophile who has been writing about A/V for Electronic House sister publication CE Pro since 2000.