Could your chair or sofa damage your system’s performance?

You may not have this problem, but you might know someone who does! As the guy who wrote the owner's manual for the ARC/Magnepan Tympani 1Ds, I thought I pretty much knew it all when it came to installing these speakers and getting the most from them in any room.

That's why I'll never forget the humbling lesson I learned in my lofty role as an audio "guru." Here's what happened:

As a high-end dealer in the early '80s, I had sold a pair of Magneplanar Tympani 1D loudspeakers for use in an Audio Research system. I went out to install the system (which I always insisted on-and your dealer should do it for you as well).

I always carried along a 1/3-octave real time analyzer so that I could quickly see where the bass standing wave problems were located in the listening area of the room.

In less than an hour, I had found the best place to locate the speakers (somewhat tricky due to the negative bass waves emanating from the rear of the speaker), and the best place for the listening seat.

Using my basic 3-step installation technique in Tips #74-77 , plus the information outlined in Tips #78-89 , I worked to get the sound to where I'd be proud to send a prospect over to my client's house to hear what the Maggies sounded like in a home.

As I left, my client was effusively thanking me for getting him better sound than he thought was possible. I was a hero.

A week later, he called me to complain about a "thickness in the midbass." I don't know if you are old enough to remember the Tympani 1Ds, but bass definition and timbre were their best qualities (Harry Pearson, writing in The Absolute Sound , adopted the T-1D as the bass unit for his soon-to-become-famous no-holds-barred hybrid Infinity QRS/Tympani 1D system).

There was NO WAY we could have a thickness in the bass! My client must have changed some component or something (in other words, I was convinced it couldn't be my installation...).

Well, I finally stopped by later that day, expecting to point out the offending component. But nothing was changed in the system. And boy, was the upper bass thick! I got out my trusty RTA.

Sure enough, there was at least a 6 dB peak at about 125 Hz. Where did this come from? Well, I couldn't figure it out, but as I moved the RTA about two or three feet in front of the listening position I'd selected (and even marked!), the bass peak gradually disappeared.

So we moved the seat forward and, just to be sure, listened to hear what

the guru (me) had fixed. Oh, no, the peak was back!

I measured the response behind the listening seat where we had originally determined was the best seat in the house (literally). Now the peak was almost gone!

Anyway, I started to think I was on Candid Camera . I was looking so foolish. Then I noticed it. My client had a new sofa. When I had set the system up, we had used an occasional chair for the listening/voicing sessions.

This sofa had a tightly stretched back panel (leather/leatherette). It was stretched so tightly, it produced its own tympanic sympathetic resonances at 125 Hz. Removing the sofa solved the mystery.

So check out any system where the seating could cause a similar effect. I'm still surprised at how many systems can be affected.

Jim Smith

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