Infinity Prelude MTS loudspeaker Page 3
Incidentally, after looking at the room data, Dr. Toole declared my new room "nearly ideal." Coming from him, that was reassuring!
Most rooms have a bass "bump" somewhere below 80Hz, which makes it difficult for designers to voice a speaker with full-range response, and for buyers to place it correctly. While careful speaker placement can sometimes reduce the bump, the placement may be impractical, or a poor choice for effective soundstaging.
Infinity has come up with R.A.B.O.S., a user-friendly, single-variable-frequency parametric equalizer that allows the buyer or dealer to find and "dial out" the bump. You're provided with a high-quality, battery-draining (keep spares handy), digital Sound Pressure Level meter, a test CD, a set of blank graphs, and a really ingenious clear-plastic Bandwidth Selector gauge to help you determine the Q of the bump you want to remove.
Using the meter and the CD's test tones, you plot the low-frequency response of the system in your room. Usually, one peak is dominant. (The excellent instruction manual also covers some common multi-peak scenarios.) Three small screwdriver-adjustable pots, accessible on the subwoofer's facade, allow you to select the peak's center frequency, approximate its bandwidth, and then flatten it electronically (footnote 1).
There's no guesswork involved. Once you've plotted the curve, used the Bandwidth Selector, and done some simple math, you refer to a chart that tells you how many clicks to turn each pot. You then re-run the test and plot the resulting curve, which now should indicate that the peak has been eliminated or greatly diminished. Finally, you set the subwoofer output level using another test tone (though you can set it any way you like), and you're done.
Unfortunately, FedEx drop-kicked one of the woofer boxes, which cracked the driver's cast basket and pulled the surround from the rim. It sounded worse than it looked, and it looked bad—though we didn't know to look until we heard it trying to reproduce 20Hz.
I was looking forward to demonstrating good analog to Toole, but it wasn't to be. We did run R.A.B.O.S. on the intact channel, and, with the exception of a pretty serious dip at 66Hz caused by an axial cancellation mode (plenty of 66Hz on either side wall opposite my listening position, but almost none where I sit!), the response from 20 to 100Hz was ±4dB with no EQ applied.
As Toole had predicted, my room was pretty much "ideal," and needed no EQ. He also predicted that the addition of the second woofer would smooth out the dip, and again he was correct. With both channels working, the response was 20-80Hz, ±3dB. I'm glad my new room measures up—if I ever move again, it'll be from here to the nursing home. I ain't moving my records again.
Can You Measure Your Way to Success?
The Prelude MTS is the only loudspeaker I know of that provides the means for you to measure and adjust its bass performance to fit your listening room, and in terms of low-frequency extension in my room, it was a complete success.
But there's more than LF extension to good bass—low distortion, for one, and high volume without dynamic compression for another. Getting quality 20Hz performance out of the Prelude's rather small sealed box requires lots of power and a rugged but light, stiff-coned driver. The trapped air is going to fight the driver; power can overcome the resistance, but if the cone isn't extremely rigid, it will flex and distort, especially given the long excursions required to move large amounts of air.
What did the trick was 850W of BASH amplification (love those acronyms) and the light, stiff, low-resonance C.M.M.D., tied to a hefty voice-coil/magnet structure. "We drive the shit out of it" was Toole's scientific explanation of how the small box could produce such prodigious, super-clean low bass. I knew it the first time I heard the foundation of this speaker. It wasn't overwhelming, just there. It rocked when it was supposed to, otherwise it shut up. The low bass was tuneful, supple, and in the right proportion.
Footnote 1: While you can try to reduce peaks, you don't fill in dips. The latter are usually due to cancellation between two interfering sounds. If you try to fill in a dip by pumping more power into the room at that frequency, the interference notch is unchanged but the system is now working much harder, to the detriment of its dynamic range.—JA