Loudspeakers: When Is Good Enough, Enough? Part 2 - page 2

Beware though; some less well designed systems which appear to play low levels with good dynamics can sometimes sound very boomy once the volume is turned up. Systems designers will generally shoot for a certain system "Q". By definition a Q of 0.5 is "critically damped". Today's designs are more likely to have Q between 1.0 and 1.3. This means the systems will have measurable but generally inaudible ripple. The latter Q range is a more common design target for mid-to-high quality system designs.

High level listening can help to define a systems upper design performance envelope. Driven hard enough any system can reach the point at which woofer or midrange drivers will "flat-top". That is, the point at which the driver's cone/voice coil assembly will get far out of the magnetic gap causing loss of magnetic and electrical control.

Assuming your amplifier still has power to spare at this point, you should be able to attribute the distorted sound you hear to the driver itself. This type of driver distortion is more common in sealed box designs which put all the low bass energy content to the driver. In ported designs the port will take the very low bass duty off of the woofer so the it's much harder to detect the driver reaching its excursion limits. It doesn't really sound bad. It just won't play any louder.

There is one other form of high level distortion which I've heard only rarely. I would describe the sound as "congested", primarily at the crossover region. This condition is puzzling at first because it doesn't seem to happen at an overly high SPL. Here's the set- up for being able to analyze this condition.

At a dealer, you may have listened to a pair of very small satellite speakers in formed plastic or extruded aluminum enclosures. The enclosures are well styled and unobtrusive (++WAF!). And the drivers, at least from the outside, appear to be made of quality parts. You listened in the dealer's small-system listening room which has quite a bit less cubic volume than your home theater room. They had seemed to play loudly enough with plenty of clarity.

You take the satellite system home and sure enough, they easily and unobtrusively blend into your home theater room. That's because your listening space is fifty percent larger than the dealers'. You turn up the system for the first time, and just when you reach a comfortable listening level you hear the distorted "congestion" right in the crossover region.

The culprit is the quality of components in the crossover. Those styled enclosures, and their tooling, can cost big time. So precious crossover dollars which were originally allocated in the bill of materials become cents. Make no mistake, systems engineers always lose to the outside cosmetic design firm and the sales department. Beautiful cosmetic designs can also leave precious little space inside for, say, an air-core inductor in the tweeter circuit when an tiny gauge iron-rod inductor will fit. (Not to mention that the iron-rod inductor is about one quarter the cost of an correct gauge air-core.) The same cost cutting happens with capacitors wherein high DF (dissipation factor) non-polar electrolytics are used instead of better quality (and larger) alternatives.

Again, it is rare to find the combination of a high quality styled cabinet with well designed drivers and a really cheesy crossover but with the proliferation of so many such designs these days it doesn't hurt to be on guard.

Part 3 of this series will address the "dynamics" of speakers and how they are intimately related to the room's size, proximity to room surfaces and the stiffness of those surfaces

* Sean Olive is Manager of Subjective Evaluation at Harman International in Northridge, CA.