Hi all,
First, remember that terms like “fast”, “tight” ,”musical” are subjective in nature. So by definition, what audio enthusiast A thinks is "fast/tight/musical"...audio enthusiast B may not. Also, human bias will enter into the equation at some point. for example, If a manufacturer goes into great detail about how “fast” their bass is...after reading enough about that...some folks will naturally have a predisposition to believe it. So when they get the product home...by god the bass IS fast..
(this is the same human workings that cause us to hear differences when painting the edge of your CD green---well, the website explained it in 5 pages with big words and they have a bunch of testimonials...so...)
When talking about "fast bass", a subwoofer reviewer once said to me..."oh yeah, the bass was so fast it got out of the room before I could measure it".
It is also important to consider the length of sound waves in this context and how that relates to their decay in a typical room. A 20hz sound wave is about 55ft long. The longer the sound wave, the more time it is going to take to decay. This longer decay time is often interpreted as “slow bass” simply because the bass doesn't *snap* to a sudden stop(as it may with a subwoofer that rolls off in the 35-50hz range). It isn't a coincidence that the majority of subwoofers that claim to be “tight/musical” are small/moderate sized sealed designs. If you look for objective performance data on these units, you'll notice there is one definite trend...they all tend to have a frequency response which rolls off in the 32-50hz range. Tom Nousaine has a lot of good FR data on a ton of subwoofers. Audio magazine and Audio Ideas Guide both used to have a lot of FR data in all their subwoofer reviews too. (AIG might still, haven't had time to pick up a copy for a few months).
Now, there is nothing wrong in preferring that type of sound...it is a personal preference after all. But the correlation between what you think is “fast/tight” often has little to do with any magic a given manufacturer knows...and more to do with the very basics of the units performance...the frequency response(and several other factors introduced below).
In the words of Bill Cosby, (paraphrased)"I told you that...so I can tell you this"...
Among SVS designs, the Ultra driver is definitely top dog. I'm not going to wax poetically about its design...it is just a very good unit(and very expensive). The dB12 (Plus driver) is also a very good design, just not up to the same standards. The same can be said for the ISD driver we use in the PCi/CS,PB-1 and PB-2isd subwoofers. At low volume levels, all three of these units are going to sound nearly identical...only as the volume levels increase will the qualities of the dB12 and then the Ultra driver manifest themselves into audible benefits. Just how MUCH the volume levels would need to increase depends on many variables...and just how audible the differences were...would depend on additional variables...including the listeners experience and insight into how these difference may present themselves during source material playback. Here is an example of how THD (total harmonic distortion) may present itself…
Let’s use 25hz and 90/95/100/105/110dB levels (just an example remember…)
The Ultra driver may produce <1% THD up to 100dBs, than 3% at 105 and 6% at 110.
The dB12 driver may produce <1% up to 95dBs, 2% at 100dBs , 6% at 105dBs and 9% at 110dBs.
The ISD driver may produce <1% at 90dBs, 2% at 95dBs, 4% at 100dBs, 8% at 105dBs and 12% at 110dBs.
Now, you can say…”well, the ISD driver has 4x (!) the distortion at 100dBs…boy that would HAVE to sound just god-awful. Well, maybe. If you were listening to a sine wave. With source material…this difference will be inaudible in just about any imaginable scenario. (the difference between 1% and 4% THD at 25hz) Most folks will notice the difference between 1% THD and 10% THD. But anything <5% (<35hz) is going to be very tough to discern with source material (and practically impossible with the LFE on DVD-films).
The interesting part about THD, since it manifests itself at multiples of the source frequency...a higher THD level can actually make the subwoofer seem more musical. A musical instruments sound is defined by its harmonics. A bass guitar playing a 42hz note will have it's THD at 84,126,168hz (ect). So a subwoofer having a higher 3rd order harmonic (126hz)...can actually give the audio presentation an "easier to follow" bass line. Since that 126hz harmonic is now louder (than it was with the subwoofer with less distortion).
Another important issue is dynamic compression. A subwoofer that doesn’t suffer from much dynamic compression is often referred to as “effortless” in subjective descriptions. One way to test for this is to loop a bassy scene on a DVD and to trun up the system in 3dB steps. Set your SPL meter at any spot and check to see when the bass stops increasing the full 3dBs. At some point…it will only get 2.5dBs louder, 2dBs louder,1dB louder and finally…it will stop getting louder. I would only recommend raising the volume until it only got 2dBs louder because by that point…the subwoofer is already straining. Pushing it further will only increase any chances of damaging the unit.
Another example, …say a 2500 cu-ft room, and a nice 26hz LFE scene(like the heart beats in the opening scene of The Professional…) Let’s say 1 meter from a corner loaded unit.
A 20-39PCi may begin to show some compression at 110dBs. Impressive, but still 11dBs under reference level with all speakers set to small. A 20-39PC+ may get to 113dBs…and a PC_Ultra may get to 115dBs. So in this context(all other factors being equal), the PC_Ultra will have about double the clean output capabilities of the 20-39PCi…if dynamic compression is one of the factors used in determining that(clean output). The driver directly relates to compression in two primary ways…excursion capabilities and thermal compression. If the driver cannot move any farther in one direction…the nass will stop getting louder. Once the voice coil heats up to a given temperature, the driver won’t be as efficient in turning electrical power into mechanical motion. In this regard, the Ultra driver has a big advantage over the ISD. It isn’t that the ISD is a substandard unit…but with a 10 layer custom would coil…the thermal capabilities of the Ultra driver are incredible.
So?
On louder, sustained bass…it wouldn’t surprise me if someone felt the dB12 driver seemed “tighter” than the ISD. And, it wouldn’t surprise me if someone felt the Ultra driver seemed “tighter” than the dB12. (I know ,I know…all that and my answer is actually two sentences long..
Tom V.
SVS
