"musical" Subs (1 Viewer)

[Edward J M]

You'll notice alot of the guys over in the DIY Forum will design a ported sub for its high output, but tune it very low(by commercial standards) because A--It will protect the sub driver when they want to play a bass heavy DVD at reference or near reference level and B--it shifts the high GD around the tuning point down where there's not alot of low frequencies being reproduced. That'll yield a sub that has the slam for Home Theater, but still is nice and tight for music reproduction.

The lower the tune point of the enclosure, the deeper the driver has to play before the resonator (in this case a port) begins to function. If you look at driver excursion vs. frequency, the excursion for the driver will always reach a peak before the tune point, and then it will drop dramatically in the region where the resonator is functioning optimally. A lower tune point requires more work from the driver (i.e., it must play deeper with more associated excursion) and is therefore less protective of the driver than a higher tune point. That is why (for example) the SVS 25-31 (25 Hz tune) series can play considerably louder than the SVS 16-46 (16 Hz tune) series with the same driver.

Below the tune point all vented enclosures lack the restoring force to properly control the driver and driver excursion always rises dramatically, to the point of bottoming the woofer fairly easily at subsonic frequencies. This is why SVS wisely employs SS filters on several of its powered models.

Anyway, I have tried (in vain) to determine which objective design criteria affect the subjective impression that a subwoofer is "tight", "fast", or "musical". It's not as easy at it might first appear.

Once might be tempted to say group delay is the primary criterion responsible for these subjective impressions, but I have found it's not that simple.

For example, group delay cannot be reliably detected under about 30 ms and only becomes significant in a vented enclosure at/near the near-subsonic tune point. In a properly designed vented alignment, GD can't make an audible difference on how a kick drum or bass guitar sounds.

Regarding transient response, here's a quote from a Secrets of Home Theater and High Fidelity article:

"I know that "fast" bass is an oxymoron, and if you really care about transient response, assuming a competent subwoofer design, you?re best off spending your energy addressing room interaction to obtain the flattest frequency response, and the transient response will follow. However, if you really want to nitpic, the fact is that aside from subwoofers that have peaky frequency responses themselves, and are doomed from the beginning, the greatest limitation of transient response is usually at the low-frequency limit of the subwoofer (and if it?s low enough so that you can?t hear it, who cares) and at the high-frequency limit, imposed by the low-pass crossover slope."

And here's a quote from Tom Vodhanel (SVS co-owner and designer) on the "musicality" of subwoofers:

"The 'musicality' of a subwoofer is dominated by several factors non related to the subwoofer at all (or related incidentally---like a built in XO). The room, the positions of the listening position and position of the subwoofer, the lowpass and highpass XOs used for both the subwoofer and the rest of the speakers, the phasing of the subwoofer relative to the rest of the speakers, and the calibration of the subwoofer.

The primary factor not related to the listening room or the setup method is simple user preference. The deepest bass has the longest soundwaves...the longer the soundwaves, the more of a bass *decay* the room will have. Deep bass takes time to decay, that is natural and inherent to the beast. Some folks mistake the longer decay for "slow" bass...when in fact, it is actually a more faithful reproduction of the source material.

Then there is an issue of harmonics. Much of the pitch of the musical bass/drum will be determined by the number and amplitude of the harmonics of the fundamentals note played. To some ears, if those harmonics are emphasized...even by a very small amount...the *pitch* of the bass guitar is made more obvious. The pacing of the bass line(s) become easier to follow...and folks can feel like that aids in the musical nature of the source playback.

So in the end, the most accurate audio reproduction may not be what someone prefers."

 

[Zack_R]

Anyway, I have tried (in vain) to determine which objective design criteria affect the subjective impression that a subwoofer is "tight", "fast", or "musical". It's not as easy at it might first appear.

I might be grasping at straws here and I'm repeating what some have said, but to me a musical sub has the ability (perceived or otherwise) to play multiple bass frequencies accurately with little THD.

For example, at 80 db and 15 feet from the sub, my BIC America 12" sub charts an almost identical frequency response curve using 1/12 octave tones as my Ultra down to the mid thirties. I can't assess THD but to my ears the tones sound pretty similar. However, play most any music containing more than a kick drum or a slow beat from a rap song, and the BIC sub blurs the notes while the Ultra plays with much greater clarity.

The speed difference is greater illustrated with the Avia subwoofer set-up guide (LF / sub, RF / Sub). My 70 - 35 region contains several peaks and dips, some large some small. When I calibrate the BIC the SPL meter (C-weighted slow) barely moves during the subwoofer bass portion. You also hear little to know variation in the bass frequencies as the pink noise moves around. When I perform the same test with the Ultra you can hear detail to the bass and the SPL meter reacts accordingly, mimicking portions of the uneven frequency response of my room.

So with the above as backgound, I think a way to objectively determine a subs speed or muciallity may be to measure it's ability to play two or more test tones simultaneously while charting THD. If the THD is significantly lower (say 10 db) then that sub will likely be musical within it's operating range.

For a ported sub you could play a tone at the tuning frequency and play another tone one half octave up, kind of how Johnson did when he tested the Ultra. This keeps it fair for subs tuned differently. I think this is a great test for ported subs as it illustrates a subs ability to play through the resonator and have the speaker moving at a higher than normal excursion while maintaing low THD.

For a sealed sub you may have to go off of what the manufacturer lists as its -3db point to determine the low end.

My thinking is if a sub can handle a test similar to the above, then higher frequencies such as 50, 60 or 70 should be a piece of cake. Doing well or poorly in a test like this could also be an indicator of a subwoofers overall design (sSpeaker, cabinet, ports etc.).

 

[Bob_M]

>So with the above as backgound, I think a way to objectively determine a subs speed or muciallity may be to measure it's ability to play two or more test tones simultaneously <

That is interesting, how does a driver produce two differnent tones at the same time? I would assume it some sort of a sum between the two tones. The cone is either moving out or in at one instant of time?

Bob

 

[Jack Gilvey]

http://www.trueaudio.com/basslst2.htm

Deep bass takes time to decay, that is natural and inherent to the beast. Some folks mistake the longer decay for "slow" bass...when in fact, it is actually a more faithful reproduction of the source material.

Yup. As I've stated a few times, most "audiophiles" would describe an actual thunderclap or passing train as "too boomy".

 

[David Judah]

Here's another link as well:
http://www.adireaudio.com/tech_papers/sub_gd.htm

DJ

 

[Michael R Price]

You guys covered this topic well. When I get the chance, I'm skipping sealed boxes and going straight from vented to dipole bass.

 

[David Judah]

A lower tune point requires more work from the driver (i.e., it must play deeper with more associated excursion) and is therefore less protective of the driver than a higher tune point

You would certainly want a driver with good excursion, and you wouldn't tune low if the driver's T/S specs weren't indicated for it anyways. What you would get is a lower point where the driver is unloading, so while you may have LF content, there would most likely be less energy there.

DJ