Subwoofer Qtc and 'tightness'

[Serow]

I'm confused (not for the first or last time ) about Qtc. Here are two conflicting quotes, but which is correct? Thanks for the help.

From Vandersteen:

" In Subwoofer engineering terms, system Q is the product of a complex mathematical equation derived from driver, electrical and enclosure parameters. In practical terms, it relates to the character of the bass response. A low Q subwoofer sounds highly damped and very tight. A high Q subwoofer produces a warm loose bass with more energy in the most audible bass range. The trick has always been to try to find the subwoofer with the Q that best matched your listening room and personal tastes.

The 2Wq is the first subwoofer to feature adjustable Q.A bass contour control on the rear panel allows you to set the system Q anywhere from ..5 (Slightly overdamped, i.e.., tight jazz sound) to 1.2 (Significantly underdamped, i.e.., typical mass-market home theater sound)."

From GRollins:

"A point that tends to get lost in the shuffle is the Q of the (sub)woofer. Critically damped (i.e. .707) is a maximally flat Butterworth alignment. Bessel will give you .5, etc. The thing is that some people go for a .5 alignment because it 'gives you more bass.' Hmmm...well, yes...and no. It rolls off more slowly, but begins rolling off sooner (i.e. higher up). A properly designed .707 cabinet will roll off with a sharper slope (which some chose to interpret as 'less bass') but stays within a tighter range in the pass band and the rolloff begins at a lower frequency. Your choice. But there's a catch. Going for a Q of .5 to get 'more bass' means a compromise--the bass you get is less tight. You want really tight bass? Try a Q of 1 or more . Of course, now you've got a pronounced hump just before rolloff, but hey, you wanted tight bass.
The stereo gods giveth and the stereo gods taketh away."

[kelticwizard]

The following thread contains the response curves for closed boxes with various values of Qtc. As you can see, A Qtc of .5 does indeed roll off quicker.

Response Shapes of Various Values of Qts and Qtc

[Serow]

I know that the low-Q design will have a gentle low-frequency roll-off starting at a high-frequency and that high-Q will have the opposite tendencies, in general.

I've always thought that high-Q=steeper roll-off slope=greater group delay= 'loose' bass. This is what Vandersteen is stating. However, GRollins seems to be indicating the opposite.

[Serow]

Here's another quote from Jon Rasmussen at vacuumstate.com:

"It has been known for some time that where-ever there is a resonance, if the Q is kept down to or below 0.5 it can be described as critically damped, so hence the title, a Critical Q Sub, short for Critically Damped Q Sub-Woofer. This Sub will have superb transient response with virtually no over-hang and will start and stop on a dime."

Again, this is in agreement with Vandersteen but opposite of Grey's statement.

Note: I'm not trying to disprove Grey's statement, I'd just like to make sure I build my box with the correct Q for my needs.

[aaroncgi]

I just modeled a few Shiva in sealed boxes. It's interesting to note that the box with Qtc of 0.7 actually has a higher value of group delay than the Qtc 1.0 box, but we are talking less than 1 ms difference, and the peak is 20 Hz lower on the box with higher group delay. As to F3, it really doesn't change all that much. Qtc of 1.0 vs Qtc of 0.5 are have virtually identical F3's, which are only about 10% higher than a Qtc 0.7 box.

ANYHOW! I personally have listened to the same woofer in a box with Qtc of 0.7 and a box with Qtc of 1.1. There was no comparison as to which one sounds tighter and more accurate, the lower Q definitely was the best.

It's difficult to explain why using group delay figures however, because even a box with a Qtc of 1.5 is going to have considerably less group delay than a vented box in the bottom two octaves, or the bottom octave anyhow.

I think it's just more intuitive to use the definition of Q, or resonance damping. A higher Q has less damping of the natural resonance, and so is less able to control the woofer cone. Less control equals less tight bass in my book. On the other hand, that seems a little counterintuitive, as you would think a smaller box has more control over the woofer, being a stiffer spring and all... grr. Oh well, least I know my subjective tests are valid.

Serow,

To address your last point about overhang, this is one of the very things I noticed quite a lot with the high Qtc box. The bass most certainly did not stop and start on a dime! More like on a silver dollar...


Aaron Gilbert

[kelticwizard]

In addition to the previous link I gave, which had the response shape at various frequencies for various values of Qtc, here is the "step response" of variious values of Qtc, (and Qts in free air as well).

The "step response" means one square pulse, I believe with the wavelength of the frequency of resonance. Anyway, you can see how transient response and overhang are different for different values of Qtc.

The graph in the previous link I gave and this following graph will give you some idea of the tradeoffs between strong response and "overhand", "tightness", etc. I think you should have the whole picture now. It is up to the listener to decide between how much amplitude he wants versus how little distortion and "overhang".

Incidentally, the following graph is taken, with some minor additional graphic work, from the article "Closed-Box Loudspeaker Systems Part 1: Analysis" in the December 1972 issue of the Journal Of The Audio Enginneering Society. It was reprinted in their Loudspeaker Anthology Part 1. The author is Richard Small, of Thiele-Small reknown.

[planet10]

Believe Joe. What Grey is saying is only part of the story. With room gain a sub designed for flat anechoic response has way too much bass. You want your sub to roll off smoothlyin an amount just opposite that of your room gain. A bessel (Q=0.5) is probably the best choice in most rooms And as far as the actual box gives the best phase response -- critical Q. The downside is that this is often a pretty big box -- and if its your thing, won't give you the over exaggerated bass some like for their movies.

dave

[planet10]

kelticwizard was posting at the same time i was... his picture very clearly shows how much more "control" there is with the low Q boxes.

dave

[kelticwizard]

I would just like to add that research has shown the farther down the frequency scale we go, the less we are able to percieve distortions and overhang. What is acceptable at 30 Hz might not be acceptable at 300 Hz.

Also, the step response of Qtc = 1 is superior to just about any vented box alignment. Transient response and "overhang" vary with vented box alignment as well, but all of the common vented box alignments have worse step response than the Qtc. = 1 response.

What Grey is saying is that Qtc = 1 is the best combination of frequency response and low distortion. It is a matter ot taste, but the output at Qtc = 1 is 6 dB higher at the resonance frequency than Qtc = .5, and the "overhang" is not that far behind.

I'll be posting the step response of some common vented alignments in another thread, but not tonight. The difference is apparent.

PS: I'm a vented box fan. I like the extra output. But I think the people on this forum should be allowed to judge for themselves.

[kelticwizard]

Most of the speaker designers from the sixties and seventies went for Qtc's between .7 and 1. But what Planet 10 is saying is indicative of a new wave of thinking, where the gradual rolloff splices into the room gain-the tendency of most rooms to emphasize lower bass frequencies.

With higher powered amps readily available and cheaper than in the sixties and seventies, plus with more low low frequency content existing on CD's now, perhaps the low Qtc path is the way to go. It's a matter of taste.

Below is a graph of typical "room gain". Some programs give higher values-Pyle Industries tech support gave me a higher number at 16 Hz than this chart indicates-9 dB as opposed to 7 dB-but it is all just an average anyway. Bass response will vary not only from room to room, but even in different spots in the same room.