Subs & Group Delay

A couple of follow up questions.

1)An SVS 13" Ultra with its ports sealed shows app 5% THD at 20hz at 100db anechoic. In a real world listening room, that's presumably a crushing spl, so could one safely assume distortion is no issue?

2) The same sub stays below 20ms group delay at 20hz. Is there any benefit to looking for a lower # here? Also, is the goal in GD minimizing the value, or matching it to the gd of your main speaker near the x-over point? (Does that question even make sense?)

3)Magfan - how does a low x-over attenuate 3rd order harmonic distortion. I assume that the sub generates this distorted output as a by-product of its response to a 20hz inpu signal. It seems that the x-over point - which attenuates inputs signals - shouldn't affect this - Am I mistaken?

Thanks in advance.

Marty

1)An SVS 13" Ultra with its ports sealed shows app 5% THD at 20hz at 100db anechoic. In a real world listening room, that's presumably a crushing spl, so could one safely assume distortion is no issue?

It is pretty impressive yes but a far cry from a crushing SPL due to the fact we can barely hear 20 Hz. For comparison it would sound as loud as a 40 db SPL note at 1 Khz...a little above the rustling of leaves. The good news is that if your sub can achieve 115 db SPL then it suddenly becomes much more audible - as 120 db SPL will sound like 80 db SPL at 1 Khz - so clearly audible even above a conversation. Also there is not much at 20 Hz anyway so the biggest benefit is the lack of distortion at higher harmonics rather than any bone crushing sound.

The reality is that a great sub will NOT sound devastating on a 20 Hz pure tone at 100db SPL - is will be mostly inaudible if it is a good high quality sub!!!

Is there any benefit to looking for a lower # here?

probably not but lower is always better - it just may not be audibly perceptible.

I assume that the sub generates this distorted output as a by-product of its response to a 20hz input signal.

Exactly - in most subs playing a 20 Hz tone most of the sound you are actually hearing is distortion.

I've never bothered about the science, never had the time, when my ears were still good, but experimented happily using my ears and from what they told me, I'd agree completely with what Shadorne has said, namely the lower the better and even if you cannot --and shouldn't-- hear it,( if you did, it was somehow funny, not right) you could feel it.

Shadorne,

Your point is taken regarding sensitivy vs. frequency - do you miss your "loudness" control?

However, when I run test sweeps on the SMS-1, the lowest audible tones generate a VERY loud rumble at 90db and up. Whatever frequencies we're talking about here (it appears to be app 25hz from the readout), it should be low enough to fall into the "insensitive" range of perception. This doesn't seem consistent with "rustling leaves" and I wonder what the explanation is. I'd also note that A0 on a piano (typically 27.5hz) registers as LOUD! well below the levels implied by your math (the room is not shaking). Any thoughts as to what I'm missing here?

Beyond this, my intended point re:crushing spl is that (it would intuitively seem that) the 100db anechoic test output should register much higher than 100db in a typical listening room. Obviously, there's lots of things going on that affect the change in output at low frequencies between anechoic and real world environments, but my assumption is that the same signal in a 15' x 20' room should be perceived as much louder than in the (quasi) anechoic measurement environment used by HTshack. Do you believe that this is a good assumption (at least below the primary cancellation frequency)?

Finally, do you have a sense as to which test sweep (they go in 5db increments from 90db to 115db) in those subwoofer tests might be the best proxy for LOUD listening - (music) in a real room. Obviously, there are too many variables for a precise answer, but, as a practical matter, there should be a reasonable answer (or range of answers) to that question. Just wondering which sweep you might look to in evaluating a sub.

Thanks in advance.

Marty