Subs & Group Delay

I'm aware of a controlled listening test in which 30% second harmonic distortion was statistically undetectable

Second harmonic is fairly benign but 3rd harmonic is bad - so it depends on the type of distortion. The other issue is that 1% distortion at the 3rd harmonic of a 20 Hz fundamental will sound nearly equally as loud as the fundamental note itself due to your increased hearing sensitivity at 60 Hz compared to 20Hz (see Equal loudness contours). So in the end, a distorting subwoofer simply populates the bandwidth where your hearing is much more highly sensitive from 60 to 180 Hz with audible response. This is may be quite the opposite of your original intention when buying the sub - which was to augment the bottom octave rather than overwhelm the 60 to 180 Hz range.

Unless you listen almost exclusively to Baroque Organ, I can't imagine much 20hz content. For HT use, where the 'musical' properties of a sub are.....less needed......it may even be a non-starter when listening to 'effects'.
Also, if Crossed-over low enough, that mitigates the effect of 20hz 3rd harmonic. I cross over at between 40/45hz and even my vented HSU produces pretty tight stand up bass. So the amplitude of 3rd/5th harmonic 20hz is pretty well attenuated.

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.