If you are asking how to tell if a sub is a good performer just from looking at measurements, there are a number of factors involved. One factor is cost; you can't expect a cheap subwoofer to perform as well as an expensive one, so the question then becomes how well does it perform for its price point? Another factor is size: small subs just can not match a large sub in terms of performance, but many people can not accommodate a large subwoofer, so the question is, how well does this sub perform for its size? Another factor is room size: the smaller the room, the more room gain will had, where sound pressure is increased from being contained in a smaller space, so the question then becomes will subwoofer X be able to meet your desired SPL in that space?
With that all being said, there are some things to look for in measurements to gauge the performance of a subwoofer. Here are some desirable subwoofer traits:
- Flat frequency response
- Dynamic range/ max SPL ability. This is a very relative metric, but I will say that, in terms of CEA-2010 measurements, 100 - 105 dB is fine, 105 - 110 dB is very good, 110 - 115 dB is excellent, and 115dB + is beastmaster mode. But this is relative to price, size, and frequency. For example, if a large, $10k subwoofer could only do 115 dB at 50 Hz, that is sadness, but if a medium sized $1k sub does 115dB at 50 Hz, that would be really good.
- Low distortion. This is relative as well, with different factors affecting what should be looked for, but generally speaking, I would say if THD is under 10%, that is very good, and if it is under 5%, that is excellent. But there are a lot of factors involved in what makes one measurement better than another. The SPL of the distortion measurement has to be considered as well as the frequency, and even the harmonic composition of the distortion.
- Low-frequency Extension. The importance of this is again very relative to personal desires, but if you want a sub that digs down to 16 Hz without problems, you should be looking for a sub that has a response that extends down to 16 Hz without rolling off too much by that point.
- High-frequency extension. The importance of this is also very relative, but it's nice to have, and it is something I look for. Some subs roll off the high end after 80 or 100 Hz or so. I'd rather have a subwoofer extend a bit higher than that, even if that high frequency extension may not ever be used.
- Time domain problems. This can be seen in the group delay measurements. If the sub manages to exceed 1.5 cycles above 40 Hz over a broad region, that would be cause for concern. I have never had a subwoofer that performed very badly. No competently built sub is likely to have a serious problem here.
- Compression behavior. Steve81 touched on this, but if a subwoofer's response shape doesn't hold for much of its dynamic range, it will change the character of the bass depending on loudness level, and that is not a good thing. The less change of a subwoofer's response thoughout its dynamic range, the better.
There is a starting point, but these are very broad outlines. There are different performance targets for different needs, so if a subwoofer doesn't do well in one of these response, that doesn't necessarily that subwoofer is bad. Performance metrics have to be viewed in context.
