What are relevant and useful specifications for a subwoofer.
A complete set of measurements, showing frequency response, capacity and distortion, is sufficient to tell how a subwoofer will perform. Then you can see how loud it can play at different frequencies, which is what you need to know for system design, you can see how low it can play and how loud. The graph also gives an indication of usable frequency range upwards.
However, most customers don't really want to see lots of measurements, they do not understand what those graphs mean, and they acknowledge that fact.
For the new Compact Horn subwoofers I did this:
The output capacity number and the frequency range gives the necessary information. You want to know the output capacity to be able to dimension your bass-system, and you want to know the usable frequency range to see if it reaches low enough and covers all the range up to the desired crossover.
The less tech-oriented customer still does not make much sense of the numbers, and is more likely to go by what I recommend. That's fine.
The tech-experts needs to be educated on the meaning of those numbers, because they make no sense to them since they are different from what other manufacturers typically publishes. They don't recognize the meaning of Output capacity, and the frequency range is not the same as frequency response with specified tolerance limits. This is labor-intensive - requires lots of time and effort to educate and show. Perhaps these customers should be ignored - it's really a question of effort vs. value. One solution could be to make additional specifications and measurements available, so they can see exactly what the performance of the subwoofer is. The graphs still require some explanation.
(The real experts usually get it, so they don't need any more education. They may ask for measurements, if they want more exact information.)
Typical subwoofer specifications are useless. They say nothing about capacity, frequency range specifications are at best unreliable. One English manufacturer speccs a small egg-shaped subwoofer with two 8" drivers as "7.5Hz" - clearly very, very far off from reality. Another manufacturer makes a hairdryer with two 6" or close to that drivers, claiming "14Hz" - I have heard it, and there is no way to get anything useful out of it at that frequency, from what i heard, it struggled hard to do normal bass frequencies.
Capacity is important to know because this tells how loud the subwoofer can play in the room. This is the number you use to determine how many units you need to achieve your desired spl at the listening position.
Frequency range is the usable range - how low it can play at still somewhat useful output level, and how high up you can set the crossover. For a subwoofer, the frequency response is largely irrelevant, you only want to know the range, and as long as the subwoofer is designed for high sound quality the response will be smooth between lower and upper limit. If the curve is flat or tilted or in some other shape does not matter, because the in-room frequency response will be dominated by the room, and will need adjustments in dsp for optimum performance.
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Since the real experts are on data-bass, this is the place to ask for opinions on this - how to specify subwoofer performance.