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 graphs 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 how the subwoofer performs. The graphs still require some explanation.
(The real experts usually get it, so 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. Capacity is never specified, instead they quote amplifier power, which is not relevant because it says nothing about acoustic sound pressure output. One 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 something 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
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.
Even though you may not be able to calculate accurately what spl you get in the room, this data can be used to compare different subwoofers, provided measured in similar conditions.
Subwoofers are measured in 2PI, or groundplane, typically at 1m or 2m distance. Groundplane means a very large space with solid floor boundary, it is the same as anechoic part from the 6dB boundary gain from the groundplane. If you have a 2m distance measurement – like from data-bass – and want to convert to 1m, then simply add 6dB to the number. Also note distortion levels and duration for max spl numbers. If one measurement is at 2% distortion continuous RMS, and another is 20% distortion short-term peak, those numbers are not comparable.
Frequency range
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.
Examples
From the presentation of the V6 we find low limit is 30Hz. The measured frequency response shows that the response rolls off around 30Hz, it is tilted slightly toward higher frequencies, and it is smooth.
In- room we get this result, after some dsp adjustments, in Room2:
In a very different room, with no custom dsp adjustments, we were lucky to get this decent response (blue), and a huge improvement from the main speaker only response (red):
We see that in both rooms the response is flat down to 22Hz, where it drops off.
With 4 V6 units stacked up front, in Room2:
Same roll-off, slight improvement in response over the 2-unit system.
Using 4 V6 distributed front and back, in Room2 (blue = 4x, green = 2x):
The 4x system fixes the midbass dip.
All those measurements indicate that extension will be 22Hz in many rooms and configurations. Looks like real performance is better than specification.
However, the output capacity of the V6 drops below 30Hz, and there is simply not enough level to do much useful below 30Hz. The specification is correct – usable range is from 30Hz and up.
Two V80 should give decent capacity, specified to 110dB/20Hz. Let us look into what this means in-room:
We see there is clean output at 114dB, distortion at 20Hz is around 2%. We also see that distortion is controlled down to 15-16Hz – which corresponds well with the specified frequency range – 16Hz and up.
Actual spl in-room will depend on room size, placement of subwoofers and listener, rigidity of walls, windows and openings. The 110dB number is for one V80 at 1m distance, measured in 2PI surroundings. Two units sum up to 116dB, and some room gain makes up for distance larger than 1m. 114dB seems reasonable. And there is more in reserve, if higher distortion levels are accepted.
Three different subwoofers in same room – V6 (orange), V80 (green), V6030 (red):
We see the V6 and V6030 rolls off very similar, but the V6030 is the one that still has usable output down to were it rolls off. A frequency response graph does not show this.
(Differences in midbass frequency range are due to different location of the subwoofer units – if the V80 and the V6030 were in the same location, they would measure similar from 30Hz and up.)
Placement affects roll-off and response. V6030 in a different location, same room:
From these frequency response charts we see that the frequency response of the subwoofer itself does not determine the in-room response we get in real-life situations. What is important to know is the range where the subwoofer has usable output and sufficiently low distortion, so that we can use equalizer in the dsp to adjust the response.
Summary
- Frequency response graph is largely irrelevant for subwoofers
- Need to know capacity
- Both usable frequency range and capacity can be expressed as simple numbers to give relevant information for comparing subwoofer performance
- Need to understand the meaning of those numbers