Test Results
Velodyne’s CHT-12Q Subwoofer proved to have a very flat and extended response in the Jazz/Classical position, which is shown in Graph 1. This trace, measured at three metres, using pink noise as the test signal, is shown unsmoothed (red trace) and smoothed to one- third octave (black trace). You can see that the frequency response extends from 30Hz to 130Hz ±3dB. Velodyne specifies response as 25Hz to 120Hz ±3dB, so you can see that Newport Test Labs’ result shows the Velodyne exceeds the specification at the high end, and comes up only 5Hz short in the bass. Al- though these differences between the spec and the measurement are so small as to be com- pletely insignificant, I should point out that the ‘Movies’ preset boosts the low bass consid- erably, so that using the ‘Movies’ equalisation preset extends the bass down to 20Hz within the ±3dB tolerance. (The lift is shown clearly on Graph 3, and this trace is of the bass driver only, without the contribution from the bass reflex port.)
The second graph (Graph 2) shows the nearfield responses of the bass driver and the slot-shaped bass reflex port. You can see that Velodyne’s engineers have got the port working beautifully, with its maximum output almost shelved between 30Hz and 50Hz. The roll-off at high frequencies is smooth and controlled and almost identical to the woofer
roll-off, so the re-inforcement between the two is perfect. The roll-off below 30Hz is also well-contoured.
Graph 3 shows the equalisation applied by using the different presets, as well as the differences in level between them. You can see that if you are listening to the Jazz/Classical preset (black trace) and switch to any of the others, you get not only different frequency contouring, but also a boost in output level. Obviously the boost varies with frequency but on average the increase is around 2.5dB. Note that all these traces are of the bass driver only: the contribution from the port has not been factored in as it has been in Graph 1.
The final graph (Graph 4) shows the effect on the frequency response of the bass driver of using the maximum and minimum settings of the low-pass filter. This shows exactly what I would have expected, so Velodyne’s specifications are spot-on. Although the port’s output is shown on this graph, you should note that its output has not been scaled to compensate for the difference in radiating area between it and the bass driver. However, the extended horizontal scale of this graph, compared to the other three graphs, has the advantage of showing that the bass reflex port does not produce any unwanted resonances or ‘leaks’ at high frequencies. Overall, an
excellent set of results. | Steve Holding |
It’s important to remember that hav- ing an onboard subwoofer calibration system such as the one that is fitted to Velodyne’s CHT-12Q is not a universal panacea. If you put any subwoofer with an automatic equaliser in a position in the room for which it’s totally unsuited, the circuit will be able to make the best of it, but you’d be far better advised to first find the position in the room where the subwoofer sounds the best, and then use the circuitry to make a good thing even better.
The two main culprits that affect the response of any subwoofer are room modes (eigentones) and room load- ing. Let’s look at loading first. If you sus- pended an EQ-equipped subwoofer in the middle of your room, well clear of any boundaries, the computer would quickly work out that this was a terrible position and would dial in maximum low bass boost. This means the internal amplifier would likely be working flat-out
all the time, with the result that dynamics would suffer. Obviously, in a real situation, the subwoofer will at least be sitting on the floor, so the low bass will be helped along by being close to one boundary. However, if you move an EQ-equipped subwoofer close to a wall, so it’s interact- ing with two boundaries, it will deliver even more bass as a result, so less equalisation would be required and the internal am- plifier would have to work less hard. If you place the subwoofer in a corner, where it would ‘see’ three boundaries, the com- puter would then very likely be able to at- tenuate volume at low frequencies, rather than boost it, which would have the effect of freeing up amplifier power, and increas- ing dynamic ability.
As for eigentones, they’re what result when low frequencies interact with the phys- ical dimensions of your room,so they’re very difficult to eliminate—or even modify—with- out resorting to moving walls and raising ceilings.
The best way of dealing with them is to use multiple subwoofers (at least two, but ideally four!) but since this solu- tion doesn’t appeal to many people for reasons of both cost and aesthet- ics, the next best option is to position the subwoofer where you don’t excite any.The only way to do this is by putting the subwoofer in all the places you’d like it to go, and choosing the position where it sounds the best, and this is the technique you’ll need to use with all subwoofers, no matter whether they’re equipped with auto-equalisation cir- cuits or not.
