As you may have noticed, no subwoofers have ever been published on these
pages and the aim of this project is to supplement a pair of smaller
speakers with subwoofer(s). Quite often we see people buying a pair of
very small speakers in the 3-6 liter range due to high-WAF or other
preferences. Next we see a subwoofer tucked
away up against the wall or in corners in order to help
in the 30-100 Hz range. This may not exactly turn a mini into a maxi,
and the minis not having a high-pass filter really
can't be played louder than before.
Otherwise there's nothing new under the sun here: A subwoofer featuring the well-known ScanSpeak 26W/4558T00 driver and the plate amp Hypex DS 4.0. Google the driver and you'll find a lot of applications. From my google search I find discussions on the significance of 22 mm versus 26 mm cone excursion, but these people are talking earthquake movie set-ups and large rooms. Not what I'm aiming at here, rather a decent response down to 30 Hz for general hifi application. Subwoofers and plate amps have been around for a long time and subwoofers even has a dedicated website, http://www.diysubwoofers.org
So, kindly sponsored by ScanSpeak, I got two 26W/4558T00 drivers and I ordered a
pair of DS 4.0 plate-amps.
Modeling the 26W sub soon makes it clear we're not going to use a ported cabinet. We need a decent port diameter and the port is going to be too long, really long. Making a slotted port soon adds one third to cabinet volume, so this was given up also. Using a passive radiator is basically the same as a port, only does not take up any volume. Downside of passive radiators is cost, but we do pay dearly in many other areas to reduce volume.
Subwoofers are rarely very sensitive, often 83-86 dB/2.8V but as we place them close to the front wall or even in corners we get some 3-9 dB room-gain, hence suitable for most main speakers. From simulation seen below you can see the impact of having the 26W some 30 cm off the floor, some 50 cm from the front wall and 180 cm to side walls. We get some 4-6 dB room gain, sometimes even more of in corners. If we didn't have room gain, many speakers would sound rather lean, even from an 8" bass driver. The tricky part for any loudspeaker constructor is to voice the speaker taking into account an estimated average room gain contribution in the low range, maybe except for very small speakers were we really do not expect much bass at all. Some people are disappointed from their 6" bass driver, which may have a decent sensitivity of e.g. 88 dB, but placed on a narrow baffle and into the room, it looses a lot of low-end power due to baffle step loss and lack of room gain.
Download driver specs here.
Passive radiators kind of falls into two groups, those for general hifi
aiming at 30-40 Hz bass extension and then dedicated subwoofer passive
radiators where we are talking moving mass of 4-500 grams. Here almost 4
times the moving mass of the subwoofer unit. We're talking deep bass
from small cabinets and the price to pay is efficiency.
Download DS4.0 specs here.
Box simulation vented
What has to be taken into account when dealing with subwoofers, is the room-gain. The room is an amplifier and adds to the low end response as can be seen from above from placing the driver 50 cm from front wall, 30 cm from the floor and 180 cm from side walls. We get some 4-5 dB extra from such placement. The reason a lot of subwoofers work at all because efficiency is sometimes appallingly low.
Box simulation passive radiator
As can be seen we get a Fb of ~20 Hz from these heavy weight passive
As always: Test cabs before cutting final panels
My test cab is 43 x 43 x 43 cm making a total of 57.5 litre from 22 mm MDF. The plate amp enclosure is 7 litre brutto, hence a net volume of ~50 litres minus drivers and bracing but I usually make the increase in volume from damping material make up for the volume of drivers and bracings. I used 30 mm egg crate foam on internal panels. From experiments on decreasing cabinet volume my final cabs will be 40 litres and with the 26W/0-00-00 passive radiator from ScanSpeak.
50 liter cabs, Vifa passive radiator
First of all: Setting up a subwoofer is anything but trivial, in
particular with the Hypex DS4 where we have the option of bass lift and
without measuring gear, it's even more
troublesome. We may take near-field measurements of the bass driver and
passive radiator and make a summation and tune the bass lift to a flat
response down to some 25 Hz, but then we place the subwoofer up against
a wall and have to do it all over again due to room-gain. Room-gain is
very much part of the equation. The good thing is that room gain is
almost linear in the range the subwoofer covers, so once tuned in for a
specific pair of satellites, we only have to deal with level.
First speakers on test were my Discovery 12W speakers. Small they are and my expectation was that this wouldn't work very well. I was expecting a serious lack of coherence due to the fact that the 12W doesn't move much air in the 100-200 Hz region. Trying out different points of crossover I ended up at 120 Hz, the highest point of crossover the DS4 allows. Now, there certainly is a lack of coherence in the low end but overall if lifts the performance considerably being able to hear an upright bass with some volume and some physical impact. When it comes to the overall enjoyment of listening to music the subwoofer certainly does its thing.
Next the Ellam 98 mkII. Here we have a bass driver close to twice the size of the 12W and things start sounding right. The point of crossover is lowered to some 60-70 Hz and overall coherence is significantly improved. Still, 95 cm^2 membrane area can't move air the way a 10-12" bass driver does, but adding a subwoofer we're certainly better off although the 100-200 Hz slam of bigger speakers is still missing.
The AudioTechnology 3-Way Classics feature an 8" bass driver, more than
twice the size of the Ellam midbass and the distance between the AT-3WC
and the Ellam is bigger than the distance between Ellam and Disc 12. All
of a sudden we have to deal with getting the deep bass right and all DS4
settings had to be changed. Lower point of crossover and fine-tuning of
bass lift as well. Actually the AT-3WC seems to favor 2 subwoofers as
efficiency is rather high and a single 10" bass driver runs into trouble
of not having enough output.
Part II will cover the construction of the final two 40 liter cabinets.