Very Small Line Array Part (concept)
This page considers a small line array loudspeaker enclosure based on two 6" drive units. The design follows on from a doodle (his description) by Gary Ford on the Harmony Central discussion board. The doodle is shown below. Gary also suggested some suitable drive units for the design and explained that he had considered the design more for a home system than sound reinforcement. One of the bass drivers was the eighteensound 6ND430. Predictions by winisd show that this drive unit could form the basis of an extremely compact loudspeaker. The graphs below show the predicted response and suggestions for a practical cabinet. The other drive units didn't look so promising. With a sensitivity of 87dB/W and a maximum power of 25 W the mid drive units were obviously not going to get very loud and the high frequency ribbon units wouldn't do much better.
Original drawing by Gary Ford.
Suggested Bass drive unit. Click on the picture to view the specifications
The graph below shows the predicted frequency response of the 6ND430.
Frequency response of 6ND430 enclosure size is for 2 drive units
For a 6" drive unit this was not bad as long as it could produce reasonable sound levels. The next thing was to check whether it would.
The graph above shows the maximum output from two drive units There is a slight excursion limitation between 70Hz and 150Hz but not much; for such small drive units this is a remarkable output. The graph below repeats the above graph but also adds the output expected from an array of four cabinets.
The above graphs don't account for any power compression but by keeping within the excursion limitation the drivers aren't being pushed to their thermal limit so I would imagine 126dB would be a realistic level.
The next step is to determine the size of the cabinet. If we stick close to the front dimensions that Gary gives we have in metric 45cm x 15cm. If the the layout is similar to the dV-dos design the V will be about 12cm deep and if a horn similar to the beyma (described in the HCLA thread) and shown below is used that will add another 21.5cm plus the drive unit (5.3cm for the beyma).
Click on the diagram for the specification sheet
The total depth of the cabinet will be about 38-40cm deep. This gives a volume of about 23 litres, which is about right for the drive units. A few additional cm might be needed to allow for space taken up by the drive units but not much.
The diagram below shows an initial drawing of a cabinet to get a basic impression of how things fit and give a more accurate idea of the enclosure volume.
The cabinet is based on a 1.2cm ply sheet. The external, imperial measurements for the cabinet are 18" x 18" x 7"
As of this moment I don't know how readily available the eighteensound drive units are. A couple of alternatives are compared below. The green plot is the 6ND430, the red plot the Ciare CW165C and the yellow plot the paudio TM-6. The cabinet size was 22 litres in all cases but the tuning is slightly changed for each drive unit.
I've not put any details of the ports because they are different for each drive unit and it is easy enough to calculate the dimensions in winisd or similar program. It also allows some flexibility in the size of port you use.
The high frequency horn bears a passing resemblance to the one shown below, the thinking behind it is completely different though. The horn below is supposed to work by reflecting the sound from the centre section and back out from the outer horn wall. I'm a bit dubious that this works.
I have simply used a similar outer wall profile to try and create a narrower dispersion pattern and the centre section simply creates the correct expansion profile. Another way of looking at it would be as a slice through Turbo Sound's original turbo horn (but smaller). For ease of construction a straight flare similar to the beyma or JBL horns could be used.
Please note that this is an idea or starting point. If you wish to discuss any of the design issues in more detail just drop me an email or post a message on the Harmony Central Line Array thread.