Loudspeaker - part 2 - the project

As I described earlier the columns are designed as 2-way system in a closed box. Now to give more details:

Used drivers are:

Tonsil GDN 20/40 8 ohm

This is an relatively old design, but I got this speakers quite cheap, and decided to use them. The thing that I like in them is that they work pretty well in the middle of the audible spectrum, around 2..3 kHz. This is why I have implemented them in this 2-way system. 

Visaton SC10N

This a dome tweeter from Visaton, also not very fresh design, although seems to be quite reliable. The mid-woofer is 40 W rated and the tweeter is 100 W rated, so it should withstand lower-than-normal cut at around 3000 kHz.

Box

18 mm thick OSB loudspeaker box is 35 cm high and 24.6 cm deep and wide. After excluding the thickness of the walls (36 mm) this gives an approximate volume of 14 liters. The final volume will be even smaller, as in this calculation I didn't exclude the volume taken by the crossover elements, inner reinforcements, tweeter etc. Finally the volume should be around 13 liters. This produces a response curve like shown below:

This gives band (-3 dB) going from 70 Hz to 3500 Hz for the mid-woofer in theory. Reasonably speaking, it should go as low as 80 Hz with ease. You can see a bit of enhancement (up to +4.5 dB) in the kick-bass region, that is intended.

Cross-over

Passive crossover network is a classical CLC third-order network for the tweeter and nothing for the mid-woofer (simplicity again). The two capacitors are 4.7 uF and 15 uF, inductor is 0.56 mH giving the cut-off frequency around 2700 Hz.

Phase issues

The third order filter, used for the changes the phase by -270 degrees - -3/4 of full period. This is not possible to be compensated by plugging the tweeter in reverse polarization as with second-order cross-over, as it would still give phase mismatch by 1/4. At 2,7 kHz (cut frequency) the full period of the wave is around 13 cm - the approximate distance between the mid and high drivers. They reach in-phase work at around 3.8 kHz. Unfortunately in the 2.7 - 3.8 kHz the mid-woofers SPL is still around 90 db/W/m. This issue will be further investigeted one the loudspeakers are done.

Making of

The boxes were assembled some time ago (few years), so I do not have any pictures from the process. When I got back to the project I started from gluing the front MDF panels using universal spray adhesive (universal meaning usable for gluing paper, wood and textiles). This was a bit problematic as both sides - OSB and MDF - were just soaking in the glue, and close to nothing remained inside. At first I have covered both sides with glue, waited a few minutes and tried to glue them together with no good effects. So I waited till the first layer of glue dry up totally, applied another thin layer as a primer and then, after 10 minutes applied the final layer, waited 3 minutes and placed in on the OSB. After that I put them under pressure (25 liters of water ;) ) and let to dry overnight. It seems it worked.

 

The OSB boxes are partially covered with acrylic filler (I really regret using this one, as this is no hard enough to get the corners right). I will apply some other filler on top of that later.

To get the speakers centers together possibly close I needed to cut out a piece of the tweeter. It didn't turn out perfect but still acceptable (tape is to block the dome of the tweeter from the dust etc):

And finally the mid-woofer (still needs to be put together):

Next part - sanding/filling/priming the boxes!