The SP93 by Graeme

See all the pictures here:

Mail #1:
I thought I would drop you a line and update you on the progress of my SP93's.  My cabinets are well on the way.  All the panels and braces are cut and the recesses are routed in. Surprisingly, they all fit together quite well. Not bad for manual routing! I've followed Finn Dahle's cabinet construction method - I don't speak Norwegian, but I got the general idea from the photographs and the few words I could understand.  The side panels, top and bottom are all 25 mm MDF. The inner front and rear panels are 18 mm MDF, and the outer front and rear panels are 25 mm MDF.  All the internal bracing is 18 mm MDF.

I've gone for all air cored inductors from Intertechnik, apart from the 3.9 mH in the notch filter which is an Intertechnik ferrite core.  The 2.2 mH inductors are HUGE! I'm using Ampohm FP-CA-AU capacitors with Vishay Roederstein MKP 1837 0.01uF bypass caps. The Ampohm Capacitors are very reasonably priced over here and have a very good 'sound'.

I'm just waiting on the drive units to arrive, so I can assemble and test.  If you like  I can send some photos of the various stages when they're assembled.

I'm going cover them in a bubinga hardwood veneer, which has a very interesting 'waterfall' grain pattern. One thing I'm not too sure about is how to attach the veneer, as it's my first veneering project.  There are so many different methods.  I see from your website that you have veneered a number of speaker projects.  Which method do you use, and can you give me some tips on how to proceed?

Best Regards,

I've finally finished the speakers.  I e-mailed
Bill Martinelli for advice on the veneering. He recommended using a PVA based glue and using a household iron to set it.  If you look on his homepage at the curved veneering there's alot of information.  As I was using raw veneer, I had to flatten it first.  I did this by spraying it with a 10% solution of glycerine in distilled water, and then sandwiching the leaves between sheets of kraft paper between some MDF boards which were weighted and clamped. I changed the paper about twice a day for over a week.  I sanded the cabinets with 80 grit paper on a random orbit sander to break the 'glaze' on the MDF.  It was then a matter of coating the cabinet one panel at a time with glue - I used Titebond II with a foam roller - and ironing the veneer on with a household iron at a moderate heat setting (just below cotton).  The heat from the iron sets the glue and locks the veneer in place. After trimming the veneer, I sanded the cabinets with 240 grade, 400 grade, 600 grade paper and then 600 grade wet 'n' dry with white spirit.  I then applied finishing oil, using a brush for the first two coats, then 00000 grade steel wool finish with a cloth for the remaining 6 coats to 8 coats in total.  I then finished off with 2 coats of creamed beeswax applied with 00000 steel wool and polished with a cloth.

The rear panel and plinths were sprayed with 4 or 5 coats of MDF primer and 4 coats of black gloss enamel.  The plinths were then coated with 4 coats of spray applied gloss polyurethane varnish.

I'm very impressed with the sound of these speakers, the bass from the 8531s is just incredible! The bottom porting seems to have worked well.  One speaker is relatively close to a wall, and the other is in free space. Both sound well balanced, there's no 'boomy' bass like that from placing a rear ported speaker near a wall. The port tuning came out at 32 Hz, with a 405 mm port length. I estimated the length from your port tuning study, and was pleasantly surprised that it came out so close. The midrange has a certain 'lushness' about it, but I rather like the sound. The 9300 tweeter is silky smooth and nicely detailed without being harsh. Soundstaging is good - reasonably wide and with good depth. The Ampohm capacitors with the MKP1837 bypass caps really help to bring out the detail and atmosphere in recordings. I used the crossover you designed for the SP95, and it works very well. The frequency response curve is quite good. I haven't mastered using the gated MLS measurement technique in Speaker Workshop yet, so the lower frequencies aren't that reliable, but overall the FR plot is very similar to the plot you measured with the SP95. Once I've figured out how to use SW properly, I'll redo the FR testing.

I've put all of the photos together on a
webpage. If you wish to put some of them on your website, feel free to use any of the photos. You can also link directly to them to save your bandwidth. My ISP provides my webspace for free, and they don't set any limits for traffic.

Thank you for sharing your designs!
Best Regards,

Mail #3:
I've been listening to some more music today (mostly jazz) and I've noticed that very low frequencies, especially acoustic double bass seem over emphasised.  I am wondering, could this be because of the bottom porting?  Am I gaining a couple of dB extra around the port frequency due to its close proximity to the floor? If so, is there an easy way to reduce it slightly? Perhaps shortening the port slightly to raise its tuning frequency? I would welcome your suggestions.

Best Regards,

Mail #4:
I've done some experimenting in the last couple of hours.  I had a 1220 x 100 mm off cut of 25 mm MDF which I cut into 16 pieces, so I could try 25 mm or 50 mm of extra lift from the floor.  25 mm didn't have much effect.  50 mm was marginally better.

I happened upon another idea - I have a granite slab which usually sits on top of my CD player.  I placed this on top of a piece of anti-slip rubber mat under the left speaker (the mesh stuff that you can use for holding stuff for routing and for lining the drawers in tool chests etc.), switched off the right monobloc power amp and played the first couple of tracks of Muddy Waters' Folk Singer.  The double bass plays almost exclusively from the left speaker. The recording set up for this 1963 recording was very simple. Muddy Waters sat in the centre with his guitar, Buddy Guy sat to the right behind him on guitar, and  Willie Dixon stood behind and to the left of Muddy on acoustic bass. Clifton James sat further to the left on drums. With the plinth under the speaker, the boomy bass was gone.  Both myself and my wife listened to it with and without the plinth, and we both agree.

Another factor which I didn't think of when building the speakers is that our house has a suspended floor which I guess is prone to resonance - the floor boards are laid on beams which are suspended between the walls.  I suspect that a similar construction is used in Denmark and probably Sweden, Norway and Finland to help with insulation for your cold Scandinavian winters.  Most new homes in the UK have concrete floors which don't resonate, so I suspect bottom porting probably works best on concrete floors.

I have to confess this idea wasn't my own.  I did some searching for bottom ported speakers and came across a high-end design which is bottom ported (If I can find it again I'll send you the link). The manufacturer offers an optional granite slab to sit under each speaker, at a cost of about $600!  Presumably the slab is intended for use on suspended floors to prevent the bass energy  from causing unwanted resonances in the floor. My granite plinth is a 12.99 worktop saver from Argos! I'm curious to find out how they sound on a concrete floor. One of my friends is keen to hear them. It just so happens that he lives in a modern, concrete floored house, so I may have to take them round to his place for an audition - I'm not looking forward to moving them, though. The cabinets with the drivers fitted weigh in at 33 kg each.

I'll let you know my findings.

Mail #5:
After my experiments last night, I did some more thinking. A more aesthetically pleasing way of decoupling the port from the floor, would be to use a solid plinth, and to have the port vent above the plinth by using spacers.  Similar venting is used by Polk on the LSi25, by Audio Physic on the Yara, and by ProAc on the D38.
A couple of hours with the table saw and router this afternoon, and I've roughed out a suitable plinth:

It works very effectively.  There's still a little room gain in the low bass, but it has reduced substantially compared to using the original plinth. All I need to do now is trim the plinth a little, fill it, sand it, apply lots of coats of primer, lots of coats of black gloss and lots of coats of varnish :-(

For those who can place their speakers far enough from a rear wall, I'd recommend using the standard rear-ported design. For others, like myself, whose room layout requires one or both speakers be closer to the rear wall, then the bottom port is certainly worth trying. A more standard sized port, say 70 mm or so may work better to minimise the little bit of extra room gain. I used 102 mm ID pipe because I had some lying around in the garage.

Best Regards,

Mail #6: 14-03-2006:
I've done some more tinkering around with plinths over the last week or so. I had a flash of inspiration and started to think about the size of the port outlet around the plinth. The speaker is 345 mm deep and 225 mm wide and with 25 mm spacers between the base of the speaker and the plinth, effective outlet area around the base of the speaker is (34.5 +22.5)*2 - 16.0 (4 times 40 mm diameter spacers)* 2.5 cm, which equals
158.75 cm^2, or the equivalent of a 14 cm diameter port.  As an experiment, I used smaller and smaller spacers to reduce the effective port outlet area. I measured the speaker iimpedance after each adjustment to see if it was having any effect on the port tuning.  The results were very interesting - 25mm made no difference compared to the original plinth, port tuning stayed at 32 Hz, I tried 12 mm, still 32 Hz, at 6.4 mm port tuning dropped to 31.5 Hz, and at 4.8 mm the port tuning dropped to 31 Hz. For the 6.4 mm and 4.8 mm spacings I used some M8 repair washers which are 1.6 mm thick. At 4.8 mm spacing, bass is exceptionally well controlled, very deep, very tight, midrange sounds much clearer and the overall tonal balance is just right.The washers are 25 mm diameter, so by my reckoning I still have ((34.5+22.5) - (4*2.5))*0.48 or 50 cm^2 of port area coupling with the room, which is equivalent to a port of 80 mm diameter.I may be talking out of turn, but my empirical thinking leads me to beleive that larger ports will give more room gain.  The bottom porting works well, but for this size of speaker and a bass rich driver like the 8531, a 102 mm port was simply too big. If I find the time in future,  maybe  I'll do a study of the effect of different port diameters tuned to the same frequency with the same driver/enclosure. If I find some time over the next week or so, I'll update my photo page to include some description of the construction, testing and tuning.
Best Regards,