By Lars Mytting
20 Jan 1998
Printed from SpeakerBuilding.com, 08 Sep 2010 22:57
URL: http:///content/diy/1012/
Building the cabinet is fairly straightforward Fig. 1. Toccata cabinet plan, front Fig. 2. Toccata cabinet plan, side. The quality of the saw used has a direct impact on solidity and thereby sound quality. Use polyurethane glue for all leakage-critical applications. This glue expands and gives a completely air-tight seal. Fasten all internal cables with hot glue to avoid rattle. Place the crossover in the port for easy access Fig. 3. Toccata at early lumber stage.
Set the lists approx. 2 mm outside each edge. When you have fitted the outer board, you can get sharp, perfectly flush edges if you remove the excess with a power sander or a wood planer.
2. With a saw, cut a 6-7 cm long, cone-shaped piece of the upper list. Cut more holes at the other sides if needed to ensure complete fill. This is where we will fill the sand Fig. 4. Preparements for sand-filling. Do not throw the bits away, they will be used to plug the holes.
3. Place short wood lists (same thickness as in step 1) evenly and asymmetric on the cabinet wall. These will form fastening points for the outer wall. Make sure the lists are positioned so that they do not block the path where the sand is to be filled.
To reduce mechanical coupling, use no more than the number needed for secure fastening of the outer board. There should be no more than needed to keep the outer wall from swelling. The amount of fastening points depends on the strength of the outer wall, and on the amount of sand. For a small cabinet you need just a few small points in the middle. The approximate number used in the Toccata is shown in blue.
4. Cut the board that is to form the outer wall. It should be a few mm larger on each side than the cabinet. When the excess is removed with a power sander you get perfect fit. As mentioned, thin walls are preferable. This is also a very simple method of getting good-looking wood finish on the speaker. By choosing a veneered board you get the wood type you prefer. The veneer on the Toccatas is teak, impregnated with a oil used for boats.
5. Place glue on all the fastening points, position the outer wall and place heavy weights on it to ensure that it gets fastened firmly. Apply as much pressure as possible!
6. With a wood planer or a power sander, remove the excess from the sides. It is better to do this before filling the sand, as the cabinets will get heavier and difficult to handle.
7. Fill sand through the hole on the top. Wiggle the speaker and knock on the side with a hammer (use a chipboard between to protect your precious veneer!!) to ensure that all areas are filled with sand. Add glue at the filling hole, position the piece of wood, and hit it down with a hammer. A layer of veneer on top will hide the holes.
As this was a prototype, I made it of lousy, 17 mm chipboard. I played a lot of music with it, and the cabinet showed massive vibrations, with heavy coloration of the sound in the 250-450 Hz region. Then I added the outer wall and sand, and supported the driver magnets. The result was astonishing. Even though the new layer added only 15 mm diameter to each wall, and I only damped two sides (not top and rear), it reduced the vibrations to great satisfaction.
From the first moment after applying sand damping everything in the sound stage became far cleaner and more precise. Bass became tighter and harder. Midrange clarity was greatly improved at high sound levels. You could now clearer distinguish the pedal kick on the skin of the bass drum. Heavy attacks on guitar strings were controlled and loud. Sounds echoed out on the stage instead of just becoming distorted fog.
Many speaker designers has pointed out that the usual, square cabinet with sharp edges is far from the best shape for a speaker. When sound waves radiate from the driver, they will first travel along the baffle, and when they reach the sharp end of the cabinet, they will "ricochet" and reach the listener delayed in time, smearing the original sound wave. If the driver chassis is not flush-mounted with the baffle, this edge will also cause reflection.
Is this a big problem? Many of the greatest speakers on this planet have flat, razor-sharp cabinet edges. Nevertheless, the problem is real, accepted and well defined by leading speaker theorists like Siegfried Linkwitz and Harry Olson. Tests by Olson and Vanderkooy have showed that the diffraction from a cube can give 5 dB ripple in frequency response. In the same tests, a sphere-shaped enclosure had only 0,5 dB ripple Fig. 5. Model of delay from reflections.
In the Toccata, these problems are attacked by making a separate midrange baffle Fig. 6. Midrange baffle at lumber stage. By making the upper part of the cabinet 8 cm shorter (seen from the side), I got a area where a second baffle for the midrange could be inset. This midrange baffle is only 16 cm wide, and the area around it is stuffed with damping material so that the edges are very smooth and round. The damping material will greatly reduce the intensity of the reflected sound. It is also easy to create a perfect spherical shape this way Fig. 7. Finished midrange baffle.
You can choose to make a box that encloses the midranges, or you can use them "unbaffled" as I have done. The shaped damping material can be used for both methods.
Except for the use of damping material, this technique has similarities to the design of the old Snell A speaker.
Using the drivers "unbaffled" gives the least internal reflections, and should be regarded as something between open baffle and aperiodic damping. It will limit the low-end response because the sound waves wider than the baffle width will cancel out. The proof that the technique gives a slightly "aperiodic" result, is that the impedance peak changed from 20 ohm in free air to 7.2 ohm in this baffle.
It is hard to define how much improvement the soft baffle gives. MLS frequency measurements show little variation between this and a standard baffle of the same width. Transients on the final speaker is very good in my opinion, but how much honour that is to be allocated to the soft baffle I cannot prove by measurements. But I would surely use this technique for other speakers in the future.
2. Fasten the midrange baffle to the cabinet with eight wooden dowels (I cut pieces from a broomstick). Use glue, and apply heavy pressure.
3. Place a piece of cloth or felt, preferably slightly elastic, over the midrange baffle. It must be so big that it will cover the baffle and the whole cavity around.
4. Cut holes in the cloth for the drivers.
5. Mount the drivers. Make sure the cloth is held firmly in place by the frames of the drivers.
6. To achieve good coupling between the magnets and the cabinet, I made a support of MDF behind the drivers, placed a asphalt plate on it, heated it and set the driver firmly in place while the asphalt was still hot. This gave a significant improvements in midrange clarity at high levels.
7. Fill the cavity with damping material like wool or house insulation. By stretching the cloth and adjusting the placement of the damping material, you will be able to create a spherical shape quite easily. Do not be afraid of stuffing the damping material quite hard. Secure the cloth to the cabinet with staples, and cut of excess.
The final result? Well, I agree that the Toccatas would not be the hot dream of every interior designer. They may give some flashbacks of the bulky TDL and Snell designs of the seventies, but should be acceptable for those with a modestly progressive relation with speakers. A black cloth front will aid the domestic properties of the Toccata greatly.
That evening was not one of the best. We hooked the Toccatas to Electrocompaniet amplifiers, and fired them up in the well-damped listening room. The speakers sure had dynamic resources, but something was... very wrong. The midrange was very badly defined. It was all over nosy and dull, but at the same time it sounded undistorted. The tonal balance was quite weird.
Measurements made the next day showed further disgrace. The frequency response was so horribly bad, so disreputable that it looked like a printout from a seismograph after a earthquake. Nearly the whole midrange area was missing in a huge, wide dip.
The speakers travelled home. After a frantic debug of the speaker, I discovered that I -- during the late-night assembly -- had connected one of the midranges out of phase. This effectively cancelled out the whole midrange, and of course, it was this speaker that had been measured.
Correct phase can be checked by connecting a 1.5 volt battery to the drivers. If drivers are in phase, they will move in the same direction. The connection must bypass any series capacitors, as these will block the DC from the battery. Do not use this test on tweeters, as they may get damaged.
The dual drivers in bass and midrange results in 4 ohm impedance, which reduces inductor values (and their costs) to the half. The high sensitivity is demonstrated by the fact that there is no damping resistor for the tweeter. By calculating from Seas' measured figures of their kits with the T25, I estimate the sensitivity to 94 dB.
Depending on room, amplifier and taste, I recommend using a resistor with a value between 1-3 ohm in series with midrange and tweeter if you find the sound too bright. This will of course reduce the average sensitivity with 2-3 dB.
The impedance reveals some flaws Fig. 12. Impedance and electrical phase. Impedance is generally flat, but also low, averagely 5 ohm. Highest impedance is 7,8 ohm at 43 Hz, and lowest is 3,0 ohm at 480 Hz. Notable are the highly damped peaks in the bass region. The amplifier meets a troublesome area from 90-200 Hz, with 3,2 ohm at 130 Hz. The steepest phase-shift is 38º at 90 Hz. However, a modern amplifier in healthy shape will be able to cope with these demands, especially since the sensitivity is high.
There are some irregularities in the soundstage when listening off axis. Some of this (especially the dip centred at 200 Hz) can be allocated to floor reflections and wavelength-related cancellations due to the many drivers, and some of to the MTM configurations.
Probably the biggest disappointment is the response in the lowest octave. Bass is quite tight, loud and "fast", but I had hoped to get more output to 35 Hz. When playing very loud, the bass may sound more or less choked, maybe due to the narrow t-line. If you are mad enough to build a Toccata, I suggest you make the t-line wider, with a ratio for two drivers of maybe 1.5. I have also tried other 10" drivers with higher Qts, but with no significant change in the lower octaves.
The bass response is not fully investigated, because I have had few well suited amplifiers connected. I have mostly used a Hiraga Le Monstre of 8 watt, but this amp meets its limits quite early due to the difficult impedance of the Toccata. A sturdy class A design of 20-30 watt would probably be optimum Fig. 13. Son of Zen. Tests showed that even a small, 20 year old Sony amplifier could provide dramatic SPL!
Impedance turned out lower than expected, with some difficult areas. The major flaw is probably the low end bass response. Some listeners may be disappointed due to the promises such a large cabinet gives. A bass reflex system with lower sensitivity is maybe what most will prefer.
But all in all, I am satisfied with the project. The Toccata may not have the detail and neutrality of a smaller two-way with top components. It likes to show off; behaving as a Anna Nicole Smith-like, full-breasted blonde, needing little to get turned on, and always great fun, but maybe overall too much for some tastes -- especially compared to the grace and refinement of a slender, Nobel-prize intelligent woman like, say Arundhati Roy, author of God of Small Things. Both are catches of the century, but you can't get it all, and in emotional fields like music it would perhaps be hard to trade the intellectual depths of Arundhati for the mad fun of Anna Nicole.
Lars Mytting
[2] Sensitivity figures must be accompanied with the room specifications too, but we will not enter that topic here.
[3] It is commonly suggested that a t-line driver should have high Q, but no complete theory for T/S parameters and t-lines has been universally accepted. Others, like Lynn Olson, claim that low Q is no drawback for a t-line. The CA25FEY has a Qts of 0.27, and does thereby correspond to Lynn's advice.
[4] The capability of the total Sd is is simplified for the clarity of the example. You must also take in account the crossover frequency. If you for example use four 18" drivers and a single 5", crossed at 70 Hz, the poor 5" will reach its limits at a stage where the 18" barely moves -- while the total Sd gives another impression.
[5] Vance Dickason, in "The Loudspeaker Cookbook"
[6] Martin Colloms, in "High Performance Loudspeakers"
[7] Every home should have a simple tone generator. They are available as kits at very low cost, and are extremely useful for a wide range of DIY-speaker purposes.
[8] Again we trust "SPL-generated excursion nomographs" by Siegfried Linkwitz, Speaker Builder 4/84.
Fig. 1. Toccata cabinet plan, front
Toccata cabinet plan seen from the front. Drawing is made by Esben Beck.
Fig. 2. Toccata cabinet plan, side
Toccata cabinet plan seen from the side. Drawing is made by Esben Beck.
Fig. 3. Toccata at early lumber stage
Fig. 4. Preparements for sand-filling
Fig. 5. Model of delay from reflections
Simplified and exaggerated model showing the principle of delay by reflections from cabinet edges and from edges on driver chassis.
Fig. 6. Midrange baffle at lumber stage
Fig. 7. Finished midrange baffle
Fig. 8. The Toccata: A flashback to the seventies
Fig. 9. Crossover schematic
The optional resistor can be located at the input terminals for easier adjustment.
Fig. 10. Frequency response and phase
Measurement made with less than 1 watt input. (Data under 500 Hz are irrelevant).
Fig. 11. Frequency response in room
Frequency response in room, 1/3 octave-smoothed.
Fig. 12. Impedance and electrical phase
Fig. 13. Son of Zen
I cannot resist showing one of the amplifiers the Toccatas was tested with: Esben Becks extremist version of the Nelson Pass "Son of Zen". I regard this as one of the most macho 6-watt amps seen by mankind. Esben's aluminium animal was also the very first Son of Zen built outside PassLabs (this was confirmed by Nelson himself!).
