TWEAKING LOUDSPEAKERS



THE TWEAKING OF LOUDSPEAKERS
written by : The Author

A further modification to loudspeakers that I found well worthwhile is to fill the cabinet with sulphur hexafluoride gas, SF6, in place of the air. This gas exhibits a sound propagation velocity which is three times less than that of air, and hence will lower the bass resonance frequency to one third, making even the smallest bookshelf loudspeaker sound monumental. This gas is inert, non-flammable and non-toxic, but please make sure it is not contaminated with traces of sulphur tetrafluoride, SF4, which is highly toxic. Do not use town gas! Uranium hexafluoride, UF6, has an even lower velocity of sound propagation, but suffers from the unfortunate side effect of being slightly radio-active. Most loudspeaker cabinets are not, however, air-tight, which means that the gas will have to be contained somehow. I tried containing the gas within party balloons, but contraceptive sheaths proved better because they are guaranteed leak-proof. With SF6 being such a large molecule, no perceptible leakage should take place. In order to get the balloons inside the cabinet, the woofer should be temporarily removed and the balloons posted through the opening. At this stage it is a good idea to spray the balloons with a proprietary branded aerosol adhesive which I shall call 'Scotch Mist'. This prevents the balloons from bouncing around inside the cabinet off the loudspeaker cone, with consequently drastic sonic results. With this modification, the improvement in low-frequency response was phenomenal, the 16-foot bass pedal note of Messiaen's 'La Nativity du Siegneur', a whimper beforehand, now came shuddering out.

Having made these improvements to the bass response of the loudspeaker, I was quite struck by the open-ness and clarity of the bass. I quickly got out all my old cathedral organ works and listened to them afresh with renewed vigour. It was only then that I noticed a slight fizz distortion, but it wasn't emanating from the loudspeakers. I tracked it down to the handles of nearby cupboards and the glass in the windows of the house which were all rattling. A quick application of blue-tack solved this.

Tap the sides of the cabinet of your loudspeaker. If it goes 'bonk' then it could do with some extra acoustic damping to reduce the amplitude of panel resonances. A sheet of lead 2mm thick applied to all sides is ideal. Of course, this does nothing to enhance the external appearance, but don't be prudent, the only criterion is sound quality. The lead must be securely fastened with a strong visco-elastic adhesive, and preferably tacked every inch or so with panel pins. I used a mathematical pattern called the Roger Penrose tiling pattern which possesses five-fold symmetry in an apparently random non-repeating pattern. This is ideal for damping the multitudinous modes of the panels. Make sure you Earth the lead panels to Ground to preclude electro-static build-up of unwanted charge. This also provides some measure of electro-magnetic shielding of the loudspeakers which can be adversely affected by the huge EMP of a nearby lightning strike. Having covered your loudspeaker in lead, do not now be tempted to now try UF6 gas instead of SF6 as the shielding against ionising radiation is in-sufficient for full radiological protection.

If, during thunderstorms, you notice a difference in sound quality from the loudspeakers, then this could well be due to over-efficient sealing of the cabinet. As a low-pressure weather front passes overhead, the woofer cone is sucked out from it's equilibrium position and is taken into a non-linear operating region. The answer is to prick the loudspeaker cone with a pin to allow equalisation of atmospheric pressure on both sides of the cone whilst presenting the frequencies above 20Hz with a high enough impedance to preserve the low-frequency response. When piercing the cone, be careful not to burst the balloons, as a sudden large pressure increase could blow the cone out.

The next experiment involved turning the bass woofer inside out, with the cone facing inwards into the cabinet. This has two beneficial effects. Firstly, the internal volume of the cabinet is increased thereby allowing room for more balloons. This reduces the bass resonance still further. Secondly, the sound is more omni-directional, being radiated away from both sides of the cone instead of being concentrated on axis like a satellite dish. The improvement in sound was distinctly ethereal, even evanescent.

It was now time to concentrate on the tweeter. The stereo soundstage was slightly fuzzy and needed focusing. Realising that the tweeter was too omni-directional, I tried making it more directional. I knew that the slight increase in pressure above atmospheric when a spherical balloon is inflated with air would focus sound rays passing through it, so I collared two friends and got them to hold a balloon each in front of the two tweeters and to adjust the positioning of them whilst I sat in the listening chair (which I had got from the Midland bank the day before). There was indeed a position where the stereo soundstage popped into sharp focus, so I rigged the balloons up permanently in these positions using coat-hangers. It may look weird, but it works.

If you have a floppy bass that needs tightening this could be achieved by attention to the loudspeaker wires. I had a look at some exotic platinum-plated gold-cored PTFE-insulated coaxial speaker leads with Kevlar kernmantle at £350 per metre, but was not impressed. I decided to experiment to find something not only cheaper, but far better. You may like to try what I found to be the most sonically satisfactory. This entails a little plumbing. The answer is to use a length of copper pipe (as used by plumbers) and attach it to the walls of the house from amplifier to loudspeaker. With the pipe being hollow inside, any skin effect is confined to the walls of the pipe. This will form the Earthy return from the loudspeaker. A similar length of welding cable rated at 400 amps is now threaded through the middle of this to form the signal path. Both conductors are so thick that there is negligible resistance so whether or not it is made out of oxygen-free copper with alignment of crystals anti-parallel to grain boundary orientation is quite immaterial. As a bonus, an amazingly high degree of shielding from extraneous radio-frequency interference is now obtained, which is especially important now that phobile moans, microwave ovens, 10GHz future-proof micro-processors and TV satellite transmissions beamed to your door are in such preponderance. As a result of the extra damping factor obtained this way, the bass became as tight as the membrane of a timpani and no longer floppy.

The viscosity of air, though slight, has a noticeable and measurable affect on the propagation of sound from the loudspeakers making the music sound slow and sluggish like treacle. If a way can be found to reduce this viscosity, a better sound would result. After a prolonged search, I managed to borrow an ultrasonic cleaner and adapted the transducer for use in air by attaching it to an exponential horn. I used an old blunderbuss barrel, but any old horn will do, except rhino. I drove it with a valved RF power amplifier fed with shot noise derived from a reverse-biassed Gunn diode. This produced wide-band RF noise (an in-audible hissing sound). With this apparatus I successfully reduced the viscosity of air by agitating the air and water molecules at ultrasonic frequencies keeping them apart for longer and preventing them from sticking to each other by Van der Waals forces. The sound now just flooded out from the speakers totally unhindered in any way. The transients were particularly clear, lacking tackiness and inertia.

But then, what was that I could hear? A slight echo perhaps? The source of this peculiar reverberant quality eluded me for months but I finally tracked it down to the sound bouncing around between my glasses and my eyes before being reflected into my ears. This was a real eye-opener! No magazine had ever mentioned removing glasses before as a way of improving sound quality, yet there was the answer staring at me right in front of my eyes. It was totally transparent. How could I have not seen it before? I removed my glasses. Suddenly everything looked blurred but the sound came into sharp focus.


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