This is a contributed blog from Jonathan Espley of Matteis Loudspeakers. He has been a long-term user of Ted Jordan’s full-range drive units and is a passionate advocate of ultra-rigid, concrete baffles. These concrete baffles appear in his own range of loudspeakers but are also available to home constructors. Jonathan describes the background to their development.
Back in April 1982, Hi-Fi News & Record Review published an article, Tone Arms And The Twist. It suggested an alternative way of making a pickup arm. The idea was to use strategically-placed mass to provide stability for the cartridge at audio frequencies. In so doing, it removed the need for an ultra rigid arm and highly engineered bearings and virtually eliminated structural resonances. I rebuilt a Hadcock Unipivot arm to test the principle and it worked superbly well.
The loudspeaker drive unit and cabinet has a lot in common with the moving coil cartridge and pickup arm. The arm/cartridge converts mechanical energy into an electrical signal whereas the loudspeaker converts the electrical signal into a sound wave. In each case, the transducer should be held absolutely stable at audio frequencies.
High end pick-up arm manufacturers go to great lengths to engineer the arm to be as rigid as possible in order to extract maximum information from the record groove. Loudspeaker manufacturers design some superb drivers only to stick them in a highly resonant wooden box.
It’s not difficult to see where the main problem lies with conventional loudspeakers. The cone is driven in a backwards/forwards motion to generate the sound wave. This puts great pressure on the baffle to resonate in the opposite direction to the movement of the cone. Any movement due to flexure must be subtracted from the cones movement. Hence there is an inevitable loss of energy.
When it comes to side panels, they are solid in the front to back dimension and any resonance does not directly affect the motion of the cone and so has minimal effect compared to the baffle. I have built speakers with concrete sides and, believe me, it’s not worth the compromise as it’s very difficult to get an acceptable finish.
I started experimenting with a concrete mould for the front baffle then tried reinforcing it to obtain greater rigidity. To cut a long story short, I ended up with a mould (shown below) that has reinforcement right across the baffle. The concrete does not take the strain as it is not stable enough at 30mm thickness. Every improvement to the reinforcement brought an improvement to sound quality. The main difference is a sense of presence to music. Stereo image is more sharply defined, giving a much more realistic presentation of a musical performance.
There is still a small residual resonance as the pressure to drive the cone against atmospheric resistance is so great that the whole enclosure moves slightly. It helps if the enclosure is deep and narrow with the base as deep as possible, with spikes to pierce the carpet.
I have a new enclosure design formulated which I hope will overcome this problem. It has some novel features and will be the basis of a future blog.