The vibro acoustic chair is a comfortable reclining chair with built in vibro acoustic units. The mounting of the units is important for two reasons. The active surface of the unit should be as flat as possible against the body. This means flexible mounting and secondly we don’t want to transmit vibrations to the body of the chair as this would create a loud noise as well as interfering with the sensation from other vibration units in the chair. I should explain that we don’t want stray sound from the vibro acoustic chair to annoy other people. Both of these can be solved by using sponge rubber mounting but the sponge rubber has to be of the type of reconstituted. Reconstituted rubber is sold commercially and consists of small pieces of sponge rubber glued together in random orientation. Unlike ordinary sponge rubber, this transmits vibration very poorly. The best place for units is on the back. I use one unit on the left back and one on the right back. With two part music, these are the only two used. With more parts I also have a vibration unit on the seat and one on the feet. When using MIDI music, which can be either from a computer or a live band, I would split up the musical parts for example as follows. Oboe on the left back, clarinet on the right back, cello on the seat, percussion on the feet or a pluck double bass on the feet. And of course with a live band, there is no necessity to use MIDI.
The units themselves consist of small loud speakers, preferably small woofers, but any sort of loud speakers do not couple vibration well into the body. To couple well, a membrane is required in front of the loud speaker, close to or in contact with the body.
The Sometron company, who I believe make the best commercially available chairs of the type which do not split up the musical parts, have a patent on this membrane. Their membrane is thin ply wood. With their permission and encouragement, I experimented extensively with different materials for the membrane and found that a membrane made from camping mats (that is dense sponge rubber about a quarter of an inch thick) were much better in that they gave a sensation that was more like music and less like a mechanical vibration, and also gave a wider frequency response in the treble where it is much required but with the disadvantage of being more easily damaged than Sometron’s plywood membrane. I also designed a double unit with two loud speakers in a pair connected in a push pull way so that when one loud speaker pulled inwards, the other pushed outwards (see photographs). This gave three benefits. A; a stronger vibratory sensation, B; less noise, and C; along with some other minor details in the design, it gave a wider frequency response. The units could be sensed about half an octave higher in pitch. Anyone can have a bass response but a response in the treble is difficult to achieve. It may not sound much but our design gave half an octave extra frequency response in the treble. The Sometron Company allowed us to use the membrane feature of their patent and said they would cover our design also with their patent and in addition they were free to use our design.
A 30-band graphic equaliser is required in the drive of the vibration unit because we want to achieve an equally strong sensation for each frequency in the usable range. Just as in music, where we want an equally loud sensation at each frequency in the range, here we want an equally strong vibratory sensation at each frequency. Only the lower half of the graphic equaliser is actually used in the range because the upper half is outside the range of frequencies that can be felt as a vibratory sensation. The body is usually more sensitive around 100Hz so the drive has to be turned down around that frequency and increased at both lower and especially higher frequencies. Considerable dynamic range compression is also required in the drive because although the ear can cope with volumes between 35 dBHL and 110 dBHL, the sensation of vibration would have to be compressed into a much smaller range by a factor of at least four.