Paul White attends an exclusive preview of the world's first commercial, high power, inflatable PA speaker system.

The days of tearing up and down the M1 in a beat up old Transit are long gone for most of us — instead we tend to turn up at gigs in our own hatchbacks, each transporting our own gear. This is fine until you get to the PA, because any PA small enough to share the back of a VW Golf with a guitar amp and a couple of guitars is likely to be too small for larger clubs and pubs. Of course, we've seen companies bring down the size of quality systems, but even then, either power is limited or there's a restriction on how much bass end you can expect.

You only have to apply a little lateral thinking to realise that even with a fairly hefty PA cab most of what you're carrying around is air — every speaker needs its enclosure, and the enclosure is really just a means of containing air. AirTec looked at this problem from a different angle and decided to experiment with a new cabinet design, comprising a rigid front baffle attached to a flexible, soft-walled enclosure, and by applying a couple of ingenious design tricks, they managed to equal, and in some cases exceed, the performance of traditional rigid-walled cabinets. The result of their endeavours is the AirTec PA1 system, a high power, full-range two-way active speaker system that can be packed flat, taking up a depth of little over six inches.

INFLATION

The idea of an inflatable speaker cabinet is not new by any means, but there are quite serious technological and practical problems to overcome before it can be made to work. And even if the system performs on an audio level, how do you inflate it at a gig? Three roadies taking shifts with a bicycle pump isn't going to impress anyone!

AirTec have obviously spent a lot of time figuring out the practicalities. Their system comprises a rigid polypropylene front baffle with solid sides around six inches deep, rather like a bakery tray stood on end. This allows the systems to be stacked flat without damage to the drivers or internal amplifiers. A flexible cabinet rear section is welded to the rigid section so that once inflated, it increases the cabinet depth to a little over 18 inches. The lower edge of the rigid section includes input connections, power connections, and a standard top-hat mounting socket. The blueprint drawing below shows the basic construction. During transit, the two cabinets can be clipped back to back.

The problem of inflation has been solved by fitting what looks like a bass port with a flap valve that only allows air into the cabinet. By playing a very low frequency tone (which is built in via the active circuitry), air is drawn into the cabinet through the port until the cabinet is fully inflated. Although the pressure inside is relatively low, it still produces a positive pressure on the rear of the loudspeaker cone, so a small DC offset is generated within the active section to counteract this. The result is that when the speaker is operating at the correct pressure, the speaker cone's resting position is exactly the same as it would be in free air. During performance, air pressure is maintained by using the music itself as a means of driving the 'port pump', although in practice, an unused pair of speakers remains fully inflated for several days. To deflate the cabinets, simply open the port flap valve.

Once the pressure problems had been sorted out, there was the question of sound leakage from the rear of the cabinet to consider. Normally a rigid cabinet is required to prevent unwanted sound leaking from the rear and sides of the box, but a thin vinyl material obviously isn't going to offer much in the way of acoustic isolation. Instead, the AirTec system uses ActivePorting (TM) technology, that involves some of the air from the front of the speakers being ducted via moulded ports in the front baffle into the airspace between two flexible membranes making up the rear of the cabinet, again clearly shown in the drawing. The outcome is that mid and high frequency energy is cancelled in the zone between the two flexible cabinet walls, thus preventing excessive spill. Low frequency spill does occur, but as low frequencies are largely omnidirectional anyway, the leakage situation is actually less severe than from conventionally constructed cabinets.

The ActivePorting system increases low frequency efficiency in much the same way as a regular porting system, but because the port opens up to a larger area between the cabinet walls, the efficiency is closer to that of a folded horn than that of a typical compact PA speaker. These technological advances, combined with injection moulded polymer speaker baskets, high efficiency voice coils, and Faradynium rare earth magnets results in a surprisingly loud, yet tight sound from such a deceptively small enclosure.

FUTURE MODELS

The current system utilises a 15-inch bass driver and a short exponential horn driven by a pair of 300 Watt switch mode amplifiers and a DSP controlled active crossover featuring a 20-bit signal path and a 102dB dynamic range. The crossover frequency is set at 1kHz, but the sharp filters and tightly controlled phase response generated by the DSP produce a virtually seamless transition from high to low. Plans are afoot for a larger three-way system of around 1200 Watts per side, and looking further into the future, experiments are being conducted with Helium filled versions for use in flown concert systems. These would be of particular interest to companies operating in larger venues where there is no suitable flying hardware; a simple tethering system and a few sandbags to maintain neutral buoyancy would be needed.

From the little time, I've had to play with these speakers, it's obvious they have a great future. They are easy to set up, light, and easy to transport, and they sound as good as the best 'plywood' systems. Repairs, should they be necessary, can be made with a basic cycle repair kit, and bands working on cruise liners can also use the speakers as flotation aids in the event of a disaster. They can even be re-inflated, if necessary, by blowing into a tube, contact with salt water starts a small battery powered transponder, and there's even a whistle for attracting attention.

PA1 £500 per pair inc VAT; subject to inflation.

AirTec, (Contact Details).