Silent flight

Aircraft cabin noise, the audible hum experienced by airline passengers, could be halved by a new system being developed in the UK by Qinetiq, Ultra Electronics and jet manufacturer Bombardier.

It is designed to address the problem of broadband noise which is caused by turbulent air hitting the fuselage. The air causes the fuselage to vibrate, which in turn makes the air inside the plane vibrate.’ said Rob McDonald, Ultra’s vice-president of marketing and support.

The system itself is a hybrid passive-active system, based on a spring on which the cabin trim panel is hung as a load, a coil, and a set of electronic controls. The spring, the passive element of the device, absorbs sound vibrations in the high frequency range, preventing their escape into the aircraft’s interior.

Meanwhile, algorithms analyse the sound vibrations and generate electrical currents in the coil, which change the characteristics of the spring, including its level of compression, allowing it to absorb low frequency sounds. As a result researchers claim the device is effective in shutting out noises down to just above 100Hz.

Broadband noise is currently addressed by rubber mounts attached to the cabin trimpanels, blanketing material behind the panels and occasionally foam or high-density films. These passive measures are effective in blocking out high frequency sounds, but addressing frequencies below 1KHz would require a large amount of material, increasing the aircraft’s weight. ‘Low frequency noise just goes straight through passive measures,’ said McDonald.

The companies expect to see particular demand for the device from manufacturers of business and regional jets, with the two markets potentially worth up to $150m (£89m) each.

Passengers on business jets pay a high premium for their seats but often experience the most harsh cabin environment. Business jet cabins are smaller, meaning passengers sit close to the sides of the aircraft, and therefore experience more noise, said McDonald.

The large commercial jet market could be worth anything from $50m-$250m, as the devices are likely to be installed in first class initially, the companies believe.

Broadband noise is predominantly carried by the aircraft structure itself, by the frames and trim panels, said Robert Coppard, business group manager for Qinetiq’s structural design group. ‘The objective is to reduce the aircraft internal noise by about 10dbA. That is effectively a halving of the noise you would experience. We are taking energy out of the cabin and stopping it in the aircraft structure.’

The team is now building a ground demonstrator to test the device, and hopes to complete construction and begin presenting it to potential customers later this year. the team will then move on to in-flight tests in 2004, and is planning to fit the devices to a business jet under development at Bombardier. These tests will allow the team to ensure the device works as effectively in the air, where windows and other factors may influence its performance, and the companies then hope to launch the device commercially in 2005.

‘The manufacturers introduced to this have been very interested. Their boundary level [for interest] is 5dbA, so this is significantly more,’ said Coppard. Each trim panel is likely to have between six and 12 mounts, while each mount uses around 5W of power.

The system could be fitted to commercial aircraft by 2005.