AES British Section lectures - January 2007

The Balanced Mode Radiator

Graham Bank, Deben Acoustics

On January 9th the UK Section of the AES hosted a lecture titled "Balanced Mode Radiators" by Graham Bank of Deben Acoustics. Graham is a well known driver designer and has previously been the Research Director at Celestion, Wharfedale and NXT.

Graham started with a brief discussion of the normal piston model prototype for a driver and its fundamental problem of what size to make the piston. He then introduced a different model, the free standing vibrating disc. Simulations of a free standing disc with a point driver show that the on-axis power and SPL don't drop off. Unfortunately, when the mass of the drive coil is taken into account the modes of the disc are heavily excited. Graham pointed out that the first mode can be balanced out by careful calculation of the location of the drive coil on the disc (prior art). Balancing the first mode significantly extends the useful range of the driver.

Graham's breakthrough occurred when he looked at the admittance of the disc for the first mode. The minima occurred at the position on the disc where the driver is. So, Graham calculated the admittance of the disc for the first two modes which showed two minima. Finite Element Analysis was used to model a disc with the drive placed at the first minima and a balancing weight placed at the second. He showed that the resulting on-axis power and SPL were restored to almost that of the vibrating disc.

Graham discussed how the technique extended to as many modes as desired, with a balancing weight at each minima of the admittance. He noted that the attached surround has to be used as one of the balances, causing it to be placed inbound of the disc's edge. Graham also pointed out that the directivity of the loudspeaker could be modified by changing which mode to balance out with the driver (i.e. varying the coil's diameter).

There followed a very convincing demonstration of some prototypes and a lively question and answer session. A recording of the lecture is available from the UK section website at www.aes.org/sections/uk/meetings/a0701.html.

Report by Tim Harris