AES British Section lectures - January 2007
The Balanced Mode Radiator
Graham Bank, Deben Acoustics
www.debenacoustics.comMany loudspeaker designers consider a loudspeaker as approximating to a "point source radiator", whilst accepting the limitations of such an approximation, for practical devices. Analysis has shown that there is a prototype for a device that behaves like a "point source", but its realisation is not obvious.
In the case of a modal diaphragm, the overall response is the sum of all the contributions, each of which is related to an individual mode of the diaphragm. The on-axis pressure of a flat diaphragm is a sum of the "pistonic" response and the contributions from these modes. For a free disc, the average volume velocity of the non-pistonic modes is zero, and therefore they will not contribute to the on-axis response. If we apply a "ideal force" to such a disc, we can turn it into an "ideal loudspeaker" with a flat on-axis response and an extended, smooth acoustic power response.
However, in the real world, "ideal forces" do not exist, and practical forces always have a finite mass (the voice-coil). This voice-coil unbalances the free disc, affecting both pressure and power responses. But, by adding extra masses at specified positions, it is feasible to restore the mode shapes of the free disc and thus restore the original response.
Results from a fully-coupled Finite Element Analysis simulation are compared to measurements from a prototype loudspeaker.
Graham Bank earned a degree in Applied Physics from the University of Bradford, in 1969. After a period as a Research Assistant in the University, he received an MSc, by research, in 1973.
Since 1974 he has worked for both Wharfedale and Celestion twice. He has held posts as Technical Director and Research Director at both companies. Following a loudspeaker research program, completed in 1997, he was awarded a PhD in Electronic Systems Engineering, at the University of Essex.
He then joined NXT as Director of Research, a post he held for seven years, researching flat panel loudspeaker technology for both the consumer and professional markets.
He left NXT in 2004 to set up his own acoustics consultancy business.
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