THEORY
PAPERS
RESONANCES IN AUDIO - THEY’RE
BAD THINGS - AREN’T THEY?
Author: G Bank Deben Acoustics Ltd
Almost every musical instrument uses resonance to generate or amplify
the sound output. Imagine listening to a violin with no body. So, it seems,
some resonances are necessary whilst others are taboo. Maybe we should
think of them as either good or bad resonances. It was Rice and Kellogg
who published a paper entitled “Notes on the Development of a New
Type of Hornless Loud Speaker” in 1925, with a description of an
instrument of the piston type they had recently developed. It consisted
of a lightweight conical diaphragm driven by a moving coil and they deduced
that it would generate a flat pressure as well as a flat power response,
at least up to the point where the diaphragm started to beam. Engineers
like simple ideas and the concept of the “rigid piston” seemed
the ideal solution for a loudspeaker – and we’ve been stuck
with it ever since. Keeping the piston rigid also meant that we would
avoid any resonances in-band, since these were regarded as bad resonances.
This paper looks at the difference between good resonances and bad ones.
The journey starts at Celestion, where the first scanning laser interferometer
in the loudspeaker industry was designed and built.
It finishes in the present day,
by describing the invention of a loudspeaker which uses resonances as
an integral part of the design – but only the good ones, of course.
Conference Proceedings: Vol. 29. Pt.7 2007
A BALANCED MODAL RADIATOR (BMR)
Authors: G Bank (NXT Consultant), Deben Acoustics Ltd., N Harris New Transducers Ltd (NXT)
It has long been a desire amongst loudspeaker engineers to be able to create a loudspeaker that behaves like a perfect point source. In its absence, the common prototype for most loudspeakers is a rigid disc, often described as a "piston". This is partly because the acoustical behaviour of such an object has been known for a long time, and was known to Rayleigh in 1896. Although many attempts have been made to make such a radiator, its realisation still leaves the question - what is the optimal size? To have good directivity the piston should be small, but to generate low frequency power it should be large.
The work described in this paper focuses on the proposition that there might be an alternative to the rigid piston prototype, and investigates whether such a prototype could be turned into a practical loudspeaker.
It is traditional to consider the loudspeaker
as approximating to "perfect point source", for the purpose
of design and analysis, whilst accepting the real limitations of such
practical devices. This often means making the diaphragm suitably stiff
in order to confine the resonances to higher frequencies and then using
appropriate means to limit their unwanted effects. However, analysis shows
that there is an alternative, which does approximate to a point source,
but its theoretical nature does not suggest an obvious practical device.
Using this prototype, a flat diaphragm loudspeaker has been developed
which has both wide directivity and a substantially flat on-axis response.
Conference Proceedings: Vol. 27. Pt.5. 2005
A Balanced Modal Radiator (BMR)
The goal of a practical loudspeaker that behaves
like the "perfect point source" has been long sought. Mathematical analysis
shows that the prototype for such a device does indeed exist, but it does
not point to an obvious embodiment. Using this prototype, a practical
flat diaphragm loudspeaker is developed, which has a substantially flat
on-axis pressure response, as well as a smooth and extended power response.
A fully-coupled FEA model is used to investigate the intrinsic characteristics
of this radiator in both the mechanical and acoustical domains. Measurements
from a real prototype loudspeaker illustrate the practicality of the method.
Authors: Neil Harris, Graham Bank
Convention Paper: 6595
Investigating the Potential Benefits to Both the Objective and Subjective
Performance of a Two-Way Loudspeaker Obtained by Using a Wide-Band “tweeter”
to Place the Cross-Over at a Lower Than Usual Frequency.
The tweeter in a two-way loudspeaker was replaced by a unit having a natural bandwidth of 300 Hz to 20 kHz. This gave a much greater degree of freedom to the choice of cross-over frequency than would normally be possible. The first part of this paper looks at the potential benefits such freedom could bring to the acoustical performance of the loudspeaker. The second part reports results of early listening tests, which were conducted to discover the most preferred cross-over frequency in the range 700 Hz to 3 kHz.
Authors: Neil Harris, Alan Hildyard, Valerie Taylor
Convention Paper: 6189