Conclusion

A compound moving-coil loudspeaker drive unit, which provides a sound source whose location does not vary with frequency can be implemented now that commercial production of high energy magnetic material has made a small tweeter magnet assembly realisable.

The impulse response of a typical co-incident driver system in Fig.15., when compared to the co-axial systems shown in Fig.6 and 7 shows the improvement in time alignment between units.

Acknowlegements

I am indebted to L.R.Fincham, M.E.Gough and A.Jones for their work on filters and system design, D.Howe and F.Low of the University of Sheffield for their assistance with magnet measurement and simulation methods and finally, R.E.Cooke, S.Kelly and G.A.V.Sowter for their background information on magnet materials and design.

Appendix

Glossary of terms:

Tweeter - a high frequency (hf) loudspeaker drive unit.

Woofer - a low frequency (lf) loudspeaker drive unit.

Frequency response - the transfer function in the frequency domain in either polar (amplitude and phase) or rectangular (real and imaginary) representations. i.e. the response of a system, as a function of frequency, to sine wave stimuli.

Impulse response, h(t) - the response of a system, as a function of time, to a delta function stimulus. The frequency response and the impulse response are a fourier transform pair.

Directivity Factor, Q(f) - the ratio of the sound intensity produced by a radiator, at a given point, to the intensity that would be produced, at that same point, by an omni-directional (point) source, radiating the same total acoustic power as the original source.

Sound Intensity, I - The average rate at which energy is transmitted through a unit area.

The intensity of of a plane wave in the direction of propagation is,
mathematical equation
where p is the sound pressure level, ro is the density of air and c is the velocity of sound.

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