Acoustics

A co-incident loudspeaker not only achieves time alignment between drivers but also provides a smooth off-axis transition by matching driver directivities over the crossover frequency range.

The directivity of a reciprocating piston in an infinite baffle has a characteristic which narrows at high frequencies, where the circumference of the piston is greater than one-half wavelength.

It follows that if a 25 mm tweeter and a 160 mm woofer are used to produce a system with a crossover frequency of about 3 kHz, their directivities will be very different at crossover.

At higher frequencies, as the motion of the lf driver becomes non-pistonic due to cone resonances, and the effective radiating area thereby reduces, there is still a wide frequency range where the total system directivity narrows.

This effect has been reported and investigated in many BBC monitoring loudspeaker reports, including the LS5/1, LS5/5, LS5/8 and the LS3/5 [15].

Around crossover, in a co-incident driver, both the inner radiating part of the woofer and the tweeter "see" the same waveguide "load", namely the woofer cone. Their directional characteristics, therefore, must be the same over this frequency range.

The directivity of a sound source is sometimes expressed by the parameter Q, a single figure related to the beamwidth. The co-incident driver, whose beamwidth shows a reduced variation with frequency compared to non-coincident systems, has therefore been named Uni-Q ® .

One of the first applications for the co-incident driver was in a low-diffraction system using a 110 mm lf unit and a 19 mm hf unit in a spherical enclosure as part of the Archimedes psychoacoustics research project [17]. Fig.14 shows the smooth on and off axis response which can be achieved in a more highly developed and
specialised enclosure [16].

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