Why Spheres?


The choice of non rectangular low resonant enclosures is strongly suggested by the fundamental research undertaken by Harry Olson of RCA Laboratories USA. Olson was a highly qualified and skilful experimenter. He understood the importance of analysing as many as possible of the factors that influence loudspeaker performance. By 1950, researchers had attended to the behaviour of drivers and to the acoustic combination of the driver and the air enclosed in the loudspeaker cabinet. Olson extended the analysis to the outside of the cabinet, observing that:

The exterior configuration of the cabinet influences the response of the loudspeaker system due to diffraction effects produced by the various surface contours of the cabinet. The diffraction effects are normally overlooked and the anomalies in response are unjustly attributed to the loudspeaker mechanism.

Olson’s experimental results for the most common loudspeaker enclosure, the rectangular parallepiped, or rectangular box, are reproduced below.


Rectangular Parallepiped

As Olson put it:

This shape is the simplest to fabricate. This is unfortunate, because the rectangular parallelepiped produces diffraction effects which adversely modify the response-frequency characteristic of a direct-radiator loudspeaker mechanism. … The pronounced minima in the response at 1000 and 2000 Hz are due to shorter distances from the mechanism to the upper and side edges. The minimum in response at 500 Hz is due to the longer distance from the mechanism to the lower edge. The variations in response due to diffraction effects by the cabinet, are of the order of 6 to 7 dB. The response frequency characteristic is typical of the response obtained with this type of enclosure. Therefore, this cabinet shape is unsuitable for housing a direct radiator loudspeaker mechanism, because of the wide variations in response produced by diffraction from the sharp edges of this cabinet.

Compare this to Olson’s results for a spherical enclosure.

Spherical Enclosure

Olson again:

It will be seen that the response is uniform and free of peaks and dips. This is due to the fact that there are no sharp edges or discontinuities to set up diffracted waves of a definite phase pattern relation with respect to the primary sound emitted by the loudspeaker. The diffracted waves are uniformly distributed as to phase and amplitude. Therefore, the transition from radiation by the loudspeaker mechanism into 4π solid angles to radiation into 2π solid angles takes place uniformly with respect to the frequency. It will be noted that the sound pressure increases uniformly in this transition frequency. The ultimate pressure is 6 dB higher than the sound pressure where the dimension of the sphere is a small fraction of the wavelength.

When Olson’s paper was first placed in my hands over 35 years ago, the rectangular enclosure, along with its and timber construction, left the equation immediately. If a sphere, either full or truncated, is the preferred shape, then, concrete was my preferred material.

All charts and quoted text are from H. F. Olson,  "Direct Radiating Loudspeaker Enclosures," first published in 1951 in Audio Engineering, reprinted in January 1969 in the Journal of the Acoustic Engineering Society, JAES Vol.17 No.1, pp.22-29.

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