Hegeman Professional Loudspeaker

This text was taken from an advertisement for the Hegeman Standard (licensed to EICO for sale as the HFS-2) Loudspeaker. In it, Hegeman describes in-detail the engineering reasoning behind his split slot-loaded conical horn. Text enclosed in [  ] is mine.

"In selecting a horn [rather than a direct radiator] for the bass element of this system, considerable weight was given to two advantages [of horns]: The horn can provide an even wider pattern of sound distribution, and it can couple acoustical energy efficiently to the listening environment from a relatively narrow source – the speaker cone. Horns have their problems, too, but these problems are subject to solution, as we shall see.

Possibly no reproducer conformation has been as much abused in the literature as the conical horn. That type, it has been fashionable to point out, has a rapid roll-off in response and is therefore unsuited for hi-fi applications. What has not been pointed out -- but what is quite apparent from even a cursory study of published curves -- is that the conical horn remains far and away the smoothest responding horn known, even when it is rolling off.

The conical horn begins to show its merits as a bass reproducer when one investigates carefully the behavior of horns near cutoff frequencies, and the use of horn mouths that are -- in theory at least -- inadequate in diameter. It is certainly true that the efficiency of a conical horn does begin to roll off at a higher frequency than other horn shapes, but that rolloff is very gradual, very linear. And it is equally true that the conical horn continues to respond albeit at lower amplitudes-an octave below an exponential horn of equivalent size!

Further, if one accepts the general proposition that sharp changes in amplitude response represent large changes in phase characteristic and that gradual changes indicate that the phase characteristic is more linear, it is obvious that the conical horn must produce less coloration of whatever signals it is reproducing than can either a hyperbolic or an exponential horn. To put it another way: the smoother rolloff in efficiency of the conical horn represents a lesser change from the desired linear phase characteristic (so essential to natural reproduction) than is possible with an exponential horn.

There still remains, of course, the problem of horn mouth size for adequate bass response, and the efficiency at the very low end. Any horn has a natural cutoff point, determined by its length, mouth diameter and rate of flare. A horn would require a mouth diameter of 7 feet to reproduce 50 cycles. This is obviously out of the question for practical applications. "Folding" the horn to make it physically shorter and using the corner of the room to lengthen it acoustically do help. But there is a better solution.

Just as in transmission practice it is entirely feasible to replace, for example, a required section of telephone line with a properly devised termination [here we see how Hegeman's work for the telephone division of Western Electric perhaps helped inspire this design concept], so acoustically it is possible to replace a section of a horn with a properly devised acoustic termination. At low frequencies, such a termination is represented by a narrow slot. Consequently, by replacing the mouth of a relatively short conical horn with a critically dimensioned slot, it is possible to cause the driver mounted at the horn throat to behave as if it were working into a horn of virtually infinite length.

The slot confines enough air within the horn to build up a suitable resistance for the driver (at the horn throat) to work against. At any point from the slot back to the driver, the air in the now shortened horn presents the same impedance to the driver as if the horn had continued to an optimum length and mouth diameter for frequencies below 30 cycles [for the HFS-2, but for the Pros, it's below 20 cycles]. Additionally, the acoustic impedance now inside the confined horn produces a greater air pressure within that horn than in the room outside the horn.

As a result of this internal pressure, the slot functions acoustically as if it were many times its own area. Behind it, every movement of the speaker cone sets up relatively high compressions of small masses of moving air. These com pressions provide the same acoustic energy as would otherwise be provided by large masses of air under less compression. The higher pressure within the horn -- higher with respect to the normal pressure of the atmosphere in a room -- results in a high particle velocity in the slot, causing the acoustic energy in the horn to be transmitted to the listening room with the effect and impact of a jet stream..."

How best to describe Hegeman's "split slot loaded conical horn" is an open question. Horn is definitely the wrong word. The design does not conform to any sort of horn theory. It seems closer to a transmission line, more specifically a "mass-loaded tuned quarter wave tube" (a Voigt Pipe).  But even then, it's a rather unique implementation of the type. What is clear is that quarter wave resonance comes into play, as does mass loading (thanks to the slots). Call it what you want. It works!

"This tweeter radiates from both sides of the free-edge steep-sided vertical cone or cornet mounted above the inch speaker. The outer surface of the cone produces a 360° horizontal radiation pattern. Radiation from the inner surface provides the vertical component for a complete hemispheric dispersion pattern.

If this were a conventional vertical cone with an edge forming a right angle to the voice coil axis, it would have a relatively narrow-band response. What's more, the lateral radiation would all be in one plane while the vertical response would be beamed, not dispersed.

In this design the edge of the cone is so cut as to provide a constantly varying distance between the cone's edge and the voice coil. This broadbands the tweeter's response while smoothing it to avoid undesirable peaks. This shaping also spreads the radiation vertically so that it is not restricted to one plane horizontally.

To enhance this “umbrella" coverage further a rigid and nonmoving "acoustical loading cone" or "plug" is inserted concentrically within the radiating cone. This plug forms an air gap between itself and the inner surface of the radiating cone. This gap acts as a ring radiator. Because of the varying distance between its effective radiating surface and the voice coil, this ring radiator produces a desirable displacement between any two more acoustical elements (air particles) in that gap. As a result of this displacement the radiation pattern from the tweeter takes on a hemispherical shape. 

An additional benefit from this plug is that it obviates any effects of response peaks or instability which might be caused by radiation from diametrically opposite points on the inner surface of the radiating cone."

“The bass crossover point is 200 cycłes. This frequency is chosen because it approximates the bass limits of the average male voice and happens also to be a natural dividing point for the fundamentals of many musical instruments. The G string, on the violin, for example, goes down about 200 cycles. You see, a prime consideration in speaker system design is not to split the apparent source of the same instrument. This can be done by keeping the fundamental and the first and second overtones on the same driver.”