Loudspeaker Enclosures & Horns: What They Do, How They Do It

How Horns Work

An acoustical horn is essentially a transformer. The horn serves to convert the high pressure and relatively small radiating area of a cone or compression driver to the vastly larger area of the external atmosphere, which by nature is a low pressure environment.

By way of its expanding flare rate, the horn transforms the high pressure, high particle velocity vibrations present at the small throat of the horn into low pressure, low particle velocity vibrations distributed over a correspondingly larger area at the mouth, thus more efficiently coupling the wavefront to the air.

The flare rate – that is, the curve that the designer has chosen for the horn design – serves to transform the driver’s energy in some preferred manner. There are many classic curves, such as exponential, hyperbolic, conical, and others, each with its own properties. Some flare rates favor LF output, others overall efficiency, and still others uniformity of frequency response and/or low distortion.

All flare rates exhibit some measure of directivity. It’s never a simple matter of saying, “This is the one perfect flare rate.” Instead, the flare rate and horn size should be chosen by the designer to best accomplish the task that the horn is intended for.

ProSoundWeb
The horn and (compression) driver relationship.

Horn designs can be “straight” (you can look down the mouth and see the throat), or folded forward, sideways, or even backwards upon themselves and then forward again. Folding conserves space within the enclosure while keeping the horn length long enough to reproduce low frequencies.

One downside of folding is that energy above a certain frequency, depending on the shape and abruptness of the folds, will reflect back on itself and cancel. Therefore, most folded horns have a limited range of operation, usually not more than two octaves.

Directivity

In addition to increasing the transfer efficiency of the driver to the atmosphere, often as much as 10 dB or more, acoustical horns provide the added benefit of pattern control of the radiated energy. A horn’s flare angle can be selected to control radiation to almost any horizontal and vertical pattern. I once saw an old horn design at the Cow Palace in San Francisco that employed a 360-degree flare angle. The compression driver was mounted on the top where the throat emerged. (It worked well for voice paging.)

Though a given flare angle may provide the desired coverage pattern, the horn’s length and mouth area must both be large enough to control the wavelengths in the lower frequencies near the crossover region. Most are not. They become something of a labyrinth in the range below which they provide effective LF control, and can also be a serious impediment in the higher frequencies.

It’s a tough gig, horn design. I remember testing a proprietary 2-inch-throat driver on a commercial horn and being very disappointed that the claim of extended HF response was not met. Nothing over 10 kHz was present. Only when I unbolted the driver from the horn and saw that suddenly the HF response above 10 kHz was almost perfectly flat to 18 kHz (fortunately, the measurement mic and analyzer were still running) did I realize how detrimental the horn flare was to propagating the driver’s HF energy.

ProSoundWeb
A cutaway view of a Renkus-Heinz ST4 self-powered loudspeaker.

Making Transitions

The long-standing domain of horns in concert loudspeakers, from mid bass to the upper extent of hearing, has now been usurped in most line arrays by the use of direct radiating cone drivers for bass and small-format cone drivers for midrange. They’re often arranged in separated pairs for LF, and in an array of four that cover MF, typically two on either side of the HF waveguide. Modern cone, former, and spider materials, supported by high-intensity magnets and durable voice coils, allow very high SPL and power handling from the relatively new breed of small cone drivers.

At various times some astoundingly novel approaches to loudspeaker design have crossed my desk, usually in the form of patent grants or patent applications. To what extent they offer performance advantages, few may ever know. The effort at innovation, however, is a tribute to the imaginative powers of the human mind. It will be interesting to see what new trends will next emerge in the pro audio sector.