Tech Talk

Comparing Horns and Waveguides

By Chris Hagen

Senior Principle System Engineer

The history of horns and horn use shares much of JBL’s own history. Although Lansing Manufacturing Corporation (LMC) started with 6-inch and 8-inch cone direct-radiating drivers, by the mid-1930’s LMC had designed the model 285 compression driver for the multicellular horn used in the historic Shearer-Lansing speaker system for movie theaters. From the experience of working on that system, smaller compression drivers and horns were created for smaller systems such as the Iconic (later D-1002) as well as coaxial drivers.

Model 285 compression driver

Shearer-Lansing System

Of great influence in the use of horns was the era: electrical signal amplification was difficult and expensive, pushing the speaker systems to be as efficient as possible. Meanwhile, the rapidly developing technologies of the phonograph, telephone, movies with sound, and more all required this efficient electrical conversion to sound technology.

So, always looking for more efficiency, the compression driver became a staple as the company evolved into JBL, Inc., with the most memorable usage of a horn and compression driver in the Hartsfield, Paragon, Everest and K2 models. But they weren’t reserved for the flagship models, with many more affordable models using them such as the L300 Summit. Countless studio monitors, professional music and reinforcement speakers used them as well, and still do, due to the higher output needs of today’s concerts and other venues.

Everest DD55000

Paragon

K2 S9500

Many innovations in the field were made by JBL engineers, including the bi-radial horn patent by Don Keele and the D2 driver patent by Dr. Alex Voishvillo, amongst others.

But what is a horn? Most people know it amplifies sound, mainly from experience of the horn systems in large public areas, like concert arenas, sports stadiums and movie theaters. A good, simple explanation is on Wikipedia: An acoustic horn or waveguide is a tapered sound guide designed to provide an acoustic impedance match between a sound source and free air. This has the effect of maximizing the efficiency with which sound waves from a particular source are transferred to the air.

The description for the horn says, “or waveguide.” But a waveguide is “… a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction.”Interestinglyenough, this sounds like a superset for the horn since no tapering is mentioned.

However, the audio industry started to use the term waveguide for a specific sound guide in the late 1980’s. This usage arose from some articles written between 1987 and 1993 on the use of “acoustic waveguides” which was shortened to simply “waveguide.” The waveguide was defined as being designed for directivity control but using a smooth and precise contour to minimize diffraction or other negative effects. The waveguide is typically used with a direct-radiating dome tweeter to control the driver’s directivity, smoothing off-axis response, and making for a smoother off-axis frequency response. There are instances of it being used with compression drivers as well, though.

JBL LE85, designed for efficiency

JBL Synthesis SCL-4, designed for d waveguide)

Question: Does one sound guide sound better than the other?

Answer: That’s difficult to answer. In a speaker system using a horn or waveguide, there are many variables affecting the sound quality. These are: the transducer, the sound guide shape, the sound guide material, the sound guide mouth termination, blend of the sound guide fascia to the baffle, and diffraction of anything on the speaker baffle. And then, there is the crossover not to mention any eq – whether in a dsp or outboard. Because of all these variables that affect both sound guides (horns and waveguides), it is very easy to make a good or poor sounding horn system or waveguide system. The difference in the sound guide is not a guaranteed positive or negative to the sound quality, and so the sound quality being more attributable to the engineering of all parts of the system. But each type of sound guide has a use – with horns still used for high output needs, such as concerts, and waveguides used for better directivity control for better off-axis smoothness.

JBL Synthesis SCL-1 with HDI horn

Revel F328Be with tweeter waveguide

Question: You mentioned “superset.” I remember from math classes that a superset was an all-encompassing group with subsets wholly inside of it. This implies that there may be other sound guides as well. Is this the case?

Answer: Yes. As a matter of fact, the design of a sound guide can be looked at as a balance of efficiency and directivity control, with some sound guides having more of one attribute than the other. Those with more efficiency are more likely to be referred to as horns, whereas those designed with more a focus on directivity control are more likely to be called waveguides. And of course, those in the middle are more likely to cause long discussions about whether they are a horn or a waveguide.

Some good examples of these sound guides in the middle area are the JBL HDI (High-Definition Imaging) sound guides used in the JBL Synthesis SCL series, the Studio Monitor series, and the HDI series. Similar sound guides, called Image Control Waveguides, are used in the JBL Professional M2 and 7 Series Master Reference Monitors, amongst others.

JBL Synthesis SCL-2, JBL 4305P, JBL Pro M2, and JBL HDI 3800

Hopefully this helps with identifying the difference between a horn and a wave guide “in the wild”, but also shows that rather than there being two types of sound guides, there is a spectrum ranging from being designed for maximum efficiency to being designed for best directivity control and off-axis blend to the system.