2. HORNS AND MIDS

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1. INTRODUCTION
2. HORNS AND MIDS
3. WOOFERS
4. SETUP
5. SUBWOOFERS
6. RESULTS
7. PARTS
8. SUBWOOFER NOTES
9. ADDENDA



 

THE MAGIC HORN FIX 

There are some tricks that are essential for eliminating horn "honk." The first is to use the cone driver placed just below the horn, all the way up to a frequency where it begins to "beam" due to the relationship of sound wavelength and cone diameter. At a frequency where the resulting Q-factor (directivity) of the cone matches that of the horn, the transition from cone to horn will be smooth, and not abrupt-as it can be in systems where the cone is too large and the horn is too small. If this condition is met, and the frequency response of the cone is good well beyond the frequency up to which it is used (a well-behaved upper end rolloff), then the horn will enjoy a seamless transition from the cone and will not honk, assuming its frequency response is good and uniform over its output angle. This latter condition is referred to as being "power-flat" and is very important to the transparent operation of the speaker system in rooms, with their concomitant acoustic implications. If the speaker system is power flat, the sound in the room will be as good as that particular room will allow it to be.

THE MIDRANGE 

The midrange driver must be a cone, unless you live in a theater and don't mind a 4-foot high horn (I crossed my mid at 300 Hz into the woofers). As it turns out a mid cone supplying 300 Hz to 1200 Hz gives the proper effortlessness with very little power, and thus has extremely small cone excursions and low distortion. As I mentioned earlier, I had to trim the 2123H mid cone back 10 dB on the amp's gain control to get the response through the band flat. The power absorbed by the mid cone driver amounts to milliwatts most of the time, which helps to hold harmonic distortion to very low levels, typically well below 1% THD up to dangerously loud volume.

I experimented with a dozen midrange drivers before I was confident that the 2123H with its high efficiency and limited excursion linearity would produce sufficiently low distortion. It is a wonderfully transparent driver and a large part of the reason this speaker system sounds like listening to live music rather than loudspeakers. (Note: as of 1995, JBL is manufacturing a new even higher power 10" driver called the 2012H. If you can obtain 2012H's, performance will increase even further.)

The driver is mounted on the baffle as close to the horn as I could get it with my inexpensive mid chamber geometry. You could do better if you are willing to cut the shape of the mid driver's frame into the lower lip of the horn and snug the mid frame up into the cutout and, of course, figure out a mid chamber arrangement that would clear the horn and driver behind the baffle, but this is not measurably better than just a touching fit.

The enclosure for the mid cone consists of a 10-inch diameter concrete casting tube made of plasticized paper. Such tubes are made by Burke Tube and Sonotube and no doubt many other regional paper products manufacturers. The tube is mounted to the baffle by gluing into a counter-bored shoulder cut-out, routed in the back of the baffle around the mounting hole. The tube is about 12 inches long (deep), it is filled completely but loosely with a "jelly roll" of unbacked fiberglass house insulation cut from a roll about 4 feet long. The back end of the tube is sealed air-tight with a disc of 1-inch thick medium density fiber board-the same material used to build the rest of the box.

Please, even if you hate handling fiberglass, don't substitute other absorbing materials for it. Fiberglass is unique in its physical properties and substitutes will not work as well. Just get some long heavy rubber gloves to handle the stuff, and shower off with cool or cold water when you're done.

One letter I received inquired about using the JBL three-inch throat midrange compression driver and horn. The horn itself is 44" wide by 42" high and with the driver attached, is 42" deep and weighs 82 pounds. The letter also asked about horn-loading the two 2227 cone drivers for greater efficiency. Let me explain why I chose the geometry I did, so that those of you inclined to even higher efficiency can decide how to proceed from an informed set of criteria: First, one of my design goals was the use of the typically small space behind the perforated theater screens in the new smaller multi-cinema complexes being built around the country and in Disney attractions that have such screen spaces. Even at Disney Imagineering, it would have been impossible to argue successfully for the space behind the screens required for horn loaded systems, and in fact, this column design (once tested and listened to) proved that horn-loaded systems were not necessary to play even the loudest ear-splitting explosion effects in theaters of 200 or 300 seats. Second is the issue of acoustic impedance. Simplistically, acoustic impedance is the ratio of radiation resistance to the acoustical load. Radiation resistance varies with the size of the acoustical aperture (source size) and the acoustical load is the air in the room in which the source is operating. The source drives the load, and so if we wish to avoid transmission line conditions where we must match the source and load to obtain proper power transfer and flat frequency response, we must provide a source of low acoustic impedance (small source size) so that the source output is sufficiently robust to essentially ignore the load conditions. What we lose doing this is some efficiency and sound pressure level capability; what we gain is flatter frequency response and freedom from such effects as the deterioration of performance when we move furniture around or close or open a door or window. The real bottom line, however, may be that this little column is simply more practical perhaps, than some other designs.

1997 Drew Daniels

 

 

 

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