Roy D's White Paper, Looking for a Copy.

[WMcD]

On the old site, there was a white paper by Roy Delgato. It concerned the use of absorbant acoustic material at the mouth of a horn.

I didn't save it and now it seems to be gone.

Does anyone have a copy, electronic or not, which they can share with me?

Thanks,

Gil

[justin_tx_16]

This is the attachment from Gil. Wordwrap is a nice thing

MINIMAL INTERFERENCE HORNS

Roy Delgado Jr.

03/27/00

INTRODUCTION

This paper describes the prototyping experiments that have been conducted to this point on one of the problems of arraying horns in systems. Most arraying speakers consists of horn components loaded into a trapezoidal cabinet. These trap cabinets are then arranged in one or in multiples so that sound can be distributed over a known area. What happens most times is that energy past the -6 dB downpoints is usually high enough to distribute acoustic energy in unwanted areas. If used in multiples, the overlap region between two cabinets has enough energy to cause ripples large enough to cause hot and dead spots in the area that is targeted.

WHAT HORNS CAN DO

Horns are excellent acoustic transformers if properly designed. Horns can also be very good coverage devices. They can have good, predictable coverage patterns and give the user useful information on how to direct the horn so that acoustic energy is placed in the right areas. Coverage patterns are determined by finding the point where the acoustic energy has diminished by 6 dB referenced to the on axis. Unfortunately, what happens after that -6 dB downpoint does not account for energy past that point. That is why the coverage pattern of the horn at the -9 and -12 dB downpoints is also important.

In the perfect world, the horn would emit a perfect sound "wedge". Then the acoustic energy of the horn would not excite unwanted areas and if arrayed, would not cause any rippling between two horns. Therefore, if our model were the sound wedge, then horns would have to begin to approach that model. One way to note how close a horn is approaching a sound wedge is to look at the coverage pattern of a horn at not only the -6 dB downpoint but also at the -9 and -12. Figure 1 shows the coverage pattern of a horn at the -6, -9 and -12 dB downpoints. The mouth area of the horn is 256 sq. in.

WHAT ABSORPTION GAINS YOU

Another way to try to arrive at a perfect sound wedge is to use sound absorption material. Sound absorption material can be sculpted and placed around a driver so that it can also have good coverage patterns; kind of making a horn out of sound absorption material. What you would lose from this kind of device is the directivity gain in SPL because the sound energy is being absorbed rather than being focused. Its directivity index would still be valid in describing how directive the device is but its SPL would not increase as coverage pattern decreased. Nevertheless, sound absorption does tend to shape the energy better.

CONFIGURATION WE ARRIVED AT

We asked ourselves if maybe there was a way to use both. What we did was to place sound absorption material past the horn. How long the material was, what kind of material it is and how thick the material is all have a direct relation to how well the material shapes the wave as it exits the horn and we are still exploring the possibilities. We need the horn in order to preserve the SPL gain we get out of horns. We use the sound absorption material to decrease the energy past the -6 dB downpoints. Using the combination of the two, we begin to arrive at a better example of a sound wedge. Figure 2 shows the -6, -9 and -12 dB coverage pattern of a prototype. The total mouth area of the horn is 256 sq. in.

COMPARISON VS HORN

A friend once said that in acoustics everything is a compromise. Horns are no exception. Comparing figures 1 and 2, what we have is two horns that have the same mouth area. One is using sound absorption material and the other is not. Figure 1 shows the horn coverage pattern at -6, -9 and -12. Typically, the -6 dB downpoints determine the coverage pattern of the horn. In this case, the horn has a 60 degree coverage pattern +/- 10 degrees. The -9 dB curve is about 85 degrees +/- 5 degrees while the -12 dB curve is about 105 degrees +/- 5 degrees. It has control down to about 800 Hz

The horn in figure 2 has a coverage pattern of 60 degrees +/- 5 degrees. The -9 dB curve is about 70 degrees +/- 5 degrees and the -12 dB curve is 85 degrees +/- 5 degrees. This horn has control down to about 2k. There is diffraction of the sound waves between 800 and 2k Hz. That is probably due to the discontinuity of the two different surfaces; fiberglass and foam. However, what is does offer is less energy past the -6 dB down points above 2k Hz.

CONCLUSION

Our first attempt at a prototype produced a horn that offers less energy past the coverage pattern of the horn. There are still other things to look at to see if we can lower the coverage cutoff of the horn. In our current prototype system, we have employed foam on the horn. We have also angled the trap box to be the same coverage angle of the horn. And of course, we are using a modified tractrix horn. We believe that all these factors add up to offer a product that will make the speaker a better coverage device by itself or in multiples.

[WMcD]

Thanks for your assistance, my friends.

It is a bit too bad we don't have the photos and the curves. But I have a vague mental image. Tweekers in the past have reported improvements by placing a moat of felt around a direct radiator tweeter, and stating that it improves diffraction problems caused the edges of the box. It is implied, or I infer, that energy going off the side of the tweeter is absorbed and doesn't reach the edge of the box.

It seems to me that the use of absortion material could improve, or at least alter, the midrange and treble horns we have. There is some comment by Ray that the on axis gain changes. So that might be a problem created by the modification.

None the less, it seems to me that the grill cloth on many recent Klipsch speakers stands about 1/3 inch away from the baffle board. So there would be room for some foam around the mouth, and perhaps it could extend into the mouth of the horn by a few inches. I recall the photos showing something like this

Gil

[lynnm]

William

Take a look at http://www.audio-ideas.com/ - They produce an interesting audio magazine and furthermore sell some devices for cone tweeters that would appear to be based on the same kind of thinking.

BTW Audio Ideas Guide and Ultra High Fidelity Magazine are a couple of Canadian Audio publications that are worth a look. The AIG site is listed above - UHF magazine's site is: http://www.uhfmag.com/