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Revisiting Horn Soundstaging & Imaging

Posted by Mark Seaton (M) on December 31, 2004 at 13:33:36

Hi All,

Last time this was mentioned we had the fortune of Earl Geddes offering up some very interesting insights and experiences. Unfortunately I was quite busy at the time and wasn't able to participate in the discussion. There is no question that many horns which have made it to market have quite erratic frequency responses for a variety of reasons, some of which will certainly have an affect on the subjective imaging and soundstaging.

If we are interested in better understanding the objections of those who have not heard well implemented horn systems, we need to understand the ramifications of our horns' directivity. On the plus side, well controlled and relatively constant directivity should yield a more intimate and clear sound through reduced early reflections. The reduction of early reflections makes sounds more distinct by reducing the "noise" the room makes in response to a sound. Subjectively, many describe a system with more directivity as sounding "quieter" with more dynamic contrast which was previously masked by reflections within the room. This is old news to fans of horn loudspeakers, yet it is important to remember when setting up systems, and explains some of the differences from more conventional speakers.

Let's consider simple left to right staging of a stereo pair. In many of the audiophile rags, we commonly read about the soundstage extending beyond the edges of the speakers and out to the width of the room. Other than the use of Q-Sound or the like, how is this possible? Simple: Side wall reflections create "image shifts" which give the perception of sound coming from an even wider space. For us to hear some directional que there must be a source of sound coming from that direction (with exception of methods like Q-Sound). If you have some fairly sizeable, constant directivity horns, this is an easy effect to demonstrate.

Start with the pair aimed such that the first reflection outside each speaker is outside the pattern of the horn. In most cases this results in the subjective sound of the stage extending speaker to speaker, maybe just outside the speakers. Now aim the speakers more forward such that the first reflection is now just inside the pattern of the horn. It may not sound as distinct and clear, but the stage is generally percieved to extend wider toward the walls which are now reflecting more energy. Obviously the better pattern control the speaker has, the more dramatic the effect.

Another subjective complaint of many audiophiles is the lack of "depth" from horn systems. Many have been romanced by the mystical effect where the soundstage seems to expand behind the location of the speakers. Again, these directional cues have to come from somewhere. Coincidentally, there was a significant push to make speakers very narrow in the interest of enhancing this effect. A narrow front baffle makes for very limited front to back directivity, making the horizontal dispersion nearly omni for much of the range. This makes for very significant early reflections from the front wall, as well as second order reflections which first strike the back wall, then side wall, and then on to the listener. Of course this is a double edged sword, as this expanding effect causes a huge increase in the reflected energy, lowering the direct to reflected ratio of sound. Clarity is lost, and the room ends up having equal or more effect on the sound as the response of the speaker.

Fortunately multi channel surround system doesn't rely so heavily on the room to provide additional signals to create a soundstage. Here we actually place speakers so engineers can deliberately place sounds and use speakers from multiple directions to create the acoustic effect desired.

The point being, once you have a nice set of horns, keep in mind exactly how they are interacting with your room, and don't fall prey to "classic" setup practices of the audiophiles.

Cheers,

Mark Seaton

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