Physical Limits

Warning: objects in rear view mirror may not be as close to a solution as their marketing may make them appear. It is often necessary to take a look at the history of audio before judging whether a new design is truly new. There are new ideas. There are useful innovations. Then there are those breaking into new markets that may believe their own hype, rather than boring old physics. By examining new products in light of what has been done (both the successful and unsuccessful) often prevents you from buying a product proved to be inadequate the last time it was tried.

No place is this more apparent than in the marketing of loudspeakers. Horns have long been held responsible for the "less than Hi-Fi" quality of sound reinforcement loudspeakers. However, there are a few companies that are innovating good horn designs that meet both the musical criteria and maintain the directional characteristics necessary for the horn to be used in the first place. It is all too easy to give up one to gain the other. The real solution requires that both criteria be met.

If you are presented with a revolutionary new sound reinforcement loudspeaker that sounds like a "Hi-Fi loudspeaker, studio monitor, real musical instrument, etc." take another look. Sometimes the basics have been downplayed. Let's focus on one example.

The mouth dimensions ultimately determine the lower and upper limits to the wavelengths a horn can control. Everyone should be familiar with the beaming effect that occurs at high frequencies, as the horn becomes too large to maintain constant directivity. However, it is the low frequency limit that will cause the most trouble. Remember this little rule of horn mouth dimensions: Wavelength -- too big to fit then the horn will quit. For instance, if the horn is only 4" high and 6" across then you can expect the device to have little control over the vertical coverage below 3250 Hz and horizontal control will be lost below 2250 Hz. Of course, this is not a big deal if you use the device in highly absorptive acoustical conditions, but in the real world…

Control over the dispersion of the sound from a horn is not simply a matter of efficiency. In poor acoustical conditions it will severely impact speech intelligibility. Throwing a lot of 2000 Hz energy at the sidewalls and ceiling may excite nasty reflections that confuse the listener. In addition, this loss of directional control is common to loudspeakers that use front-loaded 12" or 15" low-frequency drivers. These drivers are narrowing in coverage as they approach the commonly used 1.5 kHz crossover frequency, where suddenly the horn takes over and has a very broad coverage. The result is that the energy splashed around the room is not well balanced and makes the room "sound bad", when the real culprit is the loudspeaker. The off-axis response of the loudspeaker may be highly variable as the interaction between low-frequency driver and high-frequency horn can be at their worst within an octave of our most sensitive hearing range (2kHz to 4 kHz). This creates the perception that the room is responsible for the poor sound quality because the loudspeaker sounds good when you are on-axis and up close.

There are good reasons to choose a compact loudspeaker that includes a small horn and front-loaded woofer. They can be handles by one person, fit on a loudspeaker stand, don't cost too much, can get loud enough, and suit a wide variety of applications. These are some of the most commonly used sound reinforcement loudspeakers, simply due to these advantages. Just don't ignore the physics (that always supercedes a sales brochure) that clearly shows that the advantages of these small, portable loudspeakers must be traded off for lack of good acoustical performance. In fact, it is also easier to develop a musical sounding device in this format, which can achieve those illusive (Hi-Fi loudspeaker, studio monitor, real musical instrument, etc.) qualities. Unless, of course, you happen to listen to them off-axis and outside of an anechoic chamber like the rest of us.

While we see some true innovations in the acoustical interface between loudspeaker and room, so many of the latest and greatest things to come along can be found in 40 year old text books. While the power of marketing knows no bounds, we must use loudspeakers in a realm controlled by physical laws. The rear view mirror may be the best way to gain perspective on what is truly an innovation and what is the same old compromise.

United Entertainment Media Inc.
460 Park Avenue South, 9th Floor
New York, NY, 10016
Ph. 212-378-0400
FAX 212-378-2160

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