Quoting from RAM:EF 1, you said: "... A high-directivity speaker might, indeed, preclude the need for side-wall absorption as far as early reflections are concerned. As to whether that would improve the listening sound field, that is another story and I, for one, doubt it..."

This leads me to today's question: When you at Rives design room treatment, don't you treat for early reflections? If you treat early reflections, how do you treat? Early reflections are a potential source of coloration. Eliminating that source can make things only better in that you increase accuracy of reproduction - at least that's what I've learned. Textbooks like the one by Everest recommend treating reflections, and this is also the common approach when one follows discussions in Internet news groups. My question then becomes : If you have (or want) to treat reflections, why not do this by speaker design?

Klaus Rampelmann, Den Hague, The Netherlands

This is a great but also a somewhat volatile question. My reply has to delve into both the science and art of acoustical engineering. Most text books discuss the science. For this question, the science is pretty straight- forward. However, the art is based on experience and to some degree personal preferences.

Let's address the science first. I'll start with an interesting forum question I once read. Not recalling the exact phrase, the gist was: "If you're going to spend $20k on speakers, shouldn't they be made such that it doesn't matter what kind of room they go in?" The only speakers that will not interact with the room are those that don't move air within the room - headphones.

So your question is somewhat of an extension of the non-interacting speaker idea - a speaker that is directional enough to not interact with the sidewalls (and we have to include the ceiling and floor just as Everest and most text books teach us about early reflections). One generally treats the surfaces causing these early reflections with absorptive materials since it is the high frequencies that cause an overall perception of brightness in the sound. Why should that be so? Due to their different arrival times at the listening position, the high frequencies can be grossly out of phase with the direct sound field but maintain relative coherence. The best we can do is to absorb this first reflection since diffusion of these short wavelengths is nearly impossible.

Now let's consider a speaker that interacts less actively with the sidewalls. Let's imagine a speaker that is very directional above, say 8000Hz. For this speaker, what should we do on the sidewalls? The usual absorptive materials effective in this range need not be used since there is little high-frequency energy. What about the range below 8000Hz? One should probably use a combination of absorption and diffusion. It is here is where our answer begins to take a departure from the pure science and move towards the art.

Before we get into the art, however, let's take a different look at the science of reverberation time or RT-60. You've probably heard this term which relates to the time it takes for a sound to decrease by 60dB. Reverberation times are commonly used for large diffuse sound fields and they really don't apply in the classic sense to small room acoustics. However, in our experience they're still extremely useful in balancing a room. RT-60 times of .39 to .35 seconds in small rooms are generally the target range. We aim for longer reverberation times in the lower notes and shorter times in the higher frequencies. On a logarithmic scale, this gives us near equivalent energy for the diffuse or indirect sound field - in other words, a good balance.

In creating a great listening sound field, we need to balance these times as well as the frequency response. One common way is to measure the response, where the different energies are coming from, and then treat the room accordingly. This can work but doesn't account for new construction. As a method, it also doesn't take into account how we perceive different frequencies. Thus far, we've suggested that the high frequencies' first reflection should be absorbed. What's happening to the rest of the high frequency energy that is bouncing around the room? A ray tracing exercise could show us, but it's rarely necessary to go that far. What we do know is this: We want enough absorption to reduce the overall high frequency energy without killing it. For the listener, we would ideally like a diffuse non-coherent high frequency response. When we can't adequately diffuse, we must partially absorb. What does this do for highly directional speakers? It makes accomplishing this balancing act more difficult. This is where personal taste plays a major role.

Personally, I prefer a diffuse, well-balanced sound field that accounts for about 40 - 50% of the total sound at the listening position. Thus, a highly directional speaker would not likely suit my tastes because I would not be able to have the diffuse sound field in balance at all frequencies. But this is my personal taste and it is not for everyone. I know people who fancy an even greater percentage of the total sound coming from the reverberant or diffuse sound field. Conversely, I know many more who prefer less. A "near field" listening setup consists of mostly direct sound and little from the diffuse sound field. Likewise, a very dead room with plenty of absorption will have mostly direct sound except for the lower octaves where absorption is very challenging. A highly directional speaker will, by design, radiate a greater percentage of direct sound, at least at those frequencies where it behaves directionally. This would be most noticeable with speakers that have very poor off-axis response at high frequencies and a much better off- axis response at midband frequencies.

In summary to your question then: Yes, we do treat the early reflections, primarily by absorption but sometimes with a combination of diffusion and absorption. For my own personal tastes, dealing with these reflections via speaker design is not my preference. However, I have recommended such speakers to clients whose situation either doesn't allow them to treat the reflection points, or which features a very large left/right room imbalance (i.e. glass on one wall and no boundaries at all on the other). Such highly directional speaker designs then can benefit certain listeners, either because of their room limitations, or because their personal tastes prefer a predominantly direct sound.

Thank you for a really great question.
Richard Bird
