My only complaint is Wayne's cursory ellimination of my experimental evidence as invalid. The implication here is that I am incapable of making good measurements. I have been doin it for nearly thirty years and am perfectly capable of doing it well.

There are things that I agree with in Wayne's argument, like corner gain at LF. What I manily disagree with is that he attributes this to directivity. Directivity really only has meaning in free space. Now a room acts like "free space" above a certain frequency know as the Schroeder Frequency. Below that it is modal and not at all like free space. So in the kind of rooms that we are talking about below about 200 Hz directivity should not be used in the discussion. This means that the corner gain is real, but I object to calling it directivity.

Now above 200 Hz the corner will not really act like a horn because it is too wide and not symmetric and the driver cannot be placed at the apex. So to talk about the corner as acting like a horn above 200 Hz is also incorrect IMO.

I think that there are points to what Wayne is saying, but I object to his supporting arguments as invalid. He objects to my arguments because I support them with my own data.

Thats the jist of it as I see it.

Where Wayne and I would agree is that above 200 Hz the directivity should be at or below 90°, remain as constant as possible and that the sources should point inward to avoid wall refectiions. Thats a lot of common ground.

Where we disgaree is that the walls become part of the horn. This can be true only if the driver is placed in the apex of the wall corners, i.e. actuall outside of the room. Placing a horn in a corner where the driver is outside of the corner - even by a small amount, and you simply have a horn placed in a corner. The corner does not affect the function of the horn except as the sound waves reflect off of the side walls. This is best pictured as placing four horns in a circle because this is what the walls will do. If the directivity is below 90° then they don't really influence on another. If the directivity is greater than 90° then they do influence one another, but because the acoustic centers are not coincident they will have lobing errors. Hence they will not be CD.

So if the devices are less than 90° Wayne and I agree and do basically the same thing - but the walls don't enter into the picture. If the directivity is above 90° then I claim that the corner placement causes polar lobes from the wall refections interference and is not CD. I don't recommend this.

Reflections - good or bad? - depends on a lot of factors.

First is the time of arrival of a reflection. This is because of the integration time of the ear. Refelections less than 10 ms are integrated with the direct sound (first arrivalk) to form a single impression in the brain. For lateral reflections, these delayed signals cause everthing from coloration to a muddyness in the sound and a loss of image focus or transperency. All these things are basically subjective terms for the same thing.

Since our ears are lateral they react to lateral reflections differently than vertical ones. Vertical reflection can only cause coloration, not image shifts since they arrive at both ears at the same time. But they have strong coloration problems.

In the region between 10 and 20 ms. the ear is begining to desensitize to the reflection as a problem. And this too is frequency dependent. The ear is not at all sensitive to image shifts for LF reflections and only marginally sensitive to coloration efects at LF. So refections are primarily a mid to HF issue. We don't localize on LF's. Above 20 ms, reflections are perceived as a seperate auditory events which we subjectively call reverberation. If these reflections are difuse - arrive from many directions - then they create a sensory effect know as spatiousness - a very good thing almost critical in a small room.

There is one other consideration, an that is if the early refections arrive at the same ear as the direct sound or the opposite one. In other words a refcetion from the right for the speaker on the right is far worse than a reflection from the left for the speaker on the right.

So in a nut shell - we don't want ANY early refections, but we want as many later refections as we can get. If there must be some early refections, i.e. the room is small, then these should be adjusted to be from the opposite wall not the near wall. This is the crux of the room acoustics problem.

The normal small room solution is to put sound damping everywhere. This may help reduce the early reflections, but it simply kills the later reflections and so there is no spatiousness to the room at all. The direct field versus reverberant field situation cannot be cured with room treatmnet alone. SOmething must be done in the loudspeakers themselves to help to control this problem.

Now (and this is a part where I may disagree with Wayne, but he may have also mistated his point) the ear perceives both the direct (sometimes called axial, but only if one is actually on axis) and the rverberant field. Wayne stated ("On-axis, this loudspeker may sound just fine.") which I think would be better said as "This speakers will sound best on-axis". But I would contend that there will still be a perceived problem with the sound since the reverberant field is not flat. This speaker would only sound "fine" only in a reflection free environment, but then it would also sound dead - no spatiousness at all. Some contend that we get "spatiousness" from the recording, but this is not true. Why? because the reverberation in the recording is not difuse - it does not arrive at the ear from different directions. This is what multi-channel sound is trying to create - a more diffuse reverberant sound field from a recording. But the CORRECT way to get spatiuosness is with proper room acoustics.

A non-flat reverberant field will always color the sound but in a different way than an early reflection.

I don't think that Wayne and I disagee, but we might say things differently.

Finally, and this is a aspect that is seldom appreciated and that is: the higher the directivity of the source in a small room, the slower will be the build up of the reverberant field. You have to think about this a bit to see why. An omni directional source has a reverberant field that builds almost immediately because it has a flurry of very early reflections off of all nearby walls etc. A narrow directivity source has fewer early reflections and it takes a much longer time for the sound field to build into reverberation. This slower rise time of the reverbeant field is subjectively very important because it allows time for the ear to process the direct field unencumbered by the reverberant field.

These are all immensely complex factors when one takes the ear into account, because, quite frankly, the ear is immensly complicated. It is nonlinear in frequency, time and level - it couldn't be any more complicated. In fact it even works differently at LF and HF with a continuous blend in between.

Designing good audio requires a design that works best with the ear. These designs are inherently more complicated, but inherently better sounding. On this point both Watne and I agree. We are still having some disputes on the finer points however.

Eighth-space radiation is defined as having directivity factor of 8, which corresponds to directivity index of 9dB. To me, it's a no-brainer that the room's walls confine sound within them. One can quantify this by describing the radiating angle. There are other things that occur in an enclosed room also, but we're talking specifically about directivity here. I don't see any reason why you would disagree that a room corner defines its maximum radiating angle, and that bass frequencies are therefore limited by this radiating angle.

• Sound System Design Reference Manual

I've been crying the blasphemy of "the reverberant field matters" and spreading the abomination of "reflections can be our friends" over at Audiogon and the Asylum, and it sure is welcoming to read your posts on the subject here.

By the way Earl, a couple of years ago I heard one of Wayne's systems at an audio show in Ohio. As I approached the room, I was immediately struck by how natural the music sounded. I paused just to listen from out there, as this was in stark and welcome contrast to the way almost everything else sounded from out in the hall. Well I walked into the room... and didn't see any speakers! I had to walk across the room and look around yet another corner to see them - a pair of 7 Pi's in the corners. From around two 90 degree bends, they had sounded quite natural and relaxing - which speaks well for their power response. And then in the normal listening area they were not the least bit bright or edgy, which indicated the on-axis response wasn't tipped up to get the power response right. The degree of consistency in the tonal balance from the "sweet spot" to outside the room was amazing. This was among the two or three most enjoyable systems I heard at that show, and the other two were also systems that did a good job with the reverberant field, though probably not as good a job as Wayne's did.

Just for the record (not being a regular poster here and not wanting to violate the etiquette), I'm an audio dealer.

Duke

Say, maybe you could use a switch to reduce low frequency power when not in your specially prepared room? What kind of crossover do you use? What are the crossover points? Very interesting, I must say.