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In Reply to: RE: Frontal Horizontal Directivity Measurements of a speaker ?.. posted by audiophile919 on July 24, 2015 at 15:54:01
One of the main reasons loudspeaker directivity matters is, unless we listen nearfield or under approximately anechoic conditions, most of the sound that reaches our ears is reverberant energy - that is, energy that has reflected one or more times off of room surfaces. This reverberant energy can do a lot beneficial things if we get it right: It can enhance timbre, clarity (according to Toole), and sense of immersion in the acoustic space of the recording. The speaker's off-axis response is the main source of this reverberant energy, so it matters a lot.
Let me back up and state what I believe to be a reasonable goal, acknowledging that it may not be everyone's goal: I believe that a reasonable goal would be to approximate the perception of hearing a live performance. In order to do that, we need to not only get the first-arrival sound right (from a perceptual standpoint; waveform fidelity would be nice but isn't essential), but also get the reverberant field right. And by getting the reverberant field "right", I mean we want to approximate what we'd find at a decent venue - the spectral balance of the reverberant field shouldn't be skewed; the reverberant field should be highly diffuse; and the onset of significant reverberant energy should occur after a fair amount of time delay relative to the first-arrival sound.
Getting the reverberant field "right" is a cat which can in more ways than one be skinned, and I won't go into them all here, but djk's post is worth repeating:
"Many, many years ago Audio did tests to determine what kind of directivity factor sounded best in a typical room.
"They found that a Q=7 gave the best results (a typical 90°x 40° horn at the time).
"Horns have been improved a lot over the years, but I would wager that the Q=7 still holds true, the modern horns would just be a lot less colored.
"Crossing the woofer over where the directivity matches the HF also helps with realism."
So following this line of thought, a 90°x 40° low-coloration constant-directivity horn, crossed over to a midwoofer array whose radiation pattern is approximately the same as the horn's in the crossover region, would arguably be a strong candidate for getting the directivity right.
In my opinion this, or something conceptually similar, would be an excellent starting point. It arguably has some room for improvement, and a case can be made for different radiation pattern shapes, which I can do into if anyone is interested, but the basic principle is to make sure the off-axis energy is working WITH, rather than AGAINST, the first-arrival sound.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
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And AJ May they knew.
Another reason why loudspeaker directivity matters is, for the sake of long-term fatigue-free listening.What follows has not, to the best of my knowledge, been the subject of controlled blind testing, so consider this to be just my opinion, unless of course it correlates well with your own experience.
The ear is constantly examining incoming sounds to see if they are new sounds or reflections. When a new sound is detected, the ear/brain system computes its location in space and puts a copy of the sound into a short-term memory (for about 40 milliseconds or so). It then compares subsequent incoming sounds with all the sounds that are stored in this short-term memory. If an incoming sound is the same, then it's a reflection, and its directional cues are suppressed but it still contributes to perceived timbre and loudness. If it's a new sound, then its spatial cues are calculated and a copy goes into this short-term memory. See "Precedence effect" or "Haas effect" in Wikipedia for more information.
Now what the ear/brain system primarily looks at, when deciding whether an incoming sound is a new sound or a reflection, is the SPECTRAL CONTENT of that sound. So, what happens if the spectral content is "close to, but not quite" what the ear would expect after the effects of normal room acoustics? Well, the ear/brain system literally has to work harder to correctly classify this incoming sound. I believe that extra CPU usage causes that part of the brain to become fatigued, and the result is "listening fatigue" or even a head-ache.
Note that at a live, unamplified performance, listening fatigue is virtually never an issue - and the reverberant field is always right. Recall the last time you were aware of listening fatigue, make an educated guess about what was happening off-axis, and see if there's a correlation.
Here's how you make an educated guess about what's happening off-axis: If there is a significant discrepancy between the directivity of the two drivers in the crossover region (like when going from a 6.5" woofer to a 1" dome tweeter), you can expect there to be a major glitch in the off-axis response relative to the on-axis response. And ironically midwoofers that behave pistonically up to the crossover region are more likely to contribute to an off-axis glitch.
An off-axis glitch isn't the only source of listening fatigue, but mentally go back over speakers that you have found fatiguing and see if there isn't a correlation often enough to be of interest.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
Edits: 07/26/15 07/27/15
Thanks for that, Duke.A question comes to mind: Is it possible to make a "time coherent" speaker using the precepts prescribed by you in the above paragraphs?
Edits: 07/27/15
"A question comes to mind: Is it possible to make a "time coherent" speaker using the precepts prescribed by you in the above paragraphs?"
In a passive loudspeaker, imo the compromises required to get time coherence out of a horn- or waveguide-based system are prohibitive. You see, a horn inherently will have at least a second-order rolloff at the bottom of its passband. We could minimize its effects by crossing over at least an octave above the beginning of that rolloff, but now we need a horn twice as big if we want to keep the same crossover point. This bigger horn will introduce problems of its own, including reduced top-end extension. So we might now need a three-way, and if we're going to achieve time coherence in the upper crossover, we're going to have to deal with the supertweeter refecting off the main horn given its location back near the throat of the main horn.
I believe that DSP would be the way to go if time coherence combined with good radiation pattern control was the top priority. Once upon a time I was a dealer for a DSP-based speaker system and many users of the system were spending big bucks upgrading their DSP units. Not saying there isn't a price point where DSP is satisfyingly transparent, as I haven't really looked for it yet.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
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