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In Reply to: RE: and a lot of lobing. posted by Bill Fitzmaurice on April 22, 2017 at 08:10:05
"Note however that once we get to a reasonable distance from the grating, towards the right of the simulation window, the waves from each slit have added back up into a large plane wave by Huygen's principle. "
What constitutes (and how do you calculate it) a "reasonable distance"?
Don't we all listen at "close distances" in our living rooms?
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"Still Working the Problem"
> What constitutes (and how do you calculate it) a "reasonable distance"?
That depends on the frequency and the driver spacing. If it was a major issue Don Keele's CBT array wouldn't work. It does, but to function at typical living room distances there's an awful lot of very small drivers for the requisite CTC distance.
Yes, Huygens rule does show a final summation however, each source radiates independently and in the array each radiates with its own dispersion angle just as it would operating alone.
Also, the longer or more curved you make a line, the more energy is radiated to the sides.
No one here is going to buy any of our stuff at work so i will link a paper that deals with multiple sources etc with systems on a larger scale than a living room. Some of it uses a modeling program called Direct which is free and has some theoretically perfect examples one can play with as shown here.
I know that article is about 5 years old, but it says a lot for the line array vs. point source argument. I actually recommend that someone at your shop revisit that article for grammar/spelling and reducing a little wordiness, condensing it down a bit further--but keeping the excellent figures and their textual explanations. I think that might induce more people to read it. I found it most convincing, but I had to keep myself reading it to get to the end.
I know many others have mentioned the line array problem and how bad they actually sound in real life. A bunch of guys that I know have been looking for the reasons "why not line arrays" and "why point sources" to help focus on point source horn designs. That article has the meat for the big venue commercial sound crowd.
What's less known is why the point source design works so well in small rooms. I'm not sure that I've seen you clearly enumerate why point source loudspeakers designed for the large venues also do so well in small rooms. That would be an interesting discussion. There's a story there. Perhaps I'll break tradition and start a thread on that subject.
"As far as the ear can tell, consistently clean and spacious bass can be reproduced only by a driver unit coupled to a horn-type acoustic transformer..."; Jack Dinsdale, May 1974
I believe there are a number of things which enter into this. Instead of the most obvious things, the most "un-obvious" thing is that our hearing system is VERY different in several ways than a measurement system and here is one thing invisible to a single microphone but clearly audible to the ear.
For example it is possible to have two sets of loudspeakers, eq' to have the same magnitude and phase and if you listen to one, they both sound VERY similar yet when you have pairs, they produce very different results with stereo images. In one case with such recording that has a strong mono phantom (same signal to both speakers) you get a strong phantom image and are unaware of a right and left speaker while the other case, you still hear a phantom image but you can also clearly hear the right and left speakers.
In this case, you are hearing elements of "how" the loudspeaker radiates and when there is sufficient difference between what reaches the right and left ear when facing the 1 speaker, your hearing system can localize it's position. Lacking those contradicting spatial cues, the hearing system instead hears a source coming from directly in front of you in stereo or with just one, it is hard to tell how far away the speaker is when your eyes are closed, your ears choose the depth sound in the recording instead of the speakers location. This is the area I am fascinated with, so is co-worker Doug Jones and I have tried to apply what I see to our large scale loudspeakers at work.
If you have headphones handy, try the link below for a speaker I was testing on Friday which "acts like" a single driver simple source (taken at 100 meters).
Secondly, reflections from close by the speakers also impart a spatial signature on everything the speaker produces and these also conflict with the signal you want, giving you cues that weaken or conflict with the image in the recording. Often putting absorption on the side walls where the strongest reflections would be from has a marked improvement in image. One can easily hear how much or if the room has a strong effect by setting the stereo up outside at approximately the same geometry as indoors and spend some time listening, normally the stereo image is vastly better outdoors although typically the bass is less. Fwiw, this is a great excuse to have a BBQ or picnic get together.
The point is, if you have speakers large enough to have significant and consistent forward directivity, then much less energy is going to hit the side walls and the near field where the direct sound is louder than the reflected sound is larger (good juju). This factor is critical in larger scale sound where the cubic volume works against absorptive surface area and the Hopkins Stryker equation shows intelligibility goes down with increasing number of sources and up with increasing source directivity. This is why essentially all of our products at work are large horns to maximize directivity to specific angles.
The cool part of constant directivity, one can move off axis and the spectral balance doesn't change just the spl.
It has been possible to use this combination of aim angle and height in stadiums where speakers are in the scoreboard and have only a + - 2dB variation in loudness over the entire stadium and as the consultant said "it sounds the same everywhere".
The reason high output systems can work very well in the home is because nearly all of the things loudspeakers do wrong or add to the signal increase faster with level than the input signal's portion does. Loudspeakers are more like tube amplifiers than SS, here the distortion and non linearity are nearly all related to level. It has long been said "headroom is your friend" and it's especially true if/when dynamics are a concern. The success also depends on if it sounds good to begin with etc..
Hope that make sense
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