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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: Frontal Horizontal Directivity Measurements of a speaker ?.. posted by audiophile919 on July 24, 2015 at 15:54:01:
In commercial, sound polar information is much more common and required for room /sound system design and it is known that directivity is tied to intelligibility, the understanding random words.
In the home, reflected sound particularly from side wall reflections harms the delivery of a stereo image and so polar information can be an indicator of the ability to produce a mono phantom / solid stereo image as opposed to a "wall of sound" or in the worst case, obvious right and left sources.
The issue in each case is a preservation of time information and the tie in can be seen in the measurement which independent of language predicts intelligibility, the STIpa measurement.
That measurement is similar to that used in optics to show resolution and uses the audio equivalent of the optical Modulation Transfer Function, in fact the STIpa measurement uses 7 voice range MTF measurements. The optical version is easy to understand;
http://photo.net/learn/optics/mtf/
As one can see there, it is the degradation that happens when the "on and off" contrast is reduced and in the audio version it is when the off period is "filled in" with late arriving sound which can be reflected sound, noise or other signals which are not tied coherently to the direct sound.
That is why it is important for interaural crosstalk as well (the concern in the link), that late information contaminates the desired result.
But, for normal stereo, it is also very important as it effects how solid a mono phantom image is. In many recording studio's small monitors on the meter bridge are used, this is because the mix engineer now sits in the near field where the direct sound is much stronger than the wall reflections which arrive much later.
To be clear, there are other things which also effect the preservation of the stereo image like time coherency which most multi-way speakers do not do (all frequencies produced and arrive at the same time) but the directivty is the one that governs how much the room effects the results.
The cumulative effect can be easily heard playing (the same signal) a soft voice through both the right and left speakers. In a perfect system, the voice floats in front of you solidly as if a person were standing there speaking and you are not aware of the right and left sources.
At the opposite extreme where one has a large amount of incoherent sound arriving at ones ears, one plays the same signal and hears the right and left speakers as the obvious sources with a vague center phantom image which in some cases may extend the entire space between the right and left sources..
An extreme example of loss of time information is from the line arrays used in most concert venues, these are essentially incapable of producing a mono phantom image (or stereo image) because there is an arrival from each and every source arriving at ones ears but separated in time according to the path lengths for each. This is also why the voice intelligibility is so low with these systems, the same contamination of the "off" periods of the MTF is why.
On the other hand, a true coherent stereo image is so rare that most people only have a vague idea what it can be and think it's only the effect of sound coming from between the two speakers and have never heard a system that can produce a solid sense of voice location anywhere between the two.
This is why adding absorption on the walls to reduce the specular reflection is often judged to be a major improvement.
Directivity on the other hand reduces or eliminates the need for that absorption as less or much less destructive sound is projected to the sides etc.
Hope that helps.
Tom Danley
Director of R&D
Danley Sound Labs
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Follow Ups
- RE: Frontal Horizontal Directivity Measurements of a speaker ?.. - tomservo 06:29:42 07/26/15 (1)
- Very Helpful - Thanks! - briggs 08:30:01 07/26/15 (0)
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