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[Sean Thomas]

Sound Theory 101 question:

Any inherent pros/cons to either of these designs?

With a symetrical design, you have 2 points of lows and 2 points of mids.  As a listener moves to 50 degrees off axis (100 degree pattern),
the listener could be 30' from one low driver and 33' from the far low driver.  Why is this not an issue?  If it is not an issue, why are we so picky
about our sub placement, spacing, and alignment?

Thanks!

[lindsay Dean]

 You might want to peruse
https://en.wikipedia.org/wiki/Subwoofer
it goes a lot "deeper" (no pun intended) than a one paragraph explanation.

[Roland Clarke]

Sound Theory 101 question:

Any inherent pros/cons to either of these designs?

With a symetrical design, you have 2 points of lows and 2 points of mids.  As a listener moves to 50 degrees off axis (100 degree pattern),
the listener could be 30' from one low driver and 33' from the far low driver.  Why is this not an issue?  If it is not an issue, why are we so picky
about our sub placement, spacing, and alignment?

Thanks!

Simple answer, wavelength.  At low frequencies the wavelength is so long they couple, effectively as one driver.  The problem with all speakers, not just line arrays, is that these wavelengths get impossibly small the further up the frequency spectrum you go.  This inevitably leads to comb filtering.

A good thing to do is to either google this or just lurk in the forums and pick it up piece meal, then ask questions to fill in the holes.

In sound systems there are no perfect answers just different compromises, then it's a question of finding the best set of compromises for you situation, not those that fit yours or others personal philosophy.

Good luck! 

[lindsay Dean]

simply complex

[Don T. Williams]

Both symmetrical and asymmetrical boxes have some lobing and acoustical sumation"problems".  The trick is to keep them minimally audible.  Some manufacturers do a much better job of that than others.  The original research by Dr. Heil for L Acoustics explores that, and Danely has some interesting white papers on those problems and their solution.  It will be interesting how Danely implements their solutions into a "line array looking" product for those that judge sound systems with their eyes.

[Ivan Beaver]

Simple answer, wavelength.  At low frequencies the wavelength is so long they couple, effectively as one driver.  The problem with all speakers, not just line arrays, is that these wavelengths get impossibly small the further up the frequency spectrum you go.  This inevitably leads to comb filtering.

Agreed.

And just to add.

It is not the physical distance between the drivers (as many people think).

But rather it is the difference is distance to a particular listening position.

That is why, when you have spaced drivers (especially mids and highs), the sound changes (cancellation freq are different at different seats) because the distance between the drivers is different at different seats.

As a general rule, in order for drivers to couple well, (without noticable interference), they need to be within 1/4 wavelength  of the highest freq that the speaker is producing.

For example: 100Hz is 11.3'.  So the distance of arrival should be less than 2.8'.

At 1Khz, the distance is 1.13'-so less than 3.5".

At 10KHz the distance is 1.3".  So now they need to less than 3/8" of an inch to avoid cancellation.

That is why multiple HF drivers interfere with each other much more so than subs.

The wavelengths are smaller-so the distances need to get closer and closer.

[Sean Thomas]

Agreed.

And just to add.

It is not the physical distance between the drivers (as many people think).

But rather it is the difference is distance to a particular listening position.

That is why, when you have spaced drivers (especially mids and highs), the sound changes (cancellation freq are different at different seats) because the distance between the drivers is different at different seats.

As a general rule, in order for drivers to couple well, (without noticable interference), they need to be within 1/4 wavelength  of the highest freq that the speaker is producing.

For example: 100Hz is 11.3'.  So the distance of arrival should be less than 2.8'.

At 1Khz, the distance is 1.13'-so less than 3.5".

At 10KHz the distance is 1.3".  So now they need to less than 3/8" of an inch to avoid cancellation.

That is why multiple HF drivers interfere with each other much more so than subs.

The wavelengths are smaller-so the distances need to get closer and closer.

Thanks Ivan - perfect answer, and exactly what I presumed.  So if I check the specs of any major brands mid drivers highest xover point, the spacing will be or should be 1/4 wave or less?  Is that an absolute design rule?

Also, if we apply this 1/4 wave rule to our sub placement - we could just line them up any where on the ground +/- a few feet (4' @ 75hz) and be ok..... but we don't.

Back to the original question - would an asyemtrical design be any better simply becuase it could never exhibit any of the symetrical "2 points/same band" issues as you move to the widest point of coverage where the distance becomes greater - even if the drivers are 1/4 wave or less?

[Ivan Beaver]

Thanks Ivan - perfect answer, and exactly what I presumed.  So if I check the specs of any major brands mid drivers highest xover point, the spacing will be or should be 1/4 wave or less?  Is that an absolute design rule?

Also, if we apply this 1/4 wave rule to our sub placement - we could just line them up any where on the ground +/- a few feet (4' @ 75hz) and be ok..... but we don't.

Back to the original question - would an asyemtrical design be any better simply becuase it could never exhibit any of the symetrical "2 points/same band" issues as you move to the widest point of coverage where the distance becomes greater - even if the drivers are 1/4 wave or less?

There is a bit of a "grey area".

1/4" is (should) be the design target.  If it is a little larger-then there will be more interference.

How much more?  It depends.  How much is tolerable?  it depends.

When some says "turn it down".  Is 1dB enough? You "turned it down-but not enough for them to notice.

 what about 10?  Would 3 do it, or maybe 5.

As I often say, there is no simple answer that is correct.

But you need a guideline.

Many people are simply not aware of the "patterns" they are creating by placing subs where they do.

Sometimes you have no choice.

Often times there is no "right for everybody" position.  It can be good for some people, but bad for others.

That is why it is ALWAYS best (no matter the manufacturer or product) to use a SINGLE sub and SINGLE full range cabinet.

As soon as you start to add more-you start to add problems.

But sometimes the end result is better than the problems it creates.

What IS important-is to understand how the interactions work, the penalties for doing so etc.

This way at least you have an educated idea of what to do and what might be the best compromise.

If you don't understand what if going to happen with various placements, then you are just "hoping" for something that may or may not happen.

VERY OFTEN, the result of adding more speakers is NOT AT ALL what most people think (or would like) will happen.

It is MUCH more science than voodoo.  An accurate model (that is taking phase into account) will show these interactions.

A simple "amplitude" model will not show this and "assumes" (wrongly) that everything is "playing nice" with each other.

[Peter Morris]

You might want to peruse
https://en.wikipedia.org/wiki/Subwoofer
it goes a lot "deeper" (no pun intended) than a one paragraph explanation.

This is what happens with point sources and their various spacing relative to wave length.

[Peter Morris]

I believe your question is in respect to line-array design.  This is a very rough sim of what happens with your T12s at their low / mid crossover of 420Hz with the spacing they have used. The FR plots are 0 degree and 45 degrees and there is a 100Hz HP.

The Mid / High crossover of  2.0 kHz does not work quit as well - you can see what happens modelling it with Ease Focus 3 (bottom picture)

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