Quick sub array Q

[jmaudio]

Hi all,

 

I have a quick question. I am doing a project on sub arrays. And there is one thing I'm not quite sure about.

 

I am working with a very basic model. Two boxes, with a center frequency of 80Hz. Now all the places I have seen tell you to space the second box back 1/4 of a wavelength (λ/4~1.1m) behind the first speaker. Then they tell you to work out the delay amount required to make up 1/2 a wavelength, ie s=d/t so t=d/s. You then delay the back speaker by this amount and flip the polarity.

 

Now, what I don't understand is why if you, instead of using the delay, set the box back by λ/2, you don't get the same results.

 

I'm only trying this out in MAPP at the moment, and doing using λ/2 spacing gives a definite fig8 pattern.

 

Hope this all makes sense.

 

Cheers

 

J

[stevebunting]

If you put 1 box λ/2 (2.15m @ 80Hz) in front of another, the outputs will be 180 degrees out of phase at 80Hz at the front of the array. As is is symmetrical, the same will be true at the rear. You will have cancellation at the front and rear and a beam heading up into space.

 

I think the point of the array you are describing is to create a cardioid pattern at the front with a cancellation at the rear. I assume you meant to delay the rear by λ/4 instead of λ/2 as you stated?

 

The array with 1 box λ/4 in front of the other, with polarity inversion and a λ/4 delay applied to the rear box is designed to achieve this cardioid pattern. It is asymmetrical, and can create an asymmetrical beam.

 

If you model these 2 arrays in MAPP, you will see the difference.

[jmaudio]

If you put 1 box λ/2 (2.15m @ 80Hz) in front of another, the outputs will be 180 degrees out of phase at 80Hz at the front of the array. As is is symmetrical, the same will be true at the rear. You will have cancellation at the front and rear and a beam heading up into space.

 

I think the point of the array you are describing is to create a cardioid pattern at the front with a cancellation at the rear. I assume you meant to delay the rear by λ/4 instead of λ/2 as you stated?

 

The array with 1 box λ/4 in front of the other, with polarity inversion and a λ/4 delay applied to the rear box is designed to achieve this cardioid pattern. It is asymmetrical, and can create an asymmetrical beam.

 

If you model these 2 arrays in MAPP, you will see the difference.

 

 

But surely λ/4 worth of delay, and λ/4 of physical spacing gives a combined distance of λ/2? Does this make sense?

 

I have modeled the two setups in MAPP, and am getting the results you describe. However I can't see what the difference is between physical spacing, and using delay.

[mackerr]

You can also try an end fire array where you place 1 sub a quarter wave behind the other and delay the front sub by a quarter wave. This has better summation in the forward direction, and can be applied to any number of elements, delaying each element back to the farthest back one, although at 4 you are getting pretty far upstage.

 

Dave Rat has written extensively about this on his blog, and there has been a lot of discussion on the PSW forums.

 

Mac

[stevebunting]

But surely λ/4 worth of delay, and λ/4 of physical spacing gives a combined distance of λ/2? Does this make sense?

 

I have modeled the two setups in MAPP, and am getting the results you describe. However I can't see what the difference is between physical spacing, and using delay.

 

The reason they are different is that the delay on the rear sub causes different effects in different directions. At the front, it will be the same as λ/2 spacing, as you say, because the delay effectively widens the gap. In the rear, it will be the same as 0 spacing as the delay closes the gap. The polarity inversion causes the cancellation to move from the front of the array to the rear.

 

Two subs spaced λ/2 apart will not produce this asymmetry, and the physical gap will be the same whether you look at it from the front or the back.

 

Does this make it easier to visualise?

[jmaudio]

The reason they are different is that the delay on the rear sub causes different effects in different directions. At the front, it will be the same as λ/2 spacing, as you say, because the delay effectively widens the gap. In the rear, it will be the same as 0 spacing as the delay closes the gap. The polarity inversion causes the cancellation to move from the front of the array to the rear.

 

Two subs spaced λ/2 apart will not produce this asymmetry, and the physical gap will be the same whether you look at it from the front or the back.

 

Does this make it easier to visualise?

 

Brilliant! Thank you. It makes sense now, and I have just done the maths to confirm it works. Its pretty simple once you get your head round it, and can visualize what the waves are doing in the different directions.

 

Thanks again,

 

Jamie.