Stair Cardioid Subwoofer Array

Re: Stair Cardioid Subwoofer Array

Martyn ferrit Rowe said:
Hey Art,
1. "Landscape mode" refers to the orientation of he cabinet, in this case "lying down". Measurements were also made in portrait mode, i.e. "standing up" or on end.
2. This enclosure is the Martin-Audio MLX, a self-powered, dual 18", horn-loaded "Hybrid" enclosure.(Hybrid refers to Horn-loaded plus reflex-ported design).
3. Measurements are Actual, using MLSSA, performed at 5 degree intervals at 10m rotation, centered on the front of the enclosure, ground plane, indoors with the nearest boundary (ceiling) at @35m, they were then post processed to produce the 1/3rd octave data for ease of viewing.


cheers
Ferrit
Click to expand...
How far away was the mic from the "zero" point?

What did you use as the "zero" point? The front of the cabinet-the center or something else?
 
Re: Stair Cardioid Subwoofer Array

Ivan Beaver said:
How far away was the mic from the "zero" point?

What did you use as the "zero" point? The front of the cabinet-the center or something else?
Click to expand...

Ivan, did you read the post you quoted? Re-read item 3. In it Ferrit specified a 10m radius of rotation, centered on the front of the cabinet with measurements at 5º intervals.

Mac
 
Re: Stair Cardioid Subwoofer Array

Thanks Mac :)

Here's three enclosures in portrait mode, all facing forward the black trace is a single enclosure and the blue trace is for the array, again only the center cabinet is driven, other cabinets are shorted.

As to here the center of pressure or radiation is, well that is an interesting question........... :)

cheers
ferrit
 

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Re: Stair Cardioid Subwoofer Array

Hey Gang,
So where is the "Acoustic center"? Let's define it as the physical point from which the acoustic propagation emanates including the phase.
This definition is valid up to those frequencies where the acoustic wavelength is large compared to the cabinet size.
If we use a simple point source model to predict the LF acoustic radiation of an array of enclosures we need to know where this is.

General thought has put this position at the center of the piston, perhaps where the cone apex meets the voice coil?

It might come as a surprise to some that in actual fact it is situated some distance in front of the enclosure, in the case of the MLX subwoofer its actually 50cm in front!
As the frequency increases it moves in towards the enclosure, but as far as LF is concerned we can say it remains in place.

If this "offset"is not taken into account, quite large errors, in the order of 20dB, can result in the predictions. <yikes!>

In answer to the question on endfire and Cardioid setups some preliminary work has been done.

cheers
ferrit
 
Re: Article: Stair Cardioid

Martyn ferrit Rowe said:
It might come as a surprise to some that in actual fact it is situated some distance in front of the enclosure, in the case of the MLX subwoofer its actually 50cm in front!
As the frequency increases it moves in towards the enclosure, but as far as LF is concerned we can say it remains in place.



cheers
ferrit
Click to expand...


This actually matches the way I conceptualize it. There is a region in which the speaker does work on the air, creating the vibration, which then propagates as a wave.

My instinct was that for LF this region actually extends some way in front of the speaker.


Sent from my DROID RAZR HD
 
Re: Stair Cardioid Subwoofer Array

Martyn ferrit Rowe said:
Hey Gang,
So where is the "Acoustic center"? Let's define it as the physical point from which the acoustic propagation emanates including the phase.
This definition is valid up to those frequencies where the acoustic wavelength is large compared to the cabinet size.
If we use a simple point source model to predict the LF acoustic radiation of an array of enclosures we need to know where this is.

General thought has put this position at the center of the piston, perhaps where the cone apex meets the voice coil?

It might come as a surprise to some that in actual fact it is situated some distance in front of the enclosure, in the case of the MLX subwoofer its actually 50cm in front!
As the frequency increases it moves in towards the enclosure, but as far as LF is concerned we can say it remains in place.

If this "offset"is not taken into account, quite large errors, in the order of 20dB, can result in the predictions. <yikes!>

In answer to the question on endfire and Cardioid setups some preliminary work has been done.

cheers
ferrit
Click to expand...
The BIG implication here is that the traditional approach of stacking one sub on top of the other, with one reversed for a cardioid configuration could end up making the acoustic centers even further apart than what a simple measurement of your tape measure will deceive you with. It took me a little while to figure out why I ended up adding a bit of "extra" delay into my cardioid arrays to make it work optimally, but couldn't explain the rationale. It is best to use an analyzer to figure out where the true acoustic center really is.
 
Re: Stair Cardioid Subwoofer Array

Yep, I was adding a "fudge factor" to the simple delay time to get measurements to agree with predictions, the simple point source model will only get you so far :)

cheers
ferrit
 
Re: Stair Cardioid Subwoofer Array

mackerr said:
Ivan, did you read the post you quoted? Re-read item 3. In it Ferrit specified a 10m radius of rotation, centered on the front of the cabinet with measurements at 5º intervals.

Mac
Click to expand...
DUH----------------------

I looked and "thought" I read it-but I guess I had another "brain fart".

My bad :(

I did see the 5°-how did I miss the rest--------------- Oh well----
 
Re: Stair Cardioid Subwoofer Array

back to thread start: I just did an experiment with a sub configuration based on this idea and it worked quite well. Looks like I'll have try it out in reality sooner or later. I'll try to post back then how exactly I did it and how it went. So for now thanks a lot for writing this article!
 
Re: Stair Cardioid Subwoofer Array

Arthur Skudra said:
The BIG implication here is that the traditional approach of stacking one sub on top of the other, with one reversed for a cardioid configuration could end up making the acoustic centers even further apart than what a simple measurement of your tape measure will deceive you with. It took me a little while to figure out why I ended up adding a bit of "extra" delay into my cardioid arrays to make it work optimally, but couldn't explain the rationale. It is best to use an analyzer to figure out where the true acoustic center really is.
Click to expand...
It is amazing how quickly a simple 2 mic measurement can save all kinds of time and guesswork.

Simply put a mic a decent distance in front and another a decent distance in back of the "array". Then save a trace of each and adjust delay until you get the maximum output out front (including best freq response) and the maximum cancellation at the freq of interest (many people don't realize that directional sub cancellation is freq dependent-and you may not be able to cover the whole sub range-so you have to choose what is most important for a particular application).

I have run into many cases people want to "discuss" what is wrong-while a simple measurement can often point out the problem real quick. :) and more accurately than simply guessing.

Measurement trumps prediction every time.
 
Re: Stair Cardioid Subwoofer Array

I would agree that measurement trumps prediction every time, but I would move the rear mic around don't work off just one position.
The "moving mic" spectrogram technique can reveal a lot (Ferrit's first law of un-intended consequences) :)

ferrit
 
Re: Stair Cardioid Subwoofer Array

Martyn ferrit Rowe said:
Hey Gang,
As the frequency increases it moves in towards the enclosure, but as far as LF is concerned we can say it remains in place.

If this "offset"is not taken into account, quite large errors, in the order of 20dB, can result in the predictions. <yikes!>
Click to expand...

This phenomenon might lend some advantage, at least with respect to modeling if not actual performance, to arrays in which all the cabinets have the same orientation so that the relative positions of their acoustic centers are easy to predict and remain constant with frequency.

--Frank
 
Re: Stair Cardioid Subwoofer Array

Christian Güssmer said:
back to thread start: I just did an experiment with a sub configuration based on this idea and it worked quite well. Looks like I'll have try it out in reality sooner or later. I'll try to post back then how exactly I did it and how it went.
Click to expand...

I'm doing a festival in a quite challenging building - RT60 below 150Hz about 7.5 seconds. Using an endfired sub arc of 15 2x18 subs inspired by your "stair" configuration sound quality has improved a lot. Once again thanks for your article!
 
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