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[Bob McCarthy]

Ivan,
Sorry I am two years late to the dance. i will give it my best shot next week. Yes the room will have an effect. Parking lot is mo' better. Class in a Mexican parking lot? Not mo' better.

6o6

[Andy Peters]

Ivan Beaver wrote on Tue, 20 April 2010 17:05

Adam Schaible wrote on Tue, 20 April 2010 18:33 Valid point, but from my perspective it's nice to read a lot of the theory, comprehend, etc - so I think it's great. And the theory is great-but it is also a very good idea to see how the real world stands up to the theory and vice versa.

Wow, this thread is great and should be a sticky.

I agree with Adam here. The real-world measurements are great, but for the purposes of determining what configuration we'd like to use on tomorrow's show, it's nice to be able to predict how things should be arranged.

Why? Because, really, who has time to experiment on a gig?

Of course, having said that, accurate predictions of the subwoofer response in a room demands an accurate model of the room, and who has that?

-a

[Bennett Prescott]

Andy Peters wrote on Tue, 20 April 2010 20:53

Of course, having said that, accurate predictions of the subwoofer response in a room demands an accurate model of the room, and who has that?

There are rooms that I probably have a pretty accurate model of, up to 100Hz.

[John Roberts {JR}]

Ivan Beaver wrote on Tue, 20 April 2010 17:02

It ain't that hard.  I bet most people have at least 2 of the same type subs laying around and a measurement system.  OH you will also need a DSP, a couple of amps and some cables-so I guess that elimnates a lot of people Yeah it takes a little bit of effort and you have to put some clothes on.  But hey-at least you have to take your computer outside with you -so it won't get lonely. Or maybe I just see things differently.

I've only got one sub of any type, and no measurement system...  So I appreciate you sharing the fruits of your labor.

Thank you Ivan.

JR

PS: Do you perform indoor measurements au natural?

[Ivan Beaver]

Bob McCarthy wrote on Tue, 20 April 2010 20:25

Parking lot is mo' better. Class in a Mexican parking lot? Not mo' better. 6o6

Why not?  I'm sure they will be glad to help you "move" the gear around

[HarryBrillJr.]

Nick Hickman wrote on Tue, 20 April 2010 13:38

Hi Bob, As I wrote in the message you replied to, superposition predicts everything, so carry on being SuperpositionMan! It's just that to fully understand the mechanisms at work behind the scenes, you must include changes to radiation impedance or you'll draw the incorrect conclusion that you're getting a free lunch in the form of more energy out than you put in.  (If not, why should moving boxes around change the amount of radiated energy?)  It's like attempting to compute a power change in an electrical circuit without accounting for a change in the circuit impedance. Bob McCarthy wrote on Tue, 20 April 2010 18:46 So much is made of 1/4 wavelength. But as I see it, it is just a milestone in our brains. Yes, absolutely.  Everything is gradual; no sharp boundaries. Quote: But I can (and do) put speakers right next to each other. In my expereience they never add 6 dB in all directions like people are saying here. The displacement pushes the phase responses apart by SOME amount in one plane - and along that line the addition is SOME amount less than 6 dB. Maybe a tiny amount... but the amount less than 6 tends to be consistent with the superposition principle. Yes!  If the sources were truly co-located (which is impossible in the real world), you'd get +6dB, as predicted by superposition. Nick

I have yet to be convinced that ON Axis I should be looking for 3 or more dB less output from an End fire array than a 2x2 array of equal distance to the acoustic center of the array.  I do not care about total energy.  I only care that the maximum energy I can get out of that system is going in the direction I want it to go and not spraying all over the wall 15ft behind it.  I don't need to know all the details, but I think it's fun, so I seek that knowledge, when I'm off work!

Oops gate change!!!  Gotta go!

OK managed to get back on for just a minute.  When I am working, it is important to know how to get the most energy where it will do the most good, where the audience is, and keep energy that will tend to do harm, away from the audience.

[HarryBrillJr.]

Art,

A. Bob wrote the book.  He can call himself anything he wants.

B. Whether or not I measure the subs next to each other (within 1/4 wave) or apart but equal distances from the mic or my ears, I still get almost 6dB.  1/4 wave distance can apply to the distance from the HUMAN that is listening and the subs can still be apart.

C. Smaart IS and FFT Analyzer

[Guenter J. Krauss]

Hi,

on page 9 of the following paper you can see measurements of a single sub and a 2*4 stack of the same cabinet.

http://www.dynacord.com/downloads/dsp244_info_bass_en.pdf

The measurement microphone was in 10m distance on axis of the single sub and the 2*4 stack, respectively.

The measurement results clearly support the superposition approach. The SPL difference was 18dB, as expected. No significant deviation due to "mutual coupling" visible.

The  slight ripple on the SPL curve of the single sub is due to
some noise during the measurements in a large parking lot and the comparably large measurement distance of 10m.  

GJK

[Ivan Beaver]

HarryBrillJr. wrote on Wed, 21 April 2010 22:45

I have yet to be convinced that ON Axis I should be looking for 3 or more dB less output from an End fire array than a 2x2 array of equal distance to the acoustic center of the array.  I do not care about total energy.  I only care that the maximum energy I can get out of that system is going in the direction I want it to go and not spraying all over the wall 15ft behind it.  .

If you wat to REALLY see what is happening-go outside and MEASURE it yourself-and see for yourself.  That is the ONLY way you will be convinced one way or the other.  All the theory and argument by people who have never done it is not gonna do it.

For what it is worth, the measurements I gave were for a 4 box setup.  I did not do 2 boxes, so the results may be different.

I agree, that what you should be concerned with is the output out front-where the people are.

But you need to take the measurements from the FRONT of the array- to see what is really happening.

In a real world concert situation, you will not be able to move the bass array forward into the crowd-so the effective front is in the same place-so the LISTENING distance is from the front of the array-not the acoustic center.

Of course as you move far enough away the physical differences between the two will not matter.  BUT-if you don't measure at least a decent distance away (say a MINIMUM of 10M-that is whay I went about 20M away) then you are likely to get all sorts of data that is not entirely correct-or will translate into the real world experience for the majority of the audience.

And isn't that the whole purpose of measurement/alignment anyway? To translate what we measure into something that can also be perceived by our hearing?  Sure we can measure into all kinds of detail-but do our ears actually hear that?

[Nick Hickman]

Hi Harry,

HarryBrillJr. wrote on Thu, 22 April 2010 03:45

I have yet to be convinced that ON Axis I should be looking for 3 or more dB less output from an End fire array than a 2x2 array of equal distance to the acoustic center of the array.

Why would you expect that?  (I'm probably not paying sufficient attention to the thread!)  For a 2x2 array, I'd expect (in an ideal world) +12dB at low frequency relative to a single box.  For a four-deep end-fire array at any frequency, I'd expect +12dB on-axis.

Using the end-fire example posted by Bob (four boxes with 1m spacing and 2.9ms delay between boxes), simple superposition assuming omnidirectional sources gives this for far-field response:

6dB per division.  Black=single box, green=2x2 array, others four-box end-fire: cyan=30Hz, yellow=40Hz, red=50Hz, magenta=60Hz, blue=70Hz.

Nick

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