Rational Acoustics



Jeffsco
August 5th, 2010, 08:53 PM
I have a pair of Yorkville EF500P 2 way Mid/Hi boxes that I use for small to med. size rooms. This is an Active box, 15" Mid driver with 2" horn mounted driver for the Top end. It uses an Internal active crossover, signal processing and limiting. I'm not able to defeat either speaker in order to measure it independantly, nor can I change any settings to do with the Crossover.

Any advice or direction on how to go about Measuring this box checking the Phase between the components and setting up a free Field EQ setting? I've read elsewhere that I can cover up one of the drivers while taking measurements and still obtain a reasonably accurate information. Or should I just not worry about it, seeing as the internal processor has already been set?

lundbergsound
August 6th, 2010, 03:30 AM
Any advice or direction on how to go about Measuring this box checking the Phase between the components and setting up a free Field EQ setting? I've read elsewhere that I can cover up one of the drivers while taking measurements and still obtain a reasonably accurate information. Or should I just not worry about it, seeing as the internal processor has already been set?

Jeff,

Although I am not familiar with this particular loudspeaker, one of the reasons for self-powered loudspeakers is that the manufacturer takes care of the crossover settings; I would be very surprised if the phase-alignment between drivers was off. You can conduct a free-field measurement of it to confirm that there are no anomalies near the crossover frequency (the microphone should be equidistant from the horn and woofer) and to make an EQ setting, although it's possible that its free-field frequency response is already adequate for your needs. Good luck!

Regards,
Daniel

Jeffsco
August 6th, 2010, 09:26 AM
Thanks Daniel. Appreciate that. I'm trying to anticipate what I might see so as to have a heads up that something might be wrong.

How far away should I place the mic from the drivers?

Arthur Skudra
August 6th, 2010, 10:23 AM
Thanks Daniel. Appreciate that. I'm trying to anticipate what I might see so as to have a heads up that something might be wrong.

How far away should I place the mic from the drivers?+1 to what Daniel said! My "rule of thumb" says that you should place the mic so that it is at least 3 or 4 times as far from the box as the maximum dimension the box. So lets say we measure 2 feet diagonally across the face of the box (corner to corner), the mic should be at least 6 to 8 feet away from the box. Watch out for any adjacent surfaces causing a reflection in your measurement (eg. ground bounce).

Jeffsco
August 6th, 2010, 11:26 AM
Arthur: Should I consider an Ground plane measurement then to avoid Ground Bounce? Or should I pick up some Auralex or equivalent and position it in such a way as to absorb or nullify the ground bounce?

Arthur Skudra
August 6th, 2010, 01:00 PM
Arthur: Should I consider an Ground plane measurement then to avoid Ground Bounce? Or should I pick up some Auralex or equivalent and position it in such a way as to absorb or nullify the ground bounce?
The challenge in taking a ground plane measurement of the loudspeaker is that while you can get the microphone very close to the plane, the loudspeaker still has some separation from the boundary, and keep in mind the boundary effects of the loudspeaker's LF response as it gets close to a boundary as well.

Personally I prefer suspending the loudspeaker in free space (large warehouse), and using absorbers to reduce the floor or wall or ceiling bounce. If you're doing it on the cheap, get the tallest tripod stand you can find, put the loudspeaker on it, and enclose the area around the speaker and microphone with heavy velour drapes and put an absorber on the floor, hopefully you have a high enough ceiling where the ceiling bounce does not become a factor. Otherwise do it outside where the only boundary you have to worry about is the ground, which can be easily treated. :)

Or, simply recognize you will get some cancellation in your measurement due to reflections, identify where those cancellations are (move the mic, the varying path length difference will cause the cancellations to shift up or down in frequency), and then choose to ignore them in your measurement. The coherence trace is your friend!

James Woods
August 11th, 2010, 01:06 PM
Try measuring the box on itīs back looking up into the sky. This is half space, and no ground bounce, except maybe the low end where the floor is adding the omni energy of the box to the front signal.

Dr. J
August 13th, 2010, 12:05 PM
Try measuring the box on itīs back looking up into the sky. This is half space, and no ground bounce, except maybe the low end where the floor is adding the omni energy of the box to the front signal.

James -- I have thought of that very same thing. I thought of rigging up the box on a Pallet or something so I wouldn't break the speakon connector and shooting it straight up into the sky.

Would you then still do a ONAX & OFFAX average?

If I ever do it -- I will make sure I take pictures because it would be hilarious to see and most people will think I am crazy (Neighbors...):D

Jim Cousins
August 17th, 2010, 10:45 AM
I put speakers on their backs shooting skywards as well. Often in my back garden if I'm doing development work from my home office.

As James said, you need to allow for the LF phase rotation due to the speaker's baffle being off the ground by the depth of the enclosure but this technique certainly beats any other practical set-up I know - unless you have access to a very large anechoic chamber of course...

A couple of tips about measuring outdoors:

1) A strong breeze will always spring up the minute you are ready to take a measurement!

I have been known to use 1.8m windbreaks (the kind of temporary canvass barriers you see on beaches here in the chilly north) to shield the measurement mic from the worst of the wind. The windbreaks should be tilted away from the speaker to avoid reflecting sound back to the measurement mic. It's not ideal but sometimes you just cannot wait for mother nature to be kind.

2) If you're worried about disturbing the neighbours, try taking your source/ref signal from a wind-shielded location mic. Obviously, you need to place it so that a) it doesn't pick up peoples' conversations, family pets etc and b) it doesn't feed back with the system under test.

I've done this for outdoor concert tweaking where there were strict noise limits during setup. You don't have to get your measurement levels much above the normal ambient levels to get valid measurements and your source level automatically rides the background noise (including wind noise) which is great.

At home I usually do my measurements round the back of the house with my location mic round the side of the house. This gives me more than adequate isolation.

I usually add a low level of pink noise to this signal just to ensure that something is present at all frequencies.

Jeffsco
August 17th, 2010, 08:10 PM
As James said, you need to allow for the LF phase rotation due to the speaker's baffle being off the ground by the depth of the enclosure but this technique certainly beats any other practical set-up I know - unless you have access to a very large anechoic chamber of course...


Jim: What is required in allowing for the LF Phase rotation? How do I compensate for that?

Jim Cousins
August 18th, 2010, 12:42 PM
Jim: What is required in allowing for the LF Phase rotation? How do I compensate for that?

There's no need to compensate for it - simply use the delay locator/delay tracking in the normal way - but be aware that the phase display may show phase lag at low frequencies which is nothing to worry about.

In practice, assuming a reasonably sized PA speaker, this phase lag will be well below the speaker's crossover region so it shouldn't affect your driver polarity or alignment checks through crossover.

As long as the system isn't obviously cancelling through the crossover region, I'd leave crossover setting well alone and wouldn't dink with the manufacturer's settings too much - apart from, perhaps, overall eq before crossover.

Speaker alignment is always an intelligent compromise between on-axis response and polar response. Sometimes you can mess up the polar response if you tweak the on-axis response too much post crossover. It is, therefore, always advisable to use multiple mics placed around the speaker's designated coverage area if you're tempted to tweak crossovers.