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Author Topic: RTA shows big drop at 1.25khz  (Read 21138 times)

George Friedman-Jimenez

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Re: RTA shows big drop at 1.25khz
« Reply #40 on: July 23, 2012, 08:27:25 pm »

Can you describe the geometry of your measurement setup, including distance from mic to speaker, height of speaker and mic from floor? Is your measurement mic off axis either vertically or horizontally? The crossover interference is strongly affected by how far off the vertical axis your measurement mic is located. I agree with testing with one speaker at a time and no EQ in the signal path, to see what the speaker itself is doing.
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Kent Thompson

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Re: RTA shows big drop at 1.25khz
« Reply #41 on: July 24, 2012, 12:57:16 am »

Can you describe the geometry of your measurement setup, including distance from mic to speaker, height of speaker and mic from floor? Is your measurement mic off axis either vertically or horizontally? The crossover interference is strongly affected by how far off the vertical axis your measurement mic is located. I agree with testing with one speaker at a time and no EQ in the signal path, to see what the speaker itself is doing.

The measurement I took was at about 8' the microphone was positioned about mid cabinet (dual 15s and horn) on axis at about the top 15 level below the horn. Unlike the op I can really hear this dip. It would be nice to know what causes this whether I am measuring wrong or something is wrong with the speaker.

I probably should have had the microphone mid way between the top speaker and horn... I am gonna take another measurement in a couple days when its my turn to run the system again.
« Last Edit: July 24, 2012, 01:24:37 am by Kent Thompson »
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Tim McCulloch

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Re: RTA shows big drop at 1.25khz
« Reply #42 on: July 24, 2012, 03:44:08 am »

The measurement I took was at about 8' the microphone was positioned about mid cabinet (dual 15s and horn) on axis at about the top 15 level below the horn. Unlike the op I can really hear this dip. It would be nice to know what causes this whether I am measuring wrong or something is wrong with the speaker.

I probably should have had the microphone mid way between the top speaker and horn... I am gonna take another measurement in a couple days when its my turn to run the system again.


I don't doubt that you can hear this at the point of measurement, but I think there's something up that isn't electronic.

Take a look at the coherence trace and note the big-ass dip it takes at the same frequency as the magnitude trace.  Such a coherence dive is usually the result of cancellations caused by interference and reflections.  In your case it could be an architectural surface or it could be the 15" speakers interacting with each other.  Move the mic around and see what the coherence trace does.

Another interesting thing is a much smaller dip in coherence but a -9dB change in magnitude at 150Hz, a full "decade" below the area we're discussing.  The +6dB peak at 450Hz is also interesting but coherence at that freq is good; it's possible that is really the box response.

Take a few more measurements and post back.
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Mark McFarlane

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Re: RTA shows big drop at 1.25khz
« Reply #43 on: July 25, 2012, 09:32:37 am »

A couple of quick thoughts for the original OP.

You can very crudely test the mic by throwing up a few other mics (SM58, whatever you had handy). If all the mics show the same huge dip your measurement mic isn't total screwed up.  Not a scientific calibration, but I think you will see some evidence of a 10db dip at 1.25 on most any mic.

Regarding testing , someone alluded to the fact that you should only have one speaker running during the test.

I don't know this particular speaker but swapping polarity on the horns may only be a 5 minute job with a power screwdriver.  Faster than carrying your speakers outdoors.
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Tim McCulloch

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Re: RTA shows big drop at 1.25khz
« Reply #44 on: July 25, 2012, 10:11:20 am »

A couple of quick thoughts for the original OP.

You can very crudely test the mic by throwing up a few other mics (SM58, whatever you had handy). If all the mics show the same huge dip your measurement mic isn't total screwed up.  Not a scientific calibration, but I think you will see some evidence of a 10db dip at 1.25 on most any mic.

Regarding testing , someone alluded to the fact that you should only have one speaker running during the test.

I don't know this particular speaker but swapping polarity on the horns may only be a 5 minute job with a power screwdriver.  Faster than carrying your speakers outdoors.

Hi Mark-

Unless I saw some evidence that the cabinet has been opened up, I'd probably pass on swapping the polarity of the HF driver.  Why?  Because I think this box is operating the way the manufacturer intended and that the flaw in is the measurement and measurement tool.

The box is being measured in situ, so if the OP takes it down to do the horn wires, inherently his next measurement will be different unless he gets the speaker back up in the air and positions his test mic in exactly the same place.
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Chris Tsanjoures

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Re: RTA shows big drop at 1.25khz
« Reply #45 on: July 25, 2012, 01:08:15 pm »

I'm just getting into this thread at the end, so I apologize if someone has already addressed this. Because you are measuring the speaker from 8 feet away, and your massive hole is in that hi-mid/high xo point, it may be that you are just not at the same height that the manufacturer was when they factory tuned the box. I would try adjusting the height of the RTA mic and see if that hole gets filled in.
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Kevin McDonough

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Re: RTA shows big drop at 1.25khz
« Reply #46 on: July 26, 2012, 06:05:02 pm »

hey

With respect an no offence intended to any of the posters on this thread, or the OP, this seems to have been being discussed for a long period of time now, but with no one actually doing anything.  This issue could have been sorted, at least to the point of knowing for definite what the problem is, in an hour tops.

Here are the steps.....

1) Measure with another mic. This will rule out mic problems. Then....

2) Take another measurement outside, at a different height from what its at just now. This will rule out reflections and acoustic problems caused by the room. Then.....

3) Try swapping the leads on the HF. This will tell you if the HF was out of phase, even partially, and if it is more in phase once the polarity is reversed.


If these steps haven't gave you a suitable result then....


4) Disconnect the HF connections on the back of the driver and take a measurement of just the LF section, then connect back up the HF and disconnect the LF and take a measurement of just the HF. Post these on here so we can see what each driver is doing on its own and if they're cancelling at the crossover point.


It'll only take two mins with a screwdriver to pull the grill off, unscrew a driver and swap or disconnect the leads, and we can start to get to the bottom of whats going on. At this point we're just going round in circles discussing theory.


k
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Ivan Beaver

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Re: RTA shows big drop at 1.25khz
« Reply #47 on: July 26, 2012, 09:21:19 pm »

hey

With respect an no offence intended to any of the posters on this thread, or the OP, this seems to have been being discussed for a long period of time now, but with no one actually doing anything.  This issue could have been sorted, at least to the point of knowing for definite what the problem is, in an hour tops.

Here are the steps.....

1) Measure with another mic. This will rule out mic problems. Then....

2) Take another measurement outside, at a different height from what its at just now. This will rule out reflections and acoustic problems caused by the room. Then.....

3) Try swapping the leads on the HF. This will tell you if the HF was out of phase, even partially, and if it is more in phase once the polarity is reversed.


If these steps haven't gave you a suitable result then....


4) Disconnect the HF connections on the back of the driver and take a measurement of just the LF section, then connect back up the HF and disconnect the LF and take a measurement of just the HF. Post these on here so we can see what each driver is doing on its own and if they're cancelling at the crossover point.


It'll only take two mins with a screwdriver to pull the grill off, unscrew a driver and swap or disconnect the leads, and we can start to get to the bottom of whats going on. At this point we're just going round in circles discussing theory.


k
Now why would you want to go and ruin a perfectly good speculation with some practical advice???????

He just needs to DO SOMETHING!

Since the only apparent tool is an RTA-then more "data" would help.

Since phase is out of the question-at least multiple different measurement positions might start to show a "trend".
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James A. Griffin

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Re: RTA shows big drop at 1.25khz
« Reply #48 on: July 27, 2012, 08:46:30 pm »

I don't know this particular speaker but swapping polarity on the horns may only be a 5 minute job with a power screwdriver. 

Seems like reversed polarity on the horns would cause cancellation on a broader ranger of upper frequencies than just a 1.25k dip?
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Brian Wombaugh

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Re: RTA shows big drop at 1.25khz
« Reply #49 on: July 28, 2012, 07:14:44 am »

I have been able to rule out the mic as a problem.  I measured a different set ofspeakers with the same RTA and got very close to a flat response with no issue at the 1.25 level.  I also measured each individual speaker 1 at a time at different locations and still get basically the same result.  I really was not wanting to have to open the speaker up.  I think I will just leave things as they are since the sound does not seem to reflect any extreme negative affects from the problem.  Thanks to everyone for your help and ideas.
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ProSoundWeb Community

Re: RTA shows big drop at 1.25khz
« Reply #49 on: July 28, 2012, 07:14:44 am »


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