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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Audio Measurement and Testing => Topic started by: Brian Wombaugh on July 17, 2012, 05:19:04 pm

Title: RTA shows big drop at 1.25khz
Post by: Brian Wombaugh on July 17, 2012, 05:19:04 pm
I have been RTA'ing 2 Peavey PR12N speakers for a church.  I was able to get a flat response across the 31 bands with the exception of the 1.25khz which is falling out of spec by almost a negative 10db.  The bands on either side( 1khz and the 1.6khz ) all fall in line.  The problem is just with this one band.  I have tried correcting it with the EQ but still cannot get it in line with the rest of the frequencies.  Could this be an issue with the crossover point ?  Any info would be greatly appreciated.  Thanks -  Brian
Title: Re: RTA shows big drop at 1.25khz
Post by: Kevin McDonough on July 17, 2012, 06:13:05 pm
I have been RTA'ing 2 Peavey PR12N speakers for a church.  I was able to get a flat response across the 31 bands with the exception of the 1.25khz which is falling out of spec by almost a negative 10db.  The bands on either side( 1khz and the 1.6khz ) all fall in line.  The problem is just with this one band.  I have tried correcting it with the EQ but still cannot get it in line with the rest of the frequencies.  Could this be an issue with the crossover point ?  Any info would be greatly appreciated.  Thanks -  Brian


hey

Crossover point sounds like a likley candidate. First however I'd recommend taking the speaker to a different location, preferably outside, and set it at a different height from what it was measured before. This'll rule out the cancellation coming from a reflection within the room your in.

However outside of this then yeah i'd say almost certainly a slight phase problem at the crossover is causing the cancellation. How is the speaker powered?  Is it an active speaker, biamped or does it use a passive internal crossover?   If it's biamped then you should be able to correct it by measuring the impulse and phase of both sections and then adding a little delay to the compression driver to line things up and bring them into phase.

However if its a passive crossover, either on its own or as part of an active speaker, then you may not be able to change the crossover settings to do this, in which case your unfortunately stuck.

k

Title: Re: RTA shows big drop at 1.25khz
Post by: Brian Wombaugh on July 17, 2012, 06:24:52 pm
It does have a passive crossover.  The system has a BBE Sound Maximizer which suppose to allow a delay to be put on the low frequencies or the high frequencies.  I wonder if it would help to play around with that ?  Brian
Title: Re: RTA shows big drop at 1.25khz
Post by: Timo Beckman on July 17, 2012, 06:38:47 pm
It does have a passive crossover.  The system has a BBE Sound Maximizer which suppose to allow a delay to be put on the low frequencies or the high frequencies.  I wonder if it would help to play around with that ?  Brian

You might try to reverse polarity on 1 of the drivers first (probably the high's) . If this doesn't help try to apply +/- 0,4 ms on the low driver or on the high (less likely but who knowes) .
Or even better measure the response of the loudspeaker to know for shure what's going on .
Title: Re: RTA shows big drop at 1.25khz
Post by: Randall Hyde on July 17, 2012, 07:04:03 pm
I have been RTA'ing 2 Peavey PR12N speakers for a church.  I was able to get a flat response across the 31 bands with the exception of the 1.25khz which is falling out of spec by almost a negative 10db.  The bands on either side( 1khz and the 1.6khz ) all fall in line.  The problem is just with this one band.  I have tried correcting it with the EQ but still cannot get it in line with the rest of the frequencies.  Could this be an issue with the crossover point ?  Any info would be greatly appreciated.  Thanks -  Brian

Just a thought -- have you tried moving the RTA microphone around? It might be sitting in a NULL zone for that frequency (though I'd be surprised to find that other frequencies are all flat if this is the case).
Cheers,
Randy Hyde
Title: Re: RTA shows big drop at 1.25khz
Post by: Brian Wombaugh on July 17, 2012, 07:08:00 pm
I have tried the microphone in different positions and have still obtained basicially the same result.  Thanks - Brian
Title: Re: RTA shows big drop at 1.25khz
Post by: Ryan C. Davis on July 17, 2012, 07:27:36 pm
You might try to reverse polarity on 1 of the drivers first (probably the high's)...

When I was building speakers I found the same thing, usually a suck out like that was indicative of an HF driver that was out of polarity. someone recommended taking the speaker outside and measuring it, that would be a good idea just to eliminate the possibility of room interaction and interaction with the other speaker. There's always a chance someone screwed up in assembly and QA didn't catch it.
Title: RTA shows big drop at 1.25khz
Post by: Samuel Rees on July 18, 2012, 12:19:41 am
BBE Sonic Maximizer?
Title: Re: RTA shows big drop at 1.25khz
Post by: Timo Beckman on July 18, 2012, 03:14:53 am
I have tried the microphone in different positions and have still obtained basicially the same result.  Thanks - Brian

For measuring the phase response a RTA doesn't do the trick . You need to do phase measurements .
A RTA misses out on time vs frequency it's only amplitude vs frequency .
What kind of measurement device do you use ?
Title: Re: RTA shows big drop at 1.25khz
Post by: Brian Wombaugh on July 18, 2012, 04:52:11 am
I use a Phonic PAA3.  If I remember correctly I think that it will do some sort of test for phase.  I will have to get my manual out and take a look.  Thanks -  Brian
Title: Re: RTA shows big drop at 1.25khz
Post by: Tim McCulloch on July 18, 2012, 09:32:36 am
I use a Phonic PAA3.  If I remember correctly I think that it will do some sort of test for phase.  I will have to get my manual out and take a look.  Thanks -  Brian

You are confusing phase with polarity.
Title: RTA shows big drop at 1.25khz
Post by: Samuel Rees on July 18, 2012, 10:05:01 am
Is anyone else not concerned that there is a BBE Sonic Maximizer in the signal chain while RTA/tuning a speaker? Not to mention that it doesn't "delay" the high or low frequencies?
Title: Re: RTA shows big drop at 1.25khz
Post by: Brian Wombaugh on July 18, 2012, 10:22:02 am
I talked to a tech at BBE and he told me that the unit needs to be on and included in the RTA process to get an accurate reading of what is actually being heard.  I do not know a lot about how this unit processes sound, but I will tell you the sound is much clearer and crisp with the unit in the on position.  I will try bypassing the unit and see if the 1.25 drop is still there or is less severe.  Thanks -  Brian
Title: Re: RTA shows big drop at 1.25khz
Post by: Timo Beckman on July 18, 2012, 12:07:04 pm
Is anyone else not concerned that there is a BBE Sonic Maximizer in the signal chain while RTA/tuning a speaker? Not to mention that it doesn't "delay" the high or low frequencies?

I have worked for the dutch distributor for BBE a really long time ago (+15years ago) . Indeed it would be the first to get rid of but i think it's there to "fresh-up" the sound and should not have anything to do with the allignment of the loudspeakers within the cabinet .
By the way when you do the RTA thing are both sides playing or just 1 side ? And do you have the same response on both sides of your set-up ?
Title: Re: RTA shows big drop at 1.25khz
Post by: Brian Wombaugh on July 18, 2012, 12:28:44 pm
Both sides are playing and the response is about the same in various areas of the sanctuary - give or take a little either way.   Thanks - Brian
Title: Re: RTA shows big drop at 1.25khz
Post by: Kevin McDonough on July 18, 2012, 01:31:01 pm
hey

In terms of the sonic maximiser, if the OP plans to us this in his gigs all the time then its probably best he EQ's his system with this in place, so he gets a flat response with this included.

However adjusting this unfortunately wont help you with fixing your problem. Any changes you make in delay or phase will be before the crossover, and so be applied to both drivers. You need to apply a change to one driver only, AFTER the crossover. Ideally this would inside an active crossover, but as its passive you have far less options.

Someone suggested changing polarity and that's a good idea to try, but again as its a passive crossover the only way to do this is to unscrew the front from the cabinet and swap over the wires on either one of the drivers. I would suggest the compression driver because if you swap the woofer you may then run into problems down the line of its bottom crossover now being out of polarity with subs. But that's another thing you'd probably want to measure and investigate.


k


Title: Re: RTA shows big drop at 1.25khz
Post by: Doug Fowler on July 18, 2012, 02:19:12 pm
I use a Phonic PAA3.  If I remember correctly I think that it will do some sort of test for phase.  I will have to get my manual out and take a look.  Thanks -  Brian

Most likely, the popper will report negative polarity in HF if it is wired correctly.  I have seen those things mis-used SO many times, with the user blindly accepting the result.

If the HF is controlled by a passive third order crossover, the popper should report "incorrect" when it is indeed correct.
Title: Re: RTA shows big drop at 1.25khz
Post by: Timo Beckman on July 18, 2012, 02:20:08 pm
I sugested to reverse polarity (the high's most likely) but if it's a passive x-over within the cabinet he has to take the high driver out of it and reverse the wireing on it .
Again i'm guesing but al depends on what kind of filters are in there .
Again without measurement data there's no way to tell what's happening ....
Title: Re: RTA shows big drop at 1.25khz
Post by: Doug Fowler on July 18, 2012, 02:26:22 pm
I sugested to reverse polarity (the high's most likely) but if it's a passive x-over within the cabinet he has to take the high driver out of it and reverse the wireing on it .
Again i'm guesing but al depends on what kind of filters are in there .
Again without measurement data there's no way to tell what's happening ....

It's an inexpensive passive loudspeaker. I agree, swap polarity in HF.

Title: Re: RTA shows big drop at 1.25khz
Post by: Jay Barracato on July 19, 2012, 06:51:30 am

Again without measurement data there's no way to tell what's happening ....


My initial thought based on a negative result was that the OP is seeing the result of a cancellation due to mic/speaker positioning, especially since he later said he was measuring with both sides of the system on. The fact that the response does not change with eq supports that this is an interference issue. The cancellation he is seeing may be the first where the cancellation is as wide as the band measured on the RTA. The cancellations that are at higher frequencies would be narrower than the resolution of the RTA bands and would be averaged with the constructive interference before being displayed.
Title: Re: RTA shows big drop at 1.25khz
Post by: g'bye, Dick Rees on July 19, 2012, 09:00:39 am
My initial thought based on a negative result was that the OP is seeing the result of a cancellation due to mic/speaker positioning, especially since he later said he was measuring with both sides of the system on. The fact that the response does not change with eq supports that this is an interference issue. The cancellation he is seeing may be the first where the cancellation is as wide as the band measured on the RTA. The cancellations that are at higher frequencies would be narrower than the resolution of the RTA bands and would be averaged with the constructive interference before being displayed.

Or the measurement mic is broken..........
Title: Re: RTA shows big drop at 1.25khz
Post by: Kevin McDonough on July 19, 2012, 09:32:27 am
Or the measurement mic is broken..........

haha could be.

So to sum up: 

using only one speaker at a time,  step 1 move everything; speaker, mic, laptop, etc etc into a different room or preferably outside and measure again, just to rule out the null being caused by a particular reflection or standing wave within the room your currently measuring in.

Step two, try it without the sonic maximiser connected, just to double check that it isn't a problem with that unit somehow.

if this doesn't work then step three is to reverse the polarity of the HF (i.e. unscrew it and swap the wires) and measure again see if this takes the notch away, or at least reduces it).

After this, with a passive crossover these isn't really an easy step four unfortunately, short of modifying the crossover or removing it altogether and running the box two way.

k
Title: Re: RTA shows big drop at 1.25khz
Post by: Tim Weaver on July 19, 2012, 09:35:30 am
I'm going to say this clearly and plainly.

The BBE box does nothing more than apply a "smiley-face" EQ curve to the system that it is inserted into. If you are using a graphic EQ then you can get the same sound by boosting the lows and the highs.

Therefor, if you have the BBE on boosting both lows and highs, THEN use a graphic EQ to bring everything flat again, you have effectively defeated the BBE. Yes it will sound different if you bypass the BBE, because it will have a "reverse smiley-face" now that you've EQ'd it.

I would suggest taking the BBE out and selling it to a punch-drunk DJ on craigslist. They seem to be the only people who can truly appreciate the BBE "process". Then re-EQ your system and don't worry too much about 1 frequency that is not in spec. You bought bottom-of-the-line gear. Be ecstatic that you got something that is 90% correct. Use what you've learned here and convince the church that 2 years from now (when you are replacing this system again) it would be best to hire a professional instead of someones cousin at the music store.
Title: Re: RTA shows big drop at 1.25khz
Post by: Brian Wombaugh on July 19, 2012, 11:32:17 am
I bypassed the BBE and ran another RTA.  The difference was not that great.  The main thing that I saw was a slight increase in the frequencies from about 1khz and up.  It definately was not a reverse smiley face.  This leads me to believe that the unit must be doing something besides being an EQ.  The 1.25khz problem was still there.  As far as bottom of the line gear is concerned - the church has invested in decent microphones (EV767). I will say that even though this is an inexpensive system, after working on speaker placement and equalization this system sounds about 90% better than a lot of sanctuaries that I have been in. I have seen it again and again where a lot of churches invested thousands and thousands of dollars on everything high end and they have a sound system that still sounds like crap because someone didn't tune it or the operator doesn't have a clue how to properly use the board and mix sound.  I am a firm believer that system optimization along with sound operater training is as just important or even more so than the equipment itself. That does not however mean that you can take a "sows ear" and make a "silk purse" out of it.  There are several very moderatly priced microphone and speaker combinations that I have found that can be made to sound very nice.  This is a good balance for some of the smaller churches  which just don't have the finacial means for all the "high end" stuff.   Thanks - Brian   
Title: Re: RTA shows big drop at 1.25khz
Post by: David Sturzenbecher on July 19, 2012, 01:16:10 pm
I am a firm believer that system optimization along with sound operater training is as just important or even more so than the equipment itself.

Do you feel as if you are optimizing this system? 
Title: Re: RTA shows big drop at 1.25khz
Post by: Jay Barracato on July 19, 2012, 02:27:55 pm
I bypassed the BBE and ran another RTA.  The difference was not that great.  The main thing that I saw was a slight increase in the frequencies from about 1khz and up.  It definately was not a reverse smiley face.  This leads me to believe that the unit must be doing something besides being an EQ.  The 1.25khz problem was still there.  As far as bottom of the line gear is concerned - the church has invested in decent microphones (EV767). I will say that even though this is an inexpensive system, after working on speaker placement and equalization this system sounds about 90% better than a lot of sanctuaries that I have been in. I have seen it again and again where a lot of churches invested thousands and thousands of dollars on everything high end and they have a sound system that still sounds like crap because someone didn't tune it or the operator doesn't have a clue how to properly use the board and mix sound.  I am a firm believer that system optimization along with sound operater training is as just important or even more so than the equipment itself. That does not however mean that you can take a "sows ear" and make a "silk purse" out of it.  There are several very moderatly priced microphone and speaker combinations that I have found that can be made to sound very nice.  This is a good balance for some of the smaller churches  which just don't have the finacial means for all the "high end" stuff.   Thanks - Brian

Brian,

Where are you located?

If the boxes are in factory condition and wired correctly, I find it unlikely you are going to find any benefit digging into the boxes themselves.

I would:
1. Use the RTA but confirm with your ears. Get the system sounding as good as you can without worrying about what the RTA says.

2. Get someone else with experience to come in and do the same thing.

3. Compare the results to see how you boxes are performing in your space.
Title: Re: RTA shows big drop at 1.25khz
Post by: Brian Wombaugh on July 19, 2012, 02:32:17 pm
I feel I am  optimizing the current equipment they have.  I have worked on speaker placement to minimize sound bouncing off walls and the ceiling along with flattening out most of the room resonances to get rid of any feedback problems.  The system now sounds much better and has additional gain before feedback that it did not have before.  I have also made some slight eq adjustments on the vocal mics to increase clarity and remove any"mud" from the mics.  The system still sounds good as is.  I am just trying to tweak it even better by trying to figure out whats going on with this one problem frequency.  Thanks -  Brian
Title: Re: RTA shows big drop at 1.25khz
Post by: Ivan Beaver on July 19, 2012, 06:38:13 pm
Or the measurement mic is broken..........
HIGHLY possible.

I have seen a number of measurement mics have notches in the response.

Measurement mics are generally not built to be rugged-but rather for accuracy.

As with any measurement-you always have to check your MEASUREMENT system-especially if you see something that looks weird.

I can't tell the number of times that I have started to jump to conclusions-only to find out it was either human error-or an issue with the test gear.

TEST THE TESTER.
Title: Re: RTA shows big drop at 1.25khz
Post by: Arthur Skudra on July 19, 2012, 07:54:51 pm
The system still sounds good as is.  I am just trying to tweak it even better by trying to figure out whats going on with this one problem frequency.  Thanks -  Brian
There comes a point where you leave "well enough" alone, and in this case, apart from getting a higher end measurement system and being able to see things at higher resolution with the added time domain, it's hard to speculate what this cancellation is caused by.  Whether it's a crossover problem, loudspeaker problem, reflection problem, measurement mic problem, measurement system problem...you get the idea.  If this dip in the response is only 1/3 octave wide and only a few dB down, then I'd say that the human hearing mechanism is quite forgiving of problems like this.  Don't get hung up on trying to get the response curve on a RTA perfectly flat.  Often what you find sounds the best follows a shaped curve in system response.  Using dual channel FFT like Smaart or SysTune will allow you to get to the bottom of this issue.  There's only so much you can do with a RTA.
Title: Re: RTA shows big drop at 1.25khz
Post by: Brian Wombaugh on July 19, 2012, 10:17:36 pm
Thanks to everyone for all of the good advice on this issue.  I truely appreciate it !     Brian
Title: Re: RTA shows big drop at 1.25khz
Post by: David Sturzenbecher on July 19, 2012, 11:40:49 pm
If the problem is indeed the microphone, you should easily be able play a sweep or a test tone, and not be able to hear this audible 10dB drop as you pass from frequencies below, at, and above your problem frequency. Especially since you say mic position doesn't matter, your ear position shouldn't matter either.
Title: RTA shows big drop at 1.25khz
Post by: Samuel Rees on July 20, 2012, 12:55:27 am
Smiley face was of course hyperbole bur not to far off. From what it sounds like there might be an exciter in there as well to give that sparkle sound. Whatever - the nature of the device is contradictory to the concept of EQing a speaker flat. You won't find anyone here with this on the master outs of there rig.
Title: Re: RTA shows big drop at 1.25khz
Post by: Brian Wombaugh on July 20, 2012, 09:42:58 am
Thanks David.  That is a good idea.  I actually have another decibal meter that I could use along with a test tone to determine if it shows the same type of db drop.  This would help to confirm whether or not it is indeed a faulty measurement mic.
Title: RTA shows big drop at 1.25khz
Post by: Rob Spence on July 20, 2012, 11:49:35 am
Is this "problem" something you are hearing, or just seeing on an RTA?



Sent from my iPad using Tapatalk HD
Title: Re: RTA shows big drop at 1.25khz
Post by: Brian Wombaugh on July 20, 2012, 12:12:51 pm
No - Not really hearing anything that stands out.
Title: RTA shows big drop at 1.25khz
Post by: Rob Spence on July 20, 2012, 03:29:56 pm
It hasn't really been talked about as much as I expected but an RTA has very limited usefulness for measurement. Especially indoors. I would just move on. The RTA is not the right tool for fine tuning in the room to get the last little bit of improvement.

my $0.02

Sent from my iPad using Tapatalk HD
Title: Re: RTA shows big drop at 1.25khz
Post by: Kevin McDonough on July 20, 2012, 03:43:31 pm
It hasn't really been talked about as much as I expected but an RTA has very limited usefulness for measurement. Especially indoors. I would just move on. The RTA is not the right tool for fine tuning in the room to get the last little bit of improvement.

my $0.02

Sent from my iPad using Tapatalk HD


No your quite right, an RTA doesn't give any time or phase info, and thats whats really needed here, and would allow us to find out if the problem is because of the crossover.

However the problem is that with it being a passive crossover, even if the user had something more comprehensive to measure with, it would be slightly academic and it wouldn't matter much as he cant do a whole lot with that information. His only choice is to swap polarity on the HF and see if it makes it better, which has been suggested. If it does great, if not then switch it back to the way it was before and unfortunately it'll just have to be lived with.

But yes your quite right, a measurement with a more sophisticated tool would give us more information to work with.

k
Title: Re: RTA shows big drop at 1.25khz
Post by: Tim McCulloch on July 23, 2012, 11:10:46 am
Is this "problem" something you are hearing, or just seeing on an RTA?



Sent from my iPad using Tapatalk HD

+1000

I think we've been assisting the OP in beating a deceased equine.
Title: Re: RTA shows big drop at 1.25khz
Post by: g'bye, Dick Rees on July 23, 2012, 11:21:00 am
+1000

I think we've been assisting the OP in beating a deceased equine.

But we all know that for many people, it's how it looks, not how it sounds..........

Title: Re: RTA shows big drop at 1.25khz
Post by: Kent Thompson on July 23, 2012, 04:00:09 pm
I am seeing the same thing in a different pair of speakers from the same manufacturer. I saved the traces and can post them if interested.(don't really want to hijack the thread)
Title: Re: RTA shows big drop at 1.25khz
Post by: George Friedman-Jimenez 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.
Title: Re: RTA shows big drop at 1.25khz
Post by: Kent Thompson 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.
(http://i307.photobucket.com/albums/nn307/kentlowt/Trace1.jpg)
Title: Re: RTA shows big drop at 1.25khz
Post by: Tim McCulloch 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.
(http://i307.photobucket.com/albums/nn307/kentlowt/Trace1.jpg)

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.
Title: Re: RTA shows big drop at 1.25khz
Post by: Mark McFarlane 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.
Title: Re: RTA shows big drop at 1.25khz
Post by: Tim McCulloch 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.
Title: Re: RTA shows big drop at 1.25khz
Post by: Chris Tsanjoures 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.
Title: Re: RTA shows big drop at 1.25khz
Post by: Kevin McDonough 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
Title: Re: RTA shows big drop at 1.25khz
Post by: Ivan Beaver 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".
Title: Re: RTA shows big drop at 1.25khz
Post by: James A. Griffin 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?
Title: Re: RTA shows big drop at 1.25khz
Post by: Brian Wombaugh 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.
Title: Re: RTA shows big drop at 1.25khz
Post by: Ivan Beaver on July 28, 2012, 07:54:08 am
Seems like reversed polarity on the horns would cause cancellation on a broader ranger of upper frequencies than just a 1.25k dip?
That is another one of those "it depends" on the particular situation/product.

I have seen situations that reversing the polarity will cause a pretty wide cancellation notch and other times a very narrow deep notch, and other times reversing the polarity only produces a dB or two "wiggle" (that probably would not even show up on an RTA).

HOWEVER the PHASE takes a 180 shift-and that can really screw with the other cabinets in the system. 

Of course an RTA will not show that either (the phase anyway)-but the interaction it would show.

I had a situation a number of years ago that had 2 cabinets in a center cluster (passive cabinets).  There was some weird interaction going on.  So i flipped the polarity on one cabinet and the area in the middle got better-but all the bass was gone.

So I pulled out the measurement rig and noticed that one of the cabinets had a 180 phase shift right around mid/high crossover.  AH HA-THAT is the problem

So we dropped the cluster and found that one of the HF drivers was wired backwards.  PUt it back right-reflew the cabinets and all was well.  Nice and smooth down the middle.

So just because it makes sound and shows "flat" in the amplitude response-DOES NOT mean that it is correct.
Title: Re: RTA shows big drop at 1.25khz
Post by: Kevin McDonough on July 28, 2012, 11:42:41 am
Seems like reversed polarity on the horns would cause cancellation on a broader ranger of upper frequencies than just a 1.25k dip?

Well not really, as it would have nothing to cancel with.

Within the single cabinet, each driver is playing their own set of frequencies and the only place where cancellations can happen is at the crossover point, where for a brief set of frequencies they're both playing. The cancellations happen from the small set of frequencies from the LF driver combining with the same frequencies from the HF. Once you get out of that small band then its back to just a single driver playing. To me it sounds like the two drivers are out of phase, at least partially, at this point causing the cancellation.

If this were a biamped cab with an active crossover it would be a two second job to apply a little delay to the HF and bring it into line with the mids.

However as its passive, the only option it so try and swap the polarity on the HF.  Depending on how far out of phase it was to begin with it may bring things more into line, or it may make them worse. We cant measure ahead of time so the only way to know is to try and see. However it shouldnt make any difference at any other frequency as, when a single cab is playing, there will be nothing else playing the upper frequencies to cancel with.


Quote from: Ivan Beaver

HOWEVER the PHASE takes a 180 shift-and that can really screw with the other cabinets in the system. 


Very true, but I had assumed that when the swapping of the HF polarity was tested in a single cab, if it did prove to be an improvement to the speaker then the other cab(s) would have the same done to match.

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.

The only thing I would suggest, short of opening up the cab, is lifting it outside and measuring there. Even though you've tried measuring the response by moving the mic to different locations, there is still the possibility that you could be measuring a node or acoustic anamily from the position of the speaker within the room. Its unlikely at this point, but the only way to rule it out for sure would be to measure outside (or in an anechoic chamber) where there are no reflections.

While I can understand your reluctance to open the speaker up if you've never done it before, it really is a simple job. a few screws will hold the grill into place, and then a few more will hold the horn in position. Once you've undone these the whole horn and compression driver assembly will slide out and you'll be able to get to the leads and try swapping them. with a power screwdriver or drill-driver, 60 seconds tops  :)  However if your happy just to leave things as they are then all the best  :)

k
Title: Re: RTA shows big drop at 1.25khz
Post by: Ivan Beaver on July 28, 2012, 11:56:28 am
Well not really, as it would have nothing to cancel with.

Within the single cabinet, each driver is playing their own set of frequencies and the only place where cancellations can happen is at the crossover point, where for a brief set of frequencies they're both playing.
Not always true.

In some designs there is a good bit of overlapping of freq going on-sometimes as wide as an octave.  that is why is it important to look "outside" of the intended bandwidth.

One of the "misunderstood" things in audio is that the crossover "stops" the freq reproduction at a particular freq.   The "crossover freq" can mean different things-depending on how you look at it.  In many cases the electrical and acoustical crossovers are NOT the same freq.

And in any case-it is a slope (that has varying amounts of reduction)-based on both the electrical response and the acoustical response of the drivers in question.

As usual-when you start to dig a little deeper to get to the REAL answer-it often gets a bit more complicated.  And that is why so often the "simple answer" is simply wrong.
Title: Re: RTA shows big drop at 1.25khz
Post by: Steve Roth on July 28, 2012, 12:18:31 pm
I have tried the microphone in different positions and have still obtained basicially the same result.  Thanks - Brian

by different positions, if you mean different positions out in the far field, you are perhaps missing something. measure real close like 1' in front of the speaker, then compare that to 'out in the room', to get a feel for whether you are measuring the speaker or the speaker/room/nearby reflecting surfaces combination - this may help you to narrow down what you are seeing
Title: Re: RTA shows big drop at 1.25khz
Post by: Kevin McDonough on July 28, 2012, 12:34:22 pm
Not always true.

In some designs there is a good bit of overlapping of freq going on-sometimes as wide as an octave.  that is why is it important to look "outside" of the intended bandwidth.

One of the "misunderstood" things in audio is that the crossover "stops" the freq reproduction at a particular freq.   The "crossover freq" can mean different things-depending on how you look at it.  In many cases the electrical and acoustical crossovers are NOT the same freq.

And in any case-it is a slope (that has varying amounts of reduction)-based on both the electrical response and the acoustical response of the drivers in question.

As usual-when you start to dig a little deeper to get to the REAL answer-it often gets a bit more complicated.  And that is why so often the "simple answer" is simply wrong.


No, your right very true.  when I say a "small band" of frequencies in my post I should have been more clear in that I mean this as small relative to the overall operating frequency of the whole speaker. But your right the actual width of this band will vary a lot depending on the types of crossover slopes/settings/electronics used etc  :)
Title: Re: RTA shows big drop at 1.25khz
Post by: Tim McCulloch on July 28, 2012, 03:25:26 pm

No, your right very true.  when I say a "small band" of frequencies in my post I should have been more clear in that I mean this as small relative to the overall operating frequency of the whole speaker. But your right the actual width of this band will vary a lot depending on the types of crossover slopes/settings/electronics used etc  :)

The *acoustic* crossover may well extend over an octave on either side of "center."  I do out-of-band correction for at least 2 octaves above/below crossover.
Title: Re: RTA shows big drop at 1.25khz
Post by: Doug Fowler on July 28, 2012, 04:14:06 pm
Seems like reversed polarity on the horns would cause cancellation on a broader ranger of upper frequencies than just a 1.25k dip?

No, only where the interaction between pass bands is greatest, meaning at crossover and when levels are (mostly) equal.   Reduce the level on either and the null is also reduced, assuming it is indeed crossed over @ 1.25 KHz.