# Speaker Driver Replacement/Match with REW



## ksdehoff (Dec 9, 2012)

I've got a pair of B&W DM640 - I really like them except one of the 4 woofers and developed some voice coil problem and causes terrible distortion around the 200-400 hz frequencies.

I've set up a test lab using REW to chart the performance of multiple woofer replacements (The original part is ZZ 8915 and is no longer available).

I'm thinking that if I can match performance in terms of Phase and Frequency response that I would have found a good replacement. But wanted to get some expert opinions.

I've tested my first replacement candidate and it looks good (except for the fact it is 2mm larger around and doesn't quite fit the case).

Take a look at these charts - does anyone see a reason not to use this element as a replacement? I have also compared the phasing and both are exactly alike. The only thing I think I lose is that really low end performance - but I just picked up a pair of PSA XV15 so don't really excercise my mains down to 30 anymore.

Here is the frequency chart, blue is the original, green is the replacement. And the Crossover point is 300hz


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## robbo266317 (Sep 22, 2008)

I do not see any reason not to use it, the match seems very good. 
Did you have to adjust the levels to get the two curves to line up or did you measure one, unplug and measure the other with no changes?

How are you going to mount it in the cabinet if it is larger?


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## JohnM (Apr 11, 2006)

Very flat measurements, somewhat unusual even for near field. Are you using a soundcard cal file? If so, have you looked at in in REW to make sure it is correct?

As an aside, you can get REW to capture the graph images for you, just click the Capture (camera) icon at the top left of the graph.


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## ksdehoff (Dec 9, 2012)

Thanks for pointing out my problem data. I went back and redid the tests and you were right - my sound card had not been set up correctly. My new data does not look nearly as ideal, but it did highlight many problems with several of my candidate woofers. I still ended up with a reasonably close replacement. Gold is the original good DM640 and Teal is the replacement. 

Now what should I do about that huge hole at 72 hz!?


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## robbo266317 (Sep 22, 2008)

ksdehoff said:


> Now what should I do about that huge hole at 72 hz!?


That could be a room node, considering there is another at 140 Hz, if so there isn't much you can do about it.
The easiest way to tell is to do a close mic of the speaker (Within about six inches) and see if it goes away.
Or use one of the online calculators to determine your theoretical room nodes.


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## ksdehoff (Dec 9, 2012)

I upgraded my input from a RS-2055 to the UMIK-1. It has definitely improved overall resolution. I've identified an odd behavior I'm wondering if anyone can explain. It turns out that my new woofer choice skews frequency peaks lower by about 50 hz. Note the chart that compares my good DM640 to my 'repaired' DM640 - Brown is the repair and Red is the good speaker. What is this telling me? What I find completely bizarre is that the 50 hz skew is happening well above the mid's crossover of 300 hz.


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## JohnM (Apr 11, 2006)

Are you measuring with a single speaker active and comparing results measured with the speaker and mic in exactly the same locations for each measurement?


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## EarlK (Jan 1, 2010)

- I really doubt that your speakers suffer from +15db peaks ( such as the following pic shows ) .










- One explanation for the generation of a picture like that is if you have a _*feedback loop*_ occuring.
- This is where you send input back to output & where the "feedback portion" ( of the red & orange traces ) have different delay times ( samples ) . That would account for the offsets seen .

- Typically, the location of the "peaks & nulls" will be pretty static ( with a cabled loopback ) unless you change things like buffer sizes .
- OTOH, if you move a live test mic forward & backward, the "peaks & nulls"( of newly captured info ) will appear to shift against a static reference .

- Here's the trace of a "bad" loopback measurement ( due to delayed feedback being introduced & added to the original signal ) .














- Here are some locations ( within the SoundControl Panels for 2 different OS's ) were feedback can be generated ( first seen is OSX, followed by Win7 ) ;










- The "*THRU*" box must be unticked .










- The "*Listen to this Device*" box must be unticked .

:sn:


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## ksdehoff (Dec 9, 2012)

JohnEarl

Yes I am measuring a single speaker at a time - I take them out of their position and measure them from one meter out, neither the speaker nor the microphone (on a tripod) are moving. But looking at Earl's comments - there may be some small amount of play in how I put the speaker into the listening spot, perhaps +- an inch in either direction. I have set up the umik-1 correctly - it does not have listen to this device checked. The feedback theory seems to make some sense I will look in to, or perhaps I've got some processing occurring I am not aware of on my laptop/amp.


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## EarlK (Jan 1, 2010)

- Is it possible that you still have selected " Use Loopback As Timing Reference" within the "Analysis" ( tab ) of the Preferences window ? 
( JohnM has previously stated that it should be left off when using the UMIK-1 )

- ( & I don't actually know if it being selected would account for what I see in your traces, but it's worth checking out ) .

:sn:


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