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Measuring Driver Frequency Response
Measuring Driver Frequency Response
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Old 4th July 2018, 11:12 PM   #1
DreadPirate is online now DreadPirate  United States
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Default Measuring Driver Frequency Response

I have started measuring driver parameters with the DATS v2, very easy to use. I also picked up a UMIK-1 microphone with the intention of using REW to generate some frequency response curves for vintage drivers I have laying around. I would like to know what would be a good way to measure the FR, I see this arrangement from another member (in pic), is this good enough? I was planning on hanging the driver off a ladder and using a boom mic, any thoughts on that? Seems like it would be easier, not having to mount the driver. Perhaps a recommendation on a mic stand that would work well with the mic I have? If there is a standard setup people are using for this, I would appreciate a photo or some sketches, I have come across some theory on this, but surprisingly, no photos or videos on actual setups.


My goal here is to come up with replacement drivers for drivers that tend to fail in some of the vintage speakers I have. For example, the Energy Pro 22 tweeter is failure prone and there are some suggestions for replacement here and there, but none that are backed by measurements. The Thiel CS3.5 mid range, the 13m8521 Scanspeak, is an utter disaster of a driver, I have 5 failed units on hand and just discovered one on my main rig has gone south.
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Last edited by DreadPirate; 4th July 2018 at 11:41 PM.
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Old 5th July 2018, 12:15 AM   #2
OscarS is online now OscarS  United States
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That is one way, but it is only representative of what the FR would be when the final installation has a front baffle of that size. Also the bass response would not be indicative of the final result once in a specific alignment. Me personally I use test enclosures of roughly the same size and internal volume as the final actual project.
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Old 5th July 2018, 02:20 AM   #3
Michael Chua is offline Michael Chua  United States
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I'm with Oscar. Use a test box.

The plot below is my Seas ER18RNX mounted onto a 15L box with a baffle width of 8.5". The baffle step is clearly visible.
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Old 5th July 2018, 03:42 AM   #4
TMM is offline TMM  Australia
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As big a baffle as possible is best if the aim is to obtain a 'reference' measurement in order to compare drivers of different sizes. Ideally the baffle should be infinite in size, as should the room.
Another approach would be to use a very large sphere (or perhaps hemisphere with open back) - but this presents more of a construction challenge.

For obtaining a measurement to design a speaker crossover, you'll want to measure it mounted in the completed box to account for baffle edge diffraction effects:
Diffraction from baffle edges
Some people obtain a 'reference' measurement such as that in a drivers datasheet and then use their simulation software to add a simulated edge diffraction - I don't recommend this. If you have the means to accurately measure the driver mounted in the real box, that measurement is best.
If you wanted to investigate edge diffraction effects from changing the baffle dimensions or driver placement on the baffle, you could also measure it on an open baffle the same dimensions as the intended front baffle of a box. The edge diffraction effects will just be twice as severe compared to a closed box.

Finally, edge diffraction effects and room effects can be minimised by placing the microphone closer to the driver. On my 1.2x0.9m baffle shown above, when the mic is placed at around 10cm away from the driver the baffle edge diffraction effects and room effects are minimal. However, there is a limit as to how close you can place the mic to certain size speakers before the high frequency response becomes inaccurate because different parts of the cone can radiate out of phase. For accurate high frequency response I'd recommend spacing the microphone back at least twice the diameter of the cone. When designing a crossover I'd measure no closer than twice the height of the array of drivers.

Last edited by TMM; 5th July 2018 at 03:48 AM.
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Old 5th July 2018, 08:00 AM   #5
wintermute is offline wintermute  Australia
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Measuring Driver Frequency Response
The other thing that you can do if you use a large baffle such as TMM's (or even better an IEC baffle if you have the space) is that you can actually subtract the simulated baffle response from the measurement and then add back in the simulated baffle response of your proposed cabinet. This allows you to do one meter measurements and not have to worry about the baffle diffraction issues (which on an IEC baffle are fairly minimal).

I also suggest that for crossover design you should use measurements in the final box, as TMM suggests, but for initial box design using a baffle difraction simulator can be very helpful in working out what will work well and maybe not so well with a particular driver.

Another thing I have been playing with lately (from manufacturer traced curves) is subtracting the box alignment from the FR curve and then adding in the intended box alignment to see how the response looks with a different alignment. If you also measure the T/S params, and have a known volume box (say 20L) you can model the driver in 20L export the response curve and subtract that from the actual measurement (do a division).. You then model the wanted alignment (say a reflex) and add that curve back to the one you subtracted the sealed alignment from (do a multiplication).

I've not done any real world tests of this yet, but it seems to give valid results with drivers (such as peerless) where the information about the measurement setup is available (something you have when you are doing the measurements yourself).

Both of these things give you a range of options for trying different designs before you even cut any wood (other than your standard test baffle and sealed box).

Tony.
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Old 5th July 2018, 09:16 AM   #6
TBTL is offline TBTL  Germany
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To avoid picking up reflections, it is common to either use time-gating, measure near field or in an anechoic room (a parking lot + ground plane measurement also qualifies).
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Old 5th July 2018, 11:04 AM   #7
DreadPirate is online now DreadPirate  United States
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So if I am looking to find a replacement tweeter for a 2 way, disconnect the woofer and measure the tweeter mounted in the box nearfield? Next step would be looking for a tweeter of similar response by comparing manufacturing FR curves with the measured one? Purchase what might look like a good match and then test it inbox?

I do have the DATS v2 which measures all the T-S parameters and VAS, is that data sufficient to find a reasonable replacement (and then test it in place) or must I also look at the FR curves?

The drivers I am looking to replace are vintage with no published measurements.
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Old 5th July 2018, 12:24 PM   #8
Michael Chua is offline Michael Chua  United States
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Quote:
Originally Posted by DreadPirate View Post
The Thiel CS3.5 mid range, the 13m8521 Scanspeak, is an utter disaster of a driver, I have 5 failed units on hand and just discovered one on my main rig has gone south.
I don't think the problem is with the Scanspeak 13m8521. The Thiel CS3.5 is 1st order network if I'm not mistaken. Playing too loud is usually the cause of damaged drivers.
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Old 5th July 2018, 12:26 PM   #9
Michael Chua is offline Michael Chua  United States
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Quote:
Originally Posted by DreadPirate View Post
So if I am looking to find a replacement tweeter for a 2 way...
Are you planning to use a replacement tweeter with the existing crossover?
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Old 5th July 2018, 02:11 PM   #10
DreadPirate is online now DreadPirate  United States
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Quote:
Originally Posted by Michael Chua View Post
I don't think the problem is with the Scanspeak 13m8521. The Thiel CS3.5 is 1st order network if I'm not mistaken. Playing too loud is usually the cause of damaged drivers.
This is a speaker that must be played loud, sounds too good not to. The idea is to model and take measurements of the speaker with the proper drivers and crossover, and look for a substitute for the midrange, modify the crossover if need be, purchase said substitute driver and do some measurements to confirm the selection and crossover mods. Scanspeak has some similar 4" drivers, but in different ohms. Crossover I have recently rebuilt with new caps and resistors, the electrolytic on the midrange was literally blown to pieces and others have experienced same. Tweeter is the Dynaudio D28AF (the D28/2 was also used without crossover modification) and the woofer seems robust and frequently come up on eBay. Again, weakpoint here is the midrange and the reason why so many of these get parted out, in addition to difficulty of shipping (70 lbs). I figure it would be a good project to get my feet wet in speaker building and do some good.
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