MTM Measurement

ani_101

Question: How do i measure and enter the measurements in PCD? Do i measure each woofer separately and enter them as W1 and W2 in PCD or do i measure then in together in parallel and enter them as a single woofer? what works, what is best and what is easiest but acceptable?

MTMW three way? MTMWW?? WWMTMWW???

isaeagle4031

I take one measurement of the combined pair. For the x offset, set it at 0 as the acoustic center is the midpoint between them. 

ani_101

Thanks, that simplifies things....!

jhollander

I do what Chuck does for the vertical MTMs. 

For horizontal, If I want to look off axis with PCD, I still measure both woofers in parallel, then take out -3.02 db from the SPL in the Blender. 

I generate the ZMA file from Response Modeler with 1/2 the volume and TS parameters.

Kind of fun to see what a BW3 really does off axis.

ani_101

jhollander said:

I do what Chuck does for the vertical MTMs. 

For horizontal, If I want to look off axis with PCD, I still measure both woofers in parallel, then take out -3.02 db from the SPL in the Blender. 

I generate the ZMA file from Response Modeler with 1/2 the volume and TS parameters.

Kind of fun to see what a BW3 really does off axis.

Can you please explain this a bit more in detail or point me to some whitepaper or resource? Why -3.02 db?

PWRRYD

I have done it both ways and they yield the same results (vertical MTM).  So now I just measure both woofers together, on the tweeter axis, and set the woofer Y axis to 0 and treat the whole thing as a simple 2 way TM.

jhollander

Ani, sorry there's no white paper but maybe we can create one here (I'll also correct my error below).  It is a problem solving situation.

To use PCD for looking off axis we need individual driver frequency response and impedance files.  Unfortunately we built the MTM with both woofers in the same box.

So the problem is to find the individual driver responses.  

We can assume that the woofer pair will have exactly the same on baffle frequency response as the single woofer except at a higher SPL level.  The gain is 6.01 db over a single woofer as there is an increase for the efficiency from two woofers (3.01) db and a sensitivity gain (3.01).  The sensitivity gain is from the change from 8 ohms to 4 ohms.  Use the attached excel file (from Hong) to help you through the SPL change.

We can use JBs Blender program to take out 6.02 db from the FRD file.

Next is to create the individual impedance files.  Run a Dats sweep of the MTM to verify your box tuning. Use JBs Response Modeler, go to the Impedance Model section, use the single woofer TS parameters, 1/2 the box volume, and the measured FB to create a ZMA file.

You can short cut this and just measure the impedance of the single woofer. Just remember to short out the other driver so you get a response that looks like one driver in a bigger box.

Using the driver diameters and the derived individual in-box FRD and ZMA files you can use PCD to model off axis response.

The next question would be if there is value in doing this for a center channel speaker 

jhollander

Huh, can't attach excel files? trying once more here...

I guess not, the file name is: Impedance-SPL Calc.xls

jr@mac

JP is looking into the attachment issue. 

ani_101

Thanks John, i'll search for the Excel sheet....

ani_101

another question...

so, i am going with taking a combined measurement of the two woofers. For the ZMA, in box  with the both the woofers?

jhollander

Yes, combined woofer FRDs and ZMAs.  In pcd and Xsim it is just one woofer