crossover design... suspect data??

[clem_o]

Looks like the damping of the polymer disappeared huh... :-)

Regarding the amplitude rolloff - I am wondering about the phase response in the crossover region: it may look like its 4th order due to the aggregate rolloffs of the xover network and the driver natural response, but what about the phase relationship? You may get a dippy-lumpy vertical polar response in the xover frequency region...

Cheers!

[wintermute]

hmmmm now you are getting technical The drivers are time aligned so I'm hoping not to have any phase issues.... but SW should do a fairly good job of simulating it, and if there are problems they will hopefully show up in the simulation.

I noticed another thread a while back about using an Audigy II ZS for corossover prototyping, I'm going to have to dig that up, as it will be a lot easier (and cheaper) than making up multiple crossovers and seeing how they sound... will only be able to test a single speaker, but that will be ok

just setting up for a nearfield test of a midbass sitting on the floor

Tony.

[wintermute]

I know you aren't supposed to use nearfield measurements for freqencies above 200-300Hz, but I think that these results show that the issue is with the drivers themselves

though at least the dip is a very sharp one, so maybe won't be too bad..... I tested all four mw144's sitting on the floor, no box, nearfield, the attached graph shows all four.

the dip is at 2000Hz as opposed to 2300, but that is probably due to the fact that the cone doesn't have the extra loading that the sealed box gives....

Tony.

[clem_o]

Hi Tony,

Does the simulation include off-axis modeling? The on-axis response will probably look good, but the off-axis response at the crossover region could look pretty lumpy (this is what I was referring to in my previous post)...

About the dip in response - that wouldn't be the floor reflection cancelling out the response of the driver? Just curious - are you taking your measurements with the drivers in free-space (i.e. suspended)?

oops - you mentioned you have it on the floor....


Cheers!

[wintermute]

I guess it is possible, maybe I should rig it in free air and test again though the length of a wavelength at 2000Hz is about 175mm which is much greater than the distance between the back of the driver and the floor....

not sure if it includes off axis modelling, I have 30 and 60 degree off axis measurements so It possibly will let me use them.... I can always look at some other simulation packages too.... and I guess I can always use the listen and tweak approach

These drivers are 5" btw.

Tony.

[clem_o]

Could really be the drivers. I remember I was working on Vifa units of about the same size, arranged in the same MTM config. Didn't have that funny dip (I use Laud to do my frequency response tests), and wound up crossing at about 2+KHz, into a Scanspeak tweeter, although the arrangement used an electronic 3rd order crossover.

Cheers!

[wintermute]

There is one think that is bugging me.... I did a lot of tests here at home and kept getting awfull looking results, especially on the tweeter.... I did one test and got an almost perfect looking curve on the tweeter, no more than +- 1db from 2000 to 15000 Hz, then SW crashed, and I hadn't saved..... I have never been able to repeat it.... that one measurement that looked as it should has always made me doubt my other measurements.....

I emailed the distributor and he reckons it is measurement errors.....
He did say that there was a very small posibility that electrical loading of the driver could be causing it, I guess my amp comes into question again on that one

One thing I thought of this morning, is that maybe I should do a test with the bottom gate moved up to the start of the pulse.... I figured that it is possible that because there are multiple samples, that the 5ms or so before the pulse starts is recording reflections from the previous sample, and stuffing up the curve...

something for me to look at next week I think



Tony.

[clem_o]

Hi Tony - If your software can, perhaps you should be looking at the time-domain pulse response. Or, perhaps the simplest test will be to get a 'really good' driver and have a look, sort of a sanity check for the testing instrument... Somehow I would really doubt that 'electrical loading of the driver' could cause such problems. A crossover network will introduce a lot more impedance funnies than any solid-state amp would...

Cheers!

[wintermute]

Hi Clem

I just went looking for some pulse response plots, and came across this old measurement I did. Seems that I might well have an issue with my measurements (probably with the environment). This one to me looks a lot better than the recent ones.

I think that I have not been able to sucessfully get rid of floor reflections, and I'm limited to what height I can place the speaker, so it is possible I'm getting some nasty reflections that I can't eliminate from my measurements. I'd say that the peak at 5HKhz is real, and I'll probably need a notch filter on that, and I'd also say that the hump at 1Khz is real.... this curve actually looks very like the one from the baffle diffraction simulator...

This one is on the old prototype which has a different baflfle, but the later tests on this box were pretty much the same as the latest final box.

Tony.