TOPIC:

Hi All
Newbie here learning my way through the miniDSP 2x4HD. Appreciate this forum, app notes and tutorials and really enjoying learning to make and measure changes on the fly!

Hopefully my query is justified - I've tried searching for similar issues/results and haven't found anything...maybe i don't know the official term.

I've just finished an up-cycle of some drivers i've had in old cabinets and have built an MTM configuration and ditched the old crossover for the 2x4HD. I've used REW a few times about a year ago and learned enough to get setup and measure response of some bookshelf speakers i was making at the time. With this new build i've fired up REW again and have been following through the Loudspeaker measurement with UMIK-1 and REW and Designing a 2way active speaker with the 2x4HD to get the speakers setup.

• I haven't calibrated the SPL on my UMM6 so the dB isn't correct - don't think that matters but let me know if i'm wrong!
• measurements indoors at 1m with driver and mic about 110cm off the ground. 1/24 smoothing
• drivers are 5.25in and 1in dome with 10cm centre to centre spacing, cabinets are 30cm d, 18cm w, and 95cm h

I did the tweeter measurement, no filter, starting at 1kHZ and got the below response (red plot).

I thought the nulls were a bit strange (2.3 & 8.6khz) so i did a near field measurement, in case they were standing waves as i'm in a small room, but that didn't seem to change anything...
I carried on the process just to work through and not get too bogged down while I'm still working everything out and put on a few different filters. See below green plot 1800hz cut off LR48dB/oct.

Now there are more nulls than before (2, 12, 16khz)? I don't no how to interpret this? I understand that they can be there with no filter on, the cause to be something i'm still working out, but why does this change outside of the filter curve when the filter is put on?

What causes the extra nulls? Am i missing something obvious?

Regarding the original nulls are any of these likely causes?:
1. baffle diffraction
2. sympathetic mid-woofer vibrations
3. user error
4. that just normal behaviour

Thanks in advance
Hoping someone can point me in the right direction.

Jezp

Do you have sweep averaging turned on in REW? (That's the kind of result I got when I tried it, never had the need/time to investigate further).

Hi John, I am doing multiple sweeps which must be averaged to give the one plot. I'll try a single sweep next time i do a run.
Just checked on REW help and it recommends not doing multiple sweeps if the input and output are on different devices eg if the input is a usb mic. So looks like you are spot on.

thank you.

Jezp

Ah! A timing reference must be needed for averaging multiple sweeps to work. Makes sense.

Did a comparison tonight and thought I'd post back to close this out.

1 sweep, 2 sweeps and 4 sweeps attached. Usb mic and minidsp hd usb connected with no timing reference.
Thanks again John, I think i would've been scratching my head for a long time! Now to try out some EQ