HOLMImpulse: Measurements in practice

[Dr.EM]

I took the attached measurement earlier of this (not complete) speaker:

NEW mid-top for 3 way!

As my first measurement ever. I built recently a mic with WM-61A capsule (I have just realised, I actually forgot to apply the .cal file ) but to me it doesn't seem right.

The peaks in the upper ranges are explainable, I haven't set the levels perfectly yet, it's a small room with walls near the speaker and the mid and tweeter arn't recessed (and no cal). But what I find odd is basically how perfect that bass response is . It's a ported box woofer, higher than 0.7Q, tuned around 37Hz and in a small bedroom; how is it reaching 20Hz flat and with no "boom" at all? I admit, the bass sounds suprisingly good to the ear, but I'm sure it isn't reaching 20Hz. What error could cause this?

Also the phase inversion at 2k is odd, the crossovers are 820Hz and 4.65K.

I think it is feeding back at the moment too, not actually ringning or self oscillating (of course!) but the mic picks up sounds that are reaching the speaker. I think if I pan the mic to one channel only I can avoid this.

[soongsc]

Looks like the mic signal might be inverted as normal. I do beleive you might need to invert the mic input channel in the setting.

[askbojesen]

Originally posted by dlr
The problem I have is that below 1K, I don't get good agreement. It likely has to do with the window as this area changes more with the window selected. I believe that I used a half Blackman-Harris in LAUD. It would be good to have options for type of window (or have I missed that?).
Overlay of LAUD and HOLMImpulse (using Praxis for display only):

Dave

I actually think that the agreement is quite good.

Yes the time-window is essential - especial for low frequencies. I have added your feature request for symmetric classical time windows like Blackman-Harris to my list. I don't use them, because I want further flexibility.

Sorry for my late reply...

[askbojesen]

Originally posted by Dr.EM
I took the attached measurement earlier of this (not complete) speaker:
NEW mid-top for 3 way!
As my first measurement ever. I built recently a mic with WM-61A capsule (I have just realised, I actually forgot to apply the .cal file ) but to me it doesn't seem right.
I think it is feeding back at the moment too, not actually ringning or self oscillating (of course!) but the mic picks up sounds that are reaching the speaker. I think if I pan the mic to one channel only I can avoid this.

Let me see your impulse response with time window also.
Easily save both graphs to one image (file.png) by:

File > 'Save Frequency- & Impulse-Response'

Also try to measure with and without microphone calibration enabled
And keep both measurements in the graph - this way we can see what goes wrong/right with the calibration

[Dr.EM]

Thanks for replying! I've made new measurements but frankly they are even more confusing!

Blue is with cal, red is without. If I check the "invert recorded signal" box, the frequency data appears unchanged but I get 2 phase inversions. I think the way I have it here is "correct" (I don't have an absolute reference, my mixer used for connecting the mic could invert many times for all I know).

This just doesn't seem right still. The perfect flatness below 500Hz, and that large peak and lack of highs, it doesn't correlate to what I'm hearing

[Dr.EM]

Sorry to double post, but I think this may help. I prsent measurements of the drivers isolated, but connected through the crossover network.



It makes a little more sense, driver levels arn't set accurately yet (it's one of several things this software can help me with). The woofer response is a complete mess, which could very well be the box it's in (subwoofer box, no material inside), but isn't it odd how on the overall system response the woofer region shows as perfect

The mid also appears to roll off somewhat early for a 4.65kHz crossover point. The driver naturally should be flat to 10k+

I also need to measure in a much larger room. Should this make a big difference? At the moment walls are unavoidably nearer to the speaker than the mic, a bad situation. Mic is about 1m away, pointing at tweeter centre.

All taken with cal on. If these are accurate, clearly the crossover needs some adjusting to correct those peaks. Again, hopefully if I can rely on the measurements this software will help me to do it

[domtw]

Sorry, I cannot help much.
I think you had this result in the low frequencies because of your window gating. It was quite high (about 1KHz).
If you move your cursor away from the IR peak, you can get more "normal" results.
But you will also include some room effect...
I don't know exactly what is a good compromise...
Your last measurements seems believable.


I also have my questions:
I am trying to time-align the tweeter with the woofer in a 2 way speaker.
The crossover is temporary (1st order high and low pass).
Here is what I think should be the best alignment:
"when both IR peak from the woofer and from the tweeter have a top at the same time"
But I am really not sure about this assumption...
I took the 0 from the Bass as a reference and then moved the tweeter until I had this:



Woofer=blue
Tweeter=red
Total=green


Here is what I don't understand about this measurement:
1- Is my assumption about time aligning drivers right ? (matching IR's peaks)
2- Why is the woofer IR's peak so low compared to the one from the tweeter ?
3- Shouldn't be the phase of both drivers closer if they are correctly time aligned?

I have much more questions but if you can help answer these three and understand the reason, that would be great...
Thank you for your comments!

[soongsc]

I would really recommend this book
http://www.audioxpress.com/bksprods/products/bkaa45.htm
It explains lots of things in simple terms.

[domtw]

Hello,
Yes, I know I should read this book and I will as soon as possible.
I lack of time :-(

Can you just tell me about my assumptions ?
Are they totally wrong ?

[JohnPM]

This has info about alignment - subwoofer with mains, but the same principles can be applied to drive units.
http://www.synaudcon.com/site/enewsl...ooferalign.pdf