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Discussion Starter #1
Hey,

When i make a measure, my impulse response time are very short. The impulse response ends after approximately 2msec. Its hard to find the first reflex when, if i want to make a gated measurement. :coocoo:

Does anyone have any suggestions?

Regards
 

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Check the Impulse chart legend and see if the first (measurement) box was active and not the 'windowed' box.

Have you reviewed the REW help for the impulse graph?

No Joy?:
If you pad 3 more posts to get to 5, then you can post your .mdat file in this thread. From that I can take a look to see if there is any obvious problems.
 

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Discussion Starter #4
Oh, thx for your reply. I am going nuts here :)

Yes, in have reviwed the tutorial, a couple of times.


"Check the Impulse chart legend and see if the first (measurement) box was active and not the 'windowed' box."
Hmm, exactly which box are you mention?

I really appreciate your help :)
(Sry for my English)

Regards
 

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The time units on that graph you posted are seconds, not milliseconds, the response extends to about 1.7 seconds.
 

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Discussion Starter #8
Thx for your replies!

Yes, i noticed that it was 1,7s.
When iam zooming in i dont think that i see something that looks like the first reflex. The first reflex should be about 5ms.
The measure is a tweeter around 90cm in distance.
Whould be great if you could see if something is wrong.

Thx again!
 

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There are reflections at about 0.55ms, 1ms and 3.8ms. The 3.8ms is the most significant, but they are all quite low level, as you would expect from a measurement that is fairly close to the tweeter. They are easier to see if you select the Envelope trace.
 

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Discussion Starter #10
Thanks for that you take your time to help me.

Do you have any possibility to take a screenshot to show me how you see the reflection. Maybe I have the wrong boxes active or closed.
I have etc and the main window boxed.
 

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I was looking at your measurement and just decided to share my thoughts in case they are helpful.
Per Chart Below:
> Looks like a typical 80cm measurement of a horn.
> With the window as shown the effect of the first 5 ms is shown. Reducing the window gradually helped determine that:
  • IR feature #1 from about 0.5-3 ms is related to the SPL variation at ~3.9 kHz. [You will probably find that a change in mic distance and/or moving off center axis slightly will impact this feature. It is possibly due to horn diffraction?] It is a minor effect of no consequence at the LP in my opinion.
  • IR feature #2 at ~3.8 ms is related to the SPL variation below 3 kHz. [This looks to me that it may be a room reflection; possibly the floor?]
The small negative going initial peak of the IR is common in horns as is the high frequency SPL roll-off so I jumped to that thought initially. Looking now at the phase however it appear the driver is actually wired with negative polarity (or the soundcard inverts the polarity). Maybe this is not a horn? Maybe it is more likely a waveguide loaded dome?

I'm just a hobbyist so my experience is narrowly focused on open domes. I hope you don't mind my guesswork here. It's just fun for me and hopefully part of it is helpful to you.

IR Q.jpg
 

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Discussion Starter #12
@jtalden

I certainly get helped here. I am pretty new with rew. I used SW earlier.
You are absolutely right; the tweeter are mounted in a wg. And the funny thing; yes, the tweeter are mostly connected with negative polarity, from sound card! I am lacy sometimes... :)

To get the most accurate phase with the gated measurment, what "left window (ms)" do you recommed?

Great thanks!
 

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For what purpose? What are intending to do; designing a passive XO in one of the modeling programs? If so, what is your intended XO freq?
 

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Discussion Starter #14
The purpose is XO. Depending how the mid/bas looks, I was hoping to cross around, and hopefully over 3000Hz. Hopefully I can use a low-order, electric xo.
 

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Okay, if I was intending to export this into an XO modeling program and intending to use the TW with positive polarity wiring (not necessary, but it's typically done) I would:
  • Invert the IR in REW (check box in the Impulse Tab 'Controls' window).
  • Set the Window settings as shown below to add 'Frequency Dependent Windows' (FDW) and then select 'Apply Windows'.
  • Shift the IR manually to align the 20 kHz phase tail to 0°. (The shift I used below was about +0.005 ms). After doing that it is necessary to reenter a 'Window Ref Time' of 0 ms even though the box will still show 0.0 ms. The small 0.005 ms shift does not show up in the box, but will impact the phase. Just enter 0 ms there and then 'Apply Windows' again to assure the phase is displayed correctly.
  • Export the measurement as text or whatever file format is required.
Notes:
  • The shift of the IR can probably be done in the XO modeling program if you like. Maybe the IR inversion can be done there as well. I have never used one of those programs, but these are common needs.
  • It is okay to instead of using FDW to smooth the response by setting the right window to about 3 or 4 ms (333 or 250 Hz lower limit) and that will work pretty well. The resolution will be pretty good above 1000Hz. The only issue with that approach is that the higher freqs still have the impact of some room effects causing it to look a little rougher than necessary.
  • The 5 cycle FDW used instead shows the response much smoother as it filters for only the first 5 cycles at all frequencies.
These are all minor points as even the initial settings can be exported and used successfully if the ripples are just ignored. It will just look better with the FDW applied and thus a little easier to interpret in the modeling program.

[Actually, I just checked and REW will not export an IR .wav file with FDW filtering applied. If you need a .wav type file then it is better to use the 3-4 ms right window instead of FDW. The text files export fine with FDW applied.]

IR Q - 2.PNG
 

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Actually, I just checked and REW will not export an IR .wav file with FDW filtering applied.
I've fixed that for the next beta. If a frequency-dependent window is applied its effect is shown on the windowed IR trace, included in a windowed IR export and trace arithmetic will use the FDW impulse.
 
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