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REW Author
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It's done to avoid clipping, since pink noise otherwise has a crest factor of around 13 or 14 dB so it clips easily at higher levels. There's no downside I'm aware of.
 

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Discussion Starter #43
@subterFUSE: Congratulations. What head unit and what amp do you use?
2013 Audi S6

Head Unit = Audi MMI with Optical TOSlink output provided by mObridge DA1 preamp
DSP = Helix DSP Pro

Secondary Source = Audison Bit Play HD (FLAC media player with Optical TOSlink output)

Amps:

1 x Sinfoni Prestigio Class A
2 x Sinfoni Presto
1 x Sinfoni Grave

Speakers:

Eric Stevens Ultra Horn-Loaded Compression Drivers (Full-Size Horn Bodies)
Beyma 8G40 8" midbass
2 x Dynaudio Esotar 1200 subwoofers (Infinite Baffle)



 

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Pretty much. A conventional spectrogram is produced using a fixed window width, which gives it the same time resolution at all frequencies. The frequency resolution is similarly a fixed number of Hz, 10 Hz if a 100 ms window was used, for example. At low frequencies that is a big octave fraction (1/1.4 octaves at 20 Hz), at high frequencies a very, very small octave fraction (1/1386 octaves at 20 kHz). For a time-frequency plot it would be handier if the tradeoff between time and frequency resolution varied with frequency, with higher time resolution at high frequencies and lower at low frequencies. A wavelet transform can achieve that, specifically a constant Q wavelet transform. A constant Q wavelet transform is mathematically equivalent to using a frequency-dependent window to produce the spectrogram (which is what REW does). For reference, this is how a perfect impulse looks with the same settings.

View attachment 96265
nice work, a few Qs for you

1) how does this compare to a s(tockwell) transform? (superficially they seem like quite similar things)
2) what does the dotted line really mean? it is described as the "peak energy curve" but I'm not sure what that actually means?
3) should one normalise or not?
 

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REW Author
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ST and CWT with Morlet wavelet are very similar when looking at amplitude. ST retains absolute phase information, but that's not relevant for the uses these plots are put to.
The dotted line shows where the maximum level occurs at each frequency.
If the main interest is relative timing normalisation is helpful in seeing where the peak occurs vs frequency, but of course it very much distorts the amplitude since it lifts the content at each frequency up to achieve the same peak value.
 

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Discussion Starter #46
OK Guys,

I was giving this yet another try recently, this time focusing on phase aligning my subwoofer to my midbass speakers. See the attached MDAT...

Trace 1 is the Subwoofer by itself, with no time delay.

Trace 2 is the Midbass speakers playing together without the Sub.


I compared those traces in the Overlays Window using the Phase tab. I then adjusted delay on my sub and took additional measurements until I got the phase to align at the crossover range. 80Hz is where the sub and midbass are crossed over.


Trace 3 is the Sub with 1.59ms delay. This was the delay I found that aligns the phase well at the crossover. At 80 Hz, the sub and MB are showing 18 degrees phase, and the phase plot looks very stable through the crossover.

Trace 4 is the Subs w/ 1.59ms delay and the Midbass speakers playing together. I did this to observe the SPL support through the crossover, which to me looks like it was good.



For further confirmation, I applied Filtered IR @ 80Hz for the Sub w/ 1.59ms delay and the MB speakers. The filtered IR peaks line up very well in the Overlays Window for Impulse.

Let me know if this looks like I'm on the right track?


Thanks!
 

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I did invert the subwoofer by 180 degrees, and then delayed it by about 3 ms.................Here are some new measurements after the delays were applied.
subterFUSE, I was reading over these old posts as driver to driver alignment thru crossover is an area of great research and learning for me. I perform car audio system tunings at my work, therefore I have the rather unique perspective of having worked with a very wide range of different drivers, cars, processors, etc. I see that you said you inverted the subwoofer by 180 degrees, but your IR chart clearly shows otherwise. The subwoofer polarity hasn't changed from one measurement to the next. Were you by any chance using the Helix DSP when these measurements were taken? I also experienced the same thing (when using the helix) where as most processors allow a simple polarity inversion of each channel, the helix is different.
 

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Discussion Starter #50 (Edited)
Sorry.... you are correct.

I did not invert polarity. I adjusted the phase control to 180 degrees. The phase control on the Helix DSP Pro is realized by a 2nd Order Allpass filter.


Edit: Also, I think the images you linked are pretty old.... when i had even less of a clue what I was doing than I do now. LOL

I'm busy tuning my car for INAC Finals this weekend, so I'll try to post some new measurements here that might be more informative.
 

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Sorry.... you are correct. I did not invert polarity. I adjusted the phase control to 180 degrees. The phase control on the Helix DSP Pro is realized by a 2nd Order Allpass filter. Edit: Also, I think the images you linked are pretty old.... when i had even less of a clue what I was doing than I do now. LOL I'm busy tuning my car for INAC Finals this weekend, so I'll try to post some new measurements here that might be more informative.
awesome, that should be fun. I'm currently working on a way to determine how the different crossover selection, different timing, etc. affect the transient response of a system. I have come up with a way to measure, in real time, the step response, and filter the signal coming from the mic to just the range of frequencies shared by the drivers that are sharing a crossover, so, in other words, the crossover region. So far, using this method has proved fruitful. I also have begun making recordings of live instruments, (drums so far) using the same measurement mics that I use to tune systems. This way, the mics own freq. response and IR characteristics are nullified, plus, since I was the one who made the recordings, I know what the live event sounded like. So much fun and an incredible learning experience.
 
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