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.
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?
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.
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?
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.
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.
When aligning subwoofer phase/delay against mains, a loopback timing reference is crucial.
UMIK mic's latency is different than the interface's DAD latency.
How does this get reconciled or is it not possible with a USB mic?
Hi all. I was introduced to REW about a week ago. So far, I think it is within my capability to carry out measurements and generate correction filters. But there is one question which is hindering my understanding of the filter generating and export process. I see a lot of references to 'Stereo...
Running on REWv5 OSx. I want to get the frequency response of a piece of outboard hardware. The hardware has inherent latency. The response of the java sound engine on Room EQ seems to be pretty latent. I'm using an external sound card that has its own inherent latency.
When it tries to...
I just received my minidsp umik-1 and would like to know if my daughters Chromebook (c100p) will have the appropriate hardware to run REW correctly. It has two USB ports and what appears to be a micro HDMI port including your standard headphone jack. This is a pretty basic laptop and...
Je suis nouveau, j'essaie d'établir une courbe maison pour mon caisson de basses
J'obtiens 3,5 dB entre mes 30 Hz et 80 Hz
Cela vous semble-t-il juste ou ai-je raté quelque chose?
Merci pour votre aide et vos conseils.