Tweakers' Asylum

RE: Looks Very Interesting

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In Reply to: RE: Looks Very Interesting posted by Barry on September 22, 2023 at 09:26:35

The Vertical Spectrogram is basically a topological chart that you can adjust for depth contour detail.

It is very helpful because it shows the frequencies that involved in the reflections where the bottom chart does not. Both are useful, but for different details.

The bottom chart shows you the exact dB level of each peak, but not what frequencies are causing it. This one is only showing the 4ms peak, not the 9ms peak which is off the right side of the chart. I started testing with 1 poly, so could only test 1 reflection at a time, not all 4 reflections.

The Vertical Spectrogram also details time alignment of your multiple drivers and subs (black dashed line). People who align their subs by other means would be surprised to view this plot after thinking they dialed in their subs. I think it is a well worth getting familiar with.

I would have assumed the same as well, but [1-7]kHz includes the most sensitive range in human hearing so it is a critical region. Ideally, your room reflections should be down 20dB within 20ms (4 and 9ms are well within that range). The 2nd plot will easily show this range (amplitude and time).

The closer the reflections are to the direct sound, the more they smear the clarity. In my scenario, it also destabilized the soundstage by mixing the channels.

I blocked various sides of the microphone (with a tripod and heavy folded quilt), remeasured and compared to identify where the reflections were coming from using the 4 and 9 ms as hints at which reflections to try identifying first.

After the reflection sources were identified as cross and cross-side, I realized as I twisted my head, the leading ear would cross the tangential of one channel's reflection and then the other channel. On tangent would dictate the soundstage. Switching between the left and right channel with the leading ear explained the sound stage shift. The tangential effect could further be exaggerated by cupping the leading ear making it more directional. Cupping the ear to make it more directional is also a good diagnostic tool.

According to some studio designers and anechoic measuring engineers, polys have a natural sound and specify QRDs for the rear because of the phase scattering. QRDs can also make time aligning speakers harder because the measurements show altered phase from the QRDs.

John/REW has a beta thread with weekly releases of the newest REW development. I recommend grabbing the latest development version which has a lot of nice new features and bug fixes.

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2022/03/30 Historical Records CENSORED

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