Quote:
Originally Posted by
dcrhythm
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I generated a sine sweep .wav at wavtones.com from 20Hz to 500Hz. It plays at an even -6 dBFS for both L and R during the sweep. I loaded it into Virtuoso with HRTF Set A and monitored the Output Meter in Virtuoso. For the "APL listening room" preset, and even when modifying Room Size to much larger ones (up to the max 10m per dimension), and Room Damping for Low frequencies set at 100%, the output level, now coming out of Virtuoso, is no longer even across the sine sweep. Not only does the level rise and fall like there are room mode peaks and nulls (some 5-10 dB differences), but it also varies between L and R channels. What would explain this? It doesn't make sense that head shape alone, per the HRTF data, would have such a significant effect on <200Hz frequencies.
It's natural to observe such level fluctuations when monitoring sine sweep input in real time. It's like doing an FFT for all frequencies at a given sample rate, which would show very erratic peaks and dips, but they could be misleading. Our auditory system doesn't interpret the room response for each frequency individually, but rather in terms of frequency bands. Standard ways of measuring a room response are using a long-term average spectral analysis with a smoothing, or 1/3-octave band SPL analysis with pink noise as per the ITU-R BS.1116-3 recommendation for critical listening room, which are more perceptually relevant.
The real APL listening room complies with the ITU-R standard, and the "virtual" APL room in VIRTUOSO was tuned to also meet the requirement. The range 0 to 100 is perceptually motivated; it's defined to give a user control in the range from the baseline (0) to near flat (100) in the ITU 1/3 octave SPL measurement. So 100 doesn't mean no room mode but its influence is minimised. From our measurements of omni responses at the listening position before applying HRTFs, with the LF damping 100%, the room response is near flat across all bands (max ±1.5dB below 500Hz and ±1dB above). With 0%, the response is not as flat but still within the ITU-R tolerance range down to 50Hz (±3dB), with about 6dB increase below that.
Quote:
Originally Posted by
dcrhythm
➡️
The effects are audible when listening to Virtuoso's output through headphones, with the <200Hz frequency levels rising and falling during the sweep. Seems the Room Damping should take modes out of the equation, and at the max size 10m x 10m x 10m, doesn't seem like there should be any room mode issues above 50Hz, damping or not.
This is also normal. The larger the room becomes, the less the influence of low frequency room modes becomes. Also there is more air absorption of early reflections thus the less influence of early reflections on the room response.