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This isn't really the case. Longer impulse responses capture better resolution in the low-frequencies than shorter ones.Because full room reflections would require a MUCH longer IR. You can keep the IR short and have the cab, or you can go all the way to 1+ second IR to capture cab and room. Anything in the middle is generally going to be a mess. It's better to just get the cab in the cab IR and then add the room in a separate longer room reverb IR if you wan't that.
The purpose of using longer IR's is not really to do with capturing room reflections, although it can help with that. The purpose is to give finer bins when it comes to the convolution process.
In convolution, the IR length directly determines the FFT size, which determines how finely the spectrum is represented. A longer IR gives narrower frequency bins, which means:
- Tighter and more accurate low-frequency behaviour
- Less smearing and phase distortion near the bottom end
- More faithful representation of speaker resonance, cab tuning, and mic proximity effects.
The realistic sweet spot would be somewhere between 150-250ms. I think Fractal's "UltraRes" works out to be around 170ms.
So again, just to emphasize the point - IR length is related to frequency resolution, not ambience.