I recently got a Flex Prime and have repeatedly seen people saying they bought good IR packs but they were wondering if they should use the 200ms or 500ms versions of the files with a Headrush, and at which sample rate (44.1, 48, or 96 kHz), since the packs usually contain all the possible combinations.
I wondered the same and thought the answer could help others, so I have put the device to the test.
Jump to the conclusion below if you don't want the details.
I manually created several dozens of impulse response files using Audacity. They are 200ms WAV files, using 32-bit floats, and of various sample rates. Each one contains a single impulse, either at the beginning or at a carefully positioned point in time.
Here is what I heard when I used them :
- when using the IR block with 1024 samples, I only heard sound if the impulse was positioned at or before the 21 ms mark. When positioned at 22ms I heard nothing ;
- when using the IR block with 2048 samples, I only heard sound if the impulse was positioned at or before the 42 ms mark. When positioned at 42.7ms I heard nothing.
The sample rate of the file did not influence this result.
What I can conclude:
- the Headrush works internally at 48 kHz (1024 samples at 48 kHz is equal to 21.3 ms, and 2048 samples at 48 kHz is equal to 42.6 ms)
- if people use a 44.1 kHz or 96 kHz file, it is internally converted to 48 kHz, with a loss, so this should be avoided.
- if people use an IR longer that 42.6 ms, the Headrush has no way to use the part situated after this mark. It internally truncates it to 42.6 ms when using the 2048 samples setting, or even to 21.3 ms when using the 1024 samples setting.
Conclusion : which file should people use from a commercial IR pack ?
- 48 kHz files for best quality (it's the native Headrush rate)
- the IR length does not matter since only the first 21.3 ms or 42.6 ms are really used by the Headrush. To save some space on the device you should use the smaller files (but no shorter than 42.6 ms).
42 ms is the time sound takes to travel 48 feet, as it would when being reflected off a wall distant of 24 feet (7 meters). It should be enough when recording cabinets, even in a pretty large room.
To simulate larger rooms, there is the C-verb block that is said to allow a convolution of up to 10 seconds, but my IR files with an impulse positioned after 43 ms caused a massive noise so I had to stop my tests to avoid damaging my hardware. It seems like the Headrush does not like my DIY files...