The laws of nature and the laws of physics
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Sometimes you just can't do what you want in life because the laws of nature/physics will work against you. For example, if you try playing very short duration sounds to subjects in your experiment it's likely that they won't be presented correctly. This is because the speaker is a moving coil that has to move back and forth to generate sound waves. To do this it must have time to mechanically start vibrating.

Most experiment generators, or your own custom written software, will let you enter presentation figures that can't possibly be achieved in the real world. Their job is not to reality check what you are trying to accomplish. This applies across the board to TFT panels that suffer from input lag or slow pixel response times to sounds that are two short to produce using traditional moving coil speakers. Or maybe an image on a monitor is simply to dim to perceive as rapidly as might the same image at a greater intensity. These are just a couple of examples.

Any software will slavishly try to follow orders irrespective of whether those orders are sensible. Unless you self-validate your own timings with a Black Box ToolKit you will never know what has or hasn't been achieved.

One classic pitfall is the speed of sound. The speed of sound in air is roughly 1 millisecond per ft (approx 30cm). Thus if you position your speakers around the room say 10ft from your subject then there will be approximately a 10 millisecond sound lag over a visual display just because of air density. The further away your speakers the longer it will take for the sound waves to reach the subject.

The speed of sound in dry air
Image courtesy of Kwikwag Fubar

A sound pressure wave
Image courtesy of Christophe Dang Ngoc


Pneumatic headphones in MRI studies are also effected in a similar way. If you have 30ft or approx 10m or rubber tubing then the sound will take 30 milliseconds to reach your subject in which case you should start playing your sound 30 milliseconds before you present your visual image for them to be aligned. Remember how lightening works.

The speed of sound does not equal the speed of light.
So for cross modal work you should take this into account.