What is an Impulse Response?
If you just knock on something, the
material you knock on 'answers' to your knocking by making a
sound.
This sound contains a lot of information about the
material and the location you knocked on.
As you might notice, different materials produce different sounds, and different
'fade out' times.
What you hear is the
impulse response (IR) to your knocking impulse.
Obviously, the IR contains some time
related informations, like how long it takes until the sound
starts or ends.
Furthermore there are two important locations, first the location you knock
(excitation) and the location you listen (measurement).
Changing one
or both will change the IR.
Just keep in mind, that the IR changes with
the material and the point of knocking, which is a direct analogon
to acoustics, where the IR changes on different locations or
speaker/room combinations, and with different excitation and
measurement locations.
Because the IR is a recording along a time axis, we
talk about the 'time domain'.
On the right you see a part of a real world IR, where a discret reflection (echo)
is clearly visible at the cursor location.