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What is an Impulse Response?

In general terms, an impulse response is a file that records the impulse of a space and has been obtained in one of three ways:  (1)  Swept Sine usually 20 - 20k (2) Transient Method (blank 22 gun or similar) where a blank shell is fired and the impulse response is recorded and (3) Balloon Pop - by far the least desired, with vast differences in results because of thickness of balloon outer wall, the amount of air in the balloon being popped, but most practical because ease of use, balloons can go and be used virtually anywhere, and they are very portable.  There are other methods, but for the sake of general introduction, these are the most widely used, with the swept sine offering the best reproduction of a given space being recorded. 

The impulse or recording of a space, contains the recording where microphones were placed, much like taking a picture with a camera renders a jpeg wherever the picture was taken with vastly differing results depending on numerous elements.  If we record in stereo XY pattern and in ORTF (110 degrees) or NOS (90 Degrees) and place this stereo pair front facing, the impulse when recording and passing the mics, will be much different then if we take the same mic positions and turn them around 180 degrees, or rear facing.  What you get in an enclosed space, is the initial impulse passing by the mic capsules and then depending on how close we are to the back wall of the building, the impulse bouncing and then coming back directly at the microphones, because they are rear facing - this often times yields a "predelay" feel to the file when in use.    

The reason that a swept sine is drastically better than most other methods, is because of its adherence to each frequency response being equal in amplitude, thus rendering the most accurate recording in a given room or impulse response - this is of course dependent on the speaker(s) being utilized.  Another reason that a swept sine wave is preferred, is because transient noise is greatly reduced - ie you're recording a church in a downtown area and a truck rolls by and can be heard as the sine wave is sweeping, but because you are recording with a sine wave and happen to be around 6k in the sweep and truck frequencies are generally heard in the lower frequencies, the truck noise is almost non-existent - you already recorded the lower frequencies in the sweep prior to the truck rolling by. 

While transient methods like a starter pistol or a balloon pop are less desirable, they still can be quite effective if you work with software and roll out the bottom frequencies to avoid the bass multiplier effect and a general bassy, boomy sound.