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We have included several short files on the Sound Forge CD-ROM to allow you to experiment with this technique. After some experimentation, you should begin to notice a few general rules regarding this use of Acoustic Mirror:

•Impulse files that cover the entire frequency spectrum prevent the output from sounding too filtered.

•Using a frequency sweep as an impulse creates a frequency-dependent delay effect.

•Panning within the impulse causes the stereo image of the output to flutter between channels.

•Using staccato sounds (such as drum hits) creates a variety of echo effects.

Recreating spaces for foley effects and dialog replacement

Frequently, dialogue recorded in the field is rendered unusable by ambient noise. If you are shooting in the field and realize that overdubbing will be necessary, you should create an impulse in each filming location. This allows you to overdub dialog during post-production that is indistinguishable from dialog recorded on location.

If you intend to use Acoustic Mirror as a film/video post-production tool, there are some factors to keep in mind:

•Distance information is determined by the distance between the source and the microphone when creating the impulse. Record multiple impulses at various distances for each location to create realistic dialog effects when matching audio processing to approximate camera positioning.

•The frequency response of the human ear changes as the volume of a sound increases. As a result, impulses created from a significant distance may sound unusual at high volumes.

•Placing a microphone off center allows you to create directional information in the recovered impulse. For example, placing a microphone to the left of the speaker produces an impulse that approximates a source located on the left side of the screen.

Panning with head-related transfer functions

A head-related transfer function (HRTF) contains the frequency and phase response information required to make a sound seem to originate from a specific direction in a three-dimensional space. The Acoustic Mirror Impulse Files\HRTF Impulses folder on the Sound Forge CD-ROM contains a collection of impulse files that contain directional cues.

To achieve optimal results using these impulse files, the original file should be mono and playback should be monitored using headphones. To begin, convert the mono file to stereo by replicating the mono signal in each channel. After the audio is converted to stereo, choose an impulse file from the HRTF Impulses folder. You will notice that the HRTF Impulses folder is further divided into Left and Right directories. Opening the desired folder displays the available impulse files, all of which are named based on their elevation (up or down) and azimuth (left or right) angles in degrees. The following table provides some examples:

File Name

Impulse positioning

0E000L

Straight ahead

0E090L

Far left

 

 

0E090R

Far right

90E000L

Directly above your head

 

 

0E180L

Directly behind you

-20E120L

Below, behind, and to your left

 

 

Note: Refer to Readme.doc in the HRTF Impulses folder for more information.

USING ACOUSTIC MIRROR AND WAVE HAMMER

CHP. 12