user manual

196
USING ACOUSTIC MIRROR AND WAVE HAMMER CHP. 12
Recovered impulse is too noisy
To maximize the impulse’s signal-to-noise ratio, you should verify that the field recording’s noise floor is not
too high. When recording in noisy environments, increase the test tone’s amplitude until the test tone is at
least 25 dB louder than the noise floor. At least 40 dB of signal-to-noise is recommended for optimal
impulses. If you cannot avoid noise when recording in the field, Sony Pictures Digital’s Noise Reduction can
salvage a session.
Speaker nonlinear distortion can also cause noisy impulses. The most common source of nonlinear distortion
is loudspeaker harmonics. Most speakers display substantial harmonic distortion at low frequencies. For
example, when you play a 60 Hz tone, the speaker vibrates at 60 Hz, but also outputs lower-level audio at
multiples of 60 Hz (120, 180, etc.). The impulse recovery method greatly minimizes these low-frequency
distortions; however, inexpensive tweeters often display substantial high-frequency distortion that can
disrupt the recovery process. When possible, use high-quality components and do not overdrive the speakers.
Error message explanations
The following sections briefly describe error messages that may be encountered when using Acoustic Mirror.
The selected file is not a valid test file
The file specified in the Test file used box is not a test tone file included on the Sound Forge CD-ROM.
The level of the first spike is low. Do you wish to use it as a timing spike?
This typically means that no actual timing spike was detected. Verify that the first spike is within one second
of the start of the recorded file. If the recording is noisy and the spike is not very pronounced, you can aid
detection by muting the audio immediately before and after the spike.
An error occurred reading the test tone file
Either the test tone file was not found or is not a valid test tone file. Always use a test tone file provided on
the Sound Forge CD-ROM.
The selected Recorded file is much smaller than the test tone size
This may indicate that the test tone or the recorded file specified in the Recover tab is not correct. Verify that
the length of the recorded file roughly the same size as the test tone file.
The end spike was not found
Verify that the spike following the test tone is present in the recorded file when recovering impulses in Auto-
detect timing spikes
mode.