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Setting levels
After the devices are positioned, you should begin playback of the test tone. The test tone should be played as loudly as possible (or
practical) to produce the best signal-to-noise ratio. With the test tone playing at optimum volume, set the levels on the recording
device. Recording devices levels should also be set as high as possible, but not permitted to clip or distort. Safe levels are determined by
whether you are recording to an analog or digital medium.
Recording the test tone
Begin recording on the recording device and begin playback of the test tone. Remember to include the spikes at the beginning and end
of the test tone. Record the test tone several times using the initial setup, then move the microphones and record the test tones several
more times. Continue moving the microphones and recording until you have exhausted the space’s acoustic possibilities. Recording
impulses in this manner provides you with several distinct impulses for each space.
Recording the impulse through an electronic device
The recording process is similar if you are recording the output of an electronic device, but there are no speakers or microphones to be
placed.
Using the appropriate cables, connect the playback system’s outputs to the electronic device’s inputs and the electronic device’s
outputs to the recording system’s inputs. Once the devices are connected, play the test tone through the electronic device and record its
output on the recording system.
Other impulses
Any number of methods can be used to create an impulse, including starter pistols, clap boards, or even a sharp hand clap. The
drawback of these “impulse generators” is that they add their own coloration to the sound. For best results, we recommend using the
test tones included on the Sound Forge application disc.
Recovering the impulse
After you have recorded your test tones, they must be processed and converted into impulse responses. The following sections describe
the typical impulse recovery procedure.
Trimming the test tone
1.
Open your test tone file (the “room processed” output test tone) in the software.
2.
Locate the first timing spike and delete all audio before it. Cut as close to the beginning of the timing spike as possible, but do not
delete the spike itself.
3.
Locate the second timing spike and delete all data from the start of the second spike to the end of the file. Again, cut as close to the
start of the timing spike as possible.
You should now have an audio file with a spike at the beginning, a test tone, and silence.
4.
Save the test tone file.
USING ACOUSTIC MIRROR AND WAVE HAMMER | 221