The following tests are instructive of how and
from where you perceive the phantom image produced by
your stereo system. Sit in the "sweet spot" and close your eyes as you
listen. Then do it with open eyes. You can use the results to optimize your
speaker setup, toe-in and listening position. The toneburst test gives you a
subjective center image as a function of frequency. The phantom image usually
moves vertically in a straight line from low to high frequencies. You can also
find out from the toneburst test, whether your left and right ear has equal
sensitivity at all test frequencies.
1 - Pink Noise Imaging Test
Pink noise is a test signal for which the evolutionary
brain has no natural equivalent. It is not clear what it is supposed to sound
like. The closest might be the breaking ocean surf.
Pink noise is very useful for pointing out differences between left and right
speakers due to room setup or component variations. It can be very difficult,
though, to track down the cause of the sonic differences. Two different pairs of
loudspeakers will almost certainly sound different, but that does not translate
proportionally to program material. It depends highly on the spectral
content of the program material.
A stereo system should be able to create a solid center
phantom image on mono pink noise and produce pitch changes due to comb filtering
with lateral head movement. These pitch changes do not occur on program material
of familiar sounds since the brain filters them out. Stereo pink noise should be
smoothly diffuse and not change timbre when listening from different places in
the room. I have generated a one minute test track that alternates between mono
and stereo pink noise in 5 second intervals. You can check the center image for
different room locations and setups. I added three 3 kHz and three 300 Hz
ten-cycle shaped bursts at the end of the track to check the center image
location and definition for click-like signals at those frequencies. The 3 kHz
test result is very room reflection dependent.
Download and save pink-alternating3.wav
(12 MB). Then burn the 1 minute sound file to a CD-R for convenient access and
repeated play, or play the file directly from your computer. Start out with the
playback volume turned down to protect the woofer from clipping and be
careful as you increase volume.
|| Stereo = L & R
|| Left = L
|| Right = R
|| Mono = L = R
|| 3 Bursts, 10 cycles @ 3 kHz,
-3 dB FS
|| 3 Bursts, 10 cycles @ 300
Hz, -3 dB FS
2 - Toneburst Imaging Test
A rapid sequence of 22 tone bursts from 12.8 kHz down to
100 Hz can be used to check the spatial stability and focus of the stereo center image as
a function of frequency. Each burst consists of 4-cycles of a sine wave with a
raised cosine envelope. The sine waves are at 1/3rd octave frequency intervals. The
bursts are separated in time by 100 ms or 500 ms and room reflections can affect the
phantom image location.
The burst sweep can also be used to check for the onset
of signal clipping as the playback volume is cautiously increased. The
peak amplitude of each burst is constant at 0.9 FS.
Download and save phantom_12800-100Hz_spaced_100_500ms.wav
(2.7 MB). Then burn the 16 second sound file to a CD-R for convenient access and
repeated play, or play the file directly from your computer.
The test should be done from the "sweet
spot", which is normally the apex of a triangle formed by the two
loudspeakers and the listener's head. The phantom center image may shift
vertically or horizontally as the burst burst frequency changes. Note
any horizontal shift. Then turn your chair around so that the
loudspeakers are behind you and listen to the burst sequence again. If a
certain burst always appears to come from the right, then your right ear
has higher sensitivity at those frequencies than your left ear. Thus if
you listen to listen to music in stereo, then the spatial aspects of the
phantom image, which is created in your head, will be spatially
distorted and moved to the right at those frequencies. I do not know if
the brain adapts and corrects for such phantom image distortion.
3 - Voice Imaging Test
From Chesky Jazz Sampler: Vol.