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LXmini  --  LXmini+2  --  LXstudio  --  LX521.4

        

 

 

Phantom Image Tests


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.

1    Stereo = L & R 8    Mono
2    Left = L 9    Stereo
3    Right = R 10    Left
4    Mono = L = R 11    Right
5    Stereo 12    Mono
6    Mono 13    3 Bursts, 10 cycles @ 3 kHz, -3 dB FS
7    Stereo 14    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. 1-10

 

 

 

 

LX - Loudspeaker Store

LXmini  --  LXmini+2  --  LXstudio  --  LX521.4

        

 

 

What you hear is not the air pressure variation in itself 
but what has drawn your attention
in the streams of superimposed air pressure variations 
at your eardrums

An acoustic event has dimensions of Time, Tone, Loudness and Space
Have they been recorded and rendered sensibly?

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Last revised: 06/23/2020   -  1999-2019 LINKWITZ LAB, All Rights Reserved