Nuts & Volts - May 2007

SONIC REALISM

This explains why the recording sounds very different. If you were in the room, your hearing system can apply many tools to clarify the speech. Primarily there is binaural auto-correlation that can be used. Then there is sound-shadowing (from your head) to localize the person speaking. Monaural auto-correlation can be employed as well, since your brain “knows” that any identical sound coming from a different direction must be an echo. You have a visual reference as well, which certainly has the potential to assist in localization. Lastly, the sonic characteristics of the room itself can provide useable cues.

Auto-correlation cleans up the voices and makes them much more intelligible. It also explains why auto-correlation stops around 40 ms and echos are then heard. This may be because the shortest speech sound is also about 40 ms long. If an auto-correlation system was longer than that, it is possible that short speech sounds would be degraded because speech contains many repeated sounds. Obviously, the speech and hearing mechanisms must be compatible in order for them to function properly.

This finally provides a clear explanation of why phase is so important. The auto-correlation function fails with reflected multiple-driver, speaker systems because wave shape is significantly changed. Photos 1 and 2 show a distinct difference in wave shape. The ear perceives this as a different sound coming from a different direction. This is why ordinary speaker systems do not and cannot sound like the real thing.

saw that the ear’s sensitivity to phase is extremely high. Finally, it was shown that common speaker systems with more than one driver ruin the phase relationships. This is because sound reflections from the ceiling (or floor, etc.) do not maintain the same phase relationships as the direct line sound.

In the next part, two different speak-

er systems will be detailed that the provide proper phase relationships regardless of path length differences. A high-fidelity system will also be described that produces a qualitative difference in sonic reality. Ordinary stereo recordings and/or stereo FM broadcasts will be used as the source material. Recording considerations will also be discussed. NV

Conclusion

This has been a very cursory examination of the human hearing system with a focus on phase. We saw that hearing is very complicated and employs subtle and sophisticated functions associated with phase. We also