How we perceive stereo
Stereo recording was invented to bring a more realistic sound to the audio community. Stereo music uses two channels of sound. One channel for the left speaker and one channel for the right channel. Actually, the original multi-tract recording involved three channels: left, right and center. This type of recording was preferred by not possible due to the current recording medium at the time: the record. A record was able to record and playback to channels of recording by the variations in the two walls of the record grove. The stylist of the record needle, placed in the groove. The needle pickups the sound through the variations in the two walls of the groove. One wall records each channel. The playback medium is a conventional stereo system involving two speakers: a left speaker and a right speaker.
Figure of a stereo recording
The two speakers are placed equal distant from each other and from the listening position to form a equilateral triangle. When both speakers reproduce the same information, a center “Phantom Image” is created.
In Home audio situations, this is only possible because the left and right speakers are placed equal distances from the listener. In this situation the following things occur.
- Equal Time Arrival: Sound from both speakers arrive at the listening position at the same time.
- Equal Amplitude: Both speakers play at the same loudness. If one speaker was 1/2 the distance closer, it would play 6dB louder with the same power input.
- Equal Phase Response: Both Speakers will be in phase with each other.
- Equal Frequency Response: The angle of listening is the same therefore both speakers will have the same frequency response.
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topProblems with non-equal path legnths
However this is seldom true in car audio. In car audio applications, typically one of the speakers are closer to the listener than the other.
Typical Car Stereo Environment
This placement is typical of speakers located in the factory location like the doors. The result of this location is much different from the “Ideal Environment”
- NON-Equal Time Arrival: Sound from both speakers do not arrive at the listening position at the same time. The result is a bias towards the close speaker.
- ON-Equal Amplitude: Both speakers do not play at the same loudness. The closer speaker will normally play louder. This is dependent on the angle of the speaker. If the listening angle is severe, it possible that the speaker further away would sound louder.
- NON-Equal Phase Response: Both Speakers will not be in phase with each other. Constructive and destructive interference will result.
- NON-Equal Frequency Response: The angle of listening is not the same therefore both speakers will have differences in frequency response.
To best correct the situation, you must reduce the path length differences between the left and the right channel. This can be accomplished by finding locations in the vehicle that are as far away from the listening position as possible. Kick panel locations can offer good performance but do not limit yourself only to the possibility of kick panel locations. To deal with other potential location we must first understand how we determine location of a sound.
How We hear different frequencies ranges
Humans determine location by the observed difference between channels. The perceived differences we can hear are amplitude and phase. For each separate frequency range humans determine location differently. When frequencies wavelengths are small, amplitude is the determining factor for image placement. As the frequency drops, the emphasis changes from amplitude to phase. The side bar "Stereo Perception versus Frequency" lists different frequency ranges and their relationship to human hearing.
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