Phase and Car Audio
Phase and Car Audio
Like A Low Voice On A Wave Of Phase
If you are into audio, you are probably aware of what we mean by “phase” if not, it is simply the correlation in time of two sound waves. If you have noise cancelling headphones you experience the most extreme effects of putting two identical audio signals 180 degrees out of phase (they cancel each other out!). Anyone who listens to music experiences phase at many different levels. Phase can be used by musicians and sound engineers to create effects in their music however, what we are concerned with here is phase distortion or discrepancies that may mean you are not hearing music as the artist intended.
There can be a number of reasons for phase discrepancies. Sometimes these can be caused by the electronics in a system. Depending on the quality of equipment there will be differences in phase between the left and right channel of an audio system. This can be down to component tolerances in an amplifier, crossover or signal processor design. There is little or nothing that can be done about inherent electronic phase issues in an amplifier however, some signal processors offer the ability to phase correct or more accurately, falsely distort phase to compensate. Such electronic phase distortion is extremely subtle and it is unlikely that it would trouble even pretty good ears whilst on the move.
More dramatic phase distortion comes when speakers are incorrectly wired. Speaker terminals are marked with a + or -. This is to help the installer to ensure that phase is correct from the output of an amplifier to the “input” of the speakers. If one of a stereo pair of speakers is wired out of phase then the resulting audio will most typically lack bass. This is because things move less quickly but more powerfully at low frequencies. If a speaker is moving outwards in response to a low frequency impulse, its opposite number will be moving in the opposite direction. In effect, the air reaching your ears will be less “energised” by this effect.
In a car audio system, we are more likely to experience environmental phase discrepancies. These occur due to audio reflections. When sound is reflected by a piece of glass for instance it is very unlikely that it will be reflected in phase and will therefore change the sound pressure and frequency make up that reaches your ears. Furthermore, reflected sound can be accompanied by a slight delay that will then mean it affects other sounds generated by the speakers. Recording studios and other critical listening environments are treated to reduce the effects of reflected sound. Typically they will be very sonically dead with many absorbent surfaces as well as materials which diffuse audio signals. This is not possible in a car and so other interventions are required if phase is identified as an issue.
Audison has conducted the most research on phase in a car audio setting. It has already introduced Finite Impulse Response filters (FIR) as opposed to the easier to achieve Infinite Impulse Response circuitry (IIR) in its latest bit One HD product. You can read about this in slightly closer detail in a review in issue VII of Driving Sounds Magazine: ( https://www.drivingsounds.co.uk/articles/driving-sounds-issue-7/audison-bit-one-hd/)
Other Audison bit processors can be phase corrected during a bit Tune calibration process. Here a microphone is placed in a car and a number of audio signals are played which are recorded and compared to the original. The connected bit product can then vary phase to achieve the most accurate correction possible.
Alongside a lack of bass, phase discrepancies can manifest themselves in an unstable stereo image where the centre point seems to move around. Width, depth and height of image can also be compromised by phase errors. These effects are often not obvious unless compared with systems where the phenomena has been corrected. It is always a good idea to request a bit Tune when having an Audison DSP installed. When it is right, it is very obvious. When it is wrong, it is easy to blame other things!
As always, we would recommend a visit to your FOUR MASTER as they deal with such things on a daily basis and will quickly be able to assess whether or not your system is suffering from a “wave of phase”!
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