Phasers On Stun
Phasers on Stun!
This month’s final frontier is to the highly critical, planet phase. To kick off, we need to make sure you all understand how sound gets to your ears. Sound is basically the detection of movement of air entering our ears and creating sympathetic movement of the ear drum. The more violently the air is moved, the louder the sound and the more times per second that the air is moved backwards and forwards the higher the pitch of the sound we hear. All very simple if we strip away all the unnecessary scientific stuff going on! The air movement is created by the cones of a loudspeaker moving forward and backward in response to an electrical stimulus. In the age of mono where a single point source generated the air movement, phase was not a particularly discernible issue but when stereo came along things went a bit phase critical. If you have two speakers generating air movement then they are going to affect each others ability to do this with absolute integrity. The best way to demonstrate this is to reverse the + and – connections on one of your speakers on your home stereo. Depending on where you are seated (dead centre is best) you will notice that it sounds different. The reason for this is that as the right hand speaker moves forward in response to a given shared stimulus the left speaker will move backwards. The resultant sound wave or air movement will be compromised as effectively one speaker blows and the other sucks (not terms you will hear in any text book on the subject but may help with understanding the phenomena!) so, any sound which is supposed to emanate from both speakers at the same time will be reduced resulting in phase distortion. The most noticeable result of this is a lack of bass. This is because more air is moved by heavy bass than fine treble and bass tends to be delivered equally from both speakers because it is not directional. A lack of bass is always the biggest clue that phase may not be correct however, I have just given a slight clue as to why phase is also important at the top end of the audio spectrum. In a car audio system, bass is often looked after by a subwoofer and a separate amplifier. For purely practical reasons, subwoofers can sometimes be installed a long way from the listener’s ears which can cause phasing issues with lower bass coming from front mounted woofers. Most amplifiers offer some kind of phase correction by way of a 180 degree switch or in some cases a variable control which can help although perhaps not as effectively as time alignment, where the signal from the nearest speaker is delayed in order to allow the furthest mounted speaker time to move air a longer distance.
Ok, what if the phase is correct in the woofers but out at the top end? This will be harder to detect unless you are tuned into it. As things happen at 1000’s rather than 10’s of times per second at high frequencies, the phenomena manifests itself not in a deficiency at a specific frequency but in the accuracy of stereo image (see last month’s piece on separation for a more in-depth understanding here). With high-end phase distortion it will be far harder or often impossible to separate specific sounds or instruments by their position as the information which tells the brain where a sound is coming from is scrambled. High-end phase distortion is actually more common than that at lower frequencies due to the complexity of matching exactly the delays which can be introduced through performance tolerances of components in the signal circuitry. You may hear the word “linearity” used by reviewers of audio equipment and phase distortion is one of the consequences of non-linear amplifying and sound reproducing equipment.
I must end by apologising for the “Fisher Price” nature of this article but as with all things audio, research scientists spend many years looking at these phenomena and I only have 800 words in which to explain their work. Hopefully, this very top line explanation will at least make you aware enough to ask important questions if you are unhappy with the sound in your car. A FOUR MASTER will have equipment in his workshop which will tell him if the phasing of a system is correct or within the limits of the equipment employed in the system. They should use this to ensure they have wired everything correctly in the first place (we all make mistakes) but if you suspect you have a phase related issue, visit your local FOUR MASTER and have them identify whether you have a problem or simply need to revisit the equipment employed – perhaps your ears are better and deserving of higher quality equipment than you thought!
Content provided by our friends at Driving Sounds Magazine