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Subwoofers
Subs Cancelling each other?
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<blockquote data-quote="PV Audio" data-source="post: 6444240" data-attributes="member: 554493"><p>Yes they can, and it's a problem with what is called "phase". Different speakers have different response characteristics and consequently different phase delays. To make this simpler, let's use an example from mathematics: a sine and a cosine wave. All a cosine wave is, is a sine wave that is 90 degrees out of phase, or the other way, a sine wave is a cosine wave that is -90 degrees out of phase. To visualize this, look at the following plot:</p><p><img src="http://upload.wikimedia.org/wikipedia/commons/thumb/3/38/Sine_cosine_plot.svg/800px-Sine_cosine_plot.svg.png" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p></p><p>In mathematics, pi/2 radians is equal to 90 degrees, so if you look at the plot of the sine wave from pi/2 on, you'll note that it's identical to the plot of the cosine wave starting at 0.</p><p></p><p>The reason this is relevant is because all that a speaker is doing is emitting sound waves, and as you can see from the plot, sine and cosine are just waves as well. Since there are two sides to a speaker cone, front and back, if you consider the wave coming off the front to be what's called "in phase", or has a phase of 0 degrees, then the wave on the back is completely "out of phase", or has an angle of 180 degrees. A wave that is 360 degrees out of phase is actually in phase as if you might know, 0, 360, 720, etc. are all called "coterminal" angles, or angles which end at the same point on the unit circle. Now, back to the speakers //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif</p><p></p><p>If you take a 12" speaker and a 10" speaker and play them at the same time, they will have different phase responses due to the size of the cones and other various factors about them. While they might be playing the same frequency, that does not mean that their sound waves are in phase with one another. To think about this simply, imagine you start playing a 20hz tone on one 12" sub, and then start playing a 20hz tone on another 12" sub. Unless you start playing that second sub at the exact moment that the first one returns to its neutral position, the cones will not be in phase with one another, and you get interference which in this case, is likely to be destructive interference, which is where part of the waves cancel each other out. By adding speakers together, you want constructive interference which is making the amplitude of the output sound greater. That can only happen if the sound waves being emitted are in phase with one another. Note that there are a lot more factors to consider, such as enclosure type (ported enclosure make use of the back wave of the cone to make more output).</p><p></p><p>A useful application of phase is when doing show car installations where if you have multiple subwoofers, you'll mount some properly, and some inverted, as shown in the following:</p><p></p><p><img src="http://memimage.cardomain.com/ride_images/1/1056/3481/2639240015_large.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p></p><p>That one subwoofer that is inverted is wired opposite to the others, aka, it has a negative signal wire going to the positive terminal, and the positive signal wire going to the negative terminal. Why? Because if it were wired normally, when the other speakers move OUT, it would also move OUT, which in this case, is moving in the opposite direction to the other subs. By wiring that one backwards, you can ensure that they are all moving in phase.</p><p></p><p>So to sum up, no, do not use different speakers in the same installation unless they are being used as a single system as in a 2 or 3 way loudspeaker. Otherwise, since different speakers have different phase responses, you can end up, with as you suspected, cancellation due to destructive interference as the sound waves are not adding, but are subtracting.</p><p></p><p>Hope that helps and wasn't too long winded //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif</p><p></p><p>-Dave</p></blockquote><p></p>
[QUOTE="PV Audio, post: 6444240, member: 554493"] Yes they can, and it's a problem with what is called "phase". Different speakers have different response characteristics and consequently different phase delays. To make this simpler, let's use an example from mathematics: a sine and a cosine wave. All a cosine wave is, is a sine wave that is 90 degrees out of phase, or the other way, a sine wave is a cosine wave that is -90 degrees out of phase. To visualize this, look at the following plot: [IMG]http://upload.wikimedia.org/wikipedia/commons/thumb/3/38/Sine_cosine_plot.svg/800px-Sine_cosine_plot.svg.png[/IMG] In mathematics, pi/2 radians is equal to 90 degrees, so if you look at the plot of the sine wave from pi/2 on, you'll note that it's identical to the plot of the cosine wave starting at 0. The reason this is relevant is because all that a speaker is doing is emitting sound waves, and as you can see from the plot, sine and cosine are just waves as well. Since there are two sides to a speaker cone, front and back, if you consider the wave coming off the front to be what's called "in phase", or has a phase of 0 degrees, then the wave on the back is completely "out of phase", or has an angle of 180 degrees. A wave that is 360 degrees out of phase is actually in phase as if you might know, 0, 360, 720, etc. are all called "coterminal" angles, or angles which end at the same point on the unit circle. Now, back to the speakers [IMG]//content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif[/IMG] If you take a 12" speaker and a 10" speaker and play them at the same time, they will have different phase responses due to the size of the cones and other various factors about them. While they might be playing the same frequency, that does not mean that their sound waves are in phase with one another. To think about this simply, imagine you start playing a 20hz tone on one 12" sub, and then start playing a 20hz tone on another 12" sub. Unless you start playing that second sub at the exact moment that the first one returns to its neutral position, the cones will not be in phase with one another, and you get interference which in this case, is likely to be destructive interference, which is where part of the waves cancel each other out. By adding speakers together, you want constructive interference which is making the amplitude of the output sound greater. That can only happen if the sound waves being emitted are in phase with one another. Note that there are a lot more factors to consider, such as enclosure type (ported enclosure make use of the back wave of the cone to make more output). A useful application of phase is when doing show car installations where if you have multiple subwoofers, you'll mount some properly, and some inverted, as shown in the following: [IMG]http://memimage.cardomain.com/ride_images/1/1056/3481/2639240015_large.jpg[/IMG] That one subwoofer that is inverted is wired opposite to the others, aka, it has a negative signal wire going to the positive terminal, and the positive signal wire going to the negative terminal. Why? Because if it were wired normally, when the other speakers move OUT, it would also move OUT, which in this case, is moving in the opposite direction to the other subs. By wiring that one backwards, you can ensure that they are all moving in phase. So to sum up, no, do not use different speakers in the same installation unless they are being used as a single system as in a 2 or 3 way loudspeaker. Otherwise, since different speakers have different phase responses, you can end up, with as you suspected, cancellation due to destructive interference as the sound waves are not adding, but are subtracting. Hope that helps and wasn't too long winded [IMG]//content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif[/IMG] -Dave [/QUOTE]
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