TL PHASE |
This is quoted from Larry Sharp's Quick and Easy Transmission Lines.
Click on "HOME" to use the TL Calculator.
In the above figure, 1/4 wavelength = 900 phase shift, while 1/2 wavelength = 1800 phase shift, 3/4 wavelength = 2700, and 1 wavelength = 3600.
Notice that the front wave starts at the bottom of the cone and the rear wave starts at the top of the cone. This indicates the 1800 phase difference between the front and rear of the speaker cone.
If we look at this line with no damping fiber in it, then we can use the equation for the speed of sound through air.
If the woofer Fs is 28hz, then 1130 divided by 28, divided by 4 = 10.01ft for 1/4 wavelength.
Let's cut our TL to 10.08ft long for the classic 1/4 TL. Then if we stuff the line at a rate of 1/2lb per ft3, Bradbury's equations say that the speed of sound will be 408ft per second. Now by substituting 408 for 1130, we see that...
...will give us a length of 3.64ft for 1/4 wavelength. A 3/4 wavelength would be 3 times that length. So, 3.64 x 3 = 10.92ft...not much over our 10.08ft. What we have now is a TL that is acoustically just under 3/4 wavelength, and according to the figure it is basically in phase.
If we shorten the actual length of the TL to 3.64 and keep stuffing density the same, then we are back to a 1/4 wavelength, and still in phase.
So the acoustical legth of a TL is calculated in terms of phase, while actual length is measured in inches or feet.