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Time Is On Your Side
(If you want it to be…)
By Tom Young
Electroacoustic Design Services
Storrs, CT
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Driver Misalignment. In loudspeakers with both high- and low-frequency transducers that are direct radiating (not very common in sound reinforcement), there will be offset in the arrival times of the high-frequency transducer versus the low-frequency transducer. This is simply due to their required physical positioning. A high-frequency transducer consists of a smaller diaphragm with a shallower motor structure than its low-frequency counterpart. When these drivers are positioned on a flat baffle (the most common baffle shape) the high-frequency signals arrive at the ears of listeners sooner than the low frequency signals, on the order of several hundred microseconds (See Illustration 1).
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Illustrations 1 & 2 (Click on Image for Full Size)
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This misalignment is reversed when the high-frequency driver is horn loaded (the more common method used in sound reinforcement loudspeaker systems), requiring a waveguide of a certain depth to achieve the needed pattern control. This is one of two primary reasons for using such horns; the other reason is an increase in acoustic efficiency. With horns, high frequencies no longer lead low frequencies, but rather they lag behind, again on the order of several hundred microseconds (See Illustration 2).
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Illustrations 3a & 3b (Click on Image for Full Size)
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In loudspeaker systems where high-, mid- and/or low-frequency components are in various configurations of direct radiating and/or horn loading, it is almost impossible to realize horn lengths that position the drivers in physical alignment to one another (See Illustration 3a and 3b).
Note that the alignment of multiple drivers is not as simple as visualizing or physically measuring the location of each component’s diaphragm and then either positioning them to be in the same physical plane or applying electrical signal delay to the leading component.
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Every driver has varying points of acoustic origin that shift with frequency. The point of acoustic origin, as applied to driver alignment, must be measured to be derive a mean value.
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