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Attenuation characteristic with distance
- a simplified explanation
If we consider a vertical line of cabinets with closely spaced elements, the listener will
hear the vector sum of more and more cabinets as he moves further away from a straight
line array.
6-cabinet line array showing very simplified 7.5deg mid-HF coverage
As the listener moves from left to right in the illustration above, he will hear mainly
cabinet 5, then 4+5, then 4+5+6 and so on.
These increasing source contributions partially compensate for the high attenuation rate
caused by the normal inverse square law (6dB attenuation/doubling of distance). The
line array effect exhibits a lower attenuation rate that can approach 3dB
attenuation/double of distance in the near-field.
Far-field spls will increase towards the centre of the array where more elements add. The
"cylindrical" behaviour mentioned in many text books applies to theoretical line arrays
of infinite length, not to practical arrays of limited length. Cylindrical radiation patterns
are rarely found in the real-world due to practical line length limitations.
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transition distance
>>>>>>>>>low attenuation region>>>>>>>>>>>> high attenuation region>>
As our listener moves further from the array, he will eventually reach a point where he
can hear all of the cabinets summing together. This is called the transition distance. The
attenuation rate will revert to the normal inverse square law of 6dB attenuation/doubling
of distance (+ HF air absorption) beyond this point because there are no further elements
to compensate.
In practice, of course, line array characteristics are more complex. The contribution of
individual elements will depend on their amplitude, relative phase and directivity –
especially when the array is curved to cover a practical audience shape.
All material © 2007. Martin Audio Ltd. Subject to change without notice.
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