Martin Audio W8L Longbow Application Manual page 23

5.4 How many do I need?
There are four major factors to be taken into account when determining what model of
line array to use and how many:
Will I need delays? This is very imp ortant – see
Spectral balance – the minimum column length required for spectral balance over
the complete audience distance – or just beyond the first delays
Maximum spl – the number and model required to achieve maximum spl
Horizontal coverage – see
Spectral Balance
Users new to line array technology can be confused by the spectral balance requirement
because they are used to thinking in power terms only.
A straight line array's transition distance tends to increase with line length and frequency.
If we simplify the relationship between transition distance, length and frequency (by
assuming that individual elements are omni-directional - which they tend to be at very low
frequencies) we can use an industry-standard equation to estimate the effect of line length
on the low frequency transition distance:
The transition distance is proportional to the square of the line length. Reducing the line
length by approximately 30% (which would only reduce headroom by 3dB with non-line
array systems) will reduce the LF transition distance by 50%!
The minimum column length cannot be reduced simply because the band is a quiet
traditional folk combo. A short line array column would project only mid frequencies to
the far field. It would lack warmth and sparkle as it would not be long enough for the line
affect to take effect at low mid frequencies and may not have the headroom at high
frequencies. Boosting the system's LF and HF would simply cause too much bass in the
front seats and a lack of headroom at HF.
It is very important that you use a line array that is long enough for the low- mid
frequency projection to follow the superior mid and high frequency projection
out far enough for mid-high air absorption to have a balancing effect.
The following curves show the spectral balance of 4 and 12 W8L cabinets vs distance
taking air absorption into account for about 40% relative humidity.
section 5.9
section 5.6
All material © 2007. Martin Audio Ltd. Subject to change without notice.