Table 12: Frequency, Wavelength, Column Size, And Directivity Control - Community Entasys Installation & Operation Manual

INSTALLATION
Near Field / Far Field Transition
One aspect of line arrays that differ from other types of loudspeaker systems that should be kept in mind
is the transition from the near field to the far field. This is related to the size of the source and the wavelength
(frequency) that the source is radiating. The near field distance increases with increasing source size and with
increasing frequency. For "point source" type loudspeaker systems this transition usually occurs fairly close to
the loudspeaker and can usually be neglected.
For a line array, however, the source size (line length) can be very large. This can push the near field
out to a considerable distance away from the loudspeaker. When this occurs the sound system designer and/or
installer needs to be aware of it. In the near field, the sound pressure level will decrease 3 dB for every doubling
of distance away from the loudspeaker. In the far field, the sound pressure level will decrease 6 dB for every
doubling of distance away from the loudspeaker. Since the transition between the near field and far field is
frequency dependent, this may lead to changes in the frequency response of the line array at different distances.
This can be used advantageously to help mitigate the effects of air absorption of high frequencies.
Increasing Low Frequency Directivity
In order to maintain directivity control at low frequencies (large wavelength), a line array must be
physically large. In general, the length of the line must be on the order of one wavelength long to begin to yield
directivity control (i.e. approximately -6 dB at 45°). Increasing line array length compared to wavelength results
in a narrower, more well defined radiation lobe. For a very small opening aperture (beamwidth) the line length
may need to be four times as long as the wavelength to be controlled.
Table 12 shows the relationship between frequency, wavelength, the size of single and multiple column
ENTASYS arrays, and the directivity control expected. When the line length is approximately four times the
wavelength (or greater) at a given frequency, the directivity control should be good at this frequency. When the
line length is approximately two times the wavelength at a given frequency, the directivity control will be
decreasing but acceptable. If the line length is less than approximately two wavelengths there will be directivity
control but not a tight, small beamwidth lobe usually associated with a line array system.
Compare single ENTASYS and three ENTASYS columns in this table to Figures 2 and 3.
Table 12: Frequency, Wavelength, ENTASYS Column Size, and Directivity Control
1x
Frequency
Wavelength
(Hz)
(feet)
200 5.650
250 4.520
315 3.588
400 2.825
500 2.260
630 1.794
800 1.413
1,000 1.130
1,250 0.904
1,600 0.706
2,000 0.565
2x 4x
Wavelength Wavelength
(feet) (feet)
11.301 22.602
9.041 18.081
7.175 14.350
5.650 11.301
4.520 9.041
3.588 7.175
2.825 5.650
2.260 4.520
1.808 3.616
1.413 2.825
1.130 2.260
Community ENTASYS Installation/Operation Manual — Page 45
1x
Good
ENTASYS ENTASYS
Control
(feet) (feet)
3.333 No 6.667
3.333 No 6.667
3.333 No 6.667
3.333 No 6.667
3.333 No 6.667
3.333 Marginal 6.667
3.333 Marginal 6.667
3.333 Marginal 6.667
3.333 Yes 6.667
3.333 Yes 6.667
3.333 Yes 6.667
2x 3x
Good
ENTASYS
Control
(feet)
No 10.000 Marginal
No 10.000 Marginal
No 10.000 Marginal
Marginal 10.000
Marginal 10.000
Marginal 10.000
Yes 10.000
Yes 10.000
Yes 10.000
Yes 10.000
Yes 10.000
Good
Control
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes