Community ENTASYS Installation/Operation Manual — Page 45
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.
INSTALLATION
Table 12: Frequency, Wavelength, ENTASYS Column Size, and Directivity Control
Frequency
(Hz)
1x
Wavelength
(feet)
2x
Wavelength
(feet)
4x
Wavelength
(feet)
1x
ENTASYS
(feet)
Good
Control
2x
ENTASYS
(feet)
Good
Control
3x
ENTASYS
(feet)
Good
Control
200
5.650
11.301
22.602
3.333
No
6.667
No
10.000
Marginal
250
4.520
9.041
18.081
3.333
No
6.667
No
10.000
Marginal
315
3.588
7.175
14.350
3.333
No
6.667
No
10.000
Marginal
400
2.825
5.650
11.301
3.333
No
6.667
Marginal
10.000
Yes
500
2.260
4.520
9.041
3.333
No
6.667
Marginal
10.000
Yes
630
1.794
3.588
7.175
3.333
Marginal
6.667
Marginal
10.000
Yes
800
1.413
2.825
5.650
3.333
Marginal
6.667
Yes
10.000
Yes
1,000
1.130
2.260
4.520
3.333
Marginal
6.667
Yes
10.000
Yes
1,250
0.904
1.808
3.616
3.333
Yes
6.667
Yes
10.000
Yes
1,600
0.706
1.413
2.825
3.333
Yes
6.667
Yes
10.000
Yes
2,000
0.565
1.130
2.260
3.333
Yes
6.667
Yes
10.000
Yes
Содержание ENTASYS
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