Ecler BET ACOUSTICS MASS1015P Manual De Instrucciones página 6

c) Plane waves:
These are generated, for example, in the interior of pipes with a given length and diameter in which
the acoustics are maintained independently of distance covered by the wave front with respect to the
source of sound emission.
In this case the level of sound pressure is not attenuated with distance. This type of wave is
synthesised in the so-called Kundt tubes, but are infrequent in nature.
2.3. LINE ARRAY systems as sources of cylindrical waves
As was shown in the previous section the conditions under which a line array behaves as a
source of cylindrical sound waves depend on factors such as array length and the frequency used, and
can be expressed in a simplified way with the following formula:
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D<H f/2c
Where
H: array length (determined by the number of sound enclosures
used)
f: frequency
c: sound speed
D: frontal distance between the near region (Fresnel region) and the
far region (Fraunhofer region)
For distances greater than D the wave front emitted by a line array ceases to be cylindrical and
tends to become spherical, with a consequent attenuation of the sound pressure level of 6dB SPL on
doubling the distance from the array.
It is important to keep in mind that the behaviour in the near field of a line array system is in
actual fact qualitatively more complex given that each point located in this region is affected by the
pressure level contributions related to the distance to each one of the components that constitute each
of the acoustic enclosures that make up a line array.
This is why line arrays require suitably adjusted sound signal equalization and processing.
2.4. Directivity of LINE ARRAY systems
The efficiency of line array systems is based on constructive and destructive interference. The
pattern of any sound wave front begins to behave directive when the wavelength () of the sound is
comparable with the dimensions of the emitting surface.
If two acoustic boxes that emit the same frequency, phase and amplitude (see section 2) are
grouped, the resulting pattern of sound emission is different than when they emit separately:
 At points along the axis between them there will be constructive interference with a resultant sound
level 6dB greater than with only one sound source.
 At points outside the axis between them the difference in the distance travelled by the wave fronts
will cause cancellations, resulting in a lowered sound pressure level. This is the so-called
phenomenon of destructive interference or combing.
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