X-Treme Audio MISI - Manual (Page 2)

User’s manual
LINE ARRAYS
User’s manual
1. Free-field acoustics
An unlimited acoustic space without discontinuities or obsta cles
can be defined as a “
free-field” and clearly is an idealization of
the real conditions in which the sound usually propagates itself,
whether it is generated by a “natural” source or by a sound re-
inforcement system. However, this can lead to two situations in
which the free-field conditions may be approximated in a more or
less correct way. The first one refers to an open space (such as
an area where great events and rock, pop music concerts can
take place) provided that weather conditions are stable and ho-
mogeneous, and no surfaces or obstacles are found in a suitably
large area all around the source. The second one can be created
artificially in a laboratory with special conditions, known as “ane-
choic” chamber, where all the surfaces limi ting it absorb com-
pletely the sound acting on them; the sound inside the chamber
is just the one produced by the source, as no sound is reflected
by the surfaces.
The acoustic field generated by a source in free-field conditions
can be schematically divided into two regions: the so-called
near
field and the far field. In the first region of the acoustic field the
sound intensity can have a complex trend depending on the type
of source (see the following section about linear sources) and on
its dimensions, which does not necessarily follow a mo notonic
trend in relation to distance; moreover, the source directionality
characteristics should be carefully examined.
In the second region, from the near field to infinity in theoreti-
cal terms, the sound intensity shows instead a linear trend and,
as is well-know, it is inversely proportional to the square of the
distance from the source; in other words, the
sound inten-
sity level (defined as the quantity of energy that flows, per time
unit, through a unitary surface area which is perpendicular to
the wave pro pagation direction. Unit of measurement: Watt per
square metre) decreases by 6 dB for each doubling of distance
(the so called
“inverse-square” law). In addition to this, the
source directivity can be determined in an univocal and well-
defined manner.
The far-field condition occurs when the values of the distance r from
the source meet all the following conditions:
r >> λ/2π , r >> L , r >> πL 2 /2 λ,
where L is the largest linear dimension of the source, λ is the long-
est wavelength (therefore the lowest frequency) of the sound emit-
ted by the source (therefore
λ is always ≤ 17.2 m - f ≥ 20 Hz) and the
“much higher” symbol means at least 3 times higher (Bies, Hansen
1988).
Note: a free-field variant is the so-called “
free field on a reflect-
ing surface”, such as a large open space on a rigid and highly
reflecting surface (e.g. asphalt, ground) or a made-to-measure
special environment known as “semi-anechoic” chamber. An
omnidirectional source located near a reflecting surface acts as
if it was associated to an image source having the same sound
power: as a result, the intensity at every point of the acoustic
field is worth double the sound intensity generated by the same
source in a free field, therefore the intensity level will be 3 dB
higher.
CONTENTS
1. Free-field acoustics
2. Sound power and pressure levels
3. Physical-mathematical model: brief description
4. Linear sources: introduction
4.1 Infinite length linear source
4.2 Finite length linear source
5. Description of the sound field of a linear source
5.1 Directivity analysis
5.2 In-axis response analysis
6. Arc, J and progressive sources
7. Line arrays: “the state-of-the-art”
8. X-Treme Vertical Line Array: product range
9. X-Treme Vertical Line Array: system design
10. MISI TM system: from “AS IS” to “TO BE”
11. Types of installations
12. Stacking instructions
13. Suspension guidelines
13.1 X-Treme Installer (XTI)
13.2 Suspension instructions
13.3 “Straight to the… angle”
13.4 LSA: flying and lifting
13.5 MISI TM and MLA: enclosure suspension rigging
14. Subwoofers
15. Tri-amplification system configuration
16. System configurations: standard examples
16.1 Linear Source Array
16.2 MISI TM
16.3 Mini Line Array
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