Specifications

TI 323 (6.0E)
17 - 36
R
2R
A
2A
Line
source
Simulation of the directional behaviour of a discrete line
array. Note the lobes where the wavelength is shorter
than the distance [d] between the drivers
If the wave front offset [∆
∆∆
∆x] is larger than λ
λλ
λ/4, the
resulting wave front breaks up and loses its directional
property.
5.2. Line Sources
Line sources are nothing new; in the 50s it was the
only economical solution to build loudspeaker systems
that provided a specific directivity in a defined
frequency range. Without being aware, this
technology is often used today to increase the
directional behaviour of loudspeaker arrays.
The ideal line source is an infinitely long, continuous
radiator. Energy radiated from a line source is
distributed cylindrically, doubling the distance from
the source doubles the surface of the cylinder.
Therefore, the acoustic power density is halved and
according to power distribution the resulting SPL is the
√2 or –3 dB by doubling the distance.
In reality continuous and infinite line sources do not
exist, and therefore a number of limitations should be
taken into account when predicting the behaviour of
a real line array.
5.2.1. Discrete line array
A discrete line array consists of a number of drivers
arrayed in a line, of which there are two variations.
Sperical sources:
The first is an array built out of spherical sources.
Because of their physical size there will be a spacing
between the acoustical centre of the drivers, and this
distance must be smaller than the shortest transmitted
wavelength.
The wavelength is calculated by:
=
c
f
λ
Where:
λ = wavelength (m)
c = speed of sound (m/s)
f = frequency (1/s)
To transmit 10 kHz (λ = 0,034 metre / 0.11 ft) the
distance between the drivers must be 3 cm or less. As
soon as the wavelength is shorter than the distance
between them the array creates side lobes and loses
its directional behaviour.
Directional sources:
The second variation uses directional sources to
create a wave front that curves slightly and this is the
most common approach used in todays line arrays.
The wave front radius of each single source is
important to minimize the wave front offset between
sources.
The flatter the wave front the higher the frequency
that can be transmitted.
Using magnetostatic or electrostatic ribbon drivers
can improve the behaviour for very high frequencies
however at the cost of efficiency, and it is for this
reason that these types of drivers are very rarely
found in professional sound reinforcement products.