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Sound proofing by insulation
Apart from the possibilities to take directly measures
at the source of sound, already described in part before,
there is a further proven medium, the sound insulation.
Here, the sources of sound, like drive units, get completely
covered by means of sound insulation elements. When
planning such sound insulations or claddings, care has
to be taken that the sound insulation factors given by the
manufacturers for the sound insulation elements, or in the
sound insulation cladding itself, will not be reached.
The insulation property of a wall with respect to the
sound protection is expressed by the so-called sound
insulation factor R. The sound insulation factor R is the
logarithmic ratio that the sound power that impinges on
a wall bears to the sound power that passes a wall. The
sound insulation factor is dependent on the frequency and
the sound wave angle, what cannot directly be seen from
the defining equation.
If the sound insulation factor of a sound insulation
element is plotted over the frequency, there will be formed
the so-called sound insulation curve. It is a function of
the rigidity, the resonant frequency, the mass and the
coincident frequency. In the sound insulation curve there
are three different ranges. Within the range of resonant
frequency and coincident frequency, there can be clearly
seen invasions of the sound insulation factor. The two
ranges are separated from each other by a linear range to
which the mass law does apply. The mass law expresses
that a doubling of the frequency or a doubling of the basis
weight of the wall improves the sound insulation by 6dB.
The notation with which we are familiar now enables the
mass law to be formulated, as follows:
If the incident sound wave shows the same or a similar
frequency as the natural frequency of the wall, the wall
will be excited to stronger vibrations. In case of the
coincidence, the incident sound wave, or the projection of
the wave, has the same wave length as the bending wave
that is produced in the wall. So, the bending waves in the
wall become excited to a higher degree.
If the sound wave strikes a wall below an angle δ,
Issue 04 | 2010
TRANSFER OF TECHNOLOGY
and if the wave length of the incident sound wave λ 0 is
to the bending wave length of the wall λ B as the sine of
the incident angle, it will come to coincidence. The wall
becomes acoustically transparent within this area. That is
the effect called coincidence influx.
When making use of sound insulation claddings, the area
of the resonance influx is dependent on the construction
and the area of the coincidence influx on the material.
Fig. 27:
Coincidence effect
Fig. 26:
Sound insulation curve
When planning sound insulation claddings, the thermal
dissipation from the housing into the environment, while
using sound absorbers, has to be taken into account.
Furthermore, there is to be drawn up a thermal balance,
considering the environmental and climatic conditions.
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