A new face drilling rig for narrow tunnels and ... - Advanced Mining

Sound power P
Sound power P is the acoustic power that is delivered
by a vibrating element as airborne sound. It is expressed in
Nm/s or W. The sound power is, by analogy to the power
known in the physics, a relative quantity that is not able to
be measured directly.
The sound power P is an emission quantity of a plant,
a machine or a component. It can only be indirectly
determined by measuring a field size. To analyze the sound
power, there are available the sound ranging methods.
The sound ranging can take place in a dead room, in a
free field or in a live room while taking into account the
environmental conditions.
Another possibility of sound ranging is the metering
of the sound intensity on a measuring surface round the
object while eliminating the environmental influences to a
certain degree.
The mathematical link between sound pressure, sound
intensity and sound power is described following the
definitions speed of sound, sound particle velocity and
their effective values.
Link between sound pressure and sound
power
The link between sound pressure and sound power is the
most important base to be able to understand acoustics.
The sound power is not directly measurable, as already
mentioned before, but it is to be fixed by measuring the
sound pressure. While doing so, there is to be indicated,
Issue 04 | 2010
TRANSFER OF TECHNOLOGY
Fig. 2:
Überlagerung Schalldruck und
Atmosphärnedruck
among the pressure, the measuring point as well (distance
from source).
This shows the often cited analogy to the thermal
output of a source that is not directly measurable. Through
measurement of temperature at a place in a defined position
towards the source it is enabled the thermal output of a
body to be determined.
It is of importance to be always aware of the conditions
for the validity of the link, derived in the following. We
are working on the assumption that it is a homogeneous
source, isotropically radiating in an otherwise isotropic
room. Both conditions are only fulfilled in special cases.
The error resulting from the condition fixed by our end,
however, is negligible as long as we only need a „good
estimation“. All further reflections on this topic are further
based on the incoherence of the sources.
Speed of sound or sound velocity c
Speed of sound or sound velocity c is the speed at
which the sound waves propagate. It is to set apart from
the sound particle velocity. By way of illustration, the wave
motion is represented in figure 1.01. Each point may be
a gas molecule (air particle). The particle itself effects
a harmonic vibration round its neutral position (vertical
lines A – M). Due to excursion neighbouring particles get
expelled from their neutral position, and they also effect,
to the neighbouring particle, a phase-shifted, harmonic
vibration round this neutral position. That is how the wave
propagation (blue line) is created. The gradient of the
straight line is a measure for the sound velocity.
www.advanced-mining.com
69