FIELD OF INVENTION
The invention relates to a device which provides a means of faithfully
transmitting sound waves from its source to its detector such that sound level
measurements may be carried out remotely. The sound can originate from an
environment, wherein, the survival of an acoustic sensor becomes difficult due to
adverse conditions and a need arises to place the sensor at an external and / or
distant location. Such systems may include but not limited to steel plant,
chemical plant, boilers and pressure vessels industries.
BACKGROUND OF THE INVENTION
Numerous conditions are routinely encountered in industrial installations where
high temperatures, dust and moisture levels or even presence of ionizing
radiation makes for a hazardous environment. Nevertheless these locations often
need to be serviced by adequate instrumentation as a means of process control.
Over the years several sensors have been developed including detection and
measurement of sound energy. While many sensors are amenable to
ruggedization to make them withstand harsh environment, most sound sensors
are too delicate and fragile for use in industrial setup. Detection of sound waves

first involves conversion of the acoustic pressure waves into faithful vibrations of
a solid object called a diaphragm, which is subsequently measured. Since the
pressure exerted by acoustic waves is very small, they exert a very small force
on the diaphragm. For the diaphragm to appreciably move in accordance to the
acoustic force, it needs to be extremely light and flexible. This particular element
in any acoustic sensor remains the weakest link making it unsuitable for
industrial use.
SUMMARY OF THE INVENTION AND OBJECTS
The present invention proposes the use of an acoustic wave guide to faithfully
transmit the required sound energy from the point of origin across a finite
distance. While the original source of the sound may be too harsh for the sensor
to survive in, the sensor may be placed at the other end of the wave guide a
sufficient distance away for it to be placed in a suitable environment. Additionally
to stop ingress of dust into the waveguide, thin flexible diaphragms may be
transversely placed at a point sufficiently upstream of the sensor. A slow stream
of gas can also be made to flow in the wave guide to maintain sufficient positive
pressure to stop dust ingress.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a schematic setup of an acoustic wave guide and a sensor
as may be installed in practice.
Figure 2 Shows the result of sound intensity measurements from 20 Hz to 2
KHz as a function of time, with and without the proposed wave
guide.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE
INVENTION
The invention aims to provide sufficient service life for acoustic sensors utilized
for sound level measurements in harsh environments. An acoustic wave guide of
finite length would provide a lot of flexibility in placement of the sensor
sufficiently away from the source in a less harsh environment. The wave guide
itself contains two subsystems which need to be designed as per the particular
requirement.
1. Conduit – The wave guide itself should be made of rigid material with
smooth interior walls without any exposed internal seams which would
introduce artificial sound artefacts arising from multiple echoes. Required
number of bends may be added along the length of the conduit to realize

the proposed placement of the sensor. The diameter of the conduit (2)
should have an order of magnitude between 3 and 30 cm in diameter and
the radius of curvature for each bend should be at least 3 times the
diameter of the conduit. However if due to engineering constraints,
conduits of different diameter or radius of curvature be used, the system
would continue to work albeit with lower accuracy in subsequent
measurement.
The length of the wave guide while constrained by the physical
distance between the face of the sound source (1) and the sensor (6) has
an effect on the measured sound levels. For correctly determining the
length, it should be understood that due to resonance, some frequencies
of sound would be amplified by constructive interference which other
frequencies would be severely attenuated. Thus a pre-knowledge of the
target frequency band (its mean / average frequency, f in Hz) is essential.
Thus λ=V/f is the wavelength in meters of the sound waves to be
measured where V is the speed of sound in meters/second in the medium.
For the wave guide to transmit the required frequency range of sound,
the length should be an integral multiple (n) of λ/4. Thus a wave guide
designed under these principles of acoustics limits the usefulness of the
device as a narrow wavelength, and hence frequency, of sound can be

faithfully transmitted across it. Industrial instrumentation involving
measurement of sound energy often requires it be done across a wider
bandwidth than that can be offered by the wave guide. Additionally the
length of the wave guide is also restricted when the distance between the
source and sensor is not an integral multiple of the quarter wavelength.
This constraint on conduit length and the specificity of sound wavelength
and frequency can be overcome for longer pipes where values of n
become greater than 5. At these longer pipe lengths, the phenomenon of
constructive and destructive acoustic interferences become destabilized,
and the wave guide would faithfully transmit sound of all frequencies
equally, as experimentally showed in figure 2. However even a perfectly
designed acoustic wave guide may provide some degree of additional
attenuation or amplification across the frequency spectrum and the sensor
data should be used along with a static (frequency specific) correction
factor if required. For better performance the wave guide may be
wrapped with sound insulating materials like foam-wrap (5), fibreglass to
insulate it from other extraneous sound not generated by the source in
question.
2. Dust suppression system – Presence of dust in the system in question
would quickly clog the wave guide and worse, damage the sensor

attached at the other end. Ingress of dust may be attenuated by placing a
thin flexible metal diaphragm (4) transversely across wave guide at any
point upstream of the senor. The diaphragm may be placed as close to
the sound source as possible to isolate the maximum length of the conduit
from dust ingress. However the survivability of the thin metal piece very
close to the harsh environment of the sound source should be taken into
account and the placement should be sufficiently away from the source.
A slight flow, around 0.5 to 2 lpm of an inert gas (3) can be made to flow
from the region of the diaphragm towards the open end of the wave
guide in the sound source. This would provide sufficient positive pressure
to prevent dust ingress into the conduit.
It will be understood that the type of setup in accordance with present invention
has following advantages
a. Provide a means to carry out sound level measurement in a harsh
environment encountered in industry.
b. Use economical acoustic sensors for instrumentation with longer service
life.
c. Placement of sensor away from hazardous environment such that
maintenance becomes safer, quicker and economical.

Figure 2 shows the result of a test where targeted sound level measurements
were carried out both with and without a wave guide. The recorded data
suggests little attenuation in sound levels as a result. The wave guide had a
length of 4 meters and the frequency range of measurement is 2 Hz to 2 KHz.
Although particular embodiments of the invention have been shown and
described in full here, there is no intention to thereby limit the invention to the
details of such embodiments. On the contrary, the intention is to cover all
modifications, alternatives, embodiments, usages and equivalents as fall within
the spirit and scope of the present invention, specification and appended claims.

WE CLAIM
1) An acoustic wave guide (7) which transmits sound energy equally with
minimal specificity across the target bandwidth, wherein the overall length of
the device remains in excess of five times the quarter wavelength
corresponding to the lower frequency limit of the bandwidth to be measured,
the amount of transmitted sound energy bearing sufficiently correlation with
that of the source to carry out quantative estimation of the latter.
2) An device which physically separates an acoustic sensor 6 from the face of
the sound source comprising:

- conduit face (1) of sound source
- conduit pipe (2) integral to face 1, with finite length,
characterized by rigidity of the material with smooth interior wall without
any exposed internal seams, having diameter of (3-30) cm and radius of
curvature for each bend of the conduit being at least 3 times the diameter
of the conduit.
3) The device as claimed in claim 1, wherein, a flexible metal diaphragm (4),
transversely across the wave guide, at any point upstream of the sensor,
closer to sound source, isolating maximum length of the conduit from dust
ingress

4) The device as claimed in preceding claims, wherein, an inert gas inlet point
(3), allowing 0.5 to 2 Ipm gas to flow through the region of the diaphragm
towards the open end of the wave guide to provide positive pressure
preventing dust ingress in to the conduit.
5) The device as claimed in preceding claims, wherein sound insulating materials
like foam wrap/fibre glass (5) to insulate the system from other extraneous
sound
6) The device as claimed in the preceding claims, wherein, the sensor (6) to
receive the sound wave to determine the sound level within the system.
7) A method of guiding an acoustic wave in the process of determining sound
level in a harsh environment encountered in industry, by way of incorporating
conduit face (1), conduit piping (2), diaphragm (4), gas inlet point (3), sound
insulator (5), sensor(6)