FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See Section 10; rule 13)
TITLE OF THE INVENTION
A Test Setup For Swish Noise Measurement On Automotive Shock Absorbers And Method Thereof


APPLICANTS
TATA MOTORS LIMITED, an Indian company
Having its registered office at Bombay House,
24 Homi Mody Street, Hutatma Chowk,
Mumbai 400 001 Maharashtra, India
INVENTORS
Mr. Milind A Bankar, Mr. B S Venkatesha Murthy,
Mr. S B Menon and Mr. A Chandrashekhar
An are Indian Nationals
of TATA MOTORS LIMITED,
An Indian company having its registered office
at Bombay House, 24 Homi Mody Street, Hutatma Chowk,
Mumbai 400 001 Maharashtra, India

PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner
in which it is to be performed.


FIELD OF INVENTION
This invention relates to the integration of devices for the measurement of swish noise or measurement of the Sound pressure level that is generated by a shock absorber when different sinusoidal motions are applied to the shock absorber.
BACKGROUND OF INVENTION
Shock absorbers can cause noise effects when mounted in a vehicle that is driven over various road profiles. The noise problems are divided in two main groups; air borne noise and structure borne noise. The present disclosure provides a procedure for measuring air borne noise. Air borne noise can be described as noise generated by the shock absorber during its operation, due to the interaction of the internal parts of a shock absorber. This noise can be heard, directly through air as the travelling medium.
A shock absorber consists of hydraulic valves that opens and closes, generating pressure variations during its operation.
Swish noise is the kind of noise that sounds like the last two letters of its name ('sir). It is the noise that is originated from oil cavitation. Oil cavitation occurs, when fluid under high pressure travels through an orifice (small opening) and expands in a chamber with low pressure. The noise contains all kinds of frequencies but most of the energy is typically found above 1000 Hz. The swish noise is one of the irritants to the car owners and perceived as poor quality of car.
OBJECTS OF INVENTION
The main object of innovation here is the development of a setup to measure the swish noise and establish a procedure to do the measurements.
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Another object of innovation is the development of a set up to accommodate all types of shock absorbers capable of measuring a range of swish noise intensities when shock absorbers are subjected to stroking, at different velocities, amplitude of stroke and wave forms.
Yet another object of innovation is the development of a setup that shall be simple for operation, effective in measuring (especially in the range of 55 dB to 95 dB and cost effective.
SUMMARY OF INVENTION
This invention is related to a setup for objective measurement of swish noise that arises due to the movement of the piston rod over the body, The focus of this invention is to test, evaluate and measure the intensity of the swish noise.
Accordingly, the present invention discloses an apparatus for the measurement of swish noise that is generated by a automotive shock absorber comprising: a hydraulic actuator mounted on a base frame for stroking the shock absorber; an adjustable top frame supported between two vertical pillars; an upper mounting fixture attached to said top frame and a lower mounting fixture attached to said hydraulic actuator for suspending said shock absorber therebetween; a noise recording means for recording the swish noise levels of the shock absorber at various test velocities; and an anechoic chamber whose inner surface is bonded with noise insulating material, said anechoic chamber housing said upper and lower mounting fixtures, shock absorber suspended between said fixtures and said noise recording means.
The invention also discloses a method of assembling the apparatus as claimed in any one of the preceding claims comprising the steps of; mounting the anechoic chamber on a platform which in turn is mounted on the base frame; vertically mounting the top portion of the shock absorber to the top frame by means of said upper mounting fixture and the bottom portion of the shock absorber to the hydraulic actuator by means of said lower mounting fixture; mounting the noise recording means on a
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vertical stand inside the anechoic chamber; and connecting the noise recording means to a display outside the anechoic chamber.
The invention also discloses a method of measuring swish noise that is generated by a automotive shock absorber using an apparatus as claimed in claims 1 to 9 comprising the steps of: priming the shock absorber at predetermined velocity and excitation frequency before taking the actual measurement; stroking said shock absorber to record the swish noise; and measuring and displaying the swish noise produced by said shock absorber at various test velocities.
The noise measuring means according to this invention could be a microphone capable of measuring a swish noise of about 55 to 95 dB
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 shows the layout of the shock absorber mounting on the test rig along with the arrangement for mounting the anechoic chamber and noise sensor Fig.2 shows the details of the anechoic chamber used for isolating the ambient noise. Fig.3 shows the upper mounting fixture used for mounting shock absorber on the test
rig
Fig.4 shows the lower mounting fixture used for mounting the shock absorber on to
the test rig.
DETAILED DESCRIPTION OF INVENTION
Referring to the drawings, wherein the details are only for the purpose of illustration of a preferred embodiment, and not for the purpose of limiting the same.
This invention is related to the setup used for the measurement of swish noise in the automobile shock absorbers, which the passenger experiences when the vehicle passes over the speed breakers and rough patch of road.
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Test rig setup:
• Acoustic environment:
The environment in which the test is conducted should be in such that free field conditions are simulated in the anechoic chamber.
• Background noise:
For the test to be performed efficiently the chamber should be well insulated from the external noise which might interfere with the 'swish' noise (that has to be uniquely identified). So the total power of the external noises should be less than 40 db.
• Shock absorber mounting:
The shock absorber shall be mounted vertically, body actuated, piston rod fixed with mountings as on vehicle, i.e. including bushings or top mount.
• Measured parameter:
Sound pressure level (SPL) in decibels (dB) in the vicinity of the shock absorber body, by means of a good quality sound meter.
• Test condition: at 0.1, 0.3 and 0.6 m/sec shock absorber stroking velocity.
Measurement system:
• Customized software for operating the test rig.
• Customized DAQ Board hardware for acquiring and processing the measured data.
Details of rig: The test rig basically comprises of a vertical actuator (1) used
for stroking the shock absorber (2). Jt has a base frame (11) which is grouted firmly in to the ground.The vertical actuator (1) is housed in to this base frame. Top frame (3) is supported by two pillars. Top frame can be adjusted vertically upwards & downwards depending on the length of the shock absorber to be tested. It is clamped to the suitable height by hydraulic pressure. The movement of this servo hydraulic actuator is controlled through an electronic controller.
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Details of chamber: Anechoic chamber (6) is specifically fabricated for the
measurement of swish noise. An Anechoic chamber is made up of tubular steel structure. Inner surface of the chamber is bonded with special type of acoustic foam which is used to isolate the external noises entering in to the chamber. Type of the foam used is Melamine Foam with linear wedges. It is made up of open cell melamine or polyurethane. The foam is fibre free and a high performance absorption foam. The foam is natural white in colour & is fire retardant. Two Sizes of the foam can be used viz. Thickness: 2" (Standard panel) & Thickness: 6" (Max wedge). It is bonded to the inner walls of the chamber by means of an adhesive.
Details of chamber mounting: Anechoic chamber used for isolating the noise has an opening at top & bottom so as to provide accessibility for shockabsorber mounting fixtures & shock absorber on to the test rig through suitable threaded fasteners, after mounting the chamber on to the test rig. It has a door on the front side which is used to access the internal parts like mounting fixtures, shock absorber, adjustment of microphone from front side prior to test. Once all the parts are in place the door has to closed firmly so as to isolate external noises.
Fixture details: Fig. no.3 & fig no.4 shows the end fixtures which are used for mounting the shock absorber on to the rig.Consrtuction of these fixtures is made according to the end design of the test shock absorber. Mounting fixtures are fabricated out of sheet metal of thickness 3 to 5 mm.This adaptors are mounted on to the rig after the chamber mounting.They are secured to the rig with proper fasteners of the required sizes.
Shock absorber details: This is the main test item of which the swish noise is to be measured. This swish noise is the result of the cavitation effect of the hydraulic oil which is compressed & expanded in the damper body due to upward & downward motion of the damper.lt consists of circular end mountings inserted with the rubber bushes. This end mountings are welded to the damper body.The shockbbsorber is mounted on to the vehicle by means of this end mountings.The
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rubber bushes prevents metal to metal contact.The damper body houses a piston assembly, riveted to the metallic solid rod, at the bortom.The base valve is mounted at the bottom of the cylinder. Piston reciprocates within the cylinder & displaces the hydraulic oil in the cylinder through the base valve & absorbs the shocks.The performance of this shock absorber is measured in terms of force & displacement both in compression & rebound stroke.
Test procedure: Initially mount the anechoic chamber (6) on the platform (10) which in turn is mounted on the rig base frame (1 l).Place the chamber appropriately prior to the assembly of upper mounting fixture (4) & lower mounting fixture (5) on to the rig.The details of the anechoic chamber are shown in fig. no.2 & explained earlier.The details of the end mounting fixtures used for holding the shockabsorber are shown in fig.no.3 & fig.no.4 & explained earlier.
Now mount the test shock absorber at the top to a frame (3) with the help of an upper mounting fixture (4). Bottom is held to the vertical actuator (1) by means of a lower mounting fixture (5). The test shock absorber, of which the swish noise is to be measured, shall be mounted vertically on the test rig as shown in the schematic in fig.no. 1.
After mounting the shock absorber on to the rig and clamping it properly with the help of end fixtures, .the microphone (7) is mounted on the vertical stand (8) which is placed inside the noise chamber. The microphone used for measuring the swish noise can be vertically adjusted on to the stand depending on the length of the shock absorber. It should be placed approximately at the centre of the shock absorber (2) body stroke and distance by 100mm radially from the shock absorber outer surface. The microphone is connected to the display (9) which is placed outside the chamber for recording the noise levels in decibels (dB)
Test method:
a. Mount the shock absorber on to the actuator by means of top and bottom fixture after placing the anechoic chamber on the platform provided.
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b. Mount the microphone on vertical stand and adjust it vertically upwards or
downwards so that it is in the proximity of the shock absorber body (at the
centre of the shock absorber body at 100mm distance radially from the shock
absorber outer surface).
c. Connect the microphone to the display box kept outside the chamber and reset
it to zero.
d. After completing all the external mountings stroke the shock absorber with
the help of servo hydraulic actuator for limited cycles to ensure that the shock
absorber is mounted properly and other external instruments and displays are
placed properly. Ensure that they do not foul with any other part, also ensure
there is no undesired noise generated which may obstruct the measurement
activity.Before taking the measurements close the door of the anechoic
chamber firmly .Ensure that the top & bottom of the chamber are sealed
properly prior to start of the measurements.Ensure that the ambient noise is
minimum during the test.
Test conditions:
Following test conditions are used for the measurement of swish noise of a shock absorber on the test rig.
1. Priming of the shock absorber: 5 cycles at 100 mm stroke at 0.3 m/sec shock absorber velocity before taking the actual measurement.
2. Exciting amplitude: +/- 50 mm, sinusoidal waveform .This can be decided on the stroke of the shock absorber under test.
3. Test velocities: 0.1, 0.3, 0.6 m/sec: At each velocities record the noise level in db on the display mounted outside the chamber. Repeat the readings for three times to ensure the repeatability and consistency in the measurement.

WE CLAIM
1. An apparatus for the measurement of swish noise that is generated by an
automotive shock absorber comprising:
a hydraulic actuator mounted on a base frame for stroking the shock absorber;
an adjustable top frame supported between two vertical pillars;
an upper mounting fixture attached to said top frame and a lower mounting fixture attached to said hydraulic actuator for suspending said shock absorber therebetween;
a noise recording means for recording the swish noise levels of the shock absorber at various test velocities; and
an anechoic chamber whose inner surface is bonded with noise insulating material- said anechoic chamber housing said upper and lower mounting fixtures, shock absorber suspended between said fixtures and said noise recording means.
2. The apparatus as claimed in claim 1, wherein said anechoic chamber is made of tubular steel.
3. The apparatus as claimed in claim 3, wherein said foam is a Melamine foam with linear wedges preferably natural white in colour and fire retardant with a thickness of approximately 2 to 6 inches.
4. The apparatus as claimed in claim 1, wherein an opening is provided at the top, bottom and front side of said anechoic chamber to provide accessibility for upper and lower mounting fixtures, the shock absorbef and for adjustment of said noise recording means.
5. The apparatus as claimed in claim 1, wherein said noise recording means is a microphone capable of measuring a swish noise of about 55 to 95 dB located preferably at the centre of the shock absorber body preferably at 100mm distance radially from the shock absorber outer surface.
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6. A method of assembling the apparatus as claimed in any one of the preceding
claims comprising the steps of;
Mounting said upper and lower mounting fixtures at top frame and base frame respectively;
mounting the anechoic chamber on a platform which in turn is mounted on the base frame;
vertically mounting the top portion of the shock absorber to the top frame by means of said upper mounting fixture and the bottom portion of the shock absorber to the hydraulic actuator by means of said lower mounting fixture;
mounting the noise recording means on a vertical stand inside the anechoic chamber;
connecting the noise recording means to a display outside the anechoic chamber.
7. A method of measuring swish noise that is generated by a automotive shock
absorber using an apparatus as claimed in claims 1 to 6 comprising the steps of:
priming the shock absorber at predetermined velocity and excitation
frequency before taking the actual measurement;
stroking said shock absorber to record the swish noise; and
measuring and displaying the swish noise produced by said shock absorber at
at test velocities of 0.1, 0.3, 0.6 m/sec.
8. The method as claimed in claim 7, wherein said shock absorber is excited at an amplitude of+/- 50 mm with sinusoidal waveform.
9. An apparatus for the measurement of swish noise that is generated by a automotive shock absorber substantially as hereinabove described with reference to the accompanying drawings.
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10. A method of assembling the apparatus for the measurement of swish noise that is generated by a automotive shock absorber substantially as hereinabove described with reference to the accompanying drawings.
Dated this 6th day of March 2009