Format of Single Set Question paper for final Test
Birla Institute of Technology & Science, Pilani
Work Integrated Learning Programmes Division
First Semester 2024-2025
Question paper
Final Regular Test
(EC-3 )-set 1
Course No.
Course Title
Nature of Exam
Weightage
Duration
Date of Exam
: AEZG 522
: Advanced Vehicle Acoustics
: Open Book
: 30% or 35% (As per Course Handout)
: 2 & ½ Hours
: Nov/.Dec./2024
Total marks = 80
No. of Questions = 5
Q. 1 A Bus Cabin floor was resonating with average surface velocity of its vibrations measured to be 0.3
mm/sec at 60km/hr constant speed running on highways. When this Bus was offered to Govt. Authorities,
they asked for minimum 5 dB reduction inside the cabin at the same speed.
Speed of sound inside the cabin = 340 m/sec, density of air = 1.2 kg/m^3; radiation efficiency of the bus
floor =0.5; the floor length = 9 m and width = 3 m; sound power reference for dB calculations = 10^(-12)
Watts.
There are no other major sources of noise inside the cabin apart from the floor resonance at this 60 km/hr
of running. It was decided to work on the cabin-floor.
What should be % reduction in the average surface velocity of the vibrations targeted by the engineers to
get the certificate from the Govt. Authorities ?
What minimum 3 design modifications in the Bus do you suggest for achieving the above reduction in the
floor vibrations reduction if this is a diesel engine operated one?
[10 marks]
Q. 2 There are major 5 noise sources whose individual contributions to Driver’s ear level
were measured by Transfer Path Analysis for an electric Bus at its Top speed 80 km/hr on
road as below
Motor
Differential Gear box
Tyre/road
Wind noise
Cooling Fan
60 dBA
65 dBA
70 dBA
70 dBA
75 dBA
It is decided to reduce the resultant noise level at the Driver’s ear level with 3 design
modifications:
(a) Quieter tyres were developed to reduce its noise contribution from 70 to 68 dBA (b) the
Bus-body sealing got improved to reduce the wind noise contribution to 65 dBA from
70 dBA (c) cooling Fan speed was optimized to reduce its contribution from 75 to 72
dBA.
With these modifications, will the resultant E- Bus In-cab noise level will meet the
benchmarking level of 75 dBA ? As A Chief Engineer, will you be happy with the design
changes or work more on it?
(10 marks)
Q. 3 What are different orders of all possible excitations for both vibrations & noise in below 2
passenger cars?
[i] a diesel engine powered car: 4 cylinders --maximum RPM 4800, gear-box : no. of
teeth - pinion =18, no. of cooling fan blades = 32, turbo-charger RPM =90,000, no. of
turbine blades=12 [assume that the fan is fitted on the crankshaft]
[ii] an electric car: at its maximum motor RPM 9000; no. of poles = 8, no. of stator
coils = 32, no. of rotor slots = 24 ; no. of gear teeth = 20
(16 marks)
Q. 4 Are challenges of NVH refinement of Electric cars same as those of ICE powered cars?
You may Build up your argument with respect to:
[a] sound power balance between these different category of vehicles
[b] special design challenges to control their noise & vibrations
[4 marks]
Q.5 For various below noise sources of a vehicle running over road, mention minimum 2
countermeasures to mitigate them. Briefly discuss about each one of them as a Manager if you
will find any challenge in taking the NVH solutions to production while balancing other vehicle
performance attributes including cost of ownership of the vehicle!
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
IX.
X.
Combustion noise of a diesel engine
Turbo-whistle
Cabin boom induced by Driveline resonance
Exhaust noise
Valve Impact
Brake squeal
Wind noise
Piston slap
Inverter Buzz of an Electric Vehicle
Road noise inside the cabin
[40 marks]