Interior Acoustics. Lecture delivered by: Kiran V. R. Asst. Prof. Dept. of AAE MSRSAS-Bangalore. M.S Ramaiah School of Advanced Studies - Bangalore

Transcription

1 Interior Acoustics Lecture delivered by: Kiran V. R. Asst. Prof. Dept. of AAE MSRSAS-Bangalore

2 Outline of the Presentation Introduction to acoustics Different noise sources Types of vehicle noise Human Perception to sound Concepts of Source - Path - Receiver Noise reduction techniques Classification of vibration isolation Active noise cancellation Speaker localization in vehicles 2

3 What is Acoustic? Acoustic Infrasound Sound Ultrasound 20 to 20,000 Hz - Human audible range 20 to 200 Hz -Can be felt and heard 3

4 Ranges of Frequency Demo 4

5 Acoustic Classification Vehicle Acoustic Interior Noise Exterior Noise Driving Noise Constant Speed Actuation Noise Door Closing Communication/ Audio Disturbing Noise Body Resonance Gear Box Noise Idle Pass-by Acceleration Servo Actuators Engine Noise Road Noise Squeak/Rattle Perception Regulations 5

6 Variety of Vehicle noises within audible range Frequency (Hz) 6

7 What do we perceive? A Vibration under 20Hz can be felt by human beings 7

8 What do we feel and hear? Feel and hear the frequencies between 20 Hz and 200 Hz 8

9 What do we hear? Hear frequencies between 200 Hz and 20 khz 9

10 What are the typical noise sources in an automobile? 10

11 Passenger Car - Noise Sources 11

12 Typical Noise Contributions from Various Systems Other Structural Components Miscellaneous 3% Fan & Radiator Assembly 6% 10% Engine 21% Drive Line 14% Intake 14% Exhaust System 32% 12

13 Sources of Vibration in Automobiles Engine Torque fluctuation in the engine Vertical motion of the pistons and connecting rods and the associated imbalances 13

14 Sources of Vibration in Automobiles Piston Slap Piston slap is influenced by piston clearance More pronounced when the piston is cold. Overcome by offsetting the piston By adding stiffeners to the piston 14

15 Sources of Vibration in Automobiles Unbalanced Rotating Components 15

16 Sources of Vibration in Automotives Valve Train 16

17 Sources of Vibration in Automobiles Transmission 17

18 Sources of Vibration in Automobiles Engine and Transmission Mount Transmission mount Transmission mount bracket Engine mount 18

19 Sources of Vibration in Automobiles The Drive Train 19

20 Sources of Vibration in Automobiles Intake and Exhaust Noise 20

21 Sources of Vibration in Automobiles Exhaust system connected to engine directly 21

22 Sources of Vibration in Automobiles Radiator Fan Noise 22

23 Sources of Vibration in Automobiles Noise from Steering Wheel Squealing and beat noise 23

24 Sources of Vibration in Automobiles Tires and the Road 24

25 Sources of Vibration in Automobiles Brake Squeal Scraping Grinding Rattling Groaning Squealing Improper adjustment Dust or foreign material Rust 25

26 Sources of Vibration in Automobiles Brake Squeal 26

27 Sources of Vibration in Automobiles Wind Generated Noise Noise caused by protrusions of the vehicle body. 27

28 Sources of Vibration in Automobiles Wind Generated Noise Noise caused by the open windows and/or gaps in vehicle body. 28

29 Sources of Vibration in Automobiles Wind Generated Noise 29

30 Transmission of Noise 30

31 Noise Transmission Structure borne sound Vibrations transmitted through solid structures Airborne sound Sound transmitted through the air as a differential in air pressure 31

32 Passenger Car - Noise Paths 32

33 Noise Sources and Transfer Paths S S A A S S S S S A S S S A S S S S A S A S S S S S A S S A S S S 33

34 Types of Vehicle Noise Booming Rattle Beat 34

35 Booming Noise Booming from intake / exhaust system Booming caused by vibration of the drive shaft 35

36 Booming Noise Booming caused by non-uniformity of tires Booming caused by vibration of the drive line 36

37 Booming Noise Booming Noise at low speed (30 to 40 km/h) 37

38 Booming Noise at low speed (30 to 40 km/h) Sound: Frequency: 30~100 Hz Probable cause - Resonance from exhaust pipe. - When engine is running, the vibration is transmitted from engine to exhaust pipe. - If engine vibration and pipe vibration reach resonance, then the vibration from exhaust pipe is high. - Exhaust pipe vibration is transmitted to the structure through muffler hanger and muffler support. - Noise is emitted from the exhaust pipe and exhaust noise is transmitted through the body. 38

39 Booming Noise at Medium & High speed Booming Noise at Medium speed (40 to 80 km/h) and High speed (over 80 km/h) 39

40 Booming Noise at Medium & High speed Probable cause - Resonance of Drive shaft If engine vibration frequency and drive shaft vibration frequency are close to each other, then the drive shaft vibration is amplified. This amplified vibration is transmitted to the vehicle body through knuckle and suspension bush and it makes noise. Remedy Generally, long drive shaft can resonate easily at 100 km/h. In this case, the problem may be solved using a dynamic damper. 40

41 Beat Noise Beating noise is a sound synthesized from two sounds. A beating noise having a frequency 2 ~ 6 Hz is mostly noticeable. When two separate sounds with slightly different frequencies, A and B combine, their volume varies cyclically, resulting in a beating noise. The volume (amplitude) of the noise alters cyclically, coming in waves. The frequency of the noise increases with the increase in vehicle speed. 41

42 Beat Noise There are two main causes for the so-called beat-noise (which occurs during driving). 1). Noises caused by the combination of vibrations resulting from the slippage of the torque converter in automatic transaxle are combined. 2). Noises caused by the combination of vibrations from the engine and vibrations from auxiliary equipment (cooling fan, generator, power steering pump or air compressor for air conditioning system). When two high points are overlapped, they make a higher point. When two low points are overlapped, they make a lower point. 42

43 Beat Noise Vibration by slippage of the torque converter Pump Impeller Turbine Runner Stator 43

44 Rattle Noise Sway bar link connector Body or frame mounted sway bar bushing 44

45 Noise Reduction Techniques 45

46 Noise Reduction Techniques 46

47 Engine Mounts Although engine mounts are very small, they have a very important role to play in controlling noise 47

48 Sound Panel Proofing 48

49 Absorbers Absorptions of Noise inside the Vehicle 49

50 Absorbers Absorbers reduce sound and vibrations by absorbing the energy of the sound waves and dissipating them as heat (the heat generated is so small it can only be detected by very sensitive instruments). e.g. : Carpeting 50

51 Barriers Barriers perform a blocking function to the path of airborne sound 51

52 Sound Barrier Applications Foam barrier within the car body structure to suppress airborne noise transmission 52

53 Insulators Sound Insulators in a Vehicle 53

54 Damping Materials Sound Damping Materials in a Vehicle 54

55 Wind Noise Reduction Geometric changes in side mirror to reduce in wind noise A B Mirror geometry design change: Model A (above) and Model B (right) and models overlaid together (below) to show geometric differences. 55

56 Wind Noise Reduction A B Velocity magnitude variation Model A (left) has a significantly larger aerodynamic wake than Model B (right) 56

57 Wind Noise Reduction Aerodynamic/aero-acoustic simulations can quickly identify areas of wind noise created by components db map over side glass surface 57

58 Wind Noise Reduction Mercedes-Benz SLK-Class: aero-acoustics testing in wind tunnel 58

59 Windshield Wipers Aero wiper features a single-section rubber strip design (joint-free wiper ), with an integral spoiler and sprung rails positioned on the outside to curve the blade to the windscreen. 59

60 Classification of Vibration Isolation Vibration Isolation Active Isolation Passive Isolation 60

61 Classification of Vibration Isolation Active Vibration Isolation - Servomechanisms with sensor based measurement and processor based actuation methodology Passive Vibration Isolation - Metal springs, Rubber pads, Cork, Felt mats, Pneumatic spring supports 61

62 Types of Vibration Isolators Hydraulic engine mounts Elastomeric mounts Bushings Pads Hybrids (springs, pads and/or levelers) Air springs (piston + cylinder) Exotic (air tables, magnetic levitation, active) 62

63 Vibration Isolation Materials Elastomers Combinations/Laminates Steel Springs Wire rope Air Felt Cork 63

64 Active Noise Control Simple sine wave for single sound frequency + = Sum of two waves slightly out of phase One pure sound a fraction of a second after the next 64

65 Active Noise Control Sound cancellation If the sound waves were 180 or one-half a wavelength out of phase, the sum of the waveforms would be zero. They would cancel out each other and there would be no sound. 65

66 Active Noise Cancellation Noise cancellation is a method to reduce or completely cancel out undesirable sound. Active Noise Cancellation because the electronics involved actively cause the noise cancellation in real time. In the case of a single sound frequency, addition of the same frequency 180 out of phase to cancel the sound. The most common electronic noise cancellation device consists of special earphones. 66

67 Active Noise Control Noise cancellation in car interiors Acura RL The system uses a microphone to hear what the driver and passengers hear, analyzes it with an onboard computer, and pipes out a canceling sound via the stereo system. 67

68 Application Head sets High end cars Space satellite antennas Apartments 68

69 Speaker Localization in Vehicles Best listening environment : soft, no echo and sound-absorbing walls Problem area in car interiors : hard surfaces, acute angles and dead spaces 69

70 Locating Speakers in Vehicles Primary objectives in speaker placement is to achieve time alignment. A difference of even a fraction of a second reduces the realism and quality of the sound. Speakers mounted in the dash typically fire into the windshield creating all sorts of troublesome acoustic issues. Better locations in the doors and kick panels. 70

71 Interior Noise - Challenges No Regulation - But affects Product Acceptability Requirement of More absorptive Barriers - High Cost. Experimental Verification Testing, Re-Design - Expensive Analytical Predictions - Computing Resources and Material Characteristics. Perception Dependent - Jury Required 71

72 Summary/Conclusion Different noise sources and types of vehicle noise have been discussed Concepts of Source - Path - Receiver have been discussed Noise reduction techniques have been discussed 72

73 Anechoic and Reverberant Enclosures Anechoic room Reverberation room 73

74 Anechoic Enclosures