akustik dan getaran - 1 introduction

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Jurusan Teknik Mesin dan Industri, Fakultas Teknik, Universitas Gadjah Mada Akustik dan Getaran TKM 4501 / 3 SKS Dr. Indraswari Kusumaningtyas, ST., MSc.

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Page 1: Akustik Dan Getaran - 1 Introduction

Jurusan Teknik Mesin dan Industri, Fakultas Teknik, Universitas Gadjah Mada

Akustik dan GetaranTKM 4501 / 3 SKSDr. Indraswari Kusumaningtyas, ST., MSc.

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Akustik dan Getaran Mata kuliah pilihan

Tujuan: Mahasiswa memahami dasar-dasar akustik yang terkait dengan

bidang teknik, dasar dari konsep pengukuran akustik, serta aplikasi akustik dan getaran di bidang teknik dan industri.

Materi kuliah: Sebelum UTS: Dasar-dasar Akustik Ari Setelah UTS: Aplikasi Akustik dan Getaran I Made Miasa

1

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Cakupan Materi Akustik Basic concepts of acoustics (sound waves, sound pressure, acoustic

impedance, sound power, sound intensity, weighting functions, loudness, frequency analysis)

Wave equations, plane wave, spherical waves, standing waves Acoustic Measurement and Instrumentation Human Hearing and Noise Criteria Sound Radiation and Propagation Industrial Noise Sources Noise Control Techniques (sound absorbing materials, sound absorbers,

noise partitions, noise enclosures, sound barriers, silencers, vibration isolation, quiet structures, active noise control).

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Buku Referensi

Slides dibuat berdasarkan kedua buku ini: Noise Control: From Concept to Application.

By: Colin Hansen. Publisher: Taylor Francis.

Industrial Noise Control and Acoustics. By Randall F. Barron. Publisher: Marcel Dekker.

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Evaluasi

Ari = 50% Tugas = 15% (Tugas) UTS = 35% (Take Home)

Made Miasa = 50%

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The Science of Sound“If a tree falls in a forest and no one is there to hear it, does it make

a sound?”

Two approaches to define “sound”:1.Physical acoustics : sound as physical quantities

Pressure, amplitudes, frequencies, spectra2.Psychoacoustics: perception of sound

Loudness, pitch, timbre

Acoustics is the science of sound, that is, wave motion in gases, liquids and solids, and the effects of such wave motion.

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From Vibration to Acoustics Simple harmonic motion:

One mass, one spring Stiffness One natural frequency Additionally, with damping

Simple vibrating system: Pendulum (small angle) A Helmholtz resonator

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From Vibration to Acoustics Systems with two or more masses:

Two or more natural frequencies Two or more natural modes

From vibration to sound common example: musical instruments Vibrating string Vibrating membrane Vibrating bar Vibrating plates Tuning fork Air-filled pipe

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Noise Control Principles Three basic elements in any noise control system:

The source of the sound The path through which the sound travels The receiver of the sound

There may be many sources, various paths, and many receivers, but the basic principle of noise control remain the same.

Noise control is accomplished by modifying any of the elements.

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Noise Control Principles The source of noise or undesirable sound may be a vibrating surface,

a mechanical shock, mechanical friction, fluid flow, a flame burst or an explosion. It is important that the acoustic engineer identify all possible noise

sources when considering a solution for a noise problem. The path for the sound may be the air between the source and

receiver, as is the case for machinery noise transmitted directly to the operator’s ears. The path may also be indirect, such as sound being reflected by a wall

to a person in the room (or through ventilating ducts to another room). The receiver in the noise control system is usually the human ear,

although the receiver could also be sensitive equipment that would suffer impaired operation if exposed to excessively intense sound.

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Noise Control Principles The goal of a noise control procedure is normally to reduce the

acoustic noise. However, the actual application may be quite varied prevention of hearing loss for personnel, to allow effective face-to-face communication or telephone

conversation, or to reduce noise so that neighbors of the facility will not

become intensely annoyed with the sound emitted by the plant.

The engineering approach is often different in each of these cases.

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Source of Noise

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Noise Control At the Source Modifications at the sound source are usually considered to be the

best solution for a noise control problem. The major measures include reducing forces, modifying

components, replacing parts, substituting materials, stiffening structures, changing processes, and minimizing fluid flow turbulence.

Where impacts are involved, as in punch presses, any reduction of the peak impact force will dramatically reduce the noise generated. Components of a machine may be modified to reduce the

mechanical shock and produce a significant change in noise emission.

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Noise Control At the Source Noise at the source may indicate other problems, such as a need

for maintenance. For example, excessive noise from a roller bearing in a

machine may indicate wear failure of the bearing. Replacement of the defective bearing may solve the noise

problem, in addition to preventing further mechanical damage. There may be areas, such as panel coverings, that vibrate

excessively on a machine. The noise generated by large vibrating panels can be reduced

by applying damping material to the panel surface, applying stiffeners to the panel surface, or by uncoupling the panel from the vibrating force, if possible.

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Noise Control At the Source In most cases, reducing the amplitude of vibratory motion of

elements in a machine will reduce the noise generated by the machine element. For example, making a panel stiffer by increasing the panel

thickness or reducing the panel dimensions or using stiffening ribs will reduce the amplitude of vibration of the panel.

Reduction of noise resulting from the resonant vibration of structures may be achieved by ensuring that machine rotational speeds do not coincide with resonance frequencies of the supporting structure. If there is coincidence, change the machine speed or change

the resonance frequencies of the structure by changing its stiffness or mass.

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Noise Control At the Source Substitution of materials in mechanical components may help

reduce noise introduced by these components. For example, replacing metal with plastic in vibrating

structures, using plastic gears to replace metal gears. A change in the process may also be used to reduce noise.

For example, instead of using an air jet to remove debris from a manufactured part, rotating cleaning brushes may be used.

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Noise Control At the Path

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Noise Control At the Path Modifying the path through which the noise is propagated is often

used when modification of the noise source is not possible, not practical, or not economically feasible.

For noise sources located outdoors, one simple approach for noise control is to move the sound source farther away from the receiver, i.e. make the noise path longer.

Other possible measures include noise barriers, enclosures, mufflers, lined ducts, vibration isolators, absorbers and dampers, and sound absorbing materials.

For noise sources located outdoors or indoors, the transmission path may be modified by placing a wall or barrier between the source and receiver.

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Noise Control At the Path Reduction of traffic noise from vehicles on freeways passing near

residential areas and hospitals has been achieved by installation of acoustic barriers along the roadway.

The use of a barrier will not be effective in noise reduction indoors when the sound transmitted directly from the source to receiver is much less significant than the sound transmitted indirectly to the receiver through reflections on the room surfaces. For this case, the noise may be reduced by applying acoustic

absorbing materials on the walls of the room or by placing additional acoustic absorbing surfaces in the room.

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Noise Control At the Path A very effective, although sometimes expensive, noise control

procedure is to enclose the sound source in an acoustic enclosure or enclose the receiver in a personnel booth. The noise from metal cut-off saws has been reduced to

acceptable levels by enclosing the saw in an acoustically treated box.

If the equipment or process can be remotely operated, a personnel booth is usually an effective solution in reducing the workers’ noise exposure.

The exhaust noise from engines, fans, and turbines is often controlled by using mufflers or silencers in the exhaust line for the device.

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Noise Control At the Receiver

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Noise Control At the Receiver In some cases, when all else fails, it may be necessary to apply

noise control to the receiver of the excessive noise. The human ear is the usual ‘‘receiver’’ for noise, and there is a

limited amount of modification that can be done for the person’s ear. One possible approach to limit the noise exposure of a worker

to industrial noise is to limit the time during which the person is exposed to high noise levels.

Hearing protectors (earplugs, acoustic muffs, or active noise cancelling headphones) can be effective in preventing noise-induced hearing loss in an industrial environment.

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What is Sound and Noise? Sound is a travelling wave, which is an oscillation of pressure

propagating through an elastic medium (solid, liquid, or gas) at a speed characteristics of that medium. It is composed of frequencies within or outside of the range of

hearing. Noise is traditionally defined as unwanted or disturbing sound

Significant health hazard possible (noise pollution) Does not typically receive as much attention as other types of

pollution Causes stress related illnesses, high blood pressure, speech

interference, hearing loss, sleep disruption, and lost productivity.

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The Nature of Sound Sound is a pressure disturbance in an elastic medium.

Sound can be measured:

Sound pressure is the incremental variation in pressure above and below atmospheric pressure

Atmospheric pressure: about 1.013 x 105 N/m2 at sea level

Sound pressure are extremely small

Normal speech: 0.1 N/m2 above and below atmospheric pressure at a distance of 1 m from the speaker.

Fractional pressure variation of around 10-6

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The Nature of Sound Travelling wave in medium:

The medium must have inertia and elasticity Inertia: permits one element of the medium to transfer

momentum to adjacent element Related to density and, hence, mass

Elasticity: produces a force on a displaced element, tending to return it to its equilibrium position

Air has density and elasticity. Sound wave is a mechanical wave which results from the back

and forth vibration of the particles of a medium. Sound wave is a longitudinal wave

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The Nature of Sound Example:

A vibrating tuning fork is capable of creating such a longitudinal wave.

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The Nature of Sound Sound is generated by pressure

variations in a medium (fluid or solid) that propagates without particle transport.

But each particle moves back and forth around its equilibrium position with a certain particle velocity

It is characterized by fundamental measurements such as Amplitude, frequency or oscillation period, wavelength and propagation speed in the medium.

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Basic Properties of Waves Wave can transport energy and information from one place to

another, but the medium itself is not transported.

Imagine moving the end of a rope

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Basic Properties of WavesFree Progressive Waves:

Move rope up and down with frequency f

We can observe the wavelength λ

We can determine wave speed c

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A Piston Sound Generator

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Microphone and Its Components

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Microphone and Its Components

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Sound WavesBecause of the longitudinal motion of the air particles, there are

regions in the air where the air particles are compressed together and other regions where the air particles are spread apart. These regions are known as compressions and rarefactions respectively.

Wavelength

Sound speed c

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Sound WavesSound is characterized by the propagation of mechanical energy

caused by a rapid succession of compressions and rarefactions in an elastic medium.This energy, which originates from a sound source, propagates through the medium in waves travelling with finite speed.

In order for sound to occur it is thus necessary to have: a “sound source” an “elastic medium”The elastic and mass properties of the elastic medium determine

the “speed” of transmission of the perturbation.

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Sound WavesSound Source

The motion of the piston creates a plane surface having a harmonic motion at one end of an infinitely-long duct filled with an elastic medium at rest.

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Sound Waves

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WavelengthThe wavelength ( λ ) of a wave is merely the distance which a disturbance travels along the medium in one complete wave cycle.

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FrequencyThe harmonic motion of the piston is characterized by the following

quantities:f = frequency, number of oscillations in a second, measured in

“Hertz” (Hz);T = period, duration of a cycle, measured in seconds (s);ω = angular speed, measured in rad/s;

f = 1/T and f = ω/2π (Hz)

Frequency = 20 – 20.000 Hz: the perturbation will be perceivable to the human ear, and it will be defined as sound or noise.

Below 20 Hz = infrasound, Above 20.000 Hz = ultrasound

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Relationship between f and

When frequency increases, the wavelength becomes smaller and smaller…

Wavelength

Frequency

The female voice has a frequency on the order of 500 Hz. The male voice has a frequency on the order of 200 Hz.

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Speed of SoundAll sound waves are travelling at about the same speed - the speed of sound c.So waves with a longer wavelength don't arrive (at your ear, for example) as often (as frequently) as the shorter waves.

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Speed of SoundThe pressure perturbation propagates from the source in the medium, with a speed of sound c0 .For an ideal gas, the speed of sound depends can be determined with the following equation:

)K(273)C(

)J/kgK(28741.1

:airFor

o

tT

R

TRc

γ = specific heat ratioR = specific gas constantT = absolute temperature

Speed of sound in air @ 20°C = 343 m/ s

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Speed of sound in air

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Speed of sound in different mediums

Speed of sound in water:

Temperature Speed of sound

Speed of sound in some common gases at 0oC and atmospheric pressure:

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Wave NumberAnother parameter that is encountered in analysis of sound waves isthe wave number (k), which is defined by:

The wave number can be regarded as the spatial frequency of a wave, either in cycles per unit distance or radians per unit distance.It can be envisaged as the number of waves that exist over a specified distance (analogous to frequency being the number of cycles or radians per unit time).

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Example: Speed of SoundA sound wave having a frequency of 250 Hz is transmitted through air at 25oC (298.2K or 77oF). The gas constant for air is 287 J/kg-K,and the specific heat ratio is γ = 1.40. Determine the speed of sound, wavelength, and wave number for this condition.

m/s 1.3462.29828740.1

K 2.298 ,J/kgK 287 ,40.1

TRc

TR

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Sound PressureThe acoustic pressure (p) is defined as the instantaneous

difference between the local pressure (P) and the ambient pressure (Po) for a sound wave in the material.

For a simple plane harmonic sound wave moving in the positive x-direction, the sound pressure can be represented by:

pmax is the amplitude of the sound pressure wave.

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Sound Pressure Most common sound consist of a rapid, irregular series of

positive and negative pressure disturbances (compression and rarefactions) measured from the equilibrium air pressure. The mean value of sound pressure disturbance is zero. Therefore pmean is not a useful measure.

On the other hand, acoustic instruments generally do not measure the amplitude of the sound pressure.

Instead, the instruments measures the ROOT MEAN SQUARE value : prms

This value is proportional to the amplitude.

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RMS Sound Pressure

TT

T

T

rms

T

rms

dtxftT

pdtT

p

dtxftT

p

dtxftpT

p

dtpT

p

0

2max

0

2max

0

2max

0

22max

2

0

22

4cos22

4cos12

2sin1

1

2

2

max

2max2

pp

pp

rms

rms

Untuk selanjutnya, umumnya prms ditulis p saja.

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Sound Pressure

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Particle Velocity and ImpedanceWhen the acoustic wave travels in the elastic medium (air), the air particles moves. Under simple conditions (such as when plane wave propagates inside a duct), the Particle Velocity is:

where is the density of the elastic medium.

The specific acoustic impedance (Zs) :

(Pa-s/m) (= rayls)

sZpu

upZs

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Particle Velocity and ImpedanceFor plane acoustic wave, the specific acoustic impedance is a function of the fluid properties only.

The specific acoustic impedance for plane waves is called the characteristic impedance (Zo) of the plane wave:

(Pa-s/m) (= rayls)cZo

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Example: u and ZA plane sound is transmitted through air (R = 287 J/kg-K) at 25oC (298.2K or 77oF) and 101.3 kPa (14.7 psia). The speed of sound in the air is 346.1 m/s. The sound wave has an acoustic pressure (rms) of 0.20 Pa. Determine the rms acoustic particle velocity.

The particle velocity is generally much smaller than the speed of sound.