Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Fundamentals Of Acoustics > L01 > Flashcards

L01 Flashcards

1
Q

What is the wave equation and what is it used for?

A
  • Differential equation including second derivative in time and the second Derivate in space
  • Model of wave propagation
  • describes the behaviour of a wave (Continuous chain of oscillators) in space and time
  • regarding frequency, speed, Amplitude
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is acoustics and what is Sound?

A
  1. Science regarding the description, emission, control, transmission and reception of sound.
  2. Small vibrations emitted and transmitted by a fluid (gas/) or solid in a field of compressed and released density.
    - analytical: longitudinal pressure waves propagating through a medium
    (causing variations in density -> displacement of fluid particles)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What are the most important variables in acoustics?

A

Density,
Pressure,
Particle velocity,
Young’s Modulus,
Strain

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What are the assumptions we make for analytical acoustics?

A

Isentropic Environment (adiabatic, no heat exchange)
Inviscid Flow (no shear stress, only internal pressure determines the system)
The density is directly derived by the pressure and the acoustics potential
….

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is the wave vector?
What does the wave number descibe?
How does it depend on wavelength, frequency and speed of sound?

A

Vector in space informing about the frequency and the direction of travel of a wave.
It’s always perpendicular to a(n isophase) wavefront.

Spatial properties of sound waves (ref.t. spatial frequency of waves)

  • 2pi/Lambda (increasing with decreasing Lambda = c/f) increasing with increasing f
  • w/c increasing with w, decreasing with speed of sound
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the difference btw a rod/Bar and a spring vibrating in Sound emission?

A

Strings
determine the speed of sound by the Tension and the linear density,
Emit transversal waves (less speed)

Rods
emit longitudinal waves and
Determine the speed of sound by the Young’s modulus and the density.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What are normal modes?
What do they depend on?
What happens if you applying a frequency and release it in a room?
What does normal modes determine?
Which law holds at boundaries?

A
  • Harmonic oscillation at natural frequency of the System
  • dep. on boundary conditions (size, shape, material of the system)
  • it follows the oscillation in its eigenfrequencies
  • det. acoustic characterizations in room eg. T_rev, resonance frequencies, sound quality/ density distribution?)
  • there is always a fit between density and speed of sound at interfaces ( also reason for bending)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the difference between standing waves and traveling waves?
What ist the 3D traveling wave description for pressure?

A

Standing waves:
- nodes are at the same location constantly in time
- wave pattern formed when 2 equal waves travel in opposite direction:
- nodes appear to stand still (superposition: destructive interfer.)
- antinodes alternate (point of max. displacement)

Traveling waves:
- nodes and amplitudes propagate in time and space along a certain direction

3D description:
- pressure depends on angular frequency w, wave vector k, and spatial position (r vector)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

How does sound in fluids propagate?
Which principles are basic for analyzing sound propagation in fluids?
By which 3 quantities (5 variables) are acoustic fields governed?

A
  1. Isentropic (adiabatic/no heat exchange & reversible)
  2. Inviscid (no shear stresses, only internal stress is pressure p)

1.Conservation of mass (rho)
2. Conservation of inertia (velocity)

  • Rho (in simple fluids rho(p))
  • Particle Velocity v (+angular frequency)
  • Pressure p (Youngs modulus, strain)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What are the 2 basic physical principles for wave propagation in fluids?
How is the

A
  1. Newtons second law for fluids (equation of motion, F=m*a - acceleration of the fluid))
    =d(mv)/dt
    F - total force acting on the fluid
    -> describes how velocity if a fluid changes over te and space in response to various forces acting on it.
  2. Conservation of mass (continuity equation)
    - mass of closed system stays constant over time
    - density derived by time + divergence of mass flux (lapl rho v) = 0
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

How are Sound levels calculated?
(Sound intensity level, Sound pressure level)

A
  1. SIL: 10log_10(I_av/I_o)
    (reference value for SI in water)
  2. SPL: 20log_10(p_rms/p_o)
    (reference value for SP in
    water: 1mikroPa
    air: 20mikroPa)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

How is the Level for impulsive sound described?

A

By:
1. The equivalent cont SPL
L_eq = 10log_10(Integral p^2 over duration T)

  1. Sound exposure level
    (Integral of Energy system is exposed to)
  2. Peak sound level
    Used for standards (environment prot.,…)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What’s the sound speed in water and in air?
What’s the density?
What’s the reference value of sound pressure?
How does the SPL differ in air and water?

A
  1. 1500m/s & 339
  2. 1.026x10^3 kg/m^3 & 1.23
  3. 1 mikroPa & 20
  4. L_p,air = L_p,water - 61.7 dB
    f. equal sound intensity
    -> water more than 60dB louder than in air!
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What’s the wave equation for spherical waves?

A

Second derivative in time of the acoustic potential
devided by c^2
equals
2/|r| * derivative in space of acPot + 2nd der in space
(r-vector

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What’s the solution of the spherical wave equation?

A

Phi (r,t) = phi_0 /r * exp( j (wt -kr + phi_0))

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What’s the specific acoustic impedance?
How does it behave with kr»1?

A

Complex value:
Z_r = rho_0 * c * jkr/(1+jkr)

It’s tending to be real valued, the imaginary part is negligible small.

17
Q

Why do spherical waves seem locally quieter than plan waves?

A

The same energy is distributed to all dimensions, so with (phi/r) at one point its less energy observable than from a plane wave.

18
Q

How can the pressure and the velocity be derived?

A

From the solution of the spherical waves equation Phi(r, phi, Theta, …)

19
Q

Under which conditions can we assume a sound wave to be a plane wave?

A

In case the
width of the circular arc observed is
less than square root of wavelength times
distance from the source, so that the difference in distance ist assumed to be
less than 1/8 of the wavelength.

The spec. ac. Impedance in this case is assumed to be linear dependent of the speed of sound.

20
Q

What does the Doppler effect tell us?

A

The detected speed of sound and frequency of a source
varies from the emitted sos because of a
velocity either of the src or the rec or both.
In case both move, the detected frequency is calculated by (c-v_r)/(c-v_s) * f_s

Fundamentals Of Acoustics (5 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions