Physics 1251 The Science and Technology of Musical Sound - PowerPoint PPT Presentation

About This Presentation
Title:

Physics 1251 The Science and Technology of Musical Sound

Description:

Answer: A single tone of frequency 263 Hz beating with a beat frequency of 2 Hz. ... 'effective surface area' of the walls S1 , floor S2 and ceiling S3 (in sabin) etc. ... – PowerPoint PPT presentation

Number of Views:92
Avg rating:3.0/5.0
Slides: 31
Provided by: ala83
Learn more at: http://www.math.unt.edu
Category:

less

Transcript and Presenter's Notes

Title: Physics 1251 The Science and Technology of Musical Sound


1
Physics 1251The Science and Technology of
Musical Sound
  • Unit 2
  • Session 18 MWF
  • Room Acoustics

2
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Foolscap Quiz
  • Two tones of frequency 262 Hz and 264 Hz are
    sounded. What do you hear?

Answer A single tone of frequency 263 Hz
beating with a beat frequency of 2 Hz. fmean
(f 1 f2)/2 (262 264)/2 263 Hz fbeat ?f
1- f2? ?262-264 ? 2 Hz.
3
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Reflective Review
  • Wave motion is characterized by
  • Reflection
  • Refraction
  • Diffraction
  • Doppler Shift
  • Beats
  • Interference

The Reflective Ref Differed Dopily, Beating
Interference.
4
Physics 1251 Unit 2 Session 18 Room Acoustics
  • When the surface is smooth we have specular
    (mirror-like) reflection.

Reflection
Smooth Surface Roughness ? ?
5
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Refraction occurs when a wave enters a medium
    that has a different velocity?

Refraction
V1 lt V2
6
Physics 1251 Unit 2 Session 18 Room Acoustics
  • What happens when a wave is partially obstructed?

Diffraction
7
Physics 1251 Unit 2 Session 18 Room Acoustics
Doppler Shift Moving source
Higher f
Lower f
fobserver fsource v vobserver / v
vsource
8
Physics 1251 Unit 2 Session 18 Room Acoustics
Beats f1 f2
In phase
Out of phase
fmean
fbeat
9
Physics 1251 Unit 2 Session 18 Room Acoustics
Interference
Constructive
Destructive
Softer
Louder
10
Physics 1251 Unit 2 Session 18 Room Acoustics
  • 1' Lecture
  • Factors that most determine room acoustics
  • Direct sound
  • Reverberant sound
  • Freedom from echo and interference
  • Background noise level

11
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Greek Amphitheater
  • At Epidauros, Greece

12
Physics 1251 Unit 2 Session 18 Room Acoustics
Constructive Interference
13
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Greek Amphitheater
  • Acoustics

Reflected sound
Direct sound
14
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Musiksvereinssaale Vienna

15
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Royal Festival Hall
  • http//www.coxt.freeeserve.co.uk/hall.jpg

16
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Morton Meyerson Symphony Center
  • Dallas, Texas

17
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Winspear Center
  • Edmonton, Canada

18
Physics 1251 Unit 2 Session 18 Room Acoustics
  • 80/20Standards for Good Acoustics
  • Clarity
  • Uniformity
  • Envelopment
  • Smoothness
  • Reverberation
  • Performer satisfaction
  • Freedom from noise

little overlap of sounds
everywhere the same
sound from all directions
no echoes
appropriate length of time
reflected to stage
no competition
19
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Cardinal Principle of Room Acoustics
  • The temporal, spatial, intensity and phase
    relationships between the direct and reflected
    sound ultimately determine the quality of the
    acoustics in a room.
  • A room is an instrument that can dull the most
    illustrious performance by the most accomplished
    musician, or
  • it can increase the pleasure of listening.

20
Physics 1251 Unit 2 Session 18 Room Acoustics
  • 80/20 Haas or Precedence Effect
  • The earliest sound that arrives determines the
    sense of the origin of a sound, even if the later
    (lt100 ms) reflections are louder.
  • The direct sound should arrive first.

21
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Room Acoustics Reverberation

Speaker
Hearer
22
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Energy Lost in Reflections
  • The sound reflects many times, each time losing
    energy to the reflecting surface.
  • The quantity a is the absorptivity of the
    surface.
  • The intensity of the sound that is lost in a
    reflection is ?Ilost a Iin.

23
Physics 1251 Unit 2 Session 18 Room Acoustics
  • The intensity of the reflected wave is
  • Ireflected (1-a) Iin.

a
24
Physics 1251 Unit 2 Session 18 Room Acoustics
  • The intensity of the reflected wave is
  • Ireflected (1-a) Iin.
  • Values for a, the absorptivity, for many types of
    surfaces have been measured and appear in
    extensive tables.

25
Physics 1251 Unit 2 Session 18 Room Acoustics
  • The Absorptivity
  • Material a (at 500 Hz)
  • Acoustic tile 0.6
  • Plaster wall 0.1
  • Concrete 0.02
  • Person 0.8 (x1 m2)

26
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Wallace Sabine
  • (Harvard professor 1868-1919)
  • Asked How long will it take for the
  • sound to die down to 1 millionth (-60 dB)
  • of the initial value?
  • 80/20The reverberation time is the time for the
    intensity to decay by a factor of 10 6 (-
    60dB) of its initial value.

Wallace Sabine
27
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Intensity of Sound in a Room

Io
I Io ? 10 6 t / TR
t ? TR
1/10 Io
Pressure Amplitude
t ? TR
1/100 Io
Time (ms)
http//hybrid.colorado.edu/phys1240/sounds.html
28
Physics 1251 Unit 2 Session 18 Room Acoustics
  • 80/20The Sabine Equation
  • I Io ? 10 6 (t/TR)
  • TR 0.16 V/Se
  • V is the volume of the room.
  • Se is the effective surface area of the walls
    S1 , floor S2 and ceiling S3 (in sabin) etc.
  • a is the absorptivity of the surface (in table)
  • Se a1 S1 a2 S2 a3 S3 a4 S4

29
Physics 1251 Unit 2 Session 18 Room Acoustics
  • What is the reverberation time for 500 Hz sound
    in a concrete room that is 3 x 3 x3 meters?
  • The Sabine Equation
  • TR 0.16 V/Se
  • V is the volume of the room 3x3x3 27.0 m3.
  • Walls S1 4 (3x3 m2 ) 36. m2 ),
  • floor S2 9.0 m2
  • ceiling S3 9.0 m2
  • a 0.02Se (0.02) (36.) (0.02) (9.0)
    (0.02) (9.0) 1.02 sabine
  • TR 0.16 V/Se 0.16 (27)/(1.02) 4.2 sec

30
Physics 1251 Unit 2 Session 18 Room Acoustics
  • Summary
  • Direct sound should come first
  • Haas or precedence effect
  • Reverberant sound
  • TR 0.16 V/Se
  • 0.8 sec for clear speech, 1-2 for music
  • Freedom from echo and interference
  • Use diffuse and random reflectors
  • Background noise level
  • Assure good acoustic isolation
Write a Comment
User Comments (0)
About PowerShow.com