Download - Acoustics Presentation
ACOUSTICS
Acoustics The science of sound, including its
production, propagation and effects The objective study of the physical
behavior of sound in an enclosed space Sound
A wave motion consisting of a series of condensations and rarefactions in an elastic medium produced by a vibrating body
Requirements to Produce Sound
Requirements to Produce Sound
1. Presence of vibrating body2. Presence of transmitting medium3. Presence of receiving medium
AUDIBLE FREQUENCY RANGE
Infrasonic/Subsonic frequencies below the audible range
Ultrasonic/Supersonic frequencies above the audible range
Audible Range: 20 Hz– 20kHz
AUDIBLE FREQUENCY RANGE
General Interpretations of Sound
1. Physical phenomenon consisting of wave motion in a transmitting medium (objective)
2. Sensation due to outside simulation (subjective)
Physical Properties of Sound
Physical Properties of Sound
1. Amplitude – magnitude of the vibration (pressure, current, voltage)
2. Period – time it takes to complete a vibration/cycle
3. Frequency – number of vibrations / cycle per unit time
Physical Properties of Sound
4. Wavelength – physical length of a vibration
5. Velocity of PropagationVsound << VRF
(344 m/sec << 3 x 108 m/sec)
Velocity of Sound
Solids
Where:E = Young’s Modulus of elasticity, dynes/cm3
d = density of the medium, g/cm3
Velocity of Sound
Liquids
Where:E = Bulk’s Modulus of elasticity, dynes/cm3
d = density of the medium, g/cm3
Velocity of Sound
Gases
Where:k = specific heat ratio = hsp/hsvhsp = specific heat at constant pressurehsv = specific heat at constant volumep = gas pressure, dynes/cm2
d = density, g/cm3
Velocity of Sounds
Dry Air/Air (for TC ≤ 20 0C)
Velocity of Sounds
Dry Air/Air (for TC ≥ 20 0C)
where:TK = temperature in Kelvin
Velocity of Sound
Velocity of Sounds
Notes Sounds travel more slowly in gases than
in liquids, and more slowly in liquids than in solids.
Sounds travels slower with an increased altitude (elevation if you are on solid earth), primarily as a result and humidity changes.
Possibilities when a Propagated Sound is
Obstructed (3)
Possibilities when a Propagated Sound is
Obstructed (3)
Possibilities when a Propagated Sound is
Obstructed (3) Sound is Reflected
Echo Becomes apparent to the listener only when the
distance from the source and the reflecting medium is great and the difference between the original and reflected sound is greater or equal to 1/17 of a second.
Flutter Brought about by a series of reflections between two
parallel surfaces resulting to prolongation of sound Creates listening fatigue
Interference Reflection caused by two parallel surfaces, producing
standing waves
Possibilities when a Propagated Sound is
Obstructed Sound is absorbed
Conversion of sound energy to heat energy Onward transmission through
obstruction
Physiological Characteristics of Wave
Motion (3) Pitch
Physiological Characteristics of Wave
Motion (3) Pitch
Number of cycles a wave goes through in a definite interval
The higher the frequency, the higher the pitch
Mel – unit of pitch 1000 mels – pitch of 1000Hz tone at 40dB Octave – pitch interval 2:1; frequency is
twice the given tone
Physiological Characteristics of Wave
Motion (3) Tone
Timbre quality of sound
Pure Tone – a sound composed of only one frequency in which the sound pressure varies sinusoidally with time.
Musical Sound – composed of the fundamental frequency and its harmonics
Physiological Characteristics of Wave
Motion (3) Loudness
Fluctuation of air pressure created by sound waves Observer’s auditory impression of the strength of a
sound and is associated with the rate at which energy is transmitted to the ear.
Depends on the amplitude of the sound
Loudness Level – measured by the sound level of a standard pure tone or specified frequency which is assessed by normal observers as being equally loud
PHON
Phon is the unit of loudness level when:
The standard pure tone is produced by a sensibly plane sinusoidal progressive sound wave coming from directly in front of the observer and having the frequency of 1kHz
The sound pressure level in the free progressive wave is expressed in dB above 2 x 10-5 N/m2
SONE
Sone is the unit of loudness of an individual listener.
Phon = 40 + 10 log2 sone
Sound Levels
Sound Pressure (P) and Sound Pressure Level (SPL)
Sound Pressure The alternating component of the pressure
at a particular point in a sound field Expressed in N/m2 or Pa
Sound Levels
Sound Pressure Level Equal to 20 times the logarithm to the base 10
of the ratio of the RMS sound pressure to the reference sound pressure
SPL = 20 log (P/Po)Where: P = rms sound pressurePo = reference sound pressurePo = 2 x 10-5 N/m2 or Pa or 2 x 10-4 dynes/cm2
Po = 0.0002 μbar or 2.089 lb/ft2
Sound Pressure Levels
Sound Pressure Levels
Sound Pressure Level (SPL) at any unit of pressure in dB
SPL = 20log(P+N)
Where:PN = rms sound pressure expressed in any of pressure in dBN = SPL constant corresponding to the unit at which sound pressure is expressed
Sound Pressure Levels
SPL Constants
Sound Levels
Sound Intensity (I) and Sound Intensity Level (SIL)Sound Intensity
Defined as the acoustic power per unit area The basic units are W/m2 or W/cm2
The average rate of transmission of sound energy through a cross-sectional area of 1 m2 at right angles to a particular direction.
Sound Levels
Sound Levels
Sound Levels
For sound produced at ground level
Sound Levels
Sound Intensity
I = ρ2 / d v
Where: d – density of the medium (kg/m3)
v – velocity of sound in medium (m/sec)
ρ – rms pressure in Pa (N/m2)
Sound Levels
Sound Intensity in Air
I = ρ2 / 410
Where: dv – 410 ray/sec
ρ – rms pressure in Pa (N/m2)
Sound Levels
Sound Intensity Level
Where:I = sound intensity, Io = threshold intensity,
Io = 10-12 W/m2 or 10-16 W/cm2
Sound Levels
Sound Power (W) and Sound Power Level (PWL)
Sound Power (W) The total energy radiated per unit time.
Sound Levels
Sound Power Level (PWL)
Where:W = sound power , WWo = reference sound power
Wo = 10-12 w
Room Acoustics
Room Acoustics Concerned with the behavior of sound
within an enclosed space with a view to obtaining the optimum acoustic effect on the occupants
Room Acoustics
Room Acoustics
Requirements Adequate amount of sound must reach all
parts of the room. Even distribution of sound Noise must be reduced to an acceptable
level. Optimum Reverberation time, RT60
Reverberation
Reverberation Tendency for the sound to persist over a
definite period of time after it has been produced originally and stopped at the source.
Reverberation
Reverberation
Reverberation
Reverberation Time, RT60
Time taken for the density of sound energy in the room to drop to 1 millionth (60dB) below of its initial value
Optimum Periods of Reverberation
Factors Affecting Reverberation Time
Volume of the room Type of materials Surface area of material
TYPES OF ROOM
LIVE ROOM- Little absorption (RT60 > 1 sec)
DEAD ROOM- Large absorption (RT60 < 1 sec)
ANECHOIC ROOM- 100% absorption (free field conditions)
Room Acoustics
Coefficient of absorption, α Ratio of incident sound and absorbed
sound Efficiency of sound absorption
Room AcousticsCoefficient of Absorption
Room AcousticsCoefficient of Absorption
Reverberation Time Equations
a. Sabine’s Equation For actual reverberation time with average
absorption less than or equal to 0.2; (absorption coefficient, α ≤ 0.2)
Where;V = room volume,
m3
A = total absorption units
Reverberation Time Equations
Where;V = room volume, ft3
A = total absorption units
Reverberation Time Equations
Example:
Calculate the reverberation time of a broadcast studio 8 ft. high by 13 ft wide by 20 ft. long. The material used has a total absorption of 180.75 sabines.
Reverberation Time Equations
b. Norris – Eyring Equation For actual reverberation time with average
absorption greater than 0.2; ( α ≥ 0.2 )
Where;V = room volume, m3
α = average coefficient of reflecting surfaces
Reverberation Time Equations
Example:
A lecture room, 16 m. long, 12.5 m. wide and 5 m. high has a reverberation time of 0.75 sec. Calculate the average absorption coefficient of the surfaces using the Eyring formula.
Reverberation Time Equations
c. Stephens and Bate Equation For ideal reverberation time computation
Where:r = 4 for speechr = 5 for orchestrar = 6 for choir
Optimum Volume / person
Concert Halls 7.1Italian type opera houses 4.2 – 5.1Churches 7.1 – 9.9Cinemas 3.1Rooms for Speeches 2.8
Reverberation Time Equations
Example:
Suggest the optimum volume and reverberation time for a concert hall to be used mainly for orchestral music and to hold 450 people.
MICROPHONES
Microphone
An acoustic device classified as a transducer which converts sound waves into their corresponding electrical impulses
Transducer A device which when actuated by energy in
one transmission system, supplies energy in the same form or in another form, to a second transmission system
Classification of Microphones
A. General Categories
1. Passive (Generator Type) Microphone
Does not require external power source
2. Active (Amplifier Type) Microphone Needs an external power source for its
operation
Classification of Microphones
B. According to Impedance
1. High Impedance Greater than 1000 ohms
2. Low Impedance 1000 ohms and below
Classification of Microphones
C. According to Method of Coupling
Pressure Type- Actuated by thepressure of soundwaves againstthe diaphragm.
Classification of Microphones
C. According to Method of Coupling
Velocity Type
- actuated by
velocity of sound waves
Classification of Microphones
C. According to Method of Coupling
Contact Type
Classification of Microphones
D. According to Elements Used1. Dynamic
Uses the principle of electromagnetic induction
Electromagnetic moving coil microphone A medium-priced instrument of high
sensitivity
Classification of Microphones
Classification of Microphones
2. Ribbon Velocity microphone Ribbon moves as if it is a part of the air
that experiences rarefactions and condensations
Classification of Microphones
Classification of Microphones
3. Capacitor Condenser type or electrostatic
microphone
Classification of Microphones
4. Carbon Uses principle of variable resistance
Classification of Microphones
5. Crystal Uses principle of piezoelectric effect
Classification of Microphones
6. Magnetic Operated on the magnetic reluctance due
to the movable core
Classification of Microphones
E. According to directional Characteristics
Unidirectional
Classification of Microphones
E. According to directional Characteristics
Bidirectional
Classification of Microphones
E. According to directional Characteristics
Omnidirectional
Classification of Microphones
E. According to directional Characteristics
Cardioid
Characteristics of Microphone
1. Frequency Response Frequency over which the microphone will
operate normally
Magnetic : 60 – 10 000HzCrystal : 50 – 10 000HzCondenser : 50 – 15 000HzCarbon : 200 – 3 000Hz
Characteristics of Microphone
2. Sensitivity Ability that would be covered by the
microphone
3. Dynamic Range Range of sound intensity that would be
covered by the microphone
Special Types of Microphones
Line Microphone Capable of picking up sound from a great
distance at an angle of 45 degrees and is highly sensitive
Special Types of Microphones
Differential Microphone Used in noisy places; good up to 3-in
distance
LOUDSPEAKERS
Types of Loudspeakers
Direct Radiator Type Those in which the vibrating surface
(diaphragm) radiates sound directly into the air
1. Dynamic or Moving Coil Loudspeaker Makes use of a moving coil in a
magnetic field and a permanent magnet
Types of Loudspeakers
Dynamic or Moving Coil Loudspeaker
Types of Loudspeakers
Electrostatic Loudspeaker Operates on the same principle as a
condenser microphone
Types of Loudspeakers
Horn Type Those in which a horn is interposed between the
diaphragm and the air Used for efficient coupling of sound into the air
Types: Conical Horn Parabolic Horn Exponential Horn Hyperbolic Horn
Types of Loudspeakers
To cover the entire range of audible frequencies, the following speakers are used:
Types of Loudspeakers
Woofer – for low frequencies
Types of Loudspeakers
Tweeter – for high frequencies
Types of Loudspeakers
Midrange – for normal range
Types of Loudspeakers
Subwoofer – for very low frequencies
DIVIDING NETWORK
Loudspeaker Phasing
When more than one speaker is used: Phasing must be uniform Polarities and voice coils are in phase such
that the cone of all the speakers move inwards at the same instant.
Loudspeaker Enclosure (Baffle)
Loudspeaker mounting that is used to prevent the sound waves from the rear from interfering with the sound waves in the front of speaker
QUESTIONS
1. Which best describe the sound wave?a. It may be longitudinalb. It is always transversec. It is always longitudinal d. All of the above
2. Which of the following cannot travel through a vacuum?a. Electromagnetic waveb. Radio wavec. Sound waved. Light wave
3. Through which medium does sound travel fastest?a. Airb. Waterc. Steeld. Mercury
4. Speed that is faster than that of sound.a. Ultrasonicb. Supersonic c. Subsonicd. Transonic
5. What is the speed of sound in air at 20°C?a. 1087 ft/sb. 1100 ft/sc. 1126 ft/sd. 200 ft/s
6. Calculate a half wavelength sound for sound of 16000 Hza. 35 ftb. 10 ftc. 0.035 ftd. 100 ft
7. The lowest frequency that a human ear can hear isa. 5 Hzb. 20 Hzc. 30 Hzd. 20 kHz
8. Sound that vibrates at frequency too high for the human ear to hear (over 20 kHz)a. Subsonicb. Ultrasonicc. Transonicd. Stereo
9. The frequency interval between two sounds whose frequency ratio is 2.a. Octaveb. Half octavec. Third-octaved. Decade
10. A 16 KHz sound is how many octaves higher than a 500 Hz sounda. 2b. 5c. 4d. 8
11. Sound waves composed of but one frequency is a/ana. Infra soundb. Pure tonec. Structure borned. Residual sound
12. Sound wave has two main characteristics which are a. Highness and loudnessb. Tone and loudnessc. Pitch and loudnessd. Rarefactions and compressions
13. When waves bend away from straight lines of travel, it is calleda. Reflectionb. Diffractionc. Rarefactiond. Refraction
14. The amplitude of sound waves, the maximum displacement of each air particle, is the property which perceive as _____ of a sounda. Pitchb. Intensityc. Loudnessd. Harmonics
15. It is the weakest sound that average human hearing can detect.a. SPL = 0 dBb. Threshold of hearingc. Reference pressure = 2 x 10-5N/m2d. A, b, c
16. What is a device that is used to measure the hearing sensitivity of a person?a. Audiometerb. OTDRc. SLMd. Spectrum analyzer
17. What is the device used in measuring sound pressure levels incorporating a microphone, amplification, filtering and a display.a. Audiometerb. OTDRc. SLMd. Spectrum analyzer
18. It is the device used to calibrate an SLM?a. Microphoneb. Pistonphonec. Telephoned. Filter
19. _____ is the sound power measured over the area upon which is received.a. Sound pressure b. Sound energyc. Sound intensityd. Sound pressure level
20. A measure of the intensity of sound in comparison to another sound intensitya. Phonb. Decibelc. Pascald. Watts
21. Calculate the sound intensity level in dB of a sound whose intensity is 0.007 W/m2.a. 95 dBb. 91 dBc. 98 dBd. 101 dB
22. What is the sound pressure level for a given sound whose RMS pressure is 200 N/m2?a. 200 dBb. 20 dBc. 140 dBd. 14 dB
23. What is the sound intensity for an RMS pressure of 200 Pascal?a. 90 W/m2
b. 98 W/m2
c. 108 W/m2
d. 88 W/m2
24. The sound pressure level is increased by _____ dB if the pressure is doubled.a. 3b. 4c. 5d. 6
25. The sound pressure level is increased by _____ dB if the intensity is doubled.a. 3b. 4c. 5d. 6
26. If four identical sounds are added what is the increase in level in dB?a. 3b. 4c. 5d. 6
27. The transmission of sound from one room to an adjacent room, via common walls, floors or ceilings.a. Flanking transmissionb. Reflectionc. Refractiond. Reverberation
28. _____ is the continuing presence of an audible sound after the sound source has stop.a. Flutter echob. Sound concentrationc. Sound shadowd. Reverberation
29. Required time for any sound to decay to 60 dBa. Echo timeb. Reverberation timec. Delay timed. Transient time
30. A room containing relatively little sound absorption a. Dead roomb. Anechoic roomc. Live roomd. Free-field
31. A room in which the walls offer essentially 100% absorption, therefore simulating free field conditions.a. Dead roomb. Anechoic roomc. Live roomd. Closed room
32. Calculate the reverberation time of the room, which has a volume of 8700 ft3 and total sound absorption 140 sabines.a. 0.3 secb. 3.5 secc. 3 secd. 0.53 sec
33. It is an audio transducer that converts acoustic pressure in air into its equivalent electrical impulsesa. Loudspeakerb. Amplifierc. Baffled. Microphone
34. _____ is a pressure type microphone with permanent coil as a transducing element.a. Dynamicb. Condenserc. Magneticd. Carbon
35. A microphone which has an internal impedance of 25 kΩ is _____ type.a. High impedanceb. Low impedancec. Dynamicd. Magnetic
36. A microphone that uses the piezoelectric effecta. Dynamicb. Condenserc. Crystald. Carbon
37. _____ is a type of loudspeaker driver with an effective diameter of 5 inches used at midrange audio frequency.a. Tweeter b. Wooferc. Mid-ranged. A or C
38. _____ is measure of how much sound is produced from the electrical signal.a. Sensitivityb. Distortionc. Efficiencyd. Frequency response
39. It describes the output of a microphone over a range of frequencies.a. Directivityb. Sensitivityc. Frequency responsed. All of the above
40. A loudspeaker radiates an acoustic power of 1 mW if the electrical input is 10 W. What is its rated efficiency?a. -10 dBb. -20 dB c. -30 dBd. -40 dB
41. An amplifier can deliver 100 W to a loudspeaker. If the rated efficiency of the loudspeaker is -60 dB. What is the maximum intensity 300 ft from it?a. 10 dBb. 20 dBc. 30 dBd. 40 dB
42. Speaker is a device thata. Converts sound waves into current and voltageb. Converts current variations into sound wavesc. Converts electrical energy to mechanical energyd. Converts electrical energy to electromagnetic energy
43. The impedance of most drivers is about _____ ohms at their resonant frequency.a. 4b. 6c. 8d. 10
44. It is a transducer used to convert electrical energy to mechanical energy.a. Microphone b. Baffle c. Magnetic assemble d. Driver
45. It is an enclosure used to prevent front and back wave cancellation.a. Loudspeakerb. Driverc. Baffled. Frame
46. A circuit that divides the frequency components into separate bands in order to have individual feeds to the different drivers.a. Suspension systemb. Dividing networkc. Magnet assemblyd. Panel board
47. _____ is early reflection of sound.a. Echob. Pure soundc. Reverberationd. Intelligible sound
48. Noise reduction system used for film sound in movie.a. Dolbyb. DBxc. dBad. dBk
49. Using a microphone at less than the recommended working distance will create a _____ which greatly increases the low frequency signals.
a. Roll-off b. Proximity effect c. Drop out d. None of the choices
50. What is the unit of loudness of an individual listener?a. Sone b. Phonc. Decibeld. Mel
51. A unit of noisiness related to the perceived noise levela. Noyb. dBc. Soned. Phon
52. What is the loudness level of a 1KHz tone if its intensity is
1 x 10-5W/cm2?a. 100 phonsb. 105 phonsc. 110 phonsd. 100 phons
53. A transducer that converts acoustic signals into electrical signals.
a. microphoneb. loudspeakerc. both a and bd. none of these
54. A characteristic of a microphone which indicates the frequency range over which the microphone the frequency range over which the microphone will operate normally.
a. sensitivityb. frequency responsec. dynamic ranged. directional characteristic
55. An ability of the microphone to detect very slight changes of sound.
a. sensitivityb. frequency responsec. dynamic ranged. directional characteristic
56. The range of sound intensity that would be covered by the microphone.
a. sensitivityb. frequency responsec. dynamic ranged. directional characteristic
57. A special microphone characterized by a long perforated tube and high sensitivity, suitable for TV applications.
a. line microphoneb. dynamic microphonec. differential microphoned. ribbon microphone
58. A sound intensity that could cause painful sensation to the human ear.
a. threshold of senseb. threshold of painc. hearing thresholdd. sensation intensity
59. What is the speed of sound in a material having a density of 1000 kg/cu.m. and Young’s modulus of elasticity of 2.3 x 10exp 9 N/sq.m.?
a. 1517 m/secb. 1571 m/secc. 1715 m/secd. 1751 m/sec
60. In acoustics, the volume velocity component is a function of the _____ of the material.
a. densityb. volumec. diameterd. Young’s modulus
61. If the sound source radiates 1 watt, what is its sound power level?
a. 0 dBb. 60 dBc. 120 dBd. 240 dB
62. If a note has a fundamental frequency of 100Hz, what is its 5th octave?
a. 6400 Hzb. 3200 Hzc. 500 Hzd. 1600 Hz
63. A church has an internal volume of 2550 cu.m. When it contains absorption of 186 metric sabines, what will be its reverberation time in sec.?
a. 2b. 2.2c. 2.5d. 3.0