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


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


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)


Obstructed (3)


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


Obstructed Sound is absorbed

Conversion of sound energy to heat energy Onward transmission through

obstruction

Physiological Characteristics of Wave

Motion (3) Pitch


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


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


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 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


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


Where;V = room volume, ft3

A = total absorption units


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.


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


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.


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


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


B. According to Impedance

1. High Impedance Greater than 1000 ohms

2. Low Impedance 1000 ohms and below


C. According to Method of Coupling

Pressure Type- Actuated by thepressure of soundwaves againstthe diaphragm.



Velocity Type

- actuated by

velocity of sound waves



Contact Type


D. According to Elements Used1. Dynamic

Uses the principle of electromagnetic induction

Electromagnetic moving coil microphone A medium-priced instrument of high

sensitivity



2. Ribbon Velocity microphone Ribbon moves as if it is a part of the air

that experiences rarefactions and condensations



3. Capacitor Condenser type or electrostatic

microphone


4. Carbon Uses principle of variable resistance


5. Crystal Uses principle of piezoelectric effect


6. Magnetic Operated on the magnetic reluctance due

to the movable core


E. According to directional Characteristics

Unidirectional



Bidirectional



Omnidirectional



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


Dynamic or Moving Coil Loudspeaker


Electrostatic Loudspeaker Operates on the same principle as a

condenser microphone


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


To cover the entire range of audible frequencies, the following speakers are used:


Woofer – for low frequencies


Tweeter – for high frequencies


Midrange – for normal range


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.


56. The range of sound intensity that would be covered by the microphone.


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

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Acoustics Presentation