Definitions of SOUND

Definitions of SOUND

1. physical wave in an elastic medium, usually air
2. the sensation stimulated in the organs of hearing by mechanical radiant energy transmitted as longitudinal pressure waves through the air or other medium
3. a vibration in an elastic medium such as air, water, most building materials, and the earth
4. physically, sound is a rapid fluctuation of air pressure
Types of Sound
1. Wanted Sound (speech, music) – heard properly
o considered desirable
o heard properly
2. Unwanted Sound (noise) – annoyance
o annoyance
o not desired or objectionable
Characteristics of Sound
1. Audible
2. Inaudible
Basic Principles of Sound - understanding the characteristics of sound is essential in designing for good acoustics)
1. Generation
o sound is generated when an object vibrates, causing the adjacent air to move, resulting in a series of pressure waves radiating out from the moving object
o Wave – a disturbance or oscillation that transfers energy progressively from point to point in a medium or space without advance by the points themselves, as in the transmission of sound or light
o Sound Wave – a longitudinal pressure wave in air or an elastic medium esp. one producing an audible sensation
2. Frequency
o the no. of times the cycle of compression and rarefaction of air occurs in a given unit of time (e.g. 1000 cycles in the period of 1 second = 1000 cps = 1000 Hertz)
o the no. of cycles per unit time of a wave or oscillation
o the no. of complete cycles per second (frequency of vibration)
o the rate of oscillation of molecules by sound (frequency of vibration) measured in cycles per second (Hertz)
o Pitch – frequency of sound vibration; the predominant frequency of a sound as perceived by the human ear
o Vibration – the back and forth motion of a complete cycle
o Cycle – full circuit by a displaced particle
o Period – the time required for one complete cycle
o 20 – 20,000 Hz – approx. audio frequency range of human hearing;
o 600 – 40,000 Hz – critical frequency for speech communication
o Tone – composed of a fundamental frequency with multiples of the fundamental, called Harmonics
o Pure Tone – sound composed only of 1 frequency
o Musical sounds are combination of many pure tones
o Frequency bands – used to express division of sounds into sections (Octave bands are the most common)
o 9 Octave Bands – 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000 Hz
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3. Velocity
o varies according to the medium of transmission; approx. 1100 fps in air at normal temperature and pressure
4. Wavelength
o Distance between similar points on successive waves; the distance the sound travels in one cycle
o Relationship between wavelength, frequency, and velocity:
Λ = c/f
Λ = wavelength, ft.2
c = velocity of sound, fps
f = frequency of sound, Hz (cps)
o Low frequency sounds: characterized by long wavelengths
o High frequency sounds: characterized by short wavelengths
o Sounds with wavelengths ranging from ½” – 50’ can be heard by humans
5. Magnitude
o Sound power (watts) – amount of energy produced by a source
o Intensity – describes the energy level; unit is watts/cm2
o Loudness (what is perceived) – related to the Intensity Level (IL) or Sound Pressure Level (SPL)
o The Sound Pressure Level (SPL) is equivalent to the Sound Intensity Level (IL)
o Intensity Level is expressed in decibels
o Decibel – dimensionless unit used to express the ratio of two numerical values of a logarithmic scale
Comparison of decimal, exponential, and logarithmic statements of various acoustic intensities
Intensity (watts/cm2) Intensity Level
Decimal Notation Exponential Notation Logarithmic Notation Examples
0.001 10-3 130 dB Painful
0.0001 10-4 120 dB
0.00001 10-5 110 dB 75-piece orchestra
0.000001 10-6 100 dB
0.0000001 10-7 90 dB Shouting at 5 ft.
0.000000001 10-9 70 dB Speech at 3ft.
0.00000000001 10-11 50 dB Average office
0.0000000000001 10-13 30 dB Quiet unoccupied office
0.00000000000001 10-14 20 dB Rural ambient
0.000000000000001 10-15 10 dB
0.0000000000000001 10-16 0 dB Threshold of hearing

o The fundamental relationship that determines the decibel level is ten times the logarithm to the base 10 of the numerical ratio of the two intensities. For example:
IL = 10 log I
I0
IL = intensity level
I = intensity, watts/cm2
I0 = reference intensity, 10-16 watts/cm2
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6. Time Characteristics of Sound
o Some sounds remain constant with time (e.g. fan may produce a sound with constant frequency and intensity characteristics – a steady state sound)
o Sounds (traffic, voices, musical instruments) vary as a function of time
o Vehicular traffic sounds: maximum levels/levels exceeded for 90% of the time
o Speech sounds: 1% peaks
o Orchestra’s sound: long-term average levels
Behavior of Sound in a Free Field
• Sound intensity is reduced to one-quarter each time the distance doubles:

I1 and I2 are the intensities at distances d1 and d2 from the source
• The intensity of a sound at a distance from the source is expressed as the power (P) of the source divided by the area over which it has spread (A)
I = P/A
• If the sound radiates freely in all directions from a source the area is a sphere. The intensity is represented as:
I = P
4π r2
P = power in watts
r = distance from source in cm
In English units, the conversion factor is 930 cm2/ft4π r2Using this, the equation for spherical direction is:
I = P/930 x 4π r2
As the intensity is reduced to one-quarter with each doubling of distance, the sound pressure level (IL) is reduced by 6 dB. Quartering the intensity reduced the intensity level by 6 dB
• Sound attenuation is due to distance, absorption, or barriers
• Barriers, to be effective must be solid, quite large as compared to the wavelength of sound and near to the source or receiver
• Shrubs, hedges, small groves: visual barriers only; no value acoustically
• Sound Combining. Sounds from separate sources may combine. Two violins produce a louder sound than 1 instrument; 3 violins are louder still. Decibels are not added directly to make the perceived level twice as loud for 2 sources or 3 times as loud for 3. The level for combined sources is determined by adding powers, intensities or pressures; and subsequently converting to decibels. Doubling of intensities results in a 3 dB increase (e.g. 50 dB plus 50 dB is 53 dB, not 100 dB)


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