BASIC CONCEPTS IN ARCHITECTURAL ACOUSTICS ENVIRONMENTAL CONTROL III (ACOUSTICS AND NOISE CONTROL) DEPARTMENT OF ARCHITECTURE. FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE ONDO STATE.

Outline Introduction The Nature of Sounds Properties of Sound Propagation of Sound Sound Power and Intensity Effects of Barrier on Sound Conclusion

Introduction Acoustics is the science of sound and it covers two areas, those of room acoustics and control of noise. Noise is unwanted or damaging sound which interferes with what people are trying to do, or sound which has an adverse effect on health and safety.

Introduction This lecture covers basic architectural acoustics including the properties and nature of sound, the terms used to describe sound waves, and the relationship between sound pressure, sound intensity and sound power.

The Nature of Sound Sound is a disturbance, or wave which moves through a physical medium (such as air, water, or metal) from a source to cause the sensation of hearing in animals.

Sound waves Sound waves are longitudinal waves originating from a source and conveyed by a medium. They are characterized by velocity (v), frequency (f), wavelength (٨), and amplitude (a).

Sound waves Compression in sound waves is a region of raised pressure. Rarefaction in sound waves is a region of lowered pressure.

Sound waves Figure 1: Compression And Rarefaction Of Sound By A Vibrating Tuning Fork.

Sound waves Figure 2: Visualization of sound rarefaction and compression in a coiled spring.

Figure 3: Sound wave illustration. Sound waves Figure 3: Sound wave illustration.

Frequency range of sound Sounds produced by various sources can range from frequencies below 20Hz to 20,000Hz and above. Infrasound are sounds with frequencies below 20Hz.Ultrasound are sounds with frequencies above 20,000Hz.

The Audible range of sound Audible sounds range from the threshold of audibility to the threshold of pain. The threshold of audibility is the lower Limit of hearing and it has a standard value of 1 picowatt per metre square (1pW/m²).

The Audible range of sound The threshold of pain is the upper Limit of hearing and it has a standard value of 1 watt per metre square (1W/m²). Sounds below the lower limit of hearing are inaudible while the those above the upper limit may cause pain or even damage the human ear.

Figure 4: Audible range of sound. The Audible range of sound Figure 4: Audible range of sound.

Figure 5: Audible range of sound. The Audible range of sound Figure 5: Audible range of sound.

The Audible range of sound The sound level or decibel scale is the logarithm of the ratio of measured sound intensity to the intensity at the threshold of audibility.

The Audible range of sound The loudness of a sound is determined by referring to the loudness or phon scale which shows sounds of various levels and frequencies which are perceived as of the same sound loudness.

The Audible range of sound Figure 6: Equal loudness contours.

The Audible range of sound Figure 7: Psychological and physiological effects of sounds

Properties of Sound •Wave length This is the distance between two successive pressure peaks. Its symbol is ٨ and it is measured in units of metres (m).

Properties of Sound •Period This is the time taken for one vibration cycle. Its symbol is T and it is measured in units of seconds (s).

Properties of Sound •Frequency This is the number of vibration cycles per seconds. Its symbol is F and it is measured in units of Hertz (Hz). For the relationship between frequency and period, F=1/T

Properties of Sound •Speed or wave velocity This is the speed with which sound travels through a medium. Its symbol is C and it is measured in units of metres per seconds (m/s). For the relationship between the speed (C), frequency (F) and wave length (٨ ), C=F ٨

Properties of Sound Figure 8: Variation of speed of sound with the medium of transmission. .

Properties of Sound Factors that affect the speed of sound through a medium •Elasticity of the medium •Density of the medium •Temperature of the medium

Properties of Sound •Amplitude This indicates the intensity of sound Properties of Sound •Amplitude This indicates the intensity of sound. Its symbol is I and it is measured in units of watts per metres square (W/m²).

Figure 9: Amplitude illustration. Properties of Sound Figure 9: Amplitude illustration.

Properties of Sound The inverse square law of sound states that the intensity of sound in a free field is indirectly proportional to the square of the distance from the source. This infers that there is a decrease in the intensity of sound the farther the observer is from the source.

Figure 10: The inverse square law. Properties of Sound Figure 10: The inverse square law.

Figure 11: Amplitude illustration. Properties of Sound Figure 11: Amplitude illustration.

Properties of Sound •Pitch This is the property of sound that is perceived as highness and lowness depending on the rapidity of the vibrations producing it . It is measured in cycles per second (cps).

Figure 12: Pitch illustration. Properties of Sound Figure 12: Pitch illustration.

Properties of Sound •Sound pressure This is the force per unit area and it gives the magnitude of the sound wave. Its symbol is p and it is measured in units of Pascal (Pa).

Properties of Sound The pressure changes produced by a sound wave are also known as sound pressure. Compared with atmospheric pressure on which they are superimposed (about 100,000 pascals), they are very small (between 20 micropascals and 200 pascals).

Figure 13:Changes in sound pressure over time. Properties of Sound Figure 13:Changes in sound pressure over time.

Figure 14:Sound pressure superimposed on atmospheric pressure. Properties of Sound Figure 14:Sound pressure superimposed on atmospheric pressure.

Properties of Sound Figure 15:Relationship between sound pressure and sound frequency in a pure tone.

Figure 16:The characteristics of Properties of Sound Figure 16:The characteristics of Machine noise.

Propagation of Sound Inside a room, close to a source like a machine, the direct sound dominates, and the sound pressure may vary significantly with just small changes in position.

Propagation of Sound This area is called the near field and its extent is about twice the dimension of the machine or one wavelength of the sound.

Propagation of Sound The area beyond the near field is called the far field made up of two sections, •The free field •The reverberant field

Propagation of Sound In the free field the direct sound still dominates and the sound pressure level decreases by 6 dB for each doubling of distance.

Propagation of Sound In the reverberant field the reflected sound adds to the direct sound and the decrease per doubling of distance of the sound pressure level will be less than 6 dB.

Propagation of Sound Figure 17: The near field and far field of sound. Source: National Institute for Occupational Health and Safety (1988).

Propagation of Sound Figure 18: Decrease in sound intensity for a point source with doubling of distance.

Properties of Sound •Spherical wave fronts These are produced when sound spreads out from a point source in a free space. The wave fronts are spherical and the sound pressure level decreases 6 dB for each doubling of distance.

Propagation of Sound Figure 19: Decrease in sound intensity for an omnidirectional point source. source.

Properties of Sound •Cylindrical wave fronts These are produced when sound spreads out from a line source (such as a road with constant traffic or a pipe carrying fluid). The waves are cylindrical and the sound pressure level decreases 3 dB for each doubling of distance.

Propagation of Sound Figure 20: Decrease in sound intensity for a line source with doubling of distance.

Figure 21: Decrease in sound pressure level for a line source. Propagation of Sound Figure 21: Decrease in sound pressure level for a line source.

Properties of Sound •Perpendicular wave fronts These are produced when sound spreads out from a plane source (such as close to a large vibrating panel or sound travelling down a duct). The waves are perpendicular and the sound pressure does not decrease with distance.

Figure 22: Perpendicular wave fronts. Propagation of Sound Figure 22: Perpendicular wave fronts.

Properties of Sound The above relationships hold true only in ideal conditions. Decrease in sound levels depends on •Absorption by air and moisture ; •Wind and temperature gradients; •Absorption of the ground; and •Reflection and absorption by obstacles .

Sound Power and Sound Intensity •Sound Power This is the fundamental property of the source of sound that depicts the energy emitted by a sound source per unit time. Its symbol is W and it is measured in units of Watts (w).

Sound Power and Sound Intensity A source that emits power equally in all directions is called an omnidirectional source. Any other source is called a directional source.

Sound Power and Sound Intensity Figure 23: Decrease in sound pressure level for an omnidirectional point source.

Sound Power and Sound Intensity •Sound Intensity Sound intensity at a point in the surrounding medium is the power passing through a unit area. Its symbol is I and it is measured in units of watts per metre square (W/m²).

Sound Power and Sound Intensity For an omnidirectional point source the sound power is spread over the surface of the sphere. S=4pr². Hence the sound intensity is given by I=W/4r²

Sound Power and Sound Intensity The relationship between sound intensity and sound pressure is given as I=p² / rc This equation is for plane waves. However, away from a point source, spherical waves approximate plane waves.

Sound Power and Sound Intensity I is the sound intensity in watts per metre square (w/m²), p is the sound pressure in pascals (pa), r is the density of the medium in kilogram per metres cube (kg/m³), and c is the speed of sound in metres per second (m/s).

Effects of Barriers on Sound When a sound wave encounters an obstacle such as a barrier or a wall, its propagation will be affected in one of three ways •reflection •diffraction •refraction

Effects of Barriers on Sound •Reflection This occurs when the dimensions of an obstacle are larger than the wave length of the sound. In this case, the sound wave behaves like a light ray and for an obstacle with a flat surface, the reflected ray will leave the surface at the same angle as the incident wave.

Effects of Barriers on Sound Figure 24: Reflection of sound.

Effects of Barriers on Sound •Refraction This occurs when a sound wave enters a different medium at an angle. The bending of sound wave is due to the differing speed of travel of the sound wave in the two media.

Effects of Barriers on Sound Figure 25: Refraction of sound with no temperature inversion.

Effects of Barriers on Sound Figure 26: Refraction of sound with temperature inversion.

Effects of Barriers on Sound •Diffraction This occurs when the dimensions of an obstacle are of the same order or less than the wavelength of the sound. In case the edge of the obstacle acts like a source of sound itself and the sound ray appears to bend around the edge.

Effects of Barriers on Sound Figure 27: Diffraction of sound.

Transmission and Absorption of Sound •Transmission of sound This refers to the ways in which sound can be transmitted which could be in form of structure borne sound, impact sound, and airborne sound.

Transmission and Absorption of Sound Structure borne sound refers to the transmission of sound involving the re-emission of sound via vibration of the molecules of a barrier in the path of the sound.

Transmission and Absorption of Sound Impact sound refers to the transmission of sound via mechanical means. Airborne sound simply refers to the transmission of sound through air.

Transmission and Absorption of Sound •Absorption of sound This refers to the product of the absorption coefficient and the area of a given surface. It is measured in the open window unit which is equivalent to the absorption of a square metre opening with zero reflectance.

Transmission and Absorption of Sound The Absorption coefficient is an indication of the sound that is not reflected and is thus an indication of both the sound absorbed and transmitted.

Transmission and Absorption of Sound Figure 28: Transmission and absorption of sound.

Transmission and Absorption of Sound An ability of an obstacle to block the transmission of sound depends on its structure which indicates its transmission loss rating. Hence, stiff, heavy materials like concrete have high transmission loss. Soft porous materials like cell foams are not good at blocking the transmission of sound but are good absorbers.

Transmission and Absorption of Sound •Masking of sound This refers to the acoustic shadow effect of screens or barriers in the path of sound which differs in accordance with the frequency and wave length of the sound and the dimension of the barriers.

Transmission and Absorption of Sound This effect disappears when the wave length of the sound is more than the dimension of the barrier in a direction perpendicular to the sound path.

Transmission and Absorption of Sound Figure 29: Acoustic shadow at high frequencies.

Transmission and Absorption of Sound •Sound Insulation This refers to the reduction of sound transmission of airborne sounds through walls, floors, and partitions. It is achieved by using elements with an adequate transmission coefficient or sound reduction index.

Transmission and Absorption of Sound The transmission coefficient is a decimal fraction expressing the proportion of sound energy emitted. The sound reduction index or transmission loss defines the reduction effect of an element and it is expressed in decibels.

Transmission and Absorption of Sound •Reverberation This is the persistence of sound in an enclosed space as a result of repeated reflection or scattering after the sound source has stopped.

Transmission and Absorption of Sound •Reverberation Time This is number of seconds required for the energy of the reflected sound in a room to diminish to one-millionth of the original energy it had. It can also be defined as the number of seconds required for the sound pressure level to diminish to 60 decibels below its initial value.

Transmission and Absorption of Sound •Echoes This is a distinct repetition of the direct sound. Its effect may be observed by making a short sound such as a clap, in a large room.

Conclusion The basic concepts in architectural acoustics, the nature of sound and its physical properties as discussed in this lesson help in providing an understanding for dealing with the problems of noise.