Architectural Acoustics Basic Concepts

Dr. Raad Al-Bdeery

Basic Concepts• Every building acoustics consideration can

be thought of as a system of sources, paths, and receivers of sound.

• Whether a source is one we want to hear or an undesired source (i.e., noise), control can be exercised at each element of the system. In a simple auditorium both desired (speech from the lecturer as well as from and between the listeners) and undesired sounds (air-conditioning system sounds, noise from outside buildings, etc.) may be present and must be controlled.

• Naturally the building design and technology has most influence on the transmission paths.

• The selection and specification of the quietest available types of mechanical/electrical equipment may obviate the need for later design of special noise and vibration control building elements.

• Placing a noisy operation or activity within a building in a remote place from critical occupancies can save later concern and considerable cost of extraordinary sound attenuation features in the enclosing construction.

Fundamentals of sound and Its Control• Wave: is any disturbance that travels or propagates

from one region to another of a space through a certain period of time.

• Sound is generated whenever there is a disturbance of an elastic medium. Once this disturbance occurs, whether it is in air by the vibrating string of a musical instrument or in a solid floor surface by the impact of a dropped object, the sound wave will propagate away from the source at some rate depending on the elastic properties of the medium

• When a tuning fork is struck, its arms are set into vibration and the air molecules immediately adjacent to the vibrating surface are alternately compressed and rarefied as the surface goes through each complete to-and-fro movement. This cyclical disturbance (compression andrarefaction of the air molecules) is passed on the adjacent molecules and thus travels outward from the surface. The coming figure illustrates how a simple pure tone develops.

λ

Frequency• The frequency of a sound wave is simply the

number of complete vibrations (No. of cycles) occurring per unit of time. Musicians refer to this as pitch, and this basic frequency or rate of repetition of the vibration defines its character. Low frequency sounds such as a deep bass voice are classified as “boomy”. High frequency sounds such as a steam jet my have a “hissing” character.

• The unit of measure is the hertz and abbreviated Hz.

• The tuning fork generates sound at a single frequency. A simple musical tone would have a fundamental tone along with one or more harmonically related tones. All other common sounds, music, speech, and noise are more complex because they contain sound energy (i.e., vibrations) over considerable wider ranges of the human audible spectrum (about 20 to 20000 Hz for young persons with normal healthy airs). The following figure shows how these simple and more complex common sounds compare.

Frequency Bands• For Measurements purposes, the audible frequency

may be divided into convenient subdivisions. Measurements may be made over the entire range or, utilizing electronic filters in the measurement system, the frequency range may be divided into segments such as octave bands or ½, 1/3, 1/10, octave bands

• Octave bands generally yield sufficient information about a sound source.

• In some lab measurements, such as in measuring the sound transmission loss characteristics of walls , however, 1/3-octave- band measurements are made

• The sound sources commonly encountered in buildings, as well as the acoustical performance of products and materials for sound control, are frequency dependent (vary with frequency).

• It’s useful to keep in mind the range of wavelengths encountered in the audible frequency range (20-20000 Hz) range for various building acoustics problems.

• In the lab, sound transmission loss and other measurements on building components are usually made starting at the 1/3-octave band centered at 125 Hz up through the 1/3-octave band centered at 4000 Hz.

• Generally, it takes rather massive, large elements to control low-frequency sound where the wavelengths are large. On the other hand, thinner, smaller building elements can provide effective sound control by absorption, for example, at high frequencies where the wavelengths are smaller.

Properties of Sounds WavesAs discussed in previous slides, sounds waves result from the

vibration of solid objects or the separation of fluids as they pass over, around, or through holes in solid objects. The vibration and/or separation causes the surrounding air to undergo alternating compression and rarefaction, much in the same manner as a piston vibrating a tube. The compression of air molecules causes a local increase in air density and pressure. Conversely, the rarefaction causes a local decrease in density and pressure. These alternating pressure changes are the sound detected by the human ear.The rise and fall of pressure at point will follow a cyclic or wave pattern over a “period” of time. The wave pattern is called sinusoidal. The time between successive peaks or between successive troughs of the oscillation is called the period P. The reciprocal of this, the number of times a peak arrives in one second of oscillation, is called the frequency (f):

fP 1=

The distance between adjacent crests or troughs of pressure is called the wavelength λ. Therefore the speed of sound wave in air c is:

λ

λ

fcP

c

=∴

=

Sound Power and Intensity• Work is defined as the product of the displacement

of a body and the component of force in the direction of the displacement. Thus, traveling waves of sound pressure transmit energy in the direction of propagation of the waves. The rate at which this work is done is defined as the sound power (W).

• Sound intensity (I) is defined as the time-weighted average sound power per unit area normal to the direction of propagation of the sound waves. Intensity and power are related as follows:

AWI =

Where A is a unit area perpendicular to the direction of waves motion. Intensity, and hence, sound power, is related to sound pressure in the following manner:

cp

Iρ

=2

2max

Where: I = intensity, in units of , Pm= sound pressure amplitude in units of Pa

ρ = density of medium in units of

c= speed of sound in medium in units of m/s.

2mW

3mkg

The speed of sound c is a function of temperature T. The speed of sound in air at 1 atm may be determined as:

Tc 0520.=

The constant 20.05 represents , where γ = 1.4, R=8.314 J/mole.K, and M= 29 g/mole

MγR

0

10IILogLp =

Lp is measured in units called decibels, dB

Sound Intensity Level (Noise level) or Sound Pressure Level

For Noise measurements, the sound intensity level is given as:

I0 is the reference intensity ( ) which represents the faintest sound intensity that auditable to human ear, i.e. “threshold of hearing”, this corresponds to sound pressure (pressure amplitude) of 0.02 milliPascal. The maximum sound intensity level is between (120-140) dB which represents the “threshold of pain”, this corresponds to sound intensity I between (1-100) .

2mW1210−

2mW

When two different sound waves propagate in air with pressure amplitudes Pm1 and Pm2 the difference in sound level Lp at the receiving point (human ear) can be computed as:

1

2

21

22

20

)()(10

m

mp

m

mp

PPLogL

PPLogL

=

=

Combining Sound Intensity Levels

Because of their logarithmic heritage, decibels do not add and subtract the way is done to normal things or items. Adding the logarithms of numbers is the same as multiplying them. The dB is converted from to sound power level, adding them, and then converting back to dB.

The following figure provides a graphical solution of this type of problem. For noise pollution at work, results should be prepared to the nearest whole number. When there are several levels to be combined, they should be combined two at a time, starting with lower-valued levels and continuing two at a time with each successive pair until one number remains.

The previous monogram shows that the combined sound level in Decibels is always higher than the higher value. In other word the louder source dominates. Thus, whatever multiple sources are involved, the total sound output may be estimated simply by combining two sources at a time.