architectural acoustics ii

8/3/2019 Architectural Acoustics II

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CHAPTER II

Dr Hani Obeid - Applied SciencesUniversity

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ROOM ACOUSTICS

ABSORPTION IN ENCLOSED SPACES


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Perception of Sound


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Ear AnatomyThe human auditory system consists of:

1. Outer Ear

2. Middle Ear

3. Inner Ear


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OCTAVE AND DECADE


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If a ratio of two frequencies is 2:1, then these frequencies are octave apart.One octave is two to one change in frequency. = 2

= ln

ln 2

If the ratio of two frequencies is 10:1, then these frequencies are

decade apart. = 10

=ln

ln10 Where: is the highest frequency. is the lowest frequency.


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LOUDNESS


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The loudness of a sound is a subjective effect and is defined as a

Function of amplitude and frequency.

The loudness level is given in phones and is defined as beingNumerically equal to sound pressure level in dB at 1000 Hz.

The phones tells us about the subjective equality of various sounds.

Therefore, a ration scale of loudenss, the sone scale, is used.

One sone is defined as the loudness of 1 kHz tone of 40 dB (40 phon).

A sound that is judged to be twice as loud as the reference sound

Has a loudness of 2 sones.


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Dr Hani Obeid - Applied Sciences University 6

40 phons 1 sone

50 phons 2 sones

60 phons 3 sones

30 phones 0.5 sones20 phons 0.25 sones

The values of sones may be added to obtain loudness level.

= +

= + + + Where:

is loudness of the loudest band.

F = 0.15 for third octave. =1.25F= 0.2 for half octave.

=1.4

F= 0.3 for octave band. = 2


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A general equation for the relationship between loudness and

Loudness level of pure tones in the linear region of the curveAs follows:


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ACOUSTICAL ENVIRONMENT


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As previously stated the architectural acoustics is the technology of designing

Spaces, structures and mechanical systems to meet hearing needs.

The architect must:

1. Establish his acoustical objectives (how quiet? Where?)2. Include acoustical considerations in his preliminary planning & estimating.

3. Avoid acoustical pitfalls (shapes which cause echoes, focusing, standing

waves,etc

4. Solve acoustical problems not requiring a specialist.

5. Define his acoustical problems for his consultants & engineers and integrate

6. Their work with elements of design.


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10

Acoustical environment

Free Field Enclosed Field

Acoustics of rooms

Sound Absorption

Building Acoustics

Sound Transmission


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TYPES OF SOUND FIELDS


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The direct field is the zone where sound reaches the listener directlyWithout modification other than attenuation due to distance.

The reflected field is the zone where some of the sound reaches the

The listener with a slight delay compared with direct sound, after reflectionAt one of the walls.

The diffused field or reverberant field is the zone where soundReaches the listener after multiple reflections. These interfere with the

Sound Of the direct field because of different delays.


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Reflection of sound: sound can be reflected by hitting an object larger

than one quarter wavelength of sound.

> S is thickness of object.

Diffraction of sound: if the object is one quarter wavelength or slightly

smaller, the sound is diffracted (bending around the object).

Refraction of sound: refraction of sound occurs when sound passes from

one medium to another.

The laws of reflection:

1. The incident ray, the reflected ray and the normal to the

surface at the point of incidence all lie in the same plane.

2. The angle of incidence equals the angle of reflection.


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Reflection from Concave & Convex Surfaces

A. Concave surfaceB. Convex surface


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GROWTH AND DECAY OF SOUND IN A ROOM


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When a sound source is placed in a room, the sound intensity

at a particular point will increase in a series of small increments

due to the reflections arriving From walls, floor and ceiling until

an equilibrium is attained.

If the sound source is stopped suddenly, the sound will reverberate

in the room And decay will not suddenly go to zero.

The following figures show that:


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ACOUSTICS OF A ROOM


When we put a sound source in an enclosed room, the sound will reach

The listener directly from the source and via reflection from the room surfaces.


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The first sound is called direct sound and the second one is a reflected sound, which

Reinforce the direct sound.

If the sound is stopped from the source, the direct sound will fall to zero immediately,

But the reflected sound will reverberate in the room for sometime.

The reverberation process depends upon the room volume, surface area of the walls

And the absorption characteristics of the room.

If the time between direct & indirect sounds is less than 50 ms (this corresponds

To 17 m), the two sounds heard as one sound and the indirect sound reinforces

the direct. If the time is greater than 50 ms the indirect sound is heard as echo.


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Reverberation time is the period required for the sound pressure level

To decrease 60 dB after the sound source has stopped.


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Typical Reverberation Time


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Function/space use Min (sec) Max (sec)Conference room 0.6 1.3

Amphitheatre 0.6 1.6

Cinema 0.5 1.2

Theatre 1.0 1.8

Concert hall (light music) 1.4 2

Concert hall (orchestra) 1.6 3

Place of worship 1.8 3.2

Restaurant/cafeteria 1.8

Gym/swimming pool/sport hall 2.7

multi[purpose room 1.4 2

Industrial premises 3

Lecture theatre 0.7 1.0


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Reverberation time for various volumes


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Volume (cubic meters) Reverberation time350 1.1

700 1.2

1400 1.3

2400 1.4

3900 1.5

6000 1.6

9500 1.7

14500 1.8

20000 1.9

27000 2.0


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Optimum volume/person for various types of halls


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Type of hall Min Optimum maxCubic meters

Concert hall 6.5 7.1 9.9

Italian type

opera houses

4.0 4.2-5.1 5.7

Churches 5.7 7.1-9.9 11.9

Cinemas 3.1 4.2

Rooms for

speech

2.8 4.9


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Shape of halls


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There are three basic plans used for large halls:

1. Rectangular shape.

2. Fan shape.

3. Horse-shoe shape

In a hall which seats under 1000 people the shape is

not so critical.

The traditional dimensions have the ratioHeight: Width: Length

2 3 5


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Room Acoustics

Shape

Volume

Materials

Room Acoustics


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Room Acoustics

Sound re-enforcement

Reflect

Absorb


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Noise Reduction Coefficient NRC


NCR is the arithmetic average, rounded off to the nearest multiple

Of 0.05 of sound absorption coefficient at 250, 500, 1000, and 2000 Hz

for a specific material and mounting condition.

Therefore, the NCR is intended as a single number rating of sound

Absorbing efficiency at mid frequencies. It is not the difference in sound

Levels between two conditions or rooms.

= + + +

4


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Noise Reduction by Absorbing


= 10

Where:

- total absorption after treatment.

- total absorption before treatment.


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ABSORPTION


Previously, we studied the effect of volume on reverberation time.

Now, we will study the effect of absorption on reverberation time.

We can control the value of reverberation time by using different

Absorption characteristics.

Absorbent may be divided into 3 main types:

1. Porous materials.

2. Membrane absorbers.3. Helmholtz resonators.


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ABSORPTION COEFFICIENT


Absorption coefficients are used to rate a materialsEffectiveness in absorbing sound.

The absorption coefficient is a measure of the efficiency

Of a surface in absorbing sound.

= Where:

A is absorption units, sabins or metric sabins.S is surface area, sq.ft or sq.m.

is absorption coefficient.


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Mounting of Absorbents



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POROUS MATERIALS


Fiberboards, mineral wools, and insulation blankets. All have

One important thing in common-their network of interlocking

Pores. They act by converting sound energy into heat. Sound

Absorption is far more efficient at high than low frequencies.


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People as absorbent


Sound absorption in air


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MEMBRANE OR PANEL ABSORBERS


The absorption of sound at lower frequencies can be effectively

Achieved by resonant (or reactive) absorbers.

A mass suspended from a spring will vibrate at its natural

Frequency. Panels designed with an air in the cavity behind

them act similarly. The frequency of resonance for a flat,un-perforated panel can be estimated from:


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HELMHOLTZ RESONATOR

A resonator is a special device which permits very high absorptionat a given frequency known as the resonance frequency.

A resonator consists of two main parts:

1. A cavity, defined by its volume (V),

2. A neck, characterized by its section (s) and length (l).

The resonance frequency is:

=

2

architectural acoustics ii

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