architectural acoustics ii
TRANSCRIPT
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CHAPTER II
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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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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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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
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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
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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
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= 10
Where:
- total absorption after treatment.
- total absorption before treatment.
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ABSORPTION
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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
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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
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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
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Sound absorption in air
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MEMBRANE OR PANEL ABSORBERS
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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