Acoustics

BUILDING SCIENCE IIACOUSTICS AND ILLUMINATION

VII SEMESTER B.ARCHRINA SURANA

AUGUST 2011

INTRODUCTIONBASIC THEORY

SOUND ABSORPTIONROOM ACOUSTICS

SOUND ISOLATION AND NOISE CONTROLMECHANICAL SYSTEMNOISE AND VIBRATIONS

SPEECH PRIVACYELECTRONIC SOUND SYSTEMS

ACOUSTICS

INTRODUCTION

All acoustic situations can be described by three parts: SOURCE: Speech, HVAC equipment -made louder or quieter through sound absorbing/ reflecting material placement PATH: Air, earth, building materials – made to transmit more or less sound through double wall constructions etc. to interrupt sound path for isolation RECIEVER: Human/ animal ears, medical equipment- internal mechanical

equipment, outdoor noises hear better and be more comfortable if distracting noise is

controlled Best to focus on all three parts Acoustical requirements to be considered at earliest stages of building design as

later corrections may be difficult and extremely expensive

ESSENTIAL ELEMENTS OF

ARCHITECTURAL ACOUSTICS

ROOM ACOUSTICSVolume

Shape and proportionLayout: floor slope, distances from

sourceFinishes: selection and placement

FurnishingsSpecial treatments

SOUND ISOLATIONSite noise characteristics

Outdoor barriers: buildings vegetation earth-berms

Location of activities within buildings

Wall, floor and ceiling constructionBackground noise criteria

(Coordination with room acoustics)

ELECTRONIC SOUND SYSTEMSSystem compatibility with room

acousticsLoudspeaker selection, placement

and orientationSystem components and controlsBackground masking (loudspeaker

layout, sound spectra

MECHANICAL SYSTEM NOISE AND VIBRATIONS

Equipment characteristicsLocation of mech. equip.

Vibration isolation-springs, pads etc.Air duct and pipe treatment-linings

etc.Background noise from air outlets

(coordination with sound isolation)

ASSIGNMENT 1:2nd AUGUST. SUBMISSION 9th AUGUST

To explore market and collect information on acoustic materials.

Find types of materials available. The acoustic properties that are given of those materials (if any). The other physical and architectural properties such as aesthetics, durability,

weathering etc. Where they can be used Fixing details. Prices.

BASIC THEORY

Sound and noise is a vibration in an elastic medium such as air, water, earth, building material. Elastic medium returns to normal state after force is removed Amplitude, Cycle, Frequency, Time Period and wavelength. Sound energy travels but each vibrating particle of the medium moves an infinitesimal

amount and bumps against adjacent particles. It imparts most of its motion and energy to them. The maximum displacement of the particle during vibration is called Amplitude. A full circuit by a displaced particle is called a Cycle. The time required for one complete cycle is called the Time Period and is measured in

seconds per cycle. The number of complete cycles per second is called the Frequency of vibration and is

measured in cycles per second whose unit is Hertz (Hz). The distance a sound wave travels during one cycle of vibration is called the

Wavelength. The movement causes adjacent particles to push together or draw apart -

Compression and Rarefaction. Pitch is the subjective response of human hearing to frequency low frequencies are considered

boomy and high frequencies are screechy or hissy.

FREQUENCY, TONE AND BAND WIDTH

Pure Tone is vibration produced at a single frequency. A graph between sound pressure and time gives a sinusoidal curve A tone is composed of a fundamental frequency with multiples of the fundamental called Harmonics. Complex sounds consist of a variety of pressures which vary over time, most everyday sounds are

complex. Most sound sources, except for pure tones, contain energy over a wide range of frequencies. Hearing range for healthy young person is from 20 to 20,000 Hz. For measurement, analysis and specification of sound, the frequency range is divided into sections (called

bands). One common standard division is into 10 octave bands identified by their center frequencies : 31.5, 63,

125, 250, 500, 1000, 2000, 4000, 8000, 16,000. An octave band in sound analysis, represents a frequency ratio of 2:1

Wave length scales

44m 22m 11m 5.5m 2.8m 1.4m 0.7m 34cm 17cm 8.6cm 4.3cm 2.1cm 1cm

144’ 72’ 36’ 18’ 9’ 4.5’ 2.25’ 1’1/2” 6-3/4” 3-3/8” 1-3/4” 7/8” 7/16”

HEARING RANGE FOR YOUNG

HEARING RANGE FOR OLD

VOWELS CONSONANTS SPEECH

HIGH FIDELITY STEREO

8 16 20 31.5 63 125 250 500 1000 2000 4000 8000 16000 32000

20000

Frequency (Hz) Vibrations below 20Hz are not audible but can be felt.Human speech ranges between 125 to 8000 Hz.

Inverse square law for sound: Sound waves from a point source outdoors with no obstructions (free field conditions) are

virtually spherical and expand outwards from the source. Power is a basic quantity of energy flow measured in watts. Acoustic and electrical forms of energy are different and cause different responses. 10 watts of electric

energy in an incandescent bulb is very dim light, whereas 10 W of acoustic energy can produce an extremely loud sound.

The intensity from a point source outdoors at a distance d away is the sound power of the source divided by the total spherical area 4 ∏ d2 of the sound wave at the distance of interest :

I=W/4∏d2 Where I = sound intensity (W/m2 ) W = sound power (W) d = distance from sound source

(m) The Inverse square law for sound is:

I1 /I2 = (d2 /d1 )2

Where I = sound intensity (W/m2 ) d = distance from sound source (m)

Ernst Weber and Gustav Fechner discovered that: “NEARLY ALL HUMAN SENSATIONS ARE PROPORTIONAL TO THE LOGARITHM OF THE INTENSITY

OF THE STIMULUS” The unit bel was first used to relate the intensity of sound to an intensity level corresponding to the human

hearing sensation. Sound intensity level in bels equals the logarithm of the intensity ratio I/Io where Io is the minimum sound

intensity audible to the average human ear at 1000 Hz. Decibels (logarithm to be multiplied by 10) =

L1 = 10 log I/I0 where L1 = sound intensity level (dB)

I= sound intensity (W/m2) I0 = reference sound intensity = 10 -12

Human hearing ranges from the threshold of audibility at 0 dB to the threshold of pain at 130 dB. Represents a tremendous intensity ratio of 10 trillion to 1. A weighing machine of comparable range would have to be able weigh a human hair and a 30 storey building! Logarithms allow this range to be represented in convenient small numbers.

Outdoors, away from obstruction according to inverse square law the intensity ratio for doubling the distance is 22 or 4 and corresponding decibel reduction is 10 log 4 or 6 dB.

Sound from line sources like vehicles on highway drop by only 3 dB on doubling of distances as the line is a succession of point sources that reinforce each other

120 Hard rock band elect. amplification

110 Accelerating motorcycle few feet away

100 Auto horn Crowd noise at football game90 Printing press Pneumatic concrete breaker80 Cafeteria with sound reflecting surfaces.

60 Near highway traffic

50 Office activities

40 Soft stereo music in residence

30 Residence late at night

140 Jet engine 25 m away

130 Jet aircraft during takeoff 100m away

70 Aircraft cabin during flight

20 Whisper12 Rustle of leaves8 Human breathing0 Threshold of hearing/ audibility

Decibels Examples Subjective evaluation

Painful, dangerous

Deafening

Very loud

Loud

Moderate

Faint

Very faint

140

130

120

100

110

90

80

70

60

0

50

40

20

10

30

Threshold of pain

Threshold of feeling

Threshold of hearing loss (long term exposure)

Threshold of hearing /audibility

SPEECH

12010 30 60

5

10

15

20

25

0

Distance from source (ft)

Noi

se re

ducti

on (d

B)

Point source (spherical, reduction at 6dB per doubling of distance)

Line source (cylindrical, reduction at 3dB per doubling of distance)

•An area source, produced by several adjacent sources like rows of cheering spectators at a sports event or large areas of mechanical equipment has very little reduction of sound energy with distance close to the source.

CHANGE IN SOUND LEVEL

Sound intensity is not perceived directly by the ear; rather it is transferred by a complex hearing mechanism to the brain where acoustical sensations are interpreted as loudness.

Sensitivity to noise also depends on frequency content, psychological factors (emotions, expectations etc.) and duration of sound.

Properties of logs: log xy =log x + log y, log x/y =log x – log y, log xn =n log x, log 1 =0 A reasonable guide to explain increase/ decrease in sound levels is:

Change in intensity level (or noise reduction – NR) is found by:

NR =L1-L2 NR = Diff. in sound levels of 2 conditions

in Decibels (dB)

NR = 10 logI1/I2 I1, I2 = Sound intensities under the two

conditions respectively (W/m2)

NR = 10 log (d2/d1)2 by substitution (inverse square law)

NR = 20 log (d2/d1) where d’s are distances.

Change in Sound Level (dB)

Change in Apparent Loudness

1 Imperceptible

3 Just barely perceptible

6 Clearly noticeable

10 About twice (or half) as loud

20 About four times (or one-fourth) as loud

DECIBEL ADDITION

When two (or more sources) create sound the combined sound is not an algebraic addition as decibels are logarithmic values . If there are more sources, then, combine two at a time. The following table can be used to rapidly combine sound levels.

Find combined sound level of 34,41,43,58 dB 34 + 41 = 42 and 43 +58 =58 , 42 +58 = 58 dB

(Using different orders may give a difference of about 1 dB which is not significant.)

If n number of equal decibel values are to be combined, add 10 log n to the decibel value of one.

For example if n =76 trumpets each playing at an L1 of 80 dB, then the total sound is:

= 80 +10 log 76= 80 +10 (1.8808)= 99 dB for 76 trumpets

When Two dB values Differ by

Add foll. to higher value.

0 or 1 3

2 or 3 2

4 to 8 1

9 or more 0

Sound Intensity Level Sound Pressure Level Sound Power Level

Symbol LI LP LW

Express as 10 log I/ IO 20 log p/po 10 Log W/WO

Units LI measured in dB LP measured in dB LW measured in dB

I measured in W/m2 p measured in N/m2

(or pascal, Pa)W measured in Watt

Reference value* Io = 10 -12 W/m2 po = 2x10-5 N/m2 Wo = 10-12 W (1pW)

At reference value LI = 0 dB LP = 0 dB LW = 0 dB

Pain threshold value I = 10 W/m2 P = 63 N/m2

At pain threshold value LI = 130 dB LP = 130 dB

SOUND ABSORPTION

When sound hits the boundaries and other surfaces of a room, part of its energy is absorbed and transmitted, and part is reflected back into the room. Sound levels in a room can be reduced by effective use of sound absorbing treatment, such as false ceilings, curtains and carpets etc.

Free Field conditions occur when sound waves are free from the influence of reflective/ absorptive surfaces.

Reverberant Field: Indoors, sound energy drops off under free field conditions only near the source (usually < than 5 ft for small rooms). Room surfaces reflect sound so there will be little further noise reduction with distance away from the source (called reverberant field)

EFFECT OF SOUND ABSORBING TREATMENT

Close to the source the reduction will be only 3 dB. The addition of sound absorption to the ceiling of a small room (<500 ft2 ) can reduce the reverberant

sound levels by10 dB. If the ceiling and all four walls are treated then the sound level in the reverberant field drops an additional

6 dB, but sound levels near the source (free field) are not affected. No more reduction is achieved by adding further sound absorbing treatment.

The room was initially completely enclosed by sound reflecting surfaces and had few

furnishings to absorb sound energy.

Generally a reduction of 6 to 8 dB in rev. noise

is more likely the upper limit for furnished spaces

of comparable size

Soun

d le

vel (

dB)

SOUND ABSORBTION COEFFICIENT

SOUND ABSORBTION COEFFICIENT is an expression of the effectiveness of a sound absorbing material. It is the fraction of the incident sound energy that a material absorbs. Varies from >0 (=no absorption) to <1 (perfect, or all incident sound energy absorbed) and is denoted as α.

TOTAL ROOM ABSORPTION is the sum of all surface areas in a room multiplied by their respective sound absorption coefficients with a unit of Sabins in FPS

a = Σ S α where a = total room absorption (sabins)

S = surface area (ft2 )

α = sound abs. coeff. at given frequency as a decimal percent

Materials with SAC > 0.5 are referred as sound absorbing and Materials with SAC <0.2 are referred as sound reflecting. Basic types of sound absorbing materials are porous materials, vibrating or resonant panels, and

volume resonators. Sound absorption by Porous and fibrous materials is predominantly the indirect conversion of sound

energy to thermal energy mainly by frictional flow resistance. The amount of sound absorption that can be achieved depends upon physical properties of thickness,

density, and porosity for most porous materials and fiber diameter and orientation for fibrous materials

EFFICIENCY OF SOUND ABSORBERS

EFFECT OF THICKNESS ON ABSORPTION EFFICIENCY Thickness has a considerable effect on sound absorption efficiency of porous materials but the pores must

be inter-connected.

Vibrating Panels convert sound energy into vibrational energy which is dissipated by internal damping and radiation

Volume Resonators reduce sound energy by friction at openings and by inter-reflections within the cavities

These specialized types of sound absorption techniques can be used to supplement porous materials or to absorb specific low frequency sound energy like a 120 Hz ‘hum’ from electrical equipment

NOISE REDUCTION COEFFICIENT

The Noise Reduction Coefficient NRC is the arithmetic average , rounded off to the nearest multiple of 0.05, of the Sound Absorption Coefficients α’s at 250, 500, 1000, and 2000 Hz for a specific material and mounting condition.

NRC is a single-number rating of sound-absorbing efficiency at mid-frequencies. NRC is not: as the name suggests, the difference in sound levels between two conditions or

between two rooms

NRC = (α250 + α500 + α1000 + α2000 )/4 Where: NRC = noise reduction coefficient (decimal percent)

α = sound absorption coefficient ( decimal percent)

Note : Two materials may have identical NRC but very different absorption characteristics.

As NRC does not include α’s at 125 and 4000 Hz it should not be used to evaluate materials where speech or music perception is important

APPLICATIONS FOR SOUND ABSORBING MATERIALS

REVERBERATION CONTROL: So that speech is clear and not garbled. The larger the room volume, the longer the RT because sound waves will encounter room surfaces less often than in small rooms. Each doubling of the total amount of absorption in a room reduces the RT by one half. Sound absorption can make sound appear to come directly from the actual source rather than from everywhere.

NOISE REDUCTION IN ROOMS: When correctly used, they can be effective in controlling noise build-up in a room. However they have limited application for noise control and are not all-purpose solution for all noise problems. Each doubling of the total amount of absorption in a room reduces the noise level only by 3 dB. Thus it becomes an increasingly inefficient method for noise reduction.

In large open-plan rooms, sound absorbing materials can contribute to speech privacy by causing sound energy to decrease with distance.

ECHO CONTROL: Sound Absorbing Materials can be used to control Echo usually (along with reverberation control). Echoes are long delayed, distinct reflections of sufficient sound level to be clearly heard above the general reverberation as a repetition of the original sound. Flutter Echo, which can be heard as a ‘rattle’ or ‘clicking’ from a hand clap, may be present in small rooms (or narrow spaces with parallel walls) can also be controlled with SAM.

REVERBERATION TIME

Wallace Clement Sabine (beginning 1895 at 27) – criteria for good listening conditions in rooms were largely non – existing.

Asked to improve listening conditions for speech in the new lecture hall (Fogg Art Museum, Harvard Univ.) Sound in the hall would persist for about 5 ½ s due to multiple reflections from the hard surfaces of the hall. Most English speaking persons complete 15 syllables in this time, words were impossible to understand everywhere in

this hall. Sabine recognized that the problem of persistence of sound energy in the room was due to the size of the room, its

furnishings and the occupants. Size of the room affects av. length of reflections, called the Mean Free Path approx. equal to 4V/S where V =Volume and

S = surface area, (square and cubic feet). Called this persistence “duration of audibility of residual sound”. Repeated tests conducted in the hall with organ pipes as source – had an initial sound level of 60dB above a young

listeners threshold of audibility at 512 Hz. He tried to find out how much time it took for the 60 decibels sound to decay – 1 / 1,000,000 of the initial sound level.

Conducted tests late at night. Defined as REVERBERATION TIME. Cushions used (3” thick, porous sound absorbing material covered with canvas and damask). More cushions- more sound

absorption and lower RT. When he used 550 cushions (1m long) the RT became 1 sec. First unit of sound absorption was 1m length of this cushion. The result of Sabine’s work made it possible to plan RT in advance of construction. For the first time , desired RT in rooms

at 512 Hz could be the result of design and not luck or copying.

REVERBERATION TIME

• The equation which Sabine defined and proved empirically is :

T = 0.05 V/a Where

T = reverberation time, or time required for sound to decay by 60 dB after source has stopped.

V = room volume (ft3)

a = total room absorption in sabins.

This is the sabine formula and is appropriate to use in most architectural work.

It is reasonably accurate when sound field conditions are diffuse (uniformly distributed absorption) and room dimensions do not vary widely; without one extremely long dimension, deep pockets, or transepts in churches etc.

Different activities and conditions have different preferred RT’s. A classroom having a height of 15 ft is 60 ft long and 35 ft wide. Sound Absorption Coefficients α’s is 0.30

for the walls, 0.04 for ceiling, and 0.10 for floor at 500Hz . Find RT at 500 Hz for this space with no occupants and no sound absorbing treatment. Find RT if 50% of the ceiling is treated with acoustical panels with α of 0.85.

ROOM NOISE REDUCTION

The buildup of sound levels in a room is due to repeated reflections of sound from its enclosing surfaces. This buildup is affected by the size of the room and the amount of the absorption within the room.

The difference in decibels in reverberant noise levels or noise reduction , under two conditions of room absorption can be found as follows:

NR = 10 log a2/a1 Where

NR = room noise reduction (dB)

a2 = total room absorption after treatment (sabins)

a1 = total room absorption before treatment (sabins)

Since absorption efficiencies vary with frequency, the NR should be calculated at all frequencies for which sound absorption coefficients are known.

The NR is the reduction in reverberant noise level. This does not affect the noise level very near the source of sound in a room.

A reduction in the reverberant noise level of 10dB (an increase in absorption greater than 10 times the initial value before treatment) is the practical upper limit for most remedial situations.

OPTIMUM REVERBERATION TIME

PROBLEM

A small room 10 ft x 10 ft x 10 ft has all walls and floors finished in exposed concrete. How much is the noise reduction in the room if the ceiling is completely covered with sound absorbing material.

Sound Absorption Coefficients α’s are 0.02 for concrete and 0.70 for the false ceiling, both at 500Hz.

Find the NR in the room if sound absorbing panels are added to two adjacent walls . The sound absorption coefficient α is 0.85 for the panels at 500 Hz.

Find the NR if all four walls are treated and the floor is carpeted. The sound absorption coefficient α for the carpet at 500Hz is 0.50.

ROOM ACOUSTICS

BACKGROUND

DIRECTIVITY CONTOURS FOR SPEECH ANCIENT THEATRES Open air Greek and Roman theatres had good listening

conditions for drama and instrumental recitals.

Located on steep hillsides in quiet rural locations.Layouts were semi-circular so audience close to stage, thus reducing sound energy loss due to distance.Tiers were steep (>20 0)to provide good site lines, reduce attenuation caused by seated audience, permit sound energy to be reflected from orchestra floor.Actors also wore masks with conical megaphones built in the mouthpieces which reinforced their voices.

AUDITORIUM PLAN WITH SPEECH CONTOUR OVERLAY

AUDIENCE SEATING AND SIGHT LINE BASICS

AUDIENCE SEATING SIGHT LINE BASICS

•Outdoors, sound levels fall off with distance,and audience attenuation as it gets scattered and absorbed grazing against the audience.•An overhead sound reflector can provide reflected sound to reinforce the direct sound .•Providing a hard reflecting enclosure outdoors can greatly improve listening conditions.

•Unobstructed sight lines allow full view of performers and unobstructed propagation of the direct sound. •Sight lines are normally drawn to converge at a point on stage called The Arrival Point of Sight APS.•Laterally staggered seating layouts can achieve satisfactory alternate row vision for back to back dimension B of 40 inches for continental seating and 36 inches for radial and parallel aisle seating.

PLAN, DETAIL AT STAGE SECTION

•Proscenium theatres should preferably have lateral sight lines with a preferred view angle of 300. •The view angle is measured from the perpendicular at the end of the proscenium opening.

•Floor and balconies should be designed so that entire performance area, performance and scenery can be viewed by seated audience.•Balconies should not have slope greater than 260 and the top balcony should not be more than 65 ft above the stage.•A balcony view of the first few rows of seating may be desirable for a sense of congregation.•The proscenium arch should not obstruct the view of bottom 7 ft. of the backstage wall.

WHEN SOUND WAVES IMPINGE ON A HARD SURFACE

REFLECTION ( x > 4 λ ). If the surface dimension x is larger than 2-4 times the wavelength of

the sound wave, the angle of incidence <i will be equal to the angle of reflection <r . For example 1000Hz corresponds to a wavelength of 1.1ft.; therefore a surface dimension of 4 λ or 41/2 ft will reflect sound energy wavelengths of 1000 Hz and higher.

DIFFUSION ( x = λ ). Is the scattering or random redistribution of a sound wave from a

surface. It occurs when the surface depths of hard surfaced materials are comparable to the wave lengths of the sound.

This is an extremely important characteristic of rooms used for musical performances, when satisfactory diffusion is achieved, audience will have the sensation of sound coming from all directions at equal levels.

DIFFRACTION ( x < λ ). Is the bending or flowing of the sound wave around an object or

through an opening. For example a horn of an automobile located behind a building can be heard on the other side because the sound waves bend around the corners of the building.

In auditoriums, impinging sound waves will readily diffract around panels that are smaller than their wavelengths, suspended panels must carefully be designed to be large enough (length and width) to effectively reflect the desired wavelengths.

PATTERN OF REFLECTED SOUND

CONCAVE REFLECTOR Concave sound reflecting surfaces such as barrel vaults (churches)

and rear walls (auditoriums) can focus sound, causing hot spots and echoes in the seating areas.

They are poor distributers of sound and should be avoided where sound reflecting surfaces are required near source locations in a room such as the walls near the stage.

FLAT REFLECTOR Flat building elements, if large enough and properly oriented can

effectively distributer reflected sound. A slight tilt can project sound energy to the rear of an auditorium.

CONVEX REFLECTOR Can be most effective for sound distribution. The reflected waves

diverge, enhancing diffusion which is highly desirable for music listening.

Reflected sound from convex surfaces is more evenly distributed across a wide range of frequencies.

RAY DIAGRAM GRAPHICS

PATH DIFFERENCE = REFLECTED PATH - DIRECT PATH

From location 1path difference = (11+18) -12 = 17 ft.

Excellent for speech and music as it is less than 23 ft.

From location 2path difference =(16+26) -33 = 9ft.

Excellent for speech and music as it is less than 23 ft.

•Ray diagrams are a good and simple method to find the path difference between a reflected sound path and the direct sound path.•A scaled drawing is required and the principles that: < of reflection = < of incidence and Distance = velocity x time

• Useful sound reflections for speech are those which come from the same direction as the source and are delayed by less than 30 ms

Sound Path Difference

(ft)

Time delayGap (ms)

Listening conditions

<23 <20 Excellent for speech and music

23 to 34 20 to 30 Good for speech, fair for music

34 to 50 30 to 45 Marginal, (blurred)

50 to 68 45 to 60 Unsatisfactory

> 68 >60 Poor (echo if strong enough)

•The initial-time-delay-gap is the time interval between the arrival of the direct sound and the first reflected sound of sufficient loudness.•It should be less than 30 ms or the path difference should be less than 34 ft for good listening conditions because sounds within this time interval can combine to create a single impression in the listeners brain.•Early arriving reflected sound (within 80 ms of direct sound) is important for clarity of music. Auditoriums with narrow shapes support early reflected sound because the initial-time-delay-gaps will be short.•Less initial-time-delay-gap increase the listeners sense of intimacy.•The listener in the auditorium above will hear the direct sound first and then after the initial-time-delay-gap, reflections from the wall (1), Ceiling (2), stage enclosure (3), and so on.

CEILINGS

CEILINGS ECHO CONTROL PRINCIPLES

•Hard sound reflecting flat ceilings provide useful sound reflections which cover the entire seating area of the room.•However, by careful reorientation of the ceiling, as shown, the extent of useful sound reflections can be increased so that the rear seats receive reflections from both ceiling planes.•For concert halls where long RT is a design goal high ceilings are preferred and all walls should be sound reflecting

•Potential echo producing surfaces should be treated with sound absorbing materials or shaped as shown.•The front portion of the ceiling is lowered to reduce the delayed reflections from overhead and reoriented to provide useful reflections towards the rear of the auditorium.

REAR WALL

SOUND ABSORBING WALL TREATMENTS REAR WALL ECHO CONTROL TREATMENT

Deep treatment can be provided either by thick sound absorbing materials or thin sound absorbing materials installed with airspace behind.

A flat, sound reflecting rear wall can produce echoes or unwanted long delayed reflections in medium to large auditoriums. They can be treated as shown.The treatment can be concealed or protected by using perforated facings which are highly transparent to sound waves

SIDE WALLS

•Lateral reflections help create a favorable auditory spatial impression or intimacy- essential for satisfactory perception of music performances.•The initial time delay gap or ITDG is measured from a listener seated near the centerline of the hall, halfway between the conductor and the rear wall.•ITDG should be less than 20 ms•Wide fan shapes and semi-circular floor plans do not provide strong early lateral reflections .•The reverse fan shape can provide strongest lateral reflections and spatial impressions

Fan shape (for lecture rooms)

Rectangular shape (dashed shape indicates preferred orientations for lecture rooms)

Stepped shape (alternate elements of side wall are parallel to provide lateral reflections towards audience)

Reverse fan shape (side walls at rear reflect sound towards audience)

FLUTTER ECHO SMALL ROOMS

•Caused by the repetitive inter-reflections of sound energy between parallel or concave surfaces. It is normally heard as a high frequency ringing or buzzing .•Prevented by reshaping, providing deep sound absorption, and splaying (1:10 or >50 tilt) or scalloping smooth surfaces. •It can also occur in non-parallel walls- pitched roof flutter.

Sound Absorbing Surfaces Opposite Sound Reflecting SurfacesIn small rooms the reflected sound will be minimized by using sound absorbing materials on adjacent walls or on two opposite corners

CONCAVE SURFACES

Poor distribution of sound in domed spaces: The extent of seating affected by focusing on the left

will be more than shown because source location on stage will vary

Concave wall and ceiling usually require treatment as they cause reflected sound to converge at a focal point or may get reflected along smooth concave surfaces called Creep Echo or the Whispering Gallery Effect because low voice levels or whispers can be heard at considerable distances.

SOUND REFLECTIONS•A good reflector has a hard surface like plaster, acrylic, gypsum board, sealed wood.•Is significantly larger than the wavelength of the sound designed to reflect (4 λ).•The sound reflecting canopy as shown below provides useful reinforcement for the direct sound as well as prevents long delayed reflections and potential echo conditions from the high ceiling.

VARIABLE SOUND ABSORBERS

•When RT must be varied to satisfy different activity requirements in a room the sound absorbing treatment can be designed to be adjustable. •Like Retractable Curtains.• or like Sliding Facings which have two layers of perforated material- when the holes are lined up it acts as an absorber and when they are staggered they act as reflectors with sound absorbing material behind the panels.

•They can also be designed to expose either sound absorbing or reflecting surfaces.•Like Hinged Panels.•Or like Rotatable Elements.

VARIABLE VOLUME EXAMPLES

Examples of auditoriums where the cubic volume can be changed to match requirements of reflected sound energy and reverberance requirements of the intended performances and seating capacities can vary from more than 3000 to less than 1000

STAGE BASICS

•In the thrust (open) and arena stage types sound reflecting walls and ceilings or panels are extremely important to help compensate for the directivity of high frequency speech signals which are more directional and so considerably less high frequency sound is radiated behind the performer and this part of the frequency range strongly determines speech intelligibility.•Panels called Stage Enclosure (reflecting and diffusing) can be used on stage to surround or enclose the sources of sound and will help distribute balanced and blended sound uniformly in the audience area. Stage enclosures also prevent sound energy being absorbed by scenery (highly absorbing) in the fly loft and wings.•The surface surrounding the orchestra should also contain small irregularities to blend and reflect high frequency sound.

CANOPIES

Sound reflecting panels, called Forestage Canopies suspended in front of the proscenium, reflect sound energy from the stage to the audience and decrease the initial time delay gap. These panels extend the orchestra shell into the auditorium which enhances the direct sound needed for intimacy and can also reflect sound energy from the orchestra pit back to the pit.

The openings between the panels allow sound energy to flow into the upper volume so it can contribute to the low frequency reverberance in the main auditorium below.

The RT of the stage houses should be approximately equal to or less than that of the main auditorium.

BALCONIES

•Are used in large auditoriums to reduce the distance to the rear seats and to increase seating capacity.•To prevent echoes off the balcony face, apply deep sound absorbing finish, tilt or slope the surface facing the stage, so sound will be reflected towards nearby audience or use diffusing shapes (Convex) to scatter sound. Persons seated deep under a balcony

cannot receive useful reflected sound from the ceiling and are shielded from reverberant sound. As the sound is weak and dull the listening conditions are poor.

•In a Concert Hall the depth D of the under balcony should not exceed the height H of the opening for reverberant sound to reach the rear rows. (D ≤ H)•In Opera Houses D should not exceed 1.5 H. (D ≤1.5 H)•The balcony soffit should be sloped to reflect sound towards the listeners seated underneath

In Cinema halls direct reinforced sound from loudspeakers located behind the screen allow deeper balcony depths up to 3H. (D ≤ 3H).

The cantilevered or flying balcony is open at the rear, allowing reverberant sound to surround the audience underneath. D can be more than conventional balconies of the same height because reverberant energy will be greater at the rear rows.

CHECKLIST FOR LECTURE ROOMS

MULTIPURPOSE AUDITORIUMS DESIGN

WORSHIP SPACES DESIGN

ROOM ACOUSTIC DESIGN

SOUND ISOLATION