Rak-43.3415 Building Physics Design 2
ACOUSTICAL DESIGN
Autumn 2015
LECTURE 4
Room acoustics
Matias Remes, M.Sc. FISE A acoustics
Room acoustics
”Huoneen hyvällä akustiikalla tarkoitetaan
sellaisia äänisuhteitten ominaisuuksia, että
huoneessa esitetty puhe ja musiikki kuuluu
korvaan kauniina, luonnollisena ja selvänä
huoneen jokaisessa kohdassa.”
Diplomi-insinööri U. Varjo 1938
What is the significance of room
acoustics?
Significance of room acoustics
• The purpose of room acoustical design is to control the
propagation, reflection and attenuation of sound within a
space
–
–
–
–
Direct sound
Reverberant sound (reflections)
Useful and harmful reflections
Sound attenuation and absorption, diffusion
• Design goals according to the use of space, for example:
– Speech
 Good speech distinction (e.g. auditorium) / good speech privacy
(e.g. open plan office)
– Music
 Appropriate reverberation and sense of space in the audience,
stage acoustics which support music making
Significance of room acoustics
• Room acoustical design
– Design of sound reflections
– Design of sound absorption
– Design of the shape and geometry of the space
• Room acoustical design ≠ maximising the amount of
sound absorbing material
– E.g. in a lecture hall the performer must be able to speak without
restarining ones voice and so that the audience can distinguish
what is being said
 Need for both sound absorbing and reflecting surfaces!
• Succesful room acoustics is, thus, a combination of the
geometry of the space and the absorptive and reflective
properties of materials
Significance of room acoustics
Examples of design goals
•
Movie theater
– Hearing the sound track in the way the movie makers have intended it to be
heard
•
Concert hall
– Good spatial impression (sound surrounds the listener), sense of intimacy,
”warm” sound color, adequate clarity, etc.
•
Restaurant
– Peacefull acoustical environment (communication from short distance)
•
Open plan office
– Speech sound distract concentration  speech privacy between work
places
•
Factory
– Noise level may cause hearing damage  design of effective sound
absorption and noise blocking screens
Reflection of sound
• Sound reflects at simplest as light:
angle of incidence = angle of
reflection (specular reflection), this
applies when
– Sound wavelength is adequately
smaller than the dimensions of the
object causing the reflection
– The reflecting surface is even (not
sound scattering) and hard (not sound
absorbing)
• Sound reflection is complicated
phenomenon and depends on
frequency and the properties of the
reflective surface


Reflection of sound
Significance of reflections
• Example: sound suddenly stops in a large concert hall
– Number of reflections occuring within the first 1 s is about 8000
• As the number of reflections increases, there is a
reverberant sound field in the space in which the listener
cannot distinguish single reflections from one another; in
large spaces this occurs after about 100 ms
• Sound field in a space comprises of three
distinguishable parts:
– Direct sound
– Early reflections
– Reverberant sound field
Sound field in a room
1
1
1
1
3
3
1.
Direct sound from source to listener
2.
Early reflections within 20...50 ms after direct sound
3.
Gradually attenuating reverberant sound field
Refelection of sound
Sound field in a room
Direct sound, early reflections and
reverberant sound and their relations
determine how sound is perceived in a
space
Kuvat: Rossing et al .
2002, Beranek 2004
Reflection of sound
Significance of reflections
•
•
•
•
Sound perception is affected by the
level of reflections, their delay in
relation to direct sound and the
direction from which they reach the
listener
Strong reflections with adequate
delay are heard as separate
echoes (disturbance)
If the delay between direct sound
and early reflections is appropriate
(about 50...80 ms), the reflections
increase the loudness of sound
(perceived sound level) 
important in the design of speech
and music spaces
Lateral reflections (reaching the
listener`s ears from the sides) add
to the sense spatial impression
and broadening of the sound
source  crucial in the design of
concert halls
The effect of single lateral reflection to sound
perception (Barron 2003)
Reflection of sound
The effect of basic room geometry
Rectangle:
Lateral reflections occur
in the entire space
Fan-shape:
Reflections scatter and are
directed mainly to the rear
part of the space (not in the
middle)
Round:
Reflections from concave
surfaces cause sound to
strongly focus on some
parts of the space
Reflection of sound
Example: sound focusing
7
3
8
1
13
9
4
11
10
6
5
2
1
12
Reflection of sound
Effect of surface geometry
Reflection of sound
Example: reflecting surfaces in a concert hall
Reverberation time
Significance of reverberation time
• The reverberation time in a space correlates rather well
with the perceived clarity of speech or music: long
reverberation  the syllables in speech or separate
musical notes attenuate slowly and mask each other
Reverberation time
Significance of reverberation time
• Too short a reverberation time is not desirable because in an
overly damped space there are no useful reflections!
• In addition to appropriate reverberation time, good room
acoustics provides that
– The space has appropriate size and shape
– Sound absorbing and reflecting surfaces are positioned correctly
• Two viewpoints: room acoustics experienced by the audience
and by the performer
– For the audience, it is important to have useful sound reflections
form the performer to the audience
– For the performer, it is important that the stage acoustics supports
the performer`s activity
Reverberation time vs. use
Reverberation time vs. use
Tila
V / hlö
[m3]
Kokoustila
3…5
Auditorio, teatteri
4…6
Musiikkiteatteri,
ooppera
5…8
Kamarimusiikkisali
6…10
Konserttisali
8…12
Kirkko
10…14
Reverberation time
Sabine equation
• Sabine equation:
V
RT60  0,161
A
n
A  1S1   2 S 2  ...   n S n    i Si
i 1
• Sabine equation assumes that the sound field in the room is
diffuse, i.e., at any point in the room sound can arrive from
any direction and the field remains the same throughout the
room
• This is an idealisation that does not hold perfectly true in real
rooms
Reverberation time
Notes on Sabine equation
•
Sabine equation can be used with good accuracy in rooms which
are sufficiently reverberant
– Sabine equation is most accurate in a reverberant room where the average
absorption coefficient is < 0,25 1)
– In very absorbent rooms the Sabine equation gives erroneous results
•
Additional requirements for Sabine equation to yield accurate
results:
– The room geometry should be simple (cube-like) and the room should be
quite small
– The absorption material should be evenly distributed on the room surfaces
– Calculation error increases in large and complex spaces
•
In rooms where all the absorption material is positioned only on one
surface, Sabine equation yields shorter reverberation time than is
the case in practice
1) F.A.
Everest, K.C. Pohlman, Master
Handbook of Acoustics, 2009
Reverberation time
Eyring-Norris equation
• Eyring-Norris equation:
V
RT60  0,161
 S ln 1   average 
where V is room volume, S is the total surface area of
the room and αaverage is the average absorption
coefficient:
 average
S


S
i
i
i
Reverberation time
Notes on Eyring-Norris equation
• Eyring-Norris equation can be used in more absorptive rooms
where average absorption coefficient is > 0,25, e.g., studios
• However, sound absorption coefficients that are commonly
available and published by material manufacturers are
Sabine coefficients (measured in a reverberation chamber
and calculated using Sabine equation) and can, thus, be
directly applied only to the Sabine equation
• For this reason, Sabine equation is the usual choice in
acoustical design and is also used on this course
• Other researchers have suggested alternative reverberation
formulas, e.g., Fitzroy, Millington, Hopkins-Striker...
Reverberation time
Air absorption
• Taking account of air absorption, the Sabine and EyringNorris equations can be written as:
Sabine:
V
RT60  0,161
A  4mV
Eyring-Norris:
RT60  0,161
V
 S ln 1   average   4mV
where m is the air attenuation coefficient (some values: m =
0,009 at 2 kHz; m = 0,025 at 4 kHz; m = 0,080 at 8 kHz)
• Air attenuation
– is only significant in large spaces above 2 kHz
– depends on relative humidity of air, absorption increases at low
humidity
Air absorption
T = 20 °C, RH = 50 %
Room modes
•
•
•
•
•
•
Sound field within a room is comprised of room
resonances, called room modes
Room mode = characteristic resonance of the
room
Three types of modes: axial, tangential, oblique
The amount and spacing of room modes changes
with frequency
At low frequencies there are only a few room modes
and the modes are sparsely spaced, as a result of
which the reverberation time and sound level can
vary considerably in different points in the room
(consider the placement of a subwoofer in a living
room)
At high frequencies the number of room modes gets
so high and their frequencies are so close to one
another, that single room modes cannot be
distinguished  sound field approaches the
idealisation of diffusivity
Room modes
•
The frequency, below which the sound field in a room is not diffuse (socalled Schröder frequency) depends on reverberation time and volume:
T
f s  2000
V
•
Example, typical dewlling room: T = 0,5 s ja V = 30 m3  fs = 260 Hz
•
The room modes of a rectangular room can be calculated based on the
dimensions of the room (Lx, Ly, Lz):
c  l   m   n
     
f lmn  
2  LX   LY   LZ

where l, m, n are integers
2
2



2



Room modes
•
The number of modes (mode
density) increases with
increasing frequency, at low
frequencies mode density is low
•
From room acoustics point of
view, room modes are the more
problematic the smaller the size
of the room
•
Room modes must be
considered in the design of, e.g.
– Control rooms
– Recording studios
Room modes
•
Standard deviations of
reverberation times
measured in 50 empty
rooms in a dwelling
•
Each point represents
the reverberation time
calculated from
measurement
conducted in 12 points
in a room
•
In small rooms sound
field is not diffuse at
low frequencies, thus
the variance in
reverberation time
increases towads low
frequencies
Absorption vs. reflection
Sound absorbing materials
General
• Sound absorption –
three absorption
mechanisms:
– Porous materials (P)
– Resonant absorbers (R)
– Membrane / panel
absorbers (M)
• Typical absorption
behaviour in the figure
Sound absorbing materials
Porous materials – effect of placement
•
•
•
Sound absorption of porous
materials is based on thermal
losses caused by friction in
the pores of the material
At the surface of a rigid
structure (e.g. wall, roof)
sound pressure is at
maximum and particle
velocity at minimum
Maximal particle velocity
occurs at 1/4λ distance from
the surface of the structure
 to achieve effective
absorption, there should be
absorbing material at this
distance
Sound absorbing materials
Porous materials
•
•
•
Porous material absorbs sound most effectively when the thickness of the
material is at least four times the sound wavelength: d ≥ 1/4λ
Example:
mineral wool 20 mm: 20 mm ≥ 1/4λ  λ ≤ 80 mm  f ≥ 4290 Hz
Note: low-frequency sounds have long wavelength (e.g. 100 Hz  3,4 m) 
thin layers of porous material do not much absorb low frequencies!


/4
/4
Sound absorbing materials
Porous materials
•
Absorption
coefficients of
mineral wool
•
Note the effect of
suspension height
Sound absorbing materials
Porous materials – examples of use
Sound absorbing materials
Porous materials – examples of use
Sound absorbing materials
Perforated panels (resonant absorbers)
•
•
•
Perforated panels act as resonant absorbers; the absorption is
based on mass-spring resonance caused by the air in the hole
acting as mass and the air in the background airspace acting as
spring
Absorption is most effective at the resonance frequency of the
mass-spring system
Resonance frequency and absorption coefficient are affected by:
– Thickness of the airspace and filling with porous material
– Size, amount and geometry of holes
– Thickness of the panel
•
•
Resonance frequency is typically at mid frequencies (500-1000 Hz),
below and above which absorption coefficient decreases
Absorption coefficient of perforated panel absorbers can be
increased by filling the background airspace with porous absorption
material
Sound absorbing materials
Perforated panels
1,0
0,8
0,6
Reikäala 17 %, ilmaväli 30 mm
Reikäala 17 %, ilmaväli 200 mm
0,4
Reikäala 17 %, ilmaväli 200 mm,
jossa 50 mm mineraalivilla
0,2
Keskitaajuus [Hz]
4000
2000
1000
500
250
0,0
125
Absorptiosuhde 
Reikäala 7 %, ilmaväli 30 mm
Sound absorbing materials
Panel absorbers
• Structure of a panel (or membrane) absorber: a closed
airspace behind an impervious (not perforated) panel
which can be filled with porous material
• Absorption coefficient is highest at low frequencies
around the resonance frequency
• Resonance frequency depends on the surface mass of
the panel and thickness of the airspace:
60
f0 
m`d
• Note: at high and mid frequencies panel absorbers are
sound reflecting structures!
Sound absorbing materials
Panel absorbers
1,0
Kipsilevy 13 mm, mineraalivilla 50
mm
0,6
Kipsilevy 13 mm, mineraalivilla 100
mm
Kipsilevy 13 mm, ilmaväli 100 mm
0,4
2 x kipsilevy 13 mm, mineraalivilla
50 mm
0,2
Keskitaajuus [Hz]
4000
2000
1000
500
250
0,0
125
Absorptiosuhde 0,8
Sound absorbing materials
Miscellaneous materials and structures
Hard surfaces
1,0
0,6
Lakattu puu 69 mm
Maalattu betoni
Muovimatolla päällystetty betoni
Rapattu tiili
0,4
0,2
Keskitaajuus [Hz]
4000
2000
1000
500
250
0,0
125
Absorptiosuhde 
0,8
Chairs and audience
1,0
0,6
Puu- tai muovituolit
Pehmustetut istuimet
Yleisö pehmustetuilla istuimilla
0,4
0,2
Keskitaajuus [Hz]
4000
2000
1000
500
250
0,0
125
Absorptiosuhde 0,8
Air absorpion
Effect on reverberation time
2,5
2,0
1,5
1,0
0,5
Keskitaajuus [Hz]
4000
2000
1000
500
250
0,0
125
Jälkikaiunta-aika T [s]
• In large spaces
reverberation time
decreases at high
frequencies because of
air absorption
• Example curve:
Savonlinna hall
Effect of vapour barrier on absorption
Effect of vapour barrier on absorption
Classification of absorption materials
•
According to EN 11654
(classes A-E)
•
Measured absorption
coefficient is compared to
a reference curve, the
sum of unfavourable
deviations ≤ 0,10
•
Note: definition of
absorption class does not
consider frequencies
below 200 Hz!
Classification of absorption materials
Sound diffusing structures (diffusors)
[Salter et al. 1999]
Concert halls
Subjective and objective parameters
• The characteristics of sound perceived in a hall can be
describes subjectively (e.g. ”warm sound”, ”three
dimensional sound”)
– Difficulty: how to define accurately what different subjective
descriptions mean
– Common terminology to be used by acousticians and musicians
has been developed (”subjective parameters”) which define the
different sound characteristics (e.g. Beranek)
• Objective parameters are measurable quantities which
try to describe the subjective perceptions as well as
possible
Concert halls
Objective parameters
•
Reverberance (Kaiuntaisuus)
–
–
•
Clarity (Selvyys)
–
–
•
Strength G
Warmth, brilliance (Äänen lämpimyys ja kirkkaus)
–
–
•
Lateral Energy Fraction LF (describes hthe amount of reflections reaching the listener from
the sides, laterally)
Loudness or strength of sound (Äänen voimakkuus)
–
•
Clarity C80 (describes how much of the sound arrives at the listener 0...80 ms after direct
sound)
Definition D50
Spatial impression (tilan tuntu)
–
•
Reverberation time RT60
Early decay time (Varhainen jälkikaiunta-aika) EDT, has been found to correlate better with
subjective impression than RT60)
Bass Ratio
Treble Ratio
Stage support (lavatuki)
–
Support ST1 (correlates with how the performers are able to hear each other on stage)
Concert halls
Connections between parameters
Subjektiivinen ominaisuus
Objektiivinen mittaluku
Concert halls
Sound field in a concert hall
• Acoustics experienced by the audience
– Ideally the audience should be able to hear the performace in
the same way in every point of the hall (nod acoustically bad
seats)
– The balance between different players / singers should be good
and not dependent on the listening position
• Acoustics experienced by the performer(s)
– Performers should be able to hear each other
– Room acoustics shoud act as a continuation of the performers
instrument: the performer should be able to hear the acoustics
of the space properly and the acoustics should support the
performers efforts
Concert halls
Design issues
• Volume and basic shape
– What is performed?
•
•
•
•
Rock / classical?
Full-sized syphony orchestra / chamber music?
Music theatre / opera?
Multi-functionality?
– Seating capacity (audience, performers)
• Also other than room acoustical issues must be considered!
– Sound insulation to surrounding spaces
– Noise control
• HVAC
• Theater equipment, lighting etc.
Concert halls
Example: Wien, Musikverein (1870)
Concert halls
Example: Berlin Filharmonie (1963)
Concert halls
Example: Finlandiasali (1971)
Concert halls
Example: Sibeliussali (2000)
Auditoria
• Most important room acoustical consideration: speech
intelligibility
• Essential to good speech intelligibility:
– Direct sound path from speaker to listener (other audience
members must not be in the way to block the direct sound)
 raised floor on the audience area
– Directioning of early reflections towards the audience (to
strengthen the direct sound and, thus, improve speech
intelligibility)
 positioning of sound reflecting and absorbing surfaces
– Appropriate reverberation time (remember the masking effect
from previous slides…)
Auditoria
Objective parameters
• Objective parameters describing speech intelligibility:
– Speech Transmission Index STI
– Rapid Speech Transmission Index RASTI
• STI depends on the sound level of speech, reverberation
time, reflections, distance between speaker and listener and
background noise level
• STI is based on distinction of speech syllables in a space,
value range 0...1
– STI = 0  none of the syllables are heard correctly
– STI = 1  all the syllables are heard correctly
• In auditoria and other speech spaces the goal is to achieve a
high value of STI (good speech intelligibility), whereas in
spaces such as open plan offices the goal is as low STI-value
as possible (good speech privacy between work stations)
Auditoria
Significance of STI
Auditoria
Example of positioning surfaces
•
A surface above the speaker which
reflects sound to the front and rear
parts of the audience (1)
•
Sound reflecting surfaces to the mid
and rear parts of the audience so
that the entire audience area
receives reflections (2-5)
•
Front surfaces of the hall sound
reflecting (non-parallel walls on
stage to avoid flutter echoes), lower
parts of the walls sound reflecting to
achieve useful reflections from the
sides as well as from the ceiling (6)
•
Absorbing surfaces to the upper
parts of side walls and rear part of
the ceiling, rear wall sound
absorbing or scattering to avoid
distracting reflections back to the
front part of the hall and the stage
(6)
1)
2)
3)
4)
5)
6)
Absorboivaa pintaa
A
Heijastavaa
pintaa
Classrooms
Optimising speech intelligibility (SFS 5907)
•
In classrooms, it is important that speech
distinction from the treacher to pupils is good
•
Recommended reveberation time 0,5...0,8 s
•
When class A sound absorbing material is
used (absorption coefficient > 0,9), the centre
part of the ceiling should be left sound
reflecting (absorption coefficient 0...0.20),
from which sound is reflected to the mid and
rear part of the room  this improves speech
intelligibility
•
If class C material is used, the material can
be positioned on the whole ceiling surface
because the material also reflects sound
•
The amount of class A material needed is
about 70 % of the floor area and for class C
material correspondingly 100 %
•
Absorption material should also be placed on
the walls to avoid distracting flutter echoes
Screens
• Wall elements used for dividing space, do not extend to
ceiling height (as opposed to walls), thus sound is
diffracted over the screen
• Screens have many noise control applications: screens
for blocking speech in open plan offices, noise blocking
screens in factories, road noise barriers
• The sound attenuation of a screen, D [dB], is determined
as insertion loss: D = Lp,1 – Lp,2
where Lp,1 is the measured sound pressure level at
listener position without the screen and Lp,2 is the SPL
with the screen in place
Screens
• In the free-field (a space with no sound reflections, e.g.
outdoors), the sound attenuation by screen of a point
sound source is given by the equation
z

D  10 log10 1  20 


where z is the distance difference and λ is sound
wavelength (λ = c0/f)
Screens
•
•
•
•
The effect of screen height H
on sound attenuation when x1
= x2 = h1 = h2 = 1,2 m,
calculated with equation on
previous slide
The mass of the screen is
assumed to be infinitely large
so the sound insulation of
screen is insignificant
Solid lines correspond to
sound reduction index (mass
law values) of two different
walls (surface masses 5 kg/m2
and 20 kg/m2)
Note: the sound attenuation of
a solid wall is always
significantly higher than that of
a screen
Screens
Effect of sound reflections on attenuation
• The sound attenuation values on the previous slide
assume a space with no sound reflections; the values
can be achieved by, e.g., outdoor noise barriers
• In practical rooms the sound attenuation of screens is
even considerably lower because of reflections; screens
mainly attenuate the direct sound from source to listener
but have little impact on reverberant sound
• Attenuation is especially affected by sound absorption of
the ceiling and wall surfaces close to the screen
Screens
Effect of sound reflections on attenuation
•
•
•
Example: factory hall with
screen constructed around a
work station with noisy
activity
Although the screen height
was 6 m, the screen only
attenuated sound by 1dB(A)
to its surroundings
Reasons:
• Ceiling and wall surfaces
were sound reflecting,
reverberation time of the
hall was high
• Screen height was only
about 1/3 of the room
height
Open plan offices
•
Acoustics is a key component of indoor
climate in open plan offices
•
The most distracting sound is speech
heard from surrounding work stations
•
Acoustical design goal is to maximise
speech privacy between work stations,
thus minimise speech intelligibility
(note: opposite goal to e.g. classrooms)
•
In open plan offices there are no sound
insulating partitions, but only screens
 speech privacy must be mainly
achieved by means of room acoustics
[Haapakangas et al. 2007]
Open plan offices
• Good speech privacy between work stations requires
that three factors are considered:
1. Direct sound path between work stations must be truncated
using screens, preferrable scren height is > 150 cm
2. Sounds reflecting from room surfaces must be adequately
attenuated  sufficient amount of sound absorbing material
correctly positioned on the surfaces of the room (ceiling, floor,
walls, screens, furnishings)
3. Masking sound  sufficiently high background sound level in
the space to mask speech sounds, preferrably implemented
artificially using loudspeakers to produce the masking sound,
recommende sound level 40...45 dBA
Studios
Reflectionfree zone at
the mixer`s
position
Studios
Dolby
surround 7.1
speaker
layout
Studios
Requirements for
early reflections
and reverberation
time
[2005 Dolby 5.1-channel music production guidelines]