Acoustics of classrooms, restaurants and
offices
Eng.Ivaylo Hristev
Contents
1. Terms and requirements:
• Reverberation time
• Echo criteria
• Noise floor
• Sound power in a room
• Cocktail party effect
2. Classroom requirements
3. Restaurant requirements
4. Office spaces requirements
Reverberation time
T60  0,161
V
S ln 1     4mV
Reverberation time is one of the most common acoustic criteria. Requirements
are usually posted in terms of Rt.
Echo
Echo is called a single reflection arriving at listeners position
after certain time. This time delay causes the echo to be heard
as separate sound.
Echo criteria
Haas echo criteria; depends on the level and time difference.
Echo criteria
Reflections and their perceivable effect, after M.Barron.
Noise floor – criteria NR
NR (noise rating) is a noise
criteria used widely in
Europe. In USA NC (noise
criteria) is used instead. Both
criteria set maximum allowed
noise level for 1000 Hz
allowing higher SPL at low
frequencies, and lower SPL at
high frequencies.
Noise floor – criteria NR
Noise rating curve
NR 25
NR 30
NR 35
NR 40
NR 45
NR 50
NR 60
NR 70
Application
Concert halls, broadcasting and recording studios,
churches
Private dwellings, hospitals, theatres, cinemas,
conference rooms
Libraries, museums, court rooms, schools, hospitals
operating theaters and wards, flats, hotels, executive
offices
Halls, corridors, cloakrooms, restaurants, night clubs,
offices, shops
Department stores, supermarkets, canteens, general
offices
Typing pools, offices with business machines
Light engineering works
Foundries, heavy engineering works
Noise floor – criteria NR
Real life measurements in a TV studio
Sound source directivity Q
Q=1
Q=2
Q=4
Q=8
When sound source radiates at open space (4π) its directivity is said to be 1;
radiating at half space (2π) is equal to directivity of 2. Lowering the
radiation angle to ½, directivity increases twice.
Sound power
Here:
SPL – Sound pressure level
SWL – Sound Power Level
r – distance between source and receiver
Q – sound source directivity
Rc – room constant
S – total absorption area
– averaged absorption coefficient
Speech intelligibility
Speech transmission index
STI is measured with special noise signal and gives objective information
about speech intelligibility in a room.
Alcons
Cocktail party effect
Every doubling the number of people increase the reverberant
sound pressure level with 3 dB and halves the conversation
distance. In practice people may choose to talk louder, which
further raises the noise level. Solution – decrease the
reverberant SPL by adding absorption.
Acoustic requirements for classrooms
Requirements for unoccupied classroom( volume ≤ 250 м³)
USA 0,4 - 0,6 s
UK 0,5 - 0,8 s
Italy 0,6 - 0,8 s
Switzerland 0,5 - 0,7 s
France 0,4 - 0,8 s
Germany 0,45 - 0,6 s
Sweden 0,5 - 0,6 s
20% increase of Rt is allowed for 125 Hz Octave band
Noise level: below NC30.
Typical solutions for classrooms
Step 1 – provide sound absorption on ceiling
Typical solutions for classrooms
Step 2 – ensure low frequency sound absorption
Typical solutions for classrooms
Step 3 – ensure speech comfort for lecturer by providing reflections
above him
Typical solutions for classrooms
Step 3 – get rid of backwall late reflections; remove echo possibilities
Acoustic requirements for offices
Controlling reflections
DL2 , DLf and sound distribution curve in
open plan space
Sound level, dB
DL2 – rate of decay of SPL per doubling
the distance
DLf – excess SPL with respect to free field
60
55
Without acoustical treatment
50
DL2
45
Background sound
DLf
40
With acoustical treatment
35
Free field
30
2m
4m
8m
Distance (d) from the speaker, metres
16 m
Acoustic requirements for offices
Key elements of acoustic design in open plan offices:
1. Absorption in the room at least as much as the floor area
2. Space divider screens – at least 1,5 meters height
3. Noise floor – as high as NC 35-NC 40; possibly use sound masking
system.
Open plan office acoustics online calculator:
http://legacy.spa.aalto.fi/software/fioh-officemodel/
Questions ?