Architectural/Interior Applications
Factors influencing sound propagation
indoors include the physical dimensions and
geometry of the space as well as the absorptive,
reflective or diffuse characteristics of the
terminating surfaces (walls, floors and ceilings).
Overall sound level intensity and quality
within the space are defined by acoustic
phenomena such as reverberation, echoes,
sound concentrations and room resonance.
Depending on the use of the space, the sound
quality varies with the reverberation time
(RT60) in seconds. This is the time it takes for a
sound to decay 60 dB. "Dead spaces" have
low reverberation times and are ideal where
speech intelligibility is the top priority. Higher
reverberation times characterize "live spaces"
that are best for performance areas dedicated
to music. Reverberation times in-between are
best suited for multi-purpose spaces where
both speech and music are important.
Reverberation times over 3 seconds should be
avoided altogether.
Simplified Room Acoustics:
The Sabin Formula
The Sabin Formula is named after
Wallace C. Sabine, generally accepted as the
Father of Acoustics. The formula allows for
quick and easy calculations to estimate the
existing reverberation time (RT) and to calculate
how much additional treatment, using
absorption materials, is required to obtain a
lower RT value which is consistent with the
intended use of the space.
Any room or indoor space possesses
some ability to absorb and dissipate sound
waves/energy. Reverberation time calculations
using the Sabin Formula vary according
to the volume of the space and the units of
sound absorption or Sabins in the space. A
Sabin is a unit of sound absorption equivalent
to one square foot of material with an
absorption coefficient of 1.00. For example,
a 10,000 ft.2 concrete floor would yield only
150 Sabins of absorption based on the
absorption coefficient for concrete at 500 Hz
(.015 x 10,000 =150).
Coefficients Of
General Building
Materials And
Furnishings
Complete tables of coefficients of the various
materials that normally constitute the interior
finish of rooms may be found in the various
books on architectural acoustics. The following
short list will be useful in making simple calculations
of the reverberation in rooms.
MATERIALS | COEFFICIENTS |
| 125 CPS | 250 CPS | 500 CPS | 1000 CPS | 2000 CPS | 4000 CPS |
Brick | .03 | .03 | .03 | .04 | .05 | .07 |
Carpet (heavy) on concrete | .02 | .06 | .14 | .37 | .60 | .65 |
Carpet with heavy pad | .08 | .24 | .57 | .69 | .71 | .73 |
Carpet with Impermeable backing
| .08 | .27 | .39 | .34 | .48 | .63 |
Concrete block (course) | .36 | .44 | .31 | .29 | .39 | .25 |
Concrete block (painted) | .10 | .05 | .06 | .07 | .09 | .08 |
Light fabric | .03 | .04 | .11 | .17 | .24 | .35 |
Medium fabric | .07 | .31 | .49 | .75 | .70 | .60 |
Heavy fabric | .14 | .35 | .55 | .72 | .70 | .65 |
Concrete, terrazzo,marble or glazed tile
| .01 | .01 | .015 | .02 | .02 | .02 |
Wood | .15 | .11 | .10 | .07 | .06 | .07 |
Heavy glass | .18 | .06 | .04 | .03 | .02 | .02 |
Ordinary glass | .35 | .25 | .18 | .12 | .07 | .04 |
Gypsum board 1/2" | .29 | .10 | .05 | .04 | .07 | .09 |
Plaster | .013 | .015 | .02 | .03 | .04 | .05 |
Water surface | .008 | .008 | .013 | .015 | .020 | .025 |
Air, sabins/1000 cubic feet | 2.3 | 7.2 |
People | 4 sabins |
dB Reduction Guideline
Using Absorption
- To get a 3 dB reduction: add enough
absorption to equal the existing absorbtion
in the untreated room.
- To get a 6 dB reduction: add enough
absorption to equal three times the existing
absorption in the untreated room.
- To get a 9 dB reduction: add enough
absorption to equal seven times the existing
absorption in the untreated room.
Reverberation Effect
On Listening
1/2 to 1 second |
Speech | ........................GOOD |
Music | ....................TOO DEAD |
|
1 to 1 1/2 seconds Speech | | ..........................GOOD |
Music | ................................FAIR |
|
1 1/2 to 2 seconds | Speech | ..............................FAIR |
Music | ..........................GOOD |
|
Over 2 seconds
| Speech | ..........................POOR |
Music | ................FAIR to POOR |
Reverberation Time
Reverberation time is the time measured in
seconds that a sound of average loudness can be
heard before it becomes completely inaudible
under quiet ambient conditions. The time may
vary from 1/2 second in a very "dead" room to
5 or 10 seconds in an excessively live reverberant
room.
Speech And
Communication
The maximum reverberation time for clear
speech is about 2 seconds. When reverberation
time exceeds 2 seconds and moves
upward, speech becomes increasingly more
difficult to understand. Speech finally
becomes unintelligible at reverberation times
of 3 to 10 seconds. Speech intelligibility
improves as reverberation time decreases
below 2 seconds. The ideal for classrooms or
lecture spaces is actually lower than 1 second.
Music
Optimum reverberation time for orchestral,
choral and average church music generally
ranges between 1 1/2 to 2 seconds. Large
organs: 2 seconds or more and, Chamber
Music: 1 to 1 1/2 seconds.
Understanding Diffusion, Reflection
And Absorption
Diffusion
Uniform distribution of
reflected sound energy is
accomplished through the use
of diffusion to blend musical
sounds and speech over a broad
listening area. This eliminates
sharp echoes without eliminating
the sound by absorbing it.
Diffusion is also used to
create the aural illusion of a
much larger space so that
concert hall sound can be
generated and reproduced in
smaller spaces.
By spreading the reflected
sound into many directions, the
sound in any one particular
direction is thereby abated.
Diffusion should be
combined with reflection and
absorption to assure a balanced
listening room treatment. |
Reflection
Specular reflections
caused by sound waves
bouncing off non-absorptive
wall, floor and ceiling surfaces
can result in excessive reverberation,
undesirable rear wall
slap echo and flutter echo
from parallel reflective walls.
In listening room applications,
reflection of sound
waves that differ in arrival
time by more than 0.05
seconds compared to the
direct path will result in
echoes. Echoes distort the
original sound and are
responsible for poor speech
intelligibility. In critical
listening applications a
balance of reflection,
absorption and diffusion is
desirable. |
Absorption
Incident sound waves
hitting building surfaces
constructed of absorptive
materials will result in little or
no reflected sound energy.
Loudness and reverberation
are reduced but excessive
treatment with absorption
only can render the
listening space boomy and
indistinct.
Overuse of absorption
yields a lack of reflective
surfaces which can cause
loss of upper harmonics,
high frequency treble voices,
flutes, etc.
Balancing absorption
with reflection and diffusion
is the key to optimizing room
acoustics. |
Architectural
Design:
When Are
Floating Floors
Needed?
The Problem
Building designs which incorporate quiet
spaces located near noisy areas such as
mechanical equipment rooms, kitchens, sports
and other recreational spaces or manufacturing
operations will need to reduce transmission of
noise through floor, wall and ceiling constructions.
Such quiet spaces requiring a low NC
level include theatres, broadcast and recording
studios, conference rooms and the like.
Noise reduction performance of floors
usually follows the mass law which states that
a doubling of the surface weight will reduce
the transmission of sound by up to 6dB.
Increasing from a 6" to 12" concrete floor can
only translate to 6 transmission loss (TL) points
or 6dB higher performance from 40 to 46 at
500 Hz.
The problem becomes providing practical
designs for high (TL) or dB loss with less mass
and thinner profiles to "beat" the mass law.
Decoupled masses increase performance
beyond what can be expected according to the
mass law.
The Solution
Floating floor systems can "beat" the mass
law using decoupled composite construction.
High sound transmission loss (TL) is achieved
by isolating, floating or decoupling a second
poured concrete floor using a variety of
resilient materials such as high density precompressed
molded fiberglass, neoprene
blocks or other pad type systems. A permanent
pouring form (usually exterior grade plywood)
is placed on top of the isolation blocks/pads
and the concrete floating floor is poured. A
complete floating floor system must include
perimeter isolation materials, isolated floor
drains and other engineering details to decouple
any possible flanking transmission path to
the building slab, walls or ceiling.
To reduce impact sound from structureborne
transmission associated with pedestrian
foot falls, jumping, jogging, bowling, etc. on
hard surface flooring systems (concrete, tile,
hardwood, etc.), floating floor construction is a
necessity.
Floating Sound Barrier Ceilings
Benefits Of The Low Profile Design:
- Minimal ceiling space above framing
members
- Code approved support of services
and/or a secondary ceiling without a
myriad of penetrations
- Easier and less expensive to install
- Isolator static deflection ratings from
.35" to 2.35"
- Combination of sound barrier, finished
ceiling and mechanical/electrical services;
all suspended from the same isolation
hanger without multiple penetrations
- Controls noise occurring above or in a
treated room
- Installations with as little as 1/4" above
the suspension members
|