Science of acoustics
Sound, noise, vibration
Sound: effect of rapid vibrations of bodies,
propagating in material media.
Noise: set of sound without harmony.
Vibration: periodic motion of a material system
around its equilibrium position.
The sound is transmitted through the vibration of the material.
We hear through the air particles
that vibrate our tympanic canal. That's why the
sound is not transmitted in a vacuum, the air is absent and
therefore cannot make the ear-drum vibrate.
A vibration is characterized by its amplitude and frequency. A
particle follows a path which vibrates back and forth and the
amplitude is the distance between the two extreme points while the
frequency is the number of same paths (called cycle) that the
particle paths in 1 second. Just as the ambient air is
characterized by its temperature and humidity, the sound is
characterized by its intensity and frequency.
Intensity is measured in decibels (dB). We must associate the
intensity to the strength of sound. For example, for two
equal-frequency transmitted sounds, the sound of 100 dB will be
much louder than the sound of 60 dB. The decibel scale is not
linear, it is logarithmic. When the sound doubles, his measure in
decibels does not necessarily double, it increases by 10 dB. When
we increase the intensity of 20 dB, the strength of the sound is
quadrupled, etc.. In fact the sound of 100 dB is 16 times stronger
than 60 dB because from 60 dB to 100 dB there's a 4 times increase
of 10 dB, therefore the sound is doubled 4 times (2 X 2 X 2 X 2 =
16). The threshold of perception is 0 dB and the maximum is 194 dB,
but around 120 dB pain occurs. A sound of a greater force than 194
dB will result in a dysfunction of the auditory sense.
Frequency is measured in Hertz (Hz). We must associate the
frequency to the tone of sound. For two equal-frequency transmitted
tones, the sound of 1000 Hz is more acute than the sound of 100 Hz.
For humans the field of perception is between 20 Hz and
18,000 Hz frequencies. However frequencies besides this field may
also be perceived by certain animals. For example, a dog whistler
transmits a frequency too high to be perceived by the master.
By emitting a sound, the source releases a certain amount of
sound energy. Depending on whether the sound will be transmitted at
different frequencies, the ear will perceive it with different
forces to maintain equal energy. Indeed, no ear has the same
sensitivity at all frequencies. For example, a sound from 54 Hz to
3 dB will be perceived with the same aggressiveness as a sound at
50 Hz of 79 dB. Conclusion: the ear is more sensitive to low
The acoustic of the building
There are 2 types of sound transmission: airborne transmission and
impact transmission. By airborne transmission, we mean the
propagation of sound in the air, the atmosphere (airborne sound)
for example a discussion, television, etc.. Impact transmission
refers to the sound propagation in solids (structure-borne noise)
resulting from an impact on it, such as the footsteps of a person,
the vibration of a washer on spin cycle, etc. .
To ensure the acoustic efficiency at a reasonable price for an
assembly, it must deal with the following three factors: density,
resilience and absorption.
A dense material creates a barrier to sound because of the
opposition it offers to the vibration. Actually, denser is a
material, the more energy will it take to make it vibrate. The
density is provided by concrete, wood, gypsum.
A resilient material is effective against sound because it consumes
a large amount of energy transmitted by the vibrations due to the
deformability of the material. Resilience is provided by wood
fiber, resilient bar , rubber, cork and even the air space!
Absorbing material reduces the sound due to its high porosity. It
is often characterized by very low density. It is commonly said
that the sound is "lost" in the material. The absorption is
provided by mineral wool, wool fiber glass, wood fiber, cellulose
The acoustic performance indicators
The acoustic performance of an assembly is qualified by its STC
index (Sound Transmission Class) which is indicative of the
attenuation of airborne sound and in the case of a floor/ceiling
assembly by the IIC index (Impact Insulation Class) which is
indicative of the attenuation of structure-borne sound. When the
index is preceded by an F, it means that the measurement was made
on-field. On-field measurement is generally less than that measured
in laboratory because of less stringent control of the
The performance indicator gives the average sound attenuation
that the assembly develops and it is measured in decibels. The
average attenuation is mentioned because it varies depending on the
source frequency. Note that the sound changes in intensity and
keeps the same frequency as it passes through an assembly. For
example if you give an impact of 100 dB at 1000 Hz on a floor that
develops an IIC of 60, we hear about 40 dB at 1000 Hz below. If one
makes a sound of 80 dB at 500 Hz on one side of a wall that
develops an STC of 55, we hear about 25 dB at 500 Hz on the other
side of the wall.
The determination of performance indicators
STC Sound Transmission Class Index
A sound source is installed on one side of the assembly and a
receiver on the other side. Then sound attenuation provided by the
assembly at 16 specific frequencies from 125 Hz to 4000 Hz
frequency range is recorded. It is the normal frequency range
generated by human activity.
The attenuation is defined by the difference between the
intensity of the emitted signal and the received signal. The curve
of attenuation is traced depending on the frequency. Then the curve
of ITS standard is superimposed on the attenuation curve. The ITS
curve was defined by acousticians, psychologists, physicians and
physicists as the psychological criterion of the human ear to
different frequencies. Finally this contour curve is vertically
positioned to meet two criteria:
1) The sum of deviations under the contour curve should not
exceed 32 dB.
2) No deviation under the contour curve must exceed 8 dB.
The differences are read to the 16 specific frequencies
The index of transmission of sound is read on the standard
contour curve at frequency of 500 Hz
IIC Isolation Index of standardized impact
An impact source is installed over a floor and a receiver
Then the sound transmission occurred at 16 specific frequencies
from 100 Hz to 3150 Hz is record. The sound transmission is defined
as the received signal strength. The transmission curve is then
traced depending on the frequency. The IIC standard contour curve
is the traced over the transmission curve. the contour curve is
finally vertically positioned to meet two criteria:
1) The sum of deviations over the contour curve should not
exceed 32 dB.
2) No deviation above the contour curve must exceed 8 dB.
The index of impact sound isolation standard is obtained by
subtracting from 110 dB the value on the standard contour curve at
frequency of 500 Hz
Factors that influence the transmission of sound through
In the frequency range affected by the mass, the transmission loss
increases by 6 dB / octave. An octave is defined as any frequency
range where the upper frequency limit is twice the lower frequency
limit. For a specific frequency, the transmission loss increases by
6 dB every time we double the surface density of the assembly.
The frequency of coincidence
When the wavelength of oscillation of the wall approaches the
wavelength of the incident sound, the sound frequency is called the
frequency of coincidence and the result is a decrease in high
The resonance occurs when two walls define an air space. The
intensity of the phenomenon depends on the rigidity of the walls.
At a certain frequency, a harmony is settled in with the wall
motion and it's the mass-air-mass resonance. It is recommended to
maximize the lowering of this frequency. To do so, the mass of the
walls can be increased or the depth of air space between the walls
may as well be increased. This results in reduced performance at
low frequency. This phenomenon greatly affects the performance
indicators of an assembly.
The resonance occurs at low frequency while the coincidence at
high frequency. Between the two, the mass controls and the adding
of sound insulation has the effect of broadening the range of
frequencies because it re-growth resonance towards lower
frequencies. Insulation improves performance at high frequency.
The absorption coefficient
There is also the NRC index which is the absorption coefficient
factor that indicates how the material absorbs and releases
based on what it receives. Sound absorption is defined as the ratio
between the energy absorbed and the incident energy (energy
received). The acoustic absorption is mesured at 250, 500, 1000 and
2000 Hz and the NRC index is obtained by averaging the 4
Acoustic performance: the result of an
It should be understood that the acoustic efficiency of an assembly
is not obtained by adding the indices of each component as it is
the case for thermal insulation. In thermal insulation, a
R-12insulation superimposed on a R-I0insulation gives an R-22
insulation. However, if we glue two walls that develop individually
a STC 45, it doesn't result into a wall with an STC 90. It's not
that easy. In fact, the performance will be much less.
This may be explained by the fact that the vibration is not
transmitted like the heat or the cold. The same materials
differently arranged will block the vibration differently. For
example, when you place a resilient bar between two gypsum, it
reduces the acoustic efficiency because it creates a reverberation
chamber (echo). However, the same two superimposed gypsum and
assembled on a resilient bar will be much more effective on an
acoustic point of view. Acoustic construction vices as this demier
, can't increase the sound energy, they can only slow
Propagation and reduction of the sound
When the sound wave strikes the wall of an assembly, part of the
energy is reflected back into the room where the source is
situated and the rest is absorbed by the wall which starts to
vibrate. The absorbed energy is constrained to follow two paths:
the frame and the space between framing components. The energy is
then transmitted to the other wall that transmits the residual wave
vibrating on the other side of the assembly to the receiving room.
For example, a single partition when added separately either
resilient bar or mineral wool and the substantial improvement if we
add the two together.
Acoustics is a very complex science that derives from a multitude
of factors. The noise being one source that affects more and more
to the quality of our daily lives.
It is strongly suggested to appeal to professionals and to
clearly define your expectations, types of sounds and