Term
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Definition
study of sound in spaces. Acoustics also refers to the
specific study of sound as related to materials, structures, and rooms. |
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Term
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Definition
Energy produced by a vibrating medium and transmitted as a wave
through an elastic medium: air, construction materials, ground. Sound is different from light as it requires a medium, and can bend around objects
(refraction.) Light travels as straight lines, and leaves sharp shadows. |
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Term
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Definition
part of physics that deals with sound: production, transmission,
reception, control, effects |
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Term
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Definition
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Term
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Definition
10 times the common logarithm of the ratio of a quantity to a reference
quantity of the same kind, such as power, intensity, or energy density. It is often
used as the unit of sound intensity. |
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Term
| The sensitivity of the human ear |
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Definition
human ear covers a vast range. Because of this and the fact
that the sensation of hearing is proportional to the logarithm of the source intensity,
the decibel is used in acoustical descriptions and calculations. The decibel
conveniently relates actual sound intensity to the way humans experience |
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Term
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Definition
pressure – ENERGY - of the sound wave in reference to a sound in
the threshold of audibility. It is not a proportional scale. |
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Term
| three basic qualities of sound |
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Definition
| velocity, frequency, and power. |
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Term
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Definition
depends on the medium in which it is traveling and the
temperature of the medium. In air at sea level the velocity of sound is
approximately 1130 ft/sec (344 m/s). For acoustical purposes in buildings, the
temperature effect on velocity is not significant. Water: 4,500 ft/sec – Wood: 11,700
ft/sec – Steel: 18,000 ft/sec.
Compare with light speed: 186,000 miles/sec |
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Term
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Definition
number of cycles of compression and expansion completed per
second; it is measured in Hertz (Hz). One Hz equals one cycle per second. |
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Term
| Why acoustical engineers talk so much about frequency |
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Definition
Acoustic engineers tend to discuss sound pressure levels in terms of frequencies,
partly because this is how our ears interpret sound. What we experience as "higher
pitched" or "lower pitched" sounds are pressure vibrations having a higher or lower
number of cycles per second. |
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Term
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Definition
the unit of frequency; one cycle per second equals 1 Hz. We hear
frequencies that vary between 20 to 20,000 Hz |
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Term
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Definition
| acoustical energy as measured in Watts. |
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Term
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Definition
| the amount of sound energy per second across a unit area |
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Term
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Definition
10 times the common logarithm of the ratio of a sound intensity to a
2
reference intensity. See definition of decibel. |
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Term
What is the resulting level of sound that is coming out of two speakers, one
generating 60 dB and the other generating 65 dB. |
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Definition
Because decibels are logarithmic, they cannot be added directly. Use the rule of thumb shown in
the table. The difference between 76 and 70 is 6; so the next step is to add 1 dB to 76, which gives 77 dB |
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Term
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Definition
unwanted sound, sound with no intelligible content and/or broadband
sound. It is subjective, and depends on the situation. Any sound can be
noise to someone. |
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Term
| two approaches to negative effects of noise |
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Definition
a psychologicalpractical
one and a purely physiological one. The latter is concerned with the
physical impact of noise on the body, including hearing loss and other deleterious
conditions. The former is concerned with noise levels that cause annoyance and
disturbance to daily activities |
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Term
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Definition
(NC) - a set of single-number ratings of acceptable background noise
corresponding to a set of curves specifying sound pressure levels across octave
bands. Noise criteria curves can be used to specify continuous background noise,
achieve sound isolation, and evaluate existing noise situations. |
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Term
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Definition
| 8 hours max sound level 90 dB |
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Term
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Definition
a technique that is used to hide unwanted sounds by the addition of controlled
d Th sounds. These additional sounds could be natural or produced intentionally for that purpose. The are
commonly called white or pink noise. Sound masking creates speech privacy, and eliminates
awareness of unwanted sounds in a given area, and can make a work environment more
comfortable.
An example using more visible energy, light, can be used to explain how sound masking works.
Imagine a dark space with color lights turning on and off. You would be very distracted by them. Now,
imagine the same lights in the same room, but you would turn on a very high level of conventional
illumination. The color lights loose their impact, and would only be noticed when looking directly at
them. Sound masking is a similar process, but applied to sound: it covers distracting sounds with
less intrusive, more homogeneous sounds.
It is normally used in interior work spaces, but could be used outdoors. Highway noise is a potential
application.
A more technical description: Masking is the number of decibels by which the threshold of audibility
of one sound is raised by the presence of another sound. The masking effect is greatest when two
sounds are close in frequency since the ear has greater difficulty separating sounds of similar
frequencies. Also, a low frequency sound will mask high frequency sound more effectively than the
reverse for the same decibel levels. Background levels and sounds are created to mask un-wanted
sounds. |
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Term
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Definition
sounds of a broad-band continuous nature, that are non-information bearing. They
hide the content of lower magnitude that would be annoying. |
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Term
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Definition
in plenum, under floor, direct field. Dynamic-auto: The background sound is automatically
adjusted to ambient noise levels by adjusting the amplitude of the frequency levels detected. |
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Term
| quality of sound in a space |
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Definition
depend on the relationship between how much sound is reflected
and how much sound is absorbed.
If little energy is absorbed and a lot is reflected, we will be very aware of the sound condition.
Intermittent sounds will be mixed together (which may make speech less intelligible or music more
pleasant), and steady sounds will accumulate into a reverberant field, making the space noisy.
Conversely, if much energy is absorbed and little reflected, the room will sound quiet for speech and
"dead" for music.
Indoors, sound energy lingers, and this decay is called reverberation. Reverberation time (RT) is
defined as as the length of time, in seconds, it takes a sound to decay by 60 dB. Performance spaces
for music and for the spoken word have different reverberation requirements. |
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Term
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Definition
the persistence of sound after the sound source has ceased. Persistence is a
result of multiple reflections in an enclosed space during a short period of time. Reverberation time
(Tr) is defined as the time required for the sound level to decrease 60 dB after the sound source has
stopped producing sound. |
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Term
| The reflection of sound in a room causes two things to occur |
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Definition
The noise level, or volume of sound, is
greater than it would be for the same source in an open space. There is a delay factor as well, so
some sounds persist for a time in a very reflective space, even after the source has stopped.
Reverberation is similar to an echo because both are reflected sound phenomena. Reverberation
occurs when the sound source stops but the reflections continue, decreasing until they disappear.
The length of this sound decay, or reverberation time, receives special consideration in the design of
large auditoriums, which need to have specific reverberation times to achieve optimum performance
for their intended activity. |
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Term
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Definition
Reverberation is continuous reflection. Echo is a distinct sound that comes back delayed, so the
original sound is separate and distinct from its repetition. |
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Term
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Definition
the tendency of a system to vibrate at increasing amplitude at certain frequencies.
Regarding acoustics, at certain frequencies, even small repeated sound can produce large amplitude
vibrations, because the system stores vibration energy so it can return more energy that what it
appears to be receiving.
For reflection, the angle of reflection equals the incidence angle. |
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Term
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Definition
| use of sound energy mitigation mechanisms. |
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Term
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Definition
energy is dissipated to the sound disappears.
When sound energy impinges on a material, part is reflected and the remainder is
absorbed (in the sense that it is not reflected). Some of the "absorbed" energy is
transmitted, although that part is so small that for our discussion here it will be
ignored. Materials are neither perfect reflectors nor perfect absorbers. The term
used to define a material's sound absorption characteristic is its coefficient of
absorption, which is usually represented by the lowercase Greek α (Alpha) which
represents the noise reduction coefficient. |
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Term
| Noise Reduction Coefficient |
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Definition
expression of the average sound absorption at four
frequencies. |
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Term
| Control of noise generated by mechanical systems |
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Definition
Most mechanical equipment produces noise having a definite frequency or pitch
related to the rotational speed of the equipment. Of course, numerous harmonics
also exist in most cases, producing a larger band of frequencies from any given
source. Typical sources are motors, fans, pumps, and compressors. Moving fluids,
such as air and water streams, also play a role. |
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Term
| absorptivity per square foot of any given surface |
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Definition
aries from 0 (all sound is
reflected) to 1.0 sabin (all sound is absorbed) |
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Term
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Definition
the sum of the different surface areas times their
respective absorptivity, and is expressed by the equation indicated above |
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Term
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Definition
sound absorption device. They function by deforming to absorb energy, or by allowing sound to penetrate a
cavity, where a space is connected to its surroundings by a narrow cavity.
(Helmholtz resonator.) These are tuned to certain frequencies, and can be
extraordinary when a single frequency is present. |
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Term
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Definition
| panels, cylinders, and cavity |
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Term
| Transmittance, diffraction, and reflection |
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Definition
characteristics of sound that impact outdoor
ti S acoustics. Sounds can penetrate and cross through walls and other solid objects and be transmitted
to the other side. Air-borne energy comes in contact with a barrier, makes it vibrate, and the sound
energy is transmitted to the air on the opposite side |
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Term
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Definition
Refraction means that some sounds are
bounced back. |
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Term
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Definition
Diffraction refers to the bending of sound waves around obstacles or the spreading
out of waves past a barrier. Diffraction, along with transmission, allows sound to be heard around
corners. The amount of diffraction (spreading out of a sound wave in a non-linear manner) depends
on its wavelength, obstacles, and the size of an opening through which it passes. Greater diffraction
occurs as the size of the sound's wavelength increases compared to the size of an opening through
which it passes. High-frequency sounds, with shorter wavelengths and lower energy level, will have
less diffraction than low-frequency sounds, with longer wavelengths. Therefore, high-frequency
sounds will spread out over a smaller area than low-frequency sounds. Airborne sound changes
direction easily because vibrations can be communicated to air masses behind obstacles. |
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Term
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Definition
There is a need to control outdoor sounds, as from a highway near a structure. Solid barriers, as
shown above, can be effective, but they must be properly located. The best location for a barrier is
very close to the source or very close to the receiver; middle positions are significantly less
effective. The barrier must be higher than the line-of-sight between the source and the receiver; the
higher the better. Unfortunately, the attenuation of a barrier is not constant, but instead increases at 3
dB per octave. For this reason, low-pitched noises, such as truck engine sounds, are not reduced as
much as tire whine, which is at a higher frequency.
The use of trees and vegetation as an acoustical barrier was once thought to be effective. However,
experiments show that they provide almost no attenuation at all for transmitted sounds. Vegetation in
front of a barrier may reduce the reflected sound, but vegetation alone does very little to stop the
transmission of sound. |
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Term
| Impact transmitted sounds |
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Definition
result when sounds are propagated by the materials (structure, walls,
l bi ) f th plumbing….) of the building itself. Sounds of this type are very difficult to control because attenuation
is significantly less in certain construction materials like steel or concrete, and sound speed of travel
through them is faster than through air. Attenuation describes the extent to which the intensity of a
sound is reduced as it passes through a specific material. |
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Term
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Definition
erratic sounds caused by footfalls, dropped objects, the vibration of mechanical
equipment, etc. The resulting vibration of the structure is then transformed to airborne sound that we
perceive. A standardized method of measuring the degree of isolation of impact noise in the structure
has been developed. The method utilizes a special "tapping machine.“ |
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Term
| Impact insulation class (IIC) |
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Definition
Number rating that describes how well a building floor attenuates
impact sounds.
IIC values can be improved in several ways. Carpeting and resilient tile floor coverings are effective
at middle and high frequencies. Suspended ceilings are also very effective at middle and high
frequencies, especially if combined with carpeting. A concrete slab floated on compressed glass
fiberboards laid on the structural floor is very effective in improving the performance at all frequencies
and, thus, the IIC rating.
In addition to OSHA requirements for noise levels at job sites, the Uniform Building Code has
established minimum airborne and impact sound isolation values for residential occupancies |
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Term
| airborne v. strucutre borne sounds |
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Definition
All sounds are air-borne and structure-borne.
Airborne sound is generally much less disturbing than structure-borne sound , since its initial energy
is very small and it attenuates rapidly at enclosures or with distance Structure borne sounds have
12
distance. Structure-a much higher initial energy level and it dissipates slowly as it moves through solid matter, causing
problems over large sections of a building. Structure-borne sound travels much more rapidly than
airborne sound and with attenuation as low as 1 dB per 0.60 of a mile. |
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Term
| handling Loud air-borne noises |
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Definition
better handled by enclosures than by room treatment, since enclosures
reduce the amount of sound that enters a space (TL) which absorption materials cannot do. |
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Term
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Definition
Sounds can be transmitted to the other side of a solid
barrier. Air-borne energy comes in contact with it, makes it vibrate, and the sound energy is
transmitted to the air on the opposite side |
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Term
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Definition
In reference to a wall or separation, TL is the ratio between the level of
incoming sound to the level of the sound that can pass across the barrier. It depends on the property
of a material or construction system that blocks the transfer of sound energy from one side to the
other.
Transmission loss depends on mass, distance (thickness /separation) and absorption. |
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Term
| Distance to dissipate sound |
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Definition
the energy of sound waves decreases as they spread out, so that
increasing the distance between the receiver and source results in a reduction in the intensity of
sound at the receiver. In a normal three dimensional setting, the intensity of air-borne sound waves
will be attenuated according to the inverse square of the distance from the source. (Inverse Square
Law) |
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Term
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Definition
resists the transmission of sound because its inertia (resistance to vibration) and elasticity
(capacity to deform). Best materials that can have a damping effect on sound transmission are those
which are dense and soft, like lead and neoprene. Sound transmission through different layers of
material with different densities also assists in noise damping. Doubling the mass will result on a dB
loss of 6 dB. Stiffness depends on the material composition and on its connections and restraints. |
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Term
| Separation to dissipate sounds |
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Definition
continuity is a friend of sound transmission, because of structure borne sounds, as
mentioned before. Openings would allow air molecules to bear the sound across the barrier with
minimum transmission loss. Openings and cracks can defeat a massive barrier. |
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Term
| Background sounds to dissipate sounds |
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Definition
the same principles used for sound masking operate, but in spontaneous
fashion. |
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Term
| Absorption to dissipate sounds |
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Definition
Absorption room treatments reduce reverberation. Sound absorption material will affect
the reverberant noise level (will lower the energy level) within that room but will have an insignificant
effect on the noise level coming in from the exterior or adjoining spaces, as the transmission loss in
the absorbent material is very low. |
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Term
| Sound transmission class (STC) |
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Definition
a single number rating system designed to combine TL values
from many frequencies.
Sound Transmission Class (STC) is a rating of how well a building partition attenuates airborne
sound. STC the decibel reduction in noise a partition can provide.
The STC number is tested at frequencies from 125 Hz to 4000 Hz. This leaves low frequencies
(transportation or mechanical noises) with high energy levels below 125 Hz..
Typical interior walls in homes (2 sheets of 1/2" drywall on a wood stud frame) have an STC of about
33. Absorptive insulation (sound batts) in the wall cavity increases the STC to 36-39, depending on
stud and screw spacing. Doubling up the drywall in addition to insulation can yield STC 41-45,
provided the wall gaps and penetrations are sealed properly. Doubling the mass of a partition does
not double the STC. Doubling the mass (going from two total sheets of drywall to four, for instance)
typically adds 5-6 points to the STC. Breaking the vibration paths by separating the sides of the wall
from each other will increase transmission loss much more effectively than simply adding more mass
to a wall assemby that is built as a unit. This can be done by using resilient channels, a staggeredstud
wall, or a double stud wall. This can yield an STC as high as 63 or more for a double stud wall,
with good low-frequency transmission loss as well. Compared to the baseline wall of STC 33, an
STC 63 wall will transmit only 1/1000 as much sound energy, seem 88 percent quieter and will
render most frequencies inaudible.
As a result of high density, concrete and concrete block walls have good STC values in the 40s and
50s for 4-8" thickness.
QuietRock is a drywall type created to provide high STCs. |
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Term
| Typical interior walls in homes STC |
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Definition
(2 sheets of 1/2" drywall on a wood stud frame) have an STC of about
33. Absorptive insulation (sound batts) in the wall cavity increases the STC to 36-39, depending on
stud and screw spacing. Doubling up the drywall in addition to insulation can yield STC 41-45,
provided the wall gaps and penetrations are sealed properly. Doubling the mass of a partition does
not double the STC. Doubling the mass (going from two total sheets of drywall to four, for instance)
typically adds 5-6 points to the STC. |
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Term
| effect of absorptive materials |
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Definition
It is important to understand that the principal effect of absorptive material is on the
reflected sound. In this situation, the transmitted sound energy is essentially
determined by the mass of the solid airtight barrier between the two spaces. |
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