Understanding Sound Terminology & Concepts
The following terms or concepts and the associated discussions have
been provided to help you describe your particular situation.
Absorption, absorb(er), absorptive – All building
materials (e.g., carpet, drapery, drywall, etc.) absorb some amount of
sound (i.e., converts the sound energy into undetectable amounts of
heat). Soft and porous materials absorb more sound than smooth,
hard, nonporous materials. The process of absorption decreases the
loudness of the sound and the length of time the sound bounces around
inside rooms. Generally, large, loud, noisy rooms need sound
absorbing building materials added to the ceilings and or walls.
Adding sound absorbing materials to otherwise bare rooms accounts for
the majority of the fixes for rooms with bad acoustics. The most common
sound absorptive wall material is a one inch thick or two inch thick
panel made of a sheet of fiberglass wrapped in a woven fabric. The most
common sound absorptive ceiling material is acoustical ceiling tiles
(ACT) laid into in a suspended T-bar ceiling grid.
Absorption Coefficient – All building materials
absorb sound in varying amounts in relation to frequency or pitch.
Painted drywall on studs can absorb more low pitch sound and less high
pitch sound. Conversely, drapery can absorb more high pitch sound
and very little low pitch sound. Absorption coefficients allow us
to understand how much sound is absorbed by building materials at
different pitches. These coefficients range from 0% (0.0) to 100%
(1.0), and the higher the absorption coefficient, the more sound that
gets absorbed. An absorption coefficient of 0.25 means that the
material absorbs 25% of the sound (at the specified pitch) that hits it.
These absorption coefficients are available at standard intervals called
octave bands for low pitch sound (125 Hertz & 250 Hertz), mid-pitch
sound (500 Hertz & 1,000 Hertz) and high pitch sound (2,000 Hertz &
4,000 Hertz). (Also see Noise Reduction Coefficient)
Acoustics (building) – Building acoustics involves room
acoustics, sound isolation and noise control. Room acoustics deals
with the qualitative interpretation of a desired signal in the room such
as the intelligibility of speech in a lecture hall or the reverberance
of choral music in a church. Sound isolation deals with
keeping unwanted noise out of occupied spaces. Noise control deals
with controlling noise generated by the buildings systems such as the
mechanical, electrical, plumbing and conveying systems.
Air-borne sound - Sound can move from one place to
another by different paths. If something is ‘conducting’ sound
from one place to another, the sound is said to be ‘borne’ in the
medium. So, air-borne sound is using just the air to get to your
ears. This is the most common way sounds get to our ears. When you
are listening to a TV or stereo you are experiencing air-borne sound.
Air velocity – As air moves through ductwork (supply
air going to rooms and return air coming back from rooms) it can
potentially result in audible and problematic air turbulence noise as it
passes around corners, through air volume dampers and especially through
the grilles in the ceilings or walls of the room. The extent of
the noise depends mostly on the air velocity (how fast the air is moving
through the ducts). Air velocity close to the rooms should be
moving slowly, about 500 feet per minute (fpm) or less, while air
further away from rooms can be moving faster at about 1,000 to 1,500
feet per minute. Divide the air volume in the duct (in CFM - cubic feet
per minute) by the area of the duct (length times width in inches) to
get the air velocity at that point in the duct. If air is moving
too fast, increase the size of the duct until the desired air velocity
is obtained.
Air volume damper – Air traveling inside metal ducts to
rooms (supply air) needs to be regulated so that the right volume of air
gets to each room. This helps to achieve and maintain a
comfortable temperature in each room. Air volume dampers are
typically metal blades inside the ducts that can be manually adjusted
once during construction or even controlled mechanically on an ongoing
basis by the thermostats in the rooms. These air volume dampers
can generate excessive air turbulence noise when the air velocity inside
the duct is great or when the air volume damper is almost closed,
causing a pinch point. In general, duct design should be
self-balancing so that the air volume dampers are just a ‘fine’
adjustment, not the main means for controlling air volume. Also,
air volume dampers should be located ten feet or more away from rooms
and not directly in the supply grilles in the ceilings or walls.
Lastly, it is best to also use internal duct lining (fiberglass or foam)
inside the supply air ducts (rigid or flexible) between any air volume
dampers and the supply diffusers/grilles.
Ambient noise (background noise) – In all locations you
can hear some amount of background noise from wind, crickets, traffic,
even mechanical systems inside the building. All of these
background noises combined are referred to as ambient noise. An
industrial factory has a high level of ambient noise, while a hospital
patient room typically has a low level of ambient noise.
Area sound source – Sources of sound can be described
generally by their overall shape. An area sound source is one that
has significant width and length compared to the location where the
sound is being heard. For example, an amusement park is typically
considered to be an area sound source relative to a house in a
residential community adjacent to the park.
Attenuation, attenuate – As sound passes through walls,
windows, doors, etc., it becomes less loud. Attenuation is the
resulting decrease in sound loudness. If you cannot sleep due to a
T.V. being on elsewhere in the house, you close your bedroom door to
attenuate (or decrease) the loudness of the sound.
A-Weighting (dBA) – Our ears are less sensitive to low
frequency sound and most sensitive to sound in the higher frequencies.
This naturally helps us to understand speech better and communicate with
each other. Ideally, we want to measure and analyze sound levels
at each of the six main octave bands (or more), but we always have a
tendency to simplify things into a single number (sometimes losing
relevant information in the process). A-weighting first
deemphasizes low frequency sounds and then sort of combines all the
octave band sound levels into a single number (units of dBA). Many
city noise ordinances have their criteria reported in dBA as opposed to
individual octave bands. For example, a city noise ordinance might
dictate that during the day, citizens cannot generate noise greater than
55 dBA as measured at their neighbor’s property line ~ and not greater
than 45 dBA at night.
Background noise (ambient noise) – In all locations you
can hear some amount of noise from wind, crickets, traffic, even
mechanical systems inside the building. All of these noises
combined are referred to as background noise or ambient noise. An
industrial factory has a high level of background noise, while a
hospital patient room typically has a low level of background noise.
Box-in-box construction – You can increase sound
isolation from exterior and interior noises by increasing the mass of
the walls, floors and ceilings around a room. But, the benefit of
adding mass follows a law of diminishing returns. Sometimes, the
noise you are trying to block is so loud that you must use resilient
construction. In extreme situations (e.g. a theatre next to a
highway and over a subway), you may need to resort to ‘box-in-box’
construction. The outer box (a whole building shell or a single
room) is typically standard, but massive construction. The inner
box (interior room) is typically built completely inside the outer box
and does not have any rigid connections to the outer box. There is
a continuous airspace and/or an isolator all the way around the inner
box. This approach is very effective, but also very costly, and it
requires diligent construction oversight.
Broadband noise – All noises have unique frequency (or
pitch) content. Some noise is broadband, meaning that it contains
low frequency noise, mid frequency noise and high frequency noise.
An example of broadband noise is mechanical equipment noise such as that
from large fans. Broadband does not mean that noise is equally
loud at all the contained frequencies, just that some audible amount of
noise is present at a lot of different frequencies.
Solutions to control broadband noise must address all the various
frequencies present in the noise.
Buffer space – It is often a good idea to place an
unoccupied or less critical room in between two sound sensitive or noisy
rooms. For example, you might want to put music teacher offices
and an instrument storage room in between a band room and choral room in
a high school. These spaces would act as buffer spaces, thus
decreasing the mass (and cost) of the sound isolating construction if
the two rehearsal rooms shared a common wall. Early planning like
this can result in significant sound isolation cost savings for the
project.
Castling – It is very common in commercial construction
to use corrugated (undulating), structural, metal, floor decks (on which
you then pour the concrete floors). If you then try to run a sound
isolating drywall wall up to the underside of this undulating metal
deck, you have to also deal with the possible ‘holes’ if you do not
castle the tops of the walls. The wall hits the lowest part of the
undulating deck, but in between the low parts, the deck jogs up 2-6
inches (flutes) leaving little holes or tunnels over your wall. To
maintain sound isolation, the drywall has to be ‘castled’, or custom cut
to fit the shape of the deck undulations above. This is
particularly tricky when the wall is running at some diagonal to the
flutes (undulations) of the deck above.
Damp, damping compound – This has nothing to do with
water or being wet. Some surfaces like a metal locker can ring (or
resonate) when hit. At times, a damping compound is applied (via
spray or brush) to the backside of a surface to damp the noise so the
surface does not ring as much. So to damp something is to decrease
the amount the surface rings after being struck.
Decibel (dB) – The unit of measure of sound. Most
places we encounter have sound levels between 30 dB (quiet residence)
and 80 dB (school cafeteria). When exposed to loud sound over 100
dB for long durations of time, temporary or even permanent hearing loss
can occur.
Diffusion, diffuse(r), diffusive – Bumpy, curved
(convex) and deeply textured wall and ceiling surfaces will scatter (or
diffuse) the sound in many directions. In historic buildings the
ornate columns, statuary, coffers, etc. diffuse the sound. In
modern building with an abundance of flat surfaces, getting the sound to
scatter in all directions is sometimes difficult. Diffusion can be
achieved in a custom manner by the way the surfaces are constructed, or
commercially available sound diffusers can be purchased and used.
The concept of diffusing sound in a building is similar to the concepts
of diffusing light and air in a building with light and air diffusers.
All help to create uniform distribution of the sound, light or air
throughout the room.
Direct Sound – The direct sound is the sound wave that
is generated by the source and travels directly your ears without
bouncing off any other surfaces or passing though any obstructions.
The direct sound often permits you to localize the sound source (i.e.,
identify the location of the sound source).
Doors Seals/Gaskets – A lot of sound can leak through
the small gaps between a door and the wall/floor. If the wall
between two rooms is a sound isolating wall, but a door is also
necessary, then the door should have continuous neoprene (rubber)
seals/gaskets around the full perimeter of the door. They need to
be adjusted so that the neoprene compresses when the door is closed and
so that no light can get through anywhere around the door.
Double stud wall – When you need to increase the sound
isolation capabilities of a standard wall, you generally start by adding
additional mass (e.g., extra layers of drywall on a single stud wall).
However, there is still the single, solid, stud that transmits noise
from one side of the wall to the other. At some point, it becomes
more effective (acoustically and cost-wise) to build a double-stud wall.
Two rows of studs are constructed in parallel and spaced at least one
inch apart so they do not touch. The double-stud approach provides
a structural break between the two sides of the wall, and noise is not
able to pass from one side to the other through the studs themselves.
Extra thought needs to be given early in the planning stage to where
double-stud walls may need to be used. They require extra floor
space, and it is often not possible to find this additional space once
plans are done.
Duct-borne sound - Sound can move from one place to
another via different paths. If something is ‘conducting’ sound
from one place to another, the sound is said to be ‘borne’ in the
medium. So, duct-borne sound uses a metal air duct of the
building’s HVAC system to travel from one place in the building to
another. Since ducts are continuous through walls and floors,
duct-borne sound is a flanking noise that circumvents other isolation
efforts such as walls and doors. If you sit in your office and can
hear someone talking, but it seems like the sound is coming from an air
grille in the ceiling or wall, it is likely that you are hearing
duct-borne noise, and it is likely that the air duct behind the grille
you see is routed to the office of the person you are hearing.
Duct lining – Sound can travel down air ducts very
efficiently and for distances longer than one might expect. To
help prevent this, internal duct lining is placed inside the ductwork.
Typically, supply ductwork has to be insulated in some manner for
thermal reasons, either internally or outside the duct, so opting for
internal lining for the added acoustical benefit does not affect project
cost significantly. There are two main types of lining; fiberglass
and closed-cell foam. Both attenuate sound effectively, but the
closed-cell foam type is only about a third as effective as the
fiberglass type. So, if you need ten feet of fiberglass lining,
you will need thirty feet of closed-cell foam to absorb the same amount
of sound. Decades ago, there was concern about the fibers in duct
lining being carcinogenetic and promoting mold/bacteria growth.
Both of the myths have been dealt with since then. The fibers are
far too large to be carcinogenetic. And, you are either
controlling dirt and humidity in your building or you are not. If
you are not, then mold/bacteria will be everywhere in your building, not
just in the ductwork.
Duct silencer - Noise from large fans inside the
building’s main air handling units can travel down the ducts to nearby
rooms. Duct lining inside the ducts can quickly absorb mid and
high frequency fan noise. However, it is difficult to attenuate
low frequency fan noise without using duct silencers. Duct
silencers are analogous to a car’s muffler. They are large metal
boxes (about the same size and shape as the duct in which they are being
installed) that contain fiberglass insulation and a perforated metal
inner liner to hold the fiberglass in place. You must understand
that silencers can generate noise too, especially if the air velocity
through them is too fast. That is why silencers are typically
located by the fans (not close to the rooms), and then followed by about
twenty feet of internal duct lining (to absorb the mid and high
frequency air turbulence noise generated by the silencer).
Echo – An undesirable, discrete repetition of a sound
caused by sound bouncing off a distant, hard, nonporous surface.
It is common for an echo to be heard on the stage of an auditorium or
altar of a church when the rear wall of the room is not treated with
absorptive materials.
Flanking path – Even when building elements like walls
or doors are extra heavy and thick, sound can sneak around them through
air ducts, electrical outlet boxes, gaps between the bottom of the wall
and the floor, gaps around the doors and frames, etc. These
weaknesses are referred to as flanking paths. Extra care must be
taken to prevent or address possible flanking paths or unsatisfactory
results may be experienced in the end.
Flexible connector – Mechanical and electrical
equipment such as fans and transformers make noise and vibrate.
Often, these devices are placed on top of isolators to prevent the noise
and vibration from transferring into the floor of the building.
However, there are also pipes, ducts and conduits connected to these
devices. If these are connected rigidly to the equipment, then
noise and vibration can flank (bypass) the main isolators via the pipes,
ducts and conduits. To prevent this, flexible connections are used
where pipes, ducts or conduits attach to noisy equipment.
Flutes – Structural metal floor decks used in
commercial construction are not flat. For increased structural
capacity and to achieve longer clear spans, the decks are corrugated
(undulate up and down from two to six inches). These undulations
of the deck are called flutes, and make sound-tight intersections with
the upper parts of the walls below difficult. Castling and
airtight caulking is required when drywall walls intersect with the deck
flutes on the underside of the floor.
Finish(es), finish material(s) – You typically see the
finish materials in a room. Common finish materials include paint,
carpet, tile, wood, etc. The finish material is mounted on or
applied to a substrate. The finish material, the substrate and the
way the finish material is mounted on or applied to the substrate
effects how much sound is absorbed at different pitches.
Flutter Echo – When two surfaces are parallel and
smooth/hard (nonporous) there can be a rapid, repetitive, ricocheting of
sound back and forth between the surfaces. This is called a
flutter echo because the sound appears to be ‘fluttering’ between the
surfaces. It can lead to decreased speech intelligibility and less
clarity of music. To correct flutter echoes, you generally have to
treat one surface (or both) with absorptive or diffusive finish
materials or make the surfaces unparallel.
Frequency (pitch) – Sound is essentially something
vibrating the air and then our ear drums. Faster vibrations result
in a higher pitch or frequency sound. Slower vibrations result in
a lower pitch or frequency sound. If you play the ‘A’ note above
the middle ‘C’ note on a piano, the string vibrates at 440 cycles per
second. Pitch typically refers to the ‘A’ note, while frequency
refers to the 440 cycles per second vibration.
Grout-filled wall – When a sound isolating wall is
constructed of concrete masonry units (CMU), it can be made to have even
higher sound isolating capability if the inner cells of the block (which
are typically left hollow) are instead filled with grout.
Grout-filling the inner cells helps the CMU wall reduce sound more like
a solid concrete wall would.
Hertz, Hz, kHz (cycles per second) - The unit of
measure of sound frequency. The number of beats (or cycles) per
second that something is vibrating to cause the sound you are hearing.
Hz is an abbreviation for Hertz. Kilohertz (kHz) equals 1,000 Hz.
So, 4,000 Hz equals 4 kHz.
Impact noise – An impact noise is caused when one
object strikes another abruptly. Frequently, impact noise in
building acoustics refers to the noise caused by someone walking in
hard-soled shoes on a wood or tile floor above an occupied room.
Impact isolation class (IIC/FIIC) – A single number
measure of how resistant a building assembly such as a floor/ceiling
system is to impact noises such as someone walking in hard sole shoes on
a wooden floor in the apartment above yours. Higher IIC values
mean less impact noise is heard in the room below. IIC below 50 is
not very good, and most people would complain about impact noises from
above. Some codes and standards have IIC 50 as a minimum, but
people can still find impact noise from above annoying at this minimum
criterion. Carpeting (or throw rugs/runners on hard floors) is the
best way to control impact noise and get very high IICs of 60-70.
But, if no carpet or rugs are desired, then there will need to be an
impact noise control underlayment under the finish floor material (wood,
laminate, tile, etc.) as well as a ceiling (possibly even resiliently
mounted) in the room below. IIC is a laboratory measurement.
When IIC is measured in real buildings it is called field impact
isolation class (FIIC). FIIC is generally 3-5 points lower than
laboratory IIC values.
Isolator – Isolators are used to detach or separate a
noisy element acoustically (but not physically) from a nearby surface
that you want to keep quiet. Isolators are typically metal springs
in metal supports, but they can also be made of neoprene (manmade
rubber), compressed fiberglass or a combination of these. For
example, isolators are used under rooftop mechanical equipment so that
the noise and vibration from the equipment does not transfer down into
the roof and building structure. You can place some type of
isolator between just about any noisy thing and the nearby area that you
are trying to keep quiet. Isolators can be used
architecturally as well. For example, an entire ceiling can be
hung on isolators to help prevent unwanted noise from traveling down the
threaded rods that would otherwise be used to support the ceiling.
Lagging – Air and fluid moving through ducts and pipes
can cause noise that breaks out of the ducts or pipes and is then
audible and at times problematic in rooms. A lagging material is
wrapped around the duct or pipe to help decrease the amount of noise
that breaks out of it. Lagging material can be a number of
different things, but often lagging is comprised of a one inch thick
fiberglass blanket (decouples the pipe or duct from the barrier
material) and then a loaded vinyl barrier material that is between one
and two pounds per square foot (psf).
Line Sound Source - Sources of sound can be described
generally by their overall shape. A line sound source is one that
has significant length, but not significant width compared to the
location where the sound is being heard. For example, a train or
highway is typically considered to be a line sound source relative to a
house in an adjacent residential community.
Masking, masking noise – Sometimes more background
noise is actually desired. For example, someone who hears trucks
on a distant highway while trying to sleep in a hammock in his backyard
may find that adding a fountain in his yard adds the right type of
continuous background noise to cover up (or mask) the truck noise.
There are electronic sound masking systems (speakers, amplifiers, etc.)
that are used above ceilings in open office areas to increase the
background noise. The masking noise helps to increase speech
privacy for workers. Without the masking noise, each worker might
be able to hear and understand other workers’ phone conversation more
clearly.
Mass, mass law – When you are trying to increase the
sound isolating capability of a monolithic/homogenous wall or other
building element (e.g., glass, concrete, wood, etc.) the most effective
thing you can do is to increase the mass of the element by increasing
the thickness of the element. For example, you can increase the
thickness of the glass pane in a widow from 1/8 inch to 1/4 inch.
Generally, transmission loss (TL) and sound transmission class (STC)
both increases as mass increases. In fact, mass law states that
for every doubling of the mass, you increase TL and STC by about another
5 dB. That sounds great if you only have to thicken up a glass
pane to better a window by 5 dB, but not so great if you need to better
the isolation of a twelve inch thick poured, concrete wall. It is
a lot easier to gain 5 dB of TL by selecting thicker glass for your
windows then it is to increase the thickness of a concrete wall from
twelve inches to twenty-four inches! In other words, mass law is a
law of diminishing returns. At a point where is becomes
ridiculously costly to gain the extra 5 dB of TL or STC, you have to
change approaches and think double-wall construction or resilient
construction.
Mode – One of the biggest problems with trying to do
something acoustically inside a smaller room is the low frequency room
modes. Modes are resonances or standing waves that occur in
smaller rooms. The sound reflections off the room surfaces combine
with the direct sound from a loudspeaker or instrument in a way that
cancels the combined sound completely at certain places in the room and
amplifies the sound at other places in the room. Modes are a
grossly undesired room effect that has nothing to do with the intent of
the musician. However, all rooms have modes. All
smaller rooms have modes that are audible to us. The goal is to
minimize the negative effects of the modes (by optimizing the room size
and proportioning), identify the frequencies and extents of the
remaining modes and utilize solutions specifically tuned for the
remaining problematic modes. Standard porous absorption such as
fabric wrapped fiberglass panels and foam do not work at controlling low
frequency room modes, because the wavelengths are much too long compared
to the typical thicknesses of these porous materials.
Mounting (architectural) (types A, B, C, D, E, F) –
Finish materials are attached to, applied to or supported by the
substrate behind them. The way the finish material is mounted to
the substrate can significantly affect how the building surface affects
the sound that hits it. In an ‘A’ mounting, the finish is applied
directly to the substrate with no air space in between (for example,
paint on concrete block or carpeting on a concrete slab). In a ‘D’
mount, the finish is spaced off the substrate by something like a
furring strip, metal clip, etc. and there is airspace between the two.
When reviewing manufacturers’ acoustic data, be sure that the mounting
used during their test is the same mounting you plan on using in your
project.
Narrowband noise - All noises have unique frequency (or
pitch) content. Some noise is narrowband, meaning that it contains
only certain limited frequencies (not low frequency noise, mid frequency
noise and high frequency noise like broadband noise). An example
of narrowband noise is an electrical transformer that ‘buzzes’.
Noise – Sound that is heard and judged as annoying or
undesirable. What might be sound or music to some people could be
noise to others.
Noise control – An area of building acoustics that
deals with preventing, controlling or attenuating noise generated by
building systems such as mechanical, electrical, plumbing, fire
suppression and conveying systems so that the noise is not problematic
in occupied areas.
Noise criteria (NC) – A single number measure that
reports how loud or soft the ambient or background noise is in a room
(excluding any occupant noise). NC values below 30 are very quiet
and typically used in sound critical rooms like recording studios and
performing arts halls. NC values from 30 to 45 are what we
generally experience in our everyday lives. NC values above 50 are
considered quite loud and most people would be annoyed by the noise.
Noise isolation class (NIC) – A single number measure
used to describe how easily sound travels from one room in a building to
another room considering all possible paths. Because all possible
paths are considered in this measurement, it relates to what people
actually hear or experience in the building more so than field sound
transmission class (which applies more to a single element such as a
wall, door or window). NIC values below 45 are not very good and
not much sound isolation is being provided. NIC values between 50
and 60 are moderate to good in most building types. NIC values
above 65 are very good and often require resilient construction.
Noise reduction coefficient (NRC) – A single number
measure that varies from 0% (0.0) to 100% (1.0) and is the indicator of
how much sound is absorbed by a wall, floor or ceiling material.
Higher values mean greater sound absorption. Painted drywall may
have a lower NRC value of only 5% (0.05) while a piece of acoustical
ceiling tile (ACT) might have a very high NRC value near 100% (1.0).
Manufacturers typically report NRC values in their product
specifications.
Octave bands – The human ear can hear a wide range of
sound frequencies or pitches (from 20 Hz up to 20,000 Hz). We
break this rather large range into smaller ranges called octave bands.
We generally look at just six octave bands when dealing with sound in
buildings (125 Hz, 250 Hz, 500 Hz, 1,000 Hz, 2,000 Hz and 4,000 Hz).
In certain cases we may also look at sound in the lower octave bands of
32 Hz and 63 Hz or higher octave bands of 8,000 Hz or 16,000 Hz)
All of these numbers are center frequencies in the smaller broken down
ranges.
Pink noise – Pink noise is broadband noise that has
nearly the same sound level at all frequencies. Pink noise can
provide masking, but most people do not judge pink noise as being
natural sounding or pleasant. So, for example, you would not want
to play pink noise through an open office sound masking system to
promote speech privacy, because the pink noise itself would be judged
annoying by most people.
Pitch (frequency) – Sound is essentially something
vibrating the air and then our ear drums. Faster vibrations result
in higher pitch/frequency sound. Slower vibrations result in lower
pitch/frequency sounds. If you play the ‘A’ note above the middle
‘C’ note on a piano the string vibrates at 440 cycles per second
(Hertz). Pitch typically refers to the ‘A’ note, while frequency
refers to the 440 cycles per second (Hertz) vibration.
Plenum – An architectural plenum is the large space
between a room’s ceiling and the floor above it. It is usually
partially filled with ductwork, pipes and conduits. At times, all
the return air from the rooms on a floor of a building may not be ducted
with rigid metal ducts. Instead, the plenum is used to return air
any way the air can find its way back (above the ceiling) to an opening
in the mechanical equipment room wall. If this is the case, then
walls separating rooms cannot extent up into the plenum, for they will
block return air flow. Generally, the return air plenum approach
is discouraged because of the sound isolation problems that result.
Point sound source - Sources of sound can be described
generally by their overall shape. A point sound source is one that
does not have significant length or width compared to the location where
the sound is being heard. For example, an oil line compressor
sitting out in a field is typically considered to be a point sound
source relative to a remote house in an adjacent residential community.
Reflection, reflector, reflective – A room surface such
as ceiling or wall that is flat, hard, smooth and nonporous (e.g.,
painted drywall) will not absorb sound that hits it. Instead, the
sound is bounced or reflected off the surface and redirected towards
another place in the room. At times, reflectors are specifically
designed into rooms so that sound is redirected in a specific and
desired direction. For example, you often see reflective surfaces
over a location where someone stands and speaks to an audience like over
an altar in a church. The reflector over the orator redirects a
sound reflection towards the audience making the speech louder and
easier to understand. A sound reflection off a flat, hard, smooth
surface will redirect most of the sound energy in a direction equal and
opposite to the arrival angle (think cue ball bouncing off a bumper).
This type of reflection is called a specular or discrete reflection.
Resilient, resiliently mounted - Sound moves easily
through monolithic materials or rigid assemblies once borne in the
material or assembly. For example, once sound is borne in the
drywall on one side of a common wall, it moves efficiently through the
solid studs that connect one side of the wall with the other side of the
wall. Designing a structurally resilient break into the assembly
greatly decreases the amount of noise that passes through it. For
example, the drywall on one side of the wall could be mounted on
resilient channels instead of being screwed rigidly into the studs.
Resonate, resonance – Some things, especially when made
of metal, tend to ‘ring’ or resonate when they are struck. For
example, if you tap on a hollow metal drum or box, it makes noise for
seconds after you tap it. This persistence of sound or ringing is
called a resonance. You can decrease the amount of resonance by
applying a damping material.
Reverberation time – Larger rooms that do not have
sound absorbing materials in them allow sound that is generated in them
to bounce around for long periods of time (e.g., a gymnasium).
Reverberation time is the length of time (in seconds) that sound
continues to bounce around (or reflect around) after the sound source
has stopped. Most spaces we encounter in life have reverberations
times between about 0.5 seconds and 2.0 seconds. In order for
speech to be highly intelligible (for example in classrooms) the
reverberation time should be less than 1.0 second. Acoustic music
(that which is not electronically amplified) including choral music
tends to sound better in more reverberant spaces with reverberation
times between 1.5 seconds and 2.5 seconds. So, even a 0.5 second
change in reverberation time is very significant to our ears. To
decrease reverberation time, just add sound absorbing materials in the
room on the ceiling, walls or hung free in the space.
Room acoustics – An area of building acoustics that
deals with the quality of sound inside rooms. Room shape, room
size and the types and locations of finish materials in the room all
affect whether speech during a lecture will be intelligible, music
played during a church service will be clear or general chatter from a
restaurant full of diners will be loud and annoying.
Sabine – A sabine is the unit of measure of sound
absorption. The more sound absorbing materials that are in a room,
the more sound absorption (in unit sabines) is being provided. If
you multiply the length of the absorbing material by the width (or
height) to get the area of the material, and then multiple the area by
the absorption coefficient (as reported by the manufacturer), you get
the amount of absorption in sabines being provided by the material.
For example, a four foot wide by two foot high fiberglass wall panel has
an area of eight square feet. If the panel has an absorption coefficient
of 0.40 at 500 Hertz (as reported by the manufacturer), then the panel
is providing 3.2 sabines of absorption. In order to calculate
reverberation time, you have to first calculate the sabines of
absorption being provided by all the materials in the room at the
independent octave bands. When reviewing manufacturer information,
the absorption information for some products such as low frequency tube
traps (cylinders suspended in the air) will be provided in sabines per
unit as opposed to the more standard absorption coefficient.
Silencer – Noise from large fans inside the building’s
main air handling units can travel down the ducts to nearby rooms.
Duct lining inside the ducts can quickly absorb mid and high frequency
fan noise. However, it is difficult to attenuate low frequency fan
noise without using duct silencers. Duct silencers are analogous
to a car’s muffler. They are large metal boxes (about the same
size and shape as the duct in which they are being installed) that
contain fiberglass insulation and a perforated metal inner liner to hold
the fiberglass in place. You must understand that silencers can
generate noise too, especially if the air velocity through them is too
fast. That is why silencers are typically located by the fans (not
close to the rooms), and then followed by about twenty feet of internal
duct lining to absorb the mid and high frequency noise generated by the
silencer.
Signal to noise ratio (S/N) – Speech intelligibility
relates strongly to signal to noise ratio. The signal is the
speech you are trying to hear and understand. Noise is any other
sound such as ambient noise, occupant noise, etc. that can interfere
with or mask the signal you are trying to hear. Good acoustics
often corresponds with making the desired signal as loud as possible and
the ambient noise as low as possible (in other words a high signal to
noise ratio).
Slab cut – The bottom floor of a building is often a
concrete slab poured directly on the ground. In some cases when
you are trying to control the noise of something like an MRI in a
hospital, you can locate a perimeter slab cut around the noisy device.
That means that the concrete slab is not continuous. There is a
separate smaller slab under the noisy device (or conversely under a
particularly quite room) and then there is the main large slab
elsewhere. These slabs are poured at separate times with a thick
resilient material between them so the concrete from one slab does not
contact the concrete of the other slab. This decreases the amount
of noise that transfers from one slab to the other.
Sound – When an object vibrates at a rate between about
20 cycles per second (Hertz) and 20,000 cycles per second (Hertz) it
creates audible sound that our ears can hear if we are close enough and
other sounds do not mask it.
Sound isolation – An area of building acoustics that
deals with preventing or at least attenuating unwanted noise from the
outside or adjacent rooms (horizontally or vertically) so it does not
bother occupants of an interior space. Sound isolation involves
wall constructions, window and door types, floor constructions, roof
constructions, etc. as well as a lot of details to prevent unwanted
flanking paths through which noise can leak.
Sound lock – When a quiet room is located next to a
noisy room, the doors from one to the other are often the weak link in
the overall sound isolation between the two rooms. One option is
to use special doors with very high sound transmission class (STC)
ratings. But, these doors can be very expensive, heavy, hard to
open and limited in aesthetics. Another option is to plan for and
provide a sound lock, which is a small vestibule with two standard doors
in series. The finishes inside the sound lock should be
sound-absorptive (carpet, ACT ceiling, fiberglass wall panels, etc.).
Then, the sound from the noisy room has to pass through the first door,
get absorbed by the absorptive finishes inside the sound lock and then
also pass through the second door on the other side of the sound lock.
The doors on both sides of the sound lock should also have full
perimeter door seals/gaskets.
Sound transmission class (STC/FSTC) – A single number
measure of how resistant a building assembly such as a wall or
floor/ceiling is to air-borne noise such as a neighbor playing their
stereo too loudly in the condo next to yours. A higher STC value
means less noise is heard in your condo. STC values below 45 are
not very good and not much sound isolation is provided. STC values
between 50 and 60 are moderate to good in most building types.
Increasing STC above 65 requires extensive control of flanking paths.
For example, it does not improve overall isolation as much as you might
think to increase a wall construction from STC-65 to STC-80 when there
is a continuous concrete slab running under the wall and a continuous
metal roof deck running over the wall. Sound will flank the wall
through the roof and floor. STC is a laboratory measurement.
When STC is measured in real buildings it is called field sound
transmission class (FSTC). FSTC is generally 3-5 points lower than
laboratory STC values.
Structure-borne sound - Sound can move from one place
to another in building via different paths or a combination of paths.
If something is ‘conducting’ sound from one place to another, the sound
is said to be ‘borne’ in the medium. So, structure-borne noise is
traveling from one place in the building to another via the building’s
structural system. For example, it is common in commercial
construction to use continuous concrete floors that run the entire
length and width of the building. Two rooms can be separated by a
rather large distance and be well protected from air-borne noise, but if
the sound becomes borne in the floor slab (for example a loud machine
sitting directly on the floor slab) the sound can travel through the
structure under all the walls to other areas of the building.
Substrate – Often a wall, floor or ceiling finish
material must be mounted to or applied to a substrate. For example
in a painted drywall wall on studs (the most common wall construction)
the paint is the finish and the drywall is the substrate. The
effect that a building surface has on sound depends greatly on the
finish as well as the substrate. Paint on drywall will affect
sound differently than paint on concrete block (CMU).
Transmission Loss (TL/FTL) – As sound passes (or
transmits) through a building assembly like a wall, roof/ceiling or door
it is attenuated or made less loud. Transmission loss is the amount of
decrease in loudness (expressed in dB) from one side to the other.
For example, if a 100 dB stereo is being played on one side of a wall
and the sound measures 60 dB in the adjacent room, then the transmission
loss is 40 dB (100 dB – 60 dB). Transmission loss is expressed by
octave band, so for a full picture of how a building assembly is
performing acoustically, you would actually have a transmission loss
value at each of the six octave bands from 125 Hz to 4,000 Hz.
Transmission loss is a laboratory measurement. When it is measured
in real buildings it is called field transmission loss (FTL). FTL
is generally 3-5 points lower than laboratory TL values.
Vibration – There is a difference between sound and
vibration; sound is audible vibration is only tactile. In other
words you hear sound and feel vibration. Vibration is just too low
in frequency for our ears to hear.
Wavelength of sound – Sounds at different frequencies
(or pitches) have different wavelengths. To find the wavelength,
divide 1130 (speed of sound in air) by the frequency (in Hertz).
So a 1,000 Hertz sound has a wavelength of 1.13 feet.
A 125 Hertz sound has a wavelength of about nine feet! The fact
that low frequency sound has such long wavelengths is why it is much
more difficult to control low frequency sound than mid or high frequency
sound.
White noise - White noise is broadband noise that has increasingly higher sound levels as you go up in frequency. White noise can provide masking, but most people do not judge white noise as being natural sounding or pleasant (even worse than pink noise). Instead, it sounds very ‘hissy’. So, for example, you would not want to play white noise through an open office sound masking system to promote speech privacy, because the white noise itself would be judged annoying by most people.
