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Soundproofing Tutorial: IIC, STC and Acoustics
Soundproofing
Soundproofing is any means of reducing the sound pressure with respect to a specified sound source and receptor. There are several basic approaches to reducing sound: increasing the distance between source and receiver, using noise barriers to block or absorb the energy of the sound waves, using damping structures such as sound baffles, or using active antinoise sound generators.
Soundproofing affects sound in two different ways: noise reduction and noise absorption. Noise reduction simply blocks the passage of sound waves through the use of distance and intervening objects in the sound path. Noise absorption operates by transforming the sound wave. Noise absorption involves suppressing echoes, reverberation, resonance and reflection. The damping characteristics of the materials it is made out of are important in noise absorption. The wetness or moisture level in a medium can also reflect sound waves, significantly reducing and distorting the sound traveling through it, making moisture an important factor in soundproofing.
Contents:
Acoustics IIC and STC
Impact Insulation Class (or IIC) (or IIC Rating) is an integer-number rating of how well a building floor attenuates impact sounds, such as footsteps. A larger number means more attenuation. The scale, like the decibel scale for sound, is logarithmic. The IIC is derived from ASTM method E989, which in turn uses a tapping machine specified in ASTM method E492.
The IIC number is derived from sound attenuation values tested at sixteen standard frequencies from 100 to 3150 Hz. Unfortunately, "real world" footstep noise is also generated at frequencies below 100 Hz, so the IIC value may not accurately describe the complete noise attenuation profile of a floor.
To increase IIC we recommend to use a soundproofing product call QT Sound Insulation, it will help you to reduce sound transmission to soundproof floors.
Sound Transmission Class (STC)
Sound Transmission Class (or STC) (or STC Rating) is an integer rating of how well a building partition attenuates airborne sound. In the USA, it is widely used to rate interior partitions, ceilings/floors, doors, windows and exterior wall configurations (see ASTM International Classification E413 and E90). Outside the USA, the Sound Reduction Index (SRI) ISO standard is used.
The ASTM test methods have changed every few years and over many years have been changed significantly. Thus, STC results posted before 1999 may not produce the same results today, and this difference becomes wider as one goes back in time (that is the differences in test method from the 1970's to today are vast).
The STC number is derived from sound attenuation values tested at sixteen standard frequencies from 125 Hz to 4000 Hz. These transmission-loss values are then plotted on a sound pressure level graph and the resulting curve is compared to a standard reference contour. Acoustical engineers fit these values to the appropriate TL Curve (or Transmission Loss) to determine an STC rating. The measurement is accurate for speech sounds but less so for amplified music, mechanical equipment noise, transportation noise or any sound with substantial low-frequency energy below 125 Hz. Sometimes, acoustical labs will measure TL at frequencies below the normal STC boundary of 125 Hz, possibly down to 50 Hz or lower, thus giving additional valuable data to evaluate transmission loss at very low frequencies, such as a subwoofer-rich home theater system would produce. Alternatively, Outdoor-Indoor Transmission Class (OITC) is a standard used for indicating the rate of transmission of sound between outdoor and indoor spaces in a structure that considers frequencies down to 80 Hz (Aircraft/Rail/Truck traffic) and is weighted more to lower frequencies.
STC is roughly the decibel reduction in noise a partition can provide, abbreviated 'dB'. The dB scale is a logarithmic one and the human ear perceives a 10dB reduction in sound as roughly halving the volume - a 40 dB noise subjectively seems half as loud as a 50 dB one. (For more detail on equal-loudness curves see: Fletcher-Munson curves.) If an 80dB sound on one side of a wall/floor/ceiling is reduced to 50dB on the other side, that partition is said to have an STC of 30. This number does not apply across the range of frequencies, since the STC value is derived from a curve-fit of many data points. Any partition will have less TL at lower frequencies. For example, a wall with an STC of 30 may provide over 40dB of attenuation at 3000 Hz but only 10dB of attenuation at 125 Hz.
Typical interior walls in homes (2 sheets of 1/2" drywall on a wood stud frame) have an STC of about 33. When asked to rate their acoustical performance, people often describe these walls as "paper thin". They offer little in the way of privacy. Adding absorptive insulation (i.e. fiberglass 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.
Note that 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 decoupling the panels from each other will increase transmission loss much more effectively than simply adding more and more mass to a monolithic wall/floor/ceiling assembly.
Structurally decoupling the drywall panels from each other (by using resilient channels, steel studs, a staggered-stud wall, or a double stud wall) can yield an STC as high as 63 or more for a double stud wall (see table below), 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.
Due to their high density, concrete and concrete block walls have good TL values (STC's in the 40s and 50s for 4-8" thickness) but their weight, added complexity of construction and poor thermal insulation tend to limit them as viable materials in most residential wall construction, except in temperate climates and hurricane or tornado prone areas. Various Cellulose insulation installation options can result in an STC of 50 or greater.
Materials which can improve STC's in walls we recommend to use Genie Clip. Soundproof clip for drywall. .
It must be noted that acoustical performance values such as STC are measured in specially constructed acoustical chambers and field conditions such as lack of adequate sealing, outlet boxes, back-to-back electrical boxes, medicine cabinets, flanking paths and structure-borne sound can diminish acoustical performance. The as-built 'field-STC' (FSTC) is usually lower than the laboratory-measured STC.
Section 1207 of International Building Code 2006 states that separation between dwelling units and between dwelling units and public and service areas must achieve STC 50 (STC 45 if field tested) for both airborne and structure borne. However, not all jurisdictions use the IBC 2006 for their building or municipal code. In jurisdictions where IBC 2006 is used, this requirement may not apply to all dwelling units. For example, a building conversion may not need to meet this rating for all walls.
In serious cases (for instance, a bedroom adjacent to a home theater room, and an inconsiderate nocturnal neighbor, to boot) a partition to reduce sounds from high-powered home theater or stereo should ideally be STC 70 or greater, and show good attenuation at low frequencies. An STC 70 wall can require detailed design and construction and can be easily compromised by 'flanking noise', sound traveling around the partition through the contiguous frame of the structure, thus reducing the STC significantly. STC 65 to 70 walls are often designed into luxury multifamily units, dedicated home theaters, and high end hotels.
The demanding THX reference standard (a guideline for high-quality audio in movie soundtracks) requires partitions to achieve 50dB of attenuation at 63 Hz. Few walls can meet that, as that requires a wall with an STC of 80 or higher. For all practical purposes, no sound will be heard on the other side of the wall with this level of construction. However, an STC this high is not achievable in simple construction and this level of isolation is only feasible for high-end studios and theaters, where the design and construction can be carefully controlled and the additional cost is justified.
Building codes
In the case of construction of new (or remodeled) apartments, condominiums, hospitals and hotels many US states and cities have stringent building codes with requirements of acoustical analysis, in order to protect building occupants from (a) exterior noise sources and (b) sound generated within the building itself[ With regard to exterior noise, the codes usually require measurement of the exterior acoustic environment in order to determine the performance standard required for exterior building skin design. The architect can work with the acoustical scientist to arrive at the best cost effective means of creating a quiet interior (normally 50 dB). The most important elements of design of the building skin are usually: glazing (glass thickness, double pane design, etc.), roof material, caulking standards, chimney baffles, exterior door design, mail slots, attic ventilation ports and mounting of through the wall air conditioners. A special case of building skin design arises in the case of aircraft noise, where the FAA has funded extensive work in residential retrofit.
Regarding sound generated inside the building, there are two principal types of transmission. Firstly, airborne sound travels through walls or floor/ceiling assemblies and can emanate from either human activities in adjacent living spaces or from mechanical noise within the building systems. Human activities might include voice, amplified sound systems or animal noise. Mechanical systems are elevator systems, boilers, refrigeration or air conditioning systems, generators and trash compactors. Since many of these sounds are inherently loud, the principle of regulation is to require the wall or ceiling assembly to meet certain performance standards (typically Sound Transmission Class of 50), which allows considerable attenuation of the sound level reaching occupants.
The second type of interior sound is called Impact Insulation Class (IIC) transmission. This effect arises not from airborne transmission, but rather from transmission of sound through the building itself. The most common perception of IIC noise is from footfall of occupants in living spaces above. This type of noise is somewhat more difficult to abate, but consideration must be given to isolating the floor assembly above or hanging the lower ceiling on resilient channel. Commonly a performance standard of IIC equal to 50 is specified in building codes. California has generally led the U.S. in widespread application of building code requirements for sound transmission; accordingly, the level of protection for building occupants has increased markedly in the last several decades.
Architectural acoustics is the science of controlling sound within buildings. The first application of architectural acoustics was in the design of opera houses and then concert halls. More widely, noise suppression is critical in the design of multi-unit dwellings and business premises that generate significant noise, including music venues like bars. The more mundane design of workplaces has implications for noise health effects. Architectural acoustics includes room acoustics, the design of recording and broadcast studios, home theaters, and listening rooms for media playback.
a- Building skin envelope
This science analyzes noise transmission from building exterior envelope to interior and vice versa. The main noise paths are roofs, eaves, walls, windows, door and penetrations. Sufficient control ensures space functionality and is often required based on building use and local municipal codes. An example would be providing a suitable design for a home which is to be constructed close to a high volume roadway, or under the flight path of a major airport, or of the airport itself.
b- Inter-space noise control
The science of limiting and/or controlling noise transmission from one building space to another to ensure space functionality and speech privacy. The typical sound paths are room partitions, acoustic ceiling panels (Genie Clip for walls and ceilings), doors, windows, flanking, ducting and other penetrations. An example would be providing suitable party wall design in an apartment complex to minimize the mutual disturbance due to noise by residents in adjacent apartments.
The use of distance to dissipate sound is straightforward. The energy density of sound waves decreases as they spread out, so that increasing the distance between the receiver and source results in a progressively lesser intensity of sound at the receiver. In a normal three dimensional setting, the intensity of sound waves will be attenuated according to the inverse square of the distance from the source. Using mass to absorb sound is also quite straightforward, with part of the sound energy being used to vibrate the mass of the intervening object, rather than being transmitted. When this mass consists of air the extra dissipation on top of the distance effect is only significant for typically more than 1000 meters, depending also on the weather and reflections from the soil.
Residential soundproofing aims to decrease or eliminate the effects of exterior noise. The main focus of residential soundproofing in existing structures is the windows. Curtains can be used to damp sound either through use of heavy materials or through the use of air chambers known as honeycombs. Single-, double- and triple-honeycomb designs achieve relatively greater degrees of sound damping. The primary sound proofing limit of curtains is the lack of a seal at the edge of the curtain. Double-pane windows achieve somewhat greater sound damping than single pane windows. The increase in soundproofing with higher energy efficiency windows is primarily due to the better seals on double-pane than single pane windows. Significant noise reduction can be achieved by installing a second interior window. In this case the exterior window remains in place while a slider or hung window is installed within the same wall opening.
Noise barriers and external Soundproofing
Since the early 1970s it has become common practice in the United States (followed later by many other industrialized countries) to engineer noise barriers along major highways to protect adjacent residents from intruding roadway noise. The technology exists to predict accurately the optimum geometry for the noise barrier design. Noise barriers may be constructed of masonry, earth or a combination thereof. One of the earliest noise barrier designs was in Arlington, Virginia adjacent to Interstate 66, stemming from interests expressed by the Arlington Coalition on Transportation. Possibly the earliest scientifically designed and published noise barrier construction was in Los Altos, California in 1970.
Noise cancellation generators for active noise control are a relatively modern innovation. A microphone is used to pick up the sound that is then analyzed by a computer; then, sound waves with opposite polarity (180° phase at all frequencies) are output through a speaker, causing destructive interference and cancelling much of the noise.
Room within a room
A Room Within A Room (RWAR) is one method of isolating sound and stopping it from transmitting to the outside world where it may be undesirable.
Most vibration / sound transfer from a room to the outside occurs through mechanical means. The vibration passes directly through the brick, woodwork and other solid structural elements. When it meets with an element such as a wall, ceiling, floor or window, which acts as a sounding board the vibration is amplified and heard in the second space. A mechanical transmission is much faster, more efficient and may be more readily amplified than an airborne transmission of the same initial strength.
The use of acoustic foams and other absorbent means are useless against this transmitted vibration. The user is required to break the connection between the room that contains the noise source and the outside world. This is called acoustic de-coupling. Ideal de-coupling involves eliminating vibration transfer in both solid materials and in the air, so air-flow into the room is often controlled. This has safety implications, for example proper ventilation must be assured and gas heaters cannot be used inside de-coupled space.
Fundamental concepts of acoustics
The study of acoustics revolves around the generation, propagation and reception of mechanical waves and vibrations.
The steps shown in the above diagram can be found in any acoustical event or process. There are many kinds of cause, both natural and volitional. There are many kinds of transduction process that convert energy from some other form into acoustical energy, producing the acoustic wave. There is one fundamental equation that describes acoustic wave propagation, but the phenomena that emerge from it are varied and often complex. The wave carries energy throughout the propagating medium. Eventually this energy is transduced again into other forms, in ways that again may be natural and/or volitionally contrived. The final effect may be purely physical or it may reach far into the biological or volitional domains. The five basic steps are found equally well whether we are talking about an earthquake, a submarine using sonar to locate its foe, or a band playing in a rock concert.
The central stage in the acoustical process is wave propagation. This falls within the domain of physical acoustics. In fluids, sound propagates primarily as a pressure wave. In solids, mechanical waves can take many forms including longitudinal waves, transverse waves and surface waves.
Acoustics looks first at the pressure levels and frequencies in the sound wave. Transduction processes are also of special importance.
Acoustic wave propagation: pressure levels
In fluids such as air and water, sound waves propagate as disturbances in the ambient pressure level. While this disturbance is usually small, it is still noticeable to the human ear. The smallest sound that a person can hear, known as the threshold of hearing, is nine orders of magnitude smaller than the ambient pressure. The loudness of these disturbances is called the sound pressure level, and is measured on a logarithmic scale in decibels.
| Example of Common Sound | Pressure Amplitude | Decibel Level |
|---|---|---|
| Threshold of Hearing | 20*10-6 Pa | 0 dB |
| Normal talking at 1m | .002 to .02 Pa | 40 to 60 dB |
| Power lawnmower at 1m | 2 Pa | 100 dB |
| Threshold of Pain | 200 Pa | 134 dB |
Acoustic wave propagation: frequency
Physicists and 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. In a common technique of acoustic measurement, acoustic signals are sampled in time, and then presented in more meaningful forms such as octave bands or time frequency plots. Both these popular methods are used to analyze sound and better understand the acoustic phenomenon.
The entire spectrum can be divided into three sections: audio, ultrasonic, and infrasonic. The audio range falls between 20 Hz and 20,000 Hz. This range is important because its frequencies can be detected by the human ear. This range has a number of applications, including speech communication and music. The ultrasonic range refers to the very high frequencies: 20,000 Hz and higher. This range has shorter wavelengths which allows better resolution in imaging technologies. Medical applications such as ultrasonography and elastography rely on the ultrasonic frequency range. On the other end of the spectrum, the lowest frequencies are known as the infrasonic range. These frequencies can be used to study geological phenomenon such as earthquakes.
Transduction in acoustics
A transducer is a device for converting one form of energy into another. In an acoustical context, this usually means converting sound energy into electrical energy (or vice versa). For nearly all acoustic applications, some type of acoustic transducer is necessary. Acoustic transducers include loudspeakers, microphones, hydrophones and sonar projectors. These devices convert an electric signal to or from a sound pressure wave. The most widely used transduction principles are electromagnetism (at lower frequencies) and piezoelectricity (at higher frequencies).
A subwoofer, used to generate lower frequency sound in speaker audio systems, is an electromagnetic device. Subwoofers generate waves using a suspended diaphragm which oscillates, sending off pressure waves. Electret microphones are a common type of microphone which employ an effect similar to piezoelectricity. As the sound wave strikes the electret's surface, the surface moves and sends off an electrical signal.
Acoustic transmission
Acoustic transmission in building design refers to a number of processes by which sound can be transferred from one part of a building to another. Typically these are:
- Airborne transmission - a noise source in one room sends air pressure waves which induce vibration to one side of a wall or element of structure setting it moving such that the other face of the wall vibrates in an adjacent room. Structural isolation therefore becomes an important consideration in the acoustic design of buildings. Highly sensitive areas of buildings, for example recording studios, may be almost entirely isolated from the rest of a structure by constructing the studios as effective boxes supported by springs. Air tightness also becomes an important control technique. A tightly sealed door might have reasonable sound reduction properties, but if it is left open only a few millimeters its effectiveness is reduced to practically nothing. The most important acoustic control method is adding mass into the structure, such as a heavy dividing wall, which will usually reduce airborne sound transmission better than a light one.
- Impact transmission - a noise source in one room results from an impact of an object onto a separating surface, such as a floor and transmits the sound to an adjacent room. A typical example would be the sound of footsteps in a room being heard in a room below. Acoustic control measures usually include attempts to isolate the source of the impact, or cushioning it. For example carpets will perform significantly better than hard floors.
- Flanking transmission - a more complex form of noise transmission, where the resultant vibrations from a noise source are transmitted to other rooms of the building usually by elements of structure within the building. For example, in a steel framed building, once the frame itself is set into motion the effective transmission can be pronounced.
Sound pressure is the local pressure deviation from the ambient (average, or equilibrium) pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water. The SI unit for sound pressure is the Pascal (symbol: Pa). The instantaneous sound pressure is the deviation from the local ambient pressure p0 caused by a sound wave at a given location and given instant in time. The effective sound pressure is the root mean square of the instantaneous sound pressure over a given interval of time (or space). In a sound wave, the complementary variable to sound pressure is the acoustic particle velocity. For small amplitudes, sound pressure and particle velocity are linearly related and their ratio is the acoustic impedance. The acoustic impedance depends on both the characteristics of the wave and the medium. The local instantaneous sound intensity is the product of the sound pressure and the acoustic particle velocity and is, therefore, a vector quantity.
Sound pressure level
Sound pressure level (SPL) or sound level Lp is a logarithmic measure of the rms sound pressure of a sound relative to a reference value. It is measured in decibels (dB).
Sometimes variants are used such as dB (SPL), dBSPL, or dBSPL. These variants are not permitted by SI.
The unit dB (SPL) is often abbreviated to just "dB", which gives some the erroneous notion that a dB is an absolute unit by itself.
The commonly used reference sound pressure in air is pref = 20 µPa (rms), which is usually considered the threshold of human hearing (roughly the sound of a mosquito flying 3 m away). When dealing with hearing, the perceived loudness of a sound correlates roughly logarithmically to its sound pressure. See also Weber-Fechner law. Most measurements of audio equipment will be made relative to this level, meaning 1 Pascal will equal 94 dB of sound pressure.
In other media, such as underwater, a reference level of 1 µPa is more often used.
These references are defined in ANSI S1.1-1994.
The human ear is a sound pressure sensitive detector. It does not have a flat spectral response, so the sound pressure is often frequency weighted such that the measured level will match the perceived level. When weighted in this way the measurement is referred to as a sound level. The International Electro technical Commission (IEC) has defined several weighting schemes. A-weighting attempts to match the response of the human ear to pure tones, while C-weighting is used to measure peak sound levels. If the (unweighted) SPL is desired, many instruments allow a "flat" or unweighted measurement to be made.
When measuring the sound created by an object, it is important to measure the distance from the object as well, since the SPL decreases in distance from a point source with 1/r (and not with 1/r2, like sound intensity). It often varies in direction from the source, as well, so many measurements may be necessary, depending on the situation. An obvious example of a source that varies in level in different directions is a bullhorn.
Examples of sound pressure and sound pressure levels
| Source of sound | Sound pressure | Sound pressure level |
|---|---|---|
| Pascal | dB re 20 μPa | |
| Shockwave (distorted sound waves > 1 atm; waveform valleys are clipped at zero pressure) | >101,325 Pa | >194 dB |
| Loudest undistorted sound wave (1 atm) | 101,325 Pa | 194 dB |
| Krakatoa explosion at 100 miles (160 km) in air[ | 20,000 Pa | 180 dB |
| Simple open-ended thermo acoustic device | 12,619 Pa | 176 dB |
| .30-06 carbine 1 m to shooter's left side | 7,265 Pa | 171 dB (peak) |
| M1 Garand being fired at 1 m | 5,023 Pa | 168 dB |
| Jet engine at 30 m | 632 Pa | 150 dB |
| Threshold of pain | 63.2 Pa | 130 dB |
| Hearing damage (due to short-term exposure) | 20 Pa | approx. 115 dB |
| Jet at 100 m | 6.32 – 200 Pa | 110 – 140 dB |
| Jack hammer at 1 m | 2 Pa | approx. 100 dB |
| Hearing damage (due to long-term exposure) | 0.356 Pa | 78 dB |
| Major road at 10 m | 2×10−1 – 6.32×10−1 Pa | 80 – 90 dB |
| Passenger car at 10 m | 2×10−2 – 2×10−1 Pa | 60 – 80 dB |
| TV (set at home level) at 1 m | 2×10−2 Pa | approx. 60 dB |
| Normal talking at 1 m | 2×10−3 – 2×10−2 Pa | 40 – 60 dB |
| Very calm room | 2×10−4 – 6.32×10−4 Pa | 20 – 30 dB |
| Leaves rustling, calm breathing | 6.32×10−5 Pa | 10 dB |
| Auditory threshold at 1 kHz | 2×10−5 Pa | 0 dB |
Sounding board
The sounding board or soundboard is the part of a string instrument that transmits the vibrations of the strings to the air, greatly increasing the loudness of sound over that of the string alone.
The sounding board operates by the principal of forced vibration; the board is gently vibrated by the string, and despite their differences in size and composition, the board will be "forced" to vibrate at the exact same frequency, producing the same sound as the string alone, differing only in timbre. Although the same amount of energy is transmitted with or without the board present, the sounding board, due to its greater surface area, is more readily able to transform this energy into sound. In other words, the sounding board can move a much greater volume of air, therefore producing a louder sound.
Sounding boards are traditionally made of wood (see tone wood), though other materials can be used, such as skin or plastic on instruments in the banjo family. Wood sounding boards typically have sound holes in them with different shapes depending on the instrument: round in guitars, f-holes in violin family instruments, rosettes in lutes, and so on.
The sounding boards of some instruments have unique names, such as plate, or belly (the latter in a violin).
In a grand piano, the sounding board is a large horizontal plate at the bottom of the case. In an upright piano, the sounding board is a large vertical plate at the back of the instrument. The harp has a sounding board below the strings.
More generally, any hard surface can act as a sounding board. An example is when a tuning fork is struck and placed against a table top to amplify its sound.