Scope of this article about soundproofing.
This article outlines the basics of soundproofing for a dilettante. A dilettante is someone unfamiliar with soundproofing but who feels he needs to know something about it to deal with his construction project. The article is also for the know-it-all, assuming that he will read this article. A know-it-all is a person who thinks he knows to soundproof but knows too little to address the subject knowledgeably. Typically, this person would be a renovation contractor or even someone claiming to represent a soundproofing company.
The scope of information presented here is limited to basic concepts and a general overview of soundproofing. More detailed information is available elsewhere on the internet, for example, from National Research Council of Canada and Soundproofing Calculator . Any soundproofing company has access to this information.
This article will increase your understanding of soundproofing and help you make better decisions in your projects, whether you are a homeowner or a soundproofing contractor.
Do not hesitate to contact us; we are the Toronto soundproofing company and The Soundproofing Expert. We will be pleased to work with you to complete your Toronto or GTA soundproofing project. We provide consulting services and soundproofing, and construction.
What is sound, and what is noise?
Everyone thinks they understand the meaning of the word “sound”. Surprisingly, the answer to “what is sound?” is not straightforward. Sound can be considered a wave motion in air or other elastic media. Sound can also be considered as an excitation of the hearing mechanism that results in its perception. [Reference 1]
I like to use the second part of the description of the sound. In other words, sound is what humans perceive as sound. Sound can be both pleasant and unpleasant. Generally, the pleasant sound is what we consciously like to listen to, be it music, someone else talking to us, sounds of nature during a hike, etc.
On the other hand, noise is a sound that is neither pleasant nor welcomed to us. This could be the noise of machinery, loud music played by a neighbour, traffic noise, or other people's conversations in a restaurant where we are having a quiet dinner. A soundproofing company can help you to reduce unwanted noise or improve the acoustic quality of your space.
What is soundproofing?
The dictionary definition of soundproofing is: "impervious to sound; to cause to be soundproof" . I think this definition is somewhat circular and inaccurate. To illuminate the term, I am quoting from reference :
“The construction of walls, floors, and ceilings is fundamental to any architectural endeavor. When the space within will be acoustically sensitive, those elements take additional importance. In addition to structural integrity, these partitions must work as sound barriers to isolate the interior space from exterior noise and to isolate the exterior from the interior sound. To satisfy this acoustical requirement, these structural barriers must be designed and constructed in considerably different ways from typical building specifications.”
Soundproofing is a sub-specialty of building acoustics, and building acoustics is a branch of acoustics science.
In a strict semantic sense, “soundproofing” is an oxymoron. It implies eliminating sound, but that is physically impossible. We can only achieve a degree of reduction of sound level. All buildings have some inherent level of resistance to external sound entry and sound propagation from room to room. However, there is always a residual level of sound transfer into a room that may be unacceptably high in acoustically sensitive applications. A soundproofing company's engineering and construction effort aimed at reducing the level of unwanted noise or sound in a room or building is called “soundproofing”.
Therefore, a soundproofing company aims to reduce the noise sufficiently so that we can enjoy quiet or hear the sounds that we like to listen to.
How much soundproofing do I need?
The answer to this question is not straightforward. It depends on many factors; only some key factors will be discussed here. Read more about this topic in this blog article.
Before we can ask “how much,” we need to know how to measure the quantity. I am basing my simplified explanation on reference . As stated previously, at the physical level, the sound is a rapid wave motion in the air. In other words, the sound is slight, quick variations in air pressure.
As you may recall from your high school physics class, air pressure is measured in pascals (metric units) or pounds/square inch (imperial units). Because sound variations in air pressure are so minute, the numbers representing slight pressure variations are unwieldy. We, therefore, almost always measure sound in relative values as unit-less ratios. Ratios, in logarithmic representation, are expressed in decibels. Sound pressure level (SPL) quantities are stated relative to the lowest sound pressure level humans can perceive, the faintest possible sound. This is designated as SPL 0 dB. All other SPL readings are expressed as sound pressure relative to the hearing threshold sound pressure level, expressed in logarithmic units decibels. (Note for the scientifically minded: This reference sound pressure level, the hearing threshold, is 20 micropascals ). We, somewhat inaccurately, say that sound is measured in decibels, and since ratios are unitless, the pressure units (pascals or pounds/sq. inch) are omitted.
There is an additional factor to the measurement of SPL. Physically, sound occurs at a range of frequencies, perceived by humans as a range of tones. Because the perception of the loudness of sound varies with frequency, SPL measurement instruments, and sound level meters, alter the weighting of the range of frequencies to more closely represent loudness as perceived by people. This frequency weighting is designated as “A”. Therefore, the SPL values are A-weighted, abbreviated as dB(A) or dBA.
The table below from  gives examples of Sound Pressure Levels as perceived by people.
Sound Pressure Level dB(A)
Noise office or heavy traffic
Hearing threshold, excellent ears at frequency maximium response
Sound Transmission Class (STC)
STC is a single-number rating used by soundproofing companies and the construction industry to measure and compare the effectiveness of building assemblies in attenuating airborne sound transmission. The STC number is used without the dB designation. The exact definition of STC is somewhat complicated. Interested readers can find the details in the relevant standards (see ASTM International Classification E413 and E90 ) or by searching the internet, for example, one STC explanation.
As I have already mentioned, sound loudness perception depends on the frequency (or range of frequencies) of sound. STC is designed to approximate the soundproofing effectiveness for airborne speech sounds, which are sounds with the range of frequencies typical to speech (125 Hz to 4000 Hz; Hz is the metric designation for "cycles per second"). STC is not as good a representation for soundproofing effectiveness for other than speech sounds, such as music, noise generated by machinery, or other noises.
Impact Isolation Class (IIC)
There is a distinct type of noise that occurs in buildings. It is created by the impact of objects (or footfall) on a partition, typically a floor. This noise is delivered directly to the building structure and propagates as vibration. At some point, the vibrating structure transfers its energy to the air and is perceived by the listener as impact noise.
IIC is a single number rating used in the construction industry to estimate the effectiveness of construction assemblies in attenuating transmission of impact noise, which is the noise created by objects impacting on partitions (usually floors). The exact definition of IIC is somewhat complicated. Interested readers can find a detailed explanation by searching the internet; for example, here is one IIC explanation.
Quantitatively, how much soundproofing is required?
To make soundproofing effective and useful, it must reduce unwanted noise to a tolerable level, ideally to an inaudible level. The concept of soundproofing is simple. There are two basic ways of reducing noise levels at acoustically sensitive locations:
- Reduce the level of offending noise at source.
- Insert a barrier to noise between its source and the acoustically sensitive location.
Reducing noise at source is usually the preferred approach, where feasible. For example, fixing or replacing the offending machinery is the best and usually the cheapest approach if a defective air-conditioning unit is excessively noisy.
However, in many situations, the noise level at the source cannot be reduced. Then the second approach to noise control must be employed.
A barrier to sound that is placed between the source of the sound and the receiving room (the acoustically sensitive location, the room we are trying to soundproof), be it a wall, a ceiling, a door, or a window, must attenuate the sound sufficiently to meet the stated objectives. Acousticians say the barrier introduces transmission loss (TL) to the noise path. Therefore, the required TL is the difference between the SPL of the noise source and the tolerable SPL of the residual noise in the receiving room.
For example, suppose the offending noise is at SPL of 80 dB(A) (for example, traffic noise or loud shouting), and the background noise level in the receiving room is at SPL of 35 dB(A). In that case, we need to reduce the offending noise to less than the background noise level (typically by 3 dB(A) less) for the residual noise to be unnoticeable. Therefore, the required TL is the difference between the two levels, which in this example is 48 dB. This required transmission loss roughly indicates that the STC of the wall assembly should be at least 48.
The table below [from ref. 8] provides basic guidance for answering the question about the STC value required to ensure speech privacy. The table shows a subjective description of the soundproofing effectiveness of a wall assembly with a given STC number. The unwanted speaker (noise) is on one side of the wall, the listener on the other.
STC of wall assembly
Perception of listener behind the wall assembly
Normal speech can be heard quite easily and distinctly
Loud speech can be understood fairly well, normal speech can be heard but not understood
Loud speech audible but not intelligible
Loud speech audible as a murmur
Loud speech is not audible
Very loud sounds such as musical instruments or a stereo can be faintly heard
Another point to note is that an increase of 10 STC points in wall performance is subjectively perceived as a reduction of noise to about half.
Factors that make soundproofing complicated.
Different people react differently to noise. Sometimes even the same person may react differently to noise under different circumstances. An acoustician must try to understand what his customer's needs are and what type of noise is annoying to his customer.
Acoustical design must consider the background noise in the acoustically sensitive space. Noise is only noticeable if it exceeds the level of the usual background noise. Therefore, in quiet locations, such as urban settings far from traffic and other noise sources, even relatively low noise levels may be very distracting. This must be considered in soundproofing and noise control design.
Many, sometimes unforeseen factors in the vicinity of the acoustically sensitive space and in the building structure will influence noise levels and, thus, the amount of soundproofing required to meet stated objectives. Some of these factors are:
- Large sound-reflecting surfaces nearby, such as adjacent buildings, may reflect noise toward the acoustically sensitive space.
- Particularly noisy, selfish, and disrespectful neighbour.
- The poor acoustic quality of the existing structure.
- Noisy mechanical equipment close to the acoustically sensitive space.
- Flanking noise paths.
- Hidden construction defects.
- And many other factors.
A soundproofing company takes generalized approaches.
There are five approaches to reducing noise in an acoustically sensitive space that a soundproofing company may take :
- Locating the acoustically sensitive room in a quiet place.
- Reducing noise output of the offending source.
- Interposing a sound-insulating barrier between the acoustically sensitive space and the noise source.
- Reducing the noise energy within the source room and or the receiving room.
- Minimizing both the airborne and the structure-borne noise.
A soundproofing company must consider all these approaches to achieve the desired result at a minimum cost.
Some of the approaches may not apply to every situation. For example, relocating the acoustically sensitive space to a quieter location is often not feasible. However, it may be the most cost-effective solution if it can be done.
Reducing the noise output of the offending sound is the first approach a soundproofing company should consider in most situations. It is likely the best and cheapest solution if it can be done.
Interposing a sound-insulating barrier between the noise source and the acoustically sensitive location is often the most feasible approach, also called soundproofing. It is best to incorporate soundproofing into the original building design and construct it with adequate soundproofing. However, a soundproofing company is often called upon to correct inadequate acoustical design and retrofit soundproofing measures into an existing building.
Reducing the noise energy within the source room and/or the receiving room is achieved by introducing sound absorption into these spaces. However, sound absorption often only results in marginal improvement and may not be sufficient.
Minimizing both the airborne and the structure-borne noise means that both paths of noise must be addressed to achieve the desired result.
Many physical properties of building materials and assemblies affect their soundproofing effectiveness, such as:
- Material density and mass
- Stiffness and damping
- Mechanical vibration resonance effects
- Sound absorption and attenuation
- Mechanical decoupling of components and structures
- Cavities in structures
- Cracks and gaps
By carefully assembling different materials into wall or floor systems, a soundproofing company can create transmission loss of the assembly to match the required level of soundproofing.
Field measurements of soundproofing
STC and IIC ratings of material assemblies are determined in laboratory conditions designed to eliminate any external factors that are not part of the assembly under test. This standardized approach allows for quickly and roughly comparing various materials and assemblies based on STC and IIC numbers.
The performance of these assemblies in actual structures in the field is more variable because many external factors influence the soundproofing effectiveness that cannot always be eliminated in a practical field situation. Some of these factors are:
- Quality of construction
- Effect of junctions of structures (leakage and flanking noise)
- Acoustic effects in the receiving room (e.g. reverberation)
- Level of background noise.
Therefore, it is best to perform field measurements to determine the soundproofing effectiveness of the actual building structures. The STC values obtained by field measurements are designated ASTC (Apparent Sound Transmission Class). Industry standards specify how these measurements should be done to minimize external factors' effects and make the test results more accurate and repeatable. The soundproofing effectiveness as measured in an actual building (as compared to a lab test obtained results) is often lower than the performance of similar building assemblies in a lab.
Basic Architectural Acoustics Terminology
Pressure wave traveling through a medium, such as air or building structure..
Pressure wave traveling through a medium, such as air or building structure..
The science of sound propagation and transmission is a branch of physics.
A logarithmic ratio describes sound levels relative to the threshold of hearing, which is 20 micro-pascals. dB is technically not a unit of sound pressure but is often used as such.
The rate at which sound pressure wave changes repeat. Measured in hertz (Hz). 1 Hz = 1 cycle/sec.
A band of frequencies where the upper limiting frequency is twice the lower limiting frequency. Octave bands are identified by their center frequencies: 31.5, 63, 125, 250, 500, 1000, 2000, 4000 & 8000 Hz.
One-third Octave Band
A band of frequencies where the upper limiting frequency is 1.33 times the lower limiting frequency. One-third octave bands are identified by their center frequencies: 31.5, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150, 4000, 5000, 6300, 8000, 10000, 12500 Hz.
A-Weighting Network, dBA or dB(A)
A frequency-weighting electronic network intended to represent the variations in the ear’s ability to hear different frequencies. The overall sound level measured using the A-weighting network is indicated by dBA, rather than dB.
Sound Pressure Level (SPL, Lp)
A measurement of root mean square (RMS) sound pressure variations around equilibrium atmospheric pressure which is equal to 20 times the logarithm (base 10) of the ratio of RMS sound pressure of a given sound, divided by the reference RMS sound pressure of 20 microPa (0 dB, threshold of hearing). This unit-less value is reported in decibels (dB or dBA)
Equivalent sound level (Leq)
A constant sound pressure level would result in the same total sound energy as the measured time-varying sound pressure level if the constant sound pressure level persisted over an equivalent time duration. The Leq1hr, for example, describes the equivalent continuous level over a 1 hr period.
The sound that persists in an enclosed space resulting from repeated reflections or scattering after the sound source has stopped.
Reverberation Time, RT
In seconds, it would take for a sound to decay by a set amount, typically 60 dB. It is often based on measurements of 20 dB or 30 dB decay, which are extrapolated to a standardized decay of 60 dB.
Sound absorption refers to the process by which a material, structure, or object takes in sound energy when exposed to sound waves instead of reflecting the energy. Part of the absorbed energy is transformed into heat and transmitted through the absorbing body.
Sound Absorption Coefficient
Frequency-dependent fraction of incident sound power absorbed or otherwise not reflected from the surface. This coefficient is measured according to international standards ISO 354 and ASTM C423.
Transmission Loss (TL)
The measure of the airborne sound reduction provided by a partition, ceiling/floor, door, or window. Expressed in decibels (dB), it measures the ratio of the acoustic energy striking the partition relative to the energy transmitted through it. Test methodology: ASTM E90.
Sound Transmission Class (STC)
Single figure rating derived from laboratory measurements of airborne sound transmission loss. Test method standard: ASTM E90. Rating method standard: ASTM E413.
Flanking Sound Transmission
The transmission of sound from one room to another by a path other than through the separating partition (or partition under test).
Noise Isolation Class (NIC)
Single figure rating derived from the sound level difference (noise reduction) between two rooms – includes direct and flanking sound transmission. Test method standard: ASTM E336. Rating method standard: ASTM E413.
Impact Sound Level
Sound pressure level measured in a receiving room under a test floor being excited by a standardized impact sound machine. An impact source is a tapping machine consisting of a row of five equally spaced hammers, each weighing 0.5 kg and separated by 400 mm. The machine lifts and drops the hammers successively to generate 10 impacts per second at 40 mm free drop. Test methodology: ASTM E492.
Impact Isolation Class
Single number rating derived from a laboratory measurement of impact sound level. Test methodology: ASTM E942.
Useful Acoustical Calculations
This website provides a large number of acoustical engineering calculations, formulas, and references.
 Everest & Pohlmann, Master Handbook of Acoustics, Sixth Edition (2015), McGraw Hill Education, page 1
 Everest & Pohlmann, page 321
 Everest & Pohlmann, Chapter 2
 Everest & Pohlmann, page 26
 Everest & Pohlmann, page 298
 Belle, L.H., Bell, D.H., “Industrial Noise Control: Fundamentals and Applications”, Mercel Dekker, Inc. 1994.
 Table adapted from presentation by Alan Oldfield, AECOM, at IIDEXCanada 2015