Scope of this article about soundproofing

This article outlines basics of soundproofing. The article is also useful to the know-it-all, assuming that he will read this article. A know-it-all is a person who thinks he knows soundproofing, but in fact knows too little to knowledgeably address the subject. Typically, this person would be a renovation contractor, or even someone claiming to represent a soundproofing company.

Scope of information presented here is limited to basic concepts and 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.

I hope that this article will increase your understanding of soundproofing and helps 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 supply to you the most effective resilient clip for soundproof wall or ceiling assemblies. We will work with you to get your Canadian soundproofing project done right. We provide soundproofing consulting services and products.

Everyone think they understand the meaning of the word “sound”. Surprisingly, the answer to the question “what is sound?” is not straightforward. Sound can be considered as 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 sound. In other words, sound is what humans perceive as sound. Sound can be both pleasant and unpleasant. Generally, 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 sound that is not pleasant to us, or not welcomed. This could be noise of machinery, loud music played by a neighbour, traffic noise, or conversations of other people 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.

Dictionary definition of soundproofing is: “impervious to sound; to cause to be soundproof” [2].  I think this definition is somewhat circular and inaccurate. In order to illuminate the term, I am quoting from reference [3]:

“The construction of walls, floors and ceilings is fundamental to any architectural endeavour. When the space within will be acoustically sensitive, those elements take an 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 interior sound. To satisfy this acoustical requirement, these structural barriers must be designed and constructed in ways that are considerably different 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, the word “soundproofing” is an oxymoron. It implies complete elimination of 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 entry of external sound and propagation of sound from room to room. However, there is always a residual level of sound transfer into a room that, in acoustically sensitive applications, may be unacceptably high. The engineering and construction effort by a soundproofing company aimed at reducing the level of unwanted noise or sound in a room or building is called “soundproofing”.

Therefore, the goal of a soundproofing company is to reduce noise sufficiently so that we can enjoy quiet, or hear the sounds that we like to listen to.

Measurement Units

Before we can ask “how much” we need to know how to measure the quantity. I am basing my simplified explanation on reference [4]. As stated previously, sound, at the physical level, is rapid wave motion in air. In other words, sound is minute quick variations in air pressure.

As you may recall from your high school physics class, air pressure is measured in pascals (metric units), or in pounds/square inch (imperial units). Because sound variations in air pressure are so minute, the numbers to represent small 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 threshold of hearing, is 20 micropascals [5]). We, somewhat inaccurately, say that sound is measured in decibels, and since ratios are unit-less, the pressure units (pascals or pounds/sq. inch) are omitted.

There is an additional factor to measurement of SPL. Physically, sound occurs at a range of frequencies, perceived by humans as range of tones. Because perception of loudness of sound varies with frequency, SPL measurement instruments, sound level meters, alter the weighting of 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 [6] gives examples of Sound Pressure Levels as perceived by people.

Sound Source
Sound Pressure Level dB(A)

Saturn rocket


Ram jet


Propeller aircraft




Heavy truck


Noise office or heavy traffic


Conversational speech


Quiet residence


Leaves rustling


Hearing threshold, excellent ears at frequency maximium response


STC is a single number rating used by soundproofing companies in the construction industry to measure and compare effectiveness of building assemblies to attenuate transmission of airborne sound. 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, here is one STC explanation.

As I have already mentioned, sound loudness perception depends on 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 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.

There is a distinct type of noise that occurs in buildings. It is created by impact of objects (or footfall) on a partition, typically a floor. This noise is delivered directly to the building structure and propagates as vibration through the structure. At some point the vibrating structure transfers its energy to air and is perceived by listener as impact noise.

IIC is a single number rating used in the construction industry to estimate the effectiveness of construction assemblies to attenuate transmission of impact noise, which is noise created by objects impacting on partitions (usually floors). The exact definition of IIC is somewhat complicated. Interested readers can find detailed explanation by searching the internet, for example, here is one IIC explanation.

In order to make soundproofing effective and useful, it must reduce unwanted noise to tolerable level, ideally to inaudible level. The concept of soundproofing is simple. There are two basic ways of reducing noise levels at acoustically sensitive location:

  • Reduce the level of offending noise at source.
  • Insert a barrier to noise between its source and the acoustically sensitive location.

Reducing the level of noise at source is usually the preferred approach, where feasible. For example, if a defective air-conditioning unit is excessively noisy, fixing or replacing the offending machinery is the best and usually the cheapest approach.

However, in many situations, the noise level at 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 that the barrier introduces transmission loss (TL) to the path of the noise. 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, if 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), we need to reduce the offending noise to less than the background noise level (typically by 3 dB(A) less) in order 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 a level of speach privacy. The table shows subjective description of soundproofing effectiveness of a wall assembly with given STC number. The unwanted speaker (noise) is on one side of the wall, a 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 increase of 10 STC points in wall performance is subjectively perceived as reduction of noise to about half.

Human factors

Different people react differently to noise. Sometimes even the same person my react differently to noise under different circumstances. An acoustician must make the effort to understand what his customer needs are, and what type of noise is annoying to his customer.

Background noise

Acoustical design must take into consideration the background noise in the acoustically sensitive space. Noise is only noticeable if it exceeds level of the usual background noise. Therefore, in quiet locations, such as urban setting far from traffic and other noise sources, even relatively low noise level may be very distracting. This must be considered in soundproofing and noise control design.

Other factors

There are many, sometimes unforeseen factors in the vicinity of the acoustically sensitive space and in the building structure that 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 building, which may reflect noise towards the acoustically sensitive space.
  • Particularly noisy, selfish and disrespectful neighbour.
  • Poor acoustic quality of the existing structure.
  • Noisy mechanical equipment close to the acoustically sensitive space.
  • Flanking noise paths.
  • Hidden construction defects.
  • And may other factors.

There are five approaches to reducing noise in an acoustically sensitive space that a soundproofing company may take [7]:

  • Locating the acousticaly 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 structureborne noise.

A soundproofing company must consider all of these approaches to achieve the desired result at minimum cost.

Some of the approaches may not apply to every situation. For example, it is often not feasible to relocate the acoustically sensitive space to a quieter location. However, if it can be done, it may be the most cost effective solution.

Reducing the noise output of the offending sound is the first approach that a soundproofing company should consider in most situations. If it can be done, it is likely the best and the cheapest solution.

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 original building design and construct the building 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 by itself may not be sufficient.

Minimizing both the airborne and the structureborne noise simply means that both path 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 with HushFrame Raft
  • 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.

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.

Performance of these assemblies in actual structures in the field is more variable, because there are many external factors influencing the soundproofing effectiveness that cannot be always 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 (eg. reverberation)
  • Level of background noise.

It is therefore advisable to perform filed measurements to determine soundproofing effectiveness of the actual building structures. The STC values obtained by filed measurements are often designated as ASTC (Apparent Sound Transmission Class). There are industry standards specifying how these measurements should be done to minimize the effect of external factors and to 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.




Pressure wave travelling through a medium, such as air or building structure..


Pressure wave travelling through a medium, such as air or building structure..


The science of sound propagation and transmission, a branch of physics.

Decibel, dB

A logarithmic ratio used to describe sound levels relative to 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.

Octave Band

A band of frequencies where the upper limiting frequency is twice the lower limiting frequency. Octave bands are identified by their centre 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 band are identified by their centre 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. 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)

The value of a constant sound pressure level which would result in the same total sound energy as would the measured time-varying sound pressure level if the constant sound pressure level persisted over equivalent time duration. The Leq1hr, for example, describes the equivalent continuous level over 1 hr period.


Sound that persists in an enclosed space, as a result of repeated reflections or scattering, after the sound source has stopped.

Reverberation Time, RT

The length of time, in seconds, it would take for a sound to decay by a set amount, typically 60 dB. It is often based on measurement of 20 dB or 30 dB decay, which are extrapolated to a standardized decay of 60 dB.

Sound Absorption

Sound absorption refers to the process by which a material, structure or object takes in sound energy, when exposed to the sound waves, as oposed to reflecting the energy. Part of the absorpbed energy is transformed into heat and part is transmitted through the absorbing body.

Sound Absorption Coefficient

Frequency depended 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 is a measure of the ratio of the acoustic energy striking the partition relative to the energy which is 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 path other than through the separating partition (or partition under test).

Noise Isolation Class (NIC)

Single figure rating derived from 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. The impact source is a tapping machine consisting of a row of five equally spaced hammers, each weighting 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 laboratory measurement of impact sound level. Test methodology: ASTM E942.

Useful Acoustical Calculations

This website provides large number of acoustical engineering calculations, formulas and references.

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