The noise around acoustics

by Katie Daniel | November 28, 2017 9:30 am

[1]

By Andrew Schmidt, ASA, and Ajith (AJ) Rao, PhD, WELL AP
The sheer number of acoustic metrics, criteria, and associated abbreviations can be overwhelming, confusing, or sometimes misleading. This article provides an overview of key acoustic metrics and criteria commonly referred to in various building codes, design guidelines, and standards. Many of the acoustic terms described within will look familiar as they appear frequently in literature related to the Leadership in Energy and Environmental Design (LEED) program, International WELL Building Institute, the National Building Code of Canada (NBC), and numerous other standards or product documentation.

In this article, the authors hope to lend clarity as to the comparative strengths and weaknesses of these various acoustic metrics and provoke critical thought as to which are most appropriate for different building types.

Sound transmission
To understand acoustics, it can be useful to imagine waves washing up on the beach, water pushing itself forward. There are short, little waves washing up quickly on shore. There are larger waves further apart, and some medium-sized waves somewhere in between. All these waves flow in together, slowly, simultaneously, in the same medium—water.

When thinking about the physics of sound, one can take those same waves, with the same variety of size and length, and speed them up. They travel at the speed of sound (343 m [1125 ft] per second) together, simultaneously, in another colourless medium—air. Airborne sound is a pressure wave in air; when that wave strikes a surface at the speed of sound, it hits with a high amount of energy, and that pressure wave in air will go through objects, often by force.

Most people hear wavelengths as long as 17 m (55.8 ft) and as short as 17 mm (0.67 in.). Those smaller 17-mm wavelength sounds are easily stopped by a relatively giant layer of 16-mm (⁵/8-in.) gypsum wallboard. Conversely, a tremendous 17-m wave would easily pass through a comparably small 250-mm (10-in.) thick concrete wall. This is sound transmission. Sound, like water, is adept at sneaking through and around things.

The following paragraphs break down the various key terms and definitions related to sound transmission.

Transmission loss
Transmission loss (TL) is the difference between the sound energy incident upon a material or construction assembly and the sound energy transmitted through the material or assembly. It is measured in a laboratory under specific room acoustic conditions with sound energy generated by a sound source on one side of a partition (i.e. the source) and measured on the other side (i.e. the receiver). Results are published in one-third octave frequency bands.

Noise reduction
Defined as the difference between the sound pressure level produced by a sound source on one side (i.e. the source) and the other (i.e. the receiver) of the construction assembly being evaluated, noise reduction (NR) is similar to TL. However, it is measured in the field, rather than under controlled laboratory conditions. Consequently, it accounts for sound transmission via flanking paths in addition to what is transmitted directly through the material or partition.

Sound transmission class
A single-number rating that is ascribed to the transmission loss spectrum of a construction assembly under laboratory conditions (i.e. no sound flanking paths), sound transmission class (STC) values are determined by fitting a specified contour, weighted to the human hearing spectrum, to one-third octave TL data (Figure 1). Essentially, the STC rating of a material or assembly is the approximate TL of that system at 500Hz.

[2]Figure 1: Sound transmission class (STC) calculation from one-third octave transmission loss data using curve-fit method.

[3]

Noise isolation class
Noise isolation class (NIC) is similar to STC, but is determined by fitting the standard STC contour to one-third octave NR data measured in the field. NIC is a single-number value that approximates the NR of a partition at 500Hz.

Composite sound transmission class
Used to describe the sound transmission properties of a partition with multiple elements, such as a wall with doors and windows, the composite STC (STC_c) takes into account the surface area of each element and its transmission loss contribution to the entire assembly.

Apparent sound transmission class
Similar to STC, the apparent sound transmission class (ASTC) value includes the contribution of sound transmitted via flanking paths, such as common flooring, ceiling, structural, and mechanical elements, in addition to sound transmitted directly through the material or partition.

Weighted sound reduction index with spectrum adaptation term
This is a single-number value, in decibels, derived from laboratory transmission loss data (R_w). It is calculated by a curve-fit method similar to STC. R_w puts slightly more emphasis on low-frequency transmission performance of a system than STC and is often combined with a spectrum adaptation term (C_tr)—a single-number value, in decibels, accounting for the characteristics of a particular sound source spectra.

Ceiling attenuation class
The ceiling attenuation class (CAC) is a single-number rating representing a ceiling’s ability to prevent airborne sound from travelling between adjacent spaces when the common wall between those spaces does not extend full-height. It is a good measure for acoustical tile ceilings in closed office spaces with a common ceiling plenum between spaces.

Impact insulation class
A single-number value that describes the ability of a floor/ceiling assembly to reduce impact noise transmission, the impact insulation class (IIC) is determined by fitting a standard contour to one-third octave band sound pressure level data measured under laboratory conditions.

Outdoor/indoor transmission class
The outdoor/indoor transmission class (OITC) is a single-number value that describes an exterior partition’s ability to attenuate outdoor environmental noise. OITC is similar to STC in that it is derived from one-third octave band TL data, but it differs in that it is calculated by mathematical equation, rather than a curve-fit method.

Composite outdoor/indoor transmission class
Employed to describe the sound transmission properties of an exterior partition with multiple elements (e.g. a wall with doors and windows), the composite OITC (OITC_c) considers the surface area of each element and its transmission loss contribution to the entire assembly, similar to STC_c.

Sound transmission summary
Aside from TL and NR, all these criteria are single-number ratings meant to describe the acoustic behaviour or performance of various systems. However, as sound energy spans a spectrum of low to high frequencies, these ratings do not always tell the whole story and should therefore be used as a general guideline. For high-performance buildings and critical-use spaces, it is important to consider the sound transmission performance of systems across the entire spectrum of audible frequencies.

The ability for any construction assembly to mitigate unwanted sound transmission hinges on various factors, such as good workmanship and proper sealing of the wall perimeter and penetrations. Laboratory testing minimizes these types of variables, so it is important they get the same level of attention in the field. As a rule of thumb, if any amount of air can pass through a partition, then sound will be able to more easily transmit through that partition. Even the smallest hole or unsealed penetration can severely degrade the acoustic performance of an otherwise ‘hig-STC’ assembly.

[4]

Room acoustics
The term ‘room acoustics’ has slightly different meanings to different people, but a simplified version might be the behaviour and perception of sound within closed spaces. The paragraphs that follow will review reverberation, sound absorption, and steady-state background noise—all of which play leading roles in the overall room acoustic environment.

Sound absorption co-efficient
Sound absorption co-efficients (a) are a measurement of a material’s ability to absorb sound. Defined as the fraction of the randomly incident sound power absorbed by a surface or material, they are typically measured and reported in one-third octave bands on a scale from 0.0 (perfectly sound-reflective) to 1.0 (perfectly sound-absorptive.)

Noise reduction co-efficient
A single-number rating describing a material’s ability to absorb sound, the noise reduction co-efficient (NRC) is the arithmetic average of the sound absorption co-efficients at 250Hz, 500Hz, 1000Hz, and 2000Hz octave bands.

Reverberation time
The reverberation time (RT) is the period it takes, in seconds, for a sound within a room to decay by 60dB from its initial sound level. In its simplest form, reverberation time is a function of the physical volume of the room, the total surface area of the room envelope and any interior elements, and the sound absorption co-efficients of those surfaces.

A-weighted and C-weighted sound pressure levels
Decibels are the overall sound pressure level (single-number) calculated from one-third octave or octave band sound pressure levels, then weighted according to predetermined spectrum curves. A-weighting (dBA) is the most common and is intended to imitate the response of the human ear at normal sound levels, giving less weight to sound at very low and very high frequencies. The C-weighting (dBC) curve is meant to imitate the response of the human ear at very high sound levels (over 100 dB).

Noise criteria
Noise criteria (NC) is a single-number value describing the spectrum of background noise typically associated with steady-state noise sources, such as air circulation systems. Noise criteria is often assigned in preferred ranges for different types of spaces and is used as a guideline for designing HVAC systems. For example, concert halls should be designed for NC-20 or lower (very low background noise), while open plan office spaces may be designed for NC-35 to NC-40 (moderate background noise.)

Room criteria
Similar to NC, the room criteria (RC) is a single-number value for the background noise within a space with additional descriptors for the quality of noise, based on the background noise spectrum. These descriptors are neutral, rumble, hiss, and perceptible vibration. A variant on RC, referred to as RC Mark II, includes even more descriptors for the quality of background noise.

Room noise criteria
While NC and RC are most applicable to steady-state background noise conditions, room noise criteria (RNC) is a single-number value for background noise with sensitivity to low-frequency time-varying fluctuations or surging that can be produced by HVAC systems.

Room acoustics summary
Similar to the aforementioned sound transmission loss metrics, room acoustic criteria are single-number ratings (with the exception of a) that do not describe full-spectrum effects of the behaviour and perception of sound within closed spaces. For high-performance buildings and critical use spaces, it is important to consider a broader spectrum of frequencies with respect to background noise criteria, sound-absorptive materials, and reverberation times.

Background noise criteria (e.g. NC and RC) are often referenced in mechanical systems’ standards and specification documentation.

Speech intelligibility and privacy
Indices for speech intelligibility and privacy combine various elements of sound transmission and room acoustics to paint a more complete picture of the behaviour of sound in spaces and the way humans perceive it.

Articulation index
Articulation index (AI) is a single-number rating, on a scale from 0.0 to 1.0, intended to define speech privacy in open-office environments with partial-height walls. Low AI values have better speech privacy than high AI values. Calculation of AI takes into account speech spectrum level, background noise level, the time-varying quality of background noise, and reverberation.

[5]

Privacy index
Privacy index (PI) is the inverse of AI, expressed as a percentage. The higher the PI, the better the speech privacy:

• 95 to 100 per cent represents confidential privacy;
• 80 to 94 per cent represents normal privacy; and
• values less than 80 per cent represent minimal or no speech privacy.

Speech intelligibility index
Similar to AI, the speech intelligibility index (SII) is calculated using speech spectrum level, background noise level, hearing threshold level, and a modulation transfer function that better accounts for distortion of the speech signal due to reverberation.

Speech privacy class
A single-number value developed to define speech privacy in closed rooms, private offices, and conference rooms, the speech privacy class (SPC) accounts for the sound transmission loss properties of the intervening construction as well as ambient background noise levels to predict average time intervals between lapses in speech intelligibility and audibility between rooms.

Speech transmission index
Speech transmission index (STI) is a single-number value on a scale from 0.0 to 1.0. Rating speech intelligibility, it takes into account the spectrum of the sound source, reverberant conditions, and background noise. STI is especially useful in determining speech intelligibility in reverberant spaces.

Articulation class
A single-number rating of the acoustical performance of products (e.g. acoustic ceiling tiles) used in open-plan office environments with cubicle partitions, articulation class (AC) is dependent on the product’s ability to absorb sound, particularly at higher frequencies, carrying the consonant sounds that are critical for speech intelligibility.

Sound-reflective surfaces, such as gypsum board, may have an AC value of 120, while ceilings best-suited for open-plan offices may have AC values of 180 or higher for moderate levels of privacy. For maximum privacy, AC values greater than 200 are typically recommended.

Speech intelligibility and privacy summary
If the previous terms and definitions appear quite similar to one another, it is because they are. With the exception of AC, which is a simplified value assigned to a product, all the speech privacy and intelligibility criteria listed are closely related. The origins of the study of speech intelligibility go back to the 1930s with AI becoming the first standardized calculation method of its kind in 1969 (i.e. via ASTM E1130, Standard Test Method for Objective Measurement of Speech Privacy in Open Plan Spaces Using Articulation Index). Since then, it has been a steady process of iterative improvements, adjustments and adaptations with the development of STI, SPI, and SII, and other quantifiers less frequently used than those listed above.

Myriad factors playing a role in speech privacy and intelligibility, including speech sound source levels and spectra, room acoustic conditions, background noise levels and spectra, and the sound transmission properties of the wall/floor/ceiling systems between open-plan and closed spaces. Therefore, speech privacy and intelligibility should be developed as part of a holistic acoustic approach with architecture, interior furnishings and finishes, structure, and mechanical/electrical/plumbing (MEP) systems.

Conclusion
For the sake of presentation and discussion, this article’s authors have segmented different areas of acoustics: sound transmission, room acoustics, and speech privacy and intelligibility. However, these segments are all enmeshed, intimately tied together in the fast-flowing movement of those sound pressures wave travelling through a fluid medium—much like those waves in water.

Andrew Schmidt, ASA, is an acoustical consultant experienced in a wide array of commercial and residential construction types. With an engineering degree from the acoustics and music program at University of Hartford, he spent his early years as a consultant at Jaffe Holden Acoustics, under the mentorship of the late Chris Jaffe. Schmidt’s expertise spans all areas of architectural acoustics, including sound isolation, room acoustics, and mechanical systems noise and vibration control. He joined USG Corporation (the parent company of CGC Inc.) in 2017 as a senior researcher with the Building Science & Technology Commercialization group. Schmidt can be reached at aschmidt@usg.com[6].

Ajith (AJ) Rao, PhD, WELL AP, is a senior researcher with the Building Science & Technology Commercialization group at USG, based at its Corporate Innovation Center in Libertyville, Ill. He has a PhD in architectural sciences from Rensselaer. Rao’s core expertise lies in development and application of codes, standards, and technologies related to high-performance buildings. He can be reached at arao@usg.com[7].

Source URL: https://www.constructioncanada.net/noise-around-acoustics/

---