Blocking exterior noise sources

by Katie Daniel | August 27, 2015 11:37 am

[1]

by Terence Tyson, PE, CDT
Whether at work or home, indoor acoustic comfort requires freedom from unwanted sound. Exposure to elevated noise levels may disrupt sleep and interfere with activities requiring concentration. Left uncontrolled, long-term exposure to noise levels not otherwise considered hazardous can lead
to stress.

Often, high levels of indoor noise are caused by exterior sound sources, such as vehicular traffic, rail, and aircraft operation, mechanical equipment, crowd noise, and amplified sound systems as well as sources associated with construction activity. This article examines the effect common sources of exterior noise have on noise levels experienced inside commercial, residential, and institutional buildings, and discusses classification of those receiver locations in terms of relative sensitivity to noise.

This author also reviews the relevant associated guidelines, regulations, and metrics, including thresholds of acceptability and thresholds of change, and statistical sound measurements. Finally, options for mitigation to reduce indoor noise exposure caused by exterior sources will be examined.

Noise metrics
Sound is a complex phenomenon that occurs across a broad range of frequencies, from very low to very high. A complete description of a particular sound at a given instant in time would require denoting the loudness of each individual frequency across the entire audible spectrum. To avoid this, single-number simplifications of sound spectra have been developed and are used routinely to describe and regulate sound levels.

The first such measure, dBA, is a sort of corrected sound level[2] that compensates for the human ear’s relatively poor sensitivity to low frequency sound. ‘A-weighted’ sound levels are easy to measure, and tend to correlate with the annoyance factor of many environmental noise sources. It is therefore the metric most frequently referenced in codes and ordinances.

Since sound is not necessarily constant over time, reporting a sound level as a single number average of some kind requires accounting for this time-based fluctuation. One such metric is denoted L_eq, which represents the energy average of all sound measured during a particular measurement period. It is defined as the sound pressure level that, if constant for the duration of the measurement, would contain the same total sound energy as the actual time-varying sound recorded during that period. This metric is commonly used to represent the overall average sound pressure level recorded for a given measurement.

[3]

Additionally, statistical measures accounting for the fluctuation of sound over time are also useful. L₁₀ for example, denotes a sound level that is exceeded 10 percent of the time during a particular measurement, and takes into account sporadic or intermittent noise peaks. It has been found to correlate well with the disturbance people feel when close to busy roads. Similarly, L₉₀, which describes the sound levels exceeded 90 percent of the time, is generally considered to represent the residual background noise level recorded in outdoor measurements.

The day-night equivalent sound level (L_dn) is another important metric for assessing outdoor noise sources. It is defined as a 24-hour continuous L_eq with a 10-dB penalty added to all noise levels recorded between 10 p.m. and 7 a.m. The penalty accounts for the extra sensitivity people have to noise during sleeping hours.

Many design/construction professionals are familiar with sound transmission class (STC). It is a single-number rating of the airborne sound transmission loss performance of a partition, door, or window assembly. The higher the STC value, the more effective an assembly is at blocking sound. However, it is important to note STC is weighted toward the measurement of isolation of speech frequencies. It is not a reliable measure of performance at low frequency.

To account for low frequencies associated with exterior noise sources, outdoor−indoor transmission class (OITC) is used to rate sound transmission across a building envelope. It is similar to STC, but is weighted to account for lower frequencies, and as such is the performance metric considered in this article.

[4]

Typical exterior noise sources
For the purposes of building codes and ordinances, exterior noise sources are sometimes classified as mobile noise sources, stationary noise sources, or construction noise.

Planes
Aircraft noise is generally perceived as a discrete event, and although it is not always a problem for occupants, the sound of flyovers and take-offs are often audible inside buildings. The Federal Aviation Administration (FAA) produces contour maps of flyover noise expressed in terms of L_dn levels, for use in planning and assessing the impact of aircraft noise. It is important to have access to these maps when designing sound-sensitive buildings near an airport or flight path.

Trains
Train noise is treated similarly to aircraft noise, although train pass-by events are generally of longer duration than aircraft flyovers. Location of rail lines, especially for freight, must be taken into consideration when planning construction of sound sensitive buildings. The Federal Transit Administration (FTA) uses one-hour L_eq or L_dn as the principal noise descriptors for mass transit noise, depending on the adjacent land use. Both are useful for planning and assessing new building design near railways.

Automobiles
The noise signature for vehicular traffic is characterized by fluctuating levels, punctuated with periodic noise ‘spikes,’ caused by trucks and buses. Design guidelines published by the Federal Highway Administration (FHWA) allow the use of one hour L_eq or L₁₀ in the analysis of traffic noise assessment.

Construction
Construction noise sources comprise both mobile (e.g. vehicles) and stationary (e.g. compressors, piledrivers, and power tools) sources. Noise levels generated by long-term construction projects must be considered when assessing the environmental noise level of a new building site.

Mechanical
Mechanical equipment associated with industrial operations and building ventilating systems are the primary external stationary noise source that designers must take into consideration. This equipment includes condensing units, chillers, transformers, emergency generators, cooling towers, exhaust fans, and rooftop air-handling units (AHUs). As sound pressure levels may approach 100 dBA within 3 m (10 ft) of some equipment, it is vitally important to be aware of the location of such equipment when designing for new sound-sensitive buildings nearby.

Other
Other stationary sources worth noting include crowd noise related to playgrounds or spectator events and noise from amplification systems. Although people are not usually considered stationary noise sources, sound generated at outdoor gatherings may cause considerable annoyance to adjacent communities. For example, the sound pressure level generated by spectators at outdoor sporting events may exceed 90 dBA.

[5]

Code requirements regarding noise
There are various forms of noise ordinances. Some define absolute sound pressure level thresholds
of acceptability, while others provide a maximum incremental limit, or threshold of change above existing background noise levels at neighboring locations. Some noise ordinances are even source-specific.

For example, according to the Philadelphia Department of Public Health’s Noise and Excessive Vibration Regulations, amplified sounds from restaurants and bars cannot exceed 3 dBA above the ambient sound level at the neighboring property line. Similarly, sound from commercial air-conditioning systems cannot exceed ambient levels by more than 5 dBA. Elsewhere, fixed daytime and nighttime sound level limits may be established, tabulated with daytime and nighttime maximum levels similar to what is shown in Figure 1. It is also not unusual to encounter a requirement for the maximum permissible increase above ambient at the property line to be limited to 10 dBA during daytime hours, and 5 dBA at night.

An understanding of the applicable local noise ordinance sets an expectation for normal maximum environmental noise levels in an area that is otherwise not burdened by loud mobile or stationary noise sources. The new building envelope can then be designed with these limits in mind.

[6]

Guidelines and sensitivity to noise
Of course, not all building types are equally sensitive to noise, as is suggested by the higher absolute limits allowable in commercial and industrial zones. Noise sensitivity, however, is not limited to residential buildings. Occupants of commercial offices are often quite sensitive to intrusive outside noise. For example, guidelines published in the American Society of Heating and Refrigerating Engineers (ASHRAE) 2015 HVAC Applications Handbook suggest executive suites and conference rooms in commercial office buildings be designed for interior sound levels of 30 to 35 dBA. It can be difficult to achieve these levels in the vicinity of nearby external noise sources using conventional curtain wall designs.

Institutional building types, including hospitals, nursing homes, courthouses, schools, and libraries, are usually limited to 30 to 35 dBA, and luxury residential buildings will typically target an interior background noise level of 30 dBA. Traffic, train, and aircraft noise sources must be considered in the design of these building types, recognizing the lower the required interior background noise levels, the more robust the exterior envelope of the building must be to avoid intrusion of exterior noise.

Noise mitigation in building construction
Reducing indoor perception of exterior noise requires introduction of a barrier of some kind between the source and the building occupants. Although it is sometimes possible to orient or locate a building in such a way as to take advantage of external sound barriers (e.g. highway barrier walls, earth berms, or shielding from other buildings), it is more often entirely up to the materials comprising the outer skin of the building to provide sufficient impediment to the noise transfer.

As a first step, whenever possible, one should take advantage of barriers, earth berms, and even building features such as lower-level balconies on a multi-story condo, to break up the line of sight between an exterior source and a sound-sensitive building interior. Simply eliminating direct line-of-sight to a problem noise source can usually provide a significant reduction in sound transfer.

Although the intrusion of exterior noise is highly dependent on the surface mass of the exterior wall construction, when outdoor noise enters a building, it is always reduced somewhat, even if the building has open windows or air vents. The amount of that reduction depends on several factors, including the wall construction, the amount and thickness of the glazing, the ratio of window to wall area, the amount of open window area, and the amount of acoustically absorptive interior finish treatment. Further, the audibility of an intruding noise also depends on the existing interior background sound level. As a general rule, if the intruding noise can be reduced to approximately 5 dB below the existing background noise level, it will likely not be cause for complaint.

For example, whereas a windowless masonry wall may have an OITC 50, introduction of a single window with an area of one fourth of the wall can reduce that performance to OITC 29. Of course, a couple of principles apply. For example, the heavier the masonry, and the fewer windows, the more effective a wall is at reducing exterior sound. There are, however, limits to the performance of monolithic barriers, as significant improvement in performance generally requires doubling the mass of the wall.

Larger increases in OITC performance of a wall can be achieved by incorporation of a secondary interior wall decoupled and separated from the outer wall by an insulated air space. Additionally, the choice of insulation is important. Closed-cell foam insulation materials are not sound-absorbing, and do not improve sound isolation of wall assemblies. Sound-absorbing insulation materials, such as glass fiber, open-cell foam, mineral wool, and cellulose are effective for sound isolation in all partition assemblies.

[7]

Acoustics of typical glass and concrete
Figure 2 lists the acoustic performance of typical glass and concrete building materials.

Curtain wall systems provide limited isolation from exterior noise sources. Typical performance is usually limited to about OITC 30. Interior retrofit systems or secondary inboard laminated glazing designs can increase curtain wall performance to greater than OITC 35.

It is always important to also consider roof construction, particularly when the interior space of interest is directly below the roof deck. When train noise and aircraft flyovers are a major concern, a concrete roof deck with a resiliently supported secondary, insulated, two-layer gypsum board ceiling is generally required.

Where frame construction is utilized in areas near significant noise sources, insulated laminated glass windows and multi-layer insulated gypsum board interior wall construction—decoupled from the exterior framing—can be utilized to reduce transfer of exterior sound.

Conclusion
Designing sound-sensitive buildings in the vicinity of exterior noise sources requires attention to many important details, starting with a thorough understanding of required maximum interior background noise levels. Although each design must be considered individually, when building near airports and railway lines, access to FAA L_dn contour maps and onsite noise studies should be reviewed in order to understand the environmental noise level of the new building site.

Design requirements in excess of OITC 40 are not uncommon in these areas, which, in addition to suitable exterior wall and glazing, will also typically require heavy roof construction in order to meet all of the acoustics expectations of the new occupants.

Terence Tyson, PE, CDT, is a principal consultant in acoustics at Acentech, a multi-disciplinary acoustics, audiovisual systems design, and vibration consulting firm. Much of his work has involved collaborating with mechanical and electrical engineers on the control of noise from HVAC and other mechanical and electrical systems in sound-critical facilities such as concert halls, performing arts centers, and recording studios. Tyson is a member of the Institute of Noise Control Engineering (INCE), American Society of Mechanical Engineers (ASME), and American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE). He can be reached at ttyson@acentech.com[8].

Source URL: https://www.constructionspecifier.com/blocking-exterior-noise-sources/

---