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Acoustic performance measurement protocols.

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For high performing buildings, it is essential that a common set of measurements be used and results reported against meaningful benchmarks. Such protocols are needed to give practical feedback to building designers and operators when measured performance does not match design intent and expectations. Performance Measurement Protocols for Commercial Buildings, (1) a book jointly authored by ASHRAE, U.S. Green Building Council (USGBC), and the Chartered Institution of Building Services Engineers (CIBSE), is meant to fill this gap.

Performance Measurement Protocols for Commercial Buildings identifies what to measure, how to measure (instrumentation and spatial resolution), and how often to measure under the building's operation and maintenance plan. Performance categories include energy,

water, thermal comfort, IAQ, lighting, and acoustics. Measurement protocols were developed at three levels: basic (low cost and simple), intermediate (medium cost and intermediate skill level) and advanced (high cost and accuracy, requiring experts to perform). These three levels provide realistic choices for consistent performance characterization of the building and comparison to appropriate benchmarks.

Basic protocols are intended to be easy-to-perform, low-cost measures that give an initial picture of performance for the whole building. When these measurement results reveal cause for further measurements, intermediate protocols provide more data on building performance at the major system level, typically at monthly intervals. Advanced protocols offer more detailed and comprehensive analyses for building owners or managers wishing for more information on the performance of a building or to evaluate a perceived problem, typically at a daily or weekly frequency at the system or equipment level.

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Studies have shown that acoustic issues--background noise levels, speech privacy, and intrusive noise--are the number one complaint in high performing buildings. (2-5) Figure 1 compares building occupant ratings of satisfaction with nine different aspects of the working environment in a large sample of high performing buildings. Acoustics is rated the poorest of features graded at almost five times worse than the closest item, thermal comfort.

Acoustics aspects of Performance Measurement Protocols for Commercial Buildings are discussed next, including the three levels of protocols and their benchmarks.

Definition of Sound Rating Systems

A-Weighted Sound Level (dBA). System of modifying measured sound pressure levels to simulate the response of the human ear to different sound frequencies (Figure 2).

Noise Criterion (NC). Series of curves of octave band sound pressure levels from 63 Hz to 8,000 Hz (Figure 3). These curves are nominally based on equal loudness curves from the 1940s. The rating is based on a tangential reading. The highest NC curve to which an octave band level is tangent is the NC rating.

Room Criterion (RC). Series of curves of octave band sound pressure levels from 16 Hz to 4,000 Hz (Figure 3). They are based on a 5 db per octave downslope line with factors for rating a sound as "rumbly," below 250 Hz, to "hissy," above 1000 Hz.

Balanced Noise Criteria (NCB). Series of curves of octave band sound pressure levels from 16 Hz to 8,000 Hz. These curves are similar to the NC curves but have been adjusted in frequency range and nominally based on modern equal loudness curves.

Basic Level Measurements

The recommended basic methods for evaluating acoustical performance of commercial buildings include an occupant survey to identify perceived acoustical problems in the building and a survey of the A-weighted sound pressure level (Leq in dBA) in occupied spaces. The sound level survey provides simple and quick evaluation of background noise in a room. Personnel with limited training in acoustic testing can perform this survey. The purpose is to assess acoustic annoyance that could affect study and work performance as well as sleep and relaxation in hotels and similar spaces.

The occupant survey should be conducted using a questionnaire such as the CBE survey. (6) The CBE survey has several questions on acoustics. The occupant survey has the potential of covering more spaces in a building than a sound level survey since the occupants can take the survey simultaneously without assistance.

Measurements

The sound level survey should measure A-weighted sound pressure levels (in terms of Leq in dBA) in all occupied spaces. For large open areas several measurements should be obtained and arithmetically averaged, including the "noisiest" locations of those areas. The survey method of a standard under development by the American National Standards Institute (ANSI) for room noise measurements should be used. An early draft of the procedure can be found on the ASHRAE Technical Committee 2.6, Sound and Vibration website. (7)

Conditions During Sound Tests

Sound measurement tests should be performed in empty rooms with HVAC systems operating as close to normal steady-state condition as possible. All non-HVAC related sound-producing equipment should be turned off during the measurements. If the room cannot be vacated, detailed notes should be recorded of occupant numbers and activities being performed.

When intruding noise from outdoor sources (e.g., aircraft, street traffic, lawn mowers) is a concern, then testing should be scheduled when these sounds are at a maximum. If windows are designed to open for ventilation, then measurements should be performed with windows open and closed.

For more detailed information about these measurements, equipment and calibration requirements, and test report requirements, see Performance Measurement Protocols for Commercial Buildings.

Evaluation/Benchmarking

Occupant survey results should be compared with results from similar buildings as recorded in the CBE database. (8) Sound level measurements are compared with the noise criteria in Performance Measurement Protocols for Commercial Buildings's Table 3-10, (1) which are based on specific uses of spaces. A range of criteria is provided. The lowest A-weighted sound level represents an ideal acoustic environment. The highest value represents the level above which further evaluation is recommended using intermediate measurements.

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Intermediate Measurements

Intermediate evaluation of the acoustic performance of commercial buildings requires accurate assessment of the background noise and reverberation time in occupied rooms. The objective is to assess acoustic annoyance that affects study and work performance as well as speech and telephone communication listening conditions.

Measurements

Measurements must be conducted by personnel with training in acoustic testing, such as an acoustical consultant or personnel with specific training in sound pressure level measurements. These background noise measurements should be based on the engineering grade requirements of the room sound test method. (9) The procedure requires measurement of background noise in octave bands and a more rigorous approach to determining microphone locations and measurement durations. Reverberation time measurements, which are indirect measurements of acoustical absorption in a space, are required to assess speech communication issues, such as intelligibility.

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Instrumentation

Measurement instruments for obtaining noise and reverberation time measurements must meet requirements for Type 1 sound level meter as defined in IEC 616721-1 (10) or the Type 1 specifications as defined in the latest versions of ANSI S1.43, (11) and ANSI S1.11. (12)

If room use requires good speech communication, such as conference rooms, call centers, etc., reverberation time should be measured. Reverberation times may be calculated in octave or one-third octave bands from 100 to 4000 Hz. Periodic measurements are not required unless occupant surveys suggest that the environment has changed.

Conditions During Sound Tests

The requirements for the basic level should be followed including vacating the room during measurements and normal HVAC operating conditions, and the status of windows.

Measurements

Background noise should be measured at measurement points where one would expect to find the ears of the occupant (e.g., near seated positions in front of desks in private offices). At least three more measurement locations must be measured in the room.

Reverberation time [R.sub.T60] is measured with the microphone and noise source located within the room. Two methods may be used: an impulse excitation method, or the interrupted noise method in accordance with American Society for Testing and Materials (ASTM) E2235-04e1. (13) The [R.sub.T60] is the time it takes for sound in a room to drop 60 dB after the sound input ends.

Details about conducting these measurements and requirements for test reports are available in Performance Measurement Protocols for Commercial Buildings.

Evaluation/Benchmarking

Results of intermediate measurements should be compared with the range of criteria shown in Table 1. (1) The lowest NC/ RC values represent the ideal acoustic environment, and the highest values represent a maximum level above which noise of the space is not acceptable for its use.

The measured reverberation time in all octave bands from 250 to 4,000 Hz (rounded to nearest 0.1 second) should be evaluated against specified criteria.

Acoustic performance criteria vary by room type and activities in those rooms. Any commercial building will have some spaces that are acceptable and others that are not acceptable. Any rating scheme for buildings must take this into account. An appropriate rule of thumb is that when 80% to 90% of background noise measurements are found to be acceptable, then the building may be considered marginally acceptable. If 90% of the room background noise measurements are found to be acceptable, then the building may be considered acceptable.

Advanced Level Measurements

Buildings purporting to meet high levels of acoustic performance for LEED should be required to provide higher levels of proof that those design goals have been met. Accordingly, one or more advanced measurements may be required beyond the Intermediate to validate the highest level of performance. The specific objectives of advanced measurements are to ensure speech privacy and sound isolation.

Measurements

Room use determines the types of measurements required. Speech communication is important for critical-listening spaces, such as theaters, lecture halls, courtrooms, videoconference facilities, and other spaces. Speech privacy measures are important for open offices, private offices, and rooms used in hospitals and other businesses where confidential information is discussed. Sound transmission through walls and floors is important for rooms in apartments, hotels, and schools. Whenever sound separation between interior rooms is critical, such as in schools, hospitals, and private offices, sound separation between adjacent rooms should be measured. Wherever the sound isolation of the building envelope is compromised, such as by open windows, or where outside transportation noise sources may be a problem, such as next to highways, train tracks, or bus routes, acoustic separation from environmental noise should be measured.

This is not a comprehensive list of uses requiring advanced measurements but a guide to spaces that need greater attention. The need for advanced measurements requires careful consideration and judgment by building owners and design professionals. Advanced methods of evaluating the performance of commercial buildings must be performed by experienced acoustical consultants. Advanced measurements, along with intermediate measurements, provide accurate assessments of speech privacy, speech communication, and isolation from intruding noise.

Instrumentation

The instrumentation described for intermediate measurements fulfill most of the requirements described for advanced measurements. Some measurements may require PC-based software or a PC-based analysis system. Simultaneous measurements at two locations are required to evaluate facade isolation.

Room uses requiring good speech communication require measurement of reverberation time. The complexity and skill required for these advanced measurements strongly recommend the use of an acoustical consultant. Periodic or recurring measurements are generally not required unless the most recent occupant survey (see basic level, above) suggests that the acoustic environment has changed from initial assessments.

Metrics

Speech Privacy. Measurements of open plan office speech privacy are conducted according to ASTM E1130-08, (14) with reference to ASTM E1374-0615 and ASTM E1179-87. (16) Speech privacy is rated by two metrics: Articulation Index (ANSI S3.5-1997 (17)), which is directly calculated, and Privacy Index (PI), which is derived from the Articulation Index (AI). Speech privacy can be measured directly for open plan spaces following methods of ASTM E113014 or for closed plan spaces according to ASTM E336. (18)

Speech Intelligibility. There are four standard test methods measuring speech intelligibility: Speech Intelligibility Index (SII), (19) Speech Transmission Index (STI), Rapid Speech Transmission Index (RASTI), (20) and Speech Interference Level (SIL). (21)

Sound Isolation. Acoustic privacy can be differentiated into two categories: freedom from intrusive noise, such as trucks passing by outside and telephones ringing, and speech privacy, or freedom from being overheard and from overhearing others. The performance of many building elements, such as walls, windows, and doors, are laboratory tested by manufacturers. Sound transmission loss data measured using ASTM E90-2009 (22) are used to calculate Sound Transmission Class (STC) rating using ASTM E-413-04. (23)

Laboratory ratings are helpful for initial design of a building but should not be used in evaluating field-measured sound isolation (noise reduction) between interior spaces or from outdoor to indoor areas. Field measurement of sound isolation between rooms as noise reduction is performed in accordance with ASTM E336. (18) The NR data are used to calculate a Noise Isolation Class (NIC) rating. In Europe the Standardized Level Difference ([D.sub.n][T.sub.w]) as determined by ISO 140-4 (24) and ISO 717-1 (25) is used to measure airborne sound insulation between rooms in the field.

Field measurement of sound isolation from outdoors-to-indoors of buildings is performed in accordance with ASTM E966. (26) The Outdoor-Indoor Transmission Class (OITC) is found by converting the one-third octave band noise reduction through the facade elements, converting them to an A-weighted sound level and then subtracting this level from the A-weighted base curve level. The difference is the OITC.

Evaluation/Benchmarking

Advanced measurements are required for rooms with special speech communication, speech privacy, or sound isolation needs. Any space requiring advanced level acoustic measurements should meet the recommended criteria specified in Table 2 (Page 34). (1) If it does not, the room and the building are not considered appropriate for the intended use.

References

(1.) ASHRAE, USGBC, CIBSE. 2010. Performance Measurement Protocols for Commercial Buildings. Atlanta: ASHRAE.

(2.) Jensen, K.L., E. Arens, L. Zagreus. 2005. "Acoustical quality in office workstations, as assessed by occupant surveys." Proceedings of Indoor Air 2005.

(3.) Salter, C., R. Waldeck. 2006. "Designing Acoustically Successful Workplaces: A Case Study Assessment of the Speech Privacy and Sound Isolations of Spaces Having Underfloor Air Distribution Systems." Center for the Built Environment Summary Report.

(4.) Abbaszadeh, S., et al. 2006. "Occupant satisfaction with indoor environmental quality in green buildings." Proceedings of Healthy Buildings 2006.

(5.) Salter, C., et al. 2003. "Case Studies of a Method for Predicting Speech Privacy in the Contemporary Workplace." Center for the Built Environment Summary Report.

(6.) CBE. Occupant Indoor Environmental Quality Survey summary results of benchmarked LEED buildings as of July 1, 2010. Center for the Built Environment (UC Berkeley).

(7.) ASHRAE Technical Committee 2.6, Sound and Vibration. http:// tc26.ashraeregion7.org/draftashraeroomsoundmeasurement10.pdf.

(8.) CBE. 2008. Occupant Indoor Environmental Quality Survey and Database. www.cbe.berkeley.edu/research/survey.htm. Center for the Built Environment (UC Berkeley).

(9.) ASHRAE. 2007. "ASHRAE recommended procedure for measuring the HVAC system--Induced noise in a room (Draft #10)." TC 2.6, Sound and Vibration Control.

(10.) IEC 61672-1 Ed. 1.0 b:2002, Electroacoustics--Sound level meters--Part 1: Specifications.

(11.) ANSI S1.43 (R2007), Specification for Integrating-Averaging Sound Level Meters.

(12.) ANSI/ASA S1.11-2004 (R2009), Specification for Octave-Band and Fractional-Octave-Band Analog and Digital Filters.

(13.) ASTM E2235 - 04e1, Standard Test Method for Determination of Decay Rates for Use in Sound Insulation Test Methods (ASTM 2004a).

(14.) ASTM Standard E1130-08, Standard Test Method for Objective Measurement of Speech Privacy in Open Offices Using Articulation Index (ASTM 2002a).

(15.) ASTM E1374-06, Standard Guide for Open Office Acoustics and Applicable ASTM Standards.

(16.) ASTM E1179-87-03, Standard Specification for Sound Sources Used for Testing Open Office Components and Systems.

(17.) ANSI S3.5-1997 (R2007), Methods for the Calculation of the Speech Intelligibility Index.

(18.) ASTM Standard E336-2009, Standard Test Method for Measurement of Airborne Sound Attenuation Between Rooms In Buildings.

(19.) Pavlovic, C., et al. 2010. Programs, Speech Intelligibility Index. Working Group S3-79, Acoustical Society of America. www.sii.to/ html/programs.html.

(20.) IEC 60268-16 Ed. 3.0 en:2003, Sound System Equipment, Part 16: Objective Rating of Speech Intelligibility by Speech Transmission Index.

(21.) ISO/TR 3352:1974, Acoustics--Assessment of Noise With Respect to Its Effect on the Intelligibility of Speech.

(22.) ASTM Standard E90-2009, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.

(23.) ASTM E413 -04, Classification for Rating Sound Insulation.

(24.) ISO 140-4, Acoustics--Measurement of Sound Insulation in Buildings and of Building Elements--Part 4: Field Measurements of Air-Borne Sound Insulation Between Rooms.

(25.) ISO 717-1:1996, Acoustics, Rating of Sound Insulation in Buildings and of Building Elements--Part 1: Airborne Sound Insulation.

(26.) ASTME966-04, Standard Guide for Field Measurements of Airborne Sound Insulation of Building Facades and Facade Elements.

By Neil Moiseev, Member ASHRAE

Neil Moiseev was an associate principal at Shen Milsom & Wilke in New York, and a long-time member of TC 2.6, Sound and Vibration Control. He died Nov. 17, 2010.
Table 1: Recommended sound criteria for intermediate measurements. (1)

Room Types/Applications                     RC/NC/NCB     [T.sub.60], s

Outdoor Ambient   Intrusion from               n/a             n/a
                  transportation
                  vehicle noise

                  Exposure of                  n/a             n/a
                  neighboring property
                  from building louvers
                  and outdoor equipment

Apartments and                                 30             <0.6
Condominiums

Hotels/Motels     Individual rooms          25 to 35          <0.6
                  or suites

                  Meeting/banquet rooms     25 to 35          <0.8

                  Corridors and lobbies     35 to 45           n/a

                  Service/support areas     35 to 45           n/a

Office            Executive and private     25 to 35          <0.6
Buildings         offices

                  Conference rooms          25 to 35          <0.6

                  Teleconference rooms    [less than or       <0.6
                                          equal to] 25

                  Open plan offices       [less than or       <0.8
                  without sound masking   equal to] 40

                  Open plan offices       [less than or       <0.8
                  with sound masking      equal to] 35

                  Corridors and lobbies     40 to 45           n/a

Hospitals and     Private rooms             25 to 35          <0.6
Clinics
                  Wards                     30 to 40          <0.6

                  Operating rooms           25 to 35           n/a

                  Corridors and lobbies     30 to 40           n/a

Courtrooms        Unamplified speech        25 to 35          <0.7

                  Amplified speech          30 to 40          <1.0

Performing        Drama theaters,              25           Varies by
Arts Spaces       concert and recital                      application
                  halls

Laboratories      Testing/research with     45 to 55          <1.0
                  minimal speech
                  communication

                  Extensive phone use       40 to 50          <0.6
                  and speech
                  communication

Churches,         General assembly with     25 to 35        Varies by
Mosques,          critical music                           application
Synagogues        programs

Schools           Classrooms                25 to 30          <0.6

                  Large lecture             25 to 30          <0.7
                  rooms with speech
                  amplification

                  Large lecture rooms     [less than or       <1.0
                  without speech          equal to] 25
                  amplification

Libraries                                   30 to 40          <1.0

Indoor            Gymnasiums and            40 to 50          <2.0
Stadiums,         natatoriums
Gymnasiums
                  Large                     45 to 55          <1.5
                  seating-capacity
                  spaces with speech
                  amplification

Adapted from ASHRAE (2007d), ASA (2008), ANSI (2002), and CEN (2007)

Table 2: Recommended criteria for advanced measurements (1)
for speech communication.

Speech Privacy

Privacy Index (PI)            100%-95%    Confidential Speech Privacy
PI = (1 - AI)
(AI = Articulation Index)      95%-80%    Nonintrusive (Normal, Open
                                          Plan Office) Speech Privacy

                               80%-60%        Poor Speech Privacy

                                <60%       Complete Lack of Privacy

Degree of privacy needed depends on room use: Knowledge workers
need nonintrusive privacy to work effectively, managers need
confidential, etc.

Speech Intelligibility

Speech Transmission Index     1.0-0.75     Excellent Intelligibility
(STI)
                              0.75-0.60      Good Intelligibility

                              0.60-0.45      Fair Intelligibility

                                <0.45        Poor Intelligibility

Desirable level of speech intelligibility depends on space use:
Classrooms need Excellent, conference rooms need Good, general
offices can run well with Fair. In the interest of space, I
present STI only, but SII, RASTI, and SIL areas are all valid
measurements of speech intelligibility.

Acoustic Separation

Noise Isolation Class (NIC)    Varies               Varies
Standardized Level
Difference, [D.sub.nT,w]

For acoustic or speech privacy the separation needed between
rooms depends on background sound in the rooms, level of speech
or noise sources in the rooms, and degree of privacy desired.
Acoustical separation can also be subject to national or local
codes. Applicable regulations or guidelines should be consulted.
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Title Annotation:TECHNICAL FEATURE
Author:Moiseev, Neil
Publication:ASHRAE Journal
Article Type:Reprint
Geographic Code:1USA
Date:Jan 1, 2011
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