ASHRAE Recommended Procedure for Measuring the

HVAC System - Induced Noise in a Room (Draft #10)

1. Introduction and Scope This procedure is recommended by ASHRAE for the measurement of HVAC system-induced noise in rooms. This procedure is appropriate whenever evaluating non-transient HVAC noise against a specified noise criterion rating system, such as dBA, NC, RC, or NCB. This document should not be used as a guide for measuring noise outdoors. Two methods are presented in this document: 1) Survey Method

Provides a quick but only approximate noise evaluation. Generally adequate for an initial building commissioning process.

2) Engineering Method

Provides an accurate assessment of the noise level in a room. Generally sufficient to establish compliance relative to a noise criterion.

To avoid misinterpretation of the results of any noise test conducted according to this procedure, it is essential that the data be clearly marked as to whether they were obtained using the Survey or the Engineering method. If the survey method fails to conclusively determine the compliance of the space with the criterion, then the Engineering method shall be used with the guidance of an experienced acoustical consultant. 2. Instrumentation The measurements shall be made using instrumentation equivalent to an integrating sound level meter equipped with and an omnidirectional condenser microphone. The instrumentation shall meet Type 1 specifications for the engineering method as defined in the latest versions of ANSI S1.4, ANSI S1.11, and ANSI S1.43. If the applicable noise criterion requires frequency analysis, the instrumentation shall utilize parallel octave or 1/3-octave band filters, where all frequency bands are processed simultaneously. Instrumentation that utilizes serial filters (where the meter cycles through the various frequency bands one at a time) is not permitted. The measurement quantity applied to this standard is the time average equivalent sound pressure level (Leq). A handheld, Type 1 portable acoustic calibrator shall be used to calibrate the instrumentation before and after each measurement session. Both the meter (including the microphone) and the portable acoustic calibrator shall be certified to have been calibrated by an independent testing

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agency that is traceable to the National Bureau of Standards. The calibration date for the measurement system shall not be more than one year prior to the test date. 3. Test Conditions The measurements should, if possible, be conducted with the room vacated by its normal occupants. If this is not possible, the Test Report shall indicate the occupancy of the room and a general description of what the occupants were doing during the measurements. In addition, all non-HVAC related sound-producing equipment (computers, radios, vacuum cleaners, etc.) should be turned off (if possible) for the duration of the measurements. Noise from outdoor sources (e.g. aircraft, street traffic, lawn mowers, etc.) should also be minimized by closing windows and doors and/or scheduling the tests during time periods when these sounds are at a minimum. No measurement shall constitute non-compliance of the noise level with respect to a design criterion (NC, dBA, or other) unless the room is vacated, all non-HVAC related equipment is turned off, and background ambient noise is demonstrated to be insignificant (as specified in section 6). The operating condition of the HVAC equipment or system serving the room during the measurement shall be determined and reported. If possible, the measurements shall be conducted with the system operating at full capacity (e.g. during maximum cooling for a VAV system). Measurements at two or three different operating conditions (e.g., maximum cooling, maximum heating, full economizer, etc.) are strongly encouraged, but not required. In all cases the HVAC system shall be operating in a known steady-state condition during the measurements. If noise measurements are conducted with the system running in an unknown condition, the Test Report shall clearly state this fact, and the results of the test cannot be used to show compliance with the project requirements. 4. Measurement Volume All measurements shall be made within a Measurement Volume that is defined as all locations greater than 1 meter from any room boundary (wall, floor, and ceiling), but not greater than 2 meters above the floor. In rooms with multiple floor levels (e.g., a balcony in a theater), each level shall be considered a separate room. The microphone shall be positioned such that at no time is it closer than 0.5 meters from any object in the room (e.g., furniture). If possible, all doors in the room shall be closed during the measurements to minimize the potential for noise interference from other sources. In small or narrow rooms of width less than 2.5 meters, the Measurement Volume is defined as all locations more than 0.5 meter from the room boundaries.

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5. Measurement Procedure The measurement microphone may be hand-held, mounted on a tripod or fixed to a rotating boom. A moving microphone tends to provide a sound level more representative of the room average sound level, compared to a motionless microphone, especially if the background noise spectrum contains tones. However, extreme caution must be exercised when using a hand-held moving microphone due to potential for background noise generated by movement of the person holding the meter. Therefore, the Test Report must clearly state whether the microphone used was fixed or moving, and whether it was hand-held. 5.1 Measurement Locations With limited exceptions defined below, noise measurements may be taken at any point or region within the Measurement Volume. Depending on whether the Survey or Engineering method is used, measurements may be taken at one or several microphone locations within the Measurement Volume. The Test Report must define fully the location of each measurement point. A measurement point is considered valid unless:

1. The point is within 1 meter of an air distribution duct, diffuser or unit air discharge or inlet

2. The point is within 0.5 meters of any object 3. It is not at potential ear locations for room occupants

5.1.1 Survey Method For the survey method, noise may be measured at any valid measurement point where one would typically expect to find the ears of the occupant (e.g., near the seated position in front of the desk in a private office). If a high limit measurement is desired, the measurement may be obtained at the valid measurement point closest to the dominant source of HVAC noise in the room. If desired, more than one measurement point may be measured. At each measurement point, record the time average (Leq) sound pressure level. 5.5.2 Engineering Method If the floor area of the room is less than 20 square meters (215 square feet), the Measurement Volume is characterized by four measurement points as uniformly distributed over the Measurement Volume as practical. If the floor area of the Measurement Volume is greater than 20 square meters, the room is divided into two or more Sub-Volumes. The floor area in each Sub-Volume may be greater than 20 square meters, but not more than 40 square meters. None of the measurement Sub-Volumes shall overlap.

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Each Sub-Volume is characterized by no less than four separate noise measurements. No measurement point shall be closer than one meter to another measurement point. If the measurement volume is so small that one meter microphone spacing cannot be achieved, then reduce the microphone spacing accordingly, and include the minimum microphone spacing in the Test Report. At each measurement point, record the time average (Leq) sound pressure level. For each Sub-Volume, calculate the space-average noise level. The space-average is defined as: Lp_ave_SV = 10 log ( ( i 10(Lp_i / 10) ) / N ) Where N is the number of measurement points in the Sub-Volume. For each Volume, calculate the space-average noise level. The space-average is defined as: Lp_ave_V = 10 log ( ( i 10(Lp_i / 10) ) / M ) Where M is the number of Sub-Volumes. Ambient Noise Background ambient noise is defined as any noise not generated by the HVAC system. The ambient noise level is measured with all HVAC equipment turned off, using the procedure defined in Section 5. Survey Method For the survey method ambient noise measurements are recommended but not required. The ambient noise levels should be measured immediately before or after measurements are made with the HVAC system on, preferably both. Record the time average ambient (Leq) sound pressure level at each measurement location. Engineering Method Background noise measurements are required for the engineering method. The ambient noise levels should be measured immediately before or after measurements are made with the HVAC system on, preferably both. Record the time average ambient (Leq) sound pressure level at each measurement location. Then, calculate the room average ambient noise level using the procedures defined above. If the ambient noise level measurement in a given measurement band (either before or after

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measurements with the HVAC system operating) exceeds the HVAC system noise level minus 6 dB, that data point shall be clearly marked in the Test Report as ambient controlled. 6.2 Ambient Noise Correction The measured noise level with the HVAC system turned on may, in some situations defined below, be adjusted to artificially remove the effects of ambient noise. Ambient noise correction is an optional procedure not required by this standard. The ambient noise correction is made independently on an octave by octave basis. This correction is not permitted when only one background noise measurement is made. Also, it is not to be applied if either (one or both) ambient noise level exceeds the HVAC system noise level minus 6 dB. In that case, the data shall be clearly marked in the Test Report as ambient controlled. If the energy average of two independent ambient noise level measurements (obtained before and after the HVAC system noise measurement) is more than 6 dB below the HVAC noise level in any octave band, then the ambient adjusted HVAC noise level in that octave band is computed via the following equation: Lp_ambient_adjusted = 10 log [10(Lp_hvac/10) 10(Lp_ambient/10)] where:

Lp_hvac is the sound pressure level with the HVAC system operating, and

Lp_ambient is the energy average ambient sound pressure level with the HVAC system off. The ambient noise correction shall not be allowed if the difference between the two ambient noise levels in any frequency band is more than 3 dB. For example, if the measured HVAC noise level is 45 dB (including ambient noise) and the two measured ambient noise levels are 40 dB and 37 dB, then the energy average ambient noise level is 38.8 dB and the ambient-corrected HVAC noise level is 43.8 dB. 7. Criteria Evaluation The measured average (Leq) HVAC sound pressure level (ambient corrected, if allowed) in each measurement volume shall be used for evaluation against the desired room criteria. Measurements which are ambient controlled, as defined in Paragraph 6, are to be clearly identified. Ambient controlled levels may not be used to determine non-compliance of the HVAC system relative to a noise criterion.

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The instantaneous maximum (Lmax) sound pressure level shall not be compared with the specified criterion, unless that criterion specifically states that the instantaneous maximum sound pressure level must meet the criterion. If so, the time constant of the rms detector (e.g., fast or slow response) shall also be specified in the criterion. If the time constant is not specified, the Lmax criterion is invalid. In rooms with a Measurement Volume floor area greater than 20 sq. meters, the noise criterion rating value shall be independently evaluated for each Sub-Volume. It is recommended that a measurement tolerance of plus or minus 2 dB (in both the overall and individual frequency band levels) be assumed when test data are used to show compliance with noise criteria. 8. Test Report Engineering Method and Survey Method The Test Report shall include the following: 1. Facility location 2. Definition of whether the Survey of Engineering method was used in the evaluation 3. Identification (manufacturer, model number, and serial number) of the acoustic

instrumentation used, including rotating boom (if applicable)

4. Name of the person(s) conducting the measurements 5. Date and time of day of each measurement 6. Microphone location for each measurement 7. Definition of whether the measurement was fixed, hand-held moving, or rotating

microphone. If a rotating microphone was used, the boom radius and orientation shall be documented.

8. General description of the room including room name or number, approximate floor area, approximate room volume, calculated Measurement Volume and Sub-Volumes (if appropriate)

9. General description of the surface treatments and the room occupancy 10. General description of the HVAC system (including operating conditions and room

temperature at the time of the measurements)

11. Average (Leq) sound pressure levels for each measurement location (before any ambient noise correction)

12. Maximum and average sound pressure level for each Measurement Volume or Sub-Volume (before any ambient noise correction)

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13. Ambient average (Leq) sound pressure levels for each measurement location (before any ambient noise correction)

14. Ambient maximum and average sound pressure level for each Measurement Volume or Sub-Volume (before any ambient noise correction)

15. Identification of any locations and measurement bands controlled by ambient noise as defined in Section 6

16. Ambient-corrected average (Leq) sound pressure levels for each measurement location 17. Ambient-corrected maximum and average sound pressure level for each Measurement

Volume or Sub-Volume

18. Measured noise criteria rating for each Measurement Volume or Sub-Volume. 9. Measurement Precision and Bias To be determined via round robin tests.