Kam Ng | Acoustical Solutions

Kam Ng

Vice President of Engineering

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  • Virginia

Noise Propagation Path Identification (Case Study)

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Noise Propagation Path Identification

noise propagation path identification image

The Issue:

A noise assessment was conducted to assess the noise generation and transmission by the chiller installed on the first floor, specifically for noise propagation path identification purposes. It was hoped that corrective actions and acoustical treatments would be recommended and implemented to reduce the noise in the open office space to lower noise levels..

Noise Propagation Path Identification:

No specific noise objective design criteria have been established for the office, however best practice would indicate that for the office environment and the purpose of speech intelligibility, the ANSI/ASA noise standard for offices is applicable.

It has been speculated that the noise transmitted from the chiller to the office is via the building columns and floor/beam systems. In the middle of the office, one could hear and feel the chiller noise being propagating through the columns of the building.

The noise source levels measured near the chiller were between 91.4 dB(A) and 97.5 dB(A). The noise levels measured in the open office space were between 39.7 dB(A) and 43.3 dB(A). The sound pressure levels of the open office space were within NC (Noise Criteria) Curve 40 and NC Curve 25.

As shown in the figure below, the octave sound pressure levels spectra of the peak noise and lowest noise as recorded were within NC Curve 40 and NC Curve 25. In this case, Run 13 measured adjacent to Column A corresponds to the peak noise and Run 15 measured adjacent to Column B corresponds to the lowest noise. Note that during the noise measurements, the outside ambient temperature was around 80’s degree Fahrenheit and the chiller was running at low to mid power condition. Hence, the noise source levels of the chiller were not at the peak; likewise, the noise levels in the open office space were not at the peak level. At higher outside ambient temperature, it was expected that the noise levels in the open office space would be higher by about 3 dB(A). From the noise measurements and analysis, it was validated that the steel columns, beam, and floor system were the noise transmission paths.

noise propagation path identification image

Open Office Space Ambient Noise versus NC Curve 40 and NC Curve 25 (Band 4 = 500 Hz)

Noise Propagation Path Identification Recommendations:

Services Provided:

Noise measurements and analysis were conducted to determine the noise propagation paths from the chiller to the open office space upstairs. Based on the results of the noise measurements and analysis, corrective actions and noise reduction recommendations were developed and provided to reduce the noise at the office to an acceptable level.

Noise Control Recommendations:

Based on the results of noise measurements and analysis, the following acoustic treatments were recommended to reduce the vibration and noise propagated from the chiller to the office upstairs.

  • Vibration Isolation Mounts

To minimize the structure-borne noise propagated from the chiller on the first floor to the open office space on second floor via the columns and vertical structures of the building, it was recommended to install different vibration isolation mounts to the base of the chiller and additional vibration isolation pads to the associated machinery components. The vibration isolation recommended was elastomeric vibration mounts.

  • Vibration Isolation Pads

All ground connections of the piping support associated with the chiller and related machinery components were recommended to be isolated using vibration isolation pads. Specifically, these include the base on the vertical small pipe, bracket of the pipe support, and base of the associated machinery. Vibration isolation pads, such as neoprene or rubber pads which provide vibration isolation for pipes and machinery supports were recommended.

  • Vibration Isolation Hangers

To minimize the structure-borne noise propagated from the chiller to the open office space via the columns and vertical structures of the building, it was recommended to install vibration isolation hangers on the pipes and ducts attached to the chiller.

  • Vibration Pipe Isolator

To reduce the structure-borne noise propagated from the chiller to the downstream piping, it was recommended to install vibration pipe isolators on the downstream pipes connected to the chiller. A common practice to isolate the structure-borne noise and vibration propagating downstream along the pipes is to use a vibration pipe isolator. The vibration pipe isolator is usually a rubber or flexible hose.

The Results:

With the applications of vibration isolation mounts, vibration isolation pads, vibration isolation hangers, and vibration isolators on the piping of the chiller, the acoustic environment in the open office space would be reduced to meet the NC Curve 25 to NC Curve 30 noise goal.


To learn more about how Acoustical Solutions can solve your noise control problems, use our contact form, call one of our Acoustical Sales Consultants at (800) 782-5742, or visit us on the web at www.acousticalsolutions.com.