Measurement of Sound Power Level Emissions

Measurement of the sound power emissions of the Unit Under Test (UUT) in reverberant, anechoic or free-field test environments.

Equipment:

• Motors/Generators
• Rotating Machinery
• Recreational Equipment
• Air-Moving Devices
• Lawn and Garden Equipment
• Any Sound Generating Equipment

Frequency Range of Interest:
Generally the 1/3 octave bands centered between 100 Hz and 10 kHz. May be extended to include energy in the 16 kHz octave band. Information on emissions in the 31.5 Hz and 63 Hz octave bands are increasingly in demand for many types of mechanical equipment, although it may be difficult to obtain data at these frequencies in full compliance with test standard requirements.

Units:

LW or PWL in watts or decibels relative to 1x10-12 watts. May be expressed as a single A-weighted level or in 1/3-octave bands.
Approaches:

Approach 1
Direct Method in Free Field Test Environment

Approach
This measurement is based on the proportional relationship between the mean square pressure and the sound intensity in an anechoic environment.

The sound pressure levels created by the UUT are sampled at a number of positions on a measurement surface that envelops the UUT. The sound pressure levels are averaged over the measurement surface. A term that accounts for the surface area of the measurement surface is added to the average sound pressure level in order to determine the sound power level of the UUT.

The mathematical relationship is:

Where:
Lw = Sound Power of UUT (in dB re 1x10-12 watts)
Lp = Average sound pressure level on the measurement surface (in dB re 2x10-5 Pa)
S = surface area of the measurement surface (in m2)
(Note: An additional term for normalization to standard atmospheric pressure and temperature may be applied)

The measurement surface is typically spherical, hemispherical, or a rectangular parallelepiped.

Advantages/Disadvantages to Approach
+Yields precision-grade data with standard deviation of reproducibility <= 1.0 dBA
+ Yields both sound power and source directivity information
+ Time dynamics of signal preserved for sound quality analysis
- Requires fully anechoic chamber, which can be expensive
- Large sources require very large anechoic chambers

Industry Standards
ISO 3745, ANSI S12.35 (Precision Grade)

Test Environment
Anechoic or Hemi-Anechoic chamber qualified per the draw-away methods described in ISO 3745 or ANSI S12.35.

Equipment

• Microphone(s) with uniform frequency response over frequency range of interest.
• Integrating, averaging sound level meter with A-weighting and/or 1/3 octave band filtering. This sound level meter function can be created using virtual instrumentation, using an appropriate digitizer and software (see products below). The virtual instrument solution has the additional advantage that its function can be changed at will by applying new software.
• Microphone Multiplexer (optional)

NI products commonly used for this measurement:

• PCI-4461
• PCI-4462
• NI 5911 Instrument
  
• Sound and Vibration Toolset
  
• LabVIEW
  
  Approach 2
  Comparison Method in a Free Field Test Environment
  
  Approach
  The “free field” test environment is a highly absorptive, but not anechoic environment. Because of this, the relationship between mean squared pressure and sound power is not strictly proportional. Therefore, this measurement is based on the use of a calibrated reference sound source (RSS) and a comparison of the sound pressure levels generated by the RSS to the sound pressure levels generated by the UUT.
  
  The sound pressure levels created by the UUT and the RSS are sampled at a number of positions on a measurement surface that envelops the RSS and UUT. The sound power level of the UUT is determined from the known sound power output of the RSS, the sound pressure level created by the RSS in the test environment, and the sound pressure level created by the UUT in the test environment.
  
  The mathematical relationship is:
  
  Lw (UUT) = Lw (RSS) – [Lp(RSS) – Lp(UUT)]
  
  Where:
  Lw(UUT) – Sound power of the UUT (in dB re 1 x 10---12 watts)
  Lw(RSS) – Calibrated Sound power of RSS (in dB re 1 x 10---12 watts)
  Lp(RSS) – Sound pressure level generated by RSS (in dB re 2 x 10-5 Pa)
  Lp(UUT) – Sound pressure level generated by UUT (in dB re 2 x 10-5 Pa)
  
  Advantages/Disadvantages to Approach
  -Yields engineering-grade data with standard deviation of reproducibility<=1.5 dBA
  +Yields both sound power and source directivity information
  +Time dynamics of signal preserved for sound quality analysis
  + Utilizes less expensive free field chamber
  - Large sources require very large chambers
  
  Industry Standards
  ISO 3744, ANSI S12.34 (Engineering Grade)
  
  Test Environment
• Anechoic or Hemi-Anechoic chamber qualified in accordance with ISO 3745 or ANSI S12.35
• Anechoic Chamber, Hemi-Anechoic Chamber or highly absorptive enclosures qualified per ISO 3744 or ANSI S12.34.
• Outdoor test site

Equipment

• Reference Sound Source, calibrated in accordance with ISO 6926
• Microphone(s) with uniform frequency response over frequency range of interest.
• Integrating, averaging sound level meter with A-weighting and/or 1/3 octave band filtering.
• Microphone Multiplexer (optional)

NI products commonly used for this measurement:

• PCI-4461
• PCI-4462
  
• NI 5911 Instrument
  
• Sound and Vibration Toolset
  
• LabVIEW
  
  Approach 3
  Direct Method in Reverberation Chamber
  
  Approach
  This measurement is based on the relationship between the sound power level of the UUT, and the sound pressure levels it generates in a reverberant sound field with a known reverberation time.
  
  The space/time averaged sound pressure level generated by the UUT is determined by sampling the sound pressure levels in the reverberation chamber at a number of fixed positions or over a traversed path. The reverberation time in the chamber is measured. The sound power level of the UUT is determined from the average sound pressure level and the reverberation time.
  
  The mathematical relationship is:
  
  Lw(UUT) = Lp(UUT) - 10*log(T) + 10*log (V) + 10*log[1+(S/8V)]-14.0
  
  Where:
  Lw = Sound power of UUT (in dB re 1x10-12 watts)
  Lp= Sound pressure level generated by UUT (in dB re 1x10-5 Pa)
  T = Reverberation time in seconds
  V = volume of the reverberation chamber (in m3)
  S= surface areas of reverberation chamber (in m2)
  = wavelength at center frequency of one-third octave band
  
  (Note: An additional term for normalization to standard atmospheric pressure and temperature may also be applied)
  
  Advantages/Disadvantages to Approach
  +Yields precision-grade data with standard deviation of reproducibility <= 0.5 dBA
  + No source directivity information
  -Time dynamics integrated out of signal
  + Reverberation chambers usually less expensive than anechoic chambers
  - Can accommodate large sources
  
  Industry Standards
  ISO 3741, ANSI S12.31, ANSI S12.32 (Precision Grade)
  
  Test Environment
  Reverberant Chamber of at least 180 m3 (70 m3 for measurements at 250 Hz an higher) qualified in accordance with ISO 3741 or ANSI S12.31/12.32.
  
  Equipment
• Microphone with uniform frequency response over frequency range of interest.
• Integrating/averaging sound level meter with A-weighting and/or 1/3 octave band filtering
• Level versus time analyzer with reverberation time analysis

Approach 4
Comparison Method in Reverberation Chamber

Approach
This measurement is based on the use of a calibrated reference sound source (RSS), and a comparison of the sound pressure levels generated by the RSS to the sound pressure levels generated by the UUT.

The space/time averaged sound pressure level generated by the UUT and RSS are determined by sampling the sound pressure levels in the reverberation chamber at a number of fixed positions or over a traversed path. The sound power level of the UUT is determined from the known sound power output of the RSS, the sound pressure level created by the RSS in the reverberation chamber, and the sound pressure level created by the UUT in the reverberation chamber.

The mathematical relationship is:

Lw (UUT) = Lw (RSS) – [Lp(RSS) – Lp(UUT)]

Where:
Lw(UUT) – Sound power of the UUT (in dB re 1 x 10---12 watts)
Lw(RSS) – Calibrated Sound power of RSS (in dB re 1 x 10---12 watts)
Lp(RSS) – Sound pressure level generated by RSS (in dB re 2 x 10-5 Pa)
Lp(UUT) – Sound pressure level generated by UUT (in dB re 2 x 10-5 Pa)

Advantages/Disadvantages to Approach
+Yields precision-grade data with standard deviation of reproducibility <= 0.5 dBA
+ No source directivity information
- Time dynamics integrated out of signal
+ Reverberation chambers usually less expensive than anechoic chambers
- Can accommodate large sources

Industry Standards
ISO 3741, ANSI S12.31, ANSI S12.32

Test Environment
Reverberant Chamber of at least 180 m3 (70 m3 for measurements at 250 Hz an higher) qualified in accordance with ISO 3741 or ANSI S12.31/12.32.

Equipment

• Reference Sound Source, calibrated in accordance with ISO 6926
• Microphone with uniform frequency response over frequency range of interest.
• Integrating/averaging sound level meter with A-weighting and/or 1/3 octave band filtering
• Level versus time analyzer with reverberation time analysis

Approach 5
Method Using Sound Intensity

Approach
This measurement is based on the measurement of the average sound intensity at a number of points on a measurement surface that envelops the UUT. The mathematical relationship is:

Lw (UUT) = LI + 10*log S

Where:
Lw(UUT) – Sound power of the UUT (in dB re 1 x 10---12 watts)
Li – Average sound intensity over the measurement surface (in dB re 1X10-12 watts/m2)
S = surface area of the measurement surface (in m2)

Advantages/Disadvantages to Approach
-Yields engineering-grade data
- Has more limited frequency range depending on sound intensity probe configuration and can require multiple passes to cover entire frequency range
+ Can be conducted in less expensive test facilities and, in some cases, in-situ
+ Can be useful in identifying specific noise sources within a piece of equipment
- Requires more extensive instrumentation
- More difficult to learn to use intensity based measurements
- Not accepted by many international standard test codes as sound power measurement method

Industry Standards
ISO 9614-1, ISO 9614-2

Equipment

• Reference Sound Source, calibrated in accordance with ISO 6926
• Microphone with uniform frequency response over frequency range of interest.
• Integrating/averaging sound level meter with A-weighting and/or 1/3 octave band filtering
• Level versus time analyzer with reverberation time analysis

NI products commonly used for this measurement:

• Sound and Vibration Toolset
  
• PCI-4461
  
• PCI-4462
• NI 5911 Instrument
  
• LabVIEW

Related NI Products:

• Sound and Vibration Toolset
  
  Helpful Web Sites:
  
• Acoustic Test Chambers and Environments
  
• Measurement of Sound Absorption
  
  

Information Contributed By: David A. Nelson, P.E., INCE Bd. Cert. Nelson Acoustical Engineering, Inc. specializes in noise and vibration control, sound quality, laboratory facilities and test control systems, and instruction related to plants, buildings, laboratories, products, and machinery.

Reader Comments | Submit a comment »

great information
Thank you for posting - very informative document.
- jiaconis@celerityservice.com - Dec 3, 2007