Measurement of Sound Transmission Loss

Sound Transmission Loss of a specimen conducted in a pair of reverberation chambers. The sound transmission loss is 10 times the logarithm of the reciprocal of the ratio of the energy transmitted through the specimen to the energy incident on the specimen.

Frequency Range of Interest:
Generally the 1/3 octave bands centered between 100 Hz and 5,000 Hz. The frequency range of interest may be extended downward to the 63 Hz octave band in large chambers and upwards to the 8,000 Hz octave band.

Units:

Transmission Loss (TL) in decibels (dB) in octave or 1/3 octave bands. Also may be expressed in terms of the single number ratings STC, OITC or Rw.
Sound Transmission Coefficient, dimensionless.
Approaches:

Approach 1
Method using Sound Pressure Level and Reverberation Time per ASTM E-90
The measurement of the sound transmission loss of a specimen is based on:

The measurement is based on the assumption that the sound fields in the source chamber and receiving chamber are diffuse.

The mathematical relationship is:

Where:
The major source of measurement uncertainty in the transmission loss measurement are spatial variations in the sound pressure levels in the reverberation chamber, and spatial/temporal variations in the decay rate in the receive chamber. These variations are greatest at low frequencies. This uncertainty is reduced by spatial/temporal sampling of these parameters. A sufficient number of sound pressure level and decay rate samples must be obtained to limit the 95% confidence interval on the transmission loss to the following values:

Advantages/Disadvantages to Approach
+ Covered by International Standards
+ Less intensive instrumentation requirements
+ Measurements over entire frequency range of interest in one pass
- Test facilities requirements (such as two reverberation chambers) are more extensive

Single Number Ratings (STC)
The sound transmission loss of a specimen is often expressed in terms of a single number rating. The most common rating is the Sound Transmission Class (STC) of the specimen. The STC rating is determined in accordance with the calculation procedures of ASTM E413.

The STC rating of a specimen is determined by comparing the 1/3 octave band sound transmission loss data in the bands between 125Hz and 4,000 Hz to a series of standard transmission loss contours. The rating is determined by the contour with the highest vales that fits the transmission loss data as follows:


Industry Standards
ASTM E90 – Sound Transmission Loss Measurement
ASTM E413 – Calculation of STC
ASTM 1332 – Calculation of OITC
ISO 140-1; ISO 140-2; ISO 140-3 – Sound Transmission Loss Measurement and calculation of Rw

Test Environment
Two reverberation chambers, each with a minimum volume of 50 m3 coupled through a test opening equal to the size of the test specimen. The transmission loss limit of the test facility and any filler walls used to adapt the test specimen to the facility opening shall be known.

Equipment

NI products commonly used for this measurement:

• PCI-4461
  
• LabVIEW
  
• NI 5911 Instrument
  
• PCI-4462
  
• Sound and Vibration Toolset
  
  Approach 2
  Method using Sound Pressure Level and Sound Intensity
  The measurement of the sound transmission loss of a specimen is based on:

The measurement is based on the assumption that the sound fields in the source chamber are diffuse, and that the receiving chamber is a nominally free field environment.

The mathematical relationship is:

Where:
The constant 6-dB term arises from the fact that the angle of incidence of the energy in the source chamber is random in orientation, and that the sound pressure level in the source chamber is proportional to the total energy incident on the specimen. The integration of a cos term to calculate the energy normally incident on the specimen results in 6 dB difference between the source chamber sound pressure level and the incident sound intensity.

Although measurement of sound transmission loss using this technique has not been standardized, some documentation of the method can be found in the technical literature. Uncertainty in this measurement technique is not well documented and probably will exceed the uncertainty from the E-90 method.

Advantages/Disadvantages to Approach
- Not covered by International Standards
- Requires more complex instrumentation and data analysis
- May require more than one pass to cover entire frequency range of interest
+ Test facilities less extensive

Equipment




NI products commonly used for this measurement:

• PCI-4461
• PCI-4462
  
• NI 5911 Instrument
  
• LabVIEW
  Additional References

Related NI Products:

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

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 »

Dear David After I have read your notes about sound absorption coefficient (alpha) and transmission Loss (TL) I have some a doubt. When you ask for information to the acoustic foam manufacturers, they send you the third octave values of de alpha and TL. My doubt is: How can I convert these values from 1/3 octave band to 1/1 octave band? Have I to use the same procedure as if I would be converting a noise espectra from 1/3 to 1/1 octave band?(10*log(sum(10^(0.1*Li))) I ask you because if we use the same method as to sum noise spectra (10*log...) the value of TL in 1/1 would be always greater than 1/3 octave values, so we have more insulation than in reality. In the case of de alpha, the values are adimensional so I don´t know if I have to do and average y the frequency band. Can you help me please. Thank you Best regards, Aitor
- Nov 10, 2009