Acoustics Laboratory Facilities
Transmission Loss Suite
Measuring Airborne Sound Insulation
The transmission loss suite in the Department of Mechanical Engineering is a versatile facility which is used to measure sound insulation of panel systems (including doors and walls).
The traditional method of measuring airborne sound insulation relies on creating a sound field on one side of a wall and measuring the sound level spectra on the other side. An overall result is achieved.
The sound intensity method employed in the Department’s facility uses the sound intensity method. This method determines the contribution of each element of a segmented panel to the overall sound reduction. Possible flanking transmissions are excluded.
The transmission loss suite comprises a reverberation room and an adjacent semi-anechoic room with a common opening 1.08m by 1.92m. The transmission loss of different panel constructions can be determined by measuring the sound intensity transmitted through the construction, which is placed in the common opening. The sound intensity approach to transmission loss measurement shows up variations transmission loss over the area being tested which can be particularly useful.
A Note on STC
The sound transmission class (STC) can be determined from measurements made. This is a single-number rating of the airborne sound transmission loss performance of a construction measured at standard one-third octave band frequencies. The higher the STC rating, the more efficient the construction will be in reducing sound transmission within the frequency range of the test.
The STC rating method procedures are specified in the American Society for Testing and Materials (ASTM) annual book of standards. The transmission loss of a construction test specimen is measured at 16 one-third octave bands with centre frequencies from 125 to 4000 Hz. To determine the STC of a given specimen, its measured transmission loss values are plotted against frequency and compared to the reference STC contour. The STC contour is shifted vertically relative to the plotted curve test data to as high a final position as possible according to the following limiting criteria
- The maximum deviation of the test curve below the contour at any single test frequency shall not exceed 8 dB.
- The sum of the deviations below the contour at all 16 frequencies of the test curve shall not exceed 32 dB.
The STC rating is read from the vertical scale as the transmission loss value (with dB unit dropped) corresponding to the intersection of the STC contour and the 500 Hz ordinate as shown below.