Testing the Sound Transmission Class (STC) rating of a wall, floor, or ceiling involves a standardized laboratory procedure to measure how well the structure blocks airborne sound.
Understanding STC Ratings
The STC rating is a single-number metric widely used in the United States to quantify the sound insulation performance of interior building elements. A higher STC rating indicates better sound blocking capability.
The Process of Testing STC
Determining the STC rating of a building element is done following specific standards, typically in a controlled laboratory environment. The core process involves measuring the sound transmission loss across a range of frequencies and comparing the results to a standard curve.
Based on the provided reference, the key steps include:
- Measure Transmission Loss (TL) Values: This is the crucial first step. Sound is generated on one side of the test structure (e.g., a wall), and its level is measured. Simultaneously, the sound level is measured on the other side. The difference between these levels, accounting for the absorption in the receiving room, gives the Transmission Loss at a specific frequency band. This measurement process must be performed for 16 different 1/3 octave bands with center frequencies ranging from 125 Hz to 4000 Hz.
- Plot the TL Values: The measured TL values are then plotted on a graph. The x-axis represents the frequency (typically on a logarithmic scale), and the y-axis represents the Transmission Loss in decibels (dB). This creates a unique curve representing the sound blocking performance of the tested structure across the measured frequency range.
- Compare to the Standard STC Contour: The resulting plot of TL values is then compared to a standardized STC contour curve. The reference notes that transparent overlays are often used to do this. The STC rating is determined by fitting the standard contour to the measured TL curve according to specific rules outlined in the testing standards (like ASTM E413 and E90). The goal is to find the highest STC contour that fits below the measured curve, with specific allowances for deviations. The final STC rating is derived from the position of this best-fit contour.
This multi-step process ensures that the STC rating reflects the structure's ability to attenuate sound across the critical speech frequency range (typically 125 Hz to 4000 Hz).
Practical Considerations
While the fundamental principle involves measuring and plotting TL values, actual STC testing is a complex procedure governed by standards like ASTM E90 (Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements). These standards specify the size of the test chambers, the instrumentation, the test signal, and the detailed procedure for fitting the STC contour.
It's important to note that field testing of sound isolation results in a different rating, typically the Field Sound Transmission Class (FSTC), which accounts for flanking paths and real-world installation effects. The STC rating determined through the described laboratory process represents the ideal performance of the building element itself.
