Masonry Technical Library - Masonry Acoustics and STC Ratings

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	Sound Transmission Class

		Sound transmission class(STC) is a single-figure rating derived in a prescribed manner from sound transmission loss values. The rating provides an estimate of the performance of the partition in common sound insulating situations. To determine the effectiveness of a wall system as a means of sound isolation, a two room test is generally used. In this test a steady sound is generated on one side of the wall , and the sound that passes through is measured on the other side. The measurement of sound levels is then recorded at several different frequencies over a range of 125 to 400 Hz. The difference in sound levels (in dB) determines the transmission loss level. If an 80dB signal is recorded at 10dB on the other side of the wall, the transmission loss score is 70dB.

TABLE OF CONTENTS

INTRODUCTION

The Science of sound

Determining STC Ratings

Designing for optimum performance

Sound Research Data - A list of STC Ratings for several common wall systems

Precast Hollow Core Floor Systems

Special Acoustic Units

Conclusion

Arithmetic averages of sound transmission loss at selected frequency levels were widely used in the past to rate the effectiveness of walls. This averaging method was sometimes unreliable , however, because a good average could be ascribed to a wall type that performed poorly at an important frequency. For example a given wall could perform very well at low frequency levels, but could allow human voices to be transmitted through the wall unabated. Instead of using an averaging method, we now find STC ratings by comparing transmission loss curves to a set of standard curves as described below in Figure 2.

In compliance with ASTM E-90, the STC of a wall is determined by comparing its transmission loss curve with a set of standard curves, or contours. The standard curve is superimposed over a plot of the actual sound transmission loss curve, and shifted upward or downward relative to the test curve until some of the measured TL values of the test specimen fall below those of the STC contour and the following conditions are fulfilled.

1. The sum of the deficiencies (deviations below the contour) shall not be greater than 32 dB,

2. The maximum deficiency at a single test point shall not exceed 8 dB.

When the contour is adjusted to the highest value that meets the above criteria, the sound transmission class is taken to be the transmission loss value, measured in decibels (dB), corresponding to the intersection of the standard contour and the 500 cycle per second frequency line, obtaining a more accurate assessment than a raw average.

Table of Contents


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