Sound transmission through windows

(ref. Canadian Building Digest, article by J.,D.Quirt)

Introduction

In addition to their primary function as visual openings, windows also transmit sound. This is of concern not only for the exterior surfaces of a building, but also for interior applications ranging from office doors to control booths in recording studios. Sound transmitted through windows often limits the overall acoustical insulation.

Sound transmission through windows is governed by the same physical principles that affect walls, but practical noise control measures are influenced by the properties of glass and the characteristics of the window assemblies. Increasing the glass thickness, for example, gives greater noise reduction at most grequencies, but the stiffness of glass limits the imprevement. Using multiple layers (double or triple glazing) increases noise reduction at most frequencies, but this is dependent of the separation of the layers.

As with other building assemblies, transmisison of sound through cracks may drastically reduce the effective noise reduction. this is of particular concern for openable windows: even windows with good weather-stripping have reduced noise reduction because of air leakage. Most of the data presented in this report are for sealed windows.

The acoustic terms used in this report are as follows

• decibels (abbreviated to dB.)
• Sound Transmission Loss (TL) which is a standardised measures of the noise reduction in decibels for specific frequency ranges.
• Sound Transmission Class (STC) is a single figure rating of sound transmission, calculated by fitting a standard contour to the TL data. It is most commonly used in North America

Sealed single glazing

The TL for a large thin panel would theoretically increase by 6 dB for each doubling of the sound frequency or the mass if the effect of stiffness was ignored. Although single glazing does approach this 'mass law' behavior at some frequencies, the stiffness of the glass and the limited size of typical windows cause significant deviations from this prediction (see fig. 1 below).

The sharp decrease in TL at specific frequencies is called the 'coincidence dip', and is caused by bending waves in the glass panel. Abovethe coincidence frequency, laminated glass can provide much higher TL than solid glass. This is apparently due to damping (dissipation of vibrational energy) by the plastic interlayer.

Sealed double glazing

The TL of double glazing is strongly dependent on the features of the cavity between the two layers of glass. The STC rating increases as the air space increases (see fig. 2 below). For each doubling of the air space, the SC increases by approximately 3. The STC also increases with increasing glass thickness.

If the separation between the panes is small, the STC rating is only slightly higher than that for a single pane of the same glass. This occurs because the air in the space between the two panes acts like a spring, transferring vibrational energy from one pane to hte other. This resonance falls within the range of 200 to 400 Hz for a unit with a small air gap (see fig. 3 below) Most of the energy from aircraft or heavy traffic falls within this frequency range, but by increasing the air space and using heavier glass, the resonant frequency can be lowered to improve the insulation against such noise sources.

Sealed triple glazing

Despite the widespread belief that adding another layer of glass must be beneficial, triple glazing provides essentially the same noise reduction as double glazing, unless the air gap is very large. Figure 4 (below) compares TL data for a double glazed window with that for a triple glazed window of similar total thickness.

Designing for noise control

In most cases where substantial noise control is required, double glazing is the most sensible choice. The airspace should be sufficiently large to provide the desired TL.

Using different thicknesses of glass for double glazing gives greater noise reduction. The highest STV values shown in figure 2 are for double 6 mm. glass; windows with 3mm. substituted for one of the 6 mm. panes would have equal or higher STC ratings.

The use of laminated glass has also been shown to reduce sound transmission.