Glass Surfaces in the Acoustical Environment

Glass can be the trickiest break in a wall and a troublesome reflective surface within the studio.
by John Storyk

To understand glass and its effect on sound transmission, you need an understanding of Sound Transmission Class (STC)- the transmission rating calculated in accordance with ASTM classification E413. Remember: STC is a single number indicator of sound transmission. It is helpful to review detailed octave-based transmission information to get the true picture of wall and glass transmission properties. In general, the higher the STC number, the better the total transmission performance of a particular type of glass.

Like doors, glass transmission specifications need never exceed the static boundary transmission loss criteria. Increasing glass specifications (thickness and/or type) can cause per square-foot prices to sky rocket!

Two major types of glass that can be used in studio construction: plate/float, which is typical glass with thickness ranging from 1/8-inch to 1inch; and laminated, which is several layers of glass sandwiching an interior layer, or layers, of plastic. The plastic layer in this type of glass is typically very thin, e.g., .045 inches.

The reduction of noise produced by any barrier is proportional to its mass (weight per square foot), total boundary area, limpness, and airtightness (thoroughness of construction at penetrations). This statement applies to walls, doors, and glass openings! As glass thickness increases, mass will increase. At a certain point, however, there will no more increase in transmission loss due to increased limpness (acoustic decoupling). Actually, thicker glass will become stiffer (less limp). Lamination of glass increases limpness.

Studio walls vary in their rated STC values and octave band TL values. A typical single-partition gypsum board/stud wall will have an STC in the mid-'30s- depending upon exact stud width, insulation, and so on. Ideally, one would want to match this wall with a similar STC rating for the glass opening. Figures l and 2 show typical ratings for plate glass and for laminated glass. Thinner laminated glass will equal a considerably thicker plate/float glass equivalent, although cost and availability usually rule supreme. Most studios end up using the less expensive and more available plate/float glass.

Speaking of cost, typical pricing (material only) looks like this:

5/8-inch plate/float glass - $10/sq. ft.
2 1/4-inch laminate glass-$25 to 30/sq. ft.

As the glass opening gets larger, the extra thickness is more desirable, since it will vibrate less in a control room window wall. This has become less important with the popularity and acceptance of "nonconnected" main monitor assembly construction (i.e., detached concrete front control room walls).

Most studios walls consist of more than one layer of stud construction (or equivalent masonry), thus multipane window construction is necessary. In theory, the typical "room-within-room" studio/control wall should require three separate window lites and frames. This is recommended when the glass size is large since glass thicknesses are always slightly different.

Experience and economy show us that combining two of the three partitions will give us nearly the required STC-especially when the window sizes are relatively small (e.g., 20-30 sq. ft.). See Table. Keeping an air space between frames as well as the use of some sort of acoustic decoupling material such as industrial 1/4-inch felt is important.

Glass Type and Thickness	Average Sound Transmissions Loss 125-4000 Hz	Loudness Reduction Compared to 1/4" Glass	STC Rating
SS Float	23.5dB	-	26
DS Float	24.1dB	-	29
1/4" Float	26.5dB	-	29
5/16" Float	28.8dB	15%	29
3/8" Float	29.7dB	20%	30
1/2" Float	31.5dB	29%	33
5/8" Float	34.5dB	42%	30
3/4" Float	34.6dB	43%	33
7/8" Float	35.4dB	46%	32
1" Insulating Glass	30.7dB	23%	31

Above: Float/Plate glass table showing thicknesses and corresponding STC values.

Approx. Total Glass Thickness	Construction (.045" plastic interlayer)	Average Sound Transmission Loss 125-4000 Hz	Loudness Reduction Compared to 1/4" Float Glass	STC Rating
1/4"	2 plies, 1/8" float	30.2dB	23%	33
1/2"	2 plies, 1/4" float	33.6dB	39%	36
1/2"	1 ply, 3/8" 1 ply, 1/8"	35.7dB	47%	36
5/8"	4 plies, 1/8" float	36.3dB	49%	38
3/4"	2 plies, 3/8" float	38.9dB	58%	38
3/4"	3 plies, 1/4" float	38.7dB	57%	39
1"	6 plies, 1/8" float	39.8dB	60%	41

Above: Laminated glass table showing thicknesses and corresponding STC values.

GLASS IN DOORS

Simply consider doors to be moving sound barriers. If the door selected is thick enough, we recommend multiple layers of plate/float glass or one thick (3/ 4-inch) layer. In thinner sound barrier doors (prefabricated metal doors, for example), the use of laminated glass is more desirable. Figure 4 shows a typical wood studio door with a two glass-pane installation.

INSTALLATION & DETAILING

After the exact thickness and type of glass to be used are determined, careful attention must be paid to the mounting technique. The last condition of a successful glass boundary installation is its "airtightness," so the mounting of the glass is crucial!

In nonacoustic installations, glazing shims are often used with glazing compound. This is not satisfactory in the studio environment. A continuous "airtight" or "near airtight" seal is necessary. The most common detail calls for a U-shaped neoprene seal around the three sides of the glass seat, as shown in fig. 3. The glass should depress the neoprene by no more than 50 percent. Most studio glass sizes will require a medium density neoprene of between 3/16-inch and l/4-inch thickness. Any thinner, and the compressed seal will simply not do the job. Studio glass will work just as well in wood frames and stops as in metal frames and stops. Usually building codes as well as certain aesthetic considerations will determine whether a project is constructed in wood or metal. If metal is used (typically hollow metal frames), be sure to fill the frames solid with grout or an equivalent such as dense spray foam. Metal does have the advantage of not shrinking or warping. A good kilndried hardwood will accomplish the same thing. Removable stops, "airtight" seals, and frame separations are still required-wood or metal!

GLASS & INTERNAL ROOM ACOUSTICS

After choosing the correct glass type, thickness, and construction detail, the final design consideration involves the actual placement or (in some instances) nonplacement of glass. This quickly becomes a lesson in ray room acoustic analysis as well as in reverberation vs. specular and diffuse reflection. In general, glass will become a liability for internal room acoustics. If you don't need it, try and avoid it.

Figure 3.

Typical three-wall /two-glass studio /control construction
for relatively small window openings.
(Note the combining of the two control room side walls into one window frame.)

Figure 4.

Wood studio door with double glass window installation.

Glass is virtually a 100 percent nonabsorptive surface at almost all frequencies The typical large front control room glass can become quite dangerous if it is placed in a modern 500+ sq. ft. control room with a diffuse or semidiffuse rear wall. Secondary reflections (typically 25-50 ms out from direct sound) from this surface must be controlled. This can usually be accomplished by tilting the glass downwards into some sort of mid/high frequency absorption surface, such as a plush couch or rear console absorber. This can also be accomplished in plan with splayed pieces of glass. As the total glazing area gets larger, it can be more cost effective and acoustically desirable to facet the glass configuration in both planes. As the vertical dimension of the glass grows larger, glass thickness needs to increase, as do the glass pitch angles.

RELATED TIPS FROM WSDG

How many times have you seen multilayered studio glass with dirty layers! The trick is to use surgical gloves when installing and to always have an extra person to clean the inside pane prior to sealing. All other construction should be stopped that morning, as well.
When attempting to install the typical ushaped neoprene gasket around larger size glass units, it is often easier to simply cut strips of neoprene of the desired thickness from a flat sheet and create the U-channel by pre-gluing the strips on the glass. There is no difference in construction integrity if this is done correctly and it can often be considerably less expensive and easier to accomplish than trying to find odd-size gaskets.
Carefully evaluate if and where you need studio glass. On the side of the control room, it can provide similar or superior sight lines while allowing the reflective glass surface to become an asset in the room acoustics.