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You Bet Your Glass: July/August 2010

Do you hear what I hear?


July 1, 2010
By Frank Fulton

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Every few years throughout my working life, the issue of STC in windows
has raised its head, and this year is no exception. Due to a project
specification requirement, I am currently involved in getting sound
transmission testing done for one of my clients, Sunview Patio Doors
Ltd. As a result, I’ve found myself learning more about noise than I
knew before, and thought you might find some of the information useful
for you or your customers at some point in the future.

Every few years throughout my working life, the issue of STC in windows has raised its head, and this year is no exception. Due to a project specification requirement, I am currently involved in getting sound transmission testing done for one of my clients, Sunview Patio Doors Ltd. As a result, I’ve found myself learning more about noise than I knew before, and thought you might find some of the information useful for you or your customers at some point in the future.

You’ve undoubtedly heard the term “STC” used when referring to noise reduction in building materials, and more likely when referring to windows, curtainwalls, and glass partitions.  Here is some basic terminology to give you an understanding of what STC means.

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STC stands for “Sound Transmission Class.” It is an integer rating of how well a building partition attenuates, or deadens, sound, and is derived from sound attenuation values tested at sixteen standard frequencies from 125 Hz to 4000 Hz.

The hertz (Hz) is equivalent to the wavelength frequency per second. This is the measurement of the tone or musical note of the sound. A flute has a high pitch of about 2,000 Hz while a tuba could have a pitch as low as 30 Hz. Most humans can hear sounds between 20 Hz and 20,000 Hz.

A decibel, written as dB, is simply a measurement of how loud a noise is. 50 dB is quiet, while 140 dB is so loud that it can immediately injure your ears. Typical office noise is in the 50 to 60 dB range.

Transmission Loss is a measurement of the dB difference on either side of a wall. If we have a 100 dB noise on one side of a wall and can hear 75 dB of that noise on the other side of the wall, we have a 25 dB transmission loss.

Basically, to determine an STC rating for a wall or window, the amount of noise (dB) is measured on either side at sixteen different pitches (Hz). The transmission loss is determined for each frequency, compared to a standard set of values, and reported as a single number.

The higher the STC number the better when it comes to sound proofing or sound dampening in most situations.  However, STC values do not consider sounds under 125 Hz. As it happens, the types of noise most building owners would want their windows to block out occur in the 80 Hz range (aircraft/rail/truck traffic). For soundproofing in areas subjected to these types of noises, the use of a similar rating, the Outdoor – Indoor Transmission Class (OITC) may be a better indicator as the rating is weighted more to lower frequencies than the STC rating.

So now that you know what STC is, how can you use this information to assist your customers when it comes to sound sensitive project applications?

Typical interior walls in homes or cheap motels (two sheets of ½" drywall on either side of a wood stud frame) have an STC of about 33. When asked to rate their acoustical performance, people often describe these walls as “paper thin.” They offer little in the way of privacy. With a wall or window that achieves an STC of 45, loud speech cannot be heard. Single 3 mm glass has an STC of 30 and a 6 mm 1" overall insulating glass unit has an STC of 35. The best STC available in a 1" sealed unit is 42 made up of 6 mm laminated / ½" air / 6 mm laminated.

When trying to solve a potential noise issue, in order to get an STC over 50 you will likely have to propose a manually double glazed system with an air space between the glass lites of at least 4".  Generally, the greater the spacing between the glass, the better the STC. Another method of improving the STC is to vary the thickness of the glazing of the inner and outer lites. A glass make up of 3 mm and 6 mm gets an STC about 3 points better than a make up of 6 mm and 6 mm.

Frank Fulton is president of Fultech Fenestration Consulting. He has been in the industry for 30 years and can be reached via e-mail at fultech.fc@gmail.com .


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