Constructing Paritions For Sound Reduction

By: Ray Frobosilo

Silence is golden. That’s why the sound characteristics of building materials are important to architects. Noise control is critical in the design of multi-unit apartments, office spaces, and residential developments constructed near noisy locations such as high volume roadways or airports.

In accordance with building codes, walls and ceilings must meet a minimum STC rating to be in compliance with the Uniform Building Code (UBC). For privacy reasons, the offices of counselors, lawyers, and human resource departments cannot function in a space where sound will transmit through the surrounding walls. STC ratings help architects meet the needs of clients who require confidentiality in their line of work. Additionally, excessive noise, especially in urban areas, can lead to health problems including hypertension, ischemic heart disease, and sleep disorders.

Airborne sound travels through the air and spreads through material, assembly, or partition. In addition, sound passes under doorways, through ventilation systems, and around obstructions. When sound is categorized as an annoyance, it is noise. To help architects evaluate the sound properties of the materials they use and eliminate noise, scientists developed the Sound Transmission Class (STC), a single-number rating system that tests a material's or an assembly’s ability to block airborne sound. To better comprehend STC and how the system functions, it is essential to understand some specific terminology.

Decibel (dB) is a measurement of volume or how loud a noise is. Equivalent to a quiet conversation, 30 dB is classified as “faint,” whereas 110 dB is “deafening,” such as the sound of an elevated train. Transmission Loss is a measurement of the dB difference on either side of a wall. At a specific tone or pitch, a number of decibels are lost when sent through a barrier; this number is the Transmission Loss. Finally, frequency (Hz) is the measurement of the tone of musical note of a sound. An average human is born with the ability to hear frequencies from around 20 Hz to 20,000 Hz; this range decreases with age.

Combining decibel, Transmission Loss, and frequency, the STC measures how much sound a barrier stops. STC calculates the Transmission Loss values tested at 16 standard frequencies over the range of 125 Hz to 4,000 Hz. The tested frequencies must satisfy two conditions: (1) no single frequency band may have more than eight deficiencies and (2) the total deficiencies may not exceed 32. The highest number that meets both conditions is the STC. The values are then plotted on a curve graph and compared to standard STC reference curves. In general, higher STC rating blocks more noise from being transmitted.

Despite its scientific system, STC does have arguable flaws. This particular rating system only assesses isolation in the typical speech frequencies; those below 125 Hz do not register with the STC. Ironically, most common complaints fall below the STC minimum, including traffic noise produced by airplanes, trucks, and heavy equipment; bass in music; sound energy generated by home theater systems; and industrial equipment. There are specific regulations and codes to follow when a barrier must be able to block sound below STC ratings; however, more common methods are used to increase a partition’s STC.

Construction of a partition highly affects STC ratings. Four methods exist to increase the STC and decrease the amount of noise generated.

• Absorption – An absorptive material like fiberglass can be installed in the air space. Absorption increases the rating about 3 to 6 dB, but is not intended for serious soundproofing.
• Additional mass – Adding another layer or using a heavier wall will greatly affect the amount of noise penetrating a partition by about 5 dB.
• Increasing air space – Adding air space, ultimately creating an additional wall, instills an additional barrier to sound. The amount of air space greatly affects the STC, but typically ranges from 3 to 8 dB.
• Damping – Applying a damping compound between two constrained partitions transforms the partitions into dead panels that do not vibrate. Without vibration, sound is prevented from passing through.

No one likes a noisy neighbour. Whether it’s putting up with a blaring television set in an apartment complex or lying awake to the sound of traffic in a suburban home, there is no denying that noise pollution is irritating. That is why construction techniques to reduce sound are important to builders and architects who are looking to further reduce sound transmission in the structures they design.

Ray Frobosilo is a 46-year veteran of the building products industry, and is the current vice-president and founding member of the Steel Framing Industry Association (SFIA). He is also chairman of the Fire and Acoustics Task Group. Ray founded his own company, Super Stud Building Products, in 1973, which has grown to include over 250 employees. For more information visit www.buysuperstud.com.