Why STC Isn’t the Full Story

Sound privacy is an ongoing concern across all facilities.?It’s not just sound transfer between hotel rooms or speech privacy within doctor’s offices – it also overhearing your co-workers on one of their many Zoom or Teams conference calls.?

It’s essential to take noise control seriously during the design process.?One valuable tool is Sound Transmission Class (STC). These ratings quantify a partition’s sound reduction properties. Numerically, STC ratings are meant to represent the number of decibels that speech sound is reduced as it travels from one side of a partition to another. The bigger the number, the more sound levels are reduced.

STC ratings are commonly used as a form of shorthand when discussing sound isolation. That said, these ratings can’t provide all the acoustic information you need on their own. In this article, we’ll take a closer look at STC ratings, other ways to measure a partition’s sound isolation performance, and why it’s crucial to work with someone who has a thorough understanding of these concepts.?

The Limitations of STC Ratings

An STC rating can play a big part in your choice of doors, windows, and other partitions. However, it’s still a single-number rating primarily meant to deal with speech and other office noise. On their own, these ratings can’t fully describe a partition’s sound isolation properties, so you can’t rely on STC information without having anything else to back it up.

As a recent example, Thorburn Associates recommended a sound-rated door from a particular manufacturer for a project. Our client asked us to review a door from another manufacturer that happened to be supplying all the non-sound-rated doors for the facility. The two doors had identical STC ratings. However, in low and high frequencies, our original recommendation surpassed the proposed substitute by over 10 dB (a 10 dB difference is perceived as a doubling of the sound level).

These results were possible because an STC rating only considers a partition’s sound isolation properties between the 125 Hz and 4000 Hz octave bands. Effectively, a partition could provide absolutely no sound isolation in the 63 Hz octave band and still have the same STC rating as a partition that reduced noise in this range by 50 dB.

Other Tools to Use

While the STC is the laboratory rating of sound isolation properties, a number of similar tests are also worth considering for quantifying performance of existing spaces.

One rating closely related to STC is Apparent Sound Transmission Class (ASTC). An ASTC rating accounts for both a partition’s airborne sound reduction properties and other field conditions such as flanking paths. The ASTC rating quantifies how partitions perform in real-world field tests after construction is completed while accounting for the effects of the receiving room’s reverberation time.

Another valuable rating is known as Noise Isolation Class (NIC). This rating also approximates the sound reduction properties of a partition and adjacent pathways. NIC relies on measurements of transmission loss between two interior spaces that a partition separates. While NIC ratings use the same scale as ASTC ratings, they aren’t adjusted to consider the receiving room’s reverberation time.

Whether it’s STC, ASTC, or NIC, all of these values have to be reviewed with an understanding of the impact and limitations of the testing procedures.?

Why You Need an Acoustical Consultant

No matter how many products you review, it’ll be hard to put the data to use if none of your team members know how to interpret it. Because of this, an experienced acoustical consultant is invaluable. While closely examining the performance of your partition options, a skilled noise consultant will consider the use of adjacent spaces and the types of noise likely to be generated on each side. Then, they’ll be ready to develop a recommendation suitable for your situation.

?To provide accurate recommendations, acoustical consultants rely on access to complete data. Many manufacturers will provide STC ratings for their products but often lack documentation or full octave band data. Manufacturers should have their products tested by reputable laboratories and make the results of this testing available upon request, since no proprietary information is included in the test results.

Typical reports state the manufacturer’s model designation for the product, any construction details the manufacturer chooses to provide, the test method used, and the full results in octave bands or third-octave bands. Based on this data, an acoustical consultant can make accurate recommendations regarding the right product.

Thorburn Associates has the experience and expertise to help with these issues and determine the best cost-friendly solutions for your project. Our team of acoustical consultants are ready to help demystify acoustical concerns. Contact us.