If you are considering any soundproofing products, you have probably stumbled across two acronyms: STC and NRC. At a glance, these two things seem very similar, but there are important, subtle differences between the two. As you learn the differences between STC and NRC, you will know what products will help you most with your soundproofing goals. Both STC and NRC represent ratings given to soundproofing products, and they are calculated using technical tests and formulas. There are a few terms that are helpful to know when it comes to STC and NRC:

• Hertz (Hz) - the pitch of a sound, or how high or low it is. The human ear can hear from about 20Hz to 20,000Hz.

• Decibel (dB) - How loud a sound is. Humans can hear at 0dB, and there is risk of hearing damage with sound around 85dB and above.

STC

STC stands for sound transmission class. Of the two tests, this one is much more complicated to calculate. Luckily, we don’t have to get into that here. A more accessible definition of STC is how well a material or product keeps sound from leaving a room. STC measurements are concerned with keeping sound within a particular place. A suburban drummer might need material with a high STC rating to keep the drumbeats from escaping the room.

STC is calculated using common noise frequencies. These frequencies are sent toward a certain material and measured in decibels on both sides of a wall. The difference in sound on both sides of the wall is measured across 18 different frequencies and then compared to a standard STC curve. The rating is given based on how a test curve compares to a standard curve. The higher a STC rating, the better at blocking sound a material is.

NRC

NRC stands for Noise Reduction Coefficient, which is an advanced way to say how much sound an acoustic product can absorb. NRC is all about making a room quieter and less “echoey”, while STC is about keeping noise from leaving a room. NRC could be used when deciding what acoustic material to put into an auditorium, so the speakers can be heard clearly without echoes.

NRC is determined by performing a baseline test in a controlled lab—special equipment is used to measure how much sound bounces off the walls. The sound is sent through a microphone to 4 frequencies. Next, a product is placed in the lab and the exact test is performed again. Then the NRC is calculated by comparing the differences in sound absorption in the lab with and without the product.

A rating of 0 on the NRC scale means a material absorbs no sound, while a rating of 1 on the scale means a material absorbs all sound. The higher the rating, the more sound a material will absorb, and the quieter your space will be.

Why Does it Matter?

Comprehending NRC and STC will help you choose both the right kind and quality of product for your acoustic goals. Frequently it will be a combination of products with NRC and STC ratings to achieve optimal sound (or no sound) in an area.