All walls are not alike - a look at STC

What is STC?

STC is the acronym for Sound Transmission Class. It measures how a material attenuates airborne noise. The STC is roughly equivalent to the decibels that get reduced by a barrier like a wall or a window. So, a very loud external noise of 100 decibels might be reduced to approximately 55 decibels inside your home if your walls have an STC of 45. The higher the STC, the more effective the barrier.

What kind of walls do you have?

If you live in a typical American dwelling built after the 1940s, you probably have walls made of drywall (known as plasterboard in Europe). And if you live in a home with hollow drywall construction, you probably know this already — acoustically, the walls are like drum skins.

In the 1950s and 1960s, new wall construction typically consisted of a single layer of 1/2-inch drywall nailed onto hollow wooden studs. By the 1980s, builders used 5/8-inch drywall, which has a slightly higher fire rating (up to 60 minutes). It has a barely perceptible STC difference from 1/2-inch drywall.

Before the 1950s, most walls were made of lathe and plaster. These walls offered more sound and temperature insulation than modern drywall. Consider yourself lucky if you live in an older home that didn’t get its walls replaced by an overzealous remodeler.

How much noise passes through your walls?

First, measure the decibel level of your home when it’s quiet. Turn off any music, TV, or appliances (like a dishwasher) and use a decibel-reading app on your mobile phone like Decibels, the NIOSH app, or SPL Meter. It might read at about 35 to 40 decibels when it’s quiet. This gives you a baseline from which you can measure any other noise.

So, let’s say you have uninsulated drywall in your home. The STC rating for drywall on hollow wood studs is about 33 decibels — almost the same as your home when it’s totally quiet. As soon as someone turns on a TV or makes noise on the other side of that wall, some of that sound will pass through to you. If your walls offer 33 STC but your meter goes up to 50 when your neighbor watches TV with the volume turned loud, then the difference is passing through your walls. Increasing the STC of your walls might help.

According to this article in The Craftsman Blog, lathe and plaster walls might have an STC of about 52, which is considered acceptable for a multi-unit dwelling. You might not hear your neighbor’s TV, or not as much, if you have plaster walls. And if you do have noise issues in an older home, your solutions might be different.

What can you do?

If you want to reduce noise through your walls, you have several options, though all of them involve money and labor. This chart on Wikipedia offers a comparison of STC ratings for different types of wall treatments, including no treatment. What’s not on this chart: comparisons for using other types of drywall (like QuietRock), or more efficient types of insulation (like cellulose or rock wool), or layering Green Glue between a second layer of drywall, which could boost the STC.

According to The Soundproofing Company, these are the comparative STC ratings for three different drywall combinations.

Original:
Single drywall (1/2-inch) no insulation, over wood studs: STC 33 (this is typical in many homes)

Two different modifications:
Double 1/2-inch drywall on each side of the wall, fiberglass batting: 44 STC
Double 5/8-inch wall with Green Glue on one side, single on other side: 52 STC

Every situation has unique circumstances, but you usually have options to reduce noise in your home or workplace. You don’t have to resort to demolition to get small improvements.

What about windows?

STC also applies to windows. In this short Soundproofist video, you’ll see how windows can be an additional weak spot for external noise. You might see “solutions” like soundproof curtains promoted on the internet. Beware: fabric doesn’t stop bothersome, low-frequency noise from coming into your home. (Save your money!) It can help to absorb noise inside a room (anyplace where people gather), but it won’t stop external noise from coming in.

Together, walls and windows comprise a barrier against noise, and their materials and installation matter.