Acoustical Isolation 101: Your Guide to Sound Reduction in Buildings

Minimizing disruptive and undesired sound in buildings requires an understanding of acoustics, sound transmission, and sound control. This ensures a proper acoustical isolation system can be incorporated into the building’s design.

How Sound is Perceived

Acoustics is the science of sound, and sound travels in waves, like ripples in water.

When these waves reach a person’s ears, a signal is sent to the brain that’s processed and recognized as sound. Sound is perceived in two ways: level and frequency.

Sound Level

Sound level — or how loud a sound is — is measured in decibels

The annoying buzz of an insect is 10 decibels, while the thundering roar of a jet engine at takeoff is a painful 140 decibels. We can hear sound levels change by 2 to 3 decibels,. A 10-decibel addition doubles a sound level while a 10-decible reduction cuts the sound level in half.

Sound Frequency

The second way sound is perceived is based on frequency — how high or low a sound is in pitch. This is measured in Hertz. Humans can hear sounds ranging from 20 Hertz to 20,000 Hertz, although most people lose the highest frequencies as adults. Some dogs can hear sounds as high as 65,000 Hertz. To provide real-world examples of Hertz, a low machine hum is around 50 Hertz, most voices range from 90 – 500 Hertz, and a bird’s song can be over 5,000 Hertz.

The highest frequencies people can hear have wavelengths that are less than an inch, while the lowest frequencies have wavelengths over 30 feet long — imagine the length of a school bus, or since sound waves are omni-directional, the height of a three-story building. Longer wavelengths make it harder to control low frequency sounds. In buildings, knowing how to control low frequency sounds can make a big difference in occupant satisfaction.

How Sound is Transmitted

When it comes to sound transmission, commercial and residential buildings are affected by two types of sound: airborne and structureborne.

Airborne Sounds

Airborne sounds come from voices, televisions, and barking dogs, among other sources. These sounds travel through the air and can be transmitted through walls and floors. Airborne sounds are quantified using a rating known as Sound Transmission Class or STC. To measure STC, noise is emitted from a loudspeaker in one room — a source room — into a receiver room, then the difference in sound levels between the source and receiver rooms is subtracted. The STC rating standard uses frequencies ranging from 125 - 4,000 Hertz.

Structureborne Sounds

Structureborne sounds are created by direct contact with the floor. This includes footsteps, chair scrapes, and dropped items which transmit through the building structure and radiate as sound. Structureborne sound is quantified with a rating known as Impact Insulation Class or IIC. For the IIC measurement, a standardized tapping machine is used to impact the floor. The level of the resulting sound transmitted through the floor is measured in the room below using the frequencies from 100 to 3150Hz. The IIC rating is only used for floor and ceiling assemblies. It’s not applicable to walls.

Lab Tests and Field Tests

Because some buildings require minimum STC and IIC values, it’s important to check sound codes early in the design process. Both STC and IIC are lab measurements, performed under highly controlled circumstances, and they measure only the performance of the assembly being tested. Measuring an actual building is more complex because it’s impossible to isolate the interactions between walls, floors, and ceilings. However, tests taken in buildings — also called field tests — are sometimes required by code. Field tests are more affected by the conditions in the room, like curtains or furnishings. Different normalizations are used, with the letter A (“apparent”) used when we normalize to the size and conditions in the room: ASTC and AIIC. The letter N (“normalized”) is used when we normalize to normally furnished small rooms like bedrooms or offices: NNIC and NISR. NNIC and NISR are included in the 2024 IBC code revision to establish code compliance in existing buildings. The industry previously used FSTC and FIIC to identify field tests, so you may also see them referenced this way.

Minimizing Sound Transfer

In designing a building, there are four main ways to minimize sound transfer.

Increase the Mass

The greater the mass (or weight) of an assembly, the more efficient it is at blocking airborne sound transmission.

Design for Airspace

A common way to reduce structureborne sound is to take advantage of the depth of the structure by installing a ceiling, which can also help minimize airborne sounds.

Add Absorption

Adding insulation can enhance the sound isolation ability of a ceiling cavity by damping the cavity. However, thicker insulation only provides marginal benefits.

Create Structural Breaks

An efficient way to minimize structureborne sound transfer is with a structural break that reduces vibration transfer by creating a completely isolated floor. Even small structural breaks make a big difference.

Effective Sound Mitigation

To ensure each of these sound minimizing elements work most effectively, it’s important to avoid flanking paths. A flanking path is any way that sound transfers around the sound isolation system. Unintentional flanking paths can be created by direct connections through the isolation system. Even small gaps around baseboards can be a big problem because they permit unwanted sounds to leak through.

Ensure a Robust Acoustic Isolation System

As leaders in sound control for the building industry, Maxxon provides proven products and selection tools to deliver robust acoustic isolation systems for commercial and residential buildings. Our tested assemblies provide rated systems to increase mass, design for airspace, and create structural breaks, including systems that can increase the IIC rating by up to 20 points. Each robust acoustic isolation system includes a Maxxon® gypsum concrete underlayment, a Maxxon® Acousti-Mat®, and Maxxon® Acousti-Mat Perimeter Isolation Strips to optimize noise reduction and protect against the sound bleed of flanking paths. Using our Fire & Sound Manual — which features a full listing of assemblies backed by third-party sound tests and UL fire ratings by construction type — and our Interactive System Selector, you can identify which acoustical isolation system is the best fit for ensuring a quieter and more comfortable environment for your building’s occupants. 

To determine the most robust acoustic isolation system for your next project, contact your Maxxon representative.