Sound is one of those things in construction that gets treated as a finishing detail when it really belongs in the framing conversation. By the time a client notices a problem, the walls are closed and the options get expensive fast. Understanding how sound actually moves through a building, and what it takes to slow it down, is the kind of knowledge that pays off long before anyone moves in.
This piece covers the fundamentals: how sound travels, the four methods used to control it, what the ratings on a spec sheet actually measure, and where assemblies tend to fall short in the real world. It draws on a conversation recent conversation between Ben Bogie and Michael Crane, recorded for an episode of Ben’s Basement Tapes. Ben is a third-generation high-performance builder, a principal at Building Performance Cooperative in Connecticut, co-host of the BS and Beer Show, and one of the more sought-after educators in the residential construction industry. Michael is an acoustic specialist at MP Global Products with a construction background that keeps the conversation grounded in what actually happens in the field. If you want to hear them work through this material together, the episode is worth your time.
Sound is vibration, and it behaves accordingly
The starting point for understanding acoustic performance is understanding what sound actually is. Sound is vibration. When something moves, it pushes the surrounding air molecules outward, which push the next ones, creating a wave that travels in every direction from its source. When that wave reaches a surface, the surface absorbs some of that energy and vibrates in response, radiating sound on the other side.
This is why a single product rarely solves an acoustic problem on its own. Sound doesn’t arrive at a wall and stop. It moves through the material, along rigid connections, and around any gap it can find. Ben and Michael describe it in terms that tend to stick: your wall faceings behave like drum heads. When sound energy hits one side, both faces want to vibrate together, carrying that energy straight through the assembly to the space on the other side.
Builders trained in building science will recognize the air piece immediately. Sound travels through air just as freely as thermal energy does, and any gap that allows air movement will allow sound movement. A concrete wall with a small unsealed penetration will transmit sound directly through that hole regardless of how well the surrounding assembly performs. The discipline that makes a building envelope airtight is exactly the same discipline that makes a wall acoustically effective. Ben makes this connection explicitly in the episode, and it’s one of those things that reframes how you look at every detail you’re already doing.
The four methods of sound control
There are four established methods for reducing sound transmission through a building assembly. Each one addresses a different part of the problem, and the assemblies that perform best in the field combine more than one.
Mass makes a surface harder to vibrate. Heavier materials require more energy to move, which means less energy passes through to the other side. Standard drywall is the primary mass layer in most residential assemblies. Specialty drywall products incorporate additional damping compounds into the panel to increase both density and resistance to vibration. Adding a second drywall layer adds mass as well. The limit of mass alone, as both Ben and Michael point out, is that if the connection between wall faces is still rigid, vibration keeps traveling through regardless of how much weight you add.
Absorption addresses sound energy moving through the air inside a wall or floor cavity. Fiberglass and mineral wool insulation absorb that airborne energy before it can set the opposite surface vibrating. It works well in combination with other methods. On its own, it handles part of the problem and leaves the rest.
Decoupling interrupts the rigid connection between surfaces so vibration can’t travel a direct structural path. Resilient channel, isolation clips, staggered stud framing, and double stud walls all accomplish this in different ways. Michael compares it to the rubber gasket between glass panes in a window assembly: a flexible break in what would otherwise be a direct rigid connection. Ben draws the same parallel to thermal bridging, where separating surfaces interrupts energy transfer. The more thoroughly you decouple, the more effectively you reduce transmission.
Damping reduces resonance within the assembly. When sound energy reaches a wall, the wall wants to vibrate. Damping materials absorb that vibrational energy within the assembly rather than letting it radiate through. Viscoelastic compounds applied between drywall layers work on this principle. So do fiber-based board products like Quieture SoundBoard, which install between drywall and framing and dissipate energy at the point where it would otherwise transfer through the stud connection. As Michael explains it, the product doesn’t just add mass. It interrupts the path that mass alone can’t close. Damping is frequently the method left out of a standard residential spec, which is a big part of why the gap exists between how an assembly performs on paper and how it performs once people are living in it.
Combining methods is where real performance gains come from. Mass and absorption together get you further than either one alone. Add decoupling and you interrupt the structural path. Add damping and you address the resonance that the other methods leave behind.
What STC and IIC actually measure
STC stands for Sound Transmission Class. It measures how well an assembly reduces airborne sound transmission: voices, music, television, anything carried through the air. Testing runs across 16 frequency bands from roughly 80 Hz to 4,500 Hz, and the results roll up into a single number. A standard wood-framed wall with drywall on each side lands around STC 33 to 35. Add cavity insulation and you get into the low 40s. A well-built assembly that layers multiple control methods can reach STC 50 or higher.
The number is useful, and it also has a hard limit: STC measures airborne sound only. It says nothing about what happens when someone walks across the floor above.
IIC stands for Impact Insulation Class. This is the rating for impact noise, footfall being the most common source of complaints in multifamily construction. The test uses a standardized tapping machine on the floor above and measures what transmits to the space below. Impact noise is structurally transmitted, meaning it moves through the building materials themselves rather than through the air. Gypcrete adds mass to a floor system and helps, but as Michael notes, it works best alongside a decoupling underlayment. Without the isolation component, mass alone typically falls short of what’s needed to satisfy a resident living below.
The practical takeaway Ben and Michael land on is straightforward: STC and IIC measure different things, and a floor assembly in a multifamily building needs both considered. Specifying for one and assuming the other is covered is where a lot of post-occupancy complaints begin.
Where assemblies fall short
The most common acoustic performance gap in the field has less to do with the assembly spec and more to do with what happens around it. Sound will find the path of least resistance through a structure, and that path is frequently not through the carefully assembled wall. It travels through a duct shared between two rooms, through a hollow-core door at the end of an otherwise well-built wall run, through an outlet box without a putty pad, through the gap at the top plate, through the seam that wasn’t taped, through the perimeter that wasn’t caulked.
Michael shares a story in the episode about an architect he worked with on a 1970s townhome renovation. The wall assembly was carefully done: fiberboard sandwiched between two layers of drywall, cavity insulation, a thorough post-build test. The result on the neighbor’s side was remarkable. At higher frequencies, they were picking up ambient street noise more than the radio blasting at over 100 decibels on the other side of the wall. That’s what a complete assembly, well detailed and well sealed, actually does.
Ben’s framing for the air sealing piece is worth repeating here. He draws the comparison to the blower door test: you can build an assembly that performs well in theory, but if it isn’t tight, it doesn’t perform in the field. The difference with acoustics is that most builders are still learning from callbacks rather than from a test they ran before closing up the wall. Caulking perimeters, installing putty pads at electrical boxes, taping seams at board products, and sealing penetrations are standard practice in a high-performance acoustic assembly. They’re also the details most likely to get skipped when nobody has specifically called them out.
Going deeper
The fundamentals covered here are the starting point for a larger conversation. Each of the supporting articles in this series goes deeper on a specific piece of the picture: why walls underperform even when the spec looks right, what STC and IIC each measure and how to read them on a product data sheet, how the four control methods work together in a real assembly, and why air sealing is the acoustic step with the highest return on the least investment.
The conversation between Ben and Michael that informed this piece covers all of this and more, including some practical field testing techniques worth knowing about regardless of the project type.
FAQ
How does sound travel through walls in a building?
Sound moves as vibration. When airborne sound reaches a wall surface, that surface absorbs some of the energy and vibrates in response, radiating sound on the other side. It also travels along any rigid structural connection it can find, which is why a well-built assembly can still transmit noise through the framing, mechanical penetrations, or unsealed gaps at the perimeter.
What is the most effective way to soundproof a wall during construction?
The most effective approach layers multiple control methods: mass from drywall, absorption from cavity insulation, decoupling through resilient channel or isolation clips, and damping from a board product or viscoelastic compound between layers. Air sealing the perimeter and all penetrations protects the performance of everything else in the assembly. No single product replaces the system.
What does STC rating mean for walls?
STC, or Sound Transmission Class, measures how well an assembly reduces airborne sound transmission across a range of frequencies. Higher numbers indicate better performance. A standard residential wall runs around STC 33 to 35. A well-built assembly with multiple control methods can reach STC 50 or above. STC does not measure impact noise, which is rated separately by IIC.
Why does noise come through a wall even when it was built to spec?
Flanking paths are the most common reason. Sound travels through ducts, around doors, through unsealed outlet boxes, and along gaps at the floor and ceiling perimeter regardless of how the wall itself was built. A wall can perform exactly as specified and still transmit noise if the surrounding details weren’t addressed. Air sealing and perimeter detailing are what close that gap.
What is the difference between STC and IIC ratings?
STC measures how well an assembly blocks airborne sound like voices and music. IIC measures how well a floor assembly blocks impact noise like footsteps. They address different types of noise transmission and require different design strategies. In multifamily construction especially, both ratings need to be part of the spec conversation.
