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Flanking Noise & Flanking Sound Transmission EXPLAINED

Flanking Noise

So, it’s one in the morning and, finally, you feel yourself falling asleep.

All of a sudden, there’s some AMBIENT NOISE coming from God knows where.

How is this happening? You just finished soundproofing your wall. You should be fine, right?

Well, you might be experiencing flanking noise.

In this article, we’ll go over EVERYTHING you need to know about flanking noise and how you might be able to fix it!

Table of Contents

What Is Flanking Noise?

Flanking noise is the in-direct sound transmission that occurs when sound bypasses the main wall construction.

It’s the noise that happens when other noises may bounce off from another source and into your room even after soundproofing your area.

Sources of Sound

Stay with us for a bit while we go over WHY you’re having noise problems.

We’re not going to go over the way sound moves, but we all have a general understanding that sound is acoustic waves that travel through the air.

Indeed, NOTHING can stop these waves from going EVERYWHERE. That is unless you have something that absorbs it.

Enter Soundproofing Products!

These DIY soundproofing products make sure that sound is dampened or absorbed and doesn’t travel through the area the way it usually would.

But maybe you’re thinking, “Okay, I’ve soundproofed my wall. Why do I still hear the noise?”

Most of the time, regular soundproofing only addresses DIRECT SOUND.

Now, the direct sound transmission comes from the places you would NORMALLY EXPECT sound to travel through — the walls, the ceilings, or the floors via impact and airborne sound.

If you live in an apartment, you know that if you have a shared side of the wall with a neighbor, you will hear ANY loud noises that they make directly through that wall construction.

Those noises are relatively easy to treat.

However, your issue lies with IN-DIRECT SOUND, aka the sounds that come from other areas.

You would not immediately consider it as a noise source, but this is where we meet flanking noise!

Potential Flanking Noise Pathways: Walls, Windows, Doors, Ceilings, & Floors

If you’re trying to soundproof your home for more privacy, flanking noise problems can be extremely frustrating.

Unfortunately, if acoustics experts didn’t correctly check the building project or structure during the construction phase, you might find many PATHWAYS FOR FLANKING noises.

Flanking paths happen when sound reaches a wall, a ceiling, a floor, or any other structure of a lower sound transmission class, (also known as STC).

If the structure is a lower STC than a regular wall, you can bet that there will be flanking in that area.

Let’s go over some of the MOST COMMON FLANKING PATHS that you might miss in your own home so you can get the best sound isolation techniques to quiet your room!


Windows Icon

Windows are a MAJOR flanking transmission path for outdoor sounds, such as cars, planes, even people!

Standard single-pane windows will have an STC value of 26 to 28 which means the noise transfer is 8 to 10 dB LOUDER than a regular wall structure.

Some sounds may pass straight through the glass, but the sound could also get through the window pane.


Floors Icon

The sound coming through floors can cause a lot of headaches.

People who walk overhead cause vibrations that go into the walls and any adjoining rooms.

If you can imagine, a typical room construction uses common floors and built-in walls.

These vibrations are transmitted through the floor and the walls, which becomes acoustic energy or SOUND WAVES that you can hear.


Ceilings Icon

Since walls are usually only made up to the height of the ceiling, you can expect that there will be some gaps in the construction.

You might experience that sounds from one room below, quickly passes through an AIR CAVITY found in the ceiling or your floor — depending on where you are.

If you’re in an apartment building, this sound could easily travel through to another adjoining room.

Another set of noise sources from ceilings can be through ventilation, piping, ceiling lights, and electrical boxes.

Keep in mind that if there are tiny air pockets in the ceiling, even the SLIGHTEST can get through.

So, if you’re trying to figure out where the flanking is coming from, it’s highly crucial to do a SOUND TEST if you can to find as many sources of noise as possible.

Ducts and Vents

Ducts and Vents Icon

If your building has air conditioning traveling through the ceilings or floor systems, again, depending on where you are, they are likely CONNECTED through air ducts.

If this ductwork has not been correctly reinforced with sound reduction treatments, like duct liner, you may find flanking here.

These ducts will now work as flanking paths, and you’ll get sound from the air conditioning traveling through to another room.


Doors Icon

Doors are one of THE MOST COMMON flanking paths WITHIN rooms, homes, offices, buildings, etc.

Forget the very obvious air cavity you see at the bottom or side of the door, which is 100% going to be a flanking transmission path.

Instead, consider the actual material of the door.

If your doors are hollow core doors, you’re going to find an STC in the low 20s. If it were made of solid core wood, you’d have an STC in the high 20s.

Find out what your door is made of so you can find the best soundproofing products to address YOUR specific noise problems — be it flanking, impact, or airborne sound.

Electrical Outlets

Electrical Outlets Icon

You probably don’t realize how much electrical outlets account for flanking.

Electricity makes the slightest noise, but because they are made of thin plastic and metal, it is INCREDIBLY EASY for sound to come through.

So, if you ever hear random buzzing in your house, check your outlets. That might be the problem.

How to Stop Flanking Sound Transmission for Noise Control

Now that we know where the flanking might be coming from, let’s give you a few solutions that will address these issues one by one, shall we?

Once you’ve gone over where each flanking path might be, see which of these solutions can help you. You might like to make it a project to ensure the best soundproofing for all of your rooms.

#1 Structure-Borne Noise Control

A lot of flanking noise is caused by the structure of the building.

If you did not check the wall construction or the building design with a sound test, expect AT LEAST one flanking issue in your building.

For example, if the stud wall, joists, subfloors, and walls are in areas that stimulate vibration well, you’re going to get flanking.

If you have a hand in the wall design and construction phase of your building or home, take this into account.

If you know that you will need one room to be QUIETER than the others, change the design of that room.

It CANNOT be the same as the other rooms. Otherwise, whatever noise you’ll be hearing from those rooms, you will hear from your “soundproofed” room.

Another example of flanking through improper wall construction is when all the walls in the building are the same.

If you want your wall construction to be unique, you might have to change its low-frequency noise resonance.

  • Use an entirely different wall design – try a staggered wall, a stud wall, or double stud wall construction.
  • Try resilient decoupling on your studs.
  • Layer sound dampening material like Green Glue acoustic sealant onto the surface of your walls
  • DO NOT simply add a layer of drywall or soundboard to the side of the wall. It might help, but it won’t give you the highest performance walls out there.

Try and ask your contractor or the construction team to see if they have exceptional customer service that includes acoustics checks.

#2 Inspecting the Ceilings and Floors

Some HUGELY common flanking pathways are the ceiling joists and floor joist systems, which several construction sites use.

For example, if you live in a room with a shared attic floor, that noise from the other room can travel up through the joists.

One of your best bets here would be to soundproof your ceiling.

If the noise is coming through vibrations from another adjacent room, you might want to pad up the ceiling or layer it up with multiple caulk layers of Green Glue that will absorb the noise.

You can also consider creating a “room within a room” and make use of separate ceiling joists to effectively decouple unwanted flanking sounds.

You can also make use of floating floors to help mitigate any flanking. A floating floor is basically a floor that has an additional “floating surface” on top of a resilient layer like rubber or fiberglass mats.

#3 Check Your Ducts and Vents

The main reason that ductwork is a flanking problem is that sound passes through them.

Since all sound travels through the air, the ducts are a CLEAR FLANKING PATH for any ambient noise that wants to make its way into your room.

Luckily, since this is a common issue, you can find SEVERAL products that provide quick and easy solutions to soundproof your vents properly.

1. Duct Liner

Insulated ductwork means that the ducts contain sound-absorbing materials that will take in the sound as it travels through the tunnel created by the duct.

For it to be effective, the liner needs to be INSIDE the tunnel.

2. Flex Duct

A flex duct, basically a flexible duct, can be useful because it is insulated and can be placed pretty much anywhere along the path of the duct.

Make sure you DO NOT put it in areas where the duct is close to the DIRECT SOUND.

3. Using LONG and COMPLEX Pathways

The longer and more complicated the paths, the more difficult it is for sound to get through. If your duct design has many bends and breaks, you’ll find that there is less flanking.

4. Using Soffits to Cover Exposed Ductwork

Can you control the design or the construction of your ductwork? If yes, take note of where the ducts will be exposed.

If you have EXPOSED DUCTS the best idea would be to cover it with a soffit and wrap it with a few layers of drywall and Green Glue.

If you can’t hide the structure of your ducts, MAKE SURE that they are round and try to coat the duct with viscoelastic material.

#4 Reinforcing Your Doors

When soundproofing doors, try to check for the mass and seal quality of the door.

If you want to get good soundproofing performance out of your doors and make sure that they stand a chance to block a flanking path, try these:

1. A Heavy Solid Core Slab

Since mass is crucial for sound isolation, finding a heavier door will help when you’re trying to prevent any sound from coming through — whether its indirect or direct noise.

A solid core door means more mass and fewer sound vibrations into other rooms.

2. Focus on Getting Good Seal Quality

There are specialized soundproofing seals that you can use on your doors. Consider these if you’re trying to find optimal performance or a whole soundproofed experience.

REMEMBER: Heavy and sealed will get you to block out any direct sound AND flanking paths that could be invading your privacy.

3. Order Specialized ACOUSTIC DOORS

Yes, these do exist.

Several manufacturers offer a special service where you can order a door made of proper materials and special seals that give it a better soundproofing performance than other doors.

4. Communicating Doors

Communicating doors are two doors that are back-to-back. These kinds of doors usually share the same door frame.

You often see these in hotel rooms wherein it would be easy to connect next-door rooms together by opening them up.

However, if you’re not going to leave them open as a room connector, these doors are GREAT at isolating noises effectively since it creates an airlock in between the doors when closed.

#5 Checking the Electrical Outlets

Finally, we have flanking from electrical outlets.

These are tiny, but since ALL BUILDINGS need them in ALL OF THE ROOMS, you bet that they account for a large portion of the flanking you’re hearing.

Even if you have reinforced your wall with drywall and Green Glue, an electrical outlet means that there’s a hole in your wall only being covered by thin plastic and a thin sheet of metal.

But don’t despair.

Here are some things you can do to ensure proper sound isolation even with holes in the wall.

  • DO NOT place outlets back to back. ALWAYS have them in separate stud cavities.
  • Ensure that the junctions of the outlet are WELL-SEALED. Like a door, if there are any areas where sound can get through, be sure to seal it off.
  • Try to order an acoustical pad and use it around the outlet.
  • Use insulation in the walls and on the surface. Doing so will help you absorb all of the noise coming from inside. Hopefully, the sound won’t even make it through to the outside.

Now that you know how to soundproof your building from the electricity running through the wall, you can be confident that you’re going to get a peaceful and quiet environment.


Addressing these flanking issues is relatively straightforward.

If you can get in on the construction project, maybe you will be able to soundproof your place even before the issue arises.

You need to be sure that you have the proper materials and address your SPECIFIC NEEDS.

Hopefully, this article has helped you figure out where your flanking noise problems may be coming from and that you find the proper service and care to keep the noise from permeating your wall.


January 5, 2022 – updated title, updated featured image

About the Author


Andrea has always been bombarded by the hustle and bustle outside her home. Living in the city doesn’t get any quieter. The never ending noise from construction, traffic, and dogs barking on the streets day in and day out drove Andrea to a breaking point.

For 3 years, Andrea committed herself to studying DIY hacks, performing soundproofing experiments, and installing noise-free solutions. Now, she lives a quiet life free of the stress from noisy environments.

She hopes to share this knowledge so that others don’t have to endure the noise reigning in their ears and live a peaceful, stress-free life.