What is Sound Attenuation? [2022]

Soundproofing a space isn’t as easy as you may think.

It isn’t just purchasing soundproofing or sound-deadening materials and installing them. 

For a successful soundproofing project, you first need to understand how sound works.

This will allow you to come up with better soundproofing solutions to tackle the specific issues you’re facing. 

Attenuation is one word that gets thrown around a lot in the soundproofing world.

However, most people don’t know what attenuation means.

This article takes an in-depth look at what attenuation is.

By the end of the article, we hope you can have a better understanding of sound attenuation and what it means for your soundproofing project.

What does attenuation mean?

what does attenuation mean wavelength

There’s sound attenuation, audio attenuation, wave attenuation and noise attenuation. 

Attenuation means a loss or reduction of intensity or energy. 

To understand attenuation better, we need to take a look at sound waves and how they work.

Nature of sound waves

Sound is said to be a wave phenomenon. 

A sound wave is a longitudinal wave where molecules from different mediums are temporarily displaced and then returned to their original position.

Vibrating objects produce sound. 

No matter the vibrating object, the molecules of the medium vibrate in a back-and-forth motion at a given frequency. 

The frequency of a wave is measured by the number of complete back-and-forth vibrations(oscillations) per second.

This frequency is usually expressed as Hertz.

Hertz measures the frequency of the cycle.

For more on hertz, read our what are hertz post.

The wavelength of a soundwave is the distance the sound travels in one cycle.

A wave repeats its patterns once every cycle so a wavelength can also be referred to as the length of the repeating patterns.

Meanwhile, the speed of sound refers to how fast a noise is passed from molecule to molecule.

Let’s move on and have a look at how sound waves are measured.

Measuring sound waves

Sound waves are measured in decibels, sabin and hertz.

However, for attenuation, you’ll be dealing mostly with decibels.

The amount of energy transported past an area of a medium per second is known as the intensity of a sound wave.

The greater the vibrations, the greater the rate at which energy is transported through the medium.

This will create a more intense sound wave. 

The most common units for expressing the intensity of a sound wave are Watts/meter2

The human ears can detect a wide range of intensities.

However, the scale that is frequently used to measure sound intensity is based on multiples of 10.

The logarithmic scale used to measure sound intensity level is the decibel scale.

It classifies sounds in a range from 0-194dB.

The decibel scale can help to tell how loud a sound is.

Your average day consists of sounds falling between 30-100dB.

Normal conversations are around 65dB.

The intensity of sound disappears over time and distance so the further away you are from a noise source, the lower the decibel rating. 

Sound attenuation explained

wave attenuation ear hand

Now that we understand a bit more about sound and sound waves, we can take a better look at sound attenuation.

Sound attenuation is the way a sound level reduces as the listener moves away from a sound source. 

As sound travels from its source, the area where the sound is spread increases much like ripples in a pond.

Similar to how the skin of a balloon gets thinner as it gets bigger is a perfect example of how sound level reduces as the distance and area increase. 

In soundproofing, when the same amount of acoustic energy is spread over a greater area, it means the sound level at any point on this surface reduces as the distance from the source increases. 

Sound waves that are at a distance from a small source on a hard ground surface with no wall nearby will disperse in a hemispherical pattern.

If the source is suspended in the air, it disperses in a spherical pattern.

In both cases, the area is related to the square of the distance.

The area of a hemisphere is g A = 2𝝅r2

Meanwhile, for a sphere, it is A = 4𝝅r2

For both, the area quadruples with every doubling distance and increases by a factor of 100 if the distance is increased by a factor of 10.

Acoustically, this means that the sound level in these conditions will reduce at a rate of 6dB for every doubling of distance from a noise source. 

However, while the original distance has been doubled, the increased distance has to be doubled again.

This means the original distance from the noise source has to be quadrupled to achieve a further 6 dB reduction and increased to 8 times the original distance for a further 6dB reduction.

You may even be able to achieve a 20dB reduction if the original separation distance is increased by a factor of 10.

When sound is unable to disperse in this fashion, the reduction with distance is less.

This is why the level of sounds indoors may not change even if a listener is at a distance from the noise source. 

Of course, this will also depend on other factors like how absorptive or reverberant the room is.

Noise coming from a line source like a road with continuous traffic will disperse sound in a cylindrical manner along the length of the road.

The area of a half-cylinder for the length of the road(L) is given by 𝝅rL.

This means the area the sound is dispersed over is directly related to the separation distance.

Therefore, you’ll get a reduction of 3dB for every doubling distance from the source instead of 6dB from a point source.

If someone is close to the source of noise, it does not act as a point source

This is because the sound is dispersed over the entire area of the source. 

For instance, someone standing 1 meter from a large wall is 1 meter from the closest part of the wall but many meters from the majority of the wall surface. 

Even at one meter away from the wall, the sound is already dispersed over a large area.

This means the sound level at 1 meter from the wall is lower than if it were dispersed over a smaller area than 1 meter from a point source. 

So the sound level close to a large source in the near field is lower than if it was at the same distance from a small source. 

Mechanism of sound attenuation

The mechanism for reducing sound will depend on the origin of the sound.

If the sound is generated in a room, then it needs to be absorbed.

If it is airborne that means it’s coming from outside and you need to insulate.

On the other hand, if it is being transmitted through the structure then you need to isolate it from the source of vibration.

Sound Absorption

During sound absorption, a sound wave is neither transmitted or reflected.

Instead, the energy gets absorbed into the material.

Three basic categories of sound absorbers exist.

These include:

Porous absorbers

Porous absorbers often consist of matted or spun fibers.

Some common examples include:

  • Carpets
  • Draperies
  • Spray applied cellulose
  • Aerated plaster
  • Fibrous minerals like wool and glass fibers.
  • Open-cell foam
  • Cast porous ceiling tiles

These materials allow air to flow into a cellular-like structure where sound energy gets converted to heat.

Initially, there is a viscous loss as the air gets pumped in and out of the open porous structure.  

These materials also include sound-dampening properties.

Damping refers to the material’s ability to dissipate energy.

Porous absorbers are the most common sound-absorbing materials used. 

Panel absorbers 

Panel absorbers use non-rigid and non-porous materials placed over an airspace that vibrates in response to sound pressure.

Common panels include thin wood paneling over framing, lightweight waterproof floors or ceilings, glazing and other large surfaces that can resonate sound.

Panel absorbers are great for absorbing low-frequency sounds.  

Resonators

Resonators absorb sound in a narrow frequency range. 

They may include some perforated materials and material with openings like holes and slots. 

An example of a resonator is a Helmholtz resonator.

It is shaped like a bottle and the size of the opening, the length of the neck and the volume of air trapped in the chamber all control the resonant frequency.

However, most perforated materials only absorb mid-frequencies unless the facing is designed to be as acoustically transparent as possible.

Slots also have a similar acoustic response as long narrow slots can be used to absorb low frequencies. 

Insulation

Although lightweight modern buildings are good for thermal insulation, they aren’t good for sound insulation.

The heavier the material, the better it insulates. 

This is why it is always better to add concrete or bricklayers to the walls and floors of a building to improve sound insulation.

Isolation

Elastic materials and steel frames are notorious for transmitting vibrations through building structures.

This type of noise is transmitted by the continuous solid part of the structure.

Introducing a float in the floor or putting the building on resilient and cellular materials can isolate the building. 

If the whole structure is on resilient pads, it can isolate the building as well.

What is sound attenuation insulation?

attenuate sound with insulation

When looking to attenuate sound, you may find several sound attenuation insulation products on the market.

These insulation products are usually made from mineral wool, fiberglass or cotton and cellulose materials.

To control the transmission of sound, you have many options.

Most people think fiberglass is the only good option but open-cell spray polyurethane foam can also work great.

Each has its pros and cons. 

Let’s learn a bit more about these products for sound attenuation.

Sound Attenuation with Mineral Wool Insulation

Mineral wool also called Rockwool comes in rigid panels and softer batts.

They are great for soundproofing a room.

You can find them in several different sizes that can be used in ceilings, floors and residential interior walls.

These products are available in 2 types; high performance and multi-purpose.

High performance

High-performance mineral wool has some of the best low and high-frequency ratings. 

This type is considered to be the most cost-effective option.

Most brands use natural stone fibers to make this product.

They are also fire, water and moisture resistant. 

Multi-purpose

Multi-purpose mineral wool has lower ratings for sound absorption. 

However, they are the best option for thermal insulation.

Multi-purpose insulation can be placed on the inside and on top of standard thermal insulation to protect from unwanted sound.

This type of insulation is soft and flexible.

It can be purchased in different sound absorption capabilities.

This may be listed as STC rating

Attenuation with fiberglass insulation

Fiberglass insulation performs the same way as mineral wool when it comes to soundproofing.

It is available in the same thickness but costs more. 

This is because it does a better job at insulating than mineral wool.

Attenuation with cotton

Homeowners and builders may use cotton soundproofing for smaller projects. 

This is a budget-friendly option and performs well.

Cotton materials are extremely soft so they will need the support of a frame or other covering.

People who build their own acoustic panels use cotton soundproofing materials. 

How to attenuate sound 

noise attenuation hand goggles

Building a soundproof room is easier and cheaper when trying to attenuate sound than doing it in an existing room.

Attenuate sound in new construction

Walls

When soundproofing a room, you should start with the walls.

Use noise-reducing drywall.

Traditionally, people use resilient channels to reduce noise transfer between rooms.

Resilient channels are thin metal channels that are attached to the framework of the wall to isolate the drywall.

This will weaken sound waves and attenuate sound.

Today, you can use a single piece of noise-reducing drywall. 

This type of drywall is made up of two dense gypsum cores separated by a layer of viscoelastic polymer. 

This makes the drywall perfect for reducing interior noise.

Use these types of drywalls in offices, bedrooms, TV rooms and anywhere you want to block distractions.

Insulate interior walls

To stop vibrations, the proper insulation acts as a muffler against noise. 

You can fill the wall cavity with insulation.

Insulation can be placed between walls and the spaces between studs.

It will absorb noise and break the path of sound.

You can use it in both interior and exterior walls to stop noise transfers between rooms.

Floors

Now that you know how to deal with your walls, it’s time to look at the floor.

Float your floors

Traditional flooring is usually nailed to the subfloor.

Floating flooring is not nailed to the subfloor.

They are installed using a special type of adhesive instead.

Since no nails are used in floor joists, floating floors will help prevent creaks and sound from traveling between the levels of your home. 

You can achieve a floating floor.

However, the most common is using a vibration-absorbing material sandwiched between flooring. 

Attenuate sound in an existing room

If you already have an existing room and looking to attenuate sound, try these steps:

Soften the surfaces

Sounds bounce off and through hard surfaces like glass, hardwood flooring and tile so you need to cover up these surfaces. 

You can add carpet, plush furniture and window trimmings to cover hard surfaces. 

Seal it up

Any gaps or spaces under and around your door allow noise to pass through. 

Electrical outlets and vents can also allow sound to pass. 

Anywhere air enters a room, sounds will also enter.

Sealing these spaces is a great way to ensure unwanted sounds aren’t coming into the room.

Read more by clicking the link on how to soundproof a room.

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