What is the auditory system ? 

The auditory system helps us hear the world around us.

It’s an important part of how we communicate, learn, and enjoy life.

Whether it’s music, conversations or other sounds, our auditory system picks up sound and makes sense of it.

The auditory system consists of three main parts:

• the outer ear.
• the middle ear.
• and the inner ear.

Each part plays a crucial role in how we hear.

The role of the outer ear  

The outer ear includes the part of the ear you can see, called the pinna, and a canal that leads to the eardrum, known as the ear canal.

The eardrum is a thin piece of tissue that stretches across the end of the canal. 

The pinna catches sound waves from the air and funnels them into the ear canal.

These sound waves travel down the canal and hit the eardrum, causing it to vibrate.

The inner ear contains a spiral-shaped part called the cochlea, which is filled with fluid.

It is approximately 3.5 cm long, the base has a 0.9 cm diameter and the end has a 0.3 cm diameter. 

The cochlea is lined with thousands of tiny hair-like cells … called the hair cells (!!).

When the vibrations reach the cochlea, they create waves in the fluid.

These waves bend the hair cells. And as the cells bend, they turn the vibrations into electrical signals that are sent to be brain by the cochlea nerve.

High-pitched sounds make the hair cells at the base of the cochlea vibrate, while low-pitched sounds affect cells at the tip.

Loudness depends on the amplitude of the waves. Stronger waves bend the hair cells more, creating stronger electrical signals.

When the signals reach the brain, the brain processes them and tells us what we hear.

It figures out whether the type of sound. This process happens in a split second, so you can hear and understand sounds almost instantly!

References

• Architectural Acoustics – Second Edition – Marshall Long  – Academic Press
• Noise Control in Building Services  – Sound Research Laboratories (SRL) Ltd – Pergamon Press
• Sense of Hearing – Chapter: Anatomy and Physiology for Health Professionals: Special Senses (https://www.pharmacy180.com/article/sense-of-hearing-3587/)

Obviously, a specification needs to consider all factors that influence a panel’s sound absorption.

One formula estimates the resonant frequency of a simply supported panel:

with: 

• B the bending stiffness of the panel/material; 
• m the mass of the panel;
• a and b respectively the width and the length of the panel;
• i and n correspond to the harmonics of resonance, that you don’t need to understand in depth for this article. 

This formula may be daunting for you, but you can note the influence of different properties.

See below the references used to confirm the above concepts: 

• Architectural Acoustics – Second Edition – Marshall Long  – Academic Press
• Acoustic Absorbers and Diffusers – Theory, Design and Application – Third Edition –  Trevor Cox, Peter D’Antonio
• Engineering Noise Control – Sixth Edition – CRC Press – David A. Bies, Colin H. Hansen, Carl Q. Howard, Kristy L. Hansen

What are porous sound absorbers ? 

Primarily, Porous absorbers are used to diminish sound reverberation in rooms or to mitigate noise transmission when installed in cavities.

The sound absorption is achieved when sound passes through networks of interconnected pores, inducing viscous and thermal effects that dissipate acoustic energy as heat.

There are three types of porous absorbers.

Porosity and air flow resistivity of porous sound absorbers ? 

The sound absorption of a material is closely tied to its porosity and flow resistivity, representing the  resistance of the air as it traverses the material.

Hence, higher porosity correlates with greater sound absorption.

Porosity is defined as the ratio of pore volume to the matrice/medium volume.

When located on solid/hard surfaces, the sound absorption tends to increase with the thickness of the porous material, particularly at low and mid frequencies.

See below the sound absorption of fibreglass (53 kg/m³ density) samples of different thicknesses, fixed to a hard surface.

Sound absorption of fibreglass (53 kg/m³ density) samples of 25 mm, 50 mm, 75 mm and 100 mm thicknesses (fixed to a hard surface)

Similarly to the thickness, the sound absorption at low and mid frequencies typically increases with the density of a fibrous absorber.

Below are the sound absorption coefficients of mineral wool blanket samples with varying densities.

Sound absorption of mineral wool of 16 kg/m³ , 24 kg/m³, 48 kg/m³ and 60 kg/m³ densities (50 mm fixed to a hard surface)

Influence of the mounting conditions on the sound absorption? 

The sound absorption of a material greatly depends on how it is mounted.

Remember the post about Sound absorption basics? We introduced ISO EN 354 (2003 – Acoustics Measurement of sound absorption in a reverberation room mention) setting measurement standards for various mounting conditions.

For instance, consider acoustic ceiling tiles, which can be either directly fixed to a soffit or suspended.

While the sound absorption is comparable at mid and high frequencies, it is notably higher at low frequencies when the ceiling tiles are suspended (see the graph below).

Sound absorption of acoustic tiles directly applied to a hard surface or suspended 

Also, in Suspended acoutic rafts explained, we showed that the following aspect influence the sound absorption characteristics a raft systems:

• the sound absorption of the material installed in a room 
• the size of the rafts
• the spacing between the rafts
• the distance between the rafts and the hard surface above.

Influence of the cover on the sound absorption? 

In many cases, sound absorbers are covered or protected by materials such as:

• fabric
• felt
• plastic
• paint
• perforated sheet material
• slats

Some of these materials possess specific air flow resistivity and may prevent sound from passing through. Thereby reducing the sound absorption especially at the higher frequencies. Less so at low frequencies.

This is why some materials, like paint, should be avoided as they block the pores of the materials (unless it is a non-bridging paint). See below different performances for different paint applications. 

Influence of paint treatment on the absorption of a sound absorber

The same applies to glue behind cloths.

Cloth, perforated sheet materials and slats are typically preferred for protecting the sound absorbers against impact and/or small particles.

See below the references used to confirm the above concepts: 

• Noise Control in Building Services – Sound Research Laboratories Ltd – Pergamon Press
• Sound Materials, A Compendium of Sound Absorbing Materials for Architecture and Design – Tyler Adams – Frame Publishers
• Acoustic Absorbers and Diffusers – Theory, Design and Application – Third Edition –  Trevor Cox, Peter D’Antonio
• Engineering Noise Control – Sixth Edition – CRC Press – David A. Bies, Colin H. Hansen, Carl Q. Howard, Kristy L. Hansen

What are sound absorption classes ? 

The classification is a method, among others, to categorise the sound absorption performance of absorbers, particularly for construction materials.

It is simpler and easier for non-acousticians to grasp since it doesn’t involve numbers.

There are five classes ranging from Class E (lowest performance) to Class A (highest performance).

How are sound absorption classes established ? 

Establishing the sound absorption class of a material involves two main steps:

• Converting the frequency-dependent sound absorption performance to a single-number value.
• Assigning the single-number performance to the corresponding class.

From frequency-dependent performance to single number value

Sound absorption data is typically measured in third-octave bands.

(if you need to understand about frequencies read this article, and frequency bands read this article)

The initial conversion involves transitioning from the sound absorption coefficient measured in third-octave bands to the Practical Sound Absorption Coefficient (α_p) in octave bands (as introduced in this article).

From there, we calculate the Weighted Sound Absorption Coefficient (α_w), which is a single number determined as detailed below.

We begin with two curves:

• the Practical Sound Absorption Coefficient (α_p) values calculated (in yellow below).
• A curve known as the reference curve, with specific values for each octave band between 250 Hz and 4000 Hz (in black below).

The second step involves calculating the “unfavourable deviations” for each octave band.

Then, we incrementally adjust the reference curve in 0.05 steps toward the measured value until the sum of unfavorable deviations is ≤ 0.10.

Once this criterion (≤ 0.10) is met, the performance (α_w) for the material is determined by the value of the shifted reference curve at 500 Hz.

Another important point to note is if a measured value exceeds the reference value by more than 0.25 (even after the shifting process), a shape indicator is added to the weighted sound absorption performance as follows:

• L, like α_w (L), if excess occurs at 250 Hz
• M, like α_w (M), if excess occurs at 500 and 1000 Hz
• H, like α_w (H), if excess occurs at 2000 and 4000 Hz

From single number performance to sound absorption class

Once the single number is calculated, we associate it with a class using the table below.

The figure below presents the reference curves corresponding to each class (you have probably seen this type of graph before). 

What is a sound absorber ? 

A succinct definition of a sound absorptive material is:

“A sound absorptive material is a material that acts as a ‘noise sponge‘ by converting the sound energy into heat within the material.”

(from  Environmental Impact Assessment, Theory and Practice – Anji Reddy Mareddy)

When sound strikes a material, some of its energy converts to heat, reducing the reflected sound energy.

The sound absorption of construction materials typically spans across narrow or wide frequency ranges, not single frequencies.

(if you need to understand what frequencies are read this article, and frequency bands read this article)

Various factors influence a material’s sound absorption in a space:

• the material type (hard, fibre-based, porous, etc)
• the thickness
• the finish (painted, cloth-covered, plastered, etc)
• the placement in the space
• the mounting method (on/off hard surface, free-hanging, etc)

Common sound absorptive solutions in construction use porous, fibrous, or cellular materials.

Other solutions utilize these materials with additional features like cavities, perforations or microperforations to enhance the sound absorption across specific frequency ranges.

Examples of materials will be presented in future posts.

How do we rate the sound absorption capacity of materials ? 

For most construction materials, a common index to rate the sound absorption is the sound absorption coefficient , termed α (‘alpha’).

This coefficient is the ratio of: 

• the sound energy absorbed (Ea) by a material; 
• divided by the sound energy incident (Ei) on the same material.

As the absorbed sound energy (Ea) increases, the sound absorption coefficient increases too.

Conversely, a decrease in absorbed sound energy results in a lower absorption coefficient.

Thus, better sound absorption yields a higher absorption coefficient, logically.

Other methods and indices for evaluating sound absorption of building materials include:

• the Noise Reduction Coefficient (NRC) and Sound Absorption Average (SAA) as per ASTM C423 – Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method
• the Weighted Sound Absorption Coefficient (α_w) as per ISO 11654 (2023).
• the Sound absorption class (ranging from E to A, with A being the highest), as per ISO 11654 (2023)
• the Equivalent sound absorption area.

The latter two will be elaborated in future posts.

How do we measure the sound absorption coefficient of materials ? 

There are two main methods to measure the sound absorption coefficient of a material: 

• the Impedance Tube Method.
• the Reverberation Chamber Method or Room Method.

The Impedance Tube Method 

This method involves placing a sample of material at the end of a small tube.

Sound is emitted into the tube at different frequencies.

The sound absorption coefficient of the material is then derived by comparing the sound in the tube before and after it strikes the sample.

While more practical and time-efficient, this method is not preferred due to its restrictions on the type of material and mounting conditions.

The Reverberation Chamber or Room Method 

This method involves placing large samples or multiple samples of the material to be tested in a large &  highly reverberant room.

The room typically comprises hard surfaces (walls, ceiling, and floor) that reflect sound, absorbing minimal to no sound, along with large sound reflective surfaces hung from the ceiling and sometimes positioned near the walls.

This setup aims to scatter sound extensively, creating a diffusive sound field within the room. This enables simulation of sound reaching the samples from all directions.

The sound absorption coefficient is calculated by comparing:

• Sound reverberation measurements without any materials.
• Sound reverberation measurements with the material under test.

While not as practical or time-effective as the impedance tube method, this approach offers the advantage of testing materials in more realistic mounting conditions. These include (but not only):

• Mounted directly against a hard surface. 

• Hung freely.

• Positioned within a cavity.

• Split into small samples. 

• And many others

We will provide examples of sound absorption coefficients for different construction materials in future posts.

How does sound propagate? 

Assuming a single sound source, typically called a point source, emitting sound uniformly (i.e. the same way in all directions).

In the near field, it produces sound waves that spread outward from the source in a spherical pattern. 

 Note: the near field is a region close to the source

In the far field, sound continues to propagate outward from the source. However, it appears to propagate more in a line pattern.

 Note: the far field is a region far from the source

To aid visualization, we often use rays to represent how sound propagates. They clearly indicate the direction of a specific sound’s propagation.

Throughout the remainder of this post, as well as in many others, we illustrate sound propagation using rays.

Acoustic analysis 

Studying the acoustic performance of materials or the acoustic characteristics of sources/signals comes in different ways.

Frequently, acoustic experts use frequency bands for measurements or calculations.

Essentially, it is a method to reveal acoustic information across different frequency ranges.

Most commonly, frequency analysis occurs in:

• octave bands 
• third-octave bands

In third-octave bands, the precision/resolution is about three times higher than in octave bands.

The formula to obtain octave band values from third octave values depends on the acoustic metric you want to study.