How do liquid foams block sound?

April 24, 2014
How do liquid foams block sound?
This image shows a fine view of the distribution of the liquid phase in a liquid foam. The liquid channels that support the thin films, some of which can be seen here, are clearly visible. The behavior of both channels and films caused by an acoustic wave explains the unusual acoustic properties of liquid foams. Credit: Laboratoire Matière et systèmes complexes (CNRS/Université Paris Diderot

Liquid foams have a remarkable property: they completely block the transmission of sound over a wide range of frequencies. CNRS physicists working in collaboration with teams from Paris Diderot and Rennes Universities have studied how sound is attenuated in liquid foams. Their findings, published in Physical Review Letters, open the way to the development of tools called acoustic probes that could be used to monitor the quality of foams used in industry, especially in the mining and petroleum sectors.

Much research has been carried out in acoustics in order to understand how propagates through a material. One of the classic approaches is to send an through it and listen to the response, which provides key information about the material, in the same way as listening to the sound produced by tapping on a wall indicates whether it is hollow. This is why researchers analyze how various materials, from the simplest to the most complex, react when they are struck.

One of these is however keeping its secrets: liquid foam. This is a difficult material to study since it is short-lived and sound does not travel through it easily. Until now, there has been no acoustic probe for foams. The most frequently used probes rely on electrical conductivity to determine the amount of liquid contained in a foam. However, some of these substances are made up of non-conducting liquid, making it impossible to characterize them. Adding liquid foams to the list of materials that can be probed by acoustic waves is therefore central to ensuring that every type of foam used can be fully characterized.

How do liquid foams block sound? A key finding has been revealed in a recent study by researchers from the 'Matière et Systèmes Complexes' Laboratory (CNRS/Université Paris Diderot) and the Institut de Physique de Rennes (CNRS/Université Rennes 1). The study, which is part of a project funded by the French National Research Agency (ANR), brings together for the first time acoustics experts and foam specialists. Characterization of foams is essential for measuring the speed and attenuation of sound in foams of known composition. The results show that sound propagation varies greatly according to the frequency of the wave used. The researchers propose a simple interpretation of these observations. Foams are made up of 90% air plus a liquid, and this liquid is distributed between films and channels that support them (see image below). However, these two structures have very different geometries and masses: films have a large surface area and a small mass, while channels are narrower but have greater mass. The vibration of air caused by the acoustic wave displaces the films, which in turn pull on the channels. At low frequencies, the speed of sound is very low (around 30 meters per second): the sound is slowed down by the coordinated motion of the films and channels, but is not blocked. At high frequencies, the speed of sound increases (approximately 220 meters per second): only the films move, thus also allowing the sound to travel through the foam. However, at intermediate frequencies, the films behave anomalously: they move in the "wrong" direction, in other words towards the left when the air displaced by the sound pushes them to the right, which prevents the channels from moving. The sound is therefore blocked in the bubbles over a wide range of frequencies.

This work thus helps to solve the mystery of acoustics in foams. It will pave the way for the development of acoustic probes that can be applied to such materials, which are an everyday part of life as well as being widely used in industry.

Explore further: Uncovering liquid foam's bubbly acoustics

More information: "Resonant Acoustic Propagation and Negative Density in Liquid Foams." J. Pierre, B. Dollet, and V. Leroy. Physical Review Letters. 11 April 2014, DOI :

Related Stories

Uncovering liquid foam's bubbly acoustics

October 17, 2013

Liquid foams fascinate toddlers singing in a bubble bath. Physicists, too, have an interest in their acoustical properties. Borrowing from both porous material and foam science, Juliette Pierre from the Paris Diderot University, ...

Designing an acoustic diode

November 1, 2013

Most people know about ultrasound through its role in prenatal imaging: those grainy, grey outlines of junior constructed from reflected sound waves. A new technology called an "acoustic diode," envisioned by researchers ...

Levitating foam liquid under the spell of magnetic fields

November 11, 2013

Foams fascinate, partly due to their short lifespan. Foams change as fluid drains out of their structure over time. It is precisely their ephemeral nature which has, until now, prevented scientists from experimentally probing ...

The destructive power of sound waves

December 16, 2013

Researchers at the University of Arizona College of Engineering have come up with a novel way to help the U.S. Air Force dispose of stockpiles of dangerous chemicals – using nothing more than sound waves.

Scientists twist sound with metamaterials

February 25, 2014

A Chinese-U.S. research team is exploring the use of metamaterials—artificial materials engineered to have exotic properties not found in nature—to create devices that manipulate sound in versatile and unprecedented ways.

Recommended for you

More to rainbows than meets the eye

August 25, 2016

In-depth review charts the scientific understanding of rainbows and highlights the many practical applications of this fascinating interaction between light, liquid and gas.

Chemists explore outer regions of periodic table

August 25, 2016

A little known—and difficult to obtain—element on the fringes of the periodic table is broadening our fundamental understanding of chemistry. In the latest edition of the journal Science, Florida State University Professor ...

Measuring tiny forces with light

August 25, 2016

Photons are bizarre: They have no mass, but they do have momentum. And that allows researchers to do counterintuitive things with photons, such as using light to push matter around.

DNA chip offers big possibilities in cell studies

August 25, 2016

A UT Dallas physicist has developed a novel technology that not only sheds light on basic cell biology, but also could aid in the development of more effective cancer treatments or early diagnosis of disease.

Understanding nature's patterns with plasmas

August 23, 2016

Patterns abound in nature, from zebra stripes and leopard spots to honeycombs and bands of clouds. Somehow, these patterns form and organize all by themselves. To better understand how, researchers have now created a new ...


Adjust slider to filter visible comments by rank

Display comments: newest first

5 / 5 (1) Apr 24, 2014
Path-dependent dispersion.
not rated yet Apr 24, 2014
Or might it be a dispersion-dependent path?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.