Transparent Acoustic Chamber

Sound Science: How "Acoustic Experiments without Borders" is Changing Wave Research

Beau Callahan in Science & Nature August 2025 • 4 min read.

"Acoustic experiments are getting a major upgrade! Discover how new tech is removing the boundaries of lab research for clearer results."

Imagine trying to understand the Earth's deepest secrets by studying how sound waves bounce through its layers. That's precisely what geophysicists do, using seismic experiments that mimic medical ultrasounds to map the subsurface up to 50 kilometers deep. These experiments involve sending acoustic waves into the earth and recording how they scatter, much like how doctors use ultrasound to see inside the human body.

In a typical marine seismic experiment, a fleet of air guns blasts sound waves that travel through the water and penetrate the earth. A vessel towing long receiver cables then captures the echoes, which are used to reconstruct the subsurface geometry. This process, while effective, is incredibly complex and has driven decades of research and development.

To refine these techniques, researchers often turn to smaller-scale laboratory experiments to test seismic-wave-propagation models and image reconstruction methods. However, these lab setups come with their own set of challenges, primarily the reflections from the experimental setup's walls. But now, scientists at ETH Zürich have pioneered a method called immersive wave propagation, effectively making these boundaries transparent. This breakthrough eliminates unwanted reflections, making the experimental enclosure act as if it were infinitely large.

The Hurdles in Acoustic Research: Why Boundaries Matter

Seismic experiments face two major challenges: attenuation and image reconstruction. Attenuation refers to how the Earth's crust absorbs acoustic waves, which complicates data interpretation. Ideally, the Earth would act as a perfectly elastic medium where waves travel without losing energy, but this isn't the case in reality.

Image reconstruction is another tough nut to crack. It's an inverse problem, meaning researchers have to reconstruct the subsurface topography from a set of acoustic-wave measurements. To get an accurate picture, they need a massive amount of data to constrain the many unknown parameters of the subsurface structure. Even with terabytes of data from a typical 3D seismic survey covering hundreds of square kilometers, the surface parameters are often not fully constrained.

Scale Issues: Lab setups are thousands of times smaller than the Earth structures they aim to represent.
Reflection Problems: Experiments are conducted in tanks of finite size, leading to multiple, complex acoustic reflections from the walls, making it hard to correlate lab data with field data.
Absorption Limitations: Using absorbing layers to attenuate waves at the boundaries is imperfect, as no perfect absorber exists for a broad range of acoustic frequencies.

That's where laboratory experiments come in handy. They provide a controlled environment to study attenuation effects and optimize data collection strategies. However, lab setups have significant limitations due to their scale and the interference from tank walls.

Looking Ahead: The Future of Acoustic Experiments

Becker and his team have not only demonstrated the effectiveness of immersive wave propagation but also opened up new possibilities for acoustic experiments. While they've successfully tested a 1D channel, the next step is to expand this approach to 2D and 3D configurations. This advancement will require addressing several key questions: What is the optimal number of sensors needed for a 3D setup? How closely should these sensors be spaced, and will they interfere with each other? And can monopole sources generate enough energy at low frequencies for the scheme to work effectively? Despite these challenges, the potential benefits are immense. Immersive wave propagation promises to bridge the gap between field experiments and lab studies, offering a more accurate and controlled way to explore the complexities of wave behavior.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1103/physics.11.71, Alternate LINK

Title: Acoustic Experiments Without Borders

Subject: General Medicine

Journal: Physics

Publisher: American Physical Society (APS)

Authors: Martin Landrø, Nathalie Favretto-Cristini

Published: 2018-07-16

Everything You Need To Know

How does immersive wave propagation solve the problem of boundary reflections in acoustic experiments?

Immersive wave propagation addresses the issue of unwanted reflections in laboratory acoustic experiments by effectively making the boundaries of the experimental setup transparent. This allows researchers to conduct experiments without the interference of reflections from the walls, which would normally complicate the data. By eliminating these reflections, the experimental enclosure acts as if it were infinitely large, providing a more accurate representation of real-world conditions. Further research is being conducted to expand this approach to 2D and 3D configurations.

What is 'attenuation' in the context of seismic experiments, and why is it a challenge?

In seismic experiments, attenuation refers to the Earth's crust absorbing acoustic waves, which complicates the interpretation of data collected. Ideally, the Earth would act as a perfectly elastic medium, where waves travel without losing energy. However, in reality, the Earth absorbs some of the energy of the waves, leading to a weaker signal at the receivers. This absorption varies depending on the type of rock and the frequency of the wave, making it difficult to accurately determine the structure of the subsurface.

Why is image reconstruction in seismic experiments described as an 'inverse problem'?

Image reconstruction in seismic experiments is considered an inverse problem because researchers must reconstruct the subsurface topography from a set of acoustic-wave measurements. This means they're working backward from the data to create an image, which is more complex than directly imaging an object. Because many different subsurface structures can produce similar wave measurements, a massive amount of data is needed to constrain the many unknown parameters of the subsurface structure accurately.

Why might monopole sources be useful in immersive wave propagation, and what challenges do they present?

Monopole sources could be useful because they generate sound waves that radiate uniformly in all directions, unlike other types of sources that may have directional biases. This uniform radiation is particularly useful for immersive wave propagation in 3D setups, as it can ensure that waves reach all sensors evenly. However, a key challenge is ensuring that these monopole sources generate enough energy at low frequencies, which are critical for probing deeper into the experimental setup and accurately simulating real-world seismic conditions.

Why is the number and spacing of sensors important in a 3D setup for immersive wave propagation?

Researchers are considering the optimal number and spacing of sensors for 3D setups of immersive wave propagation because these factors directly affect the accuracy and completeness of the data collected. Too few sensors may result in incomplete data and a poor reconstruction of the wave field, while too many sensors spaced too closely may interfere with each other, distorting the measurements. Finding the right balance is crucial for effectively capturing the complexities of wave behavior in three dimensions and ensuring the success of the experimental method.