🔬 applied physics

Applicability of Radiowave Anechoic Chambers for Acoustic Free-Field Measurements on the Example of the Chamber at ITMO University

This paper demonstrates that the ITMO University radiowave anechoic chamber can be effectively repurposed for acoustic free-field measurements within specific frequency ranges and distances, as verified by ISO 3745:2012 standards.

Original authors: Farid Bikmukhametov, Ksenia Razrezova, Roman Smolnitsky, Yuri Shchelokov, Nikolay Kanev, Mariia Krasikova

Published 2026-03-18

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CC BY 4.0

Original authors: Farid Bikmukhametov, Ksenia Razrezova, Roman Smolnitsky, Yuri Shchelokov, Nikolay Kanev, Mariia Krasikova

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The "Double-Duty" Room: Turning a Radio Silencer into a Sound Silencer

Imagine you have a room designed to be the ultimate "quiet zone" for radio waves. In this room, if you shout a radio signal, it disappears instantly without bouncing off the walls, just like a ghost walking through a fog. Scientists call this a Radio Anechoic Chamber.

Now, imagine you need a room to test speakers or microphones, where sound waves also need to vanish without bouncing. Usually, you'd need to build a completely different, expensive room for that, lined with giant foam wedges.

This paper asks a simple, clever question: "Can we use the Radio Quiet Room to test sound, too?"

The researchers at ITMO University in Russia decided to find out. Here is the story of their experiment, explained simply.

1. The Room: A Sponge for Waves

Think of the walls of this room as being covered in thousands of tiny, sharp pyramids made of a special black foam.

For Radio Waves: These pyramids are like a sponge that soaks up radio signals, preventing them from reflecting back.
For Sound: The researchers wondered, "If this sponge eats radio waves, will it also eat sound waves?"

It turns out, the material (foam soaked with carbon and metal oxides) is a bit of a "double agent." It's good at swallowing both types of waves, but not perfectly for everything.

2. The Test: The "Perfectly Quiet" Challenge

To see if the room works for sound, the team ran a series of tests based on strict international rules (the ISO standard). They treated the room like a new car being tested for safety.

The Background Noise Check: First, they checked if the room was quiet enough when no one was talking. It was like checking if a library is truly silent. Result: Pass! The room was quiet enough; the background noise was far lower than the sound they were testing.
The Echo Check (Reverberation): They clapped their hands and measured how long the sound lingered. In a normal room, sound bounces around like a pinball. In a perfect anechoic chamber, it should stop instantly. Result: Pass! The sound died out very quickly, especially at higher pitches.
The "Invisible Wall" Test: This was the tricky part. They placed a speaker in the middle and measured the sound at different distances. In a perfect "free-field" (no reflections), the sound should get quieter in a very predictable way as you move away (like a flashlight beam getting dimmer). If the walls reflect sound, the beam gets messy.

3. The Twist: It's Not a Perfect Fit Everywhere

Here is where the story gets interesting. The room didn't work perfectly for every situation.

Imagine you are standing very close to a campfire. The heat you feel is mostly from the fire itself, not the heat bouncing off the rocks behind you. But if you walk 20 feet away, the heat bouncing off the rocks starts to mess with your measurement.

Close Range (The "Campfire Zone"): When the researchers stood close to the sound source (less than 1 meter away), the room worked beautifully. The sound behaved exactly as if the walls didn't exist. They could test frequencies up to about 3,000 Hz (a high-pitched whistle) without issues.
Far Range (The "Echo Zone"): When they moved further away, the "pyramid" walls started to act a bit like mirrors for certain sounds. The sound waves bounced back just enough to confuse the measurements, especially at very high frequencies or when looking at specific corners of the room.

4. The Verdict: A "Specialist" Room

The researchers concluded that this radio-wave room can be used for sound testing, but with a few "terms and conditions":

Stay Close: You must be relatively close to the sound source (within about 3 feet).
Watch the Frequency: It works great for low and mid-range sounds, but gets tricky for very high-pitched sounds if you are far away.
Watch the Angle: Depending on which direction you face in the room, the results change. Some corners are "quieter" than others.

The Big Picture

Think of this room like a Swiss Army Knife. It was built primarily as a "Radio Knife," but the researchers discovered it also has a "Sound Blade" attached. It's not as sharp as a dedicated Sound Knife (a purpose-built acoustic chamber), but for many jobs, it works just fine.

Why does this matter?
Building a soundproof room is expensive and takes a lot of time. If scientists can use an existing radio room for sound tests (within the limits found in this study), they save money and resources. It's a smart way to get more value out of the tools we already have.

In short: The ITMO University radio room is a "good enough" sound room, as long as you know its limits and don't expect it to be perfect for every single test.

1. Problem Statement

Acoustic anechoic chambers are essential for high-precision free-field measurements (e.g., loudspeaker directivity, microphone calibration, and noise barrier testing) but are expensive and labor-intensive to construct. Conversely, radiowave (electromagnetic) anechoic chambers are also resource-intensive. Since certain materials used for radio-wave absorption (such as carbon-impregnated foams, charcoal, or metamaterials) can also absorb acoustic waves, there is a potential for dual-use facilities.

The core problem addressed in this study is determining whether an existing radiowave anechoic chamber at ITMO University (Saint Petersburg, Russia) meets the strict acoustic standards required for free-field measurements, specifically ISO 3745:2012 / GOST-ISO 3745-2014. The authors aim to define the specific frequency ranges and spatial constraints under which this radiowave chamber can function effectively as an acoustic anechoic chamber.

2. Methodology

The study employed a combination of experimental measurements, empirical modeling, and numerical simulations to characterize the chamber.

Chamber Description: The facility is a rectangular room (4.46m × 6.96m × 3.3m) lined with pyramid-shaped elements made of open-cell foam rubber saturated with carbon particles and metal oxides. Notably, the floor has an uncovered recess (0.15m deep), and some ceiling areas use uniform layers rather than pyramids.
Acoustic Coating Characterization:
- Experimental: Absorption coefficients were measured using an impedance tube (ISO 10534-2) on disk samples cut from the pyramid material.
- Modeling: Experimental data was fitted to five empirical models (Delany-Bazley, Miki, Mechel, Garai-Pompoli, JCA) to determine flow resistivity. The Delany-Bazley model provided the best fit ( $\sigma \approx 3437$ Pa·s/m²).
- Numerical Simulation: COMSOL Multiphysics was used to simulate the absorption of the actual pyramid geometry (using Floquet periodicity conditions) to account for the enhanced absorption of the structured surface compared to flat layers.
Chamber Performance Testing (ISO 3745:2012):
- Source: An omnidirectional sound source (OED-SP360).
- Measurements: Conducted along four trajectories (I–IV) extending from the source to the room corners.
- Parameters Measured:
  - Background Noise: Compared against the source noise.
  - Reverberation Time ( $T_{60}$ ): Measured at specific points to assess reflection decay.
  - Sound Pressure Level (SPL): Measured at 11 points along each trajectory (distances 0.5m to 1.5m) to verify the inverse-square law ( $1/r$ dependence).
- Criteria: The deviation ( $\Delta L_p$ ) between measured SPL and theoretical free-field values was calculated. Acceptable deviations were set at $\pm 1.5$ dB for frequencies $\le 630$ Hz and $\ge 6300$ Hz, and $\pm 1.0$ dB for 800–5000 Hz.

3. Key Results

A. Absorption Coefficients

Flat Material: The raw foam material showed moderate absorption (0.17–0.5) in the 290–1800 Hz range.
Pyramid Structure: Numerical simulations revealed that the pyramid geometry significantly boosts performance. The absorption coefficient reached ~0.815 at 290 Hz and exceeded 0.99 above 410 Hz, confirming the coating is highly effective for acoustic damping.

B. Background Noise & Reverberation

Background Noise: The chamber's background noise was at least 65.7 dB lower than the test source across the 250–5000 Hz range, satisfying the ISO requirement of a 10 dB margin.
Reverberation Time: $T_{60}$ dropped rapidly from 1.5s at 63 Hz to 0.21s at 100 Hz, remaining between 0.05s and 0.24s for higher frequencies. This indicates strong suppression of reflections.

C. Free-Field Conditions (SPL Deviation)

The chamber's ability to maintain free-field conditions varied significantly based on distance from the source and measurement direction:

Near-Field (0.5 – 0.9 m): The chamber performs excellently. Across all four measurement directions, it satisfies ISO criteria for frequencies 50 Hz to 3150 Hz.
Far-Field (> 0.9 m): Performance becomes direction-dependent and frequency-limited due to non-uniform coating distribution (e.g., the floor recess) and scattering.
- Direction III: Valid up to 16,000 Hz (at 0.5–0.9m).
- Direction IV: Valid up to 8,000 Hz (at 0.5–0.9m).
- Direction II: Showed the most significant deviations, limiting the usable range.
Overall SPL: When considering the overall sound pressure level (integrated across frequencies), the deviation remained within $\pm 1$ dB for all directions except Direction II, and within $\pm 0.6$ dB for distances up to 0.8 m.

4. Key Contributions

Validation of Dual-Use Facilities: The study provides empirical evidence that radiowave anechoic chambers can serve as acoustic anechoic chambers, provided specific operational constraints are met.
Directional Dependency Analysis: Unlike generic assessments, this paper highlights that the "free-field" status is not uniform throughout the room; it is highly dependent on the measurement vector relative to the source and the room's geometry.
Quantitative Limits: The authors established a precise "operational map" (Table 3 in the paper) defining exactly which frequency bands and distances are valid for each of the four measurement directions.
Methodological Integration: The work successfully combines impedance tube measurements, empirical model fitting (Delany-Bazley), and full-wave numerical simulations to characterize complex absorbing geometries.

5. Significance

This research offers a practical solution for resource-constrained research institutions. It demonstrates that existing radiowave facilities can be repurposed for acoustic testing, saving the significant cost and time associated with building dedicated acoustic chambers. However, it serves as a critical warning: such repurposing is not "one-size-fits-all." Users must strictly adhere to the identified frequency and distance limits (specifically staying within 0.9m of the source for broad frequency coverage) and account for directional variations to ensure measurement accuracy according to ISO 3745 standards.