Reverberation control in indoor environments: acoustic principles and technical solutions for contemporary architecture

Why acoustics has become a design parameter

In contemporary environments, characterised by reflective surfaces, large volumes and structurally high-performance materials, the control of acoustic comfort represents an increasingly relevant design challenge. Public spaces, transport infrastructures, open-plan offices, schools and healthcare facilities now require solutions that reduce reverberation and improve speech intelligibility without compromising technical requirements such as safety, durability or ease of maintenance.
Acoustics is therefore no longer a corrective intervention applied at a later stage, but a design variable to be considered from the earliest project phases, with a direct impact on the usability and functionality of the built environment.

Reverberation and acoustic quality in indoor spaces

When a sound wave encounters a reflective surface, part of its energy is reflected back into the environment. In enclosed spaces, these reflections accumulate, generating a reverberant sound field that can reduce speech intelligibility, increase perceived background noise and lead to cognitive fatigue. In working or healthcare environments, this translates into actual acoustic discomfort. Reverberation control is based on the ability to dissipate sound energy, thereby improving perceived acoustic quality.

Sound insulation, absorption and diffusion: comparing acoustic functions

Different acoustic panels available on the market are often confused in terms of function. Sound insulating panels (or sound barriers) are used to limit sound transmission between adjacent spaces and are selected for their mass and mechanical sealing properties. Sound-absorbing panels, on the other hand, act within the same space by reducing the sound energy reflected by rigid surfaces and improving reverberation time through partial energy dissipation.

Diffractive panels modify the directionality of sound waves by breaking up their wavelength and limiting the formation of standing waves. Diffusive panels distribute sound waves more evenly, filling potential acoustic “dead zones”. Each type addresses different needs, and the selection depends on the characteristics of the environment and the project objectives. In many indoor contexts, however, reducing reverberation through sound-absorbing surfaces represents the most effective approach to improving perceived acoustic quality.

Acoustic solutions for regulated environments

In many technical contexts, such as hospitals, airports or food-processing facilities, materials must comply with specific criteria: adequate reaction to fire, low or zero smoke emissions, no flaming droplets, washability, fibre-free behaviour and suitability for sanitisation. Fibre-free metallic sandwich panels represent a versatile solution capable of combining acoustic performance with regulatory compliance.

Resonant systems: the Helmholtz principle

An effective alternative to fibrous materials is represented by resonant systems based on the physical principle of Helmholtz resonators. In such systems, sound energy is channelled into properly dimensioned air cavities, where resonance phenomena promote energy dissipation. Microperforated surfaces allow sound waves to pass through, while the backing volume acts as a resonant element. This type of system enables absorption to be tuned to specific frequency ranges without the use of fibrous materials, with the added benefit of integrating aesthetics, performance and durability.

Honeycomb sandwich panels for reverberation control

An innovative application of the Helmholtz principle is the use of sandwich panels with an aluminium honeycomb core. These systems consist of a microperforated or micro-expanded front sheet (1) that allows the sound wave to pass through, an internal honeycomb core (2) acting as a resonant cavity, and a rear sheet (3) that promotes reflection and dissipation of acoustic energy. Panel geometry can be adapted to target specific frequency bands: reduced thickness for mid- to high-frequency absorption, greater thickness for lower frequencies.

In addition to acoustic performance, these panels provide structural rigidity, light weight, durability and fully metallic configurations, making them suitable for technical or high-traffic environments.

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Acoustic metrics and standards

The performance of sound-absorbing materials is assessed through standardised parameters that enable comparison. The weighted sound absorption coefficient αw, defined by ISO 11654, summarises acoustic absorption behaviour into a single value ranging from 0 (no absorption) to 1 (total absorption). Based on performance across different frequencies, materials are classified as:

• Class A (αw 0.90–1.00): highly absorbent
• Class B (αw 0.80–0.85): highly absorbent
• Class C (αw 0.60–0.75): absorbent
• Class D (αw 0.30–0.55): moderately absorbent
• Class E (αw 0.15–0.25): low absorption
• Not classified (αw < 0.10)

Since αw is a single-number value, additional letters may indicate performance in specific frequency ranges: (L) for low, (M) for mid and (H) for high frequencies.

Another widely used index is the Noise Reduction Coefficient (NRC), defined by ASTM C423, representing the arithmetic mean of the absorption coefficients measured at 250, 500, 1000 and 2000 Hz. In laboratory conditions, values exceeding 1.0 may occur due to edge diffraction phenomena.

ISO 354 defines the reverberation chamber method used to determine these coefficients. In terms of safety, the European EN 13501-1 classification for reaction to fire is also essential, with classes such as A2-s1,d0 or B-s1,d0 being among the most commonly specified.