To design a sound-absorbing panel, it is important to identify factors that affect the maximum sound absorption of low, middle and high frequency sounds. Perforation effect is very important for the noise-reducing and noiseabsorbing panels. Perforations are often used for sound reduction. Experimental data shows that the perforation is very effective to absorb low-frequency noise. In the presented study, influence of perforation coefficient of noise reduction was analyzed with theoretical and experimental methods. The experiments were conducted in noise reduction chamber using an perforated construction with glass wool filler. Sound reductions index of 15 dB indicates good acoustic properties of the panel.

Charcoal is an environmentally friendly, biodegradable, and economical material. This material is usually produced by slow pyrolysis—the heating of wood or other substances in the absence of oxygen. The aim of this study was to investigate the acoustic efficiency of charcoal and design an acoustic diffuser that utilizes charcoal. Samples of different types of tree charcoal—birch (Betula pendula), pine (Pinus sylvestris), and oak (Quercus robur)—with different thicknesses were used for the acoustic efficiency measurements. The sound absorption and sound reflection properties of charcoal were investigated. The bulk density of charcoal was measured. In this study, an impedance tube with two microphones was employed as the measurement method. The results of the impedance tube measurements showed that the charcoal samples had high sound reflection coefficients, the highest value of which was 1. The 50 mm samples of birch had a high bulk density of 473 kg/m3. The sample of 50 mm thick oak had the best reflection coefficient at 0.99. Reflection depended on the surface’s acoustic properties, and the sound reflection coefficient increased with the increase in the density. Charcoal measurements, due to the high reflection coefficient of the material, were used for the design of a sound diffuser, which included wooden perforated plates filled with cylindrical elements of wood charcoal.

The modeling programs provide a wide range of possibilities for simulating acoustic systems. This paper describes programs used in acoustics for various purposes, such as Sound PLAN, AFMG SoundFlow, WinFLAG, Comsol multiphysics, ANSYS, Roomsim. For the purposes of the current research, the acoustic simulation of the room was carried out. Physical parameters as impedance, sound hard boundary and normal velocity were considered. The sound pressure level in rooms was investigated. Possibilities of using Comsol Multiphysics in the research of acoustics were investigated. Results of the current research show high-frequency eigenmodes located in the corners of the room and in the center of the room. Sound pressure level increased from low to medium frequency and then decreased with frequency drifts. At the frequency of 5000 Hz, minimum sound pressure is observed, which is associated with the decrease in the wavelength co-occurring with the decrease in frequency.

Wood is a sustainable renewable material that is widely used in the building sector. Due to recent trends in green initiatives in European and other countries, wood waste is becoming more popular for sound absorption and insulation because of its renewability. Recycled wood material granulated charcoal is an effective acoustic treatment. In this study, we investigated the sound absorption coefficients of raw wood and granulated charcoal species: birch (Betula pendula), pine (Pinus sylvestris), and oak (Quercus robur). The impedance tube method was used to characterize the sound absorption properties of the granulated charcoal. The maximum bulk densities of the materials investigated were 567 kg/m3 for birch (B. pendula) wood and 247 kg/m3 for granulated charcoal. The airflow resistivity of wood and granulated charcoal was found using the static airflow method according to the standard. The grain size distribution of charcoal was also determined, with the highest percentage (25–29%) of the birch and pine grain distribution of charcoal species having a size of 400 µm and 30.7% of the oak a size of 1 mm. All species had the lowest grain size value, i.e., 900 µm (1–4%). The sound absorption of granulated charcoal does not depend on the carbonization temperature at 400, 500, or 600 °C. Granular charcoal of 25 mm has a better sound absorption thickness coefficient for birch at 600 °C (0.57 at 315 Hz) compared to wood of the same thickness. The effectiveness of absorption for wood and granular charcoal depends on the type of wood (birch > pine > oak), grain size, airflow resistivity, and bulk density.

Quadratic residue diffuser (QRD) is a device that distributes the acoustic energy of intense reflections by its spatial and temporal dispersion. The quadratic residue diffuser consists of a series of wells of different depths and the same width. Optimal diffusion is very important for perceiving musical sound and eliminating unwanted acoustic effects e.g. echoes. For the purpose to determine the optimal diffusion, Schroeder used mathematical number sequences allowing to diffuse the sound in a semicircular pattern from the devices. In this paper, the presented quadratic residue diffuser modeling method is presented. Result of theoretical modeling at a frequency of 500 Hz was obtained. The estimated width of the wells is 9.4 cm. The sound scattering and sound diffusion coefficient different number of the diffusor wells were researched. It was obtained that with an increasing number of wells of the diffuser leads to an increment in sound diffusion at the design frequency for diffuser the same width sequences.