This article presents a comprehensive acoustic study of paper-based building products: cellulose wool, paperboard, corrugated cardboard, and honeycomb panels. The material configurations included the intact form as well as the various modifications, i.e., density variation, multiple-layered staking, perforation or acoustic metamaterial setup. Tests covered acoustic absorption and insulation properties, with the last examined under excitation of both a plane wave and a diffused field. Additionally, the cellulose wool is provided with the characteristic impedance and propagation wavenumber results; and the paperboard was tested for its dynamic elastic and damping properties. The paper-based products, giving their weight, prove to be a convincing replacement for conventional materials by both absorptive and insulation performance. The maximum acquired sound reduction index, for exceptionally lightweight (2.2 kg/m2) paper double-wall metamaterial structure, reached 26 dB.

Thermoacoustic converters are devices for direct conversion of acoustic energy into thermal energy in the form of temperature difference, or vice versa – for converting thermal energy into an acoustic wave. In the first case, the device is called a thermoacoustic heat pump, in the second – thermoacoustic engine. Thermoacoustic devices can use (or produce) a standing or travelling acoustic wave. This paper describes the construction and properties of a single-stage thermoacoustic engine with a travelling wave. This kind of engine works using the Stirling cycle. It uses gas as a working medium and does not contain any moving parts. The main component of the engine is a regenerator equipped with two heat exchangers. Most commonly, a porous material or a set of metal grids is used as a regenerator. An acoustic wave is created as a result of the temperature difference between a cold and a hot heat exchanger. The influence of working gas, and such parameters as static pressure and temperature at heat exchanger on the thermoacoustic properties of the engine, primarily its efficiency, was investigated. The achieved efficiency was up to 1.4% for air as the working medium, which coincides with the values obtained in other laboratories. The efficiency for argon as working gas is equal to 0.9%.

Monte Carlo Filtering (MCF) is one of the methods of Experimental Statistical Energy Analysis (E-SEA), which allows the correction of negative LFs (Loss Factors). In this article, a modification of the MCF method, called DESA (Diagonal Expansion of the Search Area), is proposed. The technique applies a non-uniform extension of the search area when generating a population of normalized energy matrices. The degree of expansion of the search area is controlled by the Diagonal Penalty Factor (DPF). The authors demonstrated the method’s effectiveness on a system that could not be identified in several frequency bands by the classical MCF method. After applying DESA, it was possible to fill in the problematic bands that were missing CLF (coupling loss factor) and DLF (damping loss factor) values. The paper also proposes a way to minimize the errors introduced by using overly high DPF values.

The paper presents results of research on an influence of listening fatigue on the detection of changes in spectrum and envelope of musical samples. The experiment was carried out under conditions which normally exist in a studio or on the stage when sound material is recorded and/or mixed. The equivalent level of presented sound samples is usually 90 dB and this is an average value of sound level existing in control room at various recording activities. Such musical material may be treated as a noise so Temporary Threshold Shift phenomenon may occur after several sessions and this may lead to a listening fatigue effect. Fourteen subjects participated in the first part of the experiment and all of them have the normal hearing thresholds. The stimuli contained the musical material with introduced changes in sound spectrum up to ±6 dB in low (100 Hz), middle (1 kHz) and high frequency (10 kHz) octave bands. In the second part of research five subjects listened to musical samples with introduced envelope changes up to ±6 dB in interval of 1 s. The time of loud music exposure was 60, 90 and 120 minutes and this material was completely different from the tested samples. It turned out that listening to the music with an L_eq= 90 dB for 1 hour influences the hearing thresholds for middle frequency region (about 1-2 kHz) and this has been reflected in a perception of spectral changes. The perceived peaks/notches of 3 dB have the detection ability at 70% and the changes of low and high ranges of spectrum were perceived at the similar level. After the longer exposure, the thresholds shifted up to 4.5 dB for the all investigated stimuli. It has been also found that hearing fatigue after 1 hour of a listening influences the perception of envelope which gets worse of 2 dB in comparison to the fresh-ear listening. When time of listening to the loud music increases, the changes in envelopes which can be detected rise to the value of 6 dB after 90-minutes exposure and it does not increase with further prolongation of listening time.