This paper presents a detailed analysis of quadcopter noise emitted in different flight regimes and suggests an innovative approach to quadcopter sound detection. Currently, quadcopters have become extremely popular and are used for various applications. In some case scenarios, these applications are not legal; therefore, they represent a threat, and early detection of quadcopters becomes imperative. However, conventional detection with radars and infrared cameras does not provide satisfactory results, since some quadcopters have relatively small dimensions, and their motors have low heat emissions. Thus, this paper focuses on the noise emitted by quadcopters, which is connected to the changing revolutions per minute of the electric quadcopter motors. Thus, the paper connects the unique quadcopter features with the emitted sounds, which can be used for detection in different acoustic environments. The suggested algorithm uses signal processing with a nonlinear function, which results in new spectral lines that correspond to rotational speed differences between the front and rear motors. The analysis shows that the processed signal mixed with a traffic noise signal can be detected when the level reaches 3 dB lower than the level of the surrounding noise.

The dynamics of an experimental electrodynamic loudspeaker is studied by using the tools of chaos theory and time series analysis. Delay time, embedding dimension, fractal dimension, and other empirical quantities are determined from experimental data. Particular attention is paid to issues of stationarity in a system in order to identify sources of uncertainty. Lyapunov exponents and fractal dimension are measured using several independent techniques. Results are compared in order to establish independent confirmation of low dimensional dynamics and a positive dominant Lyapunov exponent. We thus show that the loudspeaker may function as a chaotic system suitable for low dimensional modeling and the application of chaos control techniques.

An electrodynamic loudspeaker has been operated in a nonlinear regime with driving currents I ₀ = 2–4 A. At I ₀ = 4 A an amplitude cut-off was observed at frequency ƒ = 46 Hz. In the next step driving frequencies have been selected in the interval 45–55 Hz, and driving current was then swept until vibration amplitude produced sequences of subharmonics which terminated in the chaotic state. An attempt to explain the chaotic phenomena in terms of forced anharmonic oscillator with elastic stiffness derived from static stress-strain measurement and entering Duffing equation failed. Viscoelastic model proposed by Bennewitz and Rötger was applied, when membrane viscoelasticity and corresponding intrinsic friction term enhance the restoring force of the vibrating system. Such a model supplemented with system parameters and transferred to simulation procedure resulted in a period doubling and bifurcation diagram, which might be brought within the loudspeaker technical performances and qualitative agreements such as would be indicated by observations and measurements.

In this paper, six different acoustic environments with different loudness and sharpness levels were reproduced to two groups of listeners—control and experimental. The difference between the groups was achieved by adding different melodious singing bird sounds to the original recording of a children’s park. Therefore, the recording of the children’s park represents the recording listened to by the control group, while the experimental groups listened to the recordings of a children’s park mixed with different types of bird species. The questionnaire used in this research relies on the semantic differential method implemented by defining bipolar adjective pairs, which are selected in a way that they can significantly describe an auditory and emotional human response to a certain soundscape recording. In analyzing the results, psychological research methodology was used to determine the statistical significance of obtained human feedback, which enabled us to evaluate different acoustic environments appropriately. The calculation and correlation between the objective acoustic parameters (loudness, sharpness, roughness, and fluctuation strength) and subjective questionnaire parameters were carried out. The aim of this paper was to investigate the influence of different singing bird species, which are typical for Eastern Europe in terms of human appraisal and overall well-being, especially for participants residing in urban areas where the decline of biodiversity is already present. The results of this type of study could serve as a guideline for restoring and recovering biodiversity in urban areas across Europe; furthermore, it could benefit residents’ mood and overall health.