Ing. Vojtěch Jandák, Ph.D.

The article focuses on the determination of the acoustic properties (sound transmission loss, sound absorption and transmission coefficient under acoustic plane wave excitation) of membrane-type of specimens by means of a combination of incident plane wave sound pressure and membrane surface displacement information, measuring the sound pressure with a microphone and the membrane displacement by means of a laser Doppler vibrometer. An overview of known measurement methods and the theoretical background of the proposed so-called mobility-based method (MM) is presented. The proposed method was compared with the conventional methods for sound transmission loss and absorption measurement in the impedance tube, both numerically and experimentally. Finite element model (FEM) simulation results of two single layer membrane samples of different shape configurations were compared, amongst which six different variations of the backing wall termination. Four different approaches to determine the sound transmission loss and two methods to determine sound absorption properties of the membranes were compared. Subsequently, the proposed method was tested in a laboratory environment. The proposed MM method can be possibly used to measure the vibro-acoustic properties of building parts in situ.

The function of libraries has changed considerably in recent decades. This article presents the case study of the acoustic properties of the National Library of Technology (NTK), built in 2009, visited by more than 2000 users daily. From the acoustic point of view, these are coupled spaces, with horizontally structured spaces, which allow for dividing the space into zones according to the function of use. Through their openness, the individual floors can partially be viewed as open-space offices. Moreover, all floors are connected by a common central atrium, which is the cause of the mentioned acoustic coupling. Undesirable and annoying noise is one of the fundamental problems of these spaces. This article deals with the relationship between the number of visitors, noise in a given space and its acoustic parameters and the impact of these parameters on acoustic comfort, which is a compromise between RT, STI and noise. (C) 2020 Elsevier Ltd. All rights reserved.

Only a few bird species are known to produce low-frequency vocalizations. We analyzed the display vocalizations of Western Capercaillie males kept in breeding centers and identified harmonically structured signals with a fundamental frequency of 28.7 +/- 1.2 Hz (25.6-31.6 Hz). These low-frequency components temporally overlap with the Whetting phase (96% of its duration) and they significantly contribute to the distinct vocal expression between individuals. The resulting model of discrimination analysis classified 67.6% vocalizations (63%, cross-validated result) correctly to the specific individual in comparison to the probability by chance of 12.5%. We discuss a possible function of low-frequency components that remains unclear. The occurrence of such low frequencies is surprising as this grouse is substantially smaller than cassowaries (Southern cassowary Casuarius casuarius and Dwarf cassowary Casuarius bennetti), the species that produces similarly low frequencies. Because these low frequency components temporarily overlap with the Whetting phase, they are hardly audible from a distance larger than several meters.

Termites have evolved diverse defence strategies to protect themselves against predators, including a complex alarm communication system based on vibroacoustic and/or chemical signals. In reaction to alarm signals, workers and other vulnerable castes flee away while soldiers, the specialized colony defenders, actively move toward the alarm source. In this study, we investigated the nature of alarm communication in the pest Reticulitermes flavipes. We found that workers and soldiers of R. flavipes respond to various danger stimuli using both vibroacoustic and chemical alarm signals. Among the danger stimuli, the blow of air triggered the strongest response, followed by crushed soldier head and light flash. The crushed soldier heads, which implied the alarm pheromone release, had the longest-lasting effect on the group behaviour, while the responses to other stimuli decreased quickly. We also found evidence of a positive feedback, as the release of alarm pheromones increased the vibratory communication among workers and soldiers. Our study demonstrates that alarm modalities are differentially expressed between castes, and that the response varies according to the nature of stimuli.

This article outlines the design of a piezoelectric line moment actuator used for active structural acoustic control. Actuators produce a dynamic bending moment that appears in the controlled structure resulting from the inertial forces when the attached piezoelectric stripe actuators start to oscillate. The article provides a detailed theoretical analysis necessary for the practical realization of these actuators, including considerations concerning their placement, a crucial factor in the overall system performance. Approximate formulas describing the dependency of the moment amplitude on the frequency and the required electric voltage are derived. Recommendations applicable for the system's design based on both theoretical and empirical results are provided. (C) 2017 Elsevier Ltd. All rights reserved.

This paper describes the preliminary trial of computer keyboards sounds as the first acquaintance with the psychoacoustic listening test. The description of preparation, process and evaluation is presented as well.

This paper deals with an actuator based on piezoelectric bimorphs developed at CTU for applica- tions of ASAC on thin structures. The actuator takes the form of two pairs of bimorphs connected separately, so that the actuator can act in both moment and force configurations simultaneously. The ASAC system using this actuator was tested on a simple structure in the form of a steel strip fixed at both ends. The behavior of the actuator was compared with theoretical assumptions including efficiency of the actuator as a function of the position and power input. A good agreement has been demonstrated between the model results and the measurement.

Studying basal taxa often allows shedding a light on the evolution of advanced representatives. The most basal termite species, Mastotermes darwiniensis, possesses unique morphological and behavioural traits, of which many remain scarcely studied. For these reasons, we conducted a comprehensive study of the alarm communication in this species and compared its components to behavioural modes described in other termites. In M. darwiniensis, the alarm is communicated by substrate-borne vibrations resulting from vertical vibratory movements. Another similar behaviour consists in longitudinal movements, by which the alarm is delivered to other termites in contact with alerted individual. Both these two behavioural modes could be used in synergy to create complex movements. M. darwiniensis also uses chemical alarm signals produced by labial gland secretion, in contrast to Neoisoptera in which this function is fulfilled exclusively by the frontal gland secretion. Moreover, we demonstrated in M. darwiniensis the presence of a positive feedback mechanism thought to occur exclusively in the crown group Termitidae. This positive feedback consists in both oscillatory movements of alerted individuals in response to alarm signals and release of alarm pheromone by excited soldiers. Our results confirm that M. darwiniensis is a remarkable example of mosaic evolution, as it combines many primitive and advanced features, and its alarm communication clearly belongs to the latter category.

The present paper deals with the testing of a force-moment bimorph actuator designed for ASAC applications. The actuator was tested on the third natural mode of a steel strip fixed on both ends baffled in the rigid board. The strip was excited by a point force in its center at a frequency corresponding to the third mode. The presented results show the efficiency of the actuator as a function of the actuator’s working mode. Measurements were realized in an anechoic room to simplify measurement of the radiated sound field. The resulting efficiency was evaluated by means of sound pressure in the position of microphone 0.5 m above the strip. The behavior of the force moment actuator in general working mode is discussed.

The paper describes properties of the pneu-acoustic transducer with variable waveguide. Basic principles of its function and influence of the waveguide length are described and evaluated. Analysis of the transient after opening of the valve is studied using wavelet transform.

Alarm signalling is of paramount importance to communication in all social insects. In termites, vibroacoustic and chemical alarm signalling are bound to operate synergistically but have never been studied simultaneously in a single species. Here, we inspected the functional significance of both communication channels in Constrictotermes cyphergaster (Termitidae: Nasutitermitinae), confirming the hypothesis that these are not exclusive, but rather complementary processes. In natural situations, the alarm predominantly attracts soldiers, which actively search for the source of a disturbance. Laboratory testing revealed that the frontal gland of soldiers produces a rich mixture of terpenoid compounds including an alarm pheromone. Extensive testing led to identification of the alarm pheromone being composed of abundant monoterpene hydrocarbons (1S)-α-pinene and myrcene, along with a minor component, (E)-β-ocimene. The vibratory alarm signalling consists of vibratory movements evidenced as bursts; a series of beats produced predominantly by soldiers. Exposing termite groups to various mixtures containing the alarm pheromone (crushed soldier heads, frontal gland extracts, mixture of all monoterpenes, and the alarm pheromone mixture made of standards) resulted in significantly higher activity in the tested groups and also increased intensity of the vibratory alarm communication, with the responses clearly dose-dependent. Lower doses of the pheromone provoked higher numbers of vibratory signals compared to higher doses. Higher doses induced long-term running of all termites without stops necessary to perform vibratory behaviour. Surprisingly, even crushed worker heads led to low (but significant) increases in the alarm responses, suggesting that other unknown compound in the worker’s headis perceived and answered by termites. Our results demonstrate the existence of different alarm levels in termites.

This paper deals with a recently developed actuator based on piezoelectric bimorphs which are useful for ASAC applications. The actuator takes the form of two channels with pairs of bimorphs. Each of the bimorph transducers in the couple is connected separately so that the actuator can act in both moment and force configurations simultaneously. The control algorithm allows the ratio between the moment and force excitation to be adapted according to the desired mechanical output. The ASAC system using this actuator was tested on a simple structure in the form of a fixed beam. Configurations with maximum attenuation were selected and discussed.

The acoustic behavior of diffusers is usually described by three parameters: sound absorption co-efficient, scattering coefficient and diffusion coefficient. The first two coefficients are measured in a reverberation room and have been standardized for many years. This contribution deals with the measurement of the diffusion coefficient of surfaces in an anechoic room characterizing the directional uniformity of the sound scattering. The diffusion coefficient is defined in ISO 17497-2 issued in 2012. The measurement technique was tested on an MLS diffuser of the 4th order. Measured results were in good conformity with theoretical assumptions.

The paper describes acoustic properties of the pneu-acoustic transducers. Basic acoustic parameters as a sensitivity, frequency response and directivity were evaluated. The simple model of the finite conical horn based on Webster's equation is presented as well.

The paper deals with active structural acoustics control (ASAC) applied to the lightweight structure in order to increase transmission loss at low frequencies. Presented results show efficiency of moment actuators developed especially for ASAC mounted on a double-layer acoustic panel consisting of two metal sheets separated by air gap.

Sufficient transmission loss of lightweight structures is always a challenge, especially at low frequencies. The paper deals with application of the moment actuator developed at CTU specifically for active structural acoustic control (ASAC) to the double-layer structure consisting of two metal sheets separated by an air gap. The efficiency of the actuators and the entire system were experimentally tested using various noise signals, particularly low-frequency broadband noise. A comparison of results obtained from application of actuators to the incident plate and to the radiating plate of the double layer structure is presented. The filtered-x LMS algorithm was used in the experimental part of ASAC with promising results in the frequency range from 60 Hz to 200 Hz.

This paper deals with the measurement of the diffusion coefficient of surfaces in anechoic room characterizing the directional uniformity of the sound scattering. The diffusion coefficient is defined in ISO 17497-2. Measurement technique was tested on MLS diffuser of 4th order. Measured results were in good conformity with theoretical assumptions.

One of the dominant research areas in the field of active structural acoustics control (ASAC) is improvement of transmission loss in lightweight structures. The present paper deals with the application of the moment actuator developed for ASAC on a double-panel structure consisting of two metal sheets separated by an air gap. The first experiments show promising results in the frequency range from 60 Hz to 150 Hz.

The paper describes the design, system arrangement, and results evaluation of a dual- channel feed-forward ASAC experiment. As a testing structure, a three dimensional mock-up (machinery casing) excited by polyharmonic signal was used. Sound pressure level measured by two error microphones was employed as an error criterion. The overall system performance was evaluated by means of the total radiated power measured by intensity scanning. Finally, the results are discussed in detail.

Objective parameters for the evaluation of the Rudolfinum concert hall in Prague, Czech Republic are the focus of the present article. The measured results for Reverberation parameters, Energy parameters, Intelligibility parameters, and Spatial parameters of the building's two halls are presented and discussed including a comparison with recommended values or theory, as well as several unique architectural and acoustical qualities of the halls. The early lateral energy fraction parameter is measured by the intensity probe method discussed in the supplement. The performance is verified by tests in anechoic and reverberant rooms.

The paper deals with active structural acoustic control of the experimental mock-up (baffled thin plate with mixed boundary conditions). The aim of the experiment is to globally reduce radiated sound pressure level by actions of distributed forces and line moments used as secondary actuators.

The paper deals with an effect of a non-equidistant (non-linear) k-space sampling to the particle velocity field reconstructed by the statistically optimized near-field acoustic holography (SONAH).

For suppression of radiated power various kinds of secondary actuators as shakers, piezoelectric stacks or smart materials can be used. In addition to the force actuators moment actuators can be applied. Piezoceramics based line moment actuators are used in presented experiments. Sound radiation of simple two-dimensional vibrating structure excited by mechanical force(s) is controlled by the moment actuators minimizing radiated power. Description of moment actuator as well as tested structure and achieved experimental results are presented.

The paper deals with strategies of active structural acoustic control, method intensively developed within the last decade. Special interest is dedicated to the sound transmission through thin plate (wall). As the secondary control source moment actuators are selected. Active control is focused on control of radiating modes of the plate.

The paper deals with comparison of two sound absorption measurement methods: handheld p-u probe method and standard Kundt's tube measurement. The p-u probe method represents relatively new and unverified approach for which practical employment repeatability and reliability tests are needed.

The paper deals with particular problems of active structural acoustic control of thin plates. As primary excitation, force and incident sound wave was used. The active control system was tested with secondary actuators producing force and line moment.

The paper deals with in situ measurement of sound absorption in small enclosures, e.g. car interior. The handheld device based on p-u intensity sensor was tested. Basic characteristics of the method with respect to its practical usage were examined. Various types of surfaces were measured and results were compared with other measurement approaches. Applicability of the new method was discussed.

This paper deals with the effect of secondary actuator positioning in an active structural acoustics control (ASAC) experiment.

The paper deals with Active Control of Sound Radiation from Vibrating Bodies Using Piezoelectric Actuators

This paper deals with the active structural acoustics control (ASAC) of a thin planar structure vibrating due to an incident sound wave.

This paper deals with the influence of the active structural acoustics control (ASAC) system configuration on the radiation of sound from a vibrating surface.

The true 1/3-octave noise is not feasible. However, an approximate 1/3-octave noise is needful for analysis of the sound field. A concept of approximate expression of 1/3-octave noise, based on a fictional 1/3-octave transient sound, is outlined in this paper. Evidently, this expression is considerably uncertain. In this paper a method of uncertainty evaluation based on analysis of the 1/3-octave packets sequence is outlined.

Článek popisuje měření s modelovou hlavou.

Článek se zabývá návrhem buňky pro aktivní řízení vyzařování a vibrací.

Tento článek se věnuje jak teoretickému tak experimentálnímu stanovení tranzientní aksutické rychlosti. Akustické rychlost je vyjádřena na základě Eulerovi rovnice jako gradient akustického tlaku transientní sférické vlny.

Active control of noise and vibration is a technique suitable mainly for low frequencies for which passive methods are usually inefficient. The approach presented in this paper is based on elementary sources (cells) that change the sound radiation of a structure to the far field by active modification of structure vibrations. The elementary cell uses piezoelectric bending actuators to actuate vibrations of the secondary (active) surface of the cell. Both theoretical and experimental results are presented.

Článek popisuje směrové charakteristiky dvanáctistěnu.