Kristian Jambrošić

Assessment of urban soundscapes with the focus on an architectural installation with musical features

Urban soundscapes at five locations in the city of Zadar were perceptually assessed by on-site surveys and objectively evaluated based on monaural and binaural recordings. All locations were chosen so that they would display auditory and visual diversity as much as possible. The unique sound installation known as the Sea Organ was included as an atypical music-like environment. Typical objective parameters were calculated from the recordings related to the amount of acoustic energy, spectral properties of sound, the amount of fluctuations, and tonal properties. The subjective assessment was done on-site using a common survey for evaluating the properties of sound and visual environment. The results revealed the importance of introducing the context into soundscape research because objective parameters did not show significant correlation with responses obtained from interviewees. Excessive values of certain objective parameters could indicate that a sound environment will be perceived as unpleasant or annoying, but its overall perception depends on how well it agrees with peoples expectations. This was clearly seen for the case of Sea Organ for which the highest values of objective parameters were obtained, but, at the same time, it was evaluated as the most positive sound environment in every aspect.

Student project of building an impedance tube

This paper describes a student project of building an impedance tube for measuring the absorption coefficient using the transfer‐function method, in accordance with the standard ISO 10534‐2. This method is well‐established and has many advantages compared to the older method using standing wave ratio (ISO 10354‐1) in terms of measurement speed and accuracy. For the tube, only inexpensive materials and transducers were used. The tube was designed for the frequency range between 90 and 2000 Hz. In order to achieve this range with one tube, three microphone positions have been used. The resulting absorption coefficient has been calculated using the one‐ and two‐microphone method. Different broadband excitation signals have been used in order to compare their robustness, such as MLS, frequency sweep and white noise. Various problems with the design and construction are addressed and the optimal configuration is discussed.

Reverberation time measuring methods

In this paper different well‐established methods of reverberation time measurement are compared. Furthermore, the results obtained using these methods are compared to the results provided by some additional methods which could serve as an in situ tool if, for any reason, the reverberation time measurements cannot be carried out using the standardized methods. The methods compared in this paper include the standardized methods (EN ISO 3382:2000), namely the impulse response measured with pink noise, exponential sweep, MLS, but also pistol shots of different calibers, balloon bursts, gated external pink noise, and the B&K filtered burst method. In order to make the comparison, the measurements were performed in four acoustically very different spaces ‐ a rather small and well‐damped listening room, a much bigger damped listening room, a rather reverberant atrium, and a large and very reverberant shoebox‐shaped room. The results were evaluated according to signal‐to‐noise ratio criterion as well. Special attention has been given to the influence of room modes on measurement results.

Audio -- Visual Interaction: Multimedia Applications

The entire experience of our environment is obtained by interaction of multiple senses. The most obvious example of such interaction which we encounter in every day life is speech and its comprehension which is affected by vision and hearing. The most significant factor which affects speech comprehensibility during audio-visual interaction is by all means the time synchrony of audio and video notation. The goal of our experiment is to determine the boundaries of sound delay with respect to picture, and vice versa, within which a human being experiences the audio-visual stimuli as synchronous, and to determine to what extent asynchrony of stimuli negatively affects the subjective experience of the observer

Sound Quality Evaluation of Hand-Held Power Tools

Sound quality evaluation in the context of product sound quality has been addressed and investigated on hand-held power tools. Out of those, power drills, hand-held circular saws and jigsaws have been chosen as the products of interest. A series of listening tests has been conducted in order to ascertain how people perceive not only the sound emitted by these devices in operation, but also certain properties of the devices themselves, such as their quality, safety of handling, robustness, proper functioning and others. The values of objective parameters describing the sounds of these devices were obtained. Well known parameters such as loudness, sharpness and roughness were used, and certain new parameters were defined. Linear regression models were made for subjective parameters describing the perception of the devices using relevant objective parameters as an attempt to model the subjective grades obtained through listening tests and to predict such grades for future products to be included in the investigation. Based on individual parameters, a single-number overall grade was defined.

A comparison of impulse‐like sources to be used in reverberation time measurements

As a part of an extensive ongoing research on reverberation time measurements, efforts have been made to find an impulse‐like source that will comply with certain demands concerning its spectral content and sound pressure levels it can provide. Although the relevant standards state that such kind of sources should be avoided due to the lack of measurement repeatability, their use has proved to be necessary due to inability of conventional omni‐directional loudspeakers to provide adequate sound pressure levels, especially at the low end of the frequency range of interest, thereby ensuring sufficient dynamic range. Therefore, the emphasis of this investigation has been made on peak sound pressure levels and the amount of low frequency content each source is able to provide. The investigated sources include 6 mm and 8 mm pistols, firecrackers with different amount of explosive compound and explosive mixtures of acetylene gas.

The velocity of ultrasound propagation through brain tissue at low ultrasound frequencies

The acoustic properties of the brain tissue have been in the focus of much scientific research. These properties determine the possibilities of the application and development of the ultrasound methods, as well as the development of the equipment used by the neurosurgeon during the resection of the brain tissue. The results of the measurements of these acoustic properties done at megahertz frequency range have been published. Although the velocity of ultrasound wave propagation does not directly influence the distribution of the ultrasound field within the cranium, it enables the determination of the specific resistance, which has an important role in the distribution of the ultrasound field within the cranium. Using the published results and the conditions of the measurements that have been done, the velocity of ultrasound propagation for the frequency range at which cavitation ultrasonic surgical aspirators work has been estimated. After that the measurement of the velocity of the propagation has been d...

Improvement of acoustic resistance equations in perforated plate absorbers with thin porous layers

The absorption coefficient of perforated plate absorbers largely depends on the acoustic resistance of the backing layer, adjacent to the plate itself. The optimization process of Helmholtz resonators is focused on the maximization of their absorption coefficients, by tuning the acoustic resistance and reactance of the overall absorbing system. In this paper, the comparison of acoustic resistance of a perforated plate absorber for two characteristic cases is shown: one with a thin porous layer glued tightly to the backing of the perforated plate, and the other with a thin porous layer placed near the perforations, without tight contact. The measurements were done in the Kundts tube, by using four characteristic porosities of the plate (4.4%, 8.5%, 12.9% and 16.9%) and a 2 mm thick porous layer. The porous layer can be simply added to the acoustic resistance in the case of its tight placement to the perforated plate. On the other hand, absorber resistance is much less increased when a loosely placed porous layer is introduced to the perforations, but still shows a positive correlation with the resistance increase in the case of tight placement. Ratios of measured resistances are shown for the mentioned cases, and a range of useful values of those ratios is introduced as an improvement to the analytical expression used for calculating the acoustic impedance. A novel correction factor for calculating acoustic resistance of a perforated plate with porous layer in loose contact is proposed.

The examination of the influence of standing waves on reverberation time measurements in small reverberant rooms

The measurements of reverberation time are performed in a small and rather reverberant shoe‐box type room in order to determine the spatial distribution of reverberation time with respect to the formation of standing waves in the room itself. The measurements are made in a two‐axes raster with equidistant distribution of measurement points. The goal is to investigate the conclusions made in an earlier work by Graber et al. that the distribution of reverberation time values along the fore mentioned raster reaches maximum values at the points of minimum sound pressure level andvice versa. The room in which the measurements are made is deliberately chosen to be relatively small in order to examine the lowest modes of standing waves that form in the room.

The design of narrow-band water loaded ultrasound transmitters

The performance of thin-disk transducers made of PZT-5A ceramic using acoustic matching and air-backing is described using simplified 1-D model and experimental results. Several ultrasound transducers have been designed for studying influence of high power ultrasound energy on tissue. PZT-5A ceramic has been used as active element with thickness approximately lambda/2, depending on working frequency (500 kHz, 1 MHz, 2 MHz) and with three different diameter to thickness ratio (D/t) for obtaining different directivity patterns in radiated medium. Theoretical input impedance, conductance and bandwidth of transducer systems has been compared with experimental results. Theoretical results have been derived using Masons equivalent circuit