In several classroom activities, it is required to feed a signal simultaneously to several headsets to have students listening to the same sound signal, e.g., to show binaural effects. To drive several headphones, an appropriate headphone amplifier is needed. The aim of this work was to build a prototype of a simple headphone amplifier based on a publicly available project around different types of operational amplifiers and test its electrical and acoustical performance when used with typical headphones. Frequency response function, total harmonic distortion, and signal-to-noise ratio were evaluated. Among the different OPAMPs the OPA2134 showed the best performance, especially because of the very flat frequency response function in the hearing range, low harmonic distortion, and superior SNR (−83 dBFS). Acoustical test were therefore performed based on the amplifier with the OPA2134 only, using a Sennheiser HD 600 headphone and a Sennheiser KR4 microphone. THD measured with this configuration ranged from −86 dBC@2.5 kHz and global SPL = 70 dB to −35 dBC@100 Hz and global SPL = 110 dB. At all, the test showed that the amplifier circuit is appropriate for the required application, taking into account that it is very simple and can be mounted easily by the students.

Previous investigations have shown that green roofs provide many environmental benefits, such as thermal conditioning, air cleaning, and rain water absorption. Nevertheless, information regarding acoustic properties, such as sound absorption and transmission loss is still sparse. This work presents measurements of the sound absorption coefficient of two types of green roofs commercially available in Brazil: the alveolar and the hexa system. Measurements were made in a reverberant chamber according to ISO-354 for different variations of both systems: the alveolar system with 2.5 cm of substrate with and without grass and 4 cm of substrate only. The hexa system was measured with layers of 4 and 6 cm of substrate without vegetation and 6 cm of substrate with a layer of vegetation of the sedum type. For all systems, high absorption coefficients were found for medium and high frequency limits (α ≈ 0.7) and low absorption in low frequencies (α ≈ 0.2). This was expected due to the highly porous structure of the substrate. The results suggest that the types of green roofs evaluated in this work could be a good approach to noise control in urban areas.

Comercial hardware compatible with IEPE precision sensors normally are expensive and often coupled to proprietary and expensive software packages. commercially available sound cards are a low cost option for AD, but are incompatible with IEPE sensors. To create 4 mA constant current for IEPE transducers commercial solutions are available and labs also have created such solutions, e.g., ITA at RWTH Aachen University. Unfortunately, commercially available circuits are still to expensive for large scale classroom use in Brazil and circuits created elsewhere contain parts subject to US export restrictions or require machines for creation of circuits. Thus, based on a previous project, a new low-cost prototype was mounted on phenolic board. The circuit was tested with an IEPE microphone connected to a commercial soundcard and ITA-Toolbox software and compared to a commercial hardware/software package. The results were very similar in the frequency range between 20 Hz and 10 kHz. The difference below 20 Hz probably occurs due the different high pass filters in the AD-cards. The differences in the high frequency range are very likely due to differences in the electrical background noise. The results suggest the device works well and is a good alternative to make measurements with IEPE sensors.

Cardiac auscultation can generate important information in the diagnosis of diseases. The sounds that the cardiac system provides are understood in the frequency range of human hearing, but in a region of low sensitivity. This project aims to build a low cost didactic software/hardware set for teaching cardiac auscultation technique in Brazilian universities. The frequencies of interest to describe the human cardiac cycle were found in the range of 20 Hz to 1 kHz which includes low frequencies where available low-cost transducers usually have large errors. To create the system, an optimization of the geometry of the chestpiece is being programmed with finite element simulations; meanwhile, digital filters for specific frequencies of interest and an interface based on MATLAB are being developed. There were needed filters for the gallops (20 to 70 Hz), heart beats (20 to 100 Hz), ejection murmurs (100 to 500 Hz), mitral stenosis (30 to 80 Hz), and regurgitations (200 to 900 Hz). The FEM simulation of a chestpiece demonstrates high signaling levels on the desired frequency range, which can be used with the filters to obtain specific information. Furthermore, the ideal signal recording equipments will be defined, implemented, and tested.