Pedro Cobo

Hybrid passive–active absorption using microperforated panels

A theoretical and experimental study of a proposed hybrid passive–active plane-wave system to provide broadband acoustic absorption is presented. The passive absorber consists of a microperforated panel (MPP), used in place of conventional fibrous materials, in front of an air layer. The active system uses an active transducer (a loudspeaker), an error sensor (a microphone), and an adaptive controller. MPPs are thinner than fibrous materials and provide a better solution in hygienic environments. For two such systems, the dependence of performance on MPP parameters is studied for two control strategies: impedance matching and pressure release. The first condition is found to be better for cases where the acoustic impedance of the system approaches that of air. Otherwise, the pressure-release condition is better, and a wideband solution is the implementation of the active control system up to a frequency where the natural band of the passive system provides higher absorption. Therefore, the use of a low-pa...

Auditory discrimination therapy (ADT) for tinnitus management

Auditory discrimination training (ADT) designs a procedure to increase cortical areas responding to trained frequencies (damaged cochlear areas with cortical misrepresentation) and to shrink the neighboring over-represented ones (tinnitus pitch). In a prospective descriptive study of 27 patients with high frequency tinnitus, the severity of the tinnitus was measured using a visual analog scale (VAS) and the tinnitus handicap inventory (THI). Patients performed a 10-min auditory discrimination task twice a day during one month. Discontinuous 4 kHz pure tones were mixed randomly with short broadband noise sounds through an MP3 system. After the treatment mean VAS scores were reduced from 5.2 to 4.5 (p=0.000) and the THI decreased from 26.2% to 21.3% (p=0.000). Forty percent of the patients had improvement in tinnitus perception (RESP). Comparing the ADT group with a control group showed statistically significant improvement of their tinnitus as assessed by RESP, VAS, and THI.

Cortical reorganisation and tinnitus: principles of auditory discrimination training for tinnitus management

Scientific evidence has proved reorganisation processes in the auditory cortex after sensorineural hearing loss and overstimulation of certain tonotopic cortical areas, as we see in auditory conditioning techniques. Acoustic rehabilitation reduces the impact of these reorganisation changes. Recent theories explain tinnitus mechanisms as a negative consequence of neural plasticity in the central nervous system after a peripheral aggression. Auditory discrimination training (ADT) could partially reverse the wrong changes in tonotopic representation and improve tinnitus. We discuss different studies and their efficacy on tinnitus perception and annoyance. Indications, method, dose and sound strategy need to be implemented.

Active control of the exhaust noise radiated by an enclosed generator

Abstract The aim of this work was to design and implement a hybrid passive/active system to control the noise radiated by a small generator. Passive control is afforded by enclosing the generator with a close-fitting, leaky, rectangular box. The measured Insertion Loss is higher than 20 dB above 500 Hz. Special attention is paid to technical aspects such as air refreshing and temperature inside the enclosure. Low frequency noise escapes from the enclosure through air intake and gas exhaust openings. A single-input single-output (SISO) feedforward active noise control (ANC) system, implemented in a commercially available device, is used to reduce the radiated exhaust noise below 500 Hz. The reference signal to the SISO ANC system is supplied by an accelerometer located on the air filter case of the generator. The error signal is provided by an electret microphone along the exhaust pipe. The control source consists of a high temperature loudspeaker, positioned in a side-branch configuration to avoid direct contact with the exhaust gas. Some harmonics were attenuated more than 30 dB.

Auditory discrimination therapy (ADT) for tinnitus managment: preliminary results

Conclusion. This clinical assay has demonstrated the efficacy of auditory discrimination therapy (ADT) in tinnitus management compared with a waiting-list group. In all, 43% of the ADT patients improved their tinnitus, and its intensity together with its handicap were statistically decreased (EMB rating: B-2). Objective. To describe the effect of sound discrimination training on tinnitus. ADT designs a procedure to increase the cortical representation of trained frequencies (damaged cochlear areas with a secondary reduction of cortical stimulation) and to shrink the neighbouring over-represented ones (corresponding to tinnitus pitch). Study design. This prospective descriptive study included 14 patients with high frequency matched tinnitus. Tinnitus severity was measured according to a visual analogue scale (VAS) and the Tinnitus Handicap Inventory (THI). Patients performed a 10-min auditory discrimination task twice a day for 1 month. Discontinuous 8 kHz pure tones were randomly mixed with 500 ms ‘white noise’ sounds through a MP3 system. ADT group results were compared with a waiting-list group (n=21). Results. In all, 43% of our patients had improvement in their tinnitus. A significant improvement in VAS (p=0.004) and THI mean scores was achieved (p=0.038). Statistical differences between ADT and the waiting-list group have been proved, considering patients’ self-evaluations (p=0.043) and VAS scores (p=0.004). A non-significant reduction of THI was achieved (p=0.113).

Low-frequency absorption using a two-layer system with active control of input impedance

Broadband noise absorption, including low frequencies, may be obtained by a hybrid passive-active two-layer system. A porous layer in front of an air layer provides passive absorption, at medium and high frequencies. Active control of the input impedance of the two-layer system yields absorption at low frequencies. The active control system can implement either pressure-release or impedance-matching conditions. A simple analytical model based upon plane waves propagating in a tube permits the comparison of both control strategies. The results of this simple model show that the pressure-release condition affords higher absorption than the impedance-matching condition for some combinations of geometrical and material parameters. Experimental results corroborate the good performance of the pressure-release condition under the prescribed geometrical setup.

Optimisation of an active control system to reduce the exhaust noise radiated by a small generator

Abstract In a previous paper (Cuesta M, Cobo P. Active control of the exhaust noise radiated by an enclosed generator. Applied Acoustics 2000;61(1):83–94) the authors reported a passive/active system to control the exhaust noise radiated by a small generator. Passive control was afforded by a steel rectangular enclosure lined with a layer of absorbing material. The enclosure, designed to provide the higher Insertion Loss, supplied attenuation higher than 20 dB above 500 Hz. To reduce the noise below 400 Hz, an active control system was designed with one reference, one error input and one control output. Whilst many harmonics in this frequency band were attenuated, the ANC system was unable to reduce some of them. The aim of this paper is to identify the origin of this lack of attenuation and to alleviate it. An exhaustive analysis of both the transfer function between the secondary source and the error sensor (the plant) and the reference signal has been carried out. A new ANC system with improved performance is described which controls all the harmonics until 300 Hz.

Swept-sine noise-induced damage as a hearing loss model for preclinical assays

Mouse models are key tools for studying cochlear alterations in noise-induced hearing loss (NIHL) and for evaluating new therapies. Stimuli used to induce deafness in mice are usually white and octave band noises that include very low frequencies, considering the large mouse auditory range. We designed different sound stimuli, enriched in frequencies up to 20 kHz (“violet” noises) to examine their impact on hearing thresholds and cochlear cytoarchitecture after short exposure. In addition, we developed a cytocochleogram to quantitatively assess the ensuing structural degeneration and its functional correlation. Finally, we used this mouse model and cochleogram procedure to evaluate the potential therapeutic effect of transforming growth factor β1 (TGF-β1) inhibitors P17 and P144 on NIHL. CBA mice were exposed to violet swept-sine noise (VS) with different frequency ranges (2–20 or 9–13 kHz) and levels (105 or 120 dB SPL) for 30 min. Mice were evaluated by auditory brainstem response (ABR) and otoacoustic emission tests prior to and 2, 14 and 28 days after noise exposure. Cochlear pathology was assessed with gross histology; hair cell number was estimated by a stereological counting method. Our results indicate that functional and morphological changes induced by VS depend on the sound level and frequency composition. Partial hearing recovery followed the exposure to 105 dB SPL, whereas permanent cochlear damage resulted from the exposure to 120 dB SPL. Exposure to 9–13 kHz noise caused an auditory threshold shift (TS) in those frequencies that correlated with hair cell loss in the corresponding areas of the cochlea that were spotted on the cytocochleogram. In summary, we present mouse models of NIHL, which depending on the sound properties of the noise, cause different degrees of cochlear damage, and could therefore be used to study molecules which are potential players in hearing loss protection and repair.

Double-Layer Microperforated Panel/Porous Absorber as Liner for Anechoic Closing of the Test Section in Wind Tunnels

This work investigates the use of hybrid double-layer microperforated panel/porous systems as wideband absorbers for applications in the closing of the test area of wind tunnels. For such an application, the absorber should fulfil a twofold requirement: provide absorption in a frequency band ranging from 500 up to 14000 Hz, and the face in contact with the air vein must be free of porous material. The equations for calculating the absorption coefficient at normal incidence allows to predict the performance of such a system as a function of its constitutive parameters. A prototype providing average absorption of 94% in the frequency band (500, 14000) Hz is finally built and validated experimentally in both an impedance tube and a wind tunnel.

Modelling seismic oceanography experiments by using first- and second-order Complex Frequency Shifted Perfectly Matched Layers

This work investigates the ability of modelling seismic oceanography experiments by using underwater acoustic propagation equations. Seismic oceanography tries to retrieve the fine structure of the ocean water masses by processing the acoustic waves reflected in the low-contrast interfaces of fronts, eddies, internal waves or thermohaline intrusions. Since the reflectivity of such interfaces is of order 10 ―3 ―10 ―4 , the absorption capability of the numerical boundaries becomes crucial. Complex Frequency Shifted offers a better alternative to classical Perfectly Matched Layer formulation, but has not yet been extended to acoustic equations. Here, first- and second-order Complex Frequency Shifted Perfectly Matched Layers equations are proposed which can provide reflection coefficients of order 10 ―5 . Therefore, a numerical Finite-Difference Time-Domain (FDTD) scheme combined with the proposed CFS-PML equations is able to model such experiments.