Renata Sisto

Effect of Air Injection on the Far Field Pressure Radiated from a Jet at Subsonic Mach Numbers

This paper deals with an experimental study of the acoustic field resulting from the interaction between the air injected from 6 microjets and the flow issuing from a round jet at low and high subsonic Mach numbers. The displacement and inclination of the microjet axes with respect to the main stream direction, as well as the ratio between the microjets and the main jet flow rates, are varied parametrically and the effect on the far field sound is analyzed. In the low Mach number cases no relevant influence upon the acoustic field is observed. This is probably an effect of the limited number of microjets activated. On the contrary, when the flow becomes highly compressible, a significant alteration of the far field acoustic pressure with respect to the baseline conditions is detected. The most relevant effect is observed when the microjet axes are placed in direction perpendicular to that of the main stream. In these cases the pressure energy is considerably reduced and a significant overall noise abatement, of the order of more than 2dB, is achieved.

On the dependence of the BM gain and phase on the stimulus level

The issue of the relation between the tuning factor, the active gain, and the phase slope of the basilar membrane excitation is addressed. Different 1d-transmission line cochlear models are studied and compared. The active linear models are exactly solved in frequency domain whilst a fully nonlinear model is solved in time domain. An evident dependence of the phase slope on tuning is found in the linear models both in the passive and in the active cases. This dependence is much weaker for the nonlinear model solved in the time domain. This finding suggests that nonlinear behavior near the resonant place could play an important role in explaining the relation between tuning and phase gradient. The otoacoustic emissions generated by a linear backscattering mechanism, TEOAEs and SFOAEs, have also been studied. For a given roughness pattern, for all models, the OAE response was found to consist of a small set of place-fixed emission spots. As tuning increases, the latency of each spot is constant, whereas the...

Introducing causality violation for improved DPOAE component unmixing

The DPOAE response consists of the linear superposition of two components, a nonlinear distortion component generated in the overlap region, and a reflection component generated by roughness in the DP resonant region. Due to approximate scaling symmetry, the DPOAE distortion component has approximately constant phase. As the reflection component may be considered as a SFOAE generated by the forward DP traveling wave, it has rapidly rotating phase, relative to that of its source, which is also equal to the phase of the DPOAE distortion component. This different phase behavior permits effective separation of the DPOAE components (unmixing), using time-domain or time-frequency domain filtering. Departures from scaling symmetry imply fluctuations around zero delay of the distortion component, which may seriously jeopardize the accuracy of these filtering techniques. The differential phase-gradient delay of the reflection component obeys causality requirements, i.e., the delay is positive only, and the fine-structure oscillations of amplitude and phase are correlated to each other, as happens for TEOAEs and SFOAEs relative to their stimulus phase. Performing the inverse Fourier (or wavelet) transform of a modified DPOAE complex spectrum, in which a constant phase function is substituted for the measured one, the time (or time-frequency) distribution shows a peak at (exactly) zero delay and long-latency specular symmetric components, with a modified (positive and negative) delay, which is that relative to that of the distortion component in the original response. Component separation, applied to this symmetrized distribution, becomes insensitive to systematic errors associated with violation of the scaling symmetry in specific frequency ranges.

DPOAE generation dependence on primary frequencies ratio

Two different mechanisms are responsible for the DPOAE generation. The nonlinear distortion wave-fixed mechanism generates the DPOAE Zero-Latency (ZL) component, as a backward traveling wave from the “overlap” region. Linear reflection of the forward DP wave (IDP) generates the DPOAE Long-Latency (LL) component through a place-fixed mechanism. ZL and LL components add up vectorially to generate the DPOAE recorded in the ear canal. The 2f1 − f2 and 2f2 − f1 DPOAE intensity depends on the stimulus level and on the primary frequency ratio r = f2/f1, where f1 and f2 are the primary stimuli frequencies. Here we study the behavior of the ZL and LL DPOAE components as a function of r by both numerical and laboratory experiments, measuring DPAOEs with an equal primary levels (L1 = L2) paradigm in the range [35, 75] dB SPL, with r ranging in [1.1, 1.45]. Numerical simulations of a nonlocal nonlinear model have been performed without cochlear roughness, to suppress the linear reflection mechanism. In this way the m...