Behavior Of Sound Waves Research Articles

Geometric phase and band inversion in periodic acoustic systems

The behaviour of sound waves in phononic crystals—metamaterials with spatially varying acoustic characteristics—is similar to that of electrons in solids. Now, phononic band inversion and Zak phases have been measured for a 1D phononic system.

Helmholtz resonator experiment for Project Listen Up.

The behavior of sound waves and resonant effects can be observed using Helmholtz resonators. Resonators are built from identical round wooden boxes purchased from a craft store. The tops are glued and drilled with hole diameters from 2–3 cm. We can study the resonant behavior as a function of hole diameter only. The end effect is large compared with the thickness of the orifice (0.6 cm). In the experiment, a swept tone (100–900 Hz) from a small speaker drove the resonator. One studied the output of the microphone located near the orifice. One could use a swept spectrum analyzer, a function generator, or tuning fork with varying weights to measure the resonant frequency of the box. A plot of hole diameter versus frequency can be compared with the theoretical Helmholtz resonant frequency prediction, which depends on the volume of the box, the cross sectional area, the sound speed of air, and the effective length of the hole. One can model the effective length of the resonator as the thickness of the top plu...

Underwater sound pressure variation and bottlenose dolphin (Tursiops truncatus) hearing thresholds in a small pool

Studies of underwater hearing are often hampered by the behavior of sound waves in small experimental tanks. At lower frequencies, tank dimensions are often not sufficient for free field conditions, resulting in large spatial variations of sound pressure. These effects may be mitigated somewhat by increasing the frequency bandwidth of the sound stimulus, so effects of multipath interference average out over many frequencies. In this study, acoustic fields and bottlenose dolphin (Tursiops truncatus) hearing thresholds were compared for pure tone and frequency modulated signals. Experiments were conducted in a vinyl-walled, seawater-filled pool approximately 3.7 x 6 x 1.5 m. Acoustic signals were pure tone and linear and sinusoidal frequency modulated tones with bandwidths/modulation depths of 1%, 2%, 5%, 10%, and 20%. Thirteen center frequencies were tested between 1 and 100 kHz. Acoustic fields were measured (without the dolphin present) at three water depths over a 60 x 65 cm grid with a 5-cm spacing. Hearing thresholds were measured using a behavioral response paradigm and up/down staircase technique. The use of FM signals significantly improved the sound field without substantially affecting the measured hearing thresholds.

Acoustic field measurements and bottlenose dolphin hearing thresholds using single-frequency and frequency-modulated tones

Studies of underwater hearing are often hampered by the behavior of sound waves in small experimental tanks. At lower frequencies, tank dimensions are often not sufficient for free-field conditions, resulting in large spatial variations of sound pressure. These effects may be mitigated somewhat by increasing the frequency bandwidth of the sound stimulus, so effects of multipath interference average out over many frequencies. In this study, acoustic fields and bottlenose dolphin (Tursiops truncatus) hearing thresholds were compared for pure-tone and frequency-modulated stimuli. Experiments were conducted in a vinyl-walled, seawater-filled pool approximately 4×5×1.5 m. Sound stimuli consisted of 500-ms tones at 13 carrier frequencies between 1 and 100 kHz. Frequency-modulated stimuli featured both linear and sinusoidal modulating waveforms with 5%, 10%, and 20% bandwidths. Acoustic fields were measured (without the dolphin present) at three depths over a 60×65-cm grid with a 5-cm spacing. Hearing thresholds were measured using a behavioral response paradigm and up/down staircase technique. Frequency-modulated stimuli with a 10% bandwidth resulted in significant improvements to the sound field without substantially affecting the dolphins hearing thresholds. [Work supported by ONR.]

Unexpectedly large surface gravities for acoustic horizons?

Acoustic black holes are fluid-dynamic analogues of generalrelativistic black holes, wherein the behaviour of soundwaves in a moving fluid acts as an analogue for scalar fieldspropagating in a gravitational background. Acoustichorizons, which are intimately related to regions where thespeed of the fluid flow exceeds the local speed of sound,possess many of the properties more normally associated withthe event horizons of general relativity, up to andincluding Hawking radiation. Acoustic black holes havereceived much attention because it would seem to be mucheasier to create an acoustic horizon experimentally than tocreate an event horizon. Here we wish to point outsome potential difficulties (and opportunities) in actuallysetting up an experiment that possesses an acoustic horizon.We show that in zero-viscosity, stationary fluid flow withgeneric boundary conditions, the creation of an acoustichorizon is accompanied by a formally infinite `surfacegravity', and a formally infinite Hawking flux. Only byapplying a suitable non-constant external body force, andfor very specific boundary conditions on the flow, can thesequantities be kept finite. This problem is ameliorated inmore realistic models of the fluid. For instance, addingviscosity always makes the Hawking flux finite (andtypically large), but doing so greatly complicates thebehaviour of the acoustic radiation - viscosity istantamount to explicitly breaking `acoustic Lorentzinvariance'. Thus, this issue represents both a difficultyand an opportunity - acoustic horizons may be somewhatmore difficult to form than naively envisaged, but ifformed, they may be much easier to detect than one would atfirst suppose.

Acoustic design principles

Covers acoustic design from two main aspects: the design of acoustical devices such as microphones, loudspeakers, etc. and the principles for reduction of sound through partitions in rooms, etc. These sections are preceded by a description of the behaviour of sound waves and vibrating bodies and an outline of human response to sound. Finally, special problems in silencing and in the diagnostic use of sound are described