Sound from ultrasound refers to the generation of audible sound from modulated ultrasound. Two distinct mechanisms have been demonstrated. In the first, modulated ultrasound propagates through a nonlinear medium, which acts as a passive demodulator, producing audible sound without an active element; This is the principle underlying parametric arrays. In the second, audible sound is created from modulated ultrasound which is demodulated with a mechanically driven structure periodically changing the impedance of the acousic path in the device. This is the principle underlying active modulation.
Parametric array Since the early 1960s, researchers have been experimenting with creating directive low-frequency sound from nonlinear interaction of an aimed beam of ultrasound waves produced by a parametric array using heterodyning. Ultrasound has much shorter wavelengths than audible sound, so that it propagates in a much narrower beam than any normal loudspeaker system using audio frequencies. Most of the work was performed in liquids (for underwater sound use). The first modern device for air acoustic use was created in 1998, and is now known by the trademark name "Audio Spotlight", a term first coined in 1983 by the Japanese researchers who abandoned the technology as infeasible in the mid-1980s. A transducer can be made to project a narrow beam of modulated ultrasound that is powerful enough, at 100 to 110 dBSPL, to substantially change the speed of sound in the air that it passes through. The air within the beam behaves nonlinearly and extracts the modulation signal from the ultrasound, resulting in sound that can be heard only along the path of the beam, or that appears to radiate from any surface that the beam strikes. This technology allows a beam of sound to be projected over a long distance to be heard only in a small well-defined area; for a listener outside the beam the sound pressure decreases substantially. This effect cannot be achieved with conventional loudspeakers, because sound at audible frequencies cannot be focused into such a narrow beam. There are some limitations with this approach. Anything that interrupts the beam will prevent the ultrasound from propagating, like interrupting a spotlight's beam. For this reason, most systems are mounted overhead, like lighting.
Active Modulation A distinct mechanism for generating audible sound from ultrasound employs ''active modulation'' inside the transducer itself, contrasting with the nonlinear self-demodulation of a propagating beam as used in parametric arrays. Rather than relying on the nonlinear properties of the propagation medium, active modulation uses a mechanically driven structure to periodically vary the acoustic impedance of a fluid channel carrying the modulated ultrasound signal resulting in audible sound at the device outlet. Active modulation of modulated ultrasound operates in any viscous fluid and across scales from macroscopic channels to micron-gap microstructures. Because demodulation occurs inside the device rather than during free-space propagation, the approach operates in principle in any viscous fluid and across scales from macroscopic channels to micron-gap microstructures. Actively modulated ultrasound transducers function as ''volume velocity sources' or “pump speakers”': providing constant volumetric flow from constant flow (DC) to ultrasound frequencies. Sound pressure level in an occluded cavity such as an ear canal falls at 20 dB per decade with increasing frequency. The sound pressure of a modulated ultrasound speaker can provide stronger bass and enhanced active noise cancelation but the specific response of an earphone is determined by the acoustic structures and electronic equalization. In a MEMS speaker survey literature actively modulated ultrasound has been referred to ''ultrasound pulse-based'' speakers. The active-modulation principle has been realized in both electrostatic MEMS and piezo electric MEMS. For example, an electrostatic device with a vertical membrane architecture using polysilicon surface micromachining on a silicon substrate. This device is composed of multiple acoustic cell, where each cell has a membrane radius of approximately 40 μm and a distance between membranes of approximately 2 μm; several hundred such cells are tiled on a chip of a few square millimeters, driven by an ultrasonic carrier at approximately 400 kHz and 25 V DC bias and 25 V AC. Frequency-response measurements in an IEC 60318-4 ear simulator confirm the expected audio-band characteristic . Vertical type modulated ultrasound transducers are highly efficient since channel impedance ''Z'' is a strongly nonlinear function of membrane to membrane distance ''h'', : Z = 12 μ O L h 3 + j ω 6 ρ O L 5 h {\displaystyle Z={\frac {12\mu OL}{h^{3}}}+j\omega {\frac {6\rho OL}{5h}}} making small membrane displacements highly effective at modulating flow at the ultrasonic carrier frequency.
Applications
Commercial advertising A sound signal can be aimed so that only a particular passer-by, or somebody very close, can hear it. In commercial applications, such as a museum, self checkout station, bank automated teller machine or trade show display, it can target sound to a single person without the peripheral sound and related noise of a loudspeaker.
Personal audio It can be used for personal audio, either to have sounds audible to only one person, or that which a group wants to listen to. The navigation instructions for example are only interesting for the driver in a car, not for the passengers. Another possibility are future applications for true stereo sound, where one ear does not hear what the other is hearing.
Train signaling device Directional audio train signaling may be accomplished through the use of an ultrasonic beam which will warn of the approach of a train while avoiding the nuisance of loud train signals on surrounding homes and businesses.
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