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 1998,105th AES Conv, Preprint 4853, 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 outletC. G. Tenorio, A. Bogner, C. Bretthauer, H. Heiss, and G. Schräg, "Physics-Based Compact Modeling of an Ultrasonic Modulation MEMS Speaker Concept," in Proc. 26th Int. Conf. on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems (EuroSimE), IEEE, 2025, pp. 1–7. doi:10.1109/EuroSimE65125.2025.11006558.. 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 microstructuresS. Chen, M. Harteg, and M. Margalit, "Modulated ultrasound speakers – A common framework for analyzing air pump speakers," Sensors and Actuators A: Physical, vol. 381, art. 116086, Jan. 2025. doi:10.1016/j.sna.2024.116086.. 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 M. Klasco, "SonicEdge MEMS Ultrasound Speaker-In-Chip: Next Gen MEMS for TWS Earbuds and Hearing Aids," The Audio Voice, audioXpress, 24 Feb. 2023. https://audioxpress.com/article/sonicedge-mems-ultrasound-speaker-in-chip-next-gen-mems-for-tws-earbuds-and-hearing-aids audioxpress.com]..
Vertical type modulated ultrasound transducers are highly efficient since channel impedance ''Z'' is a strongly nonlinear function of membrane to membrane distance ''h'', : making small membrane displacements highly effective at modulating flow at the ultrasonic carrier frequency S. Chen and M. Margalit, "Comparative analysis of active modulated ultrasound speaker architectures," Physica Scripta, vol. 101, art. 045906, 2026. doi:10.1088/1402-4896/ae3991..
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.
History Parametric Array Speakers were originally developed by the US Navy and Soviet Navy for underwater sonar in the mid-1960s, and were briefly investigated by Japanese researchers in the early 1980s, but these efforts were abandoned due to extremely poor sound quality (high distortion) and substantial system cost. These problems went unsolved until a paper published by Dr. F. Joseph Pompei of the Massachusetts Institute of Technology in 1998 fully described a working device that reduced audible distortion essentially to that of a traditional loudspeaker.
Active modulation was invented by Dr. M. Margalit and first published as US patent US8861752
Products there were known to be five devices which have been marketed that use ultrasound to create an audible beam of sound.
Audio Spotlight F. Joseph Pompei of MIT developed technology he calls the "Audio Spotlight", and made it commercially available in 2000 by his company Holosonics. The system has been the subject of an independent product review in AudioTechnology magazine, which measured the device's frequency and phase response and sound pressure level at varying distances. Disney was among the first major corporations to adopt the technology, using it at the Epcot Center. Audio Spotlight has also been used in out-of-home advertising applications, including digital signage and billboard installations, and was discussed in a 2021 interview with Pompei on the Connecticut Public Radio program Audacious.
Audio Spotlight is a narrow beam of sound that can be controlled with similar precision to light from a spotlight. It uses a beam of ultrasound as a "virtual acoustic source", enabling control of sound distribution. The ultrasound has wavelengths only a few millimeters long which are much smaller than the source, and therefore naturally travel in an extremely narrow beam. The ultrasound, which contains frequencies far outside the range of human hearing, is completely inaudible. But as the ultrasonic beam travels through the air, the inherent properties of the air cause the ultrasound to change shape in a predictable way. This gives rise to frequency components in the audible band, which can be predicted and controlled.
HyperSonic Sound Elwood "Woody" Norris, founder and Chairman of American Technology Corporation (ATC), announced he had successfully created a device which achieved ultrasound transmission of sound in 1996. This device used piezoelectric transducers to send two ultrasonic waves of differing frequencies toward a point, giving the illusion that the audible sound from their interference pattern was originating at that point. ATC named and trademarked their device as "HyperSonic Sound" (HSS). In December 1997, HSS was one of the items in the Best of What's New issue of Popular Science. In December 2002, Popular Science named HyperSonic Sound the best invention of 2002. Norris received the 2005 Lemelson–MIT Prize for his invention of a "hypersonic sound". ATC (now named LRAD Corporation) spun off the technology to Parametric Sound Corporation in September 2010 to focus on their long-range acoustic device (LRAD) products, according to their quarterly reports, press releases, and executive statements.
Mitsubishi Electric Engineering Corporation Mitsubishi apparently offers a sound from ultrasound product named the "MSP-50E" and commercially available from Mitsubishi electrical engineering company.
AudioBeam German audio company Sennheiser Electronic once listed their "AudioBeam" product for about $4,500.AudioBeam There is no indication that the product has been used in any public applications. The product has since been discontinued.
Literature survey The first experimental systems were built over 30 years ago, although these first versions only played simple tones. It was not until much later (see above) that the systems were built for practical listening use.
Experimental ultrasonic nonlinear acoustics A chronological summary of the experimental approaches taken to examine Audio Spotlight systems in the past will be presented here. At the turn of the millennium working versions of an Audio Spotlight capable of reproducing speech and music could be bought from Holosonics, a company founded on Dr. Pompei's work in the MIT Media Lab.
Related topics were researched almost 40 years earlier in the context of underwater acoustics. The first article consisted of a theoretical formulation of the half pressure angle of the demodulated signal. The second article provided an experimental comparison to the theoretical predictions. Both articles were supported by the U.S. Office of Naval Research, specifically for the use of the phenomenon for underwater sonar pulses. The goal of these systems was not high directivity per se, but rather higher usable bandwidth of a typically band-limited transducer.