Patent Publication Number: US-2021162457-A1

Title: A device, system and method for generating an acoustic-potential field of ultrasonic waves

Description:
TECHNICAL FIELD 
     The present disclosure relates to devices, systems and methods for generating an acoustic-potential field of ultrasonic waves. 
     Specifically, the present disclosure relates to generating an acoustic-potential field of ultrasonic waves by devices, systems and methods implemented using micromachined ultrasonic transducer (MUT) technology. 
     BACKGROUND 
     Various interactive haptic technologies exist, which provide a user or users with tactile or kinesthetic information or feedback, often in combination with visual information displayed on an interactive screen. The majority of existing tactile displays directly stimulate the skin. Wearable tactile displays designed to a headband, wrist band, arm band, glove, vest, glasses, or belt enable to receive haptic stimuli passively, for example as disclosed in Van Erp et al., 2005; Kim et al., 2009; Kajimoto et al., 2006; Jones et al., 2006. However, in each of these technologies, a user requires physical contact with a deformable surface, a pen, or a specially adapted glove. Such requirements reduce the usability and spontaneity with which a user may interact with a system. 
     While most haptic displays rely on direct contact with body parts to stimulate the receptors, some haptic actuation principles allow for the generation of non-contact haptic stimulation. Recently, there has been an increased interest in these approaches. For instance, air-jets are a comparatively simple technical solution to generate non-contact haptic feedback. See e.g. Tsalamlal et al., 2013; Kim et al., 2008. However, it is difficult to create complex haptic sensations, and the range of haptic interaction is limited due to dissipation effects. A more advanced strategy is disclosed in Gupta et al., 2013; Sodhi et al., 2013, which discloses pneumatics to create air vortices, based on which haptic feedback is generated over a distance. 
     Another non-contact approach for human perceivable feedback is based on focused ultrasonic waves. The key idea is to employ acoustic radiation pressure to stimulate e.g. the skin (for haptic feedback) at a distance using a two dimensional array of ultrasonic transducers. in such approaches, tactile sensations on human skin can be created by using a phased array of ultrasonic transducers to exert an acoustic radiation force on a target in mid-air. Ultrasonic waves are transmitted by the transducers, with the phase emitted by each transducer adjusted such that the waves arrive concurrently at the target point in order to maximize the acoustic radiation force exerted. 
     Ultrasonic haptic feedback systems create a vibrotactile sensation upon the skin of a user of the system. The focused ultrasound creates enough force at the point of intersection, hereinafter also referred to as the common focal point, to slightly displace the skin of a user. Typically, ultrasonic haptic feedback systems use ultrasound with a frequency at or above 40 kHz, which is above the threshold for receptors in the skin to feel. Therefore, a user can only detect the onset and cessation of such focused ultrasound. In order to provide a sensation that is detectable by the receptors in skin, the focused ultrasound is modulated at a lower frequency, within the detectable range of the receptors. This range is typically from 40 Hz to 500 Hz and the receptors are typically most sensitive/receptive for tactile feedback having frequencies around 200 Hz. An example of related art disclosing method and apparatus for the modulation of an acoustic field for providing haptic feedback using ultrasound is found in the patent document WO 2016/038347 A1. 
     However, this and other existing ultrasonic haptic technologies are still expensive, are inexact/have low resolution and are not scalable to a sufficient degree. They are further limited for use in certain applications for which the hardware devices and systems have been specifically designed to fit into. The high price, the low resolution and the need for major hardware adaptations are clear hinders to ultrasonic haptic technology being commercially available and reaching the broad market. 
     The present invention seeks to mitigate the above mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved haptic feedback system. 
     SUMMARY 
     The following presents a simplified summary of the specification to provide a basic understanding of some aspects of the specification. This summary is not an extensive overview of the specification. It is intended to neither identify key or critical elements of the specification nor delineate any scope particular to any embodiments of the specification, or any scope of the claims. Its sole purpose is to present some concepts of the specification in a simplified form as a prelude to the more detailed description that is presented later. 
     In a first aspect, there is provided system for generating an acoustic-potential field of ultrasonic waves, the system comprising: an array of acoustic micromachined ultrasonic transducer, MUT, elements, the array of acoustic MUT elements being comprised in one or more micromachined ultrasonic transducer, MUT; and a controller being communicably connected to two or more of said acoustic MUT elements in said array, wherein the controller is configured to control each of the two or more acoustic MUT elements to emit a modulated ultrasonic signal comprising a plurality of ultrasound waves towards a common focal point, the plurality of ultrasound waves of the modulated ultrasonic signal each comprising a carrier wave being modulated according to a modulation signal, by: generating a respective drive signal indicative of: the modulation signal; the frequency at which the at least two acoustic MUT elements are to be controlled to emit the respective modulated ultrasonic signal, and a respective phase shift to be applied to the modulated ultrasonic signal of each of the two or more acoustic MUT elements, configured to cause the ultrasound waves of the modulated ultrasonic signals to be constructively combined at the common focal point, so as to generate an acoustic-potential field of ultrasonic waves having a focal volume around the common focal point; and sending the respective drive signal to the acoustic MUT element, wherein the two or more acoustic MUT elements of the array to which the respective drive signals are sent is each configured to: receive the respective drive signal; and emit a respective modulated ultrasonic signal in response to the respective received drive signal, thereby generating an acoustic-potential field of ultrasonic waves having a focal volume around the common focal point. 
     In a second a third aspect, there is provided a corresponding micromachined ultrasonic transducer (MUT), and a corresponding method, for generating an acoustic-potential field of ultrasonic waves. 
     According to a fourth aspect there is provided a computer program loadable into a memory communicatively connected or coupled to at least one data processor, comprising software for executing the method according to any of the embodiments presented herein when the program is run on the at least one data processor. 
     According to a fifth aspect there is provided processor-readable medium, having a program recorded thereon, where the program is to make at least one data processor execute the method according to of any of the embodiments presented herein when the program is loaded into the at least one data processor. 
     Due to the miniaturization of the transducer achieved by embodiments presented herein, and hence the miniaturization of the system in which such transducers are used, embodiments presented herein makes it possible to use actuator microsystems, e.g. enabling in air or non-contact/contactless generation of an ultrasonic acoustic-potential field in many different applications. The miniaturization further allows implementation in devices such as mobile smart phones, tablets, smart televisions, infotainment systems in vehicles etc. Within a vehicle cock-pit or interior non destructive HMI is beneficial from a safety perspective. For example, embodiments presented herein enable a driver assistant system wherein smart HMI on hands, fingers, shoulder or ear can guide and give tactile and/or audio feedback to the driver without taking away attention from the road. Such a system may also be adapted to check the health condition of the driver, by measuring pulse, stress level, etc. Other applications may include, but are not limited to, human machine interfaces (HMIs), such as interactive interface presentation devices, vehicle interior monitoring or control, safety applications, robotics, gaming, virtual reality (VR), augmented reality (AR), immersive (virtual) reality (IR), etc, and machine machine interfaces (MMI) for example for assembly and/or manufacturing of microelectronic. 
     One non-limiting example application where in air human perceivable feedback is very advantageous is in hospital and healthcare settings, or in other public settings, where reduction of touch based human machine interface (HMI) interaction can greatly reduce the risk of contamination via surfaces since there is no need for touching an actual surface or use a glove or other device. Also check-in machine at airports, railway stations, hospitals and the like would for the same reasons benefit from contactless interaction, instead of todays used keyboards or touchscreens. 
     Furthermore, all embodiments presented herein provide the advantages of being very cost effective compared to currently existing non-miniaturized in air human perceivable solutions, due to the use of MUT technology for transducers and systems. 
     Yet another advantage is that a higher resolution and precision is enabled compared to non-miniaturized solutions, since the MUT based solutions presented herein enables acoustic MUT element arrays of more than 400 channels, for example but not limited to 24×24, 36×36, 48×48, 64×64, 96×96 or 128×72 sensors, which cannot be achieved cost effectively or with the same small component or system size using the technology of the existing discrete ultrasonic transducers and systems for providing generation of an ultrasonic acoustic-potential field without the end product being very expensive and therefore not an options to develop, or for consumers to buy. The existing technologies are thereby not scalable to nearly the same degree as the MUT based embodiments presented herein. 
     Furthermore, embodiments with 96×96 or 128×72 MUT elements provide a MUT having almost 10 000 channels. This high resolution cannot be achieve using non miniaturized existing technology. Such a large resolution of the array in turn enables using a greater number of acoustic MUT elements for emitting a respective modulated ultrasonic signal towards each common focal point compared to the existing technologies, which gives improved focus capabilities and a higher maximum energy or acoustic pressure at the focal volume, and/or emitting modulated ultrasonic signals towards an increased number of common focal points, thereby enabling for example more realistic tactile stimulation feedback or auditory feedback, or simultaneous trapping and possibly acoustic levitation and manipulation of numerous objects using a single system. In an “acoustic tweezers” system, as described herein, e.g., it may further be advantageous to use more than two, for example three, four, or possibly more, acoustic lobes for levitation and/or manipulation of an object captured between the acoustic lobes. Thereby, wobbling of the captured object may be avoided or at least significantly reduced, and precision hence increased, especially in cases where the focal volume may be larger than desired where a larger number of acoustic lobes may be used for compensating for this. 
     Yet a further advantage is that devices, systems and methods according to embodiments herein using micromachined components can operate at very low power levels. Further, transducers, systems and methods of the present invention are ideal for always-on applications. 
     Furthermore, various embodiments described herein can provide increased robustness. For instance, transducers, systems and methods using ultrasound may not be or affected, or may be minimally affected, by light, temperature, characteristics of objects/surfaces, and the like. 
     The following description and the drawings set forth certain illustrative aspects of the specification. These aspects are indicative, however, of but a few of the various ways in which the principles of the specification may be employed. Other advantages and novel features of the specification will become apparent from the following detailed description of the specification when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is now to be explained more closely by means of preferred embodiments, which are disclosed as examples, and with reference to the attached drawings. 
         FIGS. 1   a,    1   b  and  1   c  show schematic overviews of a system according to one or more embodiments; 
         FIG. 2  shows a schematic overview of a system according to one or more embodiments; 
         FIG. 3  shows a schematic overview of a micromachined ultrasonic transducer (MUT) according to one or more embodiments; 
         FIGS. 4 and 5  illustrate an array with acoustic MUT elements emitting ultrasonic waves, according to one or more embodiments; 
         FIG. 6  illustrates an array with acoustic MUT elements emitting modulated ultrasonic signals towards a common focal point, according to one or more embodiments; 
         FIG. 7  illustrates a system with acoustic MUT elements emitting modulated ultrasonic signals towards two different common focal points, according to one or more embodiments; 
         FIG. 8  illustrates a system with acoustic MUT elements emitting modulated ultrasonic signals towards two different common focal points and an exemplary HMI application, according to one or more embodiments; 
         FIG. 9  illustrates a system with acoustic MUT elements emitting modulated ultrasonic signals towards two different common focal points and an exemplary application of acoustic trapping, according to one or more embodiments; 
         FIG. 10  is a flow chart of a method according to one or more embodiments; 
         FIG. 11  shows a schematic overview of generation of a modulated ultrasonic signal; 
         FIG. 12  shows a schematic overview of a system with MUTs located on a non-flat surface, according to one or more embodiments; 
         FIGS. 13 a  and 13 b    show schematic overviews of a system according for acoustic levitation and manipulation with standing waves according to one or more embodiments; 
     
    
    
     DETAILED DESCRIPTION 
     Introduction 
     The present disclosure describes a device, system and method for generating an acoustic-potential field of ultrasonic waves, using micromachined ultrasonic transducer (MUT) technology. 
     While a brief overview is provided, certain aspects of the subject disclosure are described or depicted herein for the purposes of illustration and not limitation. Thus, variations of the disclosed embodiments as suggested by the disclosed apparatuses, systems and methodologies are intended to be encompassed within the scope of the subject matter disclosed herein. 
     As a non-limiting example, the MUTs used in embodiments presented herein may be microelectromechanical systems (MEMS) devices, preferably Silicon based MEMS devices. 
     A common focal point in the context of the present disclosure may be defined as having a set of spatial coordinates according to any suitable coordinate system, e.g. as having x, y and z coordinates, spherical coordinates or Euclidian coordinates. 
     System Architecture 
     Turning now to  FIG. 1 a  to 1 c    and  2 , there is shown a system  100  for generating an acoustic-potential field  270  of ultrasonic waves according to embodiments presented herein. The system  100  comprises an array of acoustic micromachined ultrasonic transducer, MUT, elements  210   1 . . . i . The array of acoustic MUT elements  210   1 . . . i  may be a two dimensional array comprised on a planar surface, e.g. as shown in  FIG. 1 , or an array comprised on a curved surface, e.g. as shown in the non-limiting example of  FIG. 12 , or an array comprised on several connected planar or curved surfaces attached to each other at non-zero angles, e.g. as shown in the non-limiting example of  FIG. 13   a.    
     As illustrated in  FIGS. 1 a  to 1 c   , the acoustic MUT elements  210   1 . . . i  in the array may part of, or comprised in, one or more micromachined ultrasonic transducers (MUT)  200 . In  FIG. 1   a,  an embodiment is shown wherein the array of acoustic MUT elements  210   1 . . . i  is built up from a number of MUTs  200   1 . . . j  (in the figure exemplary illustrated as j=3) each comprising numerous acoustic MUT elements  210 . In  FIG. 1   b,  another embodiment is shown wherein the array of acoustic MUT elements  210   1 . . . i  is built up by a single MUT comprising numerous acoustic MUT elements  210 . In  FIG. 1   c,  yet another embodiment is shown wherein the array of acoustic MUT elements  210   1 . . . i  is built up from a number of MUTs  200   1 . . . j  each comprising a single acoustic MUT element. 
     The system  100  further comprises a controller  120  being communicably connected to two or more of the acoustic MUT elements  210  in the array. The controller  120  is configured to control each of the two or more acoustic MUT elements  210  to emit a respective modulated ultrasonic signal  220 , comprising a plurality of ultrasonic waves, towards a common focal point  230 . These two or more acoustic MUT elements  210  that are emit a respective modulated ultrasonic signal  220  may be referred to as actuating acoustic MUT elements  210 . The plurality of ultrasonic waves of the modulated ultrasonic signal  220  each comprises a carrier wave  221  being modulated according to a modulation signal  222 . 
     Depending on the application, the modulation may be generated based on amplitude modulation (AM), e.g. DSBAM, dual side band suppressed Carrie modulation, frequency modulation, pulse width modulation, Square-root Amplitude modulation (SRAM), Single SideBand (SSB) modulation, dual Single SideBand (SSB) modulation, Modified Amplitude Modulation (MAM), and/or any other suitable modulation technique known in the art. For embodiments described herein as relating to contactlessly capturing/trapping, levitating and/or manipulating and object, modulation may comprise modulating with a factor 1, wherein the modulation signal  222  comprises no information or information that the carrier wave  221  is to be modulated with a factor 1. 
     An illustration of an array of acoustic MUT elements  210  emitting such modulated ultrasonic signals  220  at a common focal point  230  is shown in  FIG. 6 . 
     Turning again to the schematic system  100  of  FIG. 2 , the controller  120  is configured to control each of the two or more acoustic MUT elements  210  to emit a modulated ultrasonic signal  220  by, for each of the two or more acoustic MUT elements  210 , generating a respective drive signal S Tx  indicative of:
         the modulation signal  222 ,   the frequency of the carrier wave  221 , and   a respective phase shift to be applied to the modulated ultrasonic signal  220  of each of the two or more acoustic MUT elements  210 , configured to cause the ultrasonic waves of the modulated ultrasonic signals  220  to be constructively combined at the common focal point  230 .       

     The drive signal may also comprise voltages and/or currents needed to enable the two or more acoustic MUT elements to operate. The controller is further configured to, still for each of the acoustic MUT elements  210 , send the respective S Tx  to the acoustic MUT element  210 . The two or more acoustic MUT elements  210  of the array, to which the respective drive signals S Tx  are sent, are in turn each configured to receive the respective drive signal S Tx  and emit a respective modulated ultrasonic signal  220  in response to the respective received drive signal S Tx . 
     As illustrated in  FIG. 6 , thereby an acoustic-potential field  270  of ultrasonic waves is generated around the common focal point  230 , wherein the acoustic-potential field  270  has a focal volume  260  with a certain extension, or spot size, directly around the common focal point  230 . The focal volume  260  may be disc shaped, but its shape depends on the emission directions of the modulated ultrasonic signal  220 . The focal volume, or spot size, should be kept as small as possible to obtain the best possible focus of the emitted acoustic energy. By using MUT technology, preferably MEMS technology, according to embodiments presented herein, enabling the use of higher frequencies/shorter wavelengths, the spot size can be reduced considerably compared to existing non-miniaturized solution. In one or more embodiments, the controller  120  may be configured to control the two or more acoustic MUT elements  210  to emit the respective modulated ultrasonic signal  220  with an electro-mechanical resonance frequency of the MUT element  210  generating the modulated ultrasonic signal  220  in the range of 20 kHz to 10 MHz, preferably in the range of 100 kHz to 500 kHz, more preferably in the range of 120 kHz to 350 kHz. This advantageously provides a small maximum cross-section diameter of the focal volume  260 , or in other words a small spot size and a large output effect within the focal volume  260 . Expressed in another manner, the controller  120  may be configured to control the two or more acoustic MUT elements  210  to emit the respective modulated ultrasonic signal  220  with a carrier wave  221  having a frequency configured to generate a focal volume  260  with a maximum cross-section diameter, or spot size, of less than 5 mm, preferably less than 3 mm, which is obtained at approximately 120 kHz electro-mechanical resonance frequency of the MUT element  210  generating the modulated ultrasonic signal  220 , or higher, more preferably less than 1 mm which is obtained at approximately 340 kHz electro-mechanical resonance frequency of the MUT element  210  generating the modulated ultrasonic signal  220 , or higher. Depending on the application, different electro-mechanical resonance frequencies are desirable. For example, for haptic stimulation feedback at a decimeter range (including 5 cm to 50 cm) distance from the system  100 , the an electro-mechanical resonance frequency should be controlled to around 200 kHz for the best possible user experience of the feedback. In another example, for auditory stimulation feedback at a meter range (including 50 cm to 200 cm) distance from the system  100 , the an electro-mechanical resonance frequency should be controlled to around 100 kHz for the best possible user experience of the feedback. In yet another example, for acoustic trapping, enabling non-contact levitation and/or manipulation, of objects of less than a millimeter up to a centimeter in diameter or maximum cross section width, the electro-mechanical resonance frequency should be controlled to above 500 kHz to obtain a sufficiently small focal volume  260  or node  1140  to capture/trap the object and enable the acoustic levitation and/or manipulation. 
     Of course, the controller  120  may be configured to control the two or more acoustic MUT elements  210  of the array to emit respective modulated ultrasonic signals  220  towards multiple common focal points  230   1 . . . k , wherein k represents the maximum number of common focal points that a particular system  100  is capable of generating during operation. In  FIGS. 7 and 8 , emission of modulated ultrasonic signals  220  towards two common focal points  230  is shown, for ease of illustration. The emitting acoustic MUT elements may be referred to as actuating acoustic MUT elements. 
     In the example of  FIG. 7 , emission of modulated ultrasonic signals  220  towards two common focal points is achieved by a first group of two or more acoustic MUT elements  210  in the array (in this example number  210   5 ,  210   6 ,  210   9 ,  210   10 ,  210   13  and  210   14 ) emits a respective modulated ultrasonic signal  220  (here  220   5 ,  220   6 ,  220   9 ,  220   10 ,  220   13  and  220   14 ) at a first common focal point  230   1 , and a second group of two or more acoustic MUT elements  210  in the array (in this example number  210   7 ,  210   8 ,  210   11 ,  210   12 ,  210   15  and  210   16 ) emits a respective modulated ultrasonic signal  220  (here  220   7 ,  220   8 ,  220   11 ,  220   12 ,  220   15  and  220   16 ) at a second common focal point  230   2 . 
     In the example of  FIG. 8 , each of the actuating acoustic MUT elements  210  in the array (in this example number  210   1 ,  210   2 ,  210   5 ,  210   6 ,  210   9 ,  210   10 ,  210   13  and  210   14 ) emits a respective modulated ultrasonic signal  220  (here  220   1 ,  220   2 ,  220   5 ,  220   6 ,  220   9 ,  220   10 ,  220   13  and  220   14 ) at both a first common focal point  230   1  and a second common focal point  230   2 . This may be achieved by each of the actuating acoustic MUT elements  210  emitting it&#39;s respective modulated ultrasonic signal  220  towards the a first common focal point  230   1 , using a first beamforming or modulation setting, and at a second common focal point  230   2 , using a first beamforming or modulation setting, at alternating time instances. This way, the modulated ultrasonic signal  220  of each actuating acoustic MUT elements  210  will reach both the first common focal point  230   1  and the second common focal point  230   2  with maximum energy. If there is a large number of common focus points, the time between emissions to each common focal point  230  will of course be longer. Alternatively, emission towards the two (or more) common focal point  230   1 ,  230   2  may be achieved by each of the actuating acoustic MUT elements  210  emitting it&#39;s respective modulated ultrasonic signal  220  towards the common focal points  230  simultaneously. Thereby there is no time delay, but the energy emitted towards each common focal point  230  is instead reduced due to the spatial division. 
     The acoustic-potential field  270  of ultrasonic waves may be used for, and the system hence be adapted for providing, non-contact or in-air human perceivable feedback to a user in a human machine interface (HMI). As some non-limiting examples, the system  100  may be adapted for providing non-contact human perceivable feedback to a user in a cockpit of a motorized vehicle, wherein the vehicle may be a car, a truck, a bus, an airplane or a train, and the user may correspondingly be a driver, pilot, co-driver, co-pilot or passenger of the motorized vehicle. In another non-limiting example, the system  100  may be adapted for providing non-contact human perceivable feedback to a user in a robotics application. In some embodiments, the controller  120  may configured to control each of the two or more acoustic MUT elements  210  to emit the modulated ultrasonic signal  220  at a frequency within a frequency spectrum configured to produce a human perceivable sensory stimuli, so as to produce human perceivable sensory feedback if sensed by a human sensory organ at the common focal point  230 . In some of these embodiments, the controller  120  may be configured to control each of the two or more acoustic MUT elements  210  to emit the modulated ultrasonic signal  220  at a frequency within a frequency spectrum configured to produce a human perceivable tactile stimuli, so as to produce human perceivable tactile feedback if sensed by a human tactile sensory organ at the common focal point  230 . This means that a human will feel the ultrasonic energy focused at the common focal point  230  on the as a tactile stimulation of the skin. An example is shown in  FIG. 8 , wherein the system  100  is used in an HMI to enable a user to receive tactile feedback stimulation on a thumb  701  at a first common focal point  230   1  and tactile feedback stimulation on an index finger  702  at a second common focal point  230   2 . By adding further common focus points  230  and/or swiftly switching between numerous common focus points, enabled thanks to the high frequency of the respective carrier wave  221 , according to embodiments herein, a great number of feedback points, or even the sensory experience of touching a surface or three-dimensional volume, in air, may be achieved. To enable the sensory experience of touching a surface or three-dimensional volume, in air, multiple common focus points  230  are needed. In other embodiments, or in combination with enabling tactile stimulation feedback, the controller  120  may be configured to control two or more acoustic MUT elements  210  to emit the modulation signal  222  at a frequency within a frequency spectrum configured to produce a human perceivable auditory stimuli when the ultrasonic waves of the modulated ultrasonic signals  220  converge at the common focal point  230 . Of course, tactile stimulation feedback and auditory stimulation feedback may be provided at different common focal points  230 , wherein the auditory stimulation is preferably directed at a common focal point close to an ear drum of the user, and the tactile stimulation feedback at a point where it can be felt on the skin of the user. 
     In some embodiments, the at least one MUT  200  and the controller  120 , as well as any optional component that may additionally be comprised in the system, may be fixedly arranged on a substrate  110 . In a preferred embodiment, the at least one MUT  200  and the controller  120 , as well as any optional component that may additionally be comprised in the system, may alternatively be embedded in a substrate  110 . This is advantageous since the resulting transducer will have a substantially flat surface without protrusions, possibly have a smaller height than a transducer where the components are arranged on the substrate, and the components are better secured to the substrate. In one or more embodiment, the at least one MUT  200  and the controller  120  may have been embedded in the substrate  110  and electrically connected using a fan out wafer level processing (FOWLP) technique, or a fan out panel level processing (FOPLP) technique, or the like, whereby the electrical connections between the system components are less prone to breaking, since they do not have any protruding parts. The substrate  110  for embedding the components may consist substantially of an epoxy material. In other embodiments, the substrate may consist substantially of silicon. 
     The substrate  110  and/or the resulting system  100  may be flat, as illustrated in  FIGS. 3 to 9 . In other embodiments, the substrate  110  and/or the resulting system  100  may have a non-flat shape that enables provision of a common focal point for the emitted ultrasonic signals  220  by mechanical beam-forming, i.e. due to the placements and direction of the MUTs in relation to each other, instead of electrical beam-forming, for example beam-forming achieved according to embodiments described herein using phase shift or time shift of emitted ultrasonic signals. An example of a non-flat surface or system, in this case in the shape of a dome, is shown in  FIG. 12 . Other non-limiting examples are to provide a bendable substrate  110 , or to provide a system  100  comprised of a number of flat surfaces combined at non-zero angles in relation to each other, for example but not limited to the shape of a box, as exemplified in  FIG. 13 a   . The non-limiting example of a dome or a whole or part of a substantially spherical enclosure is advantageous because the ultrasonic pressure at the common focus point or human perceivable feedback point is the same from all emitting acoustic MUT elements  210 , since the distance is the same, within an allowable tolerance, between the one or more common focus point  230  and each of the acoustic MUT elements  210 . 
     In combination with any of the embodiments presented herein for non-flat surface  110  or system  100 , electrical and mechanical beam-forming may be used in combination to achieve a greater number of common focus points  230 . 
     In some embodiments, the non-contact ultrasonic feedback system  100  is further configured to recognize a gesture using ultrasonic gesture recognition, and generate the acoustic-potential field  270  of ultrasonic waves in response the recognized gesture. The controller  120  may in these embodiments be configured to receive a detection signal S Rx  from at least one acoustic MUT element  210  of at least one in said two dimensional array; identify a gesture based on the received detection signal or signals S Rx ; and generate the respective drive signal S Tx  in response to identifying a gesture. 
     Turning again to  FIG. 2 , details of the controller  120  according to one or more embodiments are shown. As shown in the figure, the controller  120  may comprise an analogue frontend block  122  configured to constitute a first interface towards the acoustic MUT elements  210   1 . . . i ; an internal digital processing block  126  comprising digital processing circuitry being communicably connected to the analogue frontend block  122 ; a second interface  124  towards an external processor  130 , configured to communicate information and signals between the external processor  130  and the analogue frontend  122 ; a modulation block  127  configured to generate a modulation signal  222  and communicate the modulation signal  222  to the analogue frontend block  122 . The modulation block  127  may be integrated in the digital processing block  126 , or be implemented as a separate block or component. The controller  120  may further comprise a support block  128  communicably connected to the analogue frontend block  122 , the support block being configured to generate oscillations at a frequency corresponding to the frequency of at which the at least two acoustic MUT elements  210  are to be controlled to emit the respective modulated ultrasonic signal  220 , and voltages or currents needed to enable the at least two acoustic MUT elements  210  to operate. In some embodiments, the support block  128  comprises an oscillator  129  configured to be controlled to oscillate at different frequencies, for example in the form of a clock generator; and a power block in the form of a battery  123 , or a power management unit  125  communicably and electrically connected to a power source  140 , wherein the power source  140  is external to the system  100  or integrated in the system  100 . The battery  123  or power source  140 , via the power management unit  125 , is then configured to provide power to the at least two acoustic MUT elements  210 . 
     In  FIG. 11 , modulation of carrier waves  221 , which have been generated according to a frequency provided by the support block  128 , is illustrated. When applicable, the controller  120  may be configured to control the acoustic MUT elements  210  in an array of acoustic MUT elements to emit their respective carrier waves  221  also at different amplitudes. In these cases, the applicable amplitudes are also provided by the support block  128 . The carrier waves  221  are phase shifted using the time delays T 0  to T d . Thereafter, a modulation signal  222  according to any of the embodiments presented herein is applied to each respective carrier wave to obtain the modulated ultrasonic signals  220 . At one or more common focal points  230 , the modulated ultrasonic signals  220  are constructively combined. This is illustrated in  FIG. 11  by the higher amplitude combined signal  240 . In one or more embodiments, the analogue frontend block  122  may be configured to generate the drive signal S Tx  based on the modulation signal  222  received from the modulation block  127 , and the frequency at which the at least two acoustic MUT elements  210  are to be controlled to emit the respective modulated ultrasonic signal  220 , and voltages and/or currents, received from the support block  128 . The analogue frontend block  122  may be implemented in an analogue circuit, more preferably in an application specific integrated circuit, ASIC. The internal digital processing block  126  may be implemented in a field-programmable gate array (FPGA) or a digital signal processor (DSP). In some embodiments, the analogue frontend block  122  and the digital processing block  126  are implemented in a single mixed signal analogue/digital ASIC. 
     In different embodiments, the blocks of the controller  120  may be implemented in the form of several discrete parallel connected electrical components, or with some or all blocks implemented as a single component. By combining several blocks in one component, for example in an embodiment wherein only one or two ASICS comprise all of the blocks of the controller  120 , the total size of the system  100  will be no more than one or a couple of centimeters in length and width respectively, which enables use in devices such as displays, mobile phones, smart television apparatuses, etc. At the same time, the number of available channels is substantially higher than what is enabled by existing non-miniaturized ultrasonic technologies. 
     Embodiments disclosed herein may advantageously also be used in machine-machine interface (MMI) applications, wherein two dimensional arrays of MUT elements according to embodiments herein may be used for enabling contact-less levitation and manipulation of very small object being on a millimeter scale or even micro meter scale, by generating an acoustic-potential field of standing waves of correspondingly small wavelengths, such that one or more objects may be captured, withheld, and three dimensionally translated and/or rotated in a node created between two or more standing waves. A suitable size for objects to be manipulated is objects whose diameter is half of the wavelength of the ultrasonic emitted. Existing technology cannot achieve the high frequencies that are emitted by MUTs according to embodiments presented herein, and therefore cannot be used for manipulating correspondingly small objects. Also, objects with a diameter size of for example than 0.5 mm, which can be manipulated successfully by disclosed embodiments, are too small to be moved using mechanical tweezers or similar tools. Levitation and manipulation is sometimes popularly referred to as using “tractor beams”. Levitation and manipulation of very small objects may for example advantageously facilitate “pick-and-place” contact-less mounting of electronic components during a manufacturing process, precision robotics, and other applications wherein precision is key, and the object(s) to be manipulated is/are on a millimeter scale or even micro meter scale. Another non-limiting example embodiment is sorting or arranging of such small objects. A further non-limiting example embodiment is manipulation of one or more objects, wherein each object (e.g. the object  1110  described in connection with the embodiments of  FIGS. 9 and 13   b ) comprises a small amount of fluid, i.e. a droplet. Manipulating objects in the form of small amounts of a fluid may be very useful in applications of highly controlled and/or sensitive environments where the fluid, should it not be manipulated, may cause problems. The manipulation may in such cases comprise changing the shape and/or the speed of the object. Applications where this is useful include, but are not limited to, “pick-and-place” contact-less mounting of electronic components during a manufacturing process, or precision robotics, wherein a droplet of ink, glue, solder etc. may be slowed down and/or manipulated into a suitable shape either when jetted from a nozzle or at or after coming into contact with a surface. An example of a system for generating an acoustic-potential field  270  of ultrasonic waves and enabling three dimensional acoustic levitation and manipulation of one or more objects using the acoustic-potential field  270  of ultrasonic waves is shown in  FIGS. 13 a  and 13 b   , wherein  FIG. 13 b    shows a top-view of the schematic illustration in  FIG. 13   a.    
     To enable three dimensional manipulation, four or more MUTs  200   1 . . . j , in the figures non-limitingly illustrated as four MUTs  200   1 ,  200   2 ,  200   3 ,  200   4 , may be arranged on the inside of a three dimensional volume. In  FIG. 10 a    the three-dimensional volume illustrated as a box  1100 . For a person skilled in the art it is obvious that the embodiments presented herein may just as well be applied to volumes of other shapes, for example enclosed by spherical or at least partly curved surfaces. In one or more embodiment, the MUTs are arranged such that a first common focal line of ultrasound is generated, using at least one first MUT  200   1  and at least one second MUT  200   2  to provide a first beam  1120  of standing waves between the first and the second MUTs  200   1 ,  200   2 , wherein the first MUT and the second MUT are opposite each other along a first axis A 1 ; and such that a second common focal line of ultrasound is generated, using at least one third MUT  200   3  and at least one fourth MUT  200   4  to provide a second beam  1130  of standing waves between the third and the fourth MUTs  200   3 ,  200   4 , wherein the third MUT and the fourth MUT are opposite each other along a second axis A 2  perpendicular to the first axis A 1 . The MUTs are further arranged such that the first and second beams of standing waves intersect and provide, at the intersection of the standing waves, a node  1140  having a minimum sound pressure value at which an object  1110  of an appropriate size, with regard to the wavelength of the ultrasonic signal, and located in the common intersection point can be trapped or captured, withheld, and translated and/or rotated in three dimensions by modulation of the ultrasonic signals emitted from the MUTs  200   1 ,  200   2 ,  200   3 ,  200   4 . 
     Additional pairs of MUTs  200  may also be used, arranged on the inside of the three dimensional volume so as to generate additional common focal lines of ultrasound and correspondingly additional beams of standing waves within the three dimensional volume. One or more additional MUT pairs may be arranged such that the corresponding one or more additional standing wave intersect at the common intersection point of the first and second beams of standing waves, thereby contributing to generating the node  1140 , and improving the entrapment and enabled acoustic levitation and manipulation of the object  1110  by trapping it from several directions and at higher combined surrounding sound pressure. Thereby, wobbling of the object  1110  may be advantageously avoided. Alternatively, or in combination, two or more additional MUT pairs may be arranged such that the corresponding two or more additional standing wave intersect at one or more additional common intersection point, thereby generating an additional node or nodes configured to simultaneously entrap and acoustically levitate and manipulate an additional object or objects in the acoustic-potential field  270  of ultrasonic waves within the three dimensional volume  1100 . 
     Another example of a system for generating an acoustic-potential field  270  of ultrasonic waves and enabling three dimensional acoustic levitation and manipulation of one or more objects using the acoustic-potential field  270  of ultrasonic waves is shown in  FIG. 9 . 
     In embodiments shown in  FIG. 9 , the respective phase shift to be applied to the modulated ultrasonic signal  220  of each of the two or more acoustic MUT elements  210  is configured to cause the ultrasound waves of the modulated ultrasonic signals  220  to be constructively combined at k common focal points  230   1 . . . k  so as to generate an acoustic-potential field  270  comprising k acoustic lobes  250   1 . . . k  around the respective k common focal points  230   1 . . . k . The system may be adapted to provide contact-less levitation or manipulation of one or more objects, in the figure exemplified by a single object  1110  for ease of illustration, by capturing the object/objects to be captured in a respective node at a sound pressure minima created between two or more acoustic lobes  250   1 . . . k  of the generated acoustic-potential field. As described herein, the MUT technology, preferably MEMS technology, used in embodiments of the invention enables capturing, acoustic levitation and manipulation of very small objects, e.g. having a maximum cross-section width of less than 3 mm, preferably less than 1 mm, more preferably less than 0.5 mm. The system may be implemented as part of a machine-machine interface, MMI. 
     In some embodiments, instead of simultaneously generating the two or more acoustic lobes  250   1 . . . k  of the acoustic-potential field, i.e. spatially dividing the emitted energy, the energy may be temporally divided at a very high rate between the two or more acoustic lobes  250   1 . . . k  to generate e.g. two, three or four (or more, if suitable) differently positioned common focal points  230   1 . . . k . In other word, the emitted energy and the corresponding differently positioned common focal points  230   1 . . . k  are rapidly multiplexed. In the manner described above, the position of the differently positioned common focal points  230   1 . . . k  is controlled such that one or more sound pressure minima, wherein one or more object may be captured by the system, is created between pairs or groups of the differently positioned common focal points  230   1 . . . k . In such embodiments, the system may be adapted to provide contact-less levitation or manipulation of one or more objects, in the figure exemplified by a single object  1110  for ease of illustration, by capturing the object/objects to be captured in a respective node at a sound pressure minima created between an acoustic lobe  250  rapidly moving between two or more common focal points  230   1 . . . k , or a sound minima created between one or more acoustic lobe  250  being still in combination with one or more such rapidly moving acoustic lobe  250 . 
     In all embodiments, the acoustic lobe or lobes  250  are controlled to generate at least one acoustic node at a corresponding sound pressure minimum. To achieve a multiplexing acoustic lobe  250 , the controller  120  may be configured to, and the method according to any embodiment presented herein may comprise, controlling each of the two or more acoustic MUT elements  210  to sequentially emit a respective ultrasonic signal  220 , comprising a plurality of ultrasonic waves, towards two or more common focal points  230   1 . . . k , wherein the location of the two or more common focal points  230   1 . . . k  are selected such that at least one sound pressure minimum is created between the two or more acoustic lobes  250   1 . . . k  which are generated at the respective two or more common focal points  230   1 . . . k . The rate at which the emitted energy and the corresponding differently positioned common focal points  230   1 . . . k  are multiplexed is somewhere between 50 Hz and 5000 Hz, preferably between 100 Hz and 500 Hz, in a non-limiting example 200 Hz or close to 200 Hz. Since the acoustic lobes  250   1 . . . k , surrounding and creating between them the sound pressure minima, are generated in such rapid succession, the contactless capturing/trapping, levitation and/or manipulation of the object  1110  is enabled. 
     The advantage of dividing the emitted energy in time (multiplexing) is that less total energy is needed to achieve contactless capturing/trapping, levitation and/or manipulation of an object of a certain weight. 
     Division of emitted energy in time (multiplexing) instead of in space, or the combination of both, is also applicable to the embodiments described in connection with  FIGS. 13 a    and  13   b.    
     By the embodiments described herein, a dual trap, a quadruple trap, a vortex trap, or any other suitable formation may be generated for the purpose of capturing/trapping and levitating and/or manipulating one or more object. 
     In one or more embodiments, each of the one or more MUT in the system  100  may be a MUT  200  according to any of the embodiments presented in connection with  FIG. 3 . 
     The system  100  may further be adapted to perform the method according to any of the embodiments described in connection with  FIG. 10 . 
     It is to be appreciated that system  100  can be used in connection with implementing one or more systems or components shown and described in connection with other figures disclosed herein. It is noted that all or some aspects of system  100  can be comprised in larger systems such as servers, computing devices, smart phones, tablet computers, laptop computers, personal digital assistants, set top box, computer monitors, remote controllers, headphones, and the like. Further, it is noted that the embodiments can comprise additional components not shown for sake of brevity. 
     Furthermore, the controller  120  can control various circuitry, components, and the like, to facilitate proximity detection. For instance, the controller  120  can comprise a processing device (e.g., computer processor that controls generation of signals, modes of operation and the like. 
     Additionally, embodiments disclosed herein may be comprised in larger systems or apparatuses. For instance, aspects of this disclosure can be employed in smart televisions, smart phones or other cellular phones, tablet computers, laptop computers, desktop computers, monitors, digital recording devices, appliances, home electronics, gaming devices, automotive devices, personal electronic equipment, medical devices, industrial systems, robots, VR or AR or IR wearables, and various other devices or fields. 
     In one or more embodiment, the one or more MUTs  200  may piezoelectric micromachined ultrasonic transducers, p-MUTs, and the acoustic MUT elements  210  may be acoustic p-MUT elements. The piezoelectric material may comprise Lead zirconate titanate, PZT, or doped PZT. The piezoelectric material may comprise thin film PZT, or thin film doped PZT. In some embodiments, the piezoelectric material may comprise PNZT, or PZT doped with Niobium (Nb). In some embodiments, the PZT material may have a thickness of 0.5-5 μm, preferably 1-3 μm, more preferably around 2 μm. 
     In one or more embodiments, each acoustic MUT element  210  may comprise a membrane or a diaphragm. The membrane or diaphragm may further comprise silicon. 
     In other embodiments, the one or more MUTs  200  may be capacitive micromachined ultrasonic transducers, c-MUTs, and the acoustic MUT elements may be 210 acoustic c-MUT elements. 
     According to any of the embodiments presented herein, the array may comprise at least 400 acoustic MUT elements  210 . As some non-limiting examples, the array may comprise 96×96 or 128×72 acoustic MUT elements  210 . 
     The components of the system  100  may be configured to use any suitable communication technology known in the art for communicating with each other. 
     MUT Architecture 
     In the present context, a MUT is to be understood as a micromachined device capable of receiving as well as emitting an ultrasonic pulse or signal in the form of ultrasonic waves. The transducer or MUT  200  comprises an array of acoustic MUT elements  210   1 . . . i , for example as shown in  FIG. 3 . In the example of  FIG. 3 , the array is illustrated with an x-y row-column coordinate system for ease of illustration, but other layouts are equally possible. 
     Below, a micromachined ultrasonic transducer (MUT) according to embodiments is described in more detail, with reference to the figures. 
     Turning to  FIG. 3 , there is shown a device, in the form of a MUT  200  for generating an acoustic-potential field of ultrasonic waves, the MUT  200  comprising an array of acoustic micromachined ultrasonic transducer, MUT, elements  210 . Two or more of the acoustic MUT elements  210  in the array may each be configured to emit a respective modulated ultrasonic signal  220  comprising a plurality of ultrasound waves towards a common focal point  230 , the plurality of ultrasound waves of the modulated ultrasonic signal  220  each comprising a carrier wave  221  being modulated according to a modulation signal  222 . The two or more acoustic MUT elements  210  may further each be configured to emit the respective modulated ultrasonic signal  220  according to a respective predetermined phase shift, such that the respective modulated ultrasonic signal  220  of each of the two or more acoustic MUT elements  210  is phase shifted in relation to one another, so as to be constructively combined at the common focal point  230 , thereby generating an acoustic-potential field  270  of ultrasonic waves having a focal volume  260  around the common focal point  230 . 
     In some embodiments, the two or more acoustic MUT elements  210  are each configured to emit the respective modulated ultrasonic signal  220  at an electro-mechanical resonance frequency, of the MUT element  210 , in the range of 20 kHz to 10 MHz, preferably in the range of 100 kHz to 500 kHz, more preferably in the range of 120 kHz to 350 kHz. The two or more acoustic MUT elements  210  may each be configured to emit the respective modulated ultrasonic signal  220  at a frequency configured to generate a focal volume  260  with a maximum cross-section diameter of less than 5 mm, preferably less than 3 mm, more preferably less than 1 mm. 
     In one or more embodiments, the two or more acoustic MUT elements  210  may each be arranged to emit the respective modulated ultrasonic signal  220  at a frequency within the human perceivable frequency spectrum, so as to produce human perceivable ultrasound feedback if sensed by a human sensory organ at the focal volume  260 . In these embodiments, the two or more acoustic MUT elements  210  may be arranged to emit the respective modulated ultrasonic signal  220  at a frequency within a frequency spectrum configured to produce a human perceivable tactile stimuli if sensed by a human tactile sensory organ at the focal volume  260 . Alternatively, or in combination with the above, each of the two or more acoustic MUT elements  210  may be arranged to emit the respective modulated ultrasonic signal  220  at a frequency within a frequency spectrum configured to produce a human perceivable auditory stimuli if sensed by a human auditory organ at the focal volume  260 . 
     In one or more embodiment, the MUT  200  may be a piezoelectric micromachined ultrasonic transducer, p-MUT, and the acoustic MUT elements  210  acoustic p-MUT elements. The piezoelectric material may comprise Lead zirconate titanate, PZT, or doped PZT. The piezoelectric material may comprise thin film PZT, or thin film doped PZT. In some embodiments, the piezoelectric material may comprise PNZT, or PZT doped with Niobium (Nb). In some embodiments, the PZT material may ahve a thickness of 0.5-5 μm, preferably 1-3 μm, more preferably around 2 μm. 
     In one or more embodiments, each acoustic MUT element  210  comprises a membrane or a diaphragm. The membrane or diaphragm may further comprise silicon. 
     In other embodiments, the MUT  200  may be a capacitive micromachined ultrasonic transducer, c-MUT, and the acoustic MUT elements  210  acoustic c-MUT elements. 
     According to any of the embodiments presented herein, the array may comprise at least 400 acoustic MUT elements  210 . As some non-limiting examples, the array may comprise 96×96 or 128×72 acoustic MUT elements  210 . 
     Two or more acoustic MUT elements  210  may further be configured to receive a respective drive signal S Tx  from a controller  120 , and generate the respective modulated ultrasonic signal  220  in response to the respective received drive signal S Tx . 
     In some embodiments, wherein the MUT  200  is to be used not only for emitting ultrasonic signals, but also for recognizing whether there is an object in its vicinity, at least one of the acoustic MUT elements  210  of the MUT  200  may be configured to detect entry and exit of an object in a field of detection of the at least one acoustic MUT element  210 ; generate a detection signal S Rx  indicative of the detected entry and exit to a transducer controller  120 ; and send the detection signal S Rx  to a controller  120  being communicably coupled to the array of acoustic MUT elements  210  and being configured to identify a gesture based on the detection signal S Rx . The at least one acoustic MUT element  210  may be configured to detect entry and exit of the object in a field of detection of the at least one acoustic MUT element  210  by generating an ultrasonic signal, or ultrasonic waves, for reflection off the object. In one or more embodiments, the at least one of the MUT element may be configured to detect entry and exit of the object in a field of detection of the at least one MUT element by emitting an ultrasonic signal, or un-modulated ultrasonic waves, for reflection off the object.  FIG. 2  illustrates that the acoustic MUT elements may be configured to receive a reflected ultrasonic signal or waves  240   1 . . . j  from an object located at the common focal point  230 . The object may be a body part of a human, for example a finger, a palm of a hand, a wrist, a foot, part of the chest, the neck, or an ear. The at least one of the acoustic MUT elements configured to detect entry and exit of an object in a field of detection may be, but are not necessarily, part of the at least two MUT elements configured to generate the respective ultrasonic signal. 
     In the non-limiting examples of the figures, the MUT  200  is shown as having 8×8 (i=64) or 4×4 (i=16) MUT elements, respectively. It is evident to a person skilled in the art that any suitable number of MUT elements may be used in the array and that the representations in the figures are for illustrational purposes only. 
     The MUT technology enables use of an array of acoustic MUT elements for example having as few as 2×2, 4×4, 8×8 or 16×16 MUT elements, or as many as 96×96 or 128×72 MUT elements in the form of a two-dimensional array or other non-square array configuration including circular layouts and straggle p-MUT patterns, depending on the specific application. In some embodiments, the array comprises at least  400  MUT elements  210 . In some embodiments, the array of the MUT  200  comprises 96×96 or 128×72 MUT elements  210 . The latter embodiments with 96×96 or 128×72 MUT elements provide a MUT having almost 10 000 channels. This high resolution cannot be achieve using non miniaturized existing technology. Such a large resolution of the array in turn enables using a greater number of acoustic MUT elements  210  for emitting a respective modulated ultrasonic signal  220  towards each common focal point  230  compared to the existing technologies, which gives improved focus capabilities and a higher maximum energy or acoustic pressure at the focal volume  260 , and/or emitting modulated ultrasonic signals  220  towards an increased number of common focal points  230 , thereby enabling for example more realistic tactile stimulation feedback or auditory feedback, or simultaneous trapping and possible acoustic levitation and manipulation of numerous objects using a single system. In an “acoustic tweezers” system, as described in connection with  FIG. 9 , e.g., it may further be advantageous to use more than two, for example three, four, or possibly more, acoustic lobes  250  for levitation and/or manipulation of an object  1100  between the acoustic lobes. Thereby, wobbling of the captured object  1100  may be avoided or at least significantly reduced, and precision hence increased, especially in cases where the focal volume  260  may be larger than desired where a larger number of acoustic lobes  250  may be used for compensating for this. 
     Each of the two or more acoustic MUT elements may also be configured to generate and emit the respective ultrasonic signal according to a predetermined phase shift, such that the ultrasonic signal of each of the two or more acoustic MUT elements is phase shifted in relation to each other. 
     In one or more embodiments, the acoustic MUT elements  210   1 . . . i  of the MUT  200  may be configured or controlled to generate a plurality of common focal points  230   
     The MUT  200  may be any suitable type of MUT. In one or more embodiments, the MUT  200  may be a piezoelectric MUT (p-MUT). The use of p-MUT devices is advantageous for generating high force and displacements, e.g. high sound pressure required to generate human perceivable feedback according to embodiments herein, because p-MUT devices require a low voltage and further transform electrical energy to mechanical energy in a very efficient manner. In other embodiments, the MUT  200  may be a capacitive MUT, (c-MUT). 
     The use of c-MUT devices is advantageous for the gesture recognition embodiments, because c-MUT devices transform mechanical energy into electrical energy in a very efficient manner. In one embodiment the MUT elements can be a combination of p- and c-MUT transducer principles. 
     It is noted that MUT elements  210  can comprise one or more sensing elements. Such sensing elements can include membranes, diaphragms, or other elements capable of sensing and/or generating ultrasonic waves. For instance, one or more membranes of MUT elements  210  can be excited to transmit an ultrasonic wave. In another aspect, a plurality of membranes of MUT elements  210  can receive ultrasonic waves that induce movement of the one or more membranes. 
     Method Embodiments 
     In an example embodiment of the method, firstly at least one 3D position defining a common focal point is determined, selected or retrieved. At least one MUT  200 , comprising an array of acoustic MUT elements  210 , is arranged to create an acoustic-potential field of ultrasonic waves, with the phases and amplitudes of each MUT element  210  calculated to achieve a high pressure at the focal point and a low pressure in surrounding areas. A frequency at which to modulate the feedback is then chosen in dependence on how the human perceivable feedback is intended to be perceived, e.g. felt or heard by a human. Then a modulation signal may be generated for modulating a carrier wave into a modulated ultrasonic signal  220 , also in dependence on how the human perceivable feedback is intended to be perceived. The modulation signal may be emitted, by MUT elements  210 , to generate the acoustic-potential field of ultrasonic waves. 
       FIG. 10  shows one or more embodiment of a method for controlling at least two acoustic micromachined ultrasonic transducer, MUT, elements in an array of acoustic MUT elements to emit a respective modulated ultrasonic signal comprising a plurality of ultrasound waves with a common focal point, the plurality of ultrasound waves of the modulated ultrasonic signal each comprising a carrier wave being modulated according to a modulation signal, so as to generate an acoustic-potential field of ultrasonic waves, the method comprising: 
     In step  1010 : generating, by a controller, a respective drive signal for each of the at least two acoustic MUT elements. 
     The respective drive signal may be indicative of a frequency at which the acoustic MUT element is to be controlled to emit the respective modulated signal; the modulation signal for modulating the carrier signal to obtain a respective modulated ultrasonic signal; and a respective phase shift to be applied to the respective modulated ultrasonic signal, wherein the respective phase shift is configured such that the respective modulated ultrasonic signal of each of the two or more acoustic MUT elements is phase shifted in relation to one another, so as to be constructively combined at a common focal point. 
     The respective drive signal may further be indicative voltages and/or currents needed for the acoustic MUT elements to operate. 
     In step  1020 : sending, by the controller, the respective drive signal to the acoustic MUT element. 
     In step  1030 : receiving, in each of the two or more acoustic MUT elements, the respective drive signal. 
     In step  1040 : emitting, by each of the two or more acoustic MUT elements, a respective modulated ultrasonic signal in response to the respective received drive signal. 
     Thereby an acoustic-potential field of ultrasonic waves is generated at the common focal point. The method steps may be performed repeatedly, so as to generate a continuous emission of modulated ultrasonic signals which may be altered over time. 
     The frequency at which the at least two acoustic MUT elements are to be controlled to emit the respective modulated signal is an electro-mechanical resonance frequency of the MUT element which may according to embodiments be in the range of 20 kHz to 10 MHz, preferably in the range of 100 kHz to 500 kHz, more preferably in the range of 120 kHz to 350 kHz. The electro-mechanical resonance frequency may alternatively be configured to generate a focal volume ( 260 ) with a maximum cross-section diameter of less than 5 mm, preferably less than 3 mm, more preferably less than 1 mm. 
     In one or more embodiments, emitting the respective modulated ultrasonic signal comprises emitting the modulation signal at a frequency within a frequency spectrum configured to produce a human perceivable sensory stimuli, so as to produce human perceivable sensory feedback if sensed by a human sensory organ at the common focal point. In some embodiments, emitting the respective modulated ultrasonic signal comprises emitting the modulation signal at a frequency within a frequency spectrum configured to produce a human perceivable tactile stimuli, so as to produce human perceivable tactile feedback if sensed by a human tactile sensory organ at the common focal point. In other embodiments, or in combination, emitting the respective modulated ultrasonic signal may comprise comprises emitting the modulation signal at a frequency within a frequency spectrum configured to produce a human perceivable auditory stimuli, so as to produce human perceivable auditory feedback if sensed by a human auditory sensory organ at the common focal point  230 . 
     In some embodiments, the method further comprises receiving, by the controller, a detection signal from at least one acoustic MUT element in said array; identifying, by the controller, a gesture based on the received detection signal or signals; and generating, by the controller, the respective drive signal in response to identifying a gesture. 
     In one or more embodiments, the respective phase shift to be applied to the respective modulated ultrasonic signal of each of the two or more acoustic MUT elements may be configured to cause the ultrasound waves of the modulated ultrasonic signals to be constructively combined at the common focal point so as to generate an acoustic-potential field of standing ultrasonic waves at the common focal point. According to these embodiments, the method further comprises capturing/trapping an object with a maximum cross-section width of less than 3 mm, preferably less than 1 mm, more preferably less than 0.5 mm, in a node at a sound pressure minima created between two or more standing waves of the generated acoustic-potential field of standing waves. 
     In one or more alternative embodiments, the respective phase shift to be applied to the respective modulated ultrasonic signal of each of the two or more acoustic MUT elements is configured to cause the ultrasound waves of the modulated ultrasonic signals to be constructively combined at two or more common focal points so as to generate an acoustic-potential field comprising respective acoustic lobes around the respective two or more common focal points. According to these embodiments, the method further comprises capturing/trapping one or more objects in a respective one or more node at a sound pressure minima created between two or more acoustic lobes of the generated acoustic-potential field. 
     In any of the embodiments for capturing/trapping an object, the method may further comprise contactlessly/in air translating or rotating the captured object, by further modulating the emitted modulated ultrasonic signals. 
     Further Applications 
     The non-contact ultrasonic feedback system  100  may advantageously be adapted for providing contactless human perceivable feedback in a human machine interface (HMI). 
     In a non-limiting example, the non-contact ultrasonic feedback system  100  may be adapted for providing contactless human perceivable feedback to a user via an HMI in a cockpit of a motorized vehicle, wherein the vehicle may be a car, a truck, a bus, an airplane or a train, and the user may be the driver, pilot, co-driver, co-pilot or passenger of the motorized vehicle. 
     In another non-limiting example, the non-contact ultrasonic feedback system  100  may be adapted for providing contactless human perceivable feedback to a user via an HMI in a public setting, such as e.g. a hospital setting, where many people interact with the HMI and it is desirable to reduce the risk of contamination via surfaces. 
     In yet another non-limiting example, the non-contact ultrasonic feedback system  100  may be adapted for providing contactless human perceivable feedback to a user via an HMI in a robotics application. 
     MUTs ( 200 ) according to embodiments herein may also advantageously be used for other applications than providing human perceivable feedback in a HMI, for example in machine-machine interfaces (MMI). 
     Compared to existing technology, examples of which may be found in the patent publications US20130047728A1 and US20170004819A1, MUTs according to embodiments presented herein enable three dimensional manipulations of much smaller objects. This is due to the fact that MUTs according to embodiments described herein enable generation of ultrasonic signals  220  at an electro-mechanical resonance frequency in the range of 20 kHz to 10 MHz, preferably in the range of 100 kHz to 500 kHz, most preferably in the range of 120 kHz to 350 kHz. By generating ultrasonic signals having an electro-mechanical resonance frequency over 100 kHz, for instance, the wavelength of the signal will be less than 1 mm, whereby objects smaller than 0.5 mm can be successfully manipulated. 
     Further applications or technological areas wherein embodiments presented herein may advantageously be implemented include, but not limited to:
         Digital electronic braille for blind people   Gaming: Augmentation (AR) or full Immersive virtual reality (IR) gesture-controlled gaming   Automotive: in-air safety application for feedback and control including steering of infotainment systems and dashboard functions. Digitally controlled knobs can e.g. be personalized by virtual 3D Hologram with HMI feedback of ultrasonic waves for tactile and/or audio feedback.   Location-based entertainment: 4D and 5D cinema and studios to add directional stimuli of tactile and audio sense on top of visual 3D impression and surround sound.   AR/VR: Immersive experiences and intuitive interaction with virtual objects.   Infotainment/education and computing: An extra dimension to 3D imaging and interaction with virtual objects or real object, for example at a museum/gallery where digital surface texture of old and sensitive objects can give improved stimuli to the visitor.   Smart home: Invisible, non-contact virtual buttons, slide bars and responsive controls that can be digitally personalized   Industrial and medical: Touch-less HMI to improve health and       

     Further Embodiments 
     All of the process steps, as well as any sub-sequence of steps, described with reference to  FIG. 10  above may be controlled by means of a programmed data processor. Moreover, although the embodiments of the invention described above with reference to the drawings comprise a data processor and processes performed in at least one processor, the invention thus also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of source code, object code, a code intermediate source and object code such as in partially compiled form, or in any other form suitable for use in the implementation of the process according to the invention. The program may either be a part of an operating system, or be a separate application. The carrier may be any entity or device capable of carrying the program. For example, the carrier may comprise a storage medium, such as a Flash memory, a ROM (Read Only Memory), for example a DVD (Digital Video/Versatile Disk), a CD (Compact Disc) or a semiconductor ROM, an EPROM (Erasable Programmable Read-Only Memory), an EEPROM (Electrically Erasable Programmable Read-only Memory), or a magnetic recording medium, for example a floppy disc or hard disc. Further, the carrier may be a transmissible carrier such as an electrical or optical signal which may be conveyed via electrical or optical cable or by radio or by other means. When the program is embodied in a signal which may be conveyed directly by a cable or other device or means, the carrier may be constituted by such cable or device or means. Alternatively, the carrier may be an integrated circuit in which the program is embedded, the integrated circuit being adapted for performing, or for use in the performance of, the relevant processes. 
     In one or more embodiments, there may be provided a computer program loadable into a memory communicatively connected or coupled to at least one data processor, e.g. the controller  120  or the external processor  130 , comprising software for executing the method according any of the embodiments herein when the program is run on the at least one controller  120  or external processor  130   
     In one or more further embodiment, there may be provided a processor-readable medium, having a program recorded thereon, where the program is to make at least one data processor, e.g. the controller  120  or the external processor  130 , execute the method according to of any of the embodiments herein when the program is loaded into the at least one data processor. 
     The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.