Patent Publication Number: US-2023152433-A1

Title: Ultrasonic direct echo path reduction

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and benefit of co-pending U.S. Provisional Patent Application No. 63/279,996 filed on Nov. 16, 2021 entitled “ENGRAVED RING FOR ULTRASONIC DIRECT ECHO PATH REDUCTION” by Tony Lei et al., having Attorney Docket No. IVS-1024-PR, and assigned to the assignee of the present application, the disclosure of which is hereby incorporated herein by reference in its entirety. 
     This application claims priority to and benefit of co-pending U.S. Provisional Patent Application No. 63/279,940 filed on Nov. 16, 2021 entitled “ULTRASONIC CLIFF DETECTION AND DEPTH ESTIMATION USING TILT ANGLE,” by Daniela Hall et al., having Attorney Docket No. IVS-1023-PR, and assigned to the assignee of the present application, the disclosure of which is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     A variety of devices exist which move about on or operate on floors or other surfaces such as walls, windows, roofs, tables, countertops, sidewalks, roads, and the like. The surfaces may be indoor surfaces, outdoor surfaces, or some combination. One or more examples of such a device may be semi-autonomous, by which is meant that while operating on one or more surfaces some functions of the device are controlled by a human and some are automated. One or more examples of such a device may be robotic, by which is meant that while operating on one or more surfaces some or all functions of the device may operate autonomously under the control of one or more processors. Some examples of these devices may include, but are not limited to: remote control vehicles, telepresence robots, electric scooters, electric wheelchairs, wheeled delivery robots, flying drones operating near a surface or about to land on or take off from a surface, wheeled delivery vehicles, floor vacuums, and robotic cleaning appliances (which include robotic floor cleaners and/or robotic floor vacuums). 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the Description of Embodiments, illustrate various embodiments of the subject matter and, together with the Description of Embodiments, serve to explain principles of the subject matter discussed below. Unless specifically noted, the drawings referred to in this Brief Description of Drawings should be understood as not being drawn to scale. Herein, like items are labeled with like item numbers. 
         FIGS.  1 A and  1 B  show example block diagrams of some aspects of a device which moves about or operates on a surface, in accordance with various embodiments. 
         FIG.  2    shows an upper front perspective view of one example of a system which includes a device which moves about or operates on a surface and a base station for the device, in accordance with various embodiments. 
         FIGS.  3    shows a side elevational view of one example of the system of  FIG.  2   , which includes a device which moves about or operates on a surface and a base station for the device, in accordance with various embodiments. 
         FIG.  4 A  illustrates a side elevational view of one example sensor assembly, with debossed rings defined in an external surface, which may be utilized on a device to transmit sonic signals and receive returned sonic signals, in accordance with various embodiments. 
         FIG.  4 B  illustrates a top view of the sensor assembly of  FIG.  4 A , in accordance with various embodiments. 
         FIG.  4 C  illustrates a side sectional view of a configuration of the sensor assembly of  FIG.  4 B , according to some embodiments. 
         FIG.  4 D  illustrates a second side sectional view of a configuration of the sensor assembly of  FIG.  4 B , according to some embodiments. 
         FIG.  4 E  illustrates a bottom plan view of a configuration of the sensor assembly of  FIG.  4 A , according to some embodiments. 
         FIG.  4 F  illustrates a straight on bottom view (where the viewpoint is taken orthogonal to the bottom surface) of a configuration of the sensor assembly of  FIG.  4 A , according to some embodiments. 
         FIG.  5 A  illustrates a side elevational view of one example sensor assembly, with embossed rings defined in an external surface, which may be utilized on a device to transmit sonic signals and receive returned sonic signals, in accordance with various embodiments. 
         FIG.  5 B  illustrates a top view of the sensor assembly of  FIG.  5 A , in accordance with various embodiments. 
         FIG.  5 C  illustrates a side sectional view of a configuration of the sensor assembly of  FIG.  5 B , according to some embodiments. 
         FIG.  5 D  illustrates a straight on bottom view (where the viewpoint is taken orthogonal to the bottom surface) of a configuration of the sensor assembly of  FIG.  5 A , according to some embodiments. 
         FIG.  6 A  illustrates a side elevational view of one example sensor assembly, with debossed rings defined in an external surface, which may be utilized on a device to transmit sonic signals and receive returned sonic signals, in accordance with various embodiments. 
         FIG.  6 B  illustrates a top view of the sensor assembly of  FIG.  6 A , in accordance with various embodiments. 
         FIG.  6 C  illustrates a side sectional view of a configuration of the sensor assembly of  FIG.  6 B , according to some embodiments. 
         FIG.  6 D  illustrates a straight on bottom view (where the viewpoint is taken orthogonal to the bottom surface) of a configuration of the sensor assembly of  FIG.  6 A , according to some embodiments. 
         FIG.  7    illustrates a polar graph of several examples of beam shapes which may be implemented via some example configurations of rings defined annularly around one or more an acoustic opening ports of a sensor assembly, according to various embodiments. 
         FIG.  8    shows an illustration of the ultrasonic sensors of a sensor assembly detecting a cliff and estimating its depth, in accordance with various embodiments. 
         FIGS.  9 A and  9 B  show graphs of the amplitude of received corresponding returned signals versus distance for sensor assembly tilted at 15 degrees from orthogonal and employing no defined rings around acoustic opening ports, in accordance with an embodiment. 
         FIGS.  10 A and  10 B  show graphs of the amplitude of received corresponding returned signals versus distance for sensor assembly tilted at 15 degrees from orthogonal and employing a single defined ring around each of the emitting sensor&#39;s acoustic opening port and the receiving sensor&#39;s acoustic opening port, in accordance with an embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Reference will now be made in detail to various embodiments of the subject matter, examples of which are illustrated in the accompanying drawings. While various embodiments are discussed herein, it will be understood that they are not intended to limit to these embodiments. On the contrary, the presented embodiments are intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope the various embodiments as defined by the appended claims. Furthermore, in this Description of Embodiments, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present subject matter. However, embodiments may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the described embodiments. 
     Overview of Discussion 
     A variety of devices which move about on or operate on floors or other surfaces may benefit from being able to determine information about surface upon which they are moving or operating. Surfaces may include floors, building exteriors, windows, dirt, gravel, fabric, roofing, roads, sidewalks, trails, water (or other liquid), etc. A surface may be flat and smooth or rough and textured. In some embodiments, such as at the edge of a downward stair which meets a surface, the surface may abruptly drop off in the fashion of the edge of a cliff. 
     During the operation of a device on a surface or as it moves about on a surface, it may be advantageous for the device to detect information about the surface, such as whether it is wet (and in some instances how wet on a scale of wetness), dirty (and in some instances, how dirty and/or dirty with what), dry, hard (and in some instances, how hard on a scale of hardness), soft (and in some instances how soft on a scale of softness), etc. Likewise, it may be similarly advantageous during operation of a device on a surface for the device to detect cliff like drop offs and/or to estimate a depth of drop from the surface upon which the device is operating. For example, simply detecting a cliff edge may be sufficient to permit the device to change course to avoid the cliff. Estimating the depth of the cliff may permit the device to determine if the drop off is too deep for the device to navigate down or else if it is shallow enough for the device to cross the cliff edge to the surface below and continue operation. When performing such detection in very close proximity to a surface, such as a floor, there can be quite a bit of noise due to the high number of reflections. This is especially the case when sensing hard surfaces such as wood, vinyl, or tile flooring. Herein, techniques involving the defining of one or more rings around the a sensor horn&#39;s opening (referred to herein as an “acoustic opening port”) are described. The defining may cause the rings to be represented as being debossed/engraved-in or embossed/raised-from the external surface. The defined rings operate to reduce direct echo path reflections (noise) and increase signal amplitude by disrupting the sound pressure in the small gap between where sonic signals are emitted and a floor surface. More particularly, the defined rings reduce direct path echoes which result from transmitted sonic signals which attempt to travel across an external surface that exists between the acoustic opening of port of a transmitting sonic sensor and the acoustic opening port of the receiving electronic sensor. In some embodiments, the configuration of the defined rings may additionally be employed to shape beam patters of one or more of transmitted sonic signals and corresponding returned signals which are reflected from a surface. 
     Discussion begins with a description of notation and nomenclature. Discussion then shifts to description of some block diagrams of example components of some example devices which moves about or operate on a surface. Some example depictions of a device, in the form of robotic cleaning appliance, are discussed. Some examples depictions of sensor assembly are described. A signal path of a sonic transducer is through a sensor assembly is described. The utility of adding one or more defined rings in an annular fashion around the opening to a sensor horn is described and depicted in conjunction with several example embodiments. 
     Notation and Nomenclature 
     Some portions of the detailed descriptions which follow are presented in terms of procedures, logic blocks, processes, modules and other symbolic representations of operations on data bits within a computer memory. These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. In the present application, a procedure, logic block, process, module, or the like, is conceived to be one or more self-consistent procedures or instructions leading to a desired result. The procedures are those requiring physical manipulations of physical quantities. Usually, although not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated in an electronic device/component. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the description of embodiments, discussions utilizing terms such as “transmitting,” “receiving,” “detecting,” “estimating,” “processing,” “determining,” or the like, refer to the actions and processes of an electronic device or component such as: a host processor, a sensor processing unit, a sensor processor, a digital signal processor or other processor, a memory, a sonic sensor (e.g., a sonic transducer), a robotic cleaning appliance, a device configured to operate on or move about a surface, some combination thereof, or the like. The electronic device/component manipulates and transforms data represented as physical (electronic and/or magnetic) quantities within the registers and memories into other data similarly represented as physical quantities within memories or registers or other such information storage, transmission, processing, or display components. 
     Embodiments described herein may be discussed in the general context of processor-executable instructions residing on some form of non-transitory processor-readable medium, such as program modules or logic, executed by one or more computers, processors, or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or distributed as desired in various embodiments. 
     In the figures, a single block may be described as performing a function or functions; however, in actual practice, the function or functions performed by that block may be performed in a single component or across multiple components, and/or may be performed using hardware, using software, or using a combination of hardware and software. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure. Also, the example electronic device(s) described herein may include components other than those shown, including well-known components. 
     The techniques described herein may be implemented in hardware, or a combination of hardware with firmware and/or software, unless specifically described as being implemented in a specific manner. Any features described as modules or components may also be implemented together in an integrated logic device or separately as discrete but interoperable logic devices. If implemented in software, the techniques may be realized at least in part by a non-transitory computer/processor-readable storage medium comprising computer/processor-readable instructions that, when executed, cause a processor and/or other components of a computer or electronic device to perform one or more of the methods described herein. The non-transitory processor-readable data storage medium may form part of a computer program product, which may include packaging materials. 
     The non-transitory processor-readable storage medium (also referred to as a non-transitory computer-readable storage medium) may comprise random access memory (RAM) such as synchronous dynamic random access memory (SDRAM), read only memory (ROM), non-volatile random access memory (NVRAM), electrically erasable programmable read-only memory (EEPROM), flash memory, other known storage media, and the like. The techniques additionally, or alternatively, may be realized at least in part by a processor-readable communication medium that carries or communicates code in the form of instructions or data structures and that can be accessed, read, and/or executed by a computer or other processor. 
     The various illustrative logical blocks, modules, circuits and instructions described in connection with the embodiments disclosed herein may be executed by one or more processors, such as host processor(s) or core(s) thereof, digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), application specific instruction set processors (ASIPs), field programmable gate arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. The term “processor,” as used herein may refer to any of the foregoing structures or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated software modules or hardware modules configured as described herein. Also, the techniques could be fully implemented in one or more circuits or logic elements. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a plurality of microprocessors, one or more microprocessors in conjunction with an ASIC or DSP, or any other such configuration or suitable combination of processors. 
     In various example embodiments discussed herein, a chip is defined to include at least one substrate typically formed from a semiconductor material. A single chip may, for example, be formed from multiple substrates, where the substrates are mechanically bonded to preserve the functionality. Multiple chip (or multi-chip) includes at least two substrates, wherein the two substrates are electrically connected, but do not require mechanical bonding. 
     A package provides electrical connection between the bond pads on the chip (or for example a multi-chip module) to a metal lead that can be soldered to a printed circuit board (or PCB). A package typically comprises a substrate and a cover. An Integrated Circuit (IC) substrate may refer to a silicon substrate with electrical circuits, typically CMOS circuits but others are possible and anticipated. A MEMS substrate provides mechanical support for the MEMS structure(s). The MEMS structural layer is attached to the MEMS substrate. The MEMS substrate is also referred to as handle substrate or handle wafer. In some embodiments, the handle substrate serves as a cap to the MEMS structure. 
     Some embodiments may, for example, comprise one or more sonic sensors. This sensor may be any suitable sonic sensor operating in any suitable sonic range. For example, in some embodiments, the sonic sensor may be an ultrasonic sensor which utilizes a MEMS ultrasonic transducer. In some embodiments, the sonic sensor may include digital signal processor (DSP) which may be disposed as a part of an ASIC which may be integrated into the same package as a transducer. One example of such an ultrasonic sensor which may be utilized with various embodiments, without limitation thereto, is the CH101 ultrasonic range sensor from Chirp Microsystems, a TDK Group Company, of Berkley, Calif. The CH101 is only one example of an ultrasonic sensor, other types and/or brands of ultrasonic sensors may be similarly utilized. 
     Some embodiments may, for example, comprise one or more motion sensors. For example, an embodiment with an accelerometer, a gyroscope, and a magnetometer or other compass technology, which each provide a measurement along three axes that are orthogonal relative to each other, may be referred to as a 9-axis device. In another embodiment, a three-axis accelerometer and a three-axis gyroscope may be used to form a 6-axis device. Other embodiments may, for example, comprise an accelerometer, gyroscope, compass, and pressure sensor, and may be referred to as a 10-axis device. Other embodiments may not include all the sensors or may provide measurements along one or more axes. Some or all of the sensors may be MEMS sensors. Some or all of the sensors may be incorporated in a sensor processing unit along with a sensor processor and disposed in a single semiconductor package. 
     In some embodiments, for example, one or more sensors may, be formed on a first substrate. Various embodiments may, for example, include solid-state sensors and/or any other type of sensors. The electronic circuits in a sensor processing unit may, for example, receive measurement outputs from the one or more sensors. In various embodiments, the electronic circuits process the sensor data. The electronic circuits may, for example, be implemented on a second silicon substrate. In some embodiments, the first substrate may be vertically stacked, attached and electrically connected to the second substrate in a single semiconductor chip, while in other embodiments, the first substrate may be disposed laterally and electrically connected to the second substrate in a single semiconductor package, such as a single integrated circuit. 
     Example Device which Moves About or Operates on a Surface 
       FIGS.  1 A and  1 B  show some example components of a device  100  (e.g., device  100 A or  100 B) which moves about or operates on a surface. Some examples of a device  100  may include, but are not limited to: a remote control vehicles, a telepresence robot, an electric scooter, an electric wheelchair, a wheeled delivery robot, a flying drone operating near a surface or about to land on or take off from a surface, a wheeled delivery vehicle, a floor vacuum, and a robotic cleaning appliance (which includes: a robotic floor cleaner, a robotic floor vacuum, or combinations thereof). 
       FIG.  1 A  shows a block diagram of components of an example device  100 A which moves about or operates on a surface, in accordance with various aspects of the present disclosure. As shown, example device  100 A comprises a communications interface  105 , a host processor  110 A, host memory  111 , and at least one sonic sensor  150 . In some embodiments, device  100 A may additionally include one or more of a transceiver  113 , one or more motion sensors  160 , one or more drive wheel controllers  170 , and one or more surface treatment controllers  180  (which may control a cleaning tool and/or any surface treatment item that may take an action based upon a surface type determination). Some embodiments may include sensors used to detect motion, position, surface type, or environmental context (e.g., nearby objects and/or obstacles, whether a surface is hard or soft, whether a surface is carpeted or not carpeted, whether a surface is clean or dirty, whether a surface is wet or dry, etc.); some examples of these sensors may include, but are not limited to, infrared sensors, cameras, microphones, and global navigation satellite system sensors (i.e., a global positioning system receiver). As depicted in  FIG.  1 A , included components are communicatively coupled with one another, such as, via communications interface  105 . 
     The host processor  110  may, for example, be configured to perform the various computations and operations involved with the general function of device  100 A (e.g., sending commands to move, steer, avoid obstacles, and operate/control the operation of tools). Host processor  110  can be one or more microprocessors, central processing units (CPUs), DSPs, general purpose microprocessors, ASICs, ASIPs, FPGAs or other processors which run software programs or applications, which may be stored in host memory  111 , associated with the general and conventional functions and capabilities of device  100 A. 
     Communications interface  105  may be any suitable bus or interface, such as a peripheral component interconnect express (PCIe) bus, a universal serial bus (USB), a universal asynchronous receiver/transmitter (UART) serial bus, a suitable advanced microcontroller bus architecture (AMBA) interface, an Inter-Integrated Circuit (I2C) bus, a serial digital input output (SDIO) bus, or other equivalent and may include a plurality of communications interfaces. Communications interface  105  may facilitate communication between SPU  120  and one or more of host processor  110 , host memory  111 , transceiver  113 , sonic sensor  150 , motion sensor(s)  160 , drive wheel controller(s)  170 , and/or surface treatment controller(s)  180 . 
     Host memory  111  may comprise programs, modules, applications, or other data for use by host processor  110 . In some embodiments, host memory  111  may also hold information that that is received from or provided to sensor processing unit  120  (see e.g.,  FIG.  1 B ). Host memory  111  can be any suitable type of memory, including but not limited to electronic memory (e.g., read only memory (ROM), random access memory (RAM), or other electronic memory). 
     Transceiver  113 , when included, may be one or more of a wired or wireless transceiver which facilitates receipt of data at device  100 A from an external transmission source and transmission of data from device  100 A to an external recipient. One example of an external transmission source/external recipient may be a base station to which device  100 A returns for charging, maintenance, docking, etc. By way of example, and not of limitation, in various embodiments, transceiver  113  comprises one or more of: a cellular transceiver, a wireless local area network transceiver (e.g., a transceiver compliant with one or more Institute of Electrical and Electronics Engineers (IEEE) 802.11 specifications for wireless local area network communication), a wireless personal area network transceiver (e.g., a transceiver compliant with one or more IEEE 802.15 specifications (or the like) for wireless personal area network communication), and a wired a serial transceiver (e.g., a universal serial bus for wired communication). 
     Sonic sensor  150  may be a sonic transducer. In some embodiments, sonic sensor  150  is an ultrasonic transducer (i.e., a sonic transducer which operates in the ultrasonic frequency range). In some embodiments, where sonic sensor  150  operates in an ultrasonic range, it may operate in a range between 50 kHz and 500 kHz or in a range between 150 kHz and 200 kHz. Of course, other ultrasonic ranges are anticipated and usable. Sonic sensor  150  is configured to transmit sonic signals toward a surface and receive sonic returned signals. The sonic signals transmitted may include signals in one or more of the infrasound range, the acoustic range, and ultrasonic range. Returned signals include primary returned signals (which are transmitted, encounter a surface, and reflect from the surface directly to a receiver), secondary returned signals (which are multi-path reflected prior to arriving at a receiver), and direct path returned signals which do not reflect from a surface. In some embodiments, sonic sensor  150  may be part of a sensor assembly  350  (see e.g.,  FIGS.  4 A- 4 E ), which may include an acoustic interface housing (see e.g.,  FIGS.  4 A- 6 D ), where an acoustic interface tube, cavity, horn, some combination thereof, or the like used to direct transmitted sonic signals toward a surface and to direct sonic returned signals back to sonic sensor  150 . In some embodiments, a pair (or more) of sensors  150  may be used, for example, with one transmitting signals which are received by the other(s). 
     Motion sensor(s)  160 , when included, may be implemented as MEMS-based motion sensors, including inertial sensors such as a gyroscope  161  or accelerometer  163 , or an electromagnetic sensor such as a Hall effect or Lorentz field magnetometer  165 . In some embodiments, at least a portion of the motion sensors  160  may also, for example, be based on sensor technology other than MEMS technology (e.g., CMOS technology, etc.). As desired, one or more of the motion sensors  160  may be configured to provide raw data output measured along three orthogonal axes or any equivalent structure. 
     Drive wheel controller(s)  170  or other mechanism(s) to govern movement, when included, may include motor controllers, switches, and/or logic which operate under instruction to: drive one or more wheels or other mechanisms of movement (e.g., tank treads, propellers), change speed of rotation of a drive wheel or other mechanism of movement, moderate an amount of slip or spin permitted for a drive wheel, move a drive wheel or other mechanism of movement in a desired direction, stop a drive wheel or other mechanism of movement, and/or to steer device  100 A using a drive wheel (such as via differential speed or rotation) or other mechanism of movement. 
     Surface treatment controller(s)  180 , when included, may include motor controllers, switches, and or logic to turn on, turn off, and/or adjust the operation and/or orientation of one or more surface treatment items, such as cleaning tool(s) and/or other item(s) that take action based upon a surface type determination. 
       FIG.  1 B  shows a block diagram of components of an example device  100 B which moves about or operates on a surface, in accordance with various aspects of the present disclosure. Device  100 B is similar to device  100 A except that it includes a sensor processing unit (SPU)  120  in which sonic sensor  150  is disposed. SPU  120 , when included, comprises: a sensor processor  130 ; an internal memory  140 ; and at least one sonic sensor  150 . In some embodiments, SPU  120  may additionally include one or more motion sensors  160  (e.g., gyroscope  161 , accelerometer  163 , magnetometer  165 ) and/or one or more other sensors such a light sensor, infrared sensor, GNSS sensor, microphone, etc. In various embodiments, SPU  120  or a portion thereof, such as sensor processor  130 , is communicatively coupled with host processor  110 , host memory  111 , and other components of device  100 B through communications interface  105  or other well-known means. SPU  120  may also comprise one or more communications interfaces (not shown) similar to communications interface  105  and used for communications among one or more components within SPU  120 . SPU and/or components thereof may be communicatively coupled to other components of device  100 B such as via a coupling to bus  105 . 
     Sensor processor  130  can be one or more microprocessors, CPUs, DSPs, general purpose microprocessors, ASICs, ASIPs, FPGAs or other processors that run software programs, which may be stored in memory such as internal memory  140  (or elsewhere), associated with the functions of SPU  120 . In some embodiments, one or more of the functions described as being performed by sensor processor  130  may be shared with or performed in whole or in part by another processor of a device  100 B, such as host processor  110 . 
     Internal memory  140  can be any suitable type of memory, including but not limited to electronic memory (e.g., read only memory (ROM), random access memory (RAM), or other electronic memory). Internal memory  140  may store algorithms, routines, or other instructions for instructing sensor processor  130  on the processing of data output by one or more of the motion sensors  160  and/or one or more sensors  150 . In some embodiments, internal memory  140  may store one or more modules which may be algorithms that execute on sensor processor  130  to perform a specific function. Some examples of modules may include, but are not limited to: statistical processing modules, motion processing modules, surface type detection modules, and/or decision-making modules. 
     Sonic sensor  150  may be a sonic transducer which operates in the manner and in any of the sonic ranges previously described. In some embodiments, sonic sensor  150  is an ultrasonic transducer, such as a PMUT (piezoelectric micromachined ultrasonic transducer). Sonic sensor  150  may be a MEMS device and may be very small, such as having a facing surface of less than 4 mm by 4 mm by 1.5 mm. An ultrasonic sensor may be large or small, depending on the application and the space available. In some embodiments, sonic sensor  150  may be a SOC (system on a chip) which includes a DSP. In some embodiments, the SOC packaging of sonic sensor  150  comprises sensor processing unit  120  and includes sensor processor  130  and internal memory  140 . In some embodiments, sonic sensor  150  may be part of a of sensor (see e.g.,  FIGS.  4 A- 4 E ), which may include a tube used to direct transmitted sonic signals toward a surface and to direct sonic returned signals back to sonic sensor  150 . 
     Motion sensors  160 , when included, may be implemented as MEMS-based motion sensors, including inertial sensors such as a gyroscope  161  or accelerometer  163 , or an electromagnetic sensor such as a Hall effect or Lorentz field magnetometer  165 . In some embodiments, at least a portion of the motion sensors  160  may also, for example, be based on sensor technology other than MEMS technology (e.g., CMOS technology, etc.). As desired, one or more of the motion sensors  160  may be configured to provide raw data output measured along three orthogonal axes or any equivalent structure. Motion sensor(s)  160  are communicatively coupled with sensor processor  130  by a communications interface, bus, or other well-known communication means. 
     Example System 
       FIG.  2    shows an upper front perspective view of one example of a system  200  which includes one embodiment of a device  100  which moves about or operates on a surface and also includes a base station  202  for the device  100 , in accordance with various embodiments. Device  100  includes a housing  201  to which one or more items may be coupled. Base station  202  provides a location at which a device  100  may be positioned or parked or docked when not moving about or operating on a surface. In some embodiments, base station  202  (which may also be referred to as dock  202 ) may provide information/instructions to device  100  and/or receive information from device  100 , such as via physical and/or wireless communication coupling to transceiver  113 . In some embodiments, base station  202  may provide an electrical charger for device  100  such that, via physical or wireless (e.g., inductive) electrical coupling, base station  202  electrically charges device  100  when device  100  is coupled with or suitably oriented with base station  202 . 
       FIG.  3    shows a side elevational view of the example of a system  200  of  FIG.  2   , which includes a device  100  which moves about or operates on a surface and a base station  202  for the device  100 , in accordance with various embodiments. As illustrated in  FIG.  3   , device  100  and base station  202  are disposed on a surface  300 , upon which device  100  moves about and operates. Device  100  is depicted, by way of example and not of limitation, as a robotic cleaning appliance. Device  100  includes a sensor assembly  350  coupled with housing  201  or any suitable portion of device  100 . The sensor assembly  350  includes a sonic sensor  150  (as will be described in conjunction with  FIGS.  4 A- 6 D ). Although sensor assembly  350  is shown on the bottom of device  100  and directed toward surface  300  (e.g., toward the floor), in other embodiments, a sensor assembly  350  may additionally or alternatively be disposed on another portion for detecting vertical surfaces (e.g., walls) and/or overhanging surfaces. Device  100  may include one or more wheels  303  and/or  305 , which may be driven and/or controlled by drive wheel controller(s)  170 . As depicted, sensor assembly  350  is positioned in front of a front wheel  305  to detect surface types and cliffs (i.e., drop-offs) in a forward direction of travel  301  of device  100  prior to wheel  305  and/or surface treatment item  304  encountering the surface or drop-off. Device  100  may include one or more surface treatment items, such as surface treatment item  304 , which may be driven and/or controlled by surface treatment controller(s)  180 . The depiction of surface treatment item  304  is merely an example and is not meant to limit the type of surface treatment item, tool, or tools represented by the depiction. Without limitation thereto, surface treatment item  304  may comprise one or more or some combination of: a suction tool or suction opening with respect to a floor surface of floor  300  or other surface; a rotary tool (e.g., a roller brush or broom); a sweeping tool (e.g., a broom); a wiping tool (e.g., a cloth/cloth covered surface); a brushing tool (e.g., a fixed or movable brush head); a dusting tool; a mopping tool; and a spraying tool configured to spray a cleaner or other liquid. 
     In  FIG.  3   , device  100  is traveling in direction  301  on surface  300  and is preparing to dock with base station  202 . The direction of travel depicted is a forward traveling direction for device  100 . 
     With reference to  FIG.  3   , sonic signals transmitted by a sonic sensor  150  may be emitted from an opening in sensor assembly  350 , toward surface  300 , and corresponding returned signals received from the surface  300  by a sensor  150  ( 150 ,  150 B) may be utilized by device  100  to detect a surface type of surface  300 . In some embodiments, sonic sensing may also facilitate or assist in detecting whether a device  100  is level, unlevel, stuck, or operating without one of its wheels/surface contact points in contact with a surface and may additionally operate to detect nearby objects (to avoid); to detect information about the surface upon which device  100  operates (e.g., the type of surface (soft or hard), whether the surface is wet (and if wet, how wet) or dry, and how clean or dirty the surface is; and/or to detect information about a distance to a surface in the path of travel of device  100 . Amplitudes of reflected returned signals may be used by a host processor  110  or sensor processor  130  to make such determinations about a surface type and/or distance based on amplitude of corresponding returned signals and time to receive corresponding returned signals that are reflected from a surface after being transmitted by a sonic sensor  150 . 
     Device  100  may engage, disengage, deploy, redeploy, adjust the height, adjust the speed, or make other adjustments of a surface treatment item  304  based all or in part upon a surface type detection performed using sonic sensor(s)  150  and processing of received returned signals. 
     In embodiments where a sensor assembly  350  is located on another portion of device  100  (e.g., a side or top portion) the surface may be a horizontal surface such as a wall, or an overhang such as the underside of a coffee table or chair. 
     Processing may additionally or alternatively involve comparing amplitudes of corresponding returned signals at a particular surface distance with known amplitudes for different surface types to make a determination about a surface type. Processing may additionally or alternatively include a host processor  110  or a sensor processor  130  using amplitude and delay of corresponding returned signals to detect for drop-off/cliffs in the path of a device (i.e., the edge of a stair). For example, based on the corresponding returned signals the measured distance of a surface in the path of a device  100  may suddenly increase when a drop-off is detected and/or the amplitude of a surface at a particular distance may suddenly decrease. 
     Device  100  may engage, disengage, deploy, redeploy, adjust the height, adjust the speed, or make other adjustments of a surface treatment item  304  based all or in part upon a drop off/cliff detection performed using sonic sensor(s)  150  and processing of received returned signals. For example, in response to such detection of the type of floor dropping off like a cliff (e.g., on the edge of a stair), device  100  may take one or more actions to adjust an aspect of the operation of the device  100 . By way of example and not of limitation, in various embodiments device  100 , or a portion thereof may: adjust a speed of movement of device  100 ; regulate a drive motor of device  100 ; moderate an amount of slip or spin permitted for a drive wheel of device  100 ; and adjust the direction of travel of device  100 . 
     Although  FIG.  3    shows only a single sensor assembly  350 , some embodiments may include a plurality of transducers, which may operate at different frequencies. The nature of a surface type may influence different frequencies in a different way, and therefore, using multiple frequencies may facilitate more accurate detection/determination of the surface type. 
     Example Sensor Assembly 
       FIG.  4 A  illustrates a side elevational view of one example sensor assembly  350 A which may be utilized on a device  100  to transmit sonic signals and receive returned sonic signals, in accordance with various embodiments. It should be appreciated that only a small portion sensor assembly  350 A was visible protruding from the bottom of device  100 . External surface  475  is designed to oriented in device  100  such that it is parallel to surface  300  upon which device  100  operates. 
     Sensor assembly  350 A includes at least a sonic sensor  150  and typically includes two sonic transducers ( 150  and  150 B). An acoustic interface housing  450 A, which encloses and/or defines an acoustic interface tube  405 , in the form of a tube, cavity, horn, or some combination thereof, is included and coupled with sonic sensor  150 . In some embodiments, acoustic interface housing  450 A may position sonic sensor(s)  150  at an orthogonal angle with respect to a surface it is detecting (i.e., a floor). In some embodiments, as depicted in  FIGS.  4 A- 4 F , acoustic interface housing  450 A may position sonic sensor(s)  150  at a non-orthogonal tilt angle  489  with respect to a surface it is detecting (i.e., a floor). By tilting sensor(s)  150 , it/they may sense farther in advance of the direction of forward travel of device  100  than if it/they were orthogonal with the expected location of the floor (i.e., pointed straight down at the floor). In some embodiments, the angle of tilt  489  from orthogonal may be in the range of greater than 0 degrees from orthogonal and up to 30 degrees from orthogonal (for example between 5 degrees and 30 degrees). In some embodiments, the angle of tilt  489  may be between about 10 from orthogonal and about 20 degrees from orthogonal. In some embodiments, the angle of tilt  489  is approximately 15 degrees from orthogonal to the floor (e.g., floor  300 ) or other flat surface beneath device  100 . Within the range of tilt angles, a smaller angle from orthogonal increases the amplitude of returned signals (more accurate floor type determination), but also increases the latency associated with detecting the edge of a cliff in front of device  100  in the direction of travel of device  100 . Within the range of tilt angles, a larger angle from orthogonal decreases the amplitude of returned signals as they tend to scatter more, but also decreases the latency associated with detecting the edge of a cliff as it provides a built-in forward look during a forward direction of travel of device  100 . For example, in some embodiments where the angle of tilt is around 15 degrees, there is good balance between amplitude of returned signals from a surface below device  100  and reduced latency of cliff detection in front of device  100  in the forward direction of travel of device  100 . The tilt angle  489  for an application/device may be adapted to the desired specification of the device and to the design characteristics of the complete device (e.g., size, height, distance to floor, speed of movement, cliff detection and/or depth estimation latency desired/required). In some embodiments, the title angle may be adjustable automatically depending on application or context. 
     In some embodiments, a plurality of sensors  150  (e.g.,  150  and  150 B) may be employed in an acoustic interface housing  450 A (or different housings) which facilitate different tilt angles for at least two of the sensors  150 . In such an embodiment, a sensor with a shallow tilt angle from vertical (i.e., orthogonal to floor  300 ) affords better (greater) amplitude while a sensor with a greater angle of tilt from vertical provides better (less) latency in cliff detection. 
     In some embodiments, the sensor assembly  350 A is coupled with a device housing  201  (or other portion of device  100 ) such that it transmits the sonic signals in a direction toward which a surface is expected to be encountered or operated upon (e.g., downward toward a floor surface). For example, in a floor vacuum or robotic floor cleaning embodiment of device  100 , a sensor assembly  350 A may be disposed on or configured to sense outward from the bottom of device  100  in the manner illustrated in  FIG.  3   , such that transmitted sonic signals are directed toward a floor surface  300  below and in front of device  100  when device  100  is in operation. As described in  FIGS.  1 A and  1 B  a sonic sensor  150  (e.g.,  150 ,  150 B) may also be coupled with a host processor  110  or a sensor processor, in some embodiments, either or both of which operate to process and make determinations based upon received returned signals such as detecting surface types and/or detecting cliffs. 
       FIG.  4 B  illustrates a top view of the sensor assembly  350 A of  FIG.  4 A , in accordance with various embodiments. Section line A-A marks the location and orientation associated with sectional views illustrated in  FIG.  4 C . Section line B-B marks the location and orientation associated with sectional views illustrated in  FIG.  4 D . In the depicted embodiment, two sensors  150  are used, where sensor  150  may transmit while sensor  150 B receives, or vice versa. In other embodiments, a single sensor  150  may be utilized to both transmit and then receive its own returned signals. Likewise, in some embodiments, sensor  150  may both emit and then receive its own returned signals, with sensor  150 B both emitting and receiving its own returned signals—in such an embodiment sensor  150  and sensor  150 B may sense using different frequencies. 
       FIG.  4 C  illustrates a side sectional view A-A of a configuration of a sensor assembly  350 A, according to some embodiments. As can be seen, sonic sensor  150  includes a sonic transducer  401  (e.g., an ultrasonic transducer), which is covered by a cap  402  and separated from acoustic interface housing  450 A by a membrane  403 . Acoustic interface housing  450 A encloses and/or defines an acoustic interface tube  405  (which may also be referred to as a “horn” an “acoustic tube” or simply “tube”). Acoustic interface tube  405  facilitates the travel of sonic signals, with a direction of travel represented by directional arrow  410 , transmitted from sonic sensor  150  into upper opening  407  of the acoustic interface tube  405 , through the length of acoustic interface tube  405 , and out of acoustic opening port  406  toward surface  300 . Acoustic interface tube  405 , in some embodiments, similarly facilitates the travel of sonic signals (i.e., corresponding returned signals) with a direction of travel represented by directional arrow  411 , returned from surface  300  into acoustic opening port  406  of the acoustic interface tube  405 , through the length of acoustic interface tube  405 , and out of upper opening  407  to be received by sonic sensor  150 . It should be appreciated that the dimensions are presented only by way of example and that in other embodiments, for example, the diameter of the acoustic opening port  406  of acoustic interface tube  405  may be larger or smaller, or vary along the length (e.g., getting wider near acoustic opening port  406 ). Additionally, or alternatively, the length of acoustic interface tube  405  can also be varied (measured from upper opening  407  to acoustic opening port  406 ), in addition to the opening size of acoustic opening port  406  or the overall profile of acoustic interface tube  405 . In some embodiments, dimensions of acoustic interface tube  405  may be selected to limit or enlarge an acoustic field of view of the sonic sensor  150 . In some embodiments, the shape of acoustic opening port  406  may be other shapes besides circular. 
     In one example embodiment, the received returned signals (such as those indicated by arrow  411 ) are digitized. The digitization may be performed by a DSP on-board sonic sensor  150 , by a sensor processor  130 , by a host processor  110 , or by another processor. 
     Additionally,  FIG.  4 C  illustrates that one or more rings  497  may be defined in a slightly spaced-out distance from the acoustic opening port  406  of acoustic interface tube  405 . The ring(s) are annular with respect to acoustic opening port  406 . In some embodiments, when there is more than one defined ring  497 , the rings are concentric with respect to one another. The rings  497  may be debossed (such as defined into, engraved into, etc.) or debossed (such as such as defined above, raised from, etc.) external surface  475 . In  FIG.  4 C , the rings  497  are defined as debossed valleys  499  separated by ridges  498  which are level with external surface  475 . The defined rings  497  ( 497 - 1  and  497 - 2  in  FIG.  4 C ) operate to attenuate the acoustic pressure between device  100  (e.g., device  100 A or  100 B) and a nearby surface such as a floor and to attenuate direct path echoes. The valleys of the defined rings  497  cause them to differ in height from surface  475 . There are two main paths for signals to travel from sensor  150  to sensor  150 B, or vice versa: 1) sound exits acoustic opening port  406  which is associated with sensor  150 , takes a sharp angle, travels along the bottom surface (e.g., external surface  475 ) of device  100 , and enters the acoustic opening port  406 B (see  FIG.  4 D ) associated with sensor  150 B—this is referred to as the “direct path” or a “direct path echo;” 2) sound exits acoustic opening port  406  associated with sensor  150 , travels to the floor/surface (e.g., floor  300 ), and reflects back into acoustic opening port  406 B associated with sensor  150 B—this may be referred to as the “floor echo,” surface echo,” “indirect path,” or “indirect path echo.” The signal received via this indirect path, which includes primary returned signals and secondary (i.e., multipath) returned signals, will determine the characteristics of the floor and/or the presence/size/distance of a cliff. The defined rings  497  are designed to reduce the signal from path  1 , the direct path, thereby increasing the overall floor signal from path  2  (the indirect path) by effectively increasing the Signal to Noise Ratio (SNR) of signals received via the second path. By reducing signal path  1 , in comparison to an embodiment without defined rings, direct echo path reduction is accomplished versus the amount of direct echo experienced in a similar configuration but without use of defined rings. In some cases, the amount of reduction of direct path echoes may be as high as 25%; in some cases the amount of reduction in direct path echoes may be as high as 50%. 
     In the depicted embodiment, a plurality of defined rings  497  (e.g., each defined by a trough/valley  499  and a ridge  498 ) are illustrated. Advantages of decreased direct path echo and increased amplitude (especially when sensing hard floors such as wood, tile, or vinyl) are obtained with a single defined ring  497 - 1 . These advantages improve slightly and may be tuned by adding additional rings defined rings (e.g., defined ring  497 - 2 ) spaced annularly and concentrically outward from the first ring  497 - 1 . Two debossed rings  497  ( 497 - 1  and  497 - 2 ) are depicted, but other embodiments may employ a greater or lesser number of debossed rings. For example, in some embodiments there may be a single debossed ring while in others there may be two, five, ten, fifteen, twenty, or some other number which is limited by available space on surface  475  and the pitch of rings  497 . When two or more defined rings are employed, they may be referred to as a grate or a grating. The rings/grating (and the edges thereof) create additional reflections of the ultrasonic waves and therefore result in interference patterns that can shape the beam (outgoing and incoming). Thus, the defined rings, which may be referred to as a grating, can be designed to shape the ultrasonic beam in an optimal manner for the applications mentioned herein.  FIG.  7    illustrates a polar graph of some example beam shapes associated with particular ring configurations. Moreover, these defined rings/grating in the surrounding of the ultrasonic transducer&#39;s acoustic opening port maybe applied in any application that benefits from the beam shaping that occurs as a result of the defined rings/gratings. It should also be mentioned that although, as depicted, the transducer of sensor  150  is recessed below the surface, in other applications the transducers may be positioned in any desired way with respect to the sensor assembly  350 A. 
     Another application of the defined rings  497 , is that they can be tuned to reduce the sensor&#39;s ( 150 / 150 B) performance sensitivity to nearby features (e.g., features in the acoustic interface housing  450 A or housing  201  or the device  100 ). The physical mechanism is the same as use in the reduction in direct transmission between sensors but is more generalized. For example, nearby features such as a screw hole/step/seam/etc. in the housing  201 , or even another nearby sensor, can all alter the acoustic field of sensor  150  and therefore its performance. Design of the defined rings/grating can mitigate these types of effects by reducing received signals which have interacted with these nearby features. 
     The separation between defined rings is typically very small, such as between 0.5 mm and 2 mm—a measurement associated with the width of a ridge  498  between adjacent valleys  499 . In some embodiments, for example, the separation may be about  1 mm between defined rings and between the acoustic opening port  406  and the closest defined valley  499 . In some embodiments, depth of the valley/trough  499  of defined ring(s)  497  is very shallow, such as being a partial fraction of the separation distance. For example, the depth may be between 0.1 mm and 0.5 mm. In some embodiments, the depth of at least some of the valleys  499  may be approximately 0.3 mm. In some embodiments, the depth is at or between 0.1 and 1.0 wavelengths of the frequency of the sound emitted by the sensor  150 , which in typical ultrasonic sensors would be between 0.1 mm and 2.0 mm. The width of a valley  499  may be similar to the separation (e.g., the width of a ridge  498 ) between valleys  499 . 
     It should be appreciated that the pitch (the width of a valley  499 +a ridge  498 ), separation between defined rings  497  themselves, width of valleys, width of ridges, and/or a distance between the acoustic opening port  406  and the nearest defined ring  497  may be adjusted for different operating frequencies of sonic sensor  150 . In this manner, the grating made of a plurality of defined rings is tunable. The pitch selected for the defined rings  497  in a grating may be a function of the operating frequency of the sonic sensor  150 . For example, the pitch may be ⅛ wavelength, ¼ wavelength, ⅜ wavelength, ½ wavelength, or some other partial fraction of a wavelength of the operating frequency of the sonic sensor  150 . Different pitches may be utilized to attenuate or accentuate pressure in different axes of the sonic signal (e.g., to the front, to the sides, at desired angles). In this manner, a variety of beam patterns may be effectuated by grating composed of a plurality of defined rings. 
       FIG.  4 D  illustrates a side sectional view B-B of a configuration of a sensor assembly  350 A, according to some embodiments. As can be seen, sonic sensor  150 B includes a sonic transducer  401 B (e.g., an ultrasonic transducer), which is covered by a cap  402 B and separated from acoustic interface housing  450 A by a membrane  403 B. Acoustic interface housing  450 A encloses and/or defines an acoustic interface tube  405 B (which may also be referred to as a “horn” an “acoustic tube” or simply “tube”). In some embodiments, horn  405 B is substantially parallel (e.g., within manufacturing error) to horn  405  and has the same tilt angle and, when installed in a device  100 , both are pointed in a direction designated as the front of device  100 . Acoustic interface tube  405 B facilitates the travel of sonic signals, with a direction of travel represented by directional arrow  411 , returned from surface  300  into acoustic opening port  406 B of the acoustic interface tube  405 B, through the length of acoustic interface tube  405 B, and out of upper opening  407 B to be received by sonic sensor  150 B. It should be appreciated that the dimensions are presented only by way of example and that in other embodiments, for example, the diameter of the acoustic opening port  406 B of acoustic interface tube  405 B may be larger or smaller, or vary along the length (e.g., getting wider near acoustic opening port  406 B). Additionally, or alternatively, the length of acoustic interface tube  405 B can also be varied (measured from upper opening  407 B to acoustic opening port  406 B), in addition to the opening size of acoustic opening port  406  or the overall profile of acoustic interface tube  405 B. In some embodiments, dimensions of acoustic interface tube  405  may be selected to limit or enlarge an acoustic field of view of the sonic sensor  150 B. In some embodiments, the shape of acoustic opening port  406 B may be other shapes besides circular. 
     In  FIG.  4 D , defined rings  497  ( 497 - 1 B and  497 - 2 B) are disposed annularly around acoustic opening port  406 B in a concentric arrangement with respect to one another. Defined ring  497 - 1 B comprises a valley  499 - 1 B and a ridge  498 - 1 B, while defined ring  497 - 2  comprises a valley  499 - 2 B and a ridge  498 - 2 B. 
       FIG.  4 E  illustrates a bottom plan view of a configuration of the sensor assembly  350 A, according to some embodiments. Acoustic opening ports  406  and  406 B are circular but appear to be oval in shape due the tilt of bottom surface  475 . Likewise, defined rings  497 - 1 ,  497 - 2 ,  497 - 1 B, and  497 - 2 B appear oval in shape due to the tilt illustrated in this view. 
       FIG.  4 F  illustrates a straight on bottom view (where the viewpoint is taken orthogonal to the bottom surface  475 ) of a configuration of the sensor assembly  350 A, according to some embodiments. In this view, with the tilt removed from the view, it can be seen that openings  406  and  406 B are circular. Likewise, after removing the tilt from this view, it can be seen that defined rings  497 - 1 ,  497 - 2 ,  497 - 1 B, and  497 - 2 B are circular and spaced outward from their respective acoustic opening ports ( 406  and  406 B). Although the rings  497  depicted around opening  406  and  406 B are equal in number, this is not a requirement. In some embodiments, annular rings  497  around opening  406  and annular rings  497  around opening  406 B are equal in number, height (from valley to ridge), and pitch. 
     Though depicted as circular, various other shapes and configurations of the defined rings  497  may be utilized. The pattern of a defined ring may be symmetric or non-symmetric and may take any suitable form to achieve the desired attenuation. 
     Although the illustrated acoustic interface housing  450 A provided a tilt angle  489  (with respect to a floor surface) for sensors  150 , in some embodiments the acoustic interface housing  450 A may provide for no tilt angle away from orthogonal to surface  300 . That is, in some embodiments, the sensor(s)  150  may be aimed vertically toward the expected location of a floor. Such an example is illustrated in  FIGS.  6 A- 6 D . As is depicted in  FIGS.  6 A- 6 D , one or more defined rings may be employed in a similar fashion about the opening to the tube/horn of the acoustic interface housing  450 A to similar effect (i.e., reduced direct transmission and increased amplitude from received returned signals). 
       FIG.  5 A  illustrates a side elevational view of one example sensor assembly  350 B, with embossed rings defined in an external surface  575 , which may be utilized on a device  100  to transmit sonic signals and receive returned sonic signals, in accordance with various embodiments. It should be appreciated that sensor assembly  350 B is similar to and shows some alternative embodiments and features to those shown in sensor assembly  350 A. One difference is region  501  which is raised or embossed from surface  575  and within which one or more rings  597  (see e.g.,  FIG.  5 C ) are defined. External surface  575  is designed to oriented in device  100  such that it is parallel to surface  300  upon which device  100  operates. 
     Sensor assembly  350 B includes at least a sonic sensor  150  and typically includes two sonic transducers ( 150  and  150 B). An acoustic interface housing  450 B, which encloses and/or defines an acoustic interface tube  505 , in the form of a tube, cavity, horn, or some combination thereof, is included and coupled with sonic sensor  150 . In some embodiments, acoustic interface housing  450 B may position sonic sensor(s)  150  (e.g.,  150  and  150 B) at an orthogonal angle with respect to a surface it is detecting (i.e., a floor). In some embodiments, as depicted in  FIGS.  6 A- 6 D , acoustic interface housing  450 B may position sonic sensor(s)  150  (e.g.,  150  and  150 B) at a non-orthogonal tilt angle  589  with respect to a surface it is detecting (i.e., a floor). By tilting sensor(s)  150 , it/they may sense farther in advance of the direction of forward travel of device  100  than if it/they were orthogonal with the expected location of the floor (i.e., pointed straight down at the floor). In some embodiments, the angle of tilt  589  from orthogonal may be in the range of greater than 0 degrees from orthogonal and up to 30 degrees from orthogonal. In some embodiments, the angle of tilt  589  may be between about 10 from orthogonal and about 20 degrees from orthogonal. In some embodiments, the angle of tilt  589  is approximately 15 degrees from orthogonal to the floor (e.g., floor  300 ) or other flat surface beneath device  100 . Within the range of tilt angles, a smaller angle from orthogonal increases the amplitude of returned signals (more accurate floor type determination), but also increases the latency associated with detecting the edge of a cliff in front of device  100  in the direction of travel of device  100 . Within the range of tilt angles, a larger angle from orthogonal decreases the amplitude of returned signals as they tend to scatter more, but also decreases the latency associated with detecting the edge of a cliff. For example, in some embodiments where the angle of tilt  589  is around 15 degrees, there is good balance between amplitude of returned signals from a surface below device  100  and reduced latency of cliff detection in front of device  100  in the forward direction of travel of device  100 . The tilt angle  589  for an application/device may be adapted to the desired specification of the device and to the design characteristics of the complete device (e.g., size, height, distance to floor, speed of movement, cliff detection and/or depth estimation latency desired/required). In some embodiments, the title angle may be adjustable automatically depending on application or context. 
     In some embodiments, a plurality of sensors  150  (e.g.,  150  and  150 B) may be employed in an acoustic interface housing  450 B (or different housings) which facilitate different tilt angles for at least two of the sensors  150 . In such an embodiment, a sensor with a shallow tilt angle from vertical (i.e., orthogonal to floor  300 ) affords better (greater) amplitude while a sensor with a greater angle of tilt from vertical provides better (less) latency in cliff detection. 
     In some embodiments, the sensor assembly  350 B is coupled with a device housing  201  (or other portion of device  100 ) such that it transmits the sonic signals in a direction toward which a surface is expected to be encountered or operated upon (e.g., downward toward a floor surface). For example, in a floor vacuum or robotic floor cleaning embodiment of device  100 , a sensor assembly  350 B may be disposed on or configured to sense outward from the bottom of device  100  in the manner illustrated in  FIG.  3   , such that transmitted sonic signals are directed toward a floor surface  300  below and in front of device  100  when device  100  is in operation. As described in  FIGS.  1 A and  1 B  a sonic sensor  150  (e.g.,  150 ,  150 B) may also be coupled with a host processor  110  or a sensor processor, in some embodiments, either or both of which operate to process and make determinations based upon received returned signals such as detecting surface types and/or detecting cliffs. 
       FIG.  5 B  illustrates a top view of the sensor assembly  350 B of  FIG.  5 A , in accordance with various embodiments. Section line C-C marks the location and orientation associated with sectional views illustrated in  FIG.  5 C . In the depicted embodiment, two sensors  150  are used, where sensor  150  may transmit while sensor  150 B receives, or vice versa. In other embodiments, a single sensor  150  may be utilized to both transmit and then receive its own returned signals. Likewise, in some embodiments, sensor  150  may both emit and then receive its own returned signals, with sensor  150 B both emitting and receiving its own returned signals—in such an embodiment sensor  150  and sensor  150 B may sense using different frequencies. 
       FIG.  5 C  illustrates a side sectional view C-C of a configuration of a sensor assembly  350 B, according to some embodiments. Like numbered items are the same or similar to those previously described in conjunction with  FIG.  4 C . Acoustic interface housing  450 B is similar to housing  450 A and encloses and/or defines an acoustic interface tube  505  (which may also be referred to as a “horn” an “acoustic tube” or simply “tube”). Acoustic interface tube  505  is similar to tube  405  and acoustic opening port  506  is similar to port  406 . In general, the operation of acoustic interface housing  450 B is similar to that of housing  450 A, except that the rings  597  disposed around acoustic opening port  506  are embossed (i.e., defined in a fashion that raises them from surface  575 ) rather than debossed/engraving them into the surface as depicted in  FIG.  4 C . Three embossed rings  597  ( 597 - 1 ,  597 - 2 , and  597 - 3 ) are depicted, but other embodiments may employ a greater or lesser number of embossed rings. For example, in some embodiments there may be a single embossed ring while in others there may be two, five, ten, fifteen, twenty, or some other number which is limited by available space on surface  575  and the pitch of rings  597 . As depicted, rings  597  each have a ridge  598  and a valley  599 . For example: ring  597 - 1  comprises ridge  598 - 1  and trough/valley  599 - 1 ; ring  597 - 2  comprises ridge  598 - 2  and valley  599 - 2 ; ring  597 - 3  comprises ridge  598 - 3  and valley  599 - 3 . Additionally, the outer edge of ring  597 - 3  is further defined by ridge  598 - 4 . The ridges of the defined rings  597  cause them to differ in height from surface  575 . The pitch of the rings may be measured from a leading edge of a ridge to the leading edge of a consecutive ridge or may be measured as the width of a ridge plus the width of a valley for a ring. In the depicted example, the ridges  598  are narrower in width than the valleys  599 . In some embodiments, this relationship may be swapped with the ridges being wider than the valleys, while in other embodiments the ridges and valleys may be of the same width. It should be appreciated that a similar configuration of rings to that shown in  FIG.  5 C  may be implemented as embossed rings or as debossed rings. 
       FIG.  5 C  illustrates that one or more rings  597  may be defined in a slightly spaced-out distance from the acoustic opening port  506  of acoustic interface tube  505 . The ring(s) are annular with respect to acoustic opening port  506 . In some embodiments, when there is more than one defined ring  597 , the rings are concentric with respect to one another. 
     Another application of the defined rings  597 , is that they can be tuned to reduce the sensor&#39;s ( 150 / 150 B) performance sensitivity to nearby features (e.g., features in the acoustic interface housing  450 B or housing  201  or the device  100 ). The physical mechanism is the same as used in the reduction in direct transmission between sensors but is more generalized. For example, nearby features such as a screw hole/step/seam/etc. in the housing  201 , or even another nearby sensor, can all alter the acoustic field of sensor  150  and therefore its performance. Design of the defined rings/grating can mitigate these types of effects by reducing received signals which have interacted with these nearby features. 
     The dimensions of a defined ring and separation between defined rings (when more than one is employed around an acoustic opening port) is typically very small, as has previously been described, and dimensions are similar or in the same ranges as described with respect to rings  497  of  FIG.  4 C . 
     It should be appreciated that the pitch (the width of a valley  599 +a ridge  598 ), or separation between defined rings  597  themselves and/or between the acoustic opening port  506  and the nearest defined ring  597  may be adjusted for different operating frequencies of sonic sensor  150 . In this manner, the grating made of a plurality of defined rings  597  is tunable. The pitch selected for the defined rings  597  in a grating may be a function of the operating frequency of the sonic sensor  150 . For example, the pitch may be ⅛ wavelength, ¼ wavelength, ⅜ wavelength, ½ wavelength, or some other partial fraction of a wavelength of the operating frequency of the sonic sensor  150 . Different pitches may be utilized to attenuate or accentuate pressure in different axes of the sonic signal (e.g., to the front, to the sides). In this manner, a variety of beam patterns may be effectuated by grating composed of a plurality of defined rings. 
       FIG.  5 D  illustrates a straight on bottom view (where the viewpoint is taken orthogonal to the bottom surface  575 ) of a configuration of the sensor assembly  350 B of  FIG.  5 A , according to some embodiments. In this view, with the tilt removed from the view, it can be seen that openings  506  and  506 B are circular. Opening  506 B is the opening of a horn  505 B, which is substantially parallel (e.g., within manufacturing error) to horn  505  and has the same tilt angle, in some embodiments; and when installed in a device  100 , both are pointed in a direction designated as the front of device  100 . Likewise, after removing the tilt from this view, it can be seen that defined rings  597 - 1 ,  597 - 2 ,  597 - 3 ,  597 - 1 B,  597 - 2 B, and  597 - 3 B are circular and spaced concentrically outward from their respective acoustic opening ports ( 406  and  406 B). Ring  597 - 1 B comprises ridge  598 - 1 B and trough/valley  599 - 1 B; ring  597 - 2 B comprises ridge  598 - 2 B and valley  599 - 2 B; ring  597 - 3 B comprises ridge  598 - 3 B and valley  599 - 3 B. Additionally, in the described embodiment, the outer defining ridges (ridges  598 - 4  and  598 - 4 B) of rings  597 - 3  and  597 - 3 B are tangent to one another. In a similar fashion, the outer valleys of embossed or debossed rings may be tangent to one another, in some embodiments. 
     Though depicted as circular, various other shapes and configurations of the defined rings  597  may be utilized. The pattern of a defined ring may be symmetric or non-symmetric and may take any suitable form to achieve the desired attenuation. Although the rings  597  depicted around opening  506  and  506 A are equal in number, this is not a requirement. In some embodiments, annular rings  597  around opening  506  and annular rings  597  around opening  506 B are equal in number, height (from valley to ridge), and pitch. 
     Although the illustrated acoustic interface housing  450 B provided a tilt angle  589  (with respect to a floor surface) for sensors  150 , in some embodiments the acoustic interface housing  450 B may provide for no tilt angle away from orthogonal to surface  300 . That is, in some embodiments, the sensor(s)  150  may be aimed vertically toward the expected location of a floor. Such an example is illustrated in  FIGS.  6 A- 6 D . 
       FIG.  6 A  illustrates a side elevational view of one example sensor assembly  350 C, with debossed rings defined in an external surface  675 , which may be utilized on a device  100  to transmit sonic signals and receive returned sonic signals, in accordance with various embodiments. It should be appreciated that sensor assembly  350 C is similar to and shows some alternative embodiments and features to those shown in sensor assembly  350 A and sensor assembly  350 B. External surface  675  is designed to oriented in device  100  such that it is parallel to surface  300  upon which device  100  operates. 
     Sensor assembly  350 C includes at least a sonic sensor  150  and typically includes two sonic transducers ( 150  and  150 B). An acoustic interface housing  450 C, which encloses and/or defines an acoustic interface tube  605 , in the form of a tube, cavity, horn, or some combination thereof, is included and coupled with sonic sensor  150 . In some embodiments, as depicted in  FIGS.  6 A- 6 D , acoustic interface housing  450 C may position sonic sensor(s)  150  (e.g.,  150  and  150 B) at an orthogonal angle  689  with respect to a surface it is detecting (i.e., a floor). In some embodiments, acoustic interface housing  450 C may alternatively position sonic sensor(s)  150  (e.g.,  150  and  150 B) at a non-orthogonal tilt angle with respect to a surface it is detecting (i.e., a floor) in the manner previously described and discussed with respect to acoustic interface housings  450 A and  450 B. In some embodiments, when a tilt angle from orthogonal is employed, the tilt angle for an application/device may be adapted to the desired specification of the device and to the design characteristics of the complete device (e.g., size, height, distance to floor, speed of movement, cliff detection and/or depth estimation latency desired/required). In some embodiments, the title angle may be adjustable automatically depending on application or context. 
     In some embodiments, a plurality of sensors  150  (e.g.,  150  and  150 B) may be employed in an acoustic interface housing  450 C (or different housings) which facilitate different tilt angles for at least two of the sensors  150 . In such an embodiment, a sensor with a shallow tilt angle from vertical (i.e., orthogonal to floor  300 ) affords better (greater) amplitude while a sensor with a greater angle of tilt from vertical provides better (less) latency in cliff detection. 
     In some embodiments, the sensor assembly  350 C is coupled with a device housing  201  (or other portion of device  100 ) such that it transmits the sonic signals in a direction toward which a surface is expected to be encountered or operated upon (e.g., downward toward a floor surface). For example, in a floor vacuum or robotic floor cleaning embodiment of device  100 , a sensor assembly  350 C may be disposed on or configured to sense outward from the bottom of device  100  in the manner illustrated in  FIG.  3   , such that transmitted sonic signals are directed toward a floor surface  300  below device  100  when device  100  is in operation. As described in  FIGS.  1 A and  1 B  a sonic sensor  150  (e.g.,  150 ,  150 B) may also be coupled with a host processor  110  or a sensor processor, in some embodiments, either or both of which operate to process and make determinations based upon received returned signals such as detecting surface types and/or detecting cliffs. 
       FIG.  6 B  illustrates a top view of the sensor assembly  350 C of  FIG.  6 A , in accordance with various embodiments. Section line D-D marks the location and orientation associated with sectional views illustrated in  FIG.  6 C . In the depicted embodiment, two sensors  150  are used, where sensor  150  may transmit while sensor  150 B receives, or vice versa. In other embodiments, a single sensor  150  may be utilized to both transmit and then receive its own returned signals. Likewise, in some embodiments, sensor  150  may both emit and then receive its own returned signals, with sensor  150 B both emitting and receiving its own returned signals—in such an embodiment sensor  150  and sensor  150 B may sense using different frequencies. 
       FIG.  6 C  illustrates a side sectional view D-D of a configuration of a sensor assembly  350 C, according to some embodiments. Like numbered items are the same or similar to those previously described in conjunction with  FIG.  4 C and  5 C . Acoustic interface housing  450 C is similar to housings  450 A and  450 B and encloses and/or defines an acoustic interface tube  605  (which may also referred to as a “horn” an “acoustic tube” or simply “tube”). Acoustic interface tube  605  is similar to tubes  405  and  505  and acoustic opening port  606  is similar to ports  406  and  506 . In general, the operation of acoustic interface housing  450 C is similar to that of housings  450 A and  450 B, except that only a single defined ring  697 - 1  is disposed around acoustic opening port  606 . Although one debossed ring  697 - 1  is depicted, other embodiments may employ a greater number of debossed rings such as 2, 3, 4, 5, 10, 15, 20, or some other number limited by space on surface  675  and ring pitch if more than one ring  697  is employed. As depicted, ring  697 - 1  has a ridge  698 - 1  and a valley  699 - 1 . The pitch of the ring may be measured as the width of a ridge plus the width of a valley for a ring. In the depicted example, the ridge  698 - 1  is greater in width than the valley  699 - 1 . In some embodiments, this relationship may be swapped with the valley being wider than the ridge, while in other embodiments the ridge and valley may be of the same width. Although shown as being debossed, ring  697 - 1  may similarly be embossed. 
       FIG.  6 C  illustrates that one or more rings  697  may be defined in a slightly spaced-out distance from the acoustic opening port  606  of acoustic interface tube  605 . The ring(s) are annular with respect to acoustic opening port  606 . In some embodiments, when there is more than one defined ring  697 , the rings are concentric with respect to one another. Ring(s)  697  can be employed in the same manner as previously described with respect to rings  497  and  597 . In some embodiments, annular ring(s)  697  around opening  606  and annular ring(s)  697  around opening  606 B are equal in number, height (from valley to ridge), and pitch. 
     The dimensions of a defined ring and separation between defined rings (when more than one is employed around an acoustic opening port) is typically very small, as has previously been described, and dimensions are similar or in the same ranges as described with respect to rings  497  of  FIG.  4 C . 
     It should be appreciated that the pitch (the width of a valley  699 +a ridge  698 ), or separation between defined rings  597  themselves, and/or a distance between the acoustic opening port  606  and the nearest defined ring  697  may be adjusted for different operating frequencies of sonic sensor  150 . In this manner, the grating made of a one or a plurality of defined rings  697  is tunable. The pitch selected for the defined ring(s)  697  in a grating may be a function of the operating frequency of the sonic sensor  150 . For example, the pitch may be ⅛ wavelength, ¼ wavelength, ⅜ wavelength, ½ wavelength, or some other partial fraction of a wavelength of the operating frequency of the sonic sensor  150 . Different pitches may be utilized to attenuate or accentuate pressure in different axes of the sonic signal (e.g., to the front, to the sides). In this manner, a variety of beam patterns may be effectuated by grating composed of a plurality of defined rings. 
       FIG.  6 D  illustrates a straight on bottom view (where the viewpoint is taken orthogonal to the bottom surface  675 ) of a configuration of the sensor assembly  350 C of  FIG.  6 A , according to some embodiments. In this view openings  606  and  606 B are visibly circular. Opening  606 B is the opening of a horn  605 B which is substantially parallel (e.g., within manufacturing error) to horn  605  and has the same tilt angle, in some embodiments; and when installed in a device  100 , both are pointed in a direction designated as the front of device  100 . Likewise, defined rings  697 - 1  and  697 - 1 B are also circular spaced concentrically outward from their respective acoustic openings ( 606  and  606 B). Ring  697 - 1 B comprises ridge  698 - 1 B and trough/valley  699 - 1 B. 
     Though depicted as circular, various other shapes and configurations of the defined rings  697  may be utilized. The pattern of a defined ring may be symmetric or non-symmetric and may take any suitable form to achieve the desired attenuation. 
     Although the illustrated acoustic interface housing  450 C provided no tilt angle from orthogonal (with respect to a floor surface) for sensors  150 , in some embodiments the acoustic interface housing  450 C may provide for some tilt angle away from orthogonal to surface  300 . 
       FIG.  7    illustrates a polar graph  700  several examples of beam shapes which may be implemented via some example configurations of rings defined annularly around one or more an acoustic opening ports of a sensor assembly. Such beam shaping may be employed with a transmitting sonic sensor and/or with a receiving sonic sensor. For purposed of example, and not of limitation, consider that the graph  700  illustrates gratings which include  10  defined rings that are spaced as a function of the wavelength, A, of the sonic signal being transmitted or received as corresponding returned signals. For example: beam shape  701  is smooth and is associated with no grating (i.e., no defined rings of the type described herein); beam shape  702  has a narrow forward lobe at 0 degrees, two narrow side lobes (at about +90 degrees and −90 degrees), and two additional side lobes (centered at about +30 degrees and −30 degrees), and is associated with a grating having a pitch of λ/8; beam shape  703  has a very narrow forward lobe at 0 degrees and small side lobes (at about +25 degrees, +60 degrees, −25 degrees, and −60 degrees), and is associated with a grating having a pitch of 2λ/8; beam shape  704  has a somewhat circular forward lobe centered at  0  degrees, and is associated with a grating having a pitch of 3λ/8; and beam shape  705  has a somewhat circular forward lobe centered at 0 degrees, and is associated with a grating having a pitch of 4λ/8. It should be appreciated that other beam shapes may be utilized and that in some embodiments, a pitch may be at or greater than the wavelength rather than being a partial fractional value of the wavelength. In some embodiments, by way of example and not of limitation, a configuration of grating/defined ring(s) associated with beam shapes  703 ,  704 , and/or  705  may be employed to attenuate direct path echoes which result from a subset of the transmitted sonic signals from a first sensor, which attempt to travel across the external surface (e.g.,  475 ,  575 ,  675 ) between a first acoustic opening port (e.g.,  406 ,  506 ,  606 ) and a second acoustic opening port (e.g.  406 B,  506 B,  606 B). 
     Cliff Detection and Depth Estimation Using Tilted Sensors 
     When detecting for drop-offs/cliffs from a moving device  100 , latency is a concern. With a sensor  150  (or multiple sensors  150 ) which are pointed straight down at a surface  300 , forward speed of a device  100  is limited by the latency of sending out a sonic signal, receiving corresponding returned signals, processing the signals, and then making a determination about whether a cliff exists and/or what floor type is below the device  100 . For example, and with reference to  FIG.  3   , when a straight down (i.e., orthogonal) orientation is employed for sensor assembly  350 , forward wheel  305  has to be positioned far enough behind sensor assembly  350  to account for the latency so that the wheel  305  will not go over a cliff edge before it is detected. In this manner, cliff detection latency may dictate the separation between sensor assembly  350  and wheel  305  and may also limit forward speed (i.e., in direction  301 ) of device  100  to a speed at which cliff detection can occur and action can be taken before wheel  305  goes over a cliff edge. 
     As discussed previously herein, adding a small tilt angle to sensor assembly  350  may be employed in some embodiments to allow sensor assembly to sense both downward toward floor  300  (for floor type detection) and forward in direction of travel  301  (to see cliff edges before being directly on top of them). As discussed previously, this tilt angle may be in a range of greater than 0 degrees and up to about 30 degrees to provide a combination of forward looking and downward looking. In various embodiments, the tilt angle may be utilized with or without the defined ring(s) described herein. When used with a defined ring, crosstalk in the form of a direct path echo between sensor  150  and sensor  150 B is reduced (as has been described) and consequently signal-to-noise ratio is increased. Increasing the signal-to-noise ratio results in greater sensitivity to cliff detection, as it is less likely that a direct path echo will be interpreted as a cliff and less likely a direct path echo will obscure a signal from an actual cliff (which may have a small amplitude). Use of a tilt angle (and additionally, in some embodiments, use of one or more defined rings) allows for one or more of: earlier detection of cliffs due to forward looking, more reliable detection of cliffs due to improved signal-to-noise ratio, allowing for increased spacing between a sensor assembly  350  and a wheel  301  due to reduced latency of cliff detection (i.e., the time between detecting a cliff and being at/above its edge); and/or facilitating increased speed of device  100  due to reduced latency of cliff detection. By way of example, and not of limitation, in an embodiment of a device  100  using an 18 mm long acoustic interface tube  405  it has been noted that adjusting the tilt angle from zero (pointed straight down) by fifteen degrees in a forward direction of travel results in: cliff detection occurring 621 ms earlier for a device  100  traveling at a slow speed of 20 mm/second; and cliff detection occurring 124 ms earlier for a device  100  traveling at a medium speed of 100 mm/second. In an example where a device  100  is using a 50 mm long acoustic interface tube  405  with a 15 degree tilt in a forward direction of travel of device  100 , cliff detection occurs 176 mm sooner (than it would for a zero tilt acoustic interface tube  405 ) when the device  100  is traveling at a medium speed of 100 mm/second. 
     Cliff detection can take one or both of two forms. First, cliff detection may involve sensing an absence of a floor  300  where one was previously present or was expected to be present. Second, cliff detection may involve sensing a drop-off or lower area in or the near vicinity of the detected floor  300  upon which a device  100  is operating. Processor  110  and/or sensor processor  130  may analyze/process corresponding returned signals received (e.g., by sensor  150 B) to accomplish such cliff detections. In the second example, a depth estimation for a detected cliff may be performed. 
       FIG.  8    shows an illustration  800  of the ultrasonic sensors of a sensor assembly  450  detecting a cliff and estimating its depth, in accordance with various embodiments. For purposes of simplicity and clarity, device  100  (e.g., device  100 A or  100 B) is omitted and sensors  150  and  150 B are shown in a tilt angle as they would be held by housing  450 A of  FIG.  4 A  (which, for purposes of clarity, is not depicted). The distance (X) between the centers of the openings of the sensors  150  and  150 B is illustrated and known. In some embodiments distance X is small, such as between 1 cm and 2 cm. Sensor  150 B is operating as a receiver, while sensor  150  is operating as an emitter/transmitter. The angle of tilt (e.g., tilt angle  489  in  FIG.  4 A ) from orthogonal (theta) allows emitter  150  to project its signal forward off the edge of stair riser  801  to the tread  810  at the top of the next step downward. Similarly, the tilt angle  489  (shown as theta) of sensor  150 B allows it to readily receive returned signals that reflect from point  815  on tread  810 . In this manner, detection of the cliff drop-off of riser  801  occurs sooner than it would if both sensors  150  and  150 B were oriented orthogonal to floor  300  (e.g., in the manner illustrated in  FIG.  6 A ) rather than tilted from orthogonal with respect to floor  300 . Via time-of-flight measurement between emission and receipt of corresponding returned signals and vial the use of routine trigonometry the distances D 1 , D 2 , and X 1  can be calculated and the depth (Z) from the sensor  150 B to tread  810  can be determined. With prior knowledge of approximately how high sensors  150  and  150 B are mounted above floor  300  (such as within a device  100 ), subtraction of this known height value from height Z above the detected cliff floor (e.g., tread  810 ) can be used to estimate the cliff depth  805  (i.e., the depth of the drop from floor  300  to tread  810 ). These calculations may be performed by host processor  110  and/or sensor processor  130 . If the drop-off height  805  exceeds a preestablished threshold associated with a device  100 , a host processor  110  of device  100  may direct certain actions such as reversing direction, slowing forward travel toward the edge of the cliff/drop-off, and/or initiating a turn to avoid device  100  traveling over the cliff and dropping. 
     In some embodiments, it may only be desired to know a rough estimate of cliff height  805 , to see if it exceeds a threshold which a device  100  can handle safely. In one such embodiment, the distance between sensors  150  and  150 B is small and other distances are typically not very large (e.g., less 50 cm and sometimes less than 25 cm when dealing with stairs). Accordingly, for a rough estimate, time-of-flight may be used to calculate distance d 2  and it may be presumed (for rough estimate purposes) to be equal to distance d 1 . Then, solving for a right triangle with a height of Z and an angle of theta between sides Z and d 2 , a rough estimate of distance Z is easily calculated with basic trigonometry. The known height of sensor  150 B above floor  300  may be subtracted from distance Z to achieve a rough estimate of height  805  (i.e., the drop-off depth). These calculations may be performed by host processor  110  and/or sensor processor  130 . If the drop-off height  805  exceeds a preestablished threshold associated with a device  100 , a host processor  110  of device  100  may direct certain actions such as reversing direction, slowing forward travel toward the edge of the cliff/drop-off, and/or initiating a turn to avoid device  100  traveling over the cliff and dropping. 
     In yet other embodiments, the relationship between distance d 1 , distance Z, and drop-off height  805  may be predetermined when the tilt angle theta is fixed. Accordingly, when a distance d 2  is measured by time-of-flight of sonic signals the distance d 2  can be compared by processor  110  to a preestablished threshold value of distance d 2  that is correlates with a drop-off  805  which is expected to be harmful to the operation of device  100 . In some embodiments, a lookup table may additionally or alternatively be used to correlate a measured distance d 2  with a drop-off height  805 . When distance d 2  or the correlated drop-off height  805  exceeds such a preestablished threshold, host processor  110  of device  100  may direct certain actions such as reversing direction, slowing forward travel toward the edge of the cliff/drop-off, and/or initiating a turn to avoid device  100  traveling over the cliff and dropping. 
       FIG.  9 A and  9 B  show graphs  900 A and  900 B of the amplitude of received corresponding returned signals versus distance for sensor assembly  350  tilted at 15 degrees from orthogonal and employing no defined rings around acoustic opening ports of the sensors  150  and  150 B, in accordance with an embodiment. 
     In  FIGS.  9 A and  9 B  the x-axis is in units of a range index which may be converted to distance away from the sensors, while the y-axis is in units of amplitude of the corresponding returned signals received by a receiving sensor. 
     With reference to  FIG.  9 A , the diagramed signal  910  shows a peak  911  at range index  6  which is related to a startup/ringdown signal of the receiving sensor (e.g., sensor  150 B) and a peak  912  at range index  19  related to the detection of a static (non-moving) hard floor (e.g., tile or wood). 
     With reference to  FIG.  9 B  the hard floor has been removed and replaced with a static (non-moving) large cliff from which no corresponding returned signals are received by the receiving sensor (e.g., sensor  150 B), the diagramed signal  920  shows a peak  921  at range index  6  which is related to a startup/ringdown signal of the receiving sensor (e.g., sensor  150 B) and a small peak  922  at range index  19  related cross-talk on the direct echo path between the emitting sensor  150  and the receiving sensor  150 B. It should be noted that this peak  921  would exist as a noise component of peak  912 . 
       FIG.  10 A and  10 B  show graphs  1000 A and  1000 B of the amplitude of received corresponding returned signals versus distance for sensor assembly  350  tilted at 15 degrees from orthogonal and employing a single defined ring around each of the emitting sensor&#39;s  150  acoustic opening port and the receiving sensor&#39;s  150 B acoustic opening port, in accordance with an embodiment. 
     With reference to  FIG.  10 A , the diagramed signal  1010  shows a peak  1011  at range index  6  which is related to a startup/ringdown signal of the receiving sensor (e.g., sensor  150 B) and a peak  1012  at range index  19  related to the detection of a static (non-moving) hard floor (e.g., tile or wood). Although the static hard floor is the same as the one sensed in  FIG.  9 A , the sensed amplitude is less (at around 60%), which is still sufficiently high to detect the hard floor and/or differentiate between different floor types (hard/soft, wet/dry, etc.) 
     With reference to  FIG.  10 B  the static hard floor has been removed and replaced with a static (non-moving) large cliff from which no corresponding returned signals are received by the receiving sensor (e.g., sensor  150 B), the diagramed signal  1020  shows a peak  1021  at range index  6  which is related to a startup/ringdown signal of the receiving sensor (e.g., sensor  150 B) and is substantially the same amplitude as peak  911 ,  921 , and  1011 . A second small peak  1022  is at range index  19  and is related crosstalk in the form of direct path echo between the emitting sensor  150  and the receiving sensor  150 B. It should be noted that this peak  1021  is substantially diminished such that it is less than half the amplitude of peak  921  (approximately 40%). Accordingly, the use of the defined rings has reduced this direct echo path noise by nearly 60%. Thus, even though the amplitude of the hard floor is lower in  FIG.  10   , the noise component of the direct path echo is also much lower, meaning that the signal to noise ratio is improved by use of the defined rings. In various experiments under a variety of conditions, the use of a single defined ring around each acoustic opening port of a transmitting sensor and a receiving sensor has been found to reduce direct path echo between sensors by between 9% and 59%, with an average reduction of 29%. Accordingly, utilizing a plurality of rings around each acoustic opening port to shape the emitted and received beams of ultrasonic signals can reduce the direct echo path by at least this much and more. 
     Conclusion 
     The examples set forth herein were presented in order to best explain, to describe particular applications, and to thereby enable those skilled in the art to make and use embodiments of the described examples. However, those skilled in the art will recognize that the foregoing description and examples have been presented for the purposes of illustration and example only. The description as set forth is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims. 
     Reference throughout this document to “one embodiment,” “certain embodiments,” “an embodiment,” “various embodiments,” “some embodiments,” or similar term means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of such phrases in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics of any embodiment may be combined in any suitable manner with one or more other features, structures, or characteristics of one or more other embodiments without limitation.