Patent Publication Number: US-11651762-B2

Title: Reverberation gain normalization

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Non-Provisional patent application Ser. No. 17/020,584, filed on Sep. 14, 2020, which is a continuation of U.S. Non-Provisional patent application Ser. No. 16/442,359, filed on Jun. 14, 2019, now U.S. Pat. No. 10,810,992, which claims benefit of U.S. Provisional Patent Application No. 62/685,235, filed on Jun. 14, 2018, which are hereby incorporated by reference in their entirety. 
    
    
     FIELD 
     This disclosure relates in general to reverberation algorithms and reverberators for using the disclosed reverberation algorithms. More specifically, this disclosure relates to calculating a reverberation initial power (RIP) correction factor and applying it in series with a reverberator. This disclosure also relates to calculating a reverberation energy correction (REC) factor and applying it in series with a reverberator. 
     BACKGROUND 
     Virtual environments are ubiquitous in computing environments, finding use in video games (in which a virtual environment may represent a game world); maps (in which a virtual environment may represent terrain to be navigated); simulations (in which a virtual environment may simulate a real environment); digital storytelling (in which virtual characters may interact with each other in a virtual environment); and many other applications. Modem computer users are generally comfortable perceiving, and interacting with, virtual environments. However, users&#39; experiences with virtual environments can be limited by the technology for presenting virtual environments. For example, conventional displays (e.g., 2D display screens) and audio systems (e.g., fixed speakers) may be unable to realize a virtual environment in ways that create a compelling, realistic, and immersive experience. 
     Virtual reality (“VR”), augmented reality (“AR”), mixed reality (“MR”), and related technologies (collectively, “XR”) share an ability to present, to a user of an XR system, sensory information corresponding to a virtual environment represented by data in a computer system. Such systems can offer a uniquely heightened sense of immersion and realism by combining virtual visual and audio cues with real sights and sounds. Accordingly, it can be desirable to present digital sounds to a user of an XR system in such a way that the sounds seem to be occurring naturally, and consistently with the user&#39;s expectations of the sound in the user&#39;s real environment. Generally speaking, users expect that virtual sounds will take on the acoustic properties of the real environment in which they are heard. For instance, a user of an XR system in a large concert hall will expect the virtual sounds of the XR system to have large, cavernous sonic qualities; conversely, a user in a small apartment will expect the sounds to be more dampened, close, and immediate. 
     Digital, or artificial, reverberators may be used in audio and music signal processing to simulate perceived effects of diffuse acoustic reverberation in rooms. A system that provides accurate and independent control of reverberation loudness and reverberation decay for each digital reverberator, for example, for intuitive control for sound designers may be desired. 
     BRIEF SUMMARY 
     Systems and methods for providing accurate and independent control of reverberation properties are disclosed. In some embodiments, a system may include a reverberation processing system, a direct processing system, and a combiner. The reverberation processing system can include a reverb initial power (RIP) control system and a reverberator. The RIP control system can include a reverb initial gain (RIG) and a RIP corrector. The RIG can be configured to apply a RIG value to the input signal, and the RIP corrector can be configured to apply a RIP correction factor to the signal from the RIG. The reverberator can be configured to apply reverberation effects to the signal from the RIP control system. 
     In some embodiments, the reverberator can include one or more comb filters to filter out one or more frequencies in the system. The one or more frequencies can be filtered out to mimic environmental effects, for example. In some embodiments, the reverberator can include one or more all-pass filters. Each all-pass filter can receive a signal from the comb filters and can be configured to pass its input signal without changing its magnitude, but can change a phase of the signal. 
     In some embodiments, the RIG can include a reverb gain (RG) configured to apply a RG value to the input signal. In some embodiments, the RIG can include a REC configured to apply a RE correction factor to the signal from the RG. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates an example wearable system, according to some embodiments. 
         FIG.  2    illustrates an example handheld controller that can be used in conjunction with an example wearable system, according to some embodiments. 
         FIG.  3    illustrates an example auxiliary unit that can be used in conjunction with an example wearable system, according to some embodiments. 
         FIG.  4    illustrates an example functional block diagram for an example wearable system, according to some embodiments. 
         FIG.  5 A  illustrates a block diagram of an example audio rendering system, according to some embodiments. 
         FIG.  5 B  illustrates a flow of an example process for operating the audio rendering system of  FIG.  5 A , according to some embodiments. 
         FIG.  6    illustrates a plot of an example reverberation RMS amplitude when the reverberation time is set to infinity, according to some embodiments. 
         FIG.  7    illustrates a plot of an example RMS power that substantially follows an exponential decay after a reverberation onset time, according to some embodiments. 
         FIG.  8    illustrates an example output signal from the reverberator of  FIG.  5   , according to some embodiments. 
         FIG.  9    illustrates an amplitude of an impulse response for an example reverberator including only comb filters, according to some examples. 
         FIG.  10    illustrates an amplitude of an impulse response for an example reverberator including an all-pass filter stage, according to examples of the disclosure. 
         FIG.  11 A  illustrates an example reverberation processing system having a reverberator including a comb filter, according to some embodiments. 
         FIG.  11 B  illustrates a flow of an example process for operating the reverberation processing system of  FIG.  11 A , according to some embodiments. 
         FIG.  12 A  illustrates an example reverberation processing system having a reverberator including a plurality of all-pass filters. 
         FIG.  12 B  illustrates a flow of an example process for operating the reverberation processing system of  FIG.  12 A , according to some embodiments. 
         FIG.  13    illustrates an impulse response of the reverberation processing system of  FIG.  12   , according to some embodiments. 
         FIG.  14    illustrates a signal input and output through a reverberation processing system  510 , according to some embodiments. 
         FIG.  15 A  illustrates a block diagram of an example FDN comprising a feedback matrix, according to some embodiments. 
         FIG.  16 A  illustrates a block diagram of an example FDN comprising a plurality of all-pass filters, according to some embodiments. 
         FIG.  17 A  illustrates a block diagram of an example reverberation processing system including a REC, according to some embodiments. 
         FIG.  17 B  illustrates a flow of an example process for operating the reverberation processing system of  FIG.  17 A , according to some embodiments. 
         FIG.  18 A  illustrates an example calculated RE overtime for a virtual sound source collocated with a virtual listener, according to some embodiments. 
         FIG.  18 B  illustrates an example calculated RE with instant reverberation onset, according to some embodiments. 
         FIG.  19    illustrates a flow of an example reverberation processing system, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of examples, reference is made to the accompanying drawings which form a part hereof, and in which it is shown by way of illustration specific examples that can be practiced. It is to be understood that other examples can be used and structural changes can be made without departing from the scope of the disclosed examples. 
     Example Wearable System 
       FIG.  1    illustrates an example wearable head device  100  configured to be worn on the head of a user. Wearable head device  100  may be part of a broader wearable system that comprises one or more components, such as a head device (e.g., wearable head device  100 ), a handheld controller (e.g., handheld controller  200  described below), and/or an auxiliary unit (e.g., auxiliary unit  300  described below). In some examples, wearable head device  100  can be used for virtual reality, augmented reality, or mixed reality systems or applications. Wearable head device  100  can comprise one or more displays, such as displays  110 A and  110 B (which may comprise left and right transmissive displays, and associated components for coupling light from the displays to the user&#39;s eyes, such as orthogonal pupil expansion (OPE) grating sets  112 A/ 112 B and exit pupil expansion (EPE) grating sets  114 A/ 114 B); left and right acoustic structures, such as speakers  120 A and  120 B (which may be mounted on temple arms  122 A and  122 B, and positioned adjacent to the user&#39;s left and right ears, respectively); one or more sensors such as infrared sensors, accelerometers, GPS units, inertial measurement units (IMU)(e.g. IMU  126 ), acoustic sensors (e.g., microphone  150 ); orthogonal coil electromagnetic receivers (e.g., receiver  127  shown mounted to the left temple arm  122 A); left and right cameras (e.g., depth (time-of-flight) cameras  130 A and  130 B) oriented away from the user; and left and right eye cameras oriented toward the user (e.g., for detecting the user&#39;s eye movements)(e.g., eye cameras  128  and  128 B). However, wearable head device  100  can incorporate any suitable display technology, and any suitable number, type, or combination of sensors or other components without departing from the scope of the invention. In some examples, wearable head device  100  may incorporate one or more microphones  150  configured to detect audio signals generated by the user&#39;s voice; such microphones may be positioned in a wearable head device adjacent to the user&#39;s mouth. In some examples, wearable head device  100  may incorporate networking features (e.g., Wi-Fi capability) to communicate with other devices and systems, including other wearable systems. Wearable head device  100  may further include components such as a battery, a processor, a memory, a storage unit, or various input devices (e.g., buttons, touchpads); or may be coupled to a handheld controller (e.g., handheld controller  200 ) or an auxiliary unit (e.g., auxiliary unit  300 ) that comprises one or more such components. In some examples, sensors may be configured to output a set of coordinates of the head-mounted unit relative to the user&#39;s environment, and may provide input to a processor performing a Simultaneous Localization and Mapping (SLAM) procedure and/or a visual odometry algorithm. In some examples, wearable head device  100  may be coupled to a handheld controller  200 , and/or an auxiliary unit  300 , as described further below. 
       FIG.  2    illustrates an example mobile handheld controller component  200  of an example wearable system. In some examples, handheld controller  200  may be in wired or wireless communication with wearable head device  100  and/or auxiliary unit  300  described below. In some examples, handheld controller  200  includes a handle portion  220  to be held by a user, and one or more buttons  240  disposed along a top surface  210 . In some examples, handheld controller  200  may be configured for use as an optical tracking target; for example, a sensor (e.g., a camera or other optical sensor) of wearable head device  100  can be configured to detect a position and/or orientation of handheld controller  200  which may, by extension, indicate a position and/or orientation of the hand of a user holding handheld controller  200 . In some examples, handheld controller  200  may include a processor, a memory, a storage unit, a display, or one or more input devices, such as described above. In some examples, handheld controller  200  includes one or more sensors (e.g., any of the sensors or tracking components described above with respect to wearable head device  100 ). In some examples, sensors can detect a position or orientation of handheld controller  200  relative to wearable head device  100  or to another component of a wearable system. In some examples, sensors may be positioned in handle portion  220  of handheld controller  200 , and/or may be mechanically coupled to the handheld controller. Handheld controller  200  can be configured to provide one or more output signals, corresponding, for example, to a pressed state of the buttons  240 ; or a position, orientation, and/or motion of the handheld controller  200  (e.g., via an IMU). Such output signals may be used as input to a processor of wearable head device  100 , to auxiliary unit  300 , or to another component of a wearable system. In some examples, handheld controller  200  can include one or more microphones to detect sounds (e.g., a user&#39;s speech, environmental sounds), and in some cases provide a signal corresponding to the detected sound to a processor (e.g., a processor of wearable head device  100 ). 
       FIG.  3    illustrates an example auxiliary unit  300  of an example wearable system. In some examples, auxiliary unit  300  may be in wired or wireless communication with wearable head device  100  and/or handheld controller  200 . The auxiliary unit  300  can include a battery to provide energy to operate one or more components of a wearable system, such as wearable head device  100  and/or handheld controller  200  (including displays, sensors, acoustic structures, processors, microphones, and/or other components of wearable head device  100  or handheld controller  200 ). In some examples, auxiliary unit  300  may include a processor, a memory, a storage unit, a display, one or more input devices, and/or one or more sensors, such as described above. In some examples, auxiliary unit  300  includes a clip  310  for attaching the auxiliary unit to a user (e.g., a belt worn by the user). An advantage of using auxiliary unit  300  to house one or more components of a wearable system is that doing so may allow large or heavy components to be carried on a user&#39;s waist, chest, or back which are relatively well-suited to support large and heavy objects rather than mounted to the user&#39;s head (e.g., if housed in wearable head device  100 ) or carried by the user&#39;s hand (e.g., if housed in handheld controller  200 ). This may be particularly advantageous for relatively heavy or bulky components, such as batteries. 
       FIG.  4    shows an example functional block diagram that may correspond to an example wearable system  400 , such as may include example wearable head device  100 , handheld controller  200 , and auxiliary unit  300  described above. In some examples, the wearable system  400  could be used for virtual reality, augmented reality, or mixed reality applications. As shown in  FIG.  4   , wearable system  400  can include example handheld controller  400 B, referred to here as a “totem” (and which may correspond to handheld controller  200  described above); the handheld controller  400 B can include a totem-to-headgear six degree of freedom (6DOF) totem subsystem  404 A. Wearable system  400  can also include example wearable head device  400 A (which may correspond to wearable headgear device  100  described above); the wearable head device  400 A includes a totem-to-headgear 6DOF headgear subsystem  404 B. In the example, the 6DOF totem subsystem  404 A and the 6DOF headgear subsystem  404 B cooperate to determine six coordinates (e.g., offsets in three translation directions and rotation along three axes) of the handheld controller  400 B relative to the wearable head device  400 A. The six degrees of freedom may be expressed relative to a coordinate system of the wearable head device  400 A. The three translation offsets may be expressed as X, Y, and Z offsets in such a coordinate system, as a translation matrix, or as some other representation. The rotation degrees of freedom may be expressed as sequence of yaw, pitch, and roll rotations; as vectors; as a rotation matrix; as a quaternion; or as some other representation. In some examples, one or more depth cameras  444  (and/or one or more non-depth cameras) included in the wearable head device  400 A; and/or one or more optical targets (e.g., buttons  240  of handheld controller  200  as described above, or dedicated optical targets included in the handheld controller) can be used for 6DOF tracking. In some examples, the handheld controller  400 B can include a camera, as described above; and the headgear  400 A can include an optical target for optical tracking in conjunction with the camera. In some examples, the wearable head device  400 A and the handheld controller  400 B each include a set of three orthogonally oriented solenoids which are used to wirelessly send and receive three distinguishable signals. By measuring the relative magnitude of the three distinguishable signals received in each of the coils used for receiving, the 6DOF of the handheld controller  400 B relative to the wearable head device  400 A may be determined. In some examples, 6DOF totem subsystem  404 A can include an Inertial Measurement Unit (IMU) that is useful to provide improved accuracy and/or more timely information on rapid movements of the handheld controller  400 B. 
     In some examples involving augmented reality or mixed reality applications, it may be desirable to transform coordinates from a local coordinate space (e.g., a coordinate space fixed relative to wearable head device  400 A) to an inertial coordinate space, or to an environmental coordinate space. For instance, such transformations may be necessary for a display of wearable head device  400 A to present a virtual object at an expected position and orientation relative to the real environment (e.g., a virtual person sitting in a real chair, facing forward, regardless of the position and orientation of wearable head device  400 A), rather than at a fixed position and orientation on the display (e.g., at the same position in the display of wearable head device  400 A). This can maintain an illusion that the virtual object exists in the real environment (and does not, for example, appear positioned unnaturally in the real environment as the wearable head device  400 A shifts and rotates). In some examples, a compensatory transformation between coordinate spaces can be determined by processing imagery from the depth cameras  444  (e.g., using a Simultaneous Localization and Mapping (SLAM) and/or visual odometry procedure) in order to determine the transformation of the wearable head device  400 A relative to an inertial or environmental coordinate system. In the example shown in  FIG.  4   , the depth cameras  444  can be coupled to a SLAM/visual odometry block  406  and can provide imagery to block  406 . The SLAM/visual odometry block  406  implementation can include a processor configured to process this imagery and determine a position and orientation of the user&#39;s head, which can then be used to identify a transformation between a head coordinate space and a real coordinate space. Similarly, in some examples, an additional source of information on the user&#39;s head pose and location is obtained from an IMU  409  of wearable head device  400 A. Information from the IMU  409  can be integrated with information from the SLAM/visual odometry block  406  to provide improved accuracy and/or more timely information on rapid adjustments of the user&#39;s head pose and position. 
     In some examples, the depth cameras  444  can supply 3D imagery to a hand gesture tracker  411 , which may be implemented in a processor of wearable head device  400 A. The hand gesture tracker  411  can identify a user&#39;s hand gestures, for example, by matching 3D imagery received from the depth cameras  444  to stored patterns representing hand gestures. Other suitable techniques of identifying a user&#39;s hand gestures will be apparent. 
     In some examples, one or more processors  416  may be configured to receive data from headgear subsystem  404 B, the IMU  409 , the SLAM/visual odometry block  406 , depth cameras  444 , a microphone (not shown); and/or the hand gesture tracker  411 . The processor  416  can also send and receive control signals from the 6DOF totem system  404 A. The processor  416  may be coupled to the 6DOF totem system  404 A wirelessly, such as in examples where the handheld controller  400 B is untethered. Processor  416  may further communicate with additional components, such as an audio-visual content memory  418 , a Graphical Processing Unit (GPU)  420 , and/or a Digital Signal Processor (DSP) audio spatializer  422 . The DSP audio spatializer  422  may be coupled to a Head Related Transfer Function (HRTF) memory  425 . The GPU  420  can include a left channel output coupled to the left source of imagewise modulated light  424  and a right channel output coupled to the right source of imagewise modulated light  426 . GPU  420  can output stereoscopic image data to the sources of imagewise modulated light  424 ,  426 . The DSP audio spatializer  422  can output audio to a left speaker  412  and/or a right speaker  414 . The DSP audio spatializer  422  can receive input from processor  416  indicating a direction vector from a user to a virtual sound source (which may be moved by the user, e.g., via the handheld controller  400 B). Based on the direction vector, the DSP audio spatializer  422  can determine a corresponding HRTF (e.g., by accessing a HRTF, or by interpolating multiple HRTFs). The DSP audio spatializer  422  can then apply the determined HRTF to an audio signal, such as an audio signal corresponding to a virtual sound generated by a virtual object. This can enhance the believability and realism of the virtual sound, by incorporating the relative position and orientation of the user relative to the virtual sound in the mixed reality environment that is, by presenting a virtual sound that matches a user&#39;s expectations of what that virtual sound would sound like if it were a real sound in a real environment. 
     In some examples, such as shown in  FIG.  4   , one or more of processor  416 , GPU  420 , DSP audio spatializer  422 , HRTF memory  425 , and audio/visual content memory  418  may be included in an auxiliary unit  400 C (which may correspond to auxiliary unit  300  described above). The auxiliary unit  400 C may include a battery  427  to power its components and/or to supply power to wearable head device  400 A and/or handheld controller  400 B. Including such components in an auxiliary unit, which can be mounted to a user&#39;s waist, can limit the size and weight of wearable head device  400 A, which can in turn reduce fatigue of a user&#39;s head and neck. 
     While  FIG.  4    presents elements corresponding to various components of an example wearable system  400 , various other suitable arrangements of these components will become apparent to those skilled in the art. For example, elements presented in  FIG.  4    as being associated with auxiliary unit  400 C could instead be associated with wearable head device  400 A or handheld controller  400 B. Furthermore, some wearable systems may forgo entirely a handheld controller  400 B or auxiliary unit  400 C. Such changes and modifications are to be understood as being included within the scope of the disclosed examples. 
     Mixed Reality Environment 
     Like all people, a user of a mixed reality system exists in a real environment that is, a three-dimensional portion of the “real world,” and all of its contents, that are perceptible by the user. For example, a user perceives a real environment using one&#39;s ordinary human senses sight, sound, touch, taste, smell and interacts with the real environment by moving one&#39;s own body in the real environment. Locations in a real environment can be described as coordinates in a coordinate space; for example, a coordinate can comprise latitude, longitude, and elevation with respect to sea level; distances in three orthogonal dimensions from a reference point; or other suitable values. Likewise, a vector can describe a quantity having a direction and a magnitude in the coordinate space. 
     A computing device can maintain, for example, in a memory associated with the device, a representation of a virtual environment. As used herein, a virtual environment is a computational representation of a three-dimensional space. A virtual environment can include representations of any object, action, signal, parameter, coordinate, vector, or other characteristic associated with that space. In some examples, circuitry (e.g., a processor) of a computing device can maintain and update a state of a virtual environment; that is, a processor can determine at a first time, based on data associated with the virtual environment and/or input provided by a user, a state of the virtual environment at a second time. For instance, if an object in the virtual environment is located at a first coordinate at time, and has certain programmed physical parameters (e.g., mass, coefficient of friction); and an input received from user indicates that a force should be applied to the object in a direction vector; the processor can apply laws of kinematics to determine a location of the object at time using basic mechanics. The processor can use any suitable information known about the virtual environment, and/or any suitable input, to determine a state of the virtual environment at a time. In maintaining and updating a state of a virtual environment, the processor can execute any suitable software, including software relating to the creation and deletion of virtual objects in the virtual environment; software (e.g., scripts) for defining behavior of virtual objects or characters in the virtual environment; software for defining the behavior of signals (e.g., audio signals) in the virtual environment; software for creating and updating parameters associated with the virtual environment; software for generating audio signals in the virtual environment; software for handling input and output; software for implementing network operations; software for applying asset data (e.g., animation data to move a virtual object over time); or many other possibilities. 
     Output devices, such as a display or a speaker, can present any or all aspects of a virtual environment to a user. For example, a virtual environment may include virtual objects (which may include representations of inanimate objects; people; animals; lights; etc.) that may be presented to a user. A processor can determine a view of the virtual environment (for example, corresponding to a “camera” with an origin coordinate, a view axis, and a frustum); and render, to a display, a viewable scene of the virtual environment corresponding to that view. Any suitable rendering technology may be used for this purpose. In some examples, the viewable scene may include only some virtual objects in the virtual environment, and exclude certain other virtual objects. Similarly, a virtual environment may include audio aspects that may be presented to a user as one or more audio signals. For instance, a virtual object in the virtual environment may generate a sound originating from a location coordinate of the object (e.g., a virtual character may speak or cause a sound effect); or the virtual environment may be associated with musical cues or ambient sounds that may or may not be associated with a particular location. A processor can determine an audio signal corresponding to a “listener” coordinate for instance, an audio signal corresponding to a composite of sounds in the virtual environment, and mixed and processed to simulate an audio signal that would be heard by a listener at the listener coordinate and present the audio signal to a user via one or more speakers. 
     Because a virtual environment exists only as a computational structure, a user cannot directly perceive a virtual environment using one&#39;s ordinary senses. Instead, a user can perceive a virtual environment only indirectly, as presented to the user, for example by a display, speakers, haptic output devices, etc. Similarly, a user cannot directly touch, manipulate, or otherwise interact with a virtual environment; but can provide input data, via input devices or sensors, to a processor that can use the device or sensor data to update the virtual environment. For example, a camera sensor can provide optical data indicating that a user is trying to move an object in a virtual environment, and a processor can use that data to cause the object to respond accordingly in the virtual environment. 
     Reverberation Algorithms and Reverberators 
     In some embodiments, digital reverberators may be designed based on delay networks with feedback. In such embodiments, reverberator algorithm design guidelines may be included/available for accurate parametric decay time control and for maintaining reverberation loudness when decay time is varied. Relative adjustment of the reverberation loudness may be realized by providing an adjustable signal amplitude gain in cascade with the digital reverberator. This approach may enable a sound designer or a recording engineer to tune reverberation decay time and reverberation loudness independently, while audibly monitoring a reverberator output signal in order to achieve a desired effect. 
     Programmatic applications, such as interactive audio engines for video games or VR/AR/MR, may simulate multiple moving sound sources at various positions and distances around a listener (e.g., a virtual listener) in a room/environment (e.g., virtual room/environment), relative reverberation loudness control may not be sufficient. In some embodiments, an absolute reverberation loudness is applied that may be experienced from each virtual sound source at rendering time. Many factors may adjust this value, such as, for example, listener and sound source positions, as well as acoustic properties of the room/environment, for example, simulated by a reverberator. In some embodiments, such as in interactive audio applications, it is desirable to programmatically control the reverberation initial power (RIP), for example, as defined in “Analysis and synthesis of room reverberation based on a statistical time-frequency model” by Jean-Marc Jot, Laurent Cerveau, and Olivier Warusfel. The RIP may be used to characterize a virtual room irrespective of positions of a virtual listener or virtual sound sources. 
     In some embodiments, a reverberation algorithm (executed by a reverberator) may be configured to perceptually match acoustic reverberation properties of a specific room. Example acoustic reverberation properties can include, but are not limited to, reverberation initial power (RIP) and reverberation decay time (T60). In some embodiments, the acoustic reverberation properties of a room may be measured in a real room, calculated by a computer simulation based on geometric and/or physical description of a real room or virtual room, or the like. 
     Example Audio Rendering System 
       FIG.  5 A  illustrates a block diagram of an example audio rendering system, according to some embodiments.  FIG.  5 B  illustrates a flow of an example process for operating the audio rendering system of  FIG.  5 A , according to some embodiments. 
     Audio rendering system  500  can include a reverberation processing system  510 A, a direct processing system  530 , and a combiner  540 . Both the reverberation processing system  510 A and the direct processing system  530  can receive the input signal  501 . 
     The reverberation processing system  510 A can include a RIP control system  512  and a reverberator  514 . The RIP control system  512  can receive the input signal  501  and can output a signal to the reverberator  514 . The RIP control system  512  can include a reverb initial gain (RIG)  516  and a RIP corrector  518 . The RIG  516  can receive the first portion of the input signal  501  and can output a signal to the RIP corrector  518 . The RIG  516  can be configured to apply a RIG value to the input signal  501  (step  552  of process  550 ). Setting the RIG value can have an effect of specifying an absolute amount of RIP in output signal of the reverberation processing system  510 A. 
     The RIP corrector  518  can receive a signal from the RIG  516  and can be configured to calculate and apply a RIP correction factor to its input signal (from the RIG  516 ) (step  554 ). The RIP corrector  518  can output a signal to the reverberator  514 . The reverberator  514  can receive a signal from the RIP corrector  518  and can be configured to introduce reverberation effects in the signal (step  556 ). The reverberation effects can be based on the virtual environment, for example. The reverberator  514  is discussed in more detail below. 
     The direct processing system  530  can include a propagation delay  532  and a direct gain  534 . The direct processing system  530  and the propagation delay  532  can receive the second portion of the input signal  501 . The propagation delay  532  can be configured to introduce a delay in the input signal  501  (step  558 ) and can output the delayed signal to the direct gain  534 . The direct gain  534  can receive a signal from the propagation delay  532  and can be configured to apply a gain to the signal (step  560 ). 
     The combiner  540  can receive the output signals from both the reverberation processing system  510 A and the direct processing system  530  and can be configured to combine (e.g., add, aggregate, etc.) the signals (step  562 ). The output from the combiner  540  can be the output signal  540  of the audio rendering system  500 . 
     Example Reverberation Initial Power (RIP) Normalization 
     In the reverberation processing system  510 A, both the RIG  516  and the RIP corrector  518  can apply (and/or calculate) the RIG value and the RIP correction factor, respectively, such that when applied in series the signal output from the RIP corrector  518  can be normalized to a predetermined value (e.g., unity (1.0)). That is, the RIG value of an output signal can be controlled by applying the RIG  516  in series with the RIP corrector  518 . In some embodiments, the RIP correction factor can be applied directly after the RIG value. The RIP normalization process is discussed in more detail below. 
     In some embodiments, in order to produce a diffuse reverberation tail, a reverberation algorithm may, for instance, include parallel comb filters, followed by a series of all-pass filters. In some embodiments, a digital reverberator may be constructed as a network including one or more delay units interconnected with feedback and/or feedforward paths that may also include signal gain scaling or filter units. The RIP correction factor of a reverberation processing system such as the reverberation processing system  510 A of  FIG.  5 A  may depend on one or more parameters such as, for example, reverberator topology, number and durations of delay units included in the network, connection gains, and filter parameters. 
     In some embodiments, the RIP correction factor of the reverberation processing system may be equal to a root mean square (RMS) power of an impulse response of the reverberation system when a reverberation time is set to infinity. In some embodiments, for example, as illustrated in  FIG.  6   , when the reverberation time of a reverberator is set to infinity, the impulse response of the reverberator may be a non-decaying noise-like signal having constant RMS amplitude versus time. 
     The RMS power P rms (t) of a digital signal {x} at time t, expressed in samples, may be equal to an average of a squared signal amplitude. In some embodiments, the RMS power may be expressed as: 
                       P     rm   ⁢           ⁢   s       ⁡     (   t   )       =       1   N     ⁢       ∑     n   =   t       t   +   N   -   1       ⁢       x   ⁡     (   n   )       2                 (   1   )               
where t is the time, N is the number of consecutive signal samples, and n is the signal sample. The average may be evaluated over a signal window starting at time t and containing N consecutive signal samples.
 
     The RMS amplitude may be equal to the square root of the RMS power P rms (t). In some embodiments, the RMS amplitude may be expressed as:
 
 A   rms ( t )=√{square root over ( P   rms ( t ))}  (2)
 
     In some embodiments, in the impulse response of the reverberator (e.g., as illustrated in  FIG.  6   ), the RIP correction factor may be derived as an expected RMS power of a constant-power signal that follows reverberation onset, with the reverberation decay time set to infinity.  FIG.  8    illustrates an example output signal from running a single impulse of amplitude 1.0 into the audio rendering system  500  of  FIG.  5 A . In such instance, the reverberation decay time is set to infinity, a direct signal output is set to 1.0, and the direct signal output is delayed by a source-to-listener propagation delay. 
     In some embodiments, the reverberation time of the reverberation processing system  510 A may be set to a finite value. With the finite value, the RMS power may substantially follow an exponential decay (after a reverberation onset time), as shown in  FIG.  7   . The reverberation time (T60) of the reverberation processing system  510 A may be defined generally as the duration over which the RMS power (or amplitude) decays by 60 dB. The RIP correction factor may be defined as the power measured on the RMS power decay curve extrapolated to time t=0. Time t=0 can be the time of emission of the input signal  501  (in  FIG.  5 A ). 
     Example Reverberators 
     In some embodiments, the reverberator  514  (of  FIG.  5 A ) may be configured to operate a reverberation algorithm, such as the one described in Smith, “J. O. Physical Audio Signal Processing,” http://ccrma.stanford.edu/˜jos/pasp/, online book, 2010 edition. In these embodiments, the reverberator may contain a comb filter stage. The comb filter stage may include 16 comb filters (e.g., eight comb filters for each ear), where each comb filter can have a different feedback loop delay length. 
     In some embodiments, the RIP correction factor for the reverberator may be calculated by setting the reverberation time to infinity. Setting the reverberation time to infinity may be equivalent to assuming that the comb filters do not have any built-in attenuation. If a Dirac impulse is input through the comb filters, the output signal of the reverberator  514  may be a sequence of full scale impulses, for example. 
       FIG.  8    illustrates an example output signal from the reverberator  514  of  FIG.  5 A , according to some embodiments. The reverberator  514  may include a comb filter (not shown). If there is only one comb filter with a feedback loop delay length d, expressed in samples, then the echo density may be equal to the reciprocal of the feedback loop delay length d. The RMS amplitude may be equal to the square root of the echo density. The RMS amplitude may be expressed as: 
     
       
         
           
             
               
                 
                   
                     A 
                     
                       rm 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       s 
                     
                   
                   = 
                   
                     
                       1 
                       d 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     In some embodiments, the reverberator may have a plurality of comb filters, and the RMS amplitude may be expressed as: 
                     A     r   ⁢   m   ⁢   s       =       N     d   mean                 (   4   )               
where N is the number of comb filters in the reverberator, and d mean  is the mean feedback delay length. The mean feedback delay length d mean  may be expressed in samples and averaged across the N comb filters.
 
       FIG.  9    illustrates an amplitude of an impulse response for an example reverberator including only comb filters, according to some examples. In some embodiments, the reverberator may have a decay time set to a finite value. As shown in the figure, the RMS amplitude of a reverberator impulse response falls exponentially over time. On a dB scale, the RMS amplitude falls along a straight line and starts from a value equal to the RIP at time t=0. The time t=0 may be the time of emission of a unit impulse at an input (e.g., a time of emission of an impulse by a virtual sound source). 
       FIG.  10    illustrates an amplitude of an impulse response for an example reverberator including an all-pass filter stage, according to examples of the disclosure. The reverberator may similar to the one described in Smith, J. O. Physical Audio Signal Processing, http://ccrma.stanford.edu/˜jos/pasp/, online book, 2010 edition. Since the inclusion of an all-pass filter may not significantly affect the RMS amplitude of a reverberator impulse response (compared to the RMS amplitude of the reverberator impulse response of  FIG.  9   ), a linear decaying trend of the RMS amplitude in dB may be identical to a trend of  FIG.  9   . In some embodiments, the linear decaying trend may start from the same RIP value observed at time t=0. 
       FIG.  11 A  illustrates an example reverberation processing system having a reverberator including a comb filter, according to some embodiments.  FIG.  11 B  illustrates a flow of an example process for operating the reverberation processing system of  FIG.  11 A , according to some embodiments. 
     Reverberation processing system  510 B can include a RIP control system  512  and a reverberator  1114 . The RIP control system  512  can include a RIG  516  and a RIP corrector  518 . The RIP control system  512  and the RIP corrector  518  can be correspondingly similar to those included in the reverberation processing system  510 A (of  FIG.  5 A ). The reverberation processing system  510 B can receive the input signal  501  and output the output signals  502 A and  502 B. In some embodiments, the reverberation processing system  510 B can be included in the audio rendering system  500  of  FIG.  5 A  in lieu of the reverberation processing system  510 A (of  FIG.  5 A ). 
     The RIG  516  may be configured to apply a RIG value (step  1152  of process  1150 ), and the RIP corrector  518  can apply a RIP correction factor (step  1154 ), both in series with the reverberator  1114 . The serially configuration of the RIG  516 , the RIP corrector  518 , and the reverberator  114  may cause the RIP of the reverberation processing system  510 B to be equal to the RIG. 
     In some embodiments, the RIP correction factor can be expressed as: 
                   RIPcorrection   =         d   mean     N               (   5   )               
The application of the RIP correction factor to the signal can cause the RIP to be set to a predetermined value, such as unity (1.0), when the RIG value is set to 1.0.
 
     The reverberator  514  can receive a signal from the RIP control system  512  and can be configured to introduce reverberation effects into the first portion of the input signal (step  1156 ). The reverberator  514  can include one or more comb filters  1115 . The comb filter(s)  1115  can be configured to filter out one or more frequencies in the signal (step  1158 ). For example, the comb filter(s)  1115  can filter out (e.g., cancel) one or more frequencies to mimic environmental effects (e.g., the walls of the room). The reverberator  1114  can output two or more output signals  502 A and  502 B (step  1160 ). 
       FIG.  12 A  illustrates an example reverberation processing system having a reverberator including a plurality of all-pass filters.  FIG.  12 B  illustrates a flow of an example process for operating the reverberation processing system of  FIG.  12 A , according to some embodiments. 
     Reverberation processing system  510 C can be similar to the reverberation processing system  510 B (of  FIG.  11 A ), but its reverberator  1214  may additionally include a plurality of all-pass filters  1216 . Steps  1252 ,  1254 ,  1256 ,  1258 , and  1260  may be correspondingly similar to steps  1152 ,  1154 ,  1156 ,  1158 , and  1160 , respectively. 
     The reverberation processing system  510 C can include a RIP control system  512  and a reverberator  1214 . The RIP control system  512  can include a RIG  516  and a RIP corrector  518 . The RIP control system  512  and the RIP corrector  518  can be correspondingly similar to those included in the reverberation processing system  510 A (of  FIG.  5 A ). The reverberation processing system  510 B can receive the input signal  501  and output the output signals  502 A and  502 B. In some embodiments, the reverberation processing system  510 B can be included in the audio rendering system  500  of  FIG.  5 A  in lieu of reverberation processing system  510 A (of  FIG.  5 A ) or the reverberation processing system  510 B (of  FIG.  11   ). 
     The reverberator  1214  may additionally include all-pass filters  1215  that can receive signals from the comb filters  1115 . Each all-pass filter  1215  can receive a signal from the comb filters  1115  and can be configured to pass its input signal without changing their magnitudes (step  1262 ). In some embodiments, the all-pass filter  1215  can change a phase of the signal. In some embodiments, each all-pass filter can receive a unique signal from the comb filters. The outputs of the all-pass filters  1215  can be the output signals  502  of the reverberation processing system  510 C and the audio rendering system  500 . For example, the all-pass filter  1215 A can receive a unique signal from the comb filters  1115  and can output the signal  502 A; similarly, the all-pass filter  1215 B can receive a unique signal from the comb filters  1115  and can output the signal  502 B. 
     Comparing to  FIGS.  9  and  10   , the inclusion of the all-pass filters  1216  may not significantly affect the output RMS amplitude decay trend. 
     When applying the RIP correction factor, if the reverberation time is set to infinity, the RIG value is set to 1.0, and a single unit impulse is input through the reverberation processing system  510 C, a noise-like output with a constant RMS level of 1 maybe be obtained. 
       FIG.  13    illustrates an example impulse response of the reverberation processing system  510 C of  FIG.  12   , according to some embodiments. The reverberation time may be set to a finite number, and the RIG may be set to 1.0. On a dB scale, a RMS level may fall along a straight decay line, like as shown in  FIG.  10   . However, due to the RIP correction factor, the RIP observed in  FIG.  13    at the time t=0 may be normalized to 0 dB. 
     In some embodiments, the RIP normalization method described in connection with  FIGS.  5 ,  6 ,  7 , and  18 A  may be applied regardless of the particular digital reverberation algorithm implemented in the reverberator  514  of  FIG.  5   . For example, reverberators may be built from networks of feedback and feedforward delay elements connected with gain matrices. 
       FIG.  14    illustrates a signal input and output through a reverberation processing system  510 , according to some embodiments. For example,  FIG.  14    illustrates a flow of signals of any one of the reverberation processing systems  510  discussed above, such as the ones discussed in  FIGS.  5 A,  11 A, and  12 A . The apply RIG step  1416  can include setting the RIG value and applying it to the input signal  501 . The apply RIP correction factor step  1418  can include calculating the RIP correction factor for the chosen reverberator design and internal reverberator parameter settings. Additionally, passing the signal through the reverberator  1414  can cause the system to select a reverberator topology and set internal reverberator parameters. As shown in the figure, the output of the reverberator  1414  can be the output signal  502 . 
     Example Feedback Delay Networks 
     The embodiments disclosed herein may have a reverberator that includes a feedback delay network (FDN), according to some embodiments. The FDN may include an identity matrix, which may allow the output of a delay unit to be fed back to its input.  FIG.  15 A  illustrates a block diagram of an example FDN comprising a feedback matrix, according to some embodiments. FDN  1515  can include a feedback matrix  1520 , a plurality of combiners  1522 , a plurality of delays  1524 , and a plurality of gains  1526 . 
     The combiners  1522  can receive the input signal  1501  and can be configured to combine (e.g., add, aggregate, etc.) its inputs (step  1552  of process  1550 ). The combiners  1522  can also receive a signal from the feedback matrix  1520 . The delays  1524  can receive the combined signals from the combiners  1522  and can be configured to introduce a delay into one or more signals (step  1554 ). The gains  1526  can receive the signals from the delays  1524  and can be configured to introduce a gain into one or more signals (step  1556 ). The output signals from the gains  1526  can form the output signal  1502  and may also be input into the feedback matrix  1520 . In some embodiments, the feedback matrix  1520  may be a N×N unitary (energy-preserving) matrix. 
     In the general case where the feedback matrix  1520  is a unitary matrix, the expression of the RIP correction factor may also be given by Equation (5) because the overall energy transfer around the feedback loop of the reverberator remains unchanged and delay-free. 
     For a given arbitrary choice of reverberator design and internal parameter settings, a RIP correction factor may be calculated, for example. The calculated RIP correction factor may be such that if the RIG value is set to 1.0, then the RIP of the overall reverberation processing system  510  is also 1.0. 
     In some embodiments, the reverberator may include a FDN with one or more all-pass filters.  FIG.  16    illustrates a block diagram of an example FDN comprising a plurality of all-pass filters, according to some embodiments. 
     FDN  1615  can include a plurality of all-pass filters  1630 , a plurality of delays  1632 , and a mixing matrix  1640 B. The all-pass filters  1630  can include a plurality of gains  1526 , an absorptive delay  1632 , and another mixing matrix  1640 A. The FDN  1615  may also include a plurality of combiners (not shown). 
     The all-pass filters  1630  receive the input signal  1501  and may be configured to pass its input signal without changing its magnitude. In some embodiments, the all-pass filter  1630  can change a phase of the signal. In some embodiments, each all-pass filter  1630  can be configured such that power input to the all-pass filter  1630  can be equal to power output from the all-pass filter. In other words, each all-pass filter  1630  may have no absorption. Specifically, the absorptive delay  1632  can receive the input signal  1501  and can be configured to introduce a delay in the signal. In some embodiments, the absorptive delay  1632  can delay its input signal by a number of samples. In some embodiments, each absorptive delay  1632  can have a level of absorption such that its output signal is a certain level less than its input signal. 
     The gains  1526 A and  1526 B can be configured to introduce a gain in its respective input signal. The input signal for the gain  1526 A can be the input signal to the absorptive delay, and the output signal for the gain  1526 B can be the output signal to the mixing matrix  1640 A. 
     The output signals from the all-pass filters  1630  can be input signals to delays  1632 . The delays  1632  can receive signals from the all-pass filters  1630  and can be configured to introduce delays into its respective signals. In some embodiments, the output signals from the delays  1632  can be combined to form the output signal  1502 , or, in some embodiments, these signals may be separately taken as multiple output channels in others. In some embodiments, the output signal  1502  may be taken from other points in the network. 
     The output signals from the delays  1632  can also be input signals into the mixing matrix  1640 B. The mixing matrix  1640 B can be configured to receive multiple input signals and can output its signals to be fed back into the all-pass filters  1630 . In some embodiments, each mixing matrix can be a full mixing matrix. 
     In these reverberator topologies, the RIP correction factor may be expressed by Equation (5) because the overall energy transfer in and around the feedback loop of the reverberator can remain unchanged and delay-free. In some embodiments, the FDN  1615  may vary the input and/or output signal placement to achieve the desired output signal  1501 . 
     The FDN  1615  with the all-pass filters  1630  can be a reverberating system that takes the input signal  1501  as its input and creates a multi-channel output that can include the correct decaying reverberation signal. The input signal  1501  can be the mono-input signal. 
     In some embodiments, the RIP correction factor may be expressed as a mathematical function of a set of reverberator parameters {P} that determine the reverberation RMS amplitude A rms ({P}) when the reverberation time is set to infinity, as shown in  FIG.  6   . For example, the RIP correction factor can be expressed as:
 
RIPcorrection=1/ A   rms ({ P })  (6)
 
     For a given reverberator topology and a given setting of delay unit lengths of the reverberator, the RIP correction factor may be calculated by performing the following steps: (1) setting the reverberation time to infinity; (2) recording the reverberator impulse response (as shown in  FIG.  6   ); (3) measuring the reverberation RMS amplitude A rms ; and (4) determining the RIP correction factor according to Equation (6). 
     In some embodiments, the RIP correction factor may be calculated by performing the following steps: (1) setting the reverberation time to any finite value; (2) recording the reverberator impulse response; (3) deriving the reverberation RMS amplitude decay curve A rms (t) (as shown in  FIG.  7 A  or  FIG.  7 C ); (4) determining its value (the RMS amplitude) extrapolated at the time of emission t=0 (denoted as A rms (0) and as shown in  FIG.  10   ); and (5) determining the RIP correction factor according to Equation 7 (below).
 
RIPcorrection=1/ A   rms ({0})  (7)
 
     Example Reverberation Energy Normalization Method 
     In some embodiments, it may be desirable to provide a perceptually relevant reverberation gain control method, for example, for application developers, sound engineers, and the like. For example, in some reverberator or room simulator embodiments, it may be desirable to provide programmatic control over a measure of a power amplification factor representative of an effect of a reverberation processing system on the power of an input signal. The power of an input signal may be expressed in dB, for example. The programmatic control over the power amplification factor may allow application developers, sound engineers, and the like, for example, to determine a balance between reverberation output signal loudness and input signal loudness, or direct sound output signal loudness. 
     In some embodiments, the system can apply a reverberation energy (RE) correction factor.  FIG.  17 A  illustrates a block diagram of an example reverberation processing system including a RE corrector, according to some embodiments.  FIG.  17 B  illustrates a flow of an example process for operating the reverberation processing system of  FIG.  17 A , according to some embodiments. 
     Reverberation processing system  510 D can include a RIP control system  512  and a reverberator  514 . The RIP control system  512  can include a RIG  516  and a RIP corrector  518 . The RIP control system  512 , the reverberator  514 , and the RIP corrector  518  can be correspondingly similar to those included in the reverberation processing system  510 A (of  FIG.  5 A ). The reverberation processing system  510 D can receive the input signal  501  and can output the output signal  502 . In some embodiments, the reverberation processing system  510 D can be included in the audio rendering system  500  of  FIG.  5 A  in lieu of reverberation processing system  510 A (of  FIG.  5 A ), the reverberation processing system  510 B (of  FIG.  11 A ), or the reverberation processing system  510 C (of  FIG.  12 A ). 
     The reverberation processing system  510 D may also include a RIG  516  that comprises a reverb gain (RG)  1716  and a RE corrector  1717 . The RG  1716  can receive the input signal  501  and can output a signal to the RE corrector  1717 . The RG  1716  can be configured to apply a RG value to the first portion of the input signal  501  (step  1752  of process  1750 ). In some embodiments, the RIG can be realized by cascading the RG  1716  with the RE corrector  1717 , such that the RE correction factor is applied to the first portion of the input signal after the RG value is applied. In some embodiments, the RIG  516  can be cascaded with the RIP corrector  518 , forming the RIP control system  512  that is cascaded with the reverberator  514 . 
     The RE corrector  1717  can receive a signal from the RG  1716  and can be configured to calculate and apply a RE correction factor to its input signal (from RG  1716 ) (step  1754 ). In some embodiments, the RE correction factor may be calculated such that it represents the total energy in a reverberator impulse response when: (1) a RIP is set to 1.0, and (2) a reverberation onset time is set equal to the time of emission of a unit impulse by a sound source. Both the RG  1716  and the REC  1717  can apply (and/or calculate) the RG value and the REC correction factor, respectively, such that when applied in series, the signal output from the RE corrector  1717  can be normalized to a predetermined value (e.g., unity (1.0)). The RIP of an output signal can be controlled by applying a reverberator gain in series with the reverberator, the reverberator energy corrector factor, and the reverberator initial power factor, as shown in  FIG.  17 A . The RE normalization process is discussed in more detail below. 
     The RIP corrector  518  can receive a signal from the RIG  516  and can be configured to calculate and apply a RIP correction factor to its input signal (from the RIG  516 ) (step  1756 ). The reverberator  514  can receive a signal from the RIP corrector  518  and can be configured to introduce reverberation effects in the signal (step  1758 ). 
     In some embodiments, the RIP of a virtual room may be controlled using the reverberation processing system  510 A of  FIG.  5 A  (included in the audio rendering system  500 ), the reverberation processing system  510 B of  FIG.  11 A  (included in the audio rendering system  500 ), or both. The RIG  516  of the reverberation processing system  510 A (of  FIG.  5 A ) may specify the RIP directly, and may be interpreted physically as proportional to a reciprocal of a square root of a cubic volume of the virtual room, for example, as shown in “Analysis and synthesis of room reverberation based on a statistical time-frequency model” by Jean-Marc Jot, Laurent Cerveau, and Olivier Warusfel. 
     The RG  516  of the reverberation processing system  510 D (of  FIG.  17 A ) may control the RIP of the virtual room indirectly by specifying the RE. The RE may be a perceptually relevant quantity that is proportional to an expected energy of reverberation that a user will receive from a virtual sound source if it is collocated at the same position as a virtual listener in the virtual room. One example virtual sound source that is collocated at the same position as the virtual listener is a virtual listener&#39;s own voice or footsteps. 
     In some embodiments, the RE can be calculated and used to represent the amplification of an input signal by a reverberation processing system. The amplification may be expressed in terms of signal power. As shown in  FIG.  7   , the RE can be equal to the area under a reverb RMS power envelope integrated from a reverb onset time. In some embodiments, in an interactive audio engine for video games or virtual reality, the reverb onset time may be at least equal to a propagation delay for a given virtual sound source. Therefore, the calculation of the RE for a given virtual sound source may depend on the position of the virtual sound source. 
       FIG.  18 A  illustrates the calculated RE overtime for a virtual sound source collocated with a virtual listener, according to some embodiments. In some embodiments, it can be assumed that a reverberation onset time is equal to a time of sound emission. In this case, the RE can represent the total energy in a reverberator impulse response when a reverberation onset time is assumed to be equal to the time of emission of a unit impulse by a sound source. The RE can be equal to the area under a reverb RMS power envelop integrated from a reverb onset time. 
     In some embodiments, the RMS power curve may be expressed as a continuous function of time t. In such instance, the RE may be expressed as: 
     
       
         
           
             
               
                 
                   RE 
                   = 
                   
                     
                       ∫ 
                       
                         t 
                         = 
                         0 
                       
                       ∞ 
                     
                     ⁢ 
                     
                       
                         
                           P 
                           
                             rm 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             s 
                           
                         
                         ⁡ 
                         
                           ( 
                           t 
                           ) 
                         
                       
                       · 
                       dt 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     In some embodiment, such as discrete-time embodiments of a reverberation processing system, the RMS power curve can be expressed as a function of the discrete time t=n  . In such instance, the RE may be expressed as: 
                   RE   =       ∑     n   =   0     ∞     ⁢       P     r   ⁢   m   ⁢   s       ⁡     (     n     F   ⁢   s       )                 (   9   )               
where F S  is the same rate.
 
     In some embodiments, a RE correction factor may be calculated and applied in series with the RIP correction factor and the reverberator, so that the RE may be normalized to a predetermined value (e.g., unity (1.0)). The REC may be set equal to the reciprocal of the square root of RE, as follows: 
     
       
         
           
             
               
                 
                   
                     R 
                     ⁢ 
                     E 
                     ⁢ 
                     C 
                   
                   = 
                   
                     1 
                     
                       RE 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     In some embodiments, a RIP of an output reverberation signal may be controlled by applying a RG value in series with a RE correction factor, a RIP correction factor, and a reverberator, such as shown in the reverberation processing system  510 C of  FIG.  17 A . The RG value and RE correction may be combined to determine the RIG, as follows:
 
RIG=RG*REC  (11)
 
Therefore, the RE correction factor (REC) may be used to control the RIP correction factor in terms of the signal-domain RG quantity, instead of the RIG.
 
     In some embodiments, the RIP may be mapped to a signal power amplification measured derived by integrated RE in the system impulse response. As shown above in Equations (10)-(11), this mapping allows the control of the RIP via the familiar notion of a signal amplification factor, namely, the RG. In some embodiments, the advantage of assuming instant reverberation onset for the RE calculation, as shown in  FIG.  18 B  and Equations (8)-(9), can be that this mapping may be expressed without requiring that the user or listener position be taken into account. 
     In some embodiments, the reverb RMS power curve of an impulse response of the reverberator  514  can be expressed as a decaying function of time. The decaying function of time can start at time t=0.
 
 P   rms ( t )=RIP* e   −αt   (12)
 
     In some embodiments, the decay parameter can be expressed as a function of decay time T60, as follows:
 
α=3*log(10)/ T 60  (13)
 
     The total RE may be expressed as: 
     
       
         
           
             
               
                 
                   RE 
                   = 
                   
                     RIP 
                     / 
                     
                       ( 
                       
                         
                           10 
                           
                             6 
                             
                               T 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               60 
                               * 
                               Fs 
                             
                           
                         
                         - 
                         1 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
     In some embodiments, the RIP may be normalized to a predetermined value (e.g., unity (1.0)), and the REC may be expressed as follows: 
     
       
         
           
             
               
                 
                   REC 
                   = 
                   
                     
                       
                         10 
                         
                           6 
                           
                             T 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             60 
                             * 
                             Fs 
                           
                         
                       
                       - 
                       1 
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     In some embodiments, the REC may be approximated according to the following equation: 
     
       
         
           
             
               
                 
                   REC 
                   ≈ 
                   
                     
                       
                         6 
                         * 
                         
                           log 
                           ⁡ 
                           
                             ( 
                             10 
                             ) 
                           
                         
                       
                       
                         T 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         60 
                         * 
                         Fs 
                       
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
       FIG.  19    illustrates a flow of an example reverberation processing system, according to some embodiments. For example,  FIG.  19    can illustrate the flow of the reverberation processing system  510 D of  FIG.  17 A . For a given arbitrary choice of reverberator design and internal parameter settings, a RIP correction factor can be calculated by applying Equations (5)-(7), for example. In some embodiments, for a given run-time adjustment of the reverberation decay time T60, the total RE may be re-calculated by applying Equations (8)-(9), where it can be assumed that the RIP is normalized to 1.0. The REC factor can be derived according to Equation (10). 
     Due to the application of the REC factor, adjusting the RG value or the reverberation decay time T60 at runtime may have an effect of automatically correcting the RIP of the reverberation processing system such that the RG can operate as an amplification factor for the RMS amplitude of an output signal (e.g., output signal  502 ) relative to the RMS amplitude of an input signal (e.g., input signal  501 ). It should be noted that adjusting the reverberation decay time T60 may not require recalculating the RIP correction factor because, in some embodiments, the RIP may not be affected by a modification of the decay time. 
     In some embodiments, the REC may be defined based on measuring the RE as the energy in the reverberation tail between two points specified in time from a sound source emission, after having set the RIP to 1.0 by applying the RIP correction factor. This may be beneficial, for example, when using convolution with a measured reverberation tail. 
     In some embodiments, the RE correction factor may be defined based on measuring the RE as the energy in the reverberation tail between two points defined using energy thresholds, after having set the RIP to 1.0 by applying the RIP correction factor. In some embodiments, energy thresholds relative to the direct sound, or absolute energy thresholds, may be used. 
     In some embodiments, the RE correction factor may be defined based on measuring the RE as the energy in the reverberation tail between one point defined in time and one point defined using an energy threshold, after having set the RIP to 1.0 by applying the RIP correction factor. 
     In some embodiments, the RE correction factor may be computed by considering a weighted sum of the energy contributed by the different coupled spaces, after having set the RIP of each of the reverberation tails to 1.0 by applying the RIP correction factor to each reverb. One exemplary application of this RE correction factor computation may be where an acoustical environment includes two or more coupled spaces. 
     With respect to the systems and methods described above, elements of the systems and methods can be implemented by one or more computer processors (e.g., CPUs or DSPs) as appropriate. The disclosure is not limited to any particular configuration of computer hardware, including computer processors, used to implement these elements. In some cases, multiple computer systems can be employed to implement the systems and methods described above. For example, a first computer processor (e.g., a processor of a wearable device coupled to a microphone) can be utilized to receive input microphone signals, and perform initial processing of those signals (e.g., signal conditioning and/or segmentation, such as described above). A second (and perhaps more computationally powerful) processor can then be utilized to perform more computationally intensive processing, such as determining probability values associated with speech segments of those signals. Another computer device, such as a cloud server, can host a speech recognition engine, to which input signals are ultimately provided. Other suitable configurations will be apparent and are within the scope of the disclosure. 
     Although the disclosed examples have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. For example, elements of one or more implementations may be combined, deleted, modified, or supplemented to form further implementations. Such changes and modifications are to be understood as being included within the scope of the disclosed examples as defined by the appended claims.