Patent Description:
In some examples, brainwaves or electrical frequency activity in a brain can be modulated or changed in response to various stimuli presented to the eyes or ears, or using tactile stimuli. For example, repetitive stimuli presented to a user at a particular frequency can encourage the electrical activity of the user's brain to shift toward the same particular frequency. This phenomenon or changing the brainwave behavior of a user is referred to as entrainment.

Various examples of delivering continuous auditory stimulation for entrainment have been proposed to modulate brainwaves for therapeutic effect. The auditory stimulation can be presented to a user using headphones or earphones. In some examples, the auditory stimulation is provided by modulating various aspects of a recorded audio program, such as a music program.

One form of auditory stimulation includes "binaural beats. " Binaural beats are an auditory illusion or brain response that is created by presenting different auditory information or source signals to respective ears of a listener. The different auditory information differs in frequency. The difference between the information presents itself to the listener as an amplitude-modulated signal resulting from the phase relationship of the source signals. The resulting "beats" can be perceived by the listener as an auditory beat and it can be used to entrain different rhythms or cortical potentials in the listener's brain.

<CIT> describes a sound output device in which brain waves can be induced without imparting discomfort to a user. The sound output device outputs a control sound for inducing brain waves. The sound output device is provided with a sound collection unit for collecting an environmental sound, a generating unit for changing the phase and/or amplitude of the collected environmental sound so as to form the environmental sound and a beat sound having a predetermined frequency and generating the control sound, and an output unit for outputting the generated control sound. The environmental sound is an operating sound of a machine or the like, for example.

The invention is a system as defined in the appended claims. The present inventor has recognized, among other things, that a problem to be solved includes providing auditory stimulation for entrainment without using noises that would be distracting or disruptive for a user. In an example, a solution to the problem can include using amplitude modulation of ambient sounds. In an example, the solution can include modulating existing sounds in a user's environment, for example, at selected frequencies that may not interfere with important functions like speech processing. In an example, frequencies presented to the user can be continuously modulated in a way that mimics normal, healthy brain function.

This Summary is intended to provide a brief overview of subject matter of the present application. It is not intended to provide an exclusive or exhaustive explanation of the invention or inventions discussed herein.

The description that follows describes systems, methods, techniques, instruction sequences, and computing machine program products that illustrate example embodiments of the present subject matter. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide an understanding of various embodiments of the present subject matter. It will be evident, however, to those skilled in the art, that embodiments of the present subject matter may be practiced without some or other of these specific details. Examples merely typify possible variations. Unless explicitly stated otherwise, structures (e.g., structural components, such as modules) are optional and may be combined or subdivided, and operations (e.g., in a procedure, algorithm, or other function) may vary in sequence or be combined or subdivided.

Various systems and methods discussed herein facilitate solving a technical problem of inducing particular brain behavior, or entrainment, using sounds or information in a user's environment. As such, one or more of the systems and methods described herein may obviate a need for certain efforts or computing resources that otherwise would be involved in therapy signal generation, or processing or use of other signals that could be distracting or unpleasant for a user, such as signals unrelated to the user's environment, particularly over long periods of time. As a result, resources used by one or more machines, databases, or devices (e.g., within the environment) may be reduced. Examples of such computing resources include processor cycles, network traffic, memory usage, data storage capacity, power consumption, or network bandwidth.

Systems and methods discussed herein can be used to provide auditory stimulation without injecting distracting or disruptive noise into a user's environment. In an example, a solution to the problem can include using amplitude modulation of ambient sounds or other sounds existing in a user's environment. That is, ambient sounds or other acoustic information in the vicinity of the user can be used as a carrier signal for amplitude-modulated, therapeutic auditory stimulation signals. In an example, the amplitude modulation can be introduced at selected frequencies that may not interfere with important functions like speech processing. In some examples, amplitude-modulated signals can be presented to a user continuously, and can be modulated in a way that mimics normal, healthy brain function. The amplitude-modulated signals can be presented to a user as an auditory stimulation signal, such as using one or both ears, and can be used to induce particular brainwaves or brainwave behavior. The particular brainwave activity or frequency to be induced can depend upon the particular therapy or objective for delivering the stimulus.

The present inventor has further recognized that a problem to be solved includes providing a tailored or user-specific auditory stimulation solution. For example, solutions using loudspeakers may take a "one size fits all" approach, assuming that everyone in a given environment may benefit from stimulation by the same audio frequencies. The present inventor has recognized that a solution to the problem can include allowing each user to receive his or her optimal frequency or frequencies, even while in the same environment as other users. In an example, the solution can include using the same ambient sound or sounds as a carrier for a variety of waveforms such as for a variety of respective users.

The present solution can include or use continuous auditory stimulation that induces brainwaves to couple with, or "entrain" to, the frequency of presented stimuli. Such stimulation has been found to have an effect-and significant potential therapeutic value-in a variety of use cases, including for alertness, or for treatment of diseases or disorders such as dementias, age-related memory loss, Parkinson's Disease, sleep disorders, speech issues like stuttering, depression, or anxiety, among others, in which certain brainwave rhythms are attenuated when compared with those of "normal" or neurotypical brains. The present solution allows for substantially continuous brain stimulation over long periods without distracting or disrupting the user.

In an example, the present solution includes amplitude modulation of ambient sound for continuous brain stimulation, for example, without using generated or recorded sounds to intrude on the user's senses, such as would otherwise mask a user's experience of the world and isolate or endanger the user. Rather, the present solution can include or use ambient sound in a user's environment as a carrier for an auditory stimulation signal.

In an example, the present solution can include or use frequency-selective amplitude modulation. Some frequency bands can be relatively more important for speech or other signal comprehension. Amplitude modulating frequencies in such bands can degrade speech (or other signal) comprehension and thus detract from a user's quality of life and/or can limit deployment of a potentially beneficial therapy. This problem is addressed using filtering such as to select only particular frequencies or frequency bands for modulation. In some examples, some relatively important frequencies can be unprocessed or unmodulated. In an example, the present systems and methods can be further used to augment hearing at these or other particular frequencies or frequency bands using spectral equalization, such as similarly to the way that a hearing aid can selectively tune various frequencies or bands to enhance intelligibility of particular signals. In an example, the systems and methods discussed herein can be integrated with, or used to augment various functions of, hearing aids, and can be tailored to a particular user's audiogram.

In an example, the present invention can include modulating select frequency bands, such as bands below or above one or more frequency thresholds. For example, unmodulated acoustic information can correspond to human speech-related frequencies. Acoustic information above or below the speech range can be modulated, such as without detrimental effect to the user's experience or understanding of speech. Many of the examples discussed herein refer to modulation of low frequency information (e.g., <NUM> or less), however, modulation can similarly be applied to higher frequency information above the speech range, such as at frequencies above about <NUM>.

In an example, the systems and methods discussed herein can be used to provide therapy in the form of "binaural beats. " Binaural beats are provided by presenting respective signals of slightly different frequencies to each of a listener's ears. The signals can cause the user's brainstem to produce an oscillation at a frequency that represents the difference between the two stimulation signal frequencies. For example, ambient sound can be amplitude modulated at, e.g., <NUM> in the right ear and <NUM> in_ the left ear, causing the brainstem to produce an oscillation at <NUM>, which can be measured across the entire scalp. Binaural beats can thus be used to modulate an ambient signal for continuous auditory stimulation.

In an example, the present solution can include using added sound when ambient sound pressure levels drop below a specified threshold level that may be needed to carry stimulation frequency(ies). For example, a noise signal (e.g., white noise, pink noise, brown noise, or other noise signal) can be generated and used as a carrier signal. In an example, the noise signal can be amplitude modulated to a desired frequency when ambient sound pressure levels in a user's environment drop below the specified threshold level that may be used or needed to otherwise carry a modulation signal. In an example, the noise signal can pass through a low pass filter along with, or can be processed separately from, the rest of the amplitude modulated signal, such as according to a user-defined filter cutoff. Using this noise-insertion or noise-bridging technique, stimulation frequencies can be presented to a user without disruption of the user's daily activities, such as throughout the entirety of the user's waking hours, allowing for an unprecedented amount of stimulation. In other words, acoustic information from a user's environment can be used for auditory stimulation when the acoustic information meets or exceeds a specified minimum threshold sound pressure level and a modulated noise signal can be used to occupy or bridge periods of time when the acoustic information does not meet the specified minimum threshold level.

In an example, the noise technique can be beneficial to users who suffer from tinnitus. There are many people who suffer severe tinnitus from exposure to loud sounds (e.g. veterans) and who would benefit from auditory stimulation that is evenly maintained between loud and quiet environments (those with tinnitus experience the most discomfort in quiet areas). In an example, an auditory stimulation signal to treat tinnitus can be provided at or around <NUM>. For example, amplitude modulation of a <NUM> masking noise can be provided to treat tinnitus by changing brain rhythms. Other signals can similarly be used. In an example, filtering techniques, such as can be helpful to treat various types of tinnitus, can be applied to ambient acoustic information or to a noise signal for use in auditory stimulation. A particular filtering technique can include providing a notch-filtered and amplitude-modulated auditory stimulation signal where the notch is centered at or near a frequency corresponding to a user's particular tonal tinnitus.

In an example, the present solution can include or use stimuli that are selected or designed to resemble neurotypical brainwave patterns. Some prior techniques for auditory brain stimulation make use of highly regular, sinusoidal waveforms. These waveforms, while convenient to generate and easy to find in spectrographic imagery, may not reflect the brainwaves of healthy, neurotypical individuals. Furthermore, exposure to highly symmetrical and fixed-frequency waves over long periods could be problematic for some users. In addition, the brain tends to habituate (or become increasingly less sensitive) to perfectly regular stimuli and therefore, over time, stimulation with a sinusoidal wave of unchanging amplitude can have a decreasing effect, such as leading to diminished effects of auditory stimulation. Typical brainwaves in humans can be described as "bursty," meaning they happen in bursts of activity, frequently changing in amplitude, and quickly starting and stopping. Such typical brainwaves can also drift slightly in frequency throughout different bursts.

In an example, systems and methods presented herein can addresses these problems and others, such as by mimicking various behaviors or features of neurotypical brainwaves. These features can be described in terms of spectrographic behavior, signal morphology, "burstiness," or "drift" of typical or target brainwave activity. Burstiness can refer to brainwaves that are not continuous but tend to take place in bursts. Drift can refer to brainwaves that do not remain rigidly at a fixed frequency but instead drift somewhat throughout a burst or over the course of multiple bursts. In an example, the solution herein can include or use such naturalistic frequencies by recording brainwave patterns from neurotypical brains and using those recorded waveforms or features thereof as references for amplitude modulation of other signals, including ambient acoustic signals, for delivery to a user.

<FIG> illustrates generally an example of a first auditory stimulation system <NUM>. The first auditory stimulation system <NUM> includes various components that can be used to prepare or generate an auditory stimulation signal, such as can be used to induce or augment brainwave activity in a user. As used herein, various signals are referred to in the singular form, however, plural signals can similarly or equivalently be used. For example, references to a particular signal or signal type can be understood to encompass one or multiple signals, such as can be provided using corresponding one or multiple channels (e.g., using stereo channels).

The example of the first auditory stimulation system <NUM> includes a First audio input <NUM>, an ancillary reference signal generator <NUM>, a modulation reference signal generator <NUM>, a first audio band processor circuit <NUM>, a second audio band processor circuit <NUM>, a mixer circuit <NUM>, and an audio output <NUM>. In an example, the First audio input <NUM> can include any input that is configured to sense or receive ambient or other signal information in the vicinity of the user. In an example, the First audio input <NUM> comprises a single-channel microphone and is configured to provide a monophonic microphone signal. In other examples, the First audio input <NUM> can include a multiple-channel audio input device, such as a stereo microphone, configured to provide two or more microphone signals.

The first auditory stimulation system <NUM> can include the modulation reference signal generator <NUM>. The modulation reference signal generator <NUM> can be configured to generate or provide a modulation reference signal for use in modulation of one or more other signals. For example, the modulation reference signal generator <NUM> can be configured to generate a sinusoidal reference signal, such as can be used in combination with another signal, such as a carrier signal, to provide an amplitude modulated signal In an example that includes amplitude modulation, the modulation reference signal generator <NUM> can generate a message signal that can be combined with, or used as a reference to modulate, a carrier signal. In an example, the carrier signal can comprise ambient information from the First audio input <NUM>, a noise signal, or other signal.

The first audio band processor circuit <NUM> can comprise an audio signal processor that is configured to receive information or a signal from the First audio input <NUM> and to receive information or a signal from the modulation reference signal generator <NUM>. For example, the first audio band processor circuit <NUM> can receive an ambient audio signal from the First audio input <NUM> and a sinusoidal reference signal from the modulation reference signal generator <NUM>. The first audio band processor circuit <NUM> can further include equalization, gain adjustment, or other filtering or processing circuitry.

In an example, the first audio band processor circuit <NUM> is configured to receive the ambient audio signal from the First audio input <NUM> and process the received signal with a low-pass filter to provide a low-passed intermediate signal. The low-passed intermediate signal can be further processed by the first audio band processor circuit <NUM>, such as according to a reference signal received from the modulation reference signal generator <NUM>, to provide a modulated signal. That is, the first audio band processor circuit <NUM> can be configured to provide a modulated signal that is an amplitude-modulated version of the low-frequency information in the acoustic information from the First audio input <NUM>, and characteristics of the amplitude modulation can be defined at least in part by information from the modulation reference signal generator <NUM>. The first audio band processor circuit <NUM> can provide the modulated signal to the mixer circuit <NUM>. In an example, the modulated signal can include two or more channels of information, such as corresponding to left and right channels of a stereo pair. The information in the respective channels can be similarly or differently modulated, such as based on the same modulation reference signal from the modulation reference signal generator <NUM>.

In an example, the second audio band processor circuit <NUM> can comprise an audio processor that is configured to receive information or a signal from the First audio input <NUM>. The second audio band processor circuit <NUM>. can process the received signal with a high-pass filter to provide a high-passed intermediate signal. The high-passed intermediate signal can be further processed by the second audio band processor circuit <NUM>, such as according to various gain, equalization, or other filter parameters. For example, the second audio band processor circuit <NUM> can include a speech intelligibility augmentation filter that is configured to amplify components or signal bands of the high-passed intermediate signal, or of the signal as received from the First audio input <NUM>, that are associated with human speech intelligibility. For example, the augmentation filter can be configured to boost information in a frequency band of about <NUM> to <NUM>. In an example, the augmentation filter can be configured to boost sibilance, for example, in a frequency band of about <NUM>-<NUM>. Other filters can similarly be used. In the example of <FIG>, the second audio band processor circuit <NUM> can provide an adjusted ambience signal to the mixer circuit <NUM>. In an example, the adjusted ambience signal can include two or more channels of information, such as corresponding to left and right channels of a stereo pair.

The mixer circuit <NUM> can be configured to combine or mix the modulated signal from the first audio band processor circuit <NUM> and the adjusted ambience signal from the second audio band processor circuit <NUM> and provide an auditory stimulation signal to the audio output <NUM>. The mixer circuit <NUM> or the audio output <NUM> can include other gain, filtering, or other processing circuitry to further adjust the auditory stimulation signal. In an example, the mixer circuit <NUM> is a multiple-channel mixer that is configured to combine information from multiple different channels simultaneously. In an example, the mixer circuit <NUM> is configured to receive the modulated signal from the modulation reference signal generator <NUM> as a single-channel signal to be mixed with different left and right channels of the adjusted ambience signal from the second audio band processor circuit <NUM>.

The audio output <NUM> can include an interface configured to provide the auditory stimulation signal, or a further processed version thereof, to the user. In an example, the audio output <NUM> includes a headphone interface configured to provide the auditory stimulation signal to one or both ears of the user using headphones, earphones, bone-conduction devices, hearing aids, or other sources.

The example of <FIG> includes the ancillary reference signal generator <NUM>. In an example, the ancillary reference signal generator <NUM> includes a noise generator that can be selectively used to generate a noise signal. The noise signal can comprise one or multiple different types of noise signals, such as white noise, pink noise, brown noise, or other type of noise signal, or a combination of different types of noise signals. In an example, the ancillary reference signal generator <NUM> can be configured to generate the noise signal when an ambient sound pressure level (SPL) (e.g., an average or RMS sound pressure level) of an environment is less than a specified minimum sound pressure threshold amount. In an example, the ambient SPL can be determined by the first auditory stimulation system <NUM> using information from the First audio input <NUM> or from another source, such as from a separate SPL meter or sensor.

The ancillary reference signal generator <NUM> can provide the noise signal to the first audio band processor circuit <NUM>. In an example, the first audio band processor circuit <NUM> can prepare the modulated signal using the noise signal and a modulation reference signal. In an example, the first audio band processor circuit <NUM> can prepare the modulated signal using the noise signal and using the low-passed intermediate signal that is based on information from the First audio input <NUM>.

<FIG> illustrates generally an example of a second auditory stimulation system <NUM>. The second auditory stimulation system <NUM> can include various modules or components that are the same or similar to those discussed above in the example of <FIG>. The example of <FIG> is provided to further illustrate various features or components of some modules or components in a system for augmenting or inducing brainwave activity using auditory stimulation. In an example, one or more of the components of the second auditory stimulation system <NUM> can be implemented using a mobile device, such as a mobile telephone, tablet computer, or other purpose-built device. The second auditory stimulation system <NUM> can include or comprise one or more interfaces for receiving instructions that define an auditory stimulation program. The interfaces can be local or remote. For example, the second auditory stimulation system <NUM> can include a remote interface to receive instructions from a clinician or caregiver, and the second auditory stimulation system <NUM> can include a local interface to receive instructions from a user.

The example of <FIG> includes a second audio input <NUM> such as can include a microphone or other acoustic sensor or audio signal receiver configured to provide one or more signals indicative of an acoustic environment of the user. The signals from the second audio input <NUM> can include a first audio input signal pair <NUM>, a second audio input signal pair <NUM>, and a third audio input signal pair <NUM>, such as comprising stereo pairs of audio signals that include speech information, background noise, or other acoustic information from the environment of the user. In an example, the second audio input <NUM> can include or correspond to the first audio input <NUM> from the example of <FIG>. In an example, the second audio input <NUM> comprises a receiver for broadcasted (e.g., via television or radio) signals or other acoustic signals.

The example of the second auditory stimulation system <NUM> includes a first processor circuit <NUM> and a second processor circuit <NUM>. The first processor circuit <NUM> can include or correspond to the first audio band processor circuit <NUM> and the second processor circuit <NUM> can include or correspond to the second audio band processor circuit <NUM> from the example of <FIG>. In some examples, the processor circuits, among other circuits or functional blocks, can comprise different portions of the same processor circuit. In an example, the first processor circuit <NUM> is configured to receive the second audio input signal pair <NUM>, and a noise signal, and provide a modulated signal that is based on one or both of the second audio input signal pair <NUM> and the noise signal. In an example, the first processor circuit <NUM> is configured to receive modulation parameters or instructions from a first parameter input <NUM>. The parameters can include, for example, a modulation frequency, modulation depth, therapy duration, volume, or other parameters that can influence one or more characteristics of the modulated signal.

The first processor circuit <NUM> can include a low-pass filter <NUM>. The low-pass filter <NUM> can receive the second audio input signal pair <NUM> and provide a low-passed signal that includes or represents a low frequency portion of the information in the second audio input signal pair <NUM>. In an example, the low-pass filter <NUM> can receive the noise signal and provide a low-passed signal that includes or represents low frequency information from the noise signal.

The first processor circuit <NUM> can further include a first reference signal generator <NUM> and a first modulation filter <NUM>. In an example, the first reference signal generator <NUM> can include or correspond to the modulation reference signal generator <NUM> from the example of <FIG>. The first reference signal generator <NUM> can be configured to generate a first modulation signal, or a first information signal for an auditory stimulation signal. Parameters of the first modulation signal can be based on the first parameter input <NUM>. In an example, the first modulation filter <NUM> can be configured to use the low-passed signal from the low-pass filter <NUM>, such as comprising acoustic information from the second audio input <NUM> or the noise signal or both, together with the first modulation signal from the first reference signal generator <NUM> to provide a first modulated signal.

In an example, the first reference signal generator <NUM> can be configured to generate a sinusoidal signal at a frequency that depends, at least in part, on the first parameter input <NUM>. In an example, the first reference signal generator <NUM> can be configured to generate a reference signal configured to drive Theta band activity, such as around <NUM>, such as can be used to treat dementia-related memory loss. In an example, the first reference signal generator <NUM> can be configured to generate a reference signal configured to drive Gamma band activity, such as around <NUM>. Gamma band activity can be used to augment or affect larger-scale brain activity such as related to memory, attention, or to treat disorders such as epilepsy or anxiety. In an example, the first parameter input <NUM> can be based on user-specific information about the user's disorder or disease state. For example, the first parameter input <NUM>, such as received from a clinician, can be based on a specific target frequency that may be identified as being deficient in a particular patient or user.

The example of the first processor circuit <NUM> includes a second reference signal generator <NUM> and a second modulation filter <NUM>. The second reference signal generator <NUM> can be configured to generate a second modulation signal for use in the auditory stimulation signal. Parameters of the second modulation signal can be based on the first parameter input <NUM>. The second modulation signal can be different from the first modulation signal, such as in terms of frequency or amplitude or other characteristic. The second modulation filter <NUM> can be configured to use the first modulated signal from the first modulation filter <NUM> together with the second modulation signal from the second reference signal generator <NUM> to provide a second modulated signal. Additional instances of modulation reference signal generators and filters can similarly be used. In an example, the second reference signal generator <NUM> is configured to provide a second modulation signal that is substantially lower in frequency than the first modulation signal provided by the first reference signal generator <NUM>. In an example, the second modulation filter <NUM> can apply the second modulation signal to create a bursty output signal that is configured to mimic neurotypical brainwave activity.

In an example, the first processor circuit <NUM> includes a random number generator <NUM> or other circuit or device that can be used to change spectrographic, morphologic, burst, or drift behavior of modulation provided by the second auditory stimulation system <NUM>. For example, the first processor circuit <NUM> can be configured to use information from the random number generator <NUM> to apply a modulation algorithm that stochastically moves a center frequency of a target modulation frequency band about a user-defined bandwidth. For example, a Theta band comprises <NUM>-<NUM> signals, so <NUM> and <NUM> can be set as outer bounds for movement or modulation of the signal. The algorithm can be configured to "move" the resulting signal modulation at various increments, such as <NUM>. The resulting signal can then exhibit frequency drift similar to that of a natural Theta rhythm. In an example, a width of frequency drift or a rate of frequency drift can be adjusted or toggled on/off, such as according to a user input (e.g., using the first parameter input <NUM>).

In an example, information from the random number generator <NUM> can be used to control overall amplitude or frequency characteristics of an amplitude-modulated signal provided by the first modulation filter <NUM>, such as to provide a bursty or drifty signal that can be more immune to habituation. In an example, the random number generator <NUM> includes a quasi-random number generator, or pseudo-random number generator, that is configured to move in small increments. Small increments are generally preferred to help avoid drastic changes that could materialize as audible clicks or pops. In an example, the random number generator <NUM> is configured to provide a differently valued random output signal periodically, such as every <NUM>. The random output signal can be multiplied by a user-specified frequency (e.g., <NUM>) to provide an amplitude control signal. The amplitude control signal can then be used, such as by the first modulation filter <NUM> or the second modulation filter <NUM>, to slowly change an amplitude characteristic of the auditory stimulation signal to thereby help avoid user habituation to the stimulation signal.

In an example, the second auditory stimulation system <NUM> includes a second processor circuit <NUM>. The second processor circuit <NUM> can include, among other things, a high-pass filter <NUM>, an equalizer <NUM>, and a first gain circuit <NUM>. In an example, the second processor circuit <NUM> is configured to provide a high-passed signal, based on the first audio input signal pair <NUM>, using the high-pass filter <NUM>. The high-passed signal can be equalized using the equalizer <NUM> or gain-adjusted using the first gain circuit <NUM> and then outputted as a high frequency signal. The high frequency signal can represent relatively higher frequency information in the first audio input signal pair <NUM> received from the second audio input <NUM>. For example, the high frequency signal can include speech information.

In an example, the second processor circuit <NUM> is configured to receive instructions or parameters from a second parameter input <NUM>. The parameters can include, for example, a cutoff frequency for the high-pass filter <NUM>, equalization parameters for the equalizer <NUM>, or gain instructions for the first gain circuit <NUM>. The parameters can influence various characteristics of the high frequency signal provided by the second processor circuit <NUM>. In an example, the equalization parameters include information about a user's audiogram and can be used to configure the equalizer <NUM> to provide an adjusted signal that can help augment hearing or intelligibility for the user.

The example of <FIG> includes a noise generator circuit <NUM>. The noise generator circuit <NUM> can include a threshold detector <NUM> that is configured to receive the third audio input signal pair <NUM>. The threshold detector <NUM>, such as can comprise a sound pressure level meter, can analyze the third audio input signal pair <NUM> to determine whether a specified sound pressure level threshold is met or exceeded by acoustic information in the environment of the second audio input <NUM>. That is, the threshold detector <NUM> can determine whether there is sufficient acoustic information in the environment such that it can be modulated to deliver auditory stimulation to the user. If the threshold detector <NUM> determines that there is an insufficient amount of acoustic information, or that a sound pressure level threshold is not met, then a noise signal generator <NUM> can be activated to provide the noise signal to the first processor circuit <NUM>. The first processor circuit <NUM> can modulate the noise signal to provide an auditory stimulation signal to the user, as described above.

In an example, the noise generator circuit <NUM> is configured to receive instructions or parameters from a third parameter input <NUM>. The parameters can include, for example, a sound pressure level threshold at which to activate the noise signal generator <NUM>, or a type of noise to be generated by the noise signal generator <NUM>, among other things. In an example, the third parameter input <NUM> can include a sensitivity or hysteresis control for the threshold detector <NUM> or for the noise signal generator <NUM>.

The example of <FIG> includes an interrupt circuit <NUM>. The interrupt circuit <NUM> can produce an interrupt instruction that can interrupt one or more functions or features of the second auditory stimulation system <NUM>. For example, the interrupt circuit <NUM> can include an accelerometer <NUM>. The accelerometer <NUM> can be configured to sense an orientation or configuration of a device that includes the second audio input <NUM>. When the accelerometer <NUM> is oriented in a specified manner, for example upright or in another predefined orientation, then an interrupt generator <NUM> can be caused to generate the interrupt instruction. In an example, the interrupt circuit <NUM> can include a user input such as a button <NUM>. When the button is pressed or actuated, the interrupt circuit <NUM> can generate the interrupt instruction. In an example, the interrupt instruction comprises a binary logic signal.

In an example, a second gain circuit <NUM> can receive the interrupt instruction from the interrupt circuit <NUM>, the second modulated signal from the first processor circuit <NUM> and the high frequency signal from the second processor circuit <NUM>. The second gain circuit <NUM> can be configured to provide gain-adjusted versions of the second modulated signal and the high frequency signal to a mixer circuit <NUM>, and the mixer circuit <NUM> can be configured to provide an auditory stimulation output signal for the user. When the interrupt instruction is received from the interrupt circuit <NUM>, the second gain circuit <NUM> can adjust a magnitude relationship between the second modulated signal and the high frequency signal. In an example, when the interrupt is asserted, the second auditory stimulation system <NUM> can deprioritize or mute the second modulated signal and provide the high frequency signal to the mixer circuit <NUM>. In other examples, when the interrupt is asserted, the second auditory stimulation system <NUM> can deactivate such that no signals are permitted to pass to the mixer circuit <NUM>. In other examples, when the interrupt is asserted, a different gain-adjusted version or amplified version of the input signal--such as without modulation- -can be passed to the mixer circuit <NUM>.

<FIG> illustrates generally several charts that illustrate time-aligned examples of various audio signals. For example, <FIG> includes a modulation signal reference chart <NUM>, a first input signal chart <NUM>, a high-passed signal chart <NUM>, a first spectrogram <NUM>, a low-passed and modulated signal chart <NUM>, a second spectrogram <NUM>, a first output signal chart <NUM>, and a first output signal spectrogram <NUM>. Along the x axis, the several charts have a common time axis and common scale. The y axis of the signal charts indicates relative amplitude, and the y axis of the spectrogram charts represent frequency (e.g., from about <NUM> to about <NUM>).

The example of the modulation signal reference chart <NUM> includes a reference signal waveform <NUM>. The reference signal waveform <NUM> in <FIG> represents a <NUM> sinusoidal wave. The reference signal waveform <NUM> can be used as a modulation reference or trigger to generate various other audio signals for auditory stimulation. In an example, the reference signal waveform <NUM>. can represent an information signal or message signal that can be used together with a carrier signal to provide an amplitude-modulated auditory stimulation signal. Other reference signals, such as having different frequency or amplitude characteristics, can similarly be used.

The example of the first input signal chart <NUM> includes an input signal waveform <NUM>. The input signal waveform <NUM> represents a speech signal. The speech signal can be unfiltered, and unmodulated. In an example, the input signal waveform <NUM> can represent an acoustic signal received by an audio input, such as the first audio input <NUM> or the second audio input <NUM>. In an example, the input signal waveform <NUM> can represent a carrier signal to be modulated, such as according to a reference signal such as in the reference signal waveform <NUM>.

The example of the high-passed signal chart <NUM> includes a high-passed signal waveform <NUM>. The high-passed signal waveform <NUM> can represent a high-passed and gain-adjusted version of the input signal waveform <NUM>. In an example, the high-passed signal waveform <NUM> can represent a signal generated by the second processor circuit <NUM> based on an acoustic signal from the second audio input <NUM>.

The example of the first spectrogram <NUM> includes spectrogram information corresponding to the high-passed signal waveform <NUM>. That is, the first spectrogram <NUM> represents a relative magnitude of various frequencies in the high-passed signal waveform <NUM>. In the example of the first spectrogram <NUM>, it can be observed that the high-passed signal waveform <NUM> includes information primarily at or above <NUM>.

The example of the low-passed and modulated signal chart <NUM> includes a low-passed and modulated signal waveform <NUM>. The low-passed and modulated signal waveform <NUM> can represent a low-passed and amplitude-modulated version of the input signal waveform <NUM>. In the example, the low-passed and modulated signal waveform <NUM> thus represents an amplitude-modulated version of the lower-frequency information from the input signal waveform <NUM>, and is amplitude modulated at <NUM>, owing to the <NUM> frequency of the reference signal waveform <NUM>. The amplitude modulation processing doubles the frequency of the reference signal because the envelope of the resulting signal can be adjusted per-peak of the reference signal, that is, regardless of whether the peak is negative or positive. In an example, the low-passed and modulated signal waveform <NUM> can represent a signal generated by the first processor circuit <NUM> based on an acoustic signal from the second audio input <NUM>.

The example of the second spectrogram <NUM> includes spectrogram information corresponding to the low-passed and modulated signal waveform <NUM>. That is, the second spectrogram <NUM> represents a relative magnitude of various frequencies in the low-passed and modulated signal waveform <NUM>. In the example of the second spectrogram <NUM>, it can be observed that the low-passed and modulated signal waveform <NUM> includes information primarily at or below about <NUM>. Furthermore, the amplitude modulation can be observed as wliitespaces at intervals corresponding to the <NUM> modulation signal. That is, the periodic attenuation of the amplitude of the low frequency information can be observed particularly at zero-crossings of the reference signal waveform <NUM>, as indicated by the vertical dashed lines in the figure. The vertical dashed lines are provided to help illustrate the correspondence between the time-amplitude characteristics of zero-crossings and the reference signal waveform <NUM> and the several other signals and spectrograms.

The example of the first output signal chart <NUM> includes a first output signal waveform <NUM>. The first output signal waveform <NUM> can represent a combination of the high-passed signal shown in the high-passed signal waveform <NUM> and the low-passed signal shown in the low-passed and modulated signal waveform <NUM>. In an example, the output signal corresponding to the first output signal waveform <NUM> can be provided by the mixer circuit <NUM>. In the example of the first output signal waveform <NUM>, high frequency information, such as corresponding to speech intelligibility, can be maintained across the dips in low frequency information caused by the amplitude modulation. The first output signal spectrogram <NUM> illustrates generally the periodic nature of the amplitude of the low frequency information relative to the higher frequency information of the first output signal waveform <NUM>. The first output signal waveform <NUM> can include one or more auditory stimulation signals that can be provided to a user.

<FIG> illustrates generally several charts that illustrate time-aligned examples of various audio signals. For example, <FIG> includes a reference signal spectrogram <NUM>, a noise signal chart <NUM>, a noise signal spectrogram <NUM>, a second output signal chart <NUM>, and a second output signal spectrogram <NUM>. Along the x axis, the several charts have a common time axis and common scale. The y axis of the signal charts indicates relative amplitude, and the y axis of the spectrogram charts represent frequency (e.g., from about <NUM> to about <NUM>). In the example of <FIG>, the same modulation signal reference chart <NUM> and reference signal waveform <NUM> from the example of <FIG> can be used in amplitude modulation of a carrier signal, such as a noise signal.

The reference signal spectrogram <NUM> includes spectrogram information corresponding to low frequency components of the reference signal waveform <NUM>. That is, the reference signal spectrogram <NUM> illustrates generally the relative magnitude of various frequencies in the reference signal waveform <NUM>.

The noise signal chart <NUM> includes a noise signal waveform <NUM>, such as representative of a white noise signal. In an example, the noise signal waveform <NUM> can represent a noise signal generated by the noise generator circuit <NUM>. The noise signal spectrogram <NUM> shows a spectrogram corresponding to the noise signal waveform <NUM> and it indicates generally that the white noise signal includes approximately equal amplitude, on average, across the illustrated frequency band.

The second output signal chart <NUM> includes a second output signal waveform <NUM>. The second output signal waveform <NUM> can represent an amplitude-modulated version of the noise signal corresponding to the noise signal waveform <NUM>. The second output signal spectrogram <NUM> corresponds generally to the second output signal waveform <NUM> and illustrates the periodicity of modulated signal, at <NUM>, due to the reference signal waveform <NUM>. That is, the examples of the second output signal chart <NUM> and the second output signal spectrogram <NUM> illustrate an amplitude-modulated white noise signal, as-modulated according to the reference signal waveform <NUM>. In an example, the amplitude-modulated white noise signal can be provided by the first processor circuit <NUM> as one or more auditory stimulation signals.

<FIG> illustrate generally examples of an interface for a system or device that can provide auditory stimulation to a user. The examples include a mobile device <NUM>, such as can include a mobile phone, tablet computer, or other mobile processing device configured to receive a user input, an audio input, and in response provide an amplitude-modulated output signal. In an example, the mobile device <NUM> comprises a particular example of the machine <NUM> discussed elsewhere herein.

The example of <FIG> includes the mobile device <NUM> in communication with headphones <NUM> using a headphone communication link <NUM>, such as can include a wired or wireless link. The headphones <NUM> can be used to receive an auditory stimulation signal from the mobile device <NUM> for delivery to one or more ears of a user. In an example, the mobile device <NUM> can include an accelerometer <NUM>, such as corresponding to the accelerometer <NUM> from the interrupt circuit <NUM> of <FIG>.

The example of <FIG> illustrates generally a home interface <NUM>, such as can be provided on a display of the mobile device <NUM>. The home interface <NUM> can include various controls, icons, or images. For example, the home interface <NUM> can include an on button <NUM> and an off button <NUM> configured to control operation of an auditory stimulation program. The home interface <NUM> can include a "WHAT" button <NUM> that can be used to selectively (e.g., temporarily, or in response to a user input) disengage one or more features of the auditory stimulation system. In an example, in response to user actuation of the "WHAT" button <NUM>, the interrupt circuit <NUM> can generate the interrupt instruction. The home interface <NUM> can further include a modulation visualization icon <NUM>, such as can include a pictorial representation of operation of the system. For example, a portion of the modulation visualization icon <NUM> can change or flash in correspondence with a modulation frequency of an auditory stimulation signal provided by the system.

The example of <FIG> illustrates generally a modulation settings interface <NUM>, such as can be provided on the display of the mobile device <NUM>. The modulation settings interface <NUM> can include various inputs to receive a user input (e.g., from a user, a clinician, a caregiver, etc.) to set or define one or more aspects of an auditory stimulation signal to be provided by the system, such as using the headphones <NUM>.

In the example of <FIG>, the inputs comprise graphical dials, such as can represent various relative or absolute settings. The example includes a modulation depth input <NUM> configured to set or adjust a modulated frequency volume of the auditory stimulation signal. The example includes a first high pass channel amplitude input <NUM> and a second high pass channel amplitude input <NUM> configured to set or adjust respective amplitude characteristics of high frequency information in the auditory stimulation signal. The example includes a noise signal amplitude input <NUM> configured to set or adjust a volume of a noise signal, such as when a noise signal comprises some or all of the auditory stimulation signal. The example includes an ambient signal threshold input <NUM> that can be used to set or adjust an ambient sound pressure level threshold used by the threshold detector <NUM>. The example includes a low pass frequency input <NUM> configured to set or adjust a low-pass frequency cutoff such as can be used by the low-pass filter <NUM>. The example includes a gamma signal amplitude input <NUM> that can be used to set adjust, or toggle a Gamma wave signal (e.g., additionally or alternatively to other modulation signals). The example includes a high pass frequency input <NUM> configured to set or adjust a high-pass frequency cutoff, such as can be used by the high-pass filter <NUM>. The example further includes a modulation frequency input <NUM> configured to receive information from a user about a desired modulation frequency of the auditory stimulation signal. The example of the modulation settings interface <NUM> further includes a clear settings button <NUM> to re-set or clear the inputs, or to set one or more of the inputs to predefined or specified values.

<FIG> illustrates generally an example of a first method <NUM> not according to the claimed invention that can be used to provide an auditory stimulation signal to a user. The example can include, at block <NUM>, receiving ambient acoustic information in proximity of the user. Block <NUM> can include using a microphone or the second audio input <NUM>, to receive the acoustic information.

At block <NUM>, the first method <NUM> can include amplitude-modulating a low-frequency portion of the ambient acoustic information received at block <NUM>. In an example, block <NUM> can include using the first processor circuit <NUM> to generate a low-passed signal and to modulate the low-passed signal to provide a primary modulated output signal.

At block <NUM>, the first method <NUM> can include combining the primary modulated output signal from block <NUM> with high frequency information from the ambient acoustic information received at block <NUM>. That is, block <NUM> can include generating a high-passed signal from the acoustic information received at block <NUM>, such as using the second processor circuit <NUM>, and mixing the high-passed signal with the primary modulated output signal from block <NUM>, such as using the mixer circuit <NUM>.

The first method <NUM> can further include, at block <NUM>, providing the auditory stimulation signal to a user. In an example, block <NUM> can include using the headphones <NUM> to provide the auditory stimulation signal to the user.

<FIG> illustrates generally an example of a second method <NUM> not according to the claimed invention that can be used to provide an auditory stimulation signal to a user. The example can include, at block <NUM>, receiving ambient acoustic information in proximity of a user. In an example, block <NUM> can include using a microphone or the second audio input <NUM> from the example of <FIG>.

At decision block <NUM>, the second method <NUM> can include determining whether the acoustic information received at block <NUM> meets a threshold sound pressure level (SPL) condition. If the acoustic information is not below the SPL threshold, then the second method <NUM> can continue at block <NUM>. At block <NUM>, the second method <NUM> can include providing an auditory stimulation signal based on modulated ambient acoustic information. For example, block <NUM> can include or use the first method <NUM> to provide an auditory stimulation signal based on a combination of high-passed and modulated low-passed information from the acoustic signal.

If the acoustic information is below the SPL threshold at decision block <NUM>, then the second method <NUM> can continue at block <NUM>. At block <NUM>, the second method <NUM> can include providing an auditory stimulation signal based on a noise signal. That is, block <NUM> can include providing an amplitude-modulated noise signal as all or a portion of an auditory stimulation signal for the user.

<FIG> is a diagrammatic representation of a machine <NUM> within which instructions <NUM> (e.g., software, a program, an application, an applet, an app, or other executable code) for causing the machine <NUM> to perform any one or more of the methodologies discussed herein may be executed. For example, the instructions <NUM> may cause the machine <NUM> to execute any one or more of the methods described herein, such as can include various audio signal processing methods or algorithms, such as can be used to prepare or provide an auditory stimulation signal to a user. The instructions <NUM> transform the general, non-programmed machine <NUM> into a particular machine <NUM> programmed to carry out the described and illustrated functions in the manner described. The machine <NUM> may operate as a standalone device or may be coupled (e.g., networked) to other machines. In a networked deployment, the machine <NUM> may operate in the capacity of a server machine or a client machine in a server-client network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. The machine <NUM> may comprise, but not be limited to, a server computer, a client computer, a personal computer (PC), a tablet computer, a laptop computer, a netbook, a set-top box (STB), a PDA, an entertainment media system, a cellular telephone, a smart phone, a mobile device, a wearable device (e.g., a smart watch), a smart home device (e.g., a smart appliance), other smart devices, a web appliance, a network router, a network switch, a network bridge, or any machine capable of executing the instructions <NUM>, sequentially or otherwise, that specify actions to be taken by the machine <NUM>. Further, while only a single machine <NUM> is illustrated, the term "machine" shall also be taken to include a collection of machines that individually or jointly execute the instructions <NUM> to perform any one or more of the methodologies discussed herein.

The machine <NUM> may include processors <NUM>, memory <NUM>, and I/O components <NUM>, which may be configured to communicate with each other via a bus <NUM>. In an example embodiment, the processors <NUM> (e.g., a Central Processing Unit (CPU), a Reduced Instruction Set Computing (RISC) processor, a Complex Instruction Set Computing (CISC) processor, a Graphics Processing Unit (GPU), a Digital Signal Processor (DSP), an ASIC, a Radio-Frequency Integrated Circuit (RFIC), another processor, or any suitable combination thereof) may include, for example, a processor <NUM> and a processor <NUM> that execute the instructions <NUM>. The term "processor" is intended to include multi-core processors that may comprise two or more independent processors (sometimes referred to as "cores") that may execute instructions contemporaneously. Although <FIG> shows multiple processors <NUM>, the machine <NUM> may include a single processor with a single core, a single processor with multiple cores (e.g., a multi-core processor), multiple processors with a single core, multiple processors with multiples cores, or any combination thereof.

The memory <NUM> can include a main memory <NUM>, a static memory <NUM>, and a storage unit <NUM>, accessible to the processors <NUM> via the bus <NUM>. The instructions <NUM> may also reside, completely or partially, within the main memory <NUM>, within the static memory <NUM>, within a machine-readable medium <NUM> within the storage unit <NUM>, within at least one of the processors <NUM> (e.g., within the processor's cache memory), or any suitable combination thereof, during execution thereof by the machine <NUM>.

The I/O components <NUM> may include a wide variety of components to receive input, provide output, produce output, transmit information, exchange information, capture measurements, and so on. The specific I/O components <NUM> that are included in a particular machine will depend on the type of machine. For example, portable machines such as mobile phones may include a touch input device or other such input mechanisms (e.g., corresponding to the modulation settings interface <NUM>), while a headless server machine will likely not include such a touch input device. It will be appreciated that the I/O components <NUM> may include many other components that are not shown in <FIG>. In various example embodiments, the I/O components <NUM> may include output components <NUM> and input components <NUM>. The output components <NUM> may include visual components (e.g., a display such as a plasma display panel (PDP), a light emitting diode (LED) display, a liquid crystal display (LCD), a projector, or a cathode ray tube (CRT)), acoustic components (e.g., speakers, the headphones <NUM>, etc.), haptic components (e.g., a vibratory motor, resistance mechanisms), other signal generators, and so forth. The input components <NUM> may include alphanumeric input components (e.g., a keyboard, a touch screen configured to receive alphanumeric input, a photo-optical keyboard, or other alphanumeric input components), point-based input components (e.g., a mouse, a touchpad, a trackball, a joystick, a motion sensor, or another pointing instrument), tactile input components (e.g., a physical button, a touch screen that provides location and/or force of touches or touch gestures, or other tactile input components), audio input components (e.g., a microphone), and the like.

In further example embodiments, the I/O components <NUM> may include biometric components <NUM>, motion components <NUM>, environmental components <NUM>, or position components <NUM>, among a wide array of other components. For example, the biometric components <NUM> include components to detect expressions (e.g., hand expressions, facial expressions, vocal expressions, body gestures, or eye tracking), measure biosignals (e.g., blood pressure, heart rate, body temperature, perspiration, or brainwaves), identify a person (e.g., voice identification, retinal identification, facial identification, fingerprint identification, or electroencephalogram-based identification), and the like. The motion components <NUM> include acceleration sensor components (e.g., the accelerometer <NUM>), gravitation sensor components, rotation sensor components (e.g., gyroscope), and so forth. The environmental components <NUM> include, for example, illumination sensor components (e.g., photometer), temperature sensor components (e.g., one or more thermometers that detect ambient temperature), humidity sensor components, pressure sensor components (e.g., barometer), acoustic sensor components (e.g., one or more microphones that detect background noise), proximity sensor components (e.g., infrared sensors that detect nearby objects), gas sensors (e.g., gas detection sensors to detection concentrations of hazardous gases for safety or to measure pollutants in the atmosphere), or other components that may provide indications, measurements, or signals corresponding to a surrounding physical environment of the user. Information from any one or more of the motion components <NUM> or the environmental components <NUM> or other sensors or devices can similarly be used to update or change a modulation parameter or setting of a system that provides auditory stimulation. The position components <NUM> include location sensor components (e.g., a GPS receiver component), altitude sensor components (e.g., altimeters or barometers that detect air pressure from which altitude may be derived), orientation sensor components (e.g., magnetometers), and the like.

Communication may be implemented using a wide variety of technologies. The I/O components <NUM> further include communication components <NUM> operable to couple the machine <NUM> to a network <NUM> or devices <NUM> via a coupling <NUM> and a coupling <NUM>, respectively. For example, the communication components <NUM> may include a network interface component or another suitable device to interface with the network <NUM>. In further examples, the communication components <NUM> may include wired communication components, wireless communication components, cellular communication components, Near Field Communication (NFC) components, Bluetooth® components (e.g., Bluetooth® Low Energy), Wi-Fi® components, and other communication components to provide communication via other modalities. The devices <NUM> may be another machine or any of a wide variety of peripheral devices (e.g., a peripheral device coupled via a USB).

The various memories (e.g., memory <NUM>, main memory <NUM>, static memory <NUM>, and/or memory of the processors <NUM>) and/or storage unit <NUM> may store one or more sets of instructions and data structures (e.g., software) embodying or used by any one or more of the methodologies or functions described herein. These instructions (e.g., the instructions <NUM>), when executed by processors <NUM>, cause various operations to implement the disclosed embodiments.

The instructions <NUM> may be transmitted or received over the network <NUM>, using a transmission medium, via a network interface device (e.g., a network interface component included in the communication components <NUM>) and using any one of a number of well-known transfer protocols (e.g., hypertext transfer protocol (HTTP)). Similarly, the instructions <NUM> may be transmitted or received using a transmission medium via the coupling <NUM> (e.g., a peer-to-peer coupling) to the devices <NUM>.

The solutions discussed herein can be used to treat or improve symptoms of a variety of disorders and conditions including but not limited to dementias (including Alzheimer's Dementia), Parkinson's Disease, Chronic Traumatic Encephalopathy, anxiety, and ADHD, such as using different parameters or settings for each condition and/or for each user. The solutions discussed herein can be added to the functionality of a variety of existing hearing aids or assisted listening device, such as in combination with sound processing for improved audition. The injection of noise, or use of modulated noise, can be provided in any device with headphones or other hearing devices such as for people with tinnitus. In an example, the solution can be included in or implemented using a smartphone app such as can be used with a smartphone and pair of earbuds or other headphones. In an example, the solution can include a clinical version for use in hospitals such as with a set of headphones with built-in processing or an external processing unit. Such a device can amplitude-modulate the intense sounds in a hospital environment to either prevent hospital delirium from developing (a pervasive problem with hospitalized elderly) or to help calm anxious patients.

In an example, the auditory stimulation systems and methods discussed herein can be used together with EEG activity configured to sense electrical activity of a brain. The auditory stimulation can be triggered or adjusted, for example, depending on measured brain activity. In an example, the EEG can be configured to detect an impending seizure and, in response, the auditory stimulation device can attempt to prevent or disrupt the seizure by stimulating the brain with an alternative signal.

The above description includes references to the accompanying drawings, which form a part of the detailed description.

Claim 1:
A system for inducing brainwave activity using auditory stimulation, the system comprising:
an audio input (<NUM>, <NUM>) configured to receive ambient acoustic information from an environment of a user;
a modulation reference signal generator (<NUM>, <NUM>) configured to generate a modulation reference signal based on a modulation input;
a noise signal generator (<NUM>) configured to generate a noise signal;
a first audio band processor circuit (<NUM>, <NUM>) configured to provide a primary modulated output signal based on a first frequency portion of the ambient acoustic information and on the modulation reference signal, and to provide a secondary modulated output signal based on the noise signal and on the modulation reference signal; and
a mixer circuit (<NUM>, <NUM>) configured to provide an auditory stimulation signal, wherein, when the ambient acoustic information indicates an ambient sound pressure level, SPL, meets or exceeds a specified threshold SPL, the auditory stimulation signal includes the primary modulated output signal from the first audio band processor circuit (<NUM>, <NUM>) and a different second frequency portion of the ambient acoustic information, and when the ambient acoustic information indicates the ambient SPL is less than the specified threshold SPL, the auditory stimulation signal includes the secondary modulated output signal.