Low frequency sound field in a listening environment

The present disclosure provides methods and systems for homogenizing a low frequency listening experience for users in a plurality of locations, such as a plurality of seat positions in a vehicle. An example audio system includes a plurality of woofers configured to output low frequency sound into a listening environment to yield a homogeneous low frequency sound field. Homogeneity of the low frequency sound field in this case is for example spectrally uniform throughout an extended listening space.

TECHNICAL FIELD

Embodiments disclosed herein generally relate to audio systems having low frequency transducers for controlling low frequency sound fields in listening environments.

BACKGROUND

A conventional vehicle includes a listening environment. Inside of the listening environment, there may be several seats, such as a driver seat, a front passenger seat, a first rear seat, a second rear seat, etc. The conventional vehicle may further include an audio system having a low frequency loudspeaker. The single low frequency loudspeaker may be responsible for outputting some or all of the low frequency sound into the listening environment. Because of the single low frequency loudspeaker, the low frequency sound field in the listening environment will significantly vary seat to seat. Thus an occupant's low frequency listening experience will vary significantly depending on seat choice.

SUMMARY

The present disclosure provides methods and systems for homogenizing a low frequency listening experience for users in a plurality of locations, such as a plurality of seat positions in a vehicle. An example audio system includes a plurality of woofers configured to output low frequency sound into a listening environment to yield a homogeneous low frequency sound field in specified listening regions (e.g., regions where acoustic transfer functions are estimated and/or measured). The example audio system may further include a plurality of filters, each of the plurality of woofers having a respective filter of the plurality of filters assigned thereto to yield the homogeneous low frequency sound field. Homogeneity of the low frequency sound field in this case is for example spectrally uniform across the extended listening region or regions.

As described herein, a resulting homogenous sound field is a consequence of sound field control filters (e.g., applied to low frequency loudspeakers) that are calculated based on acoustic transfer functions from each individual low frequency loudspeaker to each point in space defining the controlled listening region(s) of concern. The filters may be determined based on constrained optimization with a specified cost function realizing a given target or set of targets (e.g., least mean square with various constraints defined such as degree of homogeneity, control loudspeaker effort or power used to realize the target, etc.). When each respective filter is applied to each respective low frequency loudspeaker, the resulting combined sound field ideally realizes the target(s) and hence creates a homogeneous low frequency sound field inside the spatial listening region(s) of concern. Accordingly, the filters are calculated and applied to the respective low frequency loudspeakers in order to control the resulting combined sound field of the low frequency loudspeakers.

An example method for producing a homogeneous low frequency sound field via a plurality of woofers of an audio system includes setting, with a processor of the audio system, a target low frequency sound field performance for the homogeneous low frequency sound field, defining, with the processor, a listening area for the homogeneous low frequency sound field, and selecting, with the processor, the plurality of woofers to output the homogeneous low frequency sound field, each of the plurality of woofers having a respective position specified relative to an environment including the listening area. The example method may further include calculating filters, with the processor, that can create the homogeneous low frequency sound field, applying one or more filters to a low frequency signal supplied to the plurality of woofers, the one or more filters being tuned based on the predicted or targeted homogeneous low frequency sound field, and outputting, via the plurality of woofers, individual contributions to the sound field that by superposition, create the homogeneous low frequency sound field.

The overall achievable performance and thus homogeneity of the low frequency sound field will depend on the spatial diversity of positions of the plurality of woofers in relation to the listening regions where homogeneity is desired. For example, a reduced spatial diversity of the plurality of woofers will reduce the achievable uniformity. In order to characterize the sound field inside of the listening environment, an initial setup may include estimating or measuring acoustic transfer functions from each individual loudspeaker to different points in space in the listening environment. The estimation and/or measurement of acoustic transfer functions may include performing simulations (e.g., using estimations or measurements of geometries of the listening environment and/or prior data regarding similar environments) and/or using microphones/microphone arrays to measure sound propagation in the environment.

An example audio system for a vehicle includes a plurality of woofers, one or more non-woofer transducers, a low frequency crossover, and a controller configured to receive target parameters for a homogeneous low frequency sound field, define a listening area for the homogeneous low frequency sound field within the vehicle, and calculate the homogeneous low frequency sound field based on the received target parameters and the defined listening area. The controller may be further configured to receive an audio signal from an audio source, direct the audio signal to the low frequency crossover, the low frequency crossover configured to split the audio signal into a first signal corresponding to a first frequency range of the audio signal and a second signal corresponding to a second frequency range of the audio signal, the first frequency range being lower than the second frequency range, and direct the first signal to each of the plurality of woofers and the second signal to at least one transducer of the one or more non-woofer transducers. The controller may be further configured to, for each woofer, apply a respective filter to the first signal to generate a respective filtered signal, the respective filter being tuned based on the calculated homogeneous low frequency sound field, and output, via the plurality of woofers, each respective filtered signal to generate the homogeneous low frequency sound field.

DETAILED DESCRIPTION

Detailed embodiments are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the disclosed features that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis.

As used herein, spatial properties of a resulting sound field reproduced by a sound system may refer to how a signal is measured or perceived across a spatially confined region defining listening positions of concern. A homogenous low frequency sound field may be defined as having low spatial deviation among the listening positions inside a target region, with targets related to spectral and temporal details and targets, as will be described below, realized across the entire spatial target region. Spectral properties of a resulting sound field may refer to the spectral density of the sound signal (e.g., the frequency response) measured or perceived (related to the timbre of the sound system) at a given point in space, as a result of control frequency impulse response filters and global tuning of the sound system (e.g., global referring to tuning filters that are common for all of the low frequency loudspeaker/woofer channels and hence will not affect the sound field control potential if global filters are of reasonable nature). Homogeneous spectral solutions may refer to a spectrally flat response, or to a spectral profile that is in accordance with a target curve (e.g., a non-flat curve). Temporal properties may refer to the time response of the system measured (e.g., impulse response) and may relate to the perception of dryness, rumbling, etc. in the bass sound reproduction. A pre-defined temporal profile according to a target time response or pre-/post-ringing shaping may be introduced to a sound system in accordance with the present disclosure.

FIG. 1illustrates a schematic diagram of a vehicle100according to one or more embodiments. The vehicle includes a listening environment or space within the vehicle101. Inside of the listening environment101, the vehicle includes a plurality of seats. The plurality of seats at least includes a first seat102and a second seat103. Beyond the first seat102and the second seat103, the plurality of seats may include additional seats, such as a third seat104and a fourth seat105.

In the vehicle100, the first seat102may be a front left FL seat. When the vehicle100is a left-hand drive vehicle, the front left FL seat may be a driver seat. The second seat103may be a front right FR seat that is adjacent to the first seat102. When the vehicle100is a left-hand drive vehicle, the front right FR seat may be a front passenger seat. The first seat102and the second seat103may be in a first row of the vehicle100. The third seat104may be a rear left RL seat located behind the first seat102. The fourth seat105may be a rear right RR seat located behind the second seat103and adjacent to the third seat104. The third seat104and the fourth seat105may be in a second row of the vehicle100.

The vehicle may include an audio system106for the listening environment101. The audio system106may include a plurality of woofers. The plurality of woofers may include a first woofer107, a second woofer108, and a third woofer109. Beyond the first, second, and third woofers107,108,109, the plurality of woofers may include additional woofers, such as a fourth woofer110and a fifth woofer111.

The first woofer107and the second woofer108may be located in the first row of the vehicle100. In the first row, the first woofer107may be proximal to first seat102and distal to the second seat103, and the second woofer108may be proximal to the second seat103and distal to the first seat102.

The first woofer107may be located in a front left door of the vehicle100. Alternatively, the first woofer107may be located in another area left of a center line CL of the vehicle100. Locating the first woofer107in another area outside of the front left door may improve an occupant's low frequency listening experience, for this may reduce unwanted structural vibrations, which may otherwise occur when the first woofer107is placed in the front left door. As an example of placement in another area, when the vehicle100includes a center tunnel aligned on the center line CL, the first woofer107may be located in the center tunnel to the left of the center line CL. As another example, the first woofer107may be located in a firewall of the vehicle100, where the placement is to the left of the center line CL. As another example, the first woofer107may be located under the first seat102. The sound field control potential is related to the spatial diversity and/or scattering in the woofer layout. For example, woofers located in close proximity to one another or clustered together may reduce the potential for producing a homogeneous low frequency sound field.

The second woofer108may be located in a front right door of the vehicle100. Alternatively, the second woofer108may be located in another area right of a center line CL of the vehicle100. Like the first woofer107, locating the second woofer108in another area outside of the front right door may improve the occupant's low frequency listening experience, for similar reasons. As an example of placement in another area, when the vehicle100includes the center tunnel aligned on the center line CL, the second woofer108may be located in the center tunnel to the right of the center line CL. As another example, the second woofer108may be located in the firewall of the vehicle100, where the placement is to the right of the center line CL. As another example, the second woofer108may be located under the second seat103.

The center line CL of the vehicle100may serve as a mirror line for the first woofer107and the second woofer108. As such, the first woofer107may mirror the second woofer108via the center line CL. Thus the orientation of the first woofer107may mirror the orientation of the second woofer108via the center line CL.

In the vehicle, the third woofer109may be located in the rear of the vehicle100, such that the third woofer is proximal to the rear of the vehicle100and distal to the front of the vehicle100. For example, when the vehicle100includes a rear trunk, the third woofer109may be located therein. As another example, when the vehicle100includes a rear deck, the third woofer109may be located thereon. As another example, the third woofer109may be located behind the third seat104and the fourth seat105, such as in a rear back support pan for the third and the fourth seats104,105. The third woofer109may be located on the center line CL of the vehicle100.

The fourth woofer110and the fifth woofer111may be located in the second row of the vehicle100. In the second row, the fourth woofer110may be proximal to third seat104and distal to the fourth seat105, and the fifth woofer111may be proximal to the fourth seat105and distal to the third seat104.

The fourth woofer110may be located in a rear left interior wall of the vehicle100, which may connect to a rear left quarter panel of the vehicle100. As another example, the fourth woofer110may be located in a rear left door of the vehicle100. As another example, the fourth woofer110may be located under the third seat104. As another example, the fourth woofer may be located in another area to the left of the center line CL. Like the first woofer107, locating the fourth woofer110in another area outside of the rear left door may improve the occupant's low frequency listening experience, for similar reasons.

The fifth woofer111may be located in a rear right interior wall of the vehicle100, which may connect to a rear right quarter panel of the vehicle100. As another example, the fifth woofer111may be located in a rear right door of the vehicle100. As another example, the fifth woofer111may be located under the fourth seat105. As another example, the fifth woofer may be located in another area to the right of the center line CL. Like the first woofer107, locating the fifth woofer111in another area outside of the rear right door may improve the occupant's low frequency listening experience, for similar reasons.

The center line CL of the vehicle100may also serve as a mirror line for the fourth woofer110and the fifth woofer111. As such, the fourth woofer110may mirror the fifth woofer111via the center line CL. Thus the orientation of the fourth woofer110may mirror the orientation of the fifth woofer111via the center line CL.

The first, second, third, fourth, and fifth woofers107,108,109,110,111, as well as any additional woofers in the plurality of woofers, may be of the same make, model, size, and thus have the same acoustical parameters, such as for frequency response, and beneficially have overlapping characteristics. Each woofer in the plurality of woofers may output into the listening environment101low frequency sound from 20 Hz to 200 Hz (i.e. a low-frequency range portion of the overall sound output by the audio system). In the audio system106, each woofer of the plurality of woofers may be assigned its own filter. For example, in the audio system106, a first finite impulse response FIR filter may be assigned to the first woofer107, a second finite impulse response FIR filter may be assigned to the second woofer108, a third finite impulse response FIR filter may be assigned to the third woofer109, a fourth finite impulse response FIR filter may be assigned to the fourth woofer110, and a fifth finite impulse response FIR filter may be assigned to the fifth woofer111.

Overall, the audio system106may operate over a frequency range of 20 Hz to 20 KHz. To do so, the audio system106may include a plurality of non-woofer transducers, such as mid-frequency transducers and high frequency transducers (e.g., to output low- and mid-frequency range portions of the overall sound output by the audio system, where the overall sound output includes the low- and mid-frequency range portions output by the non-woofer transducers of the audio system and the low-frequency range portions output by the woofers of the audio system). The audio system may include a low frequency crossover. At the low frequency crossover, there may be a seamless transition from the operation of the woofers to the operation of the mid-frequency transducers.

For example, the low frequency crossover may be configured to split an incoming audio signal into multiple frequency ranges (e.g., at least a first frequency range and a second frequency range). A signal corresponding to a first frequency range of the incoming audio signal may be directed to the plurality of woofers in the audio system106, and a signal corresponding to the second frequency range of the incoming audio signal may be directed to at least one of the one or more mid- and high-frequency transducers. The low frequency crossover may be set within the operation range of the plurality of woofers (i.e., at a point between 20 Hz to 200 Hz). Using the above example, the first frequency range may be within the operation range of the plurality of woofers, and the second frequency range may be within an operation range of at least one of the plurality of non-woofer transducers (e.g., the mid-frequency transducers). The low frequency crossover and/or another crossover circuit may be configured to further split the incoming audio signal and to direct a signal corresponding to a third frequency range of the incoming audio signal to another one or more of the plurality of non-woofer transducers (e.g., the high frequency transducers). In this way, one or more crossovers may be used to split incoming audio and direct different frequency ranges of the incoming audio to associated transducers based on the operating ranges of the transducers.

The audio system106may include a user interface. The user interface may be a display. The display may be a touch-screen display. The display may be located in a center-stack of the vehicle100. Additionally or alternatively, the user interface may include an input hardware element, such as an input switch. The user interface may be electrically connected to a controller of the audio system106. The controller may include a digital signal processor DSP with a static or adaptive algorithm solution, a graphic processing unit GPU, a system-on-a-chip SOC, and/or another integrated circuit IC. For example, the controller may include frequency impulse response (FIR) control filters and/or adaptive algorithm controls that alter or re-calculate the FIR control filters according to inputs and/or parameters quantified via transducers in or outside of the listening space (e.g., ambient conditions). The controller may be electrically connected to a tuner of the audio system106, such as an AM tuner or an FM tuner. The controller may be electrically connected to a satellite radio antenna of the audio system106. The controller may be electrically connected to a wireless antenna of the audio system106, such as a Bluetooth antenna, a Wi-Fi antenna, or a Wi-Fi Direct antenna. In the audio system106, the controller may be electrically connected a cellular antenna, a telematics control unit TCU, and/or a GPS antenna. The controller may be electrically connected to a memory device, such as random access memory RAM, read only memory ROM, electrically programmable read only memory EPROM, electrically erasable read only memory EEROM, FLASH, a hard disk drive HDD, and/or a solid state drive SDD, of the audio system106. Software may be stored in the memory device and accessible and executable by the controller. The filters assigned to the woofers may be stored in the memory device. The controller may be electrically connected to one or more input ports, one or more output ports, and/or one or more input/output I/O ports of the audio system106. A portable device, such as a smartphone, may communicate with the audio system via the one or more input/output I/O ports, input ports, and/or wireless antenna. The audio system106may be electrically connected to a power source, such as a DC battery. The controller may be electrically connected to one or more microphones in the audio system106. The audio system106may include an amplifier. The amplifier may be electrically connected to the controller. The audio system106may include analog-to-digital converters ADC, digital-to-analog DAC, additional filters in software, physical hardware filters, additional audio components, and/or additional hardware components.

In the vehicle100, the listening environment101with the plurality of low frequency loudspeakers and appropriate filters may yield a homogeneous low frequency sound field. In the homogeneous low frequency sound field, an occupant may perceive the same low frequency listening experience at each seat in the plurality of seats. As such, if the occupant moves from the first seat102to the fourth seat105, the occupant may perceive the same low frequency listening experience at the fourth seat105as he did in the first seat102. The consistent perception via the homogeneous low frequency sound field may be a desirable user experience, such as for a default low frequency listening experience of the audio system106. That may be desirable for the occupant can choose any seat without concern as to whether one seat has a better low frequency listening experience than another. As such, because of the homogeneity, the low frequency listening experience is essentially optimized at each seat in the plurality of seats in the listening environment101(e.g., a loudness of audio output in the listening environment101may be substantially the same at each seat in the plurality of seats in the listening environment, thus the loudness of the audio output in the listening environment may not vary across the listening environment).

In the vehicle100, the plurality of woofers and the filters assigned thereto may yield the homogeneous low frequency sound field. This may be due to the arrangement of the plurality of woofers in the vehicle100and the values set for the filters assigned to the plurality of woofers. To create the homogenous sound field, at least the first woofer107, the second woofer108, and the third woofer109may be needed. Additionally, the fourth woofer110and the fifth woofer111may be needed. Moreover, the first finite impulse response FIR filter, the second finite impulse response FIR filter, and the third finite impulse response FIR filter may be needed to create the homogeneous sound field. Additionally, in the vehicle100, when the fourth woofer and the fifth woofer are included, the fourth finite impulse response FIR filter and the fifth finite impulse response FIR filter may be needed to create the homogeneous sound field.

In the audio system106, the controller may receive a source signal from an audio source. The audio source may be part of the audio system106, such as an audio file stored in the memory device of the audio system106. Alternatively, the audio source may be external to but in communication with the audio system106, such as a portable device in communication with the audio system via the wireless antenna and having an audio file that may be played-back on the audio system because of the communication therewith. The source signal may be a digital source signal or an analog source signal. When the source signal is the analog source signal, an analog-to-digital converter ADC may convert the analog source signal to a digital source signal. The conversion from analog to digital may occur before reaching the controller or within the controller itself. The latter may occur when the analog-to-digital converter is included in the controller.

The controller may analyze and condition the source signal. As part of the analysis and conditioning, the controller may separate a bass signal from one or more other frequency signals, such as for a stereo left, a stereo right, a center speaker, another speaker in a multichannel system. The bass signal may be a mono signal. The mono signal may be a stereo left signal combined with a stereo right signal. The mono signal may be attributable to a certain frequency range, such as a low frequency range from 20 Hz to 200 Hz.

The controller may send the bass signal (e.g., a low-frequency signal) down the signal path of each woofer in the plurality of woofers. Each signal path includes its own filter. In a signal path, the filter may be the finite impulse response FIR filter assigned to the particular woofer in the plurality of woofers. The finite impulse response FIR filter of the signal path may filter a common bass signal. The filtered bass signal may be sent to a digital-to-analog converter DAC. The filtered bass signal may be converted from digital to analog. The analog filtered bass signal may be amplified in an amplifier. The amplified analog filtered bass signal may drive the particular woofer in the plurality of woofers. In doing so, the particular woofer may output low frequency sound into the listening environment101.

The controller may send one or more signals covering other parts of the audio spectrum to one or more of the transducers in the plurality of non-woofer transducers. Unlike the bass signals, the one or more other frequency signals may be suited for providing a suitable audio experience by reproducing the whole audible audio spectrum. Therefore, unlike the plurality of woofers, the signals sent from the controller to the plurality of non-woofer transducers may not be the same. For example, if the plurality of non-woofer transducers are setup for 5.1-multichannel surround sound, then, beyond the bass signals, the controller may send out a front left signal, a center signal, a front right signal, a surround left signal, and a surround right signal. The front left signal may be sent to one or more signal paths for a first subset of the non-woofer transducers, the center signal may be sent to one or more signal paths for a second subset of the non-woofer transducers, the front right signal may be sent to one or more signal paths for a third subset of the non-woofer transducers, the surround left may be sent to one or more signal paths for a fourth subset of the non-woofer transducers, and the surround right may be sent to one or more signals for a fifth subset of the non-woofer transducers. Each subset of the non-woofer transducers may be mutually exclusive or independent of one another. Each signal path of the non-woofer transducers may include a filter, a time-delay, a digital-to-analog converter DAC, and an amplifier.

As another example, the non-woofer transducers may be in a stereo setup. In the stereo setup, beyond the homogeneous bass signal, the controller may send out a stereo left signal and a stereo right signal. The stereo left signal may be sent to one or more signal paths for a first subset of the non-woofer transducers, and the stereo right signal may be sent to one or more signal paths for a second subset of the non-woofer transducers. Each subset of the non-woofer transducers may again be mutually exclusive or independent of one another. Other setups outside of the 5.1-multichannel surround sound setup and the stereo setup may be possible for the non-woofer transducers. The sound system in the vehicle100(e.g., including woofers107-111) may provide the homogenous low frequency sound field, a mid-frequency sound field, and/or a high-frequency sound field.

FIG. 2illustrates a schematic diagram of an audio system200for outputting sound into a listening environment according to one or more embodiments. The listening environment may be a cabin of a vehicle. The audio system200may include a plurality of woofers201. The plurality of woofers201may include N+1 woofers, where N≥1. Testing has shown that in certain listening environments, three woofers may be desirable in order to create a homogeneous low frequency sound field. Such may occur when there are packaging and cost constraints. In other listening environments, testing has shown that five woofers may be desirable in order to create a homogeneous low frequency sound field. In other listening environments, though, more or less woofers may be desirable in order to create a homogeneous low frequency sound field. In practice, the number of woofers will be determined by the target low frequency sound field performance desired, such as the desired bandwidth of homogeneity and additional factors such as the cabin geometry and the extent of the listening area in which homogeneity is desired.

The audio system200may include a plurality of non-woofer transducers202. While two are pictured, there may be additional non-woofer transducers in the plurality of non-woofer transducers202. The non-woofer transducers may differ in make, model, size, and acoustical parameters from one another. Additionally or alternatively, one or more subsets of the plurality of non-woofer transducers202may be the same make, model, size, and acoustical parameters.

The audio system may include a controller203. The controller203may receive a source signal204from an audio source. The controller204may analyze and condition the source signal204. As part of the analysis and conditioning, the controller may separate bass signals205from a first frequency signal206and a second frequency signal207, such as a mid-to-high stereo left signal and a mid-to-high stereo right signal. Beyond the bass signals205, while two frequency signals206,207are mentioned, there may be additional frequency signals beyond the two frequency signals. Alternatively, instead of two frequency signals206,207, there may only be one frequency signal. As such, the first frequency signal206may be the same or differ from the second frequency signal207.

From the controller204, each signal path of the plurality of woofers201may receive the same bass signal205. Each signal path of the plurality of woofers may include its own filter block208. Each filter block208may include a filter. In each filter block208the filter may be a finite impulse response FIR filter. The values for each of the FIR filters may differ from one another. Alternatively, two or more FIR filters may have the same value settings. The bass signals205may, therefore, be filtered according to the filter of the filter block in its respective signal path. Therefore, when the filters are FIR filters having different values from one another, the filtered bass signals will differ from one another.

Each filter block208may include a digital-to-analog converter DAC and an amplifier. The filtered bass signal of a respective signal path may be converted from digital to analog via the digital-to-analog converter DAC and thereafter be amplified by the amplifier. Each amplified analog filtered bass signal209,210,211may be supplied to its respective woofer in the plurality of woofers201. Each amplified analog filtered bass signal209,210,211may be different from one another. Alternatively, one or more of the amplified analog filtered bass signals209,210,211may be identical. In response, each woofer in the plurality of woofers201may accordingly output low frequency sound into the listening environment.

The distribution and orientation of the plurality of woofers201in or around the listening environment may by passive means and without additional filtering, yield a suitably homogeneous low frequency sound field. At times, the combination of the distribution and orientation of the plurality of woofers201and the filters in the filtering blocks208may achieve the homogeneous low frequency sound field. In such scenarios, the filters in the filter blocks may be tuned to achieve the homogeneous low frequency sound field.

From the controller203, the first audio signal may pass through a first conditioning block212. The first conditioning block may include a filter, a converter, an amplifier, or another signal conditioning element. From the first conditioning block, the first audio signal is conditioned to be passed directly to its respective non-woofer transducer(s) of the plurality of non-woofer transducer202. In response, the respective non-woofer transducer(s) output mid-to-high-frequency sound into the listening environment (e.g., higher frequency sound than that which is output by the woofers). Similarly, from the controller203, the second audio signal may pass through a second conditioning block213. The second conditioning block may include a filter, a converter, an amplifier, or another signal conditioning element. From the second conditioning block, the second audio signal is conditioned to be passed directly to its respective non-woofer transducers of the plurality of non-woofer transducers202. In response, the respective non-woofer transducer(s) output mid-to-high-frequency sound into the listening environment.

FIG. 3illustrates a flowchart diagram that describes a typical workflow when optimizing the low frequency sound field. The steps in the flowchart can be manual or more or less automated as desired and the steps in the workflow can follow a different order. Initially, the desired target low frequency sound field characteristics are specified at301. This specification includes one or more parameters that describe the desired low frequency sound field which could be, but are not limited to, the desired bandwidth of the low frequency sound, such as 20-200 Hz, and/or the standard deviation of the sound pressure level values at discrete points in space within the low frequency sound field across a range of frequencies covering the desired operating bandwidth. The listening area is defined as a spatial region at302, relevant for the listening environment101, such as400shown inFIG. 4. A plurality of woofers201is then chosen at303and may include N+1 woofers, where N≥1. For the plurality of woofers, a position is chosen at304for each woofer in relation to the vehicle100and its listening environment101.

The low frequency control filters can now be calculated at305. For the calculation of the filters a database may be accessed at306, the database including the associated acoustical transfer functions, such as impulse responses or frequency responses of the transfer path, between the plurality of woofers selected at303and the discrete points in space within the defined listening area set at302. The discrete points could be a 3-dimensional grid with a spacing of, but not limited to, 5 cm. This spacing can be coarser depending on the upper limiting frequency of the sound field under analysis. The database of transfer functions may have been gained by measurement within a real listening environment or vehicle with real woofers and microphones, but the transfer functions can also be obtained from a virtual simulation model of the listening environment or vehicle using virtual woofers and virtual microphones. Such a virtual model may be a finite element model.

FIR filters (e.g., filters208ofFIG. 2) may be applied to the low frequency signal (e.g., signal205ofFIG. 2) and fed to the plurality of woofers (e.g., woofers201ofFIG. 2). The control filters are determined/calculated based on optimization approach, seeking to achieve a given target which is defined in a cost function. The assessment (through manual/automated iterative approach according toFIG. 3) is done mathematically by convolving the transfer functions from woofer to microphones with the filter responses and thus the optimization predicts the resulting low frequency sound field. An optimization routine may be based on a Least Mean Square algorithm or similar. Subsequently, the specification criteria, such as, but not limited to, the standard deviation in the sound pressure level, from the resulting low frequency sound field can be derived at frequencies over the desired bandwidth of the low frequency sound field.

Should the criteria for the desired low frequency sound field not be met (e.g., “No” at307), a number of decisions308can be made to modify the start conditions for the optimization. When met, the resulting FIR filters (e.g., filters208ofFIG. 2) calculated can be exported at309. The data310can be stored in a data format suitable for implementation of the filters and also displayed311such as in the data presented inFIGS. 6, 7, and 8.

FIG. 4illustrates a schematic diagram of a vehicle405according to one or more embodiments including the listening area around each passenger head position401,402,403, and404for the seats FL, FR, RL, and RR as described inFIG. 1respectively, and a wider listening area400covering a range of seat positions which may include all seat positions or multiple subsets of groups of seats. The listening areas illustrated represent a range of nominal ear positions for the passengers including their nominal range of heights. The low frequency sound field in the listening areas is driven by the plurality of woofers as described inFIG. 1.

FIG. 5illustrates again a wider listening area500as400inFIG. 4but in the case of an autonomous vehicle501or similar where the seating positions may be even more flexible and the passengers are free from the task of driving the vehicle such that they may face in a different direction than the direction of travel.500may include all seat positions or multiple subsets of groups of seats. The low frequency sound field in the listening areas is driven by the plurality of woofers as described inFIG. 1.

FIG. 6illustrates an example plot600of the sound system responses in each of the areas that represent each passenger head position as shown inFIG. 4when the low frequency sound field in the listening areas is driven by the plurality of woofers as described inFIG. 1and is controlled such that each position achieves a homogeneous sound pressure.

FIG. 7illustrates a plot700of the related standard deviation of the sound pressure level calculated in the whole of the wider listening area400illustrated inFIG. 4for the controlled low frequency sound field inFIG. 6. In this example, the standard deviation is below 2 below 176 Hz meaning that 68% of the responses lie within +/−2 dB of the mean for a normally distributed dataset.

FIG. 8illustrates a surface plot800at 150 Hz for the difference to the mean sound pressure level throughout the wider listening area400illustrated inFIG. 4for the controlled low frequency sound field as also described byFIG. 6andFIG. 7. The grey scale illustrates the deviation in dB from the mean sound pressure level in the wider listening area400inFIG. 4which shows low deviation when the sound field is controlled.

FIG. 9illustrates a schematic diagram of a vehicle906where only a single woofer or subwoofer905is active and includes the listening area around each passenger head position901,902,903,904for the seats FL, FR, RL, RR as described inFIG. 1respectively, and a wider listening area900covering a range of seat positions. In this example, the single woofer or subwoofer905is placed in the trunk corner area as illustrated.

FIG. 10illustrates a plot1000of the resulting sound pressure level responses in each of the areas that represent each passenger head position as described inFIG. 9in a typical vehicle when only the single woofer or subwoofer905is driving the low frequency sound field in the listening areas. In this example, the single woofer or subwoofer905is placed in the trunk corner area as illustrated inFIG. 9and a global tuning of the sound system attempts to achieve a smooth frequency response in the rear seats.

FIG. 11illustrates a plot1100of the resulting standard deviation in the sound pressure level calculated in the whole of the wider listening area900illustrated inFIG. 9in a typical vehicle when only the single woofer or subwoofer905is driving the low frequency sound field in the listening areas. In this example, the single woofer or subwoofer905is placed in the trunk corner area as illustrated inFIG. 9and a global tuning of the sound system attempts to achieve a smooth frequency response in the rear seats.

FIG. 12illustrates a surface plot1200at 150 Hz for the difference to the mean sound pressure level throughout the wider listening area900illustrated inFIG. 9in a typical sport utility vehicle (SUV) when only the single woofer or subwoofer905is driving the low frequency sound field in the listening areas. For example, a listener in the rear seat area may experience substantially higher sound pressure levels than the front seat area, such as at 150 Hz where the rear right seat area904inFIG. 9a listener experiences a significantly higher sound pressure level than the other seats, in particular a sound pressure level 12 dB higher than the front left seat area901inFIG. 9. In this example, the single woofer or subwoofer905is placed in the trunk corner area as illustrated inFIG. 9. The grey scale illustrates the deviation in dB from the mean sound pressure level in the wider listening area900inFIG. 9.

FIG. 13illustrates an example embodiment for a larger vehicle cabin130such as, but not limited to, an SUV type cabin. The listening area131covers the required listening area and may include a third row of seats. The example embodiment includes five equal woofers132,133,134,135, and136where the front woofers132and133are located under the A-pillar area on opposite sides of the vehicle. Two woofers134and135are located on opposite sides of the vehicle such as in the rear door trim panels in the middle of the cabin area. The fifth woofer136is located on the side of the trunk area at a height above the trunk floor surface. The woofers in this embodiment may have a chassis diameter of 170 mm and a radiating area of 154 cm2in one illustrative example. The sound radiating area (front side) of all woofers radiates into the passenger area of the cabin. In this embodiment it is seen that the woofers are spatially distributed within the vehicle cabin. All five woofers132,133,134,135, and136are each mounted in 12 liter sealed enclosures such that the rear sound radiation from the back side of the woofer membrane is isolated from the passenger area of the cabin and furthermore, significantly reduces leakage of low frequency audio to the outside of the vehicle.

FIG. 14illustrates a plot1400of the infinite baffle response of one of the woofers from the embodiment shown inFIG. 13when the woofer has a sealed rear enclosure of 12 liters. The volume of the enclosure can beneficially be filled with suitable acoustic damping material. The infinite baffle response shown inFIG. 14, which is a second order high pass function, can be described by a Q-factor of 0.9 and a resonance frequency of 80 Hz in this embodiment.

FIG. 15illustrates a plot1500of the controlling filter208inFIG. 2magnitude responses for each of the five woofers deployed in the embodiment of the disclosure as shown inFIG. 13. The legend refers to the woofer locations described inFIG. 13where front left woofer FLW is132, front right woofer is133, rear left woofer RLW is134, rear right woofer RRW is135and trunk left woofer TLW is136. Implementation of the controlling filters in the example embodiment will result in a homogeneous low frequency sound field as described inFIGS. 6, 7, and 8.

The description of embodiments has been presented for purposes of illustration and description. Suitable modifications and variations to the embodiments may be performed in light of the above description or may be acquired from practicing the methods. For example, unless otherwise noted, one or more of the described methods may be performed by a suitable device and/or combination of devices, such as the controller203and the speakers201/202ofFIG. 2. The methods may be performed by executing stored instructions with one or more logic devices (e.g., processors) in combination with one or more additional hardware elements, such as storage devices, memory, hardware network interfaces/antennas, switches, actuators, clock circuits, etc. The described methods and associated actions may also be performed in various orders in addition to the order described in this application, in parallel, and/or simultaneously. The described systems are exemplary in nature, and may include additional elements and/or omit elements. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed.

As used in this application, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is stated. Furthermore, references to “one embodiment” or “one example” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. The terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements or a particular positional order on their objects. The following claims particularly point out subject matter from the above disclosure that is regarded as novel and non-obvious.