Method for managing the low frequencies of a loudspeaker and device for implementing said method

Abstract

The invention relates to a method (1) for managing the low frequencies of a loudspeaker installed in a vehicle. According to the invention, the method comprises: - a calibration step (10) of a vehicle sound system including the loudspeaker, to achieve a set of specifications; - a measurement step (11) of acoustic indicators of the loudspeaker including a distortion indicator; - a low-frequency management step (12). The invention also relates to a device for implementing the method according to the invention. Figure for the abstract: Figure 1

Description

Title of the invention: Method for managing the low frequencies of a loudspeaker and device for implementing said method Technical field

[0001] The present invention relates to a method for managing the low frequencies of a loudspeaker.

[0002] More particularly, the present invention relates to the management of low frequencies of loudspeakers on board a vehicle.

[0003] The invention also relates to a device allowing the implementation of said method. Prior art

[0004] There are in the prior art systems for managing low frequencies in audio systems comprising loudspeakers.

[0005] The international application published under number WO 2020 / 256612 A1 discloses the processing of signals broadcast by several loudspeakers to exploit the interference phenomena between the loudspeakers in order to obtain a relatively uniform frequency response of the loudspeaker system, in particular in a transition frequency band centered around a cut-off frequency of low-pass and high-pass filters applied to the signals, and with limited spatial variability in the low frequencies.

[0006] US patent application published under number US 2020 / 0351585 A1 discloses an audio system comprising a subwoofer for reproducing low frequencies, and a loudspeaker for reproducing high frequencies. The signals intended for the subwoofer and the loudspeaker are pre-filtered by means of a low-pass filter and a high-pass filter respectively. The combination of these pre-filters results in a frequency response having a wavy profile at a transition frequency band centered around a cutoff frequency of the filters. The loudspeaker for reproducing high frequencies has an integrated parameterizable filter for flattening the frequency response profile in the transition zone.

[0007] The American patent published under number US 8,842,845 B2 discloses an automatic method for equalizing the sound pressure level in low frequencies, typically for frequencies between 0 and approximately 150 Hz, in particular to standardize the sound pressure level in low frequencies, for example in a vehicle, where standing waves can appear and distort sound perception. For this, an optimal phase shift is applied to one of the signals, determined based on acoustic measurements and minimization of a deviation from a target curve of the sound pressure level. Statement of the invention

[0008] The present invention proposes an alternative solution to systems for managing low frequencies reproduced by loudspeakers.

[0009] The main objectives of the low frequency management method according to the invention are:

[0010] - extend the bandwidth of the loudspeaker by equalizing its frequency response to make it as flat as possible;

[0011] - reduce the distortion phenomenon while increasing the bass level frequencies;

[0012] - protect the loudspeaker against undesirable phenomena such as: distortion, parasitic vibrations (known as “rattle” in Anglo-Saxon terminology), etc.

[0013] The invention relates to a method for managing the low frequencies of a loudspeaker on board a vehicle and belonging to a sound system of said vehicle having an adjustable volume level. According to the invention, the method comprises:

[0014] - a step of calibrating the vehicle's audio system, during which sound system parameters are adjusted to achieve a set of specifications;

[0015] - a step of measuring acoustic indicators of the loudspeaker during which a set of acoustic indicators including a distortion indicator are measured;

[0016] - a low frequency management step, implementing said indicators acoustics, during which the reproduction of low frequencies by the loudspeaker is optimized based on acoustic indicators.

[0017] In one embodiment, the calibration step comprises:

[0018] - a step of determining an acoustic signature of a vehicle interior;

[0019] - a step of comparing said measured acoustic signature with a target acoustic signature;

[0020] - a step of determining filters to reduce a difference between said signature measured acoustics and said target acoustic signature.

[0021] In one embodiment, the calibration step comprises a preliminary pre-adjustment carried out on adjustable parameters of dimensioning components of the sound chain.

[0022] In one embodiment, the measuring step comprises a step of characterizing the loudspeaker by means of a measuring system, a step of determination of a frequency response of the sound chain at each volume level, a step of determining a distortion indicator and a parasitic vibration indicator for each volume level.

[0023] In one embodiment, the loudspeaker characterization step comprises a calibration of the measurement system and a measurement of the loudspeaker modeling parameters.

[0024] In one embodiment, the measurement system comprises a laser and a microphone.

[0025] In one embodiment, the step of determining a frequency response of the sound chain at each volume level comprises the following steps:

[0026] - adjusting the volume step of the vehicle's audio system, initially at its value minimal;

[0027] - generation of an excitation signal allowing frequency exploration;

[0028] - measurement of the frequency response by means of the microphone;

[0029] - measurement of an excursion of a loudspeaker membrane by means of a laser;

[0030] - increment of the volume step;

[0031] said steps being repeated up to and including the maximum volume step.

[0032] In one embodiment, the determination of the distortion and parasitic vibration indicators is carried out by measuring a frequency response using the microphone placed at an estimated position of a driver's head, and comprises the following steps:

[0033] - measurement of background noise;

[0034] - adjustment of the volume step, initially at its minimum value;

[0035] - generation of an excitation signal of the sinusoidal sweep signal type;

[0036] - measurement of the frequency response by means of the microphone;

[0037] - measurement of the excursion of the membrane by means of the laser;

[0038] - increment of the volume step;

[0039] said steps being repeated up to and including the maximum volume step.

[0040] In one embodiment, the measurements carried out during the step of measuring the acoustic indicators of the loudspeaker are carried out in open loop, the excitation and measurement signals being realigned in time before post-processing.

[0041] In one embodiment, the signals are post-processed in particular by means of short-term Fourier transforms.

[0042] In one embodiment, the low frequency management step comprises:

[0043] - a bandwidth extension step to bring the frequency response closer of the speaker of a target frequency response;

[0044] - a speaker protection step.

[0045] In one embodiment, during the bandwidth extension step, an acoustic potential is determined, corresponding, for a given frequency and a given operating point of the sound system, to a maximum amplitude authorized for the signal beyond which the distortion indicator is greater than a specification for said operating point, said acoustic potential being calculated for each of the volume levels, by comparing the distortion indicator measurements made during the measurement step with the specification for the volume considered.

[0046] In one embodiment, during the bandwidth extension step, an equalization is applied to the loudspeaker for each volume level, the gain of which at a given frequency is given by a difference between the acoustic potential and the frequency response measured during the measurement step.

[0047] In one embodiment, the equalization is carried out by means of four filters comprising a high-pass filter and a low-shelf filter, used essentially to correct frequency bands lower than the cutoff frequency of the loudspeaker, and two peak-type filters, used essentially to correct a frequency band centered around the cutoff frequency of the loudspeaker, a set of filter parameters being initialized then optimized according to the target frequency response.

[0048] In one embodiment, the optimization of the filter parameters is carried out iteratively.

[0049] In one embodiment, during the loudspeaker protection step, a dynamic equalizer is implemented comprising at least one band-stop type filter each having a predetermined center frequency and quality factor and a dynamic gain, each of the center frequencies being defined as a function of the acoustic phenomenon that needs to be controlled, and said dynamic gain being triggered as soon as the signal crosses a threshold value.

[0050] In one embodiment, the method further comprises a step of linearizing the loudspeaker.

[0051] The invention also relates to a device for managing the low frequencies of a loudspeaker. According to the invention, the device comprises:

[0052] - means for calibrating the vehicle's sound system;

[0053] - means for measuring acoustic indicators of the loudspeaker;

[0054] - means for extending the bandwidth of the loudspeaker;

[0055] - means for limiting the amplitude of frequencies causing phenomena unwanted acoustics.

[0056] In one embodiment, the device also comprises means for compensating for the non-linearities of the loudspeaker.

[0057] In one embodiment, the means for measuring acoustic indicators of the loudspeaker comprises a microphone and a laser. Brief description of the drawings

[0058] [Fig-1] represents a schematic diagram of the bass management process frequencies according to the invention.

[0059] [Fig.2] represents a schematic diagram of the measurement step of the low frequency management method according to the invention.

[0060] [Fig.3A] illustrates a time shift between excitation signals and those measured during measurements carried out in open loop within the framework of the method according to the invention.

[0061] [Fig.3B] illustrates the signal obtained after synchronization of the excitation and measured signals illustrated in [Fig.3A].

[0062] [Fig.4A] illustrates a typical frequency response of a loudspeaker, and an ideal frequency response.

[0063] [Fig.4B] illustrates a target frequency response, defining a target frequency response corresponding to a maximum amplitude allowing the specifications to be met, in terms of mechanical constraints or distortion generated.

[0064] [Fig.5A] illustrates the frequency response obtained after optimization of the filter parameters during the bandwidth extension step.

[0065] [Fig.5B] illustrates the equalization curve applied to obtain the frequency response of [Fig.5A]. Detailed description

[0066] With reference to [Fig.l], the method 1 for managing the low frequencies of a loudspeaker of a vehicle, according to the invention, comprises the following steps:

[0067] - a calibration step 10 of a sound system of the vehicle;

[0068] - a step 11 of measuring acoustic indicators of the loudspeaker;

[0069] - a step of managing the low frequencies 12 of the signals.

[0070] The calibration step 10 is a step prior to the implementation of the subsequent steps of the method 1.

[0071] During calibration step 10, the sound system is configured to achieve a set of specifications.

[0072] The term "sound system" means all the components involved in the transmission and production of sound in the vehicle, from the generation of a signal and its management in an audio system of the vehicle, to the ears of a user of the vehicle.

[0073] The components of the sound system include in particular a set of loudspeakers and may also include, in a non-exhaustive manner: volume functions, a digital / analog converter, digital signal processing functions (for example equalization, gains, delays), a vehicle interior modeled by an acoustic transfer function, an amplifier.

[0074] In particular, the sound system considered in the context of the invention comprises a certain number of volume levels, for example 30, each corresponding to a degree of attenuation of the signal, generally at the output of the sound system. The volume level can be modified discreetly by an operator, for example in steps of 1.

[0075] Subsequently, only one loudspeaker is considered, it being understood that the measurements carried out for one loudspeaker are applied in a similar manner to the other loudspeakers. The invention is however not limited to a single loudspeaker.

[0076] The specifications are for example defined by the vehicle manufacturer, or by an entity responsible for an audio service. The specifications define a set of audio criteria necessary to achieve sound reproduction quality objectives. The audio criteria defined in the specifications include for example: sound pressure level (SPL) at the driver's ears at a given volume step for a given incoming signal, volume law, target frequency response, etc.

[0077] The sound reproduction quality objective to be achieved is generally considered from the point of view of a driver of the vehicle, but can be considered from another point of view, for example the point of view of a passenger.

[0078] Specifications are generally defined for a limited set of operating points. “Operating point” means a use case of the vehicle audio system, and corresponds to a particular configuration of the audio system. For example, the sound pressure level may be considered only at the maximum volume level.

[0079] To carry out the calibration, a pre-adjustment is carried out on adjustable parameters of the dimensioning components of the sound chain, that is to say the components having the greatest impact on the sound reproduction quality. An amplitude of a transfer function of these components is not negligible in comparison with the other components of the sound chain, typically these functions induce a significant modification of the overall sound level, for example a modification greater than 3 dB. Once this pre-adjustment has been carried out, all the parameters of all the components of the sound chain can be slightly adjusted in order to achieve the criteria defined in the specifications.

[0080] It is not necessary at the calibration stage to carry out fine adjustment of the parameters.

[0081] In one embodiment of the method, the calibration step 10 comprises:

[0082] - a step of determining an acoustic signature of the vehicle interior, which is fixed in particular by the location of the speakers and the geometry of the passenger compartment;

[0083] - a step of comparing said measured acoustic signature with a target acoustic signature;

[0084] - determining filters to reduce a gap between said acoustic signature measured and said target acoustic signature.

[0085] Once the calibration has been carried out, measurement step 11 is implemented.

[0086] With reference to [Fig.2], the measurement step 11 includes:

[0087] - a step 110 of characterizing the loudspeaker;

[0088] - a step 111 of determining a frequency response of the sound chain to each volume level;

[0089] - a step 112 of determining a distortion indicator and a distortion indicator parasitic vibrations for each volume level.

[0090] During step 110 of characterizing the loudspeaker, measuring elements of a measuring system are first calibrated.

[0091] In the embodiment considered, the measuring elements comprise a laser and at least one microphone.

[0092] Modeling parameters are then measured, in order to be able to model the behavior of the loudspeaker.

[0093] In one embodiment, the modeling parameters include the Thiele and Small parameters, which are measured using a small amplitude excitation signal, for which the loudspeaker response is linear, and using the measuring elements, here the laser and the at least one microphone, but other measuring elements can be envisaged.

[0094] The Thiele and Small parameters include, in particular, but not limited to: electrical resistance, force factor, resonance frequency, mechanical compliance.

[0095] It is thus possible to model the linear behavior of the loudspeaker, according to a model known to those skilled in the art. The linear model is used to predict the behavior of the loudspeaker for low amplitude excitation signals.

[0096] In one embodiment, measurements are also carried out using a higher amplitude excitation signal in order to determine a change in loudspeaker parameters from which non-linearities of the loudspeaker may arise. In particular, a change is determined, as a function of an excursion x of the loudspeaker membrane, of the force factor, of the inductance of the coil, and of the mechanical compliance.

[0097] It is thus possible to model the non-linear behavior of the loudspeaker, according to a model known to those skilled in the art. The non-linear model is used to predict the behavior of the loudspeaker for high amplitude excitation signals.

[0098] Knowledge of the evolution of the loudspeaker parameters at the origin of the non-linearities makes it possible to determine a maximum excursion xmax of the loudspeaker membrane beyond which the behavior of the loudspeaker is non-linear.

[0099] Step 110 of characterizing the loudspeaker is preferably carried out outside the vehicle, in an acoustic laboratory.

[0100] During the subsequent step 111 of determining the frequency response of the vehicle's audio system, the frequency response of the vehicle's audio system, including the loudspeaker, is measured in situ for each volume step of the vehicle's audio system. By "in situ" is meant that, unlike step 110 of characterizing the loudspeaker carried out in the laboratory, step 111 of determining the frequency response of the vehicle's audio system is carried out in the passenger compartment of the vehicle.

[0101] The following diagram is followed:

[0102] - setting the volume step of the vehicle's audio system, initially at its value minimal;

[0103] - generation of the excitation signal, of the sinusoidal sweep signal type (“sine sweep » in Anglo-Saxon terminology);

[0104] - measurement of the frequency response by means of at least one microphone at the level from a listening point specified by the sound reproduction quality objectives;

[0105] - measurement of the x-excursion, or displacement, of the membrane by means of the laser;

[0106] - increment of the volume step.

[0107] This scheme is thus implemented up to and including the maximum volume step.

[0108] It is thus obtained, at the end of this step, for each volume step:

[0109] - the excursion x of the membrane as a function of the amplitude of the excitation signal;

[0110] - the frequency response of the loudspeaker.

[0111] In a preferred embodiment, the excitation signal is a swept signal acting on a frequency band of interest, for example from 20Hz to 200Hz or from 40Hz to 16,000 Hz.

[0112] The amplitude of the excitation signal is also known and depends on the volume of the audio system. The amplitude of the excitation signal is for example equal to -10 dBFS, the unit dBFS designating the full scale decibel. The frequency sweep rate of the excitation signal depends on the capacity of the measurement system to provide accurate measurements of the amplitude with the required frequency precision, and must be chosen so as to effectively reveal the acoustic phenomena studied. (parasitic vibrations for example). A scanning speed of 3 seconds / octave can for example be considered if the measurement system allows it.

[0113] During the subsequent step 112 of determining a distortion indicator and a parasitic vibration indicator, said indicators are determined for each volume level.

[0114] The distortion indicator used here is the total harmonic distortion rate THD (Total Harmony Distortion in English terminology), but other indicators can be used, for example the intermodulation distortion rate IMD (Inter-Modulation Distortion in English terminology).

[0115] The measurement of the total harmonic distortion rate THD is carried out by placing at least one microphone at an estimated position of the driver's head, using for example an excitation signal of the sinusoidal sweep signal type ("sine sweep" in English terminology) covering a frequency range from 40 Hz to 16,000 Hz at -10 dBFS.

[0116] The following diagram is followed:

[0117] - measurement of background noise for correction of future measurements;

[0118] - adjustment of the volume step, initially at its minimum value;

[0119] - generation of the excitation signal;

[0120] - measurement of the frequency response by means of the microphone, from which will be deduces the distortion indicator;

[0121] - measurement of the displacement response of the membrane by means of the laser;

[0122] - increment of the volume step.

[0123] This scheme is thus implemented up to and including the maximum volume step.

[0124] The specifications are given for example for four volume levels of the vehicle audio system, determined for example in the following manner:

[0125] - the volume level “V0” must produce a sound level of 65dB(A) at the ears of the driver when playing pink noise at -18dBFS;

[0126] - volume level “VI” must produce a sound level of 75dB(A) at the ears of the driver when playing pink noise at -18dBFS;

[0127] - the volume level “V2” must produce a sound level of 85dB(A) at the ears of the driver when playing pink noise at -18dBFS;

[0128] - the volume level “V3” corresponds to the maximum volume level of the system audio.

[0129] Generally speaking, the higher the level of sound reproduction quality targeted, the lower the distortion objective targeted.

[0130] Also, in general, a higher distortion rate can be accepted for high volume levels.

[0131] The results of the THD distortion indicator measurements are gathered in a three-dimensional distortion matrix, the dimensions of which are respectively the measurement frequencies, the digital amplitudes of the excitation signal corrected for the volume law, i.e. the attenuation at the volume step considered, and the THD distortion indicators measured by means of the frequency response.

[0132] Rattle is an audible and unwanted, broadband noise radiated by the vibrating structures connected to the loudspeaker which may vibrate when operating at high levels.

[0133] The indicator of parasitic vibrations retained here corresponds to an Erattle energy located in all frequencies beyond the N-th harmonic of the excitation frequency of the sound chain. Other indicators can be used, such as the residual distortion rate THD+N (“Total Harmony Distortion plus Noise” in English terminology).

[0134] The specifications related to parasitic vibrations are similar to those for distortion: the higher the desired level of sound reproduction quality, the lower the measured Erattle energy must be.

[0135] Similar to distortion, the following pattern is followed:

[0136] - measurement of background noise for correction of future measurements;

[0137] - adjustment of the volume step, initially at its minimum value;

[0138] - generation of the excitation signal, for example of the sinusoidal sweep signal type (“sine sweep” in Anglo-Saxon terminology);

[0139] - measurement of the frequency response by means of the microphone, from which will be deducted the parasitic vibration indicator;

[0140] - measurement of the displacement response of the membrane by means of the laser;

[0141] - increment of the volume step.

[0142] This scheme is thus implemented up to and including the maximum volume step.

[0143] The measurements carried out during measurement step 11 are generally carried out in open loop, that is to say that the excitation signal Se and the measured response S of the loudspeaker or the sound system are not synchronized.

[0144] [Fig.3A] illustrates the time shift between the excitation signal Se and the measured signal S.

[0145] In order to be able to post-process the measurement accurately, it is essential to be able to identify on the measured signal S the start of the response of the sound chain or the loudspeaker to the excitation signal.

[0146] The excitation signals Se and measurement signals S are therefore realigned with each other to obtain a resynchronized signal Sa.

[0147] For this, the excitation signals used are preceded by a countdown corresponding to a succession of beeps, in order to be able to detect the start of the signal, which is also useful for warning operators of the start of the measurements.

[0148] A time difference, or response time, between a beep and the response of the sound system, or the loudspeaker, to said beep is determined by means of a cross-correlation between the excitation signal Se and the measured signal S.

[0149] The excitation signals Se and measured S can thus be realigned, as illustrated in [Fig.3B], either by truncation of the measured signal, or by a method called “zero padding” applied to the excitation signal.

[0150] Once the signals are realigned, the portion of the signals corresponding to the "beeps," or the countdown, is deleted.

[0151] If carrying out the measurements in open loop simplifies the carrying out of the measurements, it should be noted however that, within the framework of the invention, the measurements carried out during the measurement step 11 can be carried out in closed loop, if the audio system of the vehicle allows it (presence for example of a jack socket to inject an audio signal in real time).

[0152] The frequency response of the sound chain, the distortion indicator and the parasitic vibration indicator are determined during determination step 11 by means of a time-frequency analysis.

[0153] The structure of the excitation signal being known, it is possible to extract from the measured signal the amplitude of the fundamental of the frequency response, to obtain the transfer function, harmonics to obtain the distortion indicator, and it is possible to carry out a broadband analysis of the noise to determine the indicator of parasitic vibrations.

[0154] To ensure accuracy of the measured amplitudes for the THD distortion indicator and the frequency response, a short-term Fourier transform (“STFT” or Short Time Fourier Transform in English terminology) is advantageously used. Alternatively, a wavelet or periodigram transform can be used.

[0155] In one embodiment, the analysis is performed in octave bands.

[0156] For measurements made at low SPL sound pressure levels, or in noisy environments, background noise may introduce artifacts into the frequency response, distortion indicator, and parasitic vibration indicator measurements.

[0157] Advantageously, a measurement of the background noise is carried out before each measurement, as described previously, thus making it possible to eliminate artifacts in the amplitude measurements, by correcting the measurement, or making it possible to determine a confidence level of the measurement.

[0158] It should be noted that another type of excitation signal than a sinusoidal sweep signal can be used. For example, a noise signal, a pulse signal, or more generally any signal allowing frequency exploration on the desired frequency band to carry out the measurement can be used instead of the sinusoidal sweep signal.

[0159] Once the measurement step 11 is completed, the low frequency management step 12 is implemented.

[0160] The low frequency management step 12 comprises:

[0161] - a bandwidth extension step 120;

[0162] - a step of protecting the loudspeaker 121.

[0163] During the bandwidth extension step 120, the frequency response of the sound system is amplified below a cutoff frequency fc of the loudspeaker, so as to extend the bandwidth of said sound system, and thus maintain a frequency response as flat as possible in the low frequencies. The frequency amplification is constrained in order to limit undesirable phenomena such as distortion or vibrations linked to the mechanical connection with the vehicle (“rattle”).

[0164] [Fig.4A] illustrates the typical frequency response of a loudspeaker, represented by a solid line, and an ideal frequency response, represented by dashes.

[0165] [Fig.4B] illustrates a target frequency response, represented by dotted lines, taking into account the electrical and mechanical constraints of the loudspeaker, in order to limit the phenomena of vibrations, distortion, and in order to protect the loudspeaker. The target frequency response defines the maximum amplitude allowing the specifications to be met, in terms of mechanical constraints or distortion generated, at the volume level considered.

[0166] For this, an acoustic potential is determined for each operating point. The acoustic potential corresponds, for a given frequency and at the given operating point, to a maximum amplitude authorized for the signal beyond which the distortion indicator is greater than the specification. The acoustic potential is therefore deduced from the measurements carried out during step 112 of determining a distortion indicator.

[0167] As seen above, specifications are generally defined only for a finite number of volume levels. The acoustic objectives for the other volume levels are interpolated from the known specifications, following the evolution of the volume law.

[0168] By default, the volume level below (respectively above) the minimum (respectively maximum) volume level given by the specifications is associated to the same specifications as the minimum (respectively maximum) volume level. Another convention can of course be used.

[0169] Once the specifications have been defined for each volume level, the acoustic potential is calculated for each of said volume levels, by comparing the distortion indicator measurements taken during measurement step 11 with the specification for the volume considered.

[0170] Knowledge of the acoustic potential and the amplitude of the signal at the operating point considered, which corresponds to the digital amplitude of the excitation signal corrected by the volume law, i.e. the attenuation at the volume step considered, makes it possible to deduce a gain applicable during equalization of the signal to be broadcast by the loudspeaker, by taking the difference between the acoustic potential at the volume level considered and the amplitude of the excitation signal corrected by the volume law. It should be noted that the gain can be negative if loudspeaker protection is necessary.

[0171] Indeed, the measurement of the excursion of the membrane, carried out in parallel with the measurement of the distortion indicator during step 112 of determining a distortion indicator, is also compared to the maximum excursion xmax of the loudspeaker determined during step 110 of characterizing the loudspeaker, in order to ensure that the excursion of the loudspeaker does not exceed the maximum excursion.

[0172] It should be noted that the presence of background noise during measurements is likely, especially at low volume and / or if the background noise is significant, to introduce artifacts into the frequency response of the sound chain. Consequently, a fundamental frequency and / or its harmonics are likely to be more or less masked by the background noise, thus distorting the measurement result of the distortion indicator, and thus the acoustic potential.

[0173] Background noise is therefore preferentially taken into account in the measurements, in order to correct the measured signals.

[0174] The maximum applicable gain being known, a set of filters is then implemented.

[0175] For each volume level:

[0176] - a set of filter parameters, including for example a central frequency, a cutoff frequency, a quality factor Q, a gain G, and a filter type, is initialized;

[0177] - the parameter set is then optimized, depending on the frequency response uncorrected and the target frequency response.

[0178] In the example described here, a band equalizer with four bands is used. This equalizer uses a high-pass filter, a low-shelf filter, and two peak filters. Of course, a different number and / or different types filters can be considered. Furthermore, such filters can be implemented in a known manner using an IIR (for "Infinite Impulse Response" in English) or FIR (for "Finite Impulse Response" in English) digital filter architecture.

[0179] The initialization depends on the type of filter considered, as well as on the uncorrected frequency response and the target frequency response.

[0180] High-pass and low-shelf filters are used primarily to correct frequency bands below the loudspeaker cutoff frequency, and peak filters are used primarily for a frequency band around the loudspeaker cutoff frequency.

[0181] The filter gain values, given for the cutoff frequency or the center frequency, depending on the filter type, depend on the applicable gain determined by means of the acoustic potential. The gains are for example initialized to the applicable gain values ​​determined previously.

[0182] The quality factor is initialized by default to 0.707 for high-pass and low-shelf filters, and to 3 for peak-type filters.

[0183] Of course, other values ​​can be considered for initialization.

[0184] The optimization of the parameters is then carried out iteratively.

[0185] It should be noted that before any equalization correction, the distortion indicator is overestimated due to the natural shape of the loudspeaker's frequency response. As a result, the applicable gain is underestimated.

[0186] After applying the initialized filters, the frequency response of the loudspeaker tends to flatten, resulting in a decrease in the value of the THD distortion indicator. This decrease implies that an additional gain margin is applicable, while remaining within the specifications.

[0187] It is thus possible to iteratively adjust the filter parameters, and in particular the gain, in order to exploit the additional applicable gain margin.

[0188] On the other hand, beyond a certain number of iterations, the distortion indicator THD increases rapidly, due to the non-linearities of the loudspeaker.

[0189] Figures 5A and 5B illustrate an example, respectively of the frequency response after the optimization process, and of the applied equalization curve.

[0190] The filters used in the example of [Fig.5A] are as follows:

[0191] - 2nd order high-pass filter, having a cut-off frequency of 42 Hz, and a quality factor of 0.7;

[0192] - low-shelf filter having a gain of 9dB at 29 Hz and a quality factor of 0.7;

[0193] - a first “peak” type filter having a central frequency of 11Hz, a attenuation of 1.6 dB at this frequency, and a quality factor equal to 3;

[0194] - a second “peak” type filter having a central frequency of 63Hz, a zero attenuation at this frequency, and a quality factor equal to 3.763;

[0195] The filters used in the example of [Fig.5B] are as follows:

[0196] - high-pass filter having a cut-off frequency of 23.27 Hz, and a factor of quality of 0.7;

[0197] - low-shelf filter with a gain of 7.038 dB at 25.74 Hz and a quality factor of 0.7;

[0198] - a first “peak” type filter having a central frequency of 83.82Hz, an attenuation of 3.16 dB at this frequency, and a quality factor equal to 2;

[0199] - a second “peak” type filter having a central frequency of 63.25Hz, a gain of 7.335 dB at this frequency, and a quality factor equal to 3.1;

[0200] In one embodiment, the filter parameters obtained after optimizing the filter parameters for a given volume level are reused for a lower and / or higher volume level.

[0201] The loudspeaker protection step 121 is conducted downstream of the band extension step 120, so that the gain provided in the low frequencies during the bandwidth extension is eliminated if it causes vibrations.

[0202] The purpose of this loudspeaker protection step is to limit certain excitation frequencies which cause distortion or parasitic vibration phenomena, for example.

[0203] It is known to apply static filters to control these undesirable phenomena. The disadvantage of such filters is that they alter the signal even if the amplitude levels are low and not problematic from the point of view of undesirable acoustic phenomena.

[0204] During this step, a dynamic equalizer is implemented and automatically adjusted thanks to the acoustic characterization made of these undesirable effects.

[0205] The advantage of the dynamic equalizer is that it is only active when needed, and does not alter the signal when it is not needed.

[0206] In one embodiment, the dynamic equalizer comprises three quadratic band-stop filters each having a constant center frequency and quality factor, and a dynamic gain. Those skilled in the art will understand that a different number of filters may be implemented; the types of filters used may also vary.

[0207] Each central frequency is defined according to the phenomenon which needs to be controlled, and the central frequencies therefore depend in particular on the characteristics of the loudspeaker and its environment (door panel for example).

[0208] The dynamic gain is triggered as soon as the signal crosses a threshold value, which corresponds for example to the acoustic potential calculated for the THD distortion indicator or for the parasitic vibration indicator, i.e. the maximum amplitude, at a given frequency, beyond which the specification relating to the acoustic phenomenon concerned is no longer respected.

[0209] In one embodiment, the method 1 also comprises a step of linearizing the loudspeaker 122, carried out downstream of the band extension step 120 and upstream of the loudspeaker protection step 121.

[0210] The purpose of the loudspeaker linearization step is to compensate for signal distortions introduced by the loudspeaker behavior for high amplitude signals. For this, a non-linear real-time model of the loudspeaker can for example be used.

[0211] The low frequency management method according to the invention allows:

[0212] - to extend the intrinsic bandwidth of the loudspeaker by equalizing its response in frequency in the low frequencies so that it is as flat as possible, exploiting the entire linear domain of the loudspeaker and limiting the introduction of distortion;

[0213] - to reduce distortion while increasing the level of low frequencies in compensating for loudspeaker nonlinearities with a pre-corrected signal, based on a nonlinear model of the loudspeaker;

[0214] - to provide protection against undesirable phenomena such as: distortion, parasitic vibrations, by applying a dynamic filter limiting the amplitude of certain frequencies, the adjustment of this protection can be carried out by ear or on the knowledge of acoustic indicators relating to these phenomena.

[0215] The invention also relates to a device for managing the low frequencies of a loudspeaker.

[0216] The device according to the invention comprises:

[0217] - means for calibrating the vehicle's sound system;

[0218] - means for measuring acoustic indicators of the loudspeaker;

[0219] - means for extending the bandwidth of the loudspeaker;

[0220] - means for limiting the amplitude of frequencies causing phenomena unwanted acoustics.

[0221] In one embodiment, the device also comprises means for compensating for the non-linearities of the loudspeaker.

[0222] In one embodiment, the means for measuring acoustic indicators of the loudspeaker comprises a microphone and a laser.

Claims

Claims

1. Method (1) for managing the low frequencies of a loudspeaker on board a vehicle and belonging to a sound system of said vehicle having an adjustable volume level, said method being characterized in that it comprises: - a step of calibrating (10) the sound system of the vehicle, during which parameters of the sound system are adjusted to achieve a set of specifications; - a step of measuring (11) acoustic indicators of the loudspeaker during which a set of acoustic indicators including a distortion indicator are measured, the measuring step (11) comprising a step (110) of characterizing the loudspeaker by means of a measuring system, a step (111) of determining a frequency response of the sound system at each volume level, a step (112) of determining a distortion indicator and a parasitic vibration indicator for each volume level;- a low frequency management step (12), implementing said acoustic indicators, during which a restitution of the low frequencies by the loudspeaker is optimized.;

2. Method (1) according to claim 1 characterized in that the calibration step (10) comprises: - a step of determining an acoustic signature of a passenger compartment of the vehicle; - a step of comparing said measured acoustic signature with a target acoustic signature; - a step of determining filters to reduce a difference between said measured acoustic signature and said target acoustic signature.

3. Method (1) according to claim 1 or claim 2 characterized in that the calibration step (10) comprises a prior pre-adjustment carried out on adjustable parameters of dimensioning components of the sound chain.

4. Method (1) according to claim 1 characterized in that the step (110) of characterizing the loudspeaker comprises a calibration of the measurement system and a measurement of the modeling parameters of the loudspeaker.

5. Method (1) according to claim 1 or claim 4 characterized in that the measuring system comprises a laser and a microphone.

6. Method (1) according to claim 5 characterized in that the step (111) of determining a frequency response of the audio system at each volume level comprises the following steps: - adjusting the volume step of the vehicle audio system, initially at its minimum value; - generating an excitation signal allowing frequency exploration; - measuring the frequency response by means of the microphone; - measuring an excursion of a membrane of the loudspeaker by means of the laser; - incrementing the volume step; said steps being repeated up to and including the maximum volume step.

7. Method (1) according to claim 5 or claim 6 characterized in that the determination of the indicators of distortion and parasitic vibrations is carried out by measuring a frequency response by means of the microphone placed at an estimated position of the head of a driver, and comprises the following steps: - measurement of a background noise; - adjustment of the volume step, initially at its minimum value; - generation of an excitation signal of the sinusoidal sweep signal type; - measurement of the frequency response by means of the microphone; - measurement of the excursion of the membrane by means of the laser; - incrementation of the volume step; said steps being repeated up to and including the maximum volume step.

8. Method (1) according to any one of the preceding claims, characterized in that the measurements carried out during the measurement step (11) of the acoustic indicators of the loudspeaker are carried out in open loop, the excitation (Se) and measurement (S) signals being realigned in time before post-processing.

9. Method (1) according to any one of the preceding claims, characterized in that the signals are post-processed in particular by means of short-term Fourier transforms.

10. Method (1) according to any one of the preceding claims, characterized in that the step of managing the low frequencies (12) comprises: - a bandwidth extension step (120) for bringing the frequency response of the loudspeaker closer to a target frequency response; - a loudspeaker protection step (121).

11. Method (1) according to claim 10 characterized in that, during the bandwidth extension step (120), an acoustic potential is determined, corresponding, for a given frequency and a given operating point of the sound chain, to a maximum amplitude authorized for the signal beyond which the distortion indicator is greater than a specification for said operating point, said acoustic potential being calculated for each of the volume levels, by comparing the distortion indicator measurements carried out during the measurement step (11) with the specification for the volume considered.

12. Method (1) according to claim 11 characterized in that, during the bandwidth extension step (120), an equalization is applied to the loudspeaker for each volume level, the gain of which at a given frequency is given by a difference between the acoustic potential and the frequency response measured during the measurement step (11).

13. Method (1) according to claim 12 characterized in that the equalization is carried out by means of four filters comprising a high-pass filter and a "low-shelf" filter, used essentially to correct frequency bands lower than the cut-off frequency of the loudspeaker, and two "peak" type filters, used essentially to correct a frequency band centered around the cut-off frequency of the loudspeaker, a set of filter parameters being initialized then optimized according to the target frequency response.

14. Method (1) according to claim 13 characterized in that the optimization of the filter parameters is carried out iteratively.

15. Method (1) according to any one of claims 10 to 14 characterized in that, during the step of protecting the loudspeaker (121), a dynamic equalizer is implemented comprising at least one band-stop type filter each having a predetermined central frequency and quality factor and a dynamic gain, each of the central frequencies being defined in

16. function of the acoustic phenomenon which must be controlled, and said dynamic gain being triggered as soon as the signal crosses a threshold value. Method according to any one of claims 10 to 15, characterized in that it further comprises a step of linearizing the loudspeaker (122).

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