EP3000109B1 - Acoustic device capable of producing active noise reduction - Google Patents

Description

The present invention relates to an acoustic device able to achieve active noise reduction, positionable on the head of a user.

The invention lies in the field of active noise reduction.

There are various acoustic devices for reducing ambient noise, in particular helmets comprising an active noise reduction system. In general, such a headset has two headphones, conventionally positioned on the ears of a user. Each earphone is equipped with a microphone capable of picking up an audio signal representative of the ambient noise, called a noise signal. The active noise reduction is then achieved by emission by the headphones, at the input of the auditory canal of the user, of an airborne sound signal which is calculated to compensate for the noise signal picked up, also called “counter- noise ”.

The document WO 2010/052720 A1 describes a method for providing an audio signal to two ears of a user using a single earphone. The document WO 2007/107985 A1 describes a method and system for propagating sound by bone conduction.

The object of the invention is to propose an acoustic device for reducing ambient noise allowing better noise attenuation without bulk for the user.

To this end, the invention provides an acoustic device according to claim 1.

The acoustic device according to the invention can also have one or more of the characteristics of claims 2 to 4, taken independently or in combination.

• la figure 1 est une vue d'ensemble d'un dispositif acoustique selon un mode de réalisation de l'invention ;
• la figure 2 représente schématiquement une carte électronique du dispositif acoustique selon l'invention ;
• la figure 3 est un schéma de principe de réduction active de bruit selon un premier mode de réalisation ;
• la figure 4 représente schématiquement un protocole de mesure de fonction de transfert de conduction osseuse directe ;
• la figure 5 est un schéma de principe de réduction active de bruit selon un deuxième mode de réalisation ;
• la figure 6 représente schématiquement un protocole de mesure de fonction de transfert de conduction osseuse transverse, et ;
• la figure 7 est un ordinogramme des principales étapes d'un procédé de détermination de fonctions de transfert de réduction active de bruit par voie ostéophonique.

Other characteristics and advantages of the invention will emerge from the description which is given below, by way of indication and in no way limiting, with reference to the appended figures, among which:

• the figure 1 is an overall view of an acoustic device according to one embodiment of the invention;
• the figure 2 schematically represents an electronic card of the acoustic device according to the invention;
• the figure 3 is a block diagram of active noise reduction according to a first embodiment;
• the figure 4 schematically represents a protocol for measuring a direct bone conduction transfer function;
• the figure 5 is a block diagram of active noise reduction according to a second embodiment;
• the figure 6 schematically represents a protocol for measuring a transverse bone conduction transfer function, and;
• the figure 7 is a flowchart of the main steps of a method for determining transfer functions for active reduction of noise by osteophonic means.

The acoustic device 2 of the figure 1 comprises two lateral acoustic modules 4 for active noise reduction by osteophonic pathway which are similar. The acoustic device 2 is adapted to be positioned on the head of a user (not shown), the lateral acoustic modules 4 being positioned in contact with the skull of the use, preferably at the level of his temples.

An acoustic module 4 comprises an osteophonic transducer 8 and a housing 10, comprising a microphone capable of picking up a sound signal representative of an ambient sound, typically an ambient noise.

The osteophonic transducer 8 comprises an emitting element, not shown, capable of transforming a sound signal into a vibratory signal, transmitted to the auditory nerve of the user by bone conduction. Thus, a sound signal transformed by bone conduction into an acoustic signal perceptible by the user at the level of his inner ear. The emitting element is protected by a protective shell 12, which is preferably composed of two nested half-shells. The half-shells are, for example, made of plastic and injection molded.

The housing 10 also comprises an electronic card, not shown, which will be described in more detail with reference to figure 2 , and which is connected to the ambient sound capture microphone and which comprises an electronic filtering circuit making it possible to generate, from the picked up sound signal, an electrical signal called "counter-noise", transformed into a vibratory signal by the osteophonic transducer 8 and capable of reducing or completely canceling the perception of the ambient sound signal at the level of the auditory nerve of the user. Thus, in one embodiment, an ambient sound wave is canceled by bone conduction.

The acoustic device 2 also comprises a mechanical holding member 14, which is in this example a rigid band capable of supporting the acoustic modules 4 in the appropriate position, resting against the temples of the user. Preferably, the rigid band 14 is of adjustable length.

In addition, optionally, the acoustic device 2 also comprises a complementary holding member 16, which is in this exemplary embodiment a flexible band 16, preferably of adjustable length, which can be positioned on top of the user's head in order to ensure reliable maintenance.

An articulation 20, between a fastener 22 of an acoustic module 4 to the retaining members 14, 16 is also provided. The articulation 20 is suitable for allowing adequate positioning of the acoustic modules on the head of a user. In one embodiment, the articulation 20 is equipped with a spring, not shown, suitable for ensuring a return of the acoustic module 4 to a rest position.

According to a simplified variant not shown, the acoustic device according to the invention is equipped with a single lateral acoustic module for active noise reduction, positioned on only one side of the user's skull.

It is understood that the acoustic modules 4 for osteophonic noise reduction are shown and described in detail.

However, any acoustic device or helmet, comprising such acoustic modules, forms part of the invention.

According to one variant, one or two acoustic modules 4 for reducing noise by osteophonics are integrated into a conventional noise reduction device, of the noise-canceling headset type, in order to combine the reduction of noise by air by generating a airborne acoustic signal for counter-noise and osteophonic noise reduction.

An electronic card 30 according to the invention is shown on figure 2 . An ambient noise signal Sb is picked up by the microphone. A filtering module 32 is connected to the microphone, this module implementing a transfer function H _FO making it possible to determine the electrical signal, equivalent to the gain and to the phase, to the osteophonic signal to be transmitted by bone conduction to cancel the signal from noise Sb.

According to a first embodiment, the transfer function H _FO is determined for an acoustic device 2 provided with a single acoustic module 4 for osteophonic noise reduction. The transfer function H _FO thus determined also applies in the case of an acoustic device 2 provided with acoustic modules 4 right and left, but assuming equivalent ambient noise conditions at the level of the right and left ears of the user. , and considering direct bone conduction only.

In order to explain how to determine the transfer function H _FO in this first embodiment, the figure 3 schematically illustrates the principle of osteophonic noise reduction in this embodiment.

In this example, the case of an acoustic module 4 placed on the left lateral side of the user is considered. It is understood that the principle described below applies symmetrically when an acoustic module 4 is placed on the right lateral side of the user's skull.

Only the transducer 8 forming part of the acoustic module 4 is shown. Point I _G represents the user's left inner ear, and point E _{G represents} the entry point of the left ear canal or outer ear.

Noise sound waves 38 are transmitted by air, and picked up by the microphone 40. According to a modeling principle, a transfer function Hco defines the conduction through the bone between the emission of the vibratory signal by the transducer 8 and the inner ear I _G. The overall transfer function between the input of the microphone 40 and the emission of the osteophonic vibratory signal is denoted H _G. Similarly, a transfer function H _CA represents the conduction of an airborne sound signal between the outer ear E _G and the inner ear I _G , it is the transfer function of the inner and middle ear. The transfer function H _b ^G is the transfer function characteristic of the noisy environment, also used in the context of conventional noise reduction.

In order to cancel the noise at the user's inner ear I _G , the following relationship must be verified: $$H_G\cdot H_{\mathit{CO}}=-H_{\mathit{IT}}\cdot H_b^G$$

It follows that the overall transfer function H _G depends on the ratio R between the transfer function H _CA of the outer and middle ear and on the transfer function Hco of bone conduction: $$H_G=-\frac{H_{\mathit{IT}}}{H_{\mathit{CO}}}\cdot H_b^G$$

Moreover, if we denote by H _m the transfer function of the microphone 40 and H _TO the transfer function of the transducer 8, the following relation is also verified: $$H_G=H_m\cdot H_{\mathit{FO}}\cdot H_{\mathit{TO}}$$

By combining the relations (Eq 2) and (Eq 3) above, it follows: $$H_{\mathit{FO}}=-\frac{H_{\mathit{IT}}}{H_{\mathit{CO}}}\cdot \frac{H_b^G}{H_m\cdot H_{\mathit{TO}}}=-R\cdot \frac{H_b^G}{H_m\cdot H_{\mathit{TO}}}$$

In order to determine the transfer function H _FO to be applied, it is therefore useful, in a preliminary phase, to determine the ratio R = H _CA / H _CO , which amounts, in other words, to determining the airborne sound signal equivalent, except for the gain and phase, to the osteophonic vibratory signal.

Such a determination is carried out, in one embodiment, according to the protocol shown schematically in figure 4 . A human operator intervenes in this experimental determination.

As shown on the figure 4 , the operator under test is equipped with a transducer 8, positioned laterally, substantially in the region of the temple, and an earpiece 42 positioned on one ear, for example the left ear as in the example of figure 3 . The earpiece 42 is a conventional earpiece, making it possible to transmit an airborne acoustic signal at the level of the operator's left outer ear. Preferably, the right outer ear is obstructed, for example by an ear plug, in order to avoid possible hearing interference.

A generator 44 of sinusoidal signals makes it possible to successively generate signals for a set of frequencies varying from 20 Hz to 20 kHz. A generated sinusoidal signal is transmitted to both the earpiece 42 and to a filter 46, the gain Go of which and the phase ΔΦo are adjustable by the operator. The operator has the possibility of adjusting the gain and the phase of the filter 46 for a sinusoidal signal of given frequency f until he notices a cancellation of the sound perceived at the level of his inner ear I. The operator therefore provides the gain and the phase of filter 46 for each frequency f, allowing cancellation of the sound perceived at the level of the inner ear. By noting Hg the transfer function of the filter 46, H _TO the transfer function of the transducer 8 and H _TA the transfer function of the listener 42, the following relation is verified: $$H_g\cdot H_{\mathit{TO}}\cdot H_{\mathit{CO}}=-H_{\mathit{YOUR}}\cdot H_{\mathit{IT}}$$

Thus, the ratio R = H _CA / H _CO is deduced from it: $$\frac{H_{\mathit{IT}}}{H_{\mathit{CO}}}=-H_g\cdot \frac{H_{\mathit{TO}}}{H_{\mathit{YOUR}}}$$

The transfer function Hg is obtained by measurement as explained above and stored, and the respective transfer functions of the transducer 8 and of the earpiece 42 are known. Therefore, it is possible to calculate the ratio R.

As a variant, the experimental protocol is repeated for a plurality of operators, thus making it possible to obtain a plurality of subjective measurements for the transfer function Hg in the set of frequencies, and to deduce therefrom an average transfer function.

Thus, equation (Eq 4) makes it possible to determine the transfer function H _FO of the filter 32 to be applied in order to achieve cancellation of the ambient sound signal Sb, picked up by the microphone 40, by bone conduction via a transducer 8.

It should be noted that from a computational point of view, it is possible to combine the equations (Eq 4) and (Eq 6), which makes it possible to obtain the following simplified relation for obtaining the transfer function H _FO of filter 32: $$H_{\mathit{FO}}=\frac{H_g}{H_m\cdot H_{\mathit{YOUR}}}\cdot H_b^G$$

sont déterminées de manière connue dans le domaine de la réduction active de bruit, à l'aide d'un mannequin acoustique dans une chambre acoustique, avec un balayage de fréquence de 20Hz à 20000Hz.The transfer function H _FO can therefore be calculated by applying the equation (Eq 7) above, in which H _g is the transfer function measured as above. The transfer functions H _m H _TA and $H_b^G$

are determined in a manner known in the field of active noise reduction, using an acoustic mannequin in an acoustic chamber, with a frequency sweep from 20Hz to 20,000Hz.

According to a second embodiment, in an acoustic noise reduction device comprising two acoustic noise reduction modules 4, a suitable transfer function is applied, taking into account a transverse bone propagation, that is to say a conduction bone of the vibratory signal emitted by the transducer located on the left to the user's right inner ear, and vice versa, a bone conduction of the vibratory signal emitted by the right transducer to the left inner ear.

Analogously to figure 3 , the figure 5 schematically illustrates the principle of noise reduction by osteophonic pathway in this second embodiment.

Two similar transducers 8, 8 'and having the same transfer function H _TO are considered, respectively annotated G for the left transducer positioned on the left lateral part of the user's skull and D for the right transducer positioned on the right lateral part of the user's skull. Points I _G and I _D denote the entry points of the user's left and right inner ear, respectively, and points E _G and E _{D are} the respective entry points of the entrances to the left and right outer ear canals. . It is assumed that the internal auditory canals on the one hand and the bone conduction on the other hand are symmetrical for an average user, so a single transfer function Hco of direct bone conduction, Hco 'of transverse bone conduction and H _CA of conduction middle ear and inner ear are considered.

Externally, in the greatest generality, it is considered that the respective left and right transfer functions corresponding to the noisy environment may be different, and that a different filtering H _FO ^G and H _FO ^D is applied at the input of the transducers 8 , 8 'respective. A microphone 48 is connected to the left transducer 8 and a microphone 50 is connected to the right transducer 8 '.

$$H_D\cdot H_{\mathit{CO}}+H_b^D\cdot H_{\mathit{CA}}+H_{\mathit{CO}}^{\prime }\cdot H_G=0$$

In order to obtain simultaneous cancellation of the noise perceived at the level of the two internal ears, the following relationships are verified: $$H_G\cdot H_{\mathit{CO}}+H_b^G\cdot H_{\mathit{IT}}+H_{\mathit{CO}}^{\prime }\cdot H_D=0$$

$$H_D\cdot H_{\mathit{CO}}+H_b^D\cdot H_{\mathit{IT}}+H_{\mathit{CO}}^{\prime }\cdot H_G=0$$

$$H_G=\frac{R}{\left(1-P^2\right)}\cdot \left[H_b^D\cdot P-H_b^G\right]$$

We then obtain by calculation: $$H_D=\frac{R}{\left(1-P^2\right)}\cdot \left[H_b^G\cdot P-H_b^D\right]$$

$$H_G=\frac{R}{\left(1-P^2\right)}\cdot \left[H_b^D\cdot P-H_b^G\right]$$

comme précédemment, et $P=\frac{H_{\mathit{CO}}^{\prime }}{H_{\mathit{CO}}}$

est le rapport entre fonction de transfert de conduction osseuse transverse et fonction de transfert de conduction osseuse directe.Or $R=\frac{H_{\mathit{IT}}}{H_{\mathit{CO}}}$

as before, and $P=\frac{H_{\mathit{CO}}^{\prime }}{H_{\mathit{CO}}}$

is the ratio between transverse bone conduction transfer function and direct bone conduction transfer function.

et $H_b^D$

sont identiques : $H_b^G=H_b^D=H_b,$

les équations (Eq 10) et (Eq 11) se simplifient comme suit : $$H_D=H_G=-\frac{R}{1+P}\cdot H_b$$

If we consider that the transfer functions $H_b^G$

and $H_b^D$

are the same : $H_b^G=H_b^D=H_b,$

the equations (Eq 10) and (Eq 11) are simplified as follows: $$H_D=H_G=-\frac{R}{1+P}\cdot H_b$$

To determine the transfer functions, it is useful to determine the ratio P of the respective transfer functions of direct and transverse bone conduction. In one embodiment, the following relationship is used: $$P=\frac{H_{\mathit{CO}}^{\prime }}{H_{\mathit{IT}}}\cdot R$$

rapport qui peut être mesuré de manière expérimentale, de manière analogue au protocole expérimental décrit ci-dessus en référence à la figure 4.Thus, it suffices to determine $P\prime =H_{\mathit{CO}}^{\prime }/H_{\mathit{IT}},$

ratio which can be measured experimentally, analogously to the experimental protocol described above with reference to the figure 4 .

A schematic representation of a protocol for determining the ratio P 'is shown in figure 6 . In the embodiment of the figure 6 , the listening 52 is placed on the opposite side of the osteophonic transducer 8. The duct of the external ear on the same side as the osteophonic transducer 8 is blocked by an earplug 58 for example, in order to avoid any interference.

A generator 54 of sinusoidal signals, analogous to the generator 44 of the figure 4 , makes it possible to successively generate signals for a set of frequencies varying from 20 Hz to 20kHz. A generated sinusoidal signal is transmitted to both the earpiece 52 and to a filter 56, the gain G ' _o and the phase ΔΦ'o of which are adjustable by the operator. The operator has the possibility of adjusting the gain and the phase of the filter 56 for a sinusoidal signal of given frequency f until he observes a cancellation of the sound perceived at the level of his inner ear I. The operator therefore provides the gain and the filter phase 56 for each frequency f, allowing cancellation of the sound perceived at the level of the inner ear, between the sound signal supplied via the earpiece 52 and the vibratory signal transmitted by bone conduction from the transducer 8. Noting H ' _g the function of transfer of the filter 56, H _TO the transfer function of the transducer 8 and H _TA the transfer function of the earpiece 52, the following relationship is verified: $$H_{\mathit{YOUR}}\cdot H_{\mathit{IT}}=-H_g^{\prime }\cdot H_{\mathit{TO}}\cdot H_{\mathit{CO}}^{\prime }$$

The ratio P 'is deduced from this by: $$P\prime =\frac{H_{\mathit{YOUR}}}{H_g^{\prime }\cdot H_{\mathit{TO}}}$$

The function H ' _g is provided by experimental measurements, for a set of frequency values in the frequency band considered. The ratio P ′ can then be calculated for this set of frequencies thanks to the relation provided by the equation (Eq 15), knowing the respective transfer functions of the earpiece 52 and of the transducer 8. Then, it is possible to deduce therefrom the transfer functions H _FO ^G and H _FO ^D to be implemented by the respective filters of the electronic cards associated with each acoustic module 4.

H_g et H_g' étant les fonctions de transfert des filtres respectifs 42 et 52, déterminées expérimentalement comme exposé ci-dessus. $$H_{\mathit{FO}}^D=\left[\frac{-H_g}{H_m\cdot H_{\mathit{TA}}\left(1-Q^2\right)}\right]\cdot \left[H_b^{G}Q-H_b^D\right]$$

$$H_{\mathit{FO}}^G=\left[\frac{-H_g}{H_m\cdot H_{\mathit{TA}}\left(1-Q^2\right)}\right]\cdot \left[H_b^{D}Q-H_b^G\right]$$

It should be noted that the transfer functions H _FO ^G and H _FO ^D can be calculated directly by the following formulas, using the ratio $Q=\frac{H_g}{H_g^{\prime }},$

H _g and H _g 'being the transfer functions of the respective filters 42 and 52, determined experimentally as described above. $$H_{\mathit{FO}}^D=\left[\frac{-H_g}{H_m\cdot H_{\mathit{YOUR}}\left(1-Q^2\right)}\right]\cdot \left[H_b^{G}Q-H_b^D\right]$$

$$H_{\mathit{FO}}^G=\left[\frac{-H_g}{H_m\cdot H_{\mathit{YOUR}}\left(1-Q^2\right)}\right]\cdot \left[H_b^{D}Q-H_b^G\right]$$

The transfer functions H _FO ^G and H _FO ^D are obtained, according to one embodiment of the invention, by the implementation of a determination method, the main steps of which are illustrated on figure 7 .

In a first step 70, the transfer functions H _b ^G and H _b ^D representative of the characteristics of the environment are calculated, according to a standard measurement protocol in an acoustic chamber as briefly explained above.

Then, the transfer functions H _g and H _g 'are evaluated in the following step 72, according for example to the protocols described with reference to figures 4 and 6 .

In step 74, the combined transfer function H _m · H _TA is evaluated, according to a standard measurement protocol in an acoustic chamber as briefly explained above.

Steps 70, 72 and 74 can be performed in a different order. The determined transfer functions are stored in a memory associated with a calculation processor for the set of frequencies of the desired frequency interval.

Then, in step 76, the transfer functions H _FO ^G and H _FO ^D are determined by calculation, using the relationships (Eq 16) and (Eq 17) above.

The respective transfer functions thus determined are each implemented in an electronic filtering circuit of a filter card of an acoustic module for osteophonic noise reduction.

Claims (4)

1. An acoustic device (2) capable of producing active noise reduction, which may be positioned on the head of a user, comprising at least one microphone able to sense a representative sound signal of ambient noise, including at least one acoustic module (4) for active noise reduction comprising an osteophonic transducer (8, 8'), capable of being positioned on a side flank of the head of the user, and of transmitting a vibratory signal transformed by bone conduction into an acoustic signal which may be perceived by the user, connected to said microphone, said at least one acoustic module (4) including an electronic circuit (30) capable of generating a vibratory signal giving the possibility of attenuating the perception of said ambient noise by the user, the acoustic device (2) being characterized in that said electronic circuit (30) implements a filter (32) defined by a transfer function (H_FO) for noise reduction, said transfer function (H_FO) being determined in order to produce noise attenuation according to direct bone conduction between the osteophonic transducer (8, 8') and the inner ear of a user located on the same side as said transducer (8, 8'), said noise reduction transfer function (H_FO) depending on a ratio of a direct bone conduction transfer function (H_CO) representative of said bone conduction and of an airborne conduction transfer function (H_CA) representative of the conduction of an airborne acoustic signal between an outer ear and an inner ear located on a same side,
   and in that said transfer function is defined by the formula: $$H_{\mathit{FO}}=-R\cdot \frac{H_b^G}{H_m.H_{\mathit{TO}}}$$

   

   wherein R is said ratio of a direct bone conduction transfer function (H_CO) and of the airborne conduction transfer function (H_CA), $H_b^G$

   

   is a characteristic transfer function of ambient noise, H_m is the transfer function of said microphone and H_TO is the transfer function of said osteophonic transducer (8,8').
2. The acoustic device according to claim 1, characterized in that it includes two so called active noise reduction acoustic modules (4) capable of being positioned on the opposite lateral sides of the head of a user.
3. The acoustic device according to claim 2, characterized in that each active noise reduction acoustic module (4) includes an osteophonic transducer (8,8') and an electronic circuit (30) implementing a filter (32) defined by a noise reduction transfer function (H_FO), said transfer function (H_FO) being determined according to direct bone conduction (H_CO) and transverse bone conduction (H_CO') of a vibratory signal from the opposite osteophonic transducer.
4. The acoustic device according to one of the preceding claims, characterized in that it further includes an active noise reduction module for transmitting an airborne counter-noise signal.

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