CN102905208A - Noise reducing sound reproduction - Google Patents

Abstract

Disclosed are a system and a method for noise reducing sound reproduction. An input signal supplied to a loudspeaker by which it is acoustically radiated. The signal radiated by the loudspeaker may be received by a microphone that is acoustically coupled to the loudspeaker via a secondary path and that provides a microphone output signal. From the microphone output signal a useful-signal can be subtracted to generate a filter input signal. The filter input signal is filtered in an active noise reduction filter to generate an error signal, and the useful-signal is subtracted from the error signal to generate the loudspeaker input signal. In addition, the useful-signal is filtered by one or more spectrum shaping filters prior to subtraction from the microphone output signal or the loudspeaker input signal or both.

Description

The audio reproduction of reducing noise

Technical field

Herein disclosed is a kind of sound reproduction system of reducing noise, especially, disclose a kind of noise reduction system that comprises earphone, wherein earphone be used for to allow the user to enjoy the sound of the ambient noise with reduction, such as the music of reproducing etc.

Background technology

Reduce system at active noise, be also referred to as active noise and eliminate/control in (ANC) system, often use identical loud speaker, especially, be configured in the loud speaker in two earphones of headphone, carry out the sound of reducing noise and expectation such as the reproduction of music or speech.But, because common noise reduction system also reduces the fact of desired audio to a certain extent, using active noise to reduce the sound effect that produces and do not using active noise to reduce significantly difference of existence between the sound effect that produces.Therefore, need advanced electronic signal process to compensate this impact, perhaps the audience must accept to open or close and different sound effects according to reducing noise.Therefore, need a kind of improved noise reduction system to overcome this shortcoming.

Summary of the invention

In a first aspect of the present invention, a kind of sound reproduction system of reducing noise is disclosed, comprising: loud speaker is connected with the loud speaker input path; Microphone is coupled to described loud speaker by secondary path by acoustics, and is connected with the microphone outgoing route; The first subtracter is with downstream and the first useful signal Path Connection of described microphone outgoing route; Active noise reduces filter, is connected with the downstream of described the first subtracter; The second subtracter is connected to described active noise and reduces between filter and the described loud speaker input path, and with the second useful signal Path Connection; Wherein, two useful signal paths all provide reproduced useful signal, and in the described useful signal path at least one comprises one or more spectrum shape filters.

In a second aspect of the present invention, a kind of sound goodbye method of reducing noise is disclosed, wherein: input signal is offered loud speaker, propagate described input signal by described loud speaker acoustics; Receive the signal of propagating by described loud speaker by microphone, described microphone is coupled to described loud speaker by secondary path by acoustics, and described microphone provides microphone output signal; Described microphone output signal deducts useful signal to generate filter input signal; In active noise reduction filter, described filter input signal is carried out filtering with the generated error signal; And described useful signal deducts described error signal to generate described loud speaker input signal; Deduct described microphone output signal or described loud speaker input signal or above-mentioned before both at described useful signal, by one or more spectrum shape filters described useful signal is carried out filtering.

Description of drawings

The below describes various specific embodiments in detail based on the exemplary shown in the accompanying drawing.Except as otherwise noted, use identical assembly reference numbers designate similar or identical otherwise in all accompanying drawings.

Fig. 1 is the block diagram that the active noise of total feedback kind reduces system, and wherein useful signal offers the loudspeaker signal path;

Fig. 2 is the block diagram that the active noise of total feedback kind reduces system, and wherein useful signal offers microphone signal path;

Fig. 3 is the block diagram that the active noise of total feedback kind reduces system, and wherein useful signal offers loudspeaker signal path and microphone signal path;

Fig. 4 is the block diagram that the active noise of Fig. 3 reduces system, and wherein useful signal offers speaker path by spectrum shape filter;

Fig. 5 is the block diagram that the active noise of Fig. 3 reduces system, and wherein useful signal offers microphone path by spectrum shape filter;

Fig. 6 is the schematic diagram that can reduce in conjunction with the active noise of Fig. 3-6 earphone of system applies;

Fig. 7 is the block diagram that the active noise of Fig. 5 reduces system, and wherein useful signal offers microphone path by two spectrum shape filters;

Fig. 8 is the amplitude frequency response figure that expression can be applicable to the transmission characteristic of the slope filter in the system of Fig. 7;

Fig. 9 is the amplitude frequency response figure that expression can be applicable to the transmission characteristic of the equalization filter in the system of Fig. 7;

Figure 10 is the block diagram that the active noise of Fig. 5 reduces system, and wherein useful signal offers microphone path and speaker path by spectrum shape filter.

Embodiment

Feedback ANC system intention by provide in listening point noise reduction signal with reduce or even eliminate interference signal, wherein in time passage, but noise reduction signal is compared with noise signal, has ideally identical amplitude opposite phase place.By superimposed noise signal and noise reduction signal, the signal that produces is also referred to as error signal, levels off in theory zero.The quality of reducing noise depends on so-called secondary path, namely loud speaker and represent the quality of the acoustic path between audience's the microphone of ear.The quality of reducing noise further depends on the quality that is connected to the so-called ANC filter between microphone and the loud speaker, the error signal that the filtering of ANC filter provides by microphone, thereby when the error signal of filtering is passed through loudspeaker reproduction, further reduce error signal.But when in listening point, the loud speaker of the error signal by also reproducing filtering especially when providing extraly useful signal such as music or speech for the error signal of filtering, has problems.Then, as previously mentioned, useful signal can be by system deterioration.

For simplicity, between electronics and acoustic signal, do not make differentiation herein.But all signals that provide by loud speaker or receive by microphone in fact all have acoustic properties.Every other signal all is electronics in essence.Loud speaker and microphone can be to have the input phase that formed by loud speaker 3 and the part of the acoustics subsystem (that is, loud speaker-room-microphone system) of the output stage that formed by microphone.Subsystem is provided with electronic input signal and electronic output signal is provided.In this regard, " path " expression can comprise that the electronics of more element such as signal conduction unit, amplifier, filter etc. or acoustics connect.Spectrum shape filter is the frequency spectrum of input signal and output signal different filter on each Frequency point.

With reference now to Fig. 1,, the active noise that shows common feedback kind reduces the block diagram of (ANC) system, wherein, interference signal d[n], be also referred to as noise signal, transmit (propagation) to listening point by primary path 1, for example, audience's ear.Primary path 1 has transmission characteristic P (z).In addition, input signal v[n] transmit (propagation) to listening point by secondary path 2 from loud speaker 3.Secondary path 2 has transmission characteristic S (z).

The microphone 4 that is arranged on listening point receives interference signal d[n] and the signal that produced by loud speaker 3.Microphone 4 provides the microphone output signal y[n of the summation of these signals that receive of expression].Microphone output signal y[n] as filter input signal u[n] offer ANC filter 5, filter 5 output error signal e[n] to adder 6.ANC filter 5 can be sef-adapting filter, has transmission characteristic W (z).Adder 6 also receives useful signal x[n], for example music or speech, and input signal v[n is provided] to loud speaker 3, useful signal x[n wherein] randomly by pre-filtering, for example by the spectrum shape filter (not shown).

Signal x[n], y[n], e[n], u[n] and v[n] at Disgrete Time Domain.For following consideration, use their frequency spectrum designation X (z), Y (z), E (z), U (z) and V (z).The differential equation of describing the system shown in Fig. 1 is as follows:

Y(z)＝S(z)·V(z)＝S(z)·(E(z)+X(z)) (1)

E(z)＝W(z)·U(z)＝W(z)·Y(z) (2)

Therefore, in the system of Fig. 1, useful signal transmission characteristic M (z)=Y (z)/X (z) is M (z)=S (z)/(1-W (z) S (z)) (3)

Suppose W (z)=1, so

$\lim [S\left(z\right)\→1]M\left(z\right)\⇒M\left(z\right)\→\∞---\left(4\right)$

$\lim [S\left(z\right)\→\±\∞]M\left(z\right)\⇒M\left(z\right)\→1---\left(5\right)$

$\lim [S\left(z\right)\→0]M\left(z\right)\⇒S(z---\left(6\right)$

Suppose W (z)=∞, so

$\lim [S\left(z\right)\→1]M\left(z\right)\⇒M\left(z\right)\→0.---\left(7\right)$

Shown in formula (4)-(7), when the transmission characteristic W of filter 5 (z) increase, useful signal transmission characteristic M (z) is close to 0, and inferior path transmission function S (z) remains neutral, that is, and and the level about 1, that is, and 0[dB].For this reason, must correspondingly adjust useful signal x[n] to guarantee no matter ANC opens or closes, the useful signal x[n that the audience experiences] be identical.And, useful signal transmission characteristic M (z) also depends on the transmission characteristic S (z) of secondary path 2, the meaning is, useful signal x[n] the adjustment fluctuation also depending on transmission characteristic S (z) and cause owing to old and feeble, temperature and audience's change etc., because described fluctuation, the difference between " opening " and " closing " will become obvious.

Although in the system of Fig. 1, useful signal x[n] offer the acoustics subsystem (loud speaker, room, microphone) at adder 6 places, wherein adder 6 is connected with the upstream of loud speaker 3, but in the system of Fig. 2,4 places provide useful signal x[n at microphone].Therefore, in the system of Fig. 2, omit adder 6, and in the downstream of microphone 4 configuration adder 7 with summation as by the useful signal x[n of pre-filtering] and microphone output signal y[n].Therefore, loud speaker input signal v[n] be error signal [e], that is, v[n]=[e], and filter input signal u[n] be useful signal x[n] and microphone output signal y[n] and, that is, u[n]=x[n]+y[n].

The differential equation of describing the system shown in Fig. 2 is as follows:

Y(z)＝S(z)·V(z)＝S(z)·E(z) (8)

E(z)＝W(z)·U(z)＝W(z)·(X(z)+Y(z)) (9)

Therefore, do not consider interference signal d[n], the useful signal transmission characteristic M (z) in the system of Fig. 2 is

M(z)＝(W(z)·S(z))/(1-W(z)·S(z)) (10)

$\lim [(W\left(z\right)\·S\left(z\right))\→1]M\left(z\right)\⇒M\left(z\right)\→\∞---\left(11\right)$

$\lim [(W\left(z\right)\·S\left(z\right))\→0]M\left(z\right)\⇒M\left(z\right)\→0---\left(12\right)$

$\lim [(W\left(z\right)\·S\left(z\right))\→\±\∞]M\left(z\right)\⇒M\left(z\right)\→1.---\left(13\right)$

Can find out from formula (11)-(13), when open-loop transmission characteristic (W (z) S (z)) increases or reduces, useful signal transmission characteristic M (z) is near 1, when open-loop transmission characteristic (W (z) S (z)) near 0 the time, useful signal transmission characteristic M (z) is near 0.For this reason, in higher spectral range, must adjust extraly useful signal x[n] to guarantee no matter ANC opens or closes the useful signal x[n that the audience experiences] be identical.But the compensation in higher spectral range is very difficult, thereby the difference between " opening " and " closing " will become obvious.On the other hand, useful signal transmission characteristic M (z) does not depend on transmission characteristic and because the fluctuation that aging, temperature and audience's change etc. cause thereof of secondary path 2.

Fig. 3 is the block diagram that the active noise reduction system of common feedback kind is shown, and wherein useful signal offers microphone path and speaker path.For simplicity, the below omits primary path 1, although noise (interference signal x[n]) still exist.Especially, the system of Fig. 3 is based on the system of Fig. 1, but, have extra subtracter 8 and subtracter 9, subtracter 8 is from microphone output signal y[n] deduct useful signal x[n] to form ANC filter input signal u[n], subtracter 9 substitutes adders 6 and deducts useful signal x[n from error signal e [n]].

The differential equation of describing the system shown in Fig. 3 is as follows:

Y(z)＝S(z)·V(z)＝S(z)·(E(z)-X(z)) (14)

E(z)＝W(z)·U(z)＝W(z)·(Y(z)-X(z)) (15)

Thus, the useful signal transmission characteristic M (z) in the system of Fig. 3 is:

M(z)＝(S(z)-W(z)·S(z))/(1-W(z)·S(z)) (16)

$\lim [(W\left(z\right)\·S\left(z\right))\→1]M\left(z\right)\⇒M\left(z\right)\→\∞---\left(17\right)$

$\lim [(W\left(z\right)\·S\left(z\right))\→0]M\left(z\right)\⇒M\left(z\right)\→S\left(z\right)---\left(18\right)$

$\lim [(W\left(z\right)\·S\left(z\right))\±\∞]M\left(z\right)\⇒M\left(z\right)\→1.---\left(19\right)$

Can find out that from formula (17)-(19) behavior of the system of Fig. 3 is similar to the behavior of the system of Fig. 2.Unique difference is, when open-loop transmission characteristic (W (z) S (z)) near 0 the time, useful signal transmission characteristic M (z) is near S (z).With the system class of Fig. 1 seemingly, the system of Fig. 3 depends on transmission characteristic S (z) and because the fluctuation that old and feeble, temperature and audience's change etc. cause thereof of secondary path 2.

System shown in Figure 4 is based on the system of Fig. 3, and comprises extraly equalization filter 10, thus equalization filter 10 be connected with the downstream of subtracter 9 use reverse secondary path transfer function 1/S (z) filtering useful signal x[n].The differential equation of describing the system shown in Fig. 4 is as follows:

Y(z)＝S(z)·V(z)＝S(z)·(E(z)-X(z)/S(z)) (20)

E(z)＝W(z)·U(z)＝W(z)·(Y(z)-X(z)) (21)

Thus, the useful signal transmission characteristic M (z) in the system of Fig. 4 is:

M(z)＝(1-W(z)·S(z))/(1-W(z)·S(z))＝1 (22)

Can find out microphone output signal y[n from formula (22)] with useful signal x[n] identical, this means if equalization filter just in time is the inverse of secondary path transmission characteristic S (z), so signal x[n] be not changed.Equalization filter 10 can for minimum phase filter with optimum results, that is, optimize its approximate inverse to theoretic minimum phase, secondary path transmission characteristic S (z) of actual transmissions characteristic, thus y[n]=x[n].The effect of desirable linearizer is served as in this configuration, that is, its compensation is transferred to any deterioration that the microphone 4 of the ear that represents the audience causes because of useful signal from loud speaker 3.Therefore, it is useful signal x[n] compensation or the interference effect of linearisation secondary path S (z), thereby arrive audience's useful signal x[n] with the source provide the same, do not have the caused negative influence of any acoustical behavior because of headphone, that is, y[z]=x[z].Like this, by the help of such linearized filter, can so that the sound picture of the headphone of poor design by the perfect sound of adjusting in acoustics ground, namely linear voice is the same.

System shown in Figure 5 is based on the system of Fig. 3, and comprises extraly equalization filter 10, thus equalization filter 10 be connected with the upstream of subtracter 8 use secondary path transfer function S (z) filtering useful signal x[n].

The differential equation of describing the system shown in Fig. 5 is as follows:

Y(z)＝S(z)·V(z)＝S(z)·(E(z)-X(z)) (23)

E(z)＝W(z)·U(z)＝W(z)·(Y(z)-S(z)·X(z)) (24)

Thus, the useful signal transmission characteristic M (z) in the system of Fig. 5 is:

M(z)＝S(z)·(1+W(z)·S(z))/(1+W(z)·S(z))＝S(z) (25)

Can find out that from formula (25) when ANC system when being active, useful signal transmission characteristic M (z) is identical with secondary path transmission characteristic S (z).When ANC system when being passive, useful signal transmission characteristic M (z) is also identical with secondary path transmission characteristic S (z).Thus, for the audience near the position of microphone 4, no matter reducing noise is active or passive, the acoustic efficiency of useful signal is identical.

ANC filter 5 and equalization filter 10 and 11 can be the sef-adapting filters that has the fixed filters of constant transmissions characteristic or have controllable transmission characteristic.In the accompanying drawings, represent the adaptive structure of filter itself by the arrow under each square, represent by a dotted line the alternative of adaptive structure.

System shown in Fig. 5 for example can be applicable in the headphone, wherein in different situations, reproduce useful signal according to noise, such as music or speech, and the audience can close the ANC system, ought not have especially noise to exist, the active state of ANC system and passive states and between when not existing any zone of audibility other.But system disclosed herein not only only can be applicable in the headphone, also can be applicable in the every other field of reducing noise that need to be once in a while.

Fig. 6 illustrates exemplary earphone, wherein can use active noise of the present invention to lower system.The earphone that this earphone is identical with another can be the part of headphone (not shown), and can be connected to audience's ear 12 acoustics.In the present embodiment, ear 12 touches interference signal d[n by primary path 1], ambient noise for example.Earphone comprises the cup-shaped housing 14 with porose 15, and this cup-shaped housing 14 can be covered by the permeable overcover of sound, for example, and grid, grid or the permeable structure of any other sound or material.Loud speaker 3 is propagated sound to ear 12 and is disposed in 15 places, hole of housing 14, jointly forms earphone chamber 13.Chamber 13 can be airtight or opening by any method, such as by methods such as interface, perforate, openings.Microphone 4 is arranged on the front of loud speaker 3.Acoustic path 17 extends to ear 12 from speaker 3, and for the purpose of noise control, has the transmission characteristic approximate with the transmission characteristic of secondary path 2, and wherein secondary path 2 extends to microphone 4 from loud speaker 3.

In mobile device such as headphone, space and the energy that can be utilized by the ANC system are very limited.Digital circuit may too time-consuming and energy, the therefore frequent preferred analog circuit that uses in the design of the ANC of mobile device system.But analog circuit only allows the very limited complexity of ANC system, therefore is difficult to only correctly simulate secondary path by analog element.Especially, the analog filter that uses in the ANC system, has low-energy-consumption and needs less space because they are easy to make up often as fixed filters or very simple sef-adapting filter.When using analog circuit, above-mentionedly also provide gratifying result in conjunction with the system shown in Fig. 4,5 and 7 because its less (Fig. 4) even do not rely on (Fig. 5 and 7) secondary path behavior.And, Fig. 5 and 7 system allow based on ANC filter transmission characteristic W (z) and secondary path filter characteristic S (z) the essential transmission characteristic of equalization filter to be estimated preferably, wherein ANC filter transmission characteristic W (z) and secondary path filter characteristic S (z) form open-loop transmission characteristic W (z) S (z) jointly, it only has less fluctuation in principle, and when being connected to audience's head based on the estimation of the acoustical behavior of headphone.

ANC filter 5 has such transmission characteristic usually: be tending towards having lower gain at lower frequency place, along with the increase of frequency, increase to maxgain value and be decreased to loop gain again.Use the high-gain of ANC filter 5, the loop keeping system of ANC system inherence is linear in the frequency range below the 1KHz for example, abandons thus any unnecessary equilibrium in this frequency range.In the frequency range greater than 3KHz, ANC filter 5 has the effect that strengthens or eliminate hardly, does not therefore have the linearisation effect.ANC filter gain in this frequency range is near loop gain, and useful signal transmission characteristic M (z) strengthens in higher frequency place experience, therefore except equalization filter, must compensate by independent filter such as slope filter.In the frequency range between 1KHz and 3KHz, enhancing and elimination all may occur.According to acoustic efficiency, strengthen than eliminating and have more interference, the therefore enhancing of the elimination filter by respective design in can the adequate remedy transmission characteristic.If the ANC filter gain is 0dB more than 3KHz, then there is not the linearisation effect, therefore except the first equalization filter, may use the second equalization filter to substitute slope filter.

From above-mentioned consideration, can to find out, can to use at least two filters in order compensating.Fig. 7 illustrates the exemplary ANC system of two filters 18 of use (at least) and 19 (subfilters), rather than as using single filter 11 in the system of Fig. 5.For example, has transmission characteristic S ₁(z) high pitch is eliminated slope filter (for example, filter 18) and is had transmission characteristic S ₂(z) high pitch is eliminated equalization filter (for example, filter 19), wherein, and S (z)=S ₁(z) S ₂(z).Alternatively, can use high pitch to strengthen equalization filter as for example filter 18, use high pitch to eliminate equalization filter as for example filter 19.If useful signal transmission characteristic M (z) shows more complicated structure, can use three filters, for example, a high pitch elimination slope filter and two high pitchs strengthen/eliminate equalization filter.The number of the filter that uses depends on a lot of other factors, such as the noise behavior of cost, filter, the acoustical behavior of headphone, the time of delay of system, the space that executive system can be used etc.

Fig. 8 is the schematic diagram that can be applicable to the transmission characteristic of slope filter a, b in the system of Fig. 7.Especially, first order high pitch is shown and strengthens (+9dB) slope filter (a) and bass elimination (3dB) slope filter (b).Fig. 9 is the schematic diagram that can be applicable to the transmission characteristic of equalization filter c, d in the system of Fig. 7.One (c) of equalization filter provides the enhancing of 9dB at the 1KHz place, another (d) provides the elimination of 6dB at the 100Hz place with higher Q, and bandwidth becomes sharper keen thus.

Although the scope of frequency spectrum shaping function is subjected to the theoretical domination of linear filter, the requirement that can controlled flexibility satisfy according to topology and their needs of circuit the adjustment of these functions and they and difference.The normally simple first order filter of slope filter changes than the higher frequency of angular spectrum with than the relative gain between the lower frequency of angular frequency.Adjust low or bass grid and do not affect gain more than the angular frequency with the gain that affects lower frequency.Height or high pitch grid are only adjusted the gain of upper frequency.On the other hand, single equalization filter is carried out second level filter function.This comprises three adjustment: the selection of centre frequency, determine the adjustment of quality factor (Q) of the sharpness of bandwidth, and determine selected centre frequency with respect on the centre frequency or under frequency (a lot) be enhanced or eliminate what level or the adjustment of gain.

Figure 10 is the combination of the system shown in Fig. 4 and Fig. 5, wherein useful signal x[n] respectively by having transmission characteristic S ₅(z) filter 20 or have transmission characteristic S ₆(z) filter 21 offers microphone path and speaker path, wherein, and for example,

S(z)=S ₅(z)·S ₆(z)。

Realize various embodiment of the present invention although disclose, but for those skilled in the art, be appreciated that without departing from the principles and spirit of the present invention and can make various changes and modifications these embodiment, thereby realize advantages more of the present invention.For those skilled in the art, clearly can suitably replace other assemblies of carrying out identical function.These modifications to concept of the present invention also should comprise within the scope of the appended claims.

Claims (15)

1. the sound reproduction system of a reducing noise comprises:

Loud speaker is connected with the loud speaker input path;

Microphone is coupled to described loud speaker by secondary path by acoustics, and is connected with the microphone outgoing route;

The first subtracter is with downstream and the first useful signal Path Connection of described microphone outgoing route;

Active noise reduces filter, is connected with the downstream of described the first subtracter;

The second subtracter is connected to described active noise and reduces between filter and the described loud speaker input path, and with the second useful signal Path Connection; Wherein,

Two useful signal paths all provide reproduced useful signal, and

In the described useful signal path at least one comprises one or more spectrum shape filters.

2. system according to claim 1, wherein said secondary path has the secondary path transmission characteristic, and in the described spectrum shape filter at least one has the described secondary path transmission characteristic of simulation or about described useful signal at described microphone outgoing route Linear microphone signal.

3. system according to claim 1 and 2, wherein said the first useful signal path comprises the first spectrum shape filter, described the first spectrum shape filter has the transmission characteristic identical with described secondary path transmission characteristic.

4. system according to claim 3, wherein said the first spectrum shape filter has at least two subfilters.

5. according to claim 3 or 4 described systems, at least one in the subfilter of wherein said the first filter or described the first filter is equalization filter.

6. each described system according to claim 3-5, wherein said at least one equalization filter is that high pitch is eliminated equalization filter.

7. according to claim 4 or 5 described systems, one in the subfilter of wherein said the first filter or described the first filter is slope filter.

8. system according to claim 7, wherein said slope filter is that high pitch is eliminated slope filter.

9. system according to claim 2, wherein said the second useful signal path comprises the second spectrum shape filter, described the second spectrum shape filter has the transmission characteristic identical with described reverse secondary path transmission characteristic.

10. each described system according to claim 1-9, at least one in wherein said active noise reduction filter, the first spectrum shape filter and the second shaping filter is sef-adapting filter.

11. the sound reproducing method of a reducing noise, wherein:

Input signal is offered loud speaker, propagate described input signal by described loud speaker acoustics;

Receive the signal of propagating by described loud speaker by microphone, described microphone is coupled to described loud speaker by secondary path by acoustics, and described microphone provides microphone output signal;

Described microphone output signal deducts useful signal to generate filter input signal;

In active noise reduction filter, described filter input signal is carried out filtering with the generated error signal; And

Described useful signal deducts described error signal to generate described loud speaker input signal; And

Deduct described microphone output signal or described loud speaker input signal or above-mentioned before both at described useful signal, by one or more spectrum shape filters described useful signal is carried out filtering.

12. method according to claim 11, wherein said secondary path has the secondary path transmission characteristic, and described secondary path transmission characteristic is simulated in all spectrum shape filter stack ups.

13. method according to claim 12 wherein before described useful signal deducts described microphone output signal, uses the transmission characteristic identical with described secondary path transmission characteristic that described useful signal is carried out filtering.

14. method according to claim 13, the filtering of wherein said useful signal comprise equilibrium and/or slope filtering.

15. method according to claim 12 wherein before described useful signal deducts described loud speaker input signal, uses the transmission characteristic identical with described reverse secondary path transmission characteristic that described useful signal is carried out filtering.

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