JPH0690075B2 - Measuring method of transfer function used for active noise control - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial field of application) The present invention is a measurement of a transfer function used for active control of noise that actively cancels noise from a machine room accommodating a compressor. Regarding the method.

(Prior Art) A cooling device using a compressor, such as a refrigerator, is often installed in a living room of a general household.
Moreover, since it is operated continuously regardless of the season, reducing the noise is an issue. In this case, the most problematic noise source of the refrigerator is noise from the machine room in which the compressor and the piping system connected to the compressor are stored. That is, in the machine room, the compressor itself generates relatively large noise (operation noise of the compressor motor, fluid noise due to compressed gas, mechanical noise in movable mechanical elements of the compression mechanism portion, etc.) and the compressor is connected to the compressor. The piping system also generates noise due to its vibration, and such machine room noise accounts for the majority of refrigerator noise. Therefore, controlling the noise from the machine room greatly contributes to the noise reduction of the entire refrigerator.

Therefore, in the past, in order to reduce noise from the machine room, in addition to noise reduction of the compressor itself (for example, adoption of a rotary type compressor), improvement of the vibration isolation support structure of the compressor and improvement of the shape of the piping system etc. By doing so, the vibration can be damped in the vibration propagation path, or
By arranging the sound absorbing member and the sound insulating member around the compressor and the piping system, it is attempted to increase the sound absorption volume and the noise transmission loss in the machine room.

However, in general, the machine room of a refrigerator is provided with a plurality of openings for heat dissipation because it is necessary to release the heat generated by the driving of the compressor to the outside, and noise leaks to the outside from these openings. It will be. Therefore, the conventional noise reduction measures as described above are naturally limited, and the noise level reduction effect can be expected to be only about 2 dB (A).

On the other hand, in recent years, along with the development of electronics application technologies, especially acoustic data processing circuits and acoustic control technologies, the application of active noise control technology that reduces noise by using sound wave interference has attracted attention. Has been done. That is, in this active control, basically, a sound from a noise source is converted into an electric signal by a control sound receiver (for example, a microphone) provided at a specific position, and this electric signal is processed by an arithmetic unit. By operating a control sounder (for example, a speaker) based on the signal, an artificial sound having the same wavelength and the same amplitude as that of the original sound (sound from the noise source) is generated from the sounder. The original sound is attenuated by causing the sound and the original sound to interfere with each other. Hereinafter, the silencing principle by such active control will be schematically described with reference to FIG.

That is, in FIG. 8, the sound generated by the compressor S that is a noise source is Xs, the sound generated by the speaker A that is the control sounder is Xa, the sound that is received by the microphone M that is the control sound receiver is Xm, and the control target is When the sound at the point O is Xo, and the first to fourth acoustic transfer functions between the output and input points of the sound as described above are G _AM , G _AO , G _SM , and G _SO , 2 The following equation holds as an input / output system. The acoustic transfer functions G _AM ,
The meaning of G _AO , G _SM , G _SO is that the subscript in the front stage corresponds to the input side and the subscript in the rear stage corresponds to the output side (response side).
_AM indicates an acoustic transfer function when the input signal to the speaker A is measured on the input side and the output signal from the microphone M is measured on the output side.

Therefore, the sound Xa speaker A to be generated from the above _{equation, Xa = (- G SO ·} Xm + G SM · Xo) / (G SM · G AO -G SO ·
_Go ), but in this case, since the target is to make the sound level at the control target point O zero, it is possible to set Xo = 0. As a result, _{Xa = Xm · G SO / (} G SO · G AM -G SM · G AO). As can be understood from this equation, in order to reduce the sound Xo at the control target point O to zero, the sound received by the microphone M is
Xm is G = G _SO / (G _SO · G _AM −G _SM · G _AO ) .... For example, it is possible to perform active control in which the acoustic level at the control target point O is theoretically set to zero, and the calculator H is provided to perform such processing.

Therefore, in order to determine the transfer function G, it is necessary to measure the first to fourth acoustic transfer functions G _AM , G _AO , G _SM , and G _SO , and the fast Fourier transform is used for this measurement. Conversion (FF
A transfer function measuring instrument using T) is used. Further, in this case, the measurement of the first and second acoustic transfer functions G _AM and G _AO is performed in a state where the speaker A is driven by the white noise signal of the frequency bandwidth to be actively controlled, and the third and third acoustic transfer functions are measured. The acoustic transfer functions G _SM and G _SO of _{No. 4} are measured with the compressor S actually driven. For such measurement, an auxiliary microphone M'as a sound receiver for measurement is provided at the control target point O. Then, in this case, the equation (1), G = 1 / - and _{(G AM (G SM / G} SO) G AO) = 1 / (G AM -G OM · G AO) ...... (2) Since the third and fourth acoustic transfer functions G _SM and G _SO can be modified, the equivalent acoustic transfer function G _OM , that is, the output signal from the auxiliary microphone M ′ provided at the controlled point O When the equivalent acoustic transfer function G _{OM in} which the output signal from the microphone M is output side is measured, the equivalent acoustic transfer functions G _SM and G _SO are obtained. The acoustic transfer function G _AM , G thus measured
The transfer function G of the arithmetic unit H is determined based on _OM and G _AO .

(Problems to be Solved by the Invention) A noise spectrum distribution generated by driving the compressor S is as shown in FIG. 9, and the noise spectrum has frequencies of an integer multiple of the rotation speed of the compressor S and an integer multiple of the power supply frequency. Exists only in the band. Therefore, in the conventional measurement method, the acoustic transfer function G _OM is in a situation in which correct measurement data can be obtained only in the portion corresponding to the frequency at which the spectrum exists. Therefore, when performing the active control by the transfer function G determined based on the acoustic transfer function G _OM obtained in this way,
When the rotational speed of the compressor S fluctuates during operation (when the rotational speed differs between when the acoustic transfer function GOM is measured and when it is actually operated), its active control is meaningless. Therefore, the muffling effect cannot be obtained at all.

Further, when measuring the acoustic transfer function as described above, at a frequency where the noise spectrum from the compressor S does not exist, the input side signal (output signal from the auxiliary microphone M ′) is also output side signal (from the microphone M). The output signal) also becomes a noise signal. For this reason, in the transfer function measuring device, the calculation using the numerical data close to zero as the denominator is performed, and the measurement result of the acoustic transfer function may become abnormally high in some cases. The measurement data obtained in this way is
In reality, it is meaningless that is not involved in the determination of the transfer function G, but since the dynamic range of the transfer function measuring device is constant, it is possible to use other meaningless measured data because of the meaningless measurement data. The accuracy of the acoustic transfer function measurement data may be carelessly reduced, which may result in insufficient silencing effect during active control.

The present invention has been made in view of the above circumstances, and an object thereof is to measure the accuracy of a transfer function required when noise from a compressor is silenced by active control, in a wide frequency band that allows fluctuations in the rotational speed of the compressor. This provides a method for measuring the transfer function used for active control of noise, which has the effect of always being able to obtain the optimum silencing effect even when the number of revolutions of the compressor fluctuates. There is.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides a control sound receiver for converting a sound from a compressor provided in a machine room into an electric signal, and the above electric The transfer function of the arithmetic unit, which is necessary when active control of noise is performed by a combination of an arithmetic unit that processes a signal with a predetermined transfer function and a control sounding device that is operated based on the processing signal, is measured. In the method, a measuring sound receiver for monitoring a sound at a control target point by the active control is provided, and a first acoustic transfer function between the control sound generator and the control sound receiver and the control sound receiver are also provided. The second acoustic transfer function between the sounder and the sound receiver for measurement is measured with the white noise signal of a predetermined frequency bandwidth input to the sounder for control, respectively, and the compressor and the sound receiver for control are measured. Third acoustic transfer between A fourth acoustic transfer function between the number and the compressor and the measuring sound receiving device, as well as measured in a state of driving the compressor at a predetermined power source frequency,
Among the measurement data obtained by this, at least one of the measurement data group corresponding to the frequency of the integral multiple of the rotation speed of the compressor and the frequency of the integral multiple of the power supply frequency is validated, and those valid measurement data are interpolated. The transfer function of the arithmetic unit is determined on the basis of the third and fourth acoustic transfer functions interpolated with the above and the first and second acoustic transfer functions.

(Operation) As is clear from the equation (1) shown in the section of (Conventional example), the transfer function of the computing unit can be determined based on the first to fourth acoustic transfer functions. In this case, since the first and second acoustic transfer functions are measured with the white noise signal having the predetermined frequency bandwidth input to the control sounder, the measurement accuracy is wide over a wide frequency band. It will be good. On the other hand, since the third and fourth acoustic transfer functions are measured in a state where the compressor is driven at a predetermined power supply frequency, the frequency band in which the noise spectrum associated with the driving exists (an integer of the compressor rotation speed) Data other than double and the frequency band corresponding to an integral multiple of the power supply frequency) will be inaccurate. However, in this case, of the acoustic transfer function data measured as described above, a group of measurement data corresponding to the frequency of an integral multiple of the rotation speed of the compressor and the frequency of an integral multiple of its power supply frequency, that is, from the compressor. Since the third and fourth acoustic transfer functions are obtained by activating at least one of the data groups in the frequency band in which the noise spectrum is present and interpolating the effective measurement data thereof, the third after the interpolation data is obtained.
And the fourth acoustic transfer function is relatively accurate even in a frequency band in which the noise spectrum from the compressor is not distributed. Therefore, the calculation determined based on the third and fourth acoustic transfer functions thus obtained and the first and second acoustic transfer functions that have good measurement accuracy over a wide frequency band as described above. The transfer function of the device has improved accuracy over a wide frequency band that allows fluctuations in the rotational speed of the compressor.

(Embodiment) An embodiment of the present invention will be described below with reference to FIGS. 1 to 7. Here, a case where a refrigerator is taken as an example of active noise control target will be described.

First, in FIG. 3 showing the entire configuration of the refrigerator, 1 is a refrigerator main body, and inside thereof is a freezer compartment 2,
A refrigerator compartment 3 and a vegetable compartment 4 are provided. 5 is a freezer 2
A cooler disposed on the back of the cooling machine 6 is a fan for directly supplying the cool air generated by the cooler 5 to the freezing compartment 2 and the refrigerating compartment 3. 7 is a machine room formed in the lower part of the back side of the refrigerator main body 1, inside of which a rotary type compressor 8, a condenser pipe 9 and a defrosting water evaporator 10 using so-called ceramic fins are housed. .

Now, as shown in FIG. 4 (the condenser pipe 9 and the defrosting water evaporator 10 are not shown here), the machine room 7 has a shape in which only the back surface is opened in a rectangular shape. The opening is closed by the machine room cover 11. At this time, the machine room cover 11 is hermetically attached to the opening edge of the machine room 7 at its peripheral edge, and has an elongated rectangular heat dissipation opening extending in the vertical direction at the left edge in the figure. The portion 11a is formed. In other words, when the machine room cover 11 is attached, the machine room 7 has a heat dissipation opening.
It remains closed except for 11a. The machine chamber cover 11 is made of a material (for example, metal such as iron) that has excellent thermal conductivity and large sound transmission loss.

Further, in FIG. 4, reference numeral 12 is a control sound receiver, for example, a microphone arranged in the machine room 7, which is on the opposite side of the compressor 8 from the heat dissipation opening 11a (right side in the figure). Are arranged so as to face each other, so that the sound from the compressor 8, which is a noise source, is converted into an electric signal. Reference numeral 13 denotes a speaker, which is a control sound generator arranged in the machine room 7, and is located, for example, in a region near the heat dissipation opening 11a in the back wall portion of the machine room 7 (corresponding to the bottom wall portion of the refrigerator body 1). It is mounted and supported in a buried form.

Then, as shown in FIG. 6, the speaker 13 is operated by a signal obtained by processing the electric signal from the microphone 12 by the calculator 14, and the processing of the electric signal as described above is performed as follows. This is performed based on the silencing principle by active control as described in the section (conventional example).

Here, in the case of the refrigerator configured as described above, the level of noise generated in the machine room 7 in response to the driving of the compressor 8 has a band of about 700 Hz or less and 1.5 to 5 KHz as shown in FIG. It becomes a state that it has the property of becoming larger in each band. Of the noises corresponding to each of these bands, the noise on the high frequency side can be attenuated by the transmission loss in the machine room cover 11 or the like, and can be easily provided by installing an appropriate sound absorbing member in the machine room 7. Since the noise can be silenced, the active control of noise by the microphone 12, the speaker 13, and the calculator 14 as described above may be performed with a target frequency of 700 Hz or less.

Further, in the case of performing the active control of the noise as described above, the noise in the machine room 7 may be configured to be a one-dimensional plane traveling wave, which is theoretically and technically controlled. It becomes important for easy and accurate operation.
Therefore, in the present embodiment, each dimension D in the depth, width and height directions, which is the three-dimensional direction in the machine room 7 shown in FIG. 5,
Of W and H, for example, the dimension W in the width direction is set larger than the other dimensions D and H (specifically, W = 600 mm, D = H = 200 mm
By setting), the standing wave of the sound in the machine room 7 is constituted only in the primary mode. That is, for example, assuming that the machine room 7 is a rectangular cavity,
The following equation holds.

Here, f is the resonance frequency (Hz), Nx, Ny, Nz are the th modes in the X, Y, Z directions, and Lx, Ly, Lz are the dimensions (D, D in the machine room 7) in the X, Y, Z directions. W, H) and C are sound speeds. Therefore, from the above equation,
The frequency of the first standing wave fx, fy, fz for each X, Y, Z direction
Can be asked.

That is, as described above, when the depth dimension D = 200 mm, the width dimension W = 600 mm, and the height dimension H = 200 mm are established, the frequency fx of the first standing wave in the X direction is Ny =
Nz = 0, sound velocity C = 340 m / sec, Similarly, the frequencies fy and fz of the first standing wave in the Y and Z directions are Becomes As a result, the target frequency (= 700Hz)
In the following, the standing wave of noise in the machine room 7 is established only in the Y-direction (width direction) mode.
The noise inside can be regarded as a one-dimensional plane traveling wave. Therefore, at the time of noise reduction by active control of noise using the speaker 13 or the like, theoretical treatment of the wavefront becomes easy, and the noise reduction control can be performed easily and accurately.

Now, in the following, a method of measuring the transfer function G of the arithmetic unit 14 necessary for the active control as described above will be described with reference to FIGS. 1 and 2. That is, in FIG.
In the machine room of the refrigerator to be measured, the compressor 8,
In addition to the microphone 12 and the speaker 13, an auxiliary microphone 15 as a sound receiver for measurement is provided to monitor the sound at the heat dissipation opening 11a, which is a control target point during active control. Further, the power source of the compressor 8 is connected so as to be obtained from the inverter device 16 which is a variable frequency power source, so that the rotation speed of the compressor 8 can be continuously adjusted by the inverter device 16. Further, 17 is a noise signal generating circuit, which is provided so as to generate a white noise signal having a similar power over the entire frequency bandwidth to be measured. And, 18 is a transfer function measuring instrument using, for example, a fast Fourier transform (FFT) by a CPU,
It has an input signal terminal Ta and an output signal terminal Tb, and measures the transfer function between the input signal and the output signal (between the terminals Ta and Tb) based on the signals input to these terminals. There is.

Here, the data necessary for determining the transfer function G of the calculator 14 is, as is clear from the equation (1) shown in the section (conventional example), the data between the speaker 13 and the microphone 12. 1 acoustic transfer function G _AM , speaker 13 and auxiliary microphone
A second acoustic transfer function G _AO between the compressor 8 and the microphone 12, a third acoustic transfer function G _SM between the compressor 8 and the microphone 12, and a fourth acoustic transfer function G _SO between the compressor 8 and the auxiliary microphone 15. Is. At this time, as is clear from the equation (2) shown in the section of (Conventional example), the equivalent acoustic transfer function G with respect to the third and fourth acoustic transfer functions G _SM and G _SO.
OM , that is, the equivalent acoustic transfer function G _OM with the output signal from the auxiliary microphone 15 as the input side and the output signal from the microphone 12 as the output side (response side) is measured, and these acoustic transfer functions G _SM , It is equivalent to measuring G _SO . Therefore, the transfer function measuring device 18 is used to measure the first and second acoustic transfer functions G
_It suffices to measure _AM , G _AO and the equivalent acoustic transfer function G _OM .

When measuring the first acoustic transfer function G _AM , the white noise signal from the noise signal generation circuit 17 is input to the speaker 13 and the input signal terminal Ta of the transfer function measuring device 18, and the microphone is used. The output signal from 12 is connected so as to be input to the output signal terminal Tb of the transfer function measuring device 18, and the measurement data by the transfer function measuring device 18 when the noise signal generating circuit 17 is driven in such a connection state As a first acoustic transfer function GAM. When measuring the second acoustic transfer function G _AO , the connection function between the noise signal generating circuit 17 and the speaker 13 and the input signal terminal Ta of the transfer function measuring device 18 is left unchanged and the transfer function is kept. Connected so that the output signal from the auxiliary microphone 15 is input to the output signal terminal Tb of the measuring device 18, and by the transfer function measuring device 18 when the noise signal generating circuit 17 is driven in such a connection state. The measurement data is obtained as the second acoustic transfer function G _AO .
It is needless to say that the compressor 8 is stopped at the time of measuring these first and second acoustic transfer functions G _AM and G _AO .

On the other hand, when measuring the equivalent acoustic transfer function G _OM , the output signal from the auxiliary microphone 15 is input to the input signal terminal Ta of the transfer function measuring device 18, and the output signal from the microphone 12 is measured by the transfer function. It is connected so as to be input to the output signal terminal Tb of the instrument 18, and measurement is performed in this state by the procedure shown in the flowchart of FIG.

That is, the output frequency f of the inverter device 16 is set to the lower limit frequency f _0.
(For example, a frequency lower than the rated power supply frequency FR of the compressor 8 by a predetermined amount), and the compressor 8 is rotated at a rotation speed N _0.
Driven by (step a), the transfer function measuring instrument in this state
The measurement data by 18 is measured as the equivalent acoustic transfer function G _OM at the rotational speed N ₀ (step b), and then the output frequency f of the inverter device 16 is Δf (where Δf is equal to or lower than the frequency resolution of the transfer function measuring device 18). (Equation is set to)) and the compressor 8 is driven at a rotation speed N ₁ by sweeping (step c), and the measured data by the transfer function measuring device 18 after this sweep is converted into an equivalent acoustic transmission at a rotation speed N _1. It is measured as a function G _OM (step d). Then, after this, the sweep of the output frequency f and the measurement of the equivalent acoustic transfer function G _OM are repeatedly performed, and the inverter device
When the output frequency f of 16 reaches the upper limit frequency fn (for example, a frequency higher than the rated power supply frequency FR of the compressor 8 by a predetermined amount), the repeated measurement of the equivalent acoustic transfer function G _OM ends (step e). Next, of the measurement data acquired in steps b and d, the rated speed of the compressor 8
An integer multiple of NR NR × n (n is a natural number) and each measurement data group corresponding to a frequency FR × n that is an integral multiple of its rated power supply frequency FR are both validated, and linear interpolation of the validation data is executed. (Step f) Thus, the final measurement of the equivalent acoustic transfer function G _OM is completed.

Incidentally, in FIG. 7, the equivalent acoustic transfer function G is shown in (a) and (b).
_{In addition} to showing the schematic measurement data of _OM , (c) and (d) show the final equivalent acoustic transfer function G _OM data linearly interpolated based on the above measurement data.

In short, since the first and second acoustic transfer functions G _AM and G _AO are measured with the white noise signal having the predetermined frequency bandwidth input to the speaker 13, the measurement accuracy is in a wide frequency range. It will be good across. On the other hand, the equivalent acoustic transfer function G _OM (thus, the third and fourth acoustic transfer functions G _SM , G _SO ) is measured while the compressor 8 is driven at the predetermined power supply frequency f. Data other than the frequency band in which the noise spectrum associated with driving exists (frequency band corresponding to an integer multiple of the compressor rotation speed and an integer multiple of the power supply frequency) becomes inaccurate.

However, in this case, of the data of the acoustic transfer function G _OM measured as described above, the frequency NR × n that is an integral multiple of the rated speed NR of the compressor 8 and the integral multiple of the rated power supply frequency FR × n The equivalent acoustic transfer function G _{OM is} obtained by validating the measurement data groups corresponding to the frequencies, that is, the data groups in the frequency band in which the noise spectrum from the compressor 8 exists and linearly interpolating the valid measurement data.
Is obtained, the acoustic transfer function G _OM after the interpolation data is obtained.
Is relatively accurate even in a frequency band in which the noise spectrum from the compressor 8 is not distributed. Therefore, it is determined on the basis of the equivalent acoustic transfer function G _OM thus obtained and the first and second acoustic transfer functions G _AM and G _AO which have good measurement accuracy over a wide frequency band as described above. The transfer function G of the computing unit can be handled as an accurate measured value with improved accuracy over a wide frequency band that allows fluctuations in the rotation speed of the compressor 8. As a result, when performing active control of noise based on the transfer function G, active control is performed even when the rotational speed when the compressor 8 is actually operated is different from that when the transfer function G is measured. Therefore, there is no possibility that the sound deadening effect due to becomes insufficient as in the conventional case.

In the above embodiment, the acoustic transfer function G _OM data measured while sweeping the power supply frequency f of the compressor 8 corresponds to a frequency that is an integer multiple of the rotation speed of the compressor 8 and a frequency that is an integer multiple of the power supply frequency. Although both of the measured data groups are enabled and interpolation (linear interpolation) is performed, interpolation may be performed on at least one of the measured data groups (for example, one that has a large effect on the decrease in the accuracy of the transfer function G). Is.

Besides, the present invention is not limited to the embodiments described above and shown in the drawings. For example, the subject of noise reduction is not limited to the refrigerator, and an outdoor unit of an air conditioner or a refrigerated showcase may be applied. However, various modifications can be made without departing from the scope of the invention.

[Effects of the Invention] According to the present invention, as described above, the artificial sound from the control sounder operated by the signal processed by the arithmetic unit is generated from the sound generated by the driving of the compressor housed in the machine room. It is possible to improve the measurement accuracy of the transfer function of the arithmetic unit required when performing active control of actively canceling by interference with, over a wide frequency band that allows fluctuations in the rotational speed of the compressor, As a result, it is possible to obtain an excellent effect such that an optimum silencing effect can always be obtained even when the rotation speed of the compressor changes.

[Brief description of drawings]

1 to 7 show an embodiment of the present invention, and FIG. 1 is an arrangement diagram schematically showing a transfer function measuring method, and FIG.
FIG. 3 is a flow chart showing the contents of the above measuring method, FIG. 3 is a longitudinal sectional view of the refrigerator, FIG. 4 is a perspective view showing the essential parts of the refrigerator in an exploded state, and FIG. 5 is a dimensional relationship of the essential parts of the refrigerator. FIG. 6 is a schematic perspective view for doing so, FIG. 6 is a diagram schematically showing a configuration for active control of noise in the refrigerator, and FIG. 7 is a waveform diagram showing an example of measurement of an acoustic transfer function. Also,
FIG. 8 is a schematic configuration diagram showing a silencing principle by active control, and FIG. 9 is a diagram showing an example of noise level characteristics in a refrigerator. In the figure, 1 is a refrigerator main body, 7 is a machine room, 8 is a compressor, 10 is a defrosting water evaporator, 11 is a machine room cover, 11a is a heat dissipation opening, 12 is a microphone (control sound receiver), 13 Is a speaker (speaker for control), 14 is a calculator, 15 is an auxiliary microphone (sound receiver for measurement), 16 is an inverter device, 17 is a noise signal generating circuit, and 18 is a transfer function measuring device.

Claims (1)

[Claims] 1. A sound produced by driving a compressor provided in a machine room is converted into an electric signal by a control sound receiver, and the control sound is produced based on a signal processed by the arithmetic unit. In the method of measuring the transfer function of the arithmetic unit, which is used for active control of actively canceling the sound radiated from the machine room to the outside by operating the device, in the control target point by the active control, A measurement sound receiver for monitoring sound is provided, and a first acoustic transfer function between the control sounder and the control sound receiver and a second sound transfer function between the control sounder and the measurement sound receiver are provided. The acoustic transfer function is measured with the white noise signal having a predetermined frequency bandwidth input to the control sounder, and the third acoustic transfer function between the compressor and the control sound receiver and the compressor and the measurement sound function are measured. Between the receiver 4 of the acoustic transfer function, as well as measured in a state of driving the compressor at a predetermined power source frequency,
Among the measurement data obtained by this, at least one of the measurement data group corresponding to the frequency of the integral multiple of the rotation speed of the compressor and the frequency of the integral multiple of the power supply frequency is validated, and those valid measurement data are interpolated. Used for active control of noise, characterized in that it is configured to determine the transfer function of the arithmetic unit based on the third and fourth acoustic transfer functions interpolated into and the first and second acoustic transfer functions. Method of measuring the transfer function.