TWI794059B - Audio signal processing method and audio signal processing device - Google Patents

Abstract

An audio signal processing method includes: obtaining an audio frequency spectrum of an acoustic wave; determining a target frequency corresponding to a target acoustic pressure value higher than an acoustic pressure threshold value in the audio frequency spectrum; obtaining a frequency range corresponding to the target frequency and including an upper-frequency limit and a lower-frequency limit; determining whether a first acoustic pressure difference corresponding to the target frequency and the upper-frequency limit and a second acoustic pressure difference corresponding to the target frequency and the lower-frequency limit are greater than an acoustic pressure difference threshold value or not; recording the target frequency as an abnormal frequency when determining the first acoustic pressure difference and the second acoustic pressure difference are greater than the acoustic pressure difference threshold value; filtering the acoustic wave according to the abnormal frequency and a preset bandwidth.

Description

Sound processing method and sound processing device

The invention relates to the technical field of sound processing, in particular to a sound processing method and a sound processing device for judging abnormal peak frequencies in sound waveforms.

Recently, the efficiency of electronic products has gradually improved, and the heat generated by electronic products has also increased accordingly. Fans are required to dissipate heat when electronic products generate heat. Therefore, the quality of fans determines the smooth operation of electronic products. However, when the fan is running at high speed, it will generate noise and cause discomfort to users. Operators (such as engineers) need to conduct noise tests on the fans before shipment of electronic products (such as notebook computers, all-in-one computers, etc.) to confirm the performance of the fans. noise situation.

Currently, the fan noise test includes the fan noise objective listening test and the fan noise subjective listening test. The objective listening test and analysis of fan noise is done in an anechoic room, while the subjective listening test and analysis of fan noise is done in a conference room or other locations, resulting in the difference between the analysis results of objective listening of fan noise and the analysis results of subjective listening of fan noise. For example, the analysis result of objective listening of fan noise is no noise, and the analysis result of subjective listening of fan noise is uncomfortable noise.

According to the above, the present invention provides a sound processing method and a sound processing device, which analyze the sound spectrum of the sound waveform to find and filter the noise, which can help the operator to clarify the noise problem.

A sound processing method according to an embodiment of the present invention, comprising: obtaining a sound spectrum of a sound waveform; judging a target frequency corresponding to a target sound pressure value higher than a sound pressure critical value in the sound spectrum; obtaining a frequency corresponding to the target frequency Range, the frequency range includes the upper limit frequency and the lower limit frequency; judge the first sound pressure value difference corresponding to the target frequency and the upper limit frequency and the target frequency and Whether the second sound pressure value difference corresponding to the lower limit frequency is greater than the sound pressure difference critical value; when it is judged that both the first sound pressure value difference and the second sound pressure value difference are greater than the sound pressure difference critical value, the recording target frequency is the abnormal peak frequency ; Filter the sound waveform according to the abnormal peak frequency and preset bandwidth.

An audio processing device according to an embodiment of the present invention includes a recorder and a processor. The recorder records sound waveforms. The processor is connected to the recorder, and generates a sound spectrum according to the sound waveform, and the processor performs the following steps: judging the target frequency corresponding to the target sound pressure value higher than the sound pressure critical value in the sound spectrum; obtaining the frequency range corresponding to the target frequency, The frequency range includes the upper limit frequency and the lower limit frequency; judge whether the first sound pressure value difference corresponding to the target frequency and the upper limit frequency and the second sound pressure value difference corresponding to the target frequency and the lower limit frequency are greater than the sound pressure difference critical value; When both the sound pressure difference and the second sound pressure difference are greater than the sound pressure difference critical value, the recording target frequency is the abnormal peak frequency; and the sound waveform is filtered according to the abnormal peak frequency and the preset bandwidth.

In summary, the sound processing method and sound processing device of the present invention use the sound pressure critical value and the sound pressure difference critical value to judge the abnormal peak frequency and filter the sound waveform according to the abnormal peak frequency and the preset bandwidth. This improves the accuracy of judging the abnormal peak frequency, thereby improving the effect of filtering, and can assist operators to clarify noise problems. Furthermore, the sound processing device of the present invention uses a recorder to record the subjectively listened sound, and uses a processor to analyze the abnormal peak frequency of the recorded sound, so that real-time recording and sound quality analysis can be achieved.

1: Sound processing device

10:Recorder

20: Processor

P1~P4: peak point

S11~S17, S131~S132, S21~S23: steps

FIG. 1 is a functional block diagram of an audio processing device according to an embodiment of the present invention.

FIG. 2 is a flowchart of a sound processing method according to an embodiment of the present invention.

FIG. 3 is a flow chart of a step of obtaining a frequency range in a sound processing method according to an embodiment of the present invention.

FIG. 4 is a flow chart of defining a sound pressure difference threshold in a sound processing method according to another embodiment of the present invention.

FIG. 5 is a measurement interface of an audio processing device according to an embodiment of the present invention.

FIG. 6 is an audio filter playback program interface before filtering according to an embodiment of the present invention.

FIG. 7 is a program interface for filtering and playing back audio after filtering according to an embodiment of the present invention.

The detailed features and advantages of the present invention are described in detail below in the implementation mode, and its content is enough to make any person familiar with the related art understand the technical content of the present invention and implement it accordingly, and according to the content disclosed in this specification, the scope of the patent application and the drawings , anyone skilled in the art can easily understand the purpose and advantages of the present invention. The following examples are to further describe the concept of the present invention in detail, but not to limit the scope of the present invention in any way.

It should be understood that although the terms "first", "second" and the like may be used in the present invention to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections Should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer and/or section from another element, component, region, layer and/or section.

In addition, the terms "comprising" and/or "comprising" refer to the presence of stated features, regions, integers, steps, operations, elements and/or parts, but do not exclude one or more other features, regions, integers, steps, operations , the presence or addition of elements, parts and/or combinations thereof.

Please refer to FIG. 1 , which is a functional block diagram of an audio processing device according to an embodiment of the present invention. As shown in FIG. 1 , the audio processing device 1 includes a recorder 10 and a processor 20 . The recorder 10 records sound waveforms, and the sound waveforms can be the sound waveforms recorded by the device under test (for example, an electronic device with a fan such as a notebook computer and an all-in-one computer). The processor 20 can be connected to the recorder 10 in a wired or wireless manner, and generates a sound spectrum according to the sound waveform, and analyzes the sound spectrum to find and filter noise. The detailed action mechanism of finding and filtering noise will be described later. The processor 20 can be a central processing unit, a graphics processing unit or other types of processors, and the foregoing are merely examples, not limiting the scope of the present invention. Further, processor 20 may provide bandstop filtering wave function. In addition, the sound processing device 1 may further include a display. The display is connected to the processor 20 in a wired or wireless manner, and is used to display the measurement interface and the sound filter playback program interface, and the format of the interface will be described with an example later.

Please refer to FIG. 1 and FIG. 2 . FIG. 2 is a flowchart of a sound processing method according to an embodiment of the present invention. As shown in FIG. 2 , the sound processing method includes steps S11 to S17. The sound processing method shown in FIG. 2 is applicable to the sound processing device 1 shown in FIG. 1 , but is not limited thereto. The operation of the sound processing device 1 shown in FIG. 1 is exemplarily described below as step S11 to step S17.

Step S11: Obtain the sound spectrum of the sound waveform. Specifically, the processor 20 obtains a sound waveform from the recorder 10, and generates a sound spectrum according to the sound waveform.

Step S12: Determine the target frequency corresponding to the target sound pressure value higher than the sound pressure critical value in the sound spectrum. Specifically, the sound spectrum includes the correspondence between a plurality of frequencies and sound pressure values (especially a one-to-one relationship), for example, a plurality of frequencies corresponding to a plurality of frequencies within the range of 100 to 20,000 Hz (Hz). The pressure value, the sound spectrum can be a Fourier transform spectrum or a Prominence ratio spectrum. The sound pressure threshold is pre-stored in the memory of the processor 20 . Processor 20 compares each sound pressure value with a sound pressure threshold. If the sound pressure value is greater than the sound pressure critical value, the processor 20 judges that the sound pressure value is the target sound pressure value and uses its corresponding frequency as the target frequency; if the sound pressure value is not greater than the sound pressure critical value, the processor 20 No action or the frequency corresponding to the recorded sound pressure value is not the target frequency. Specifically, the frequency points included in the salient rate spectrum and the frequency points included in the Fourier spectrum are different from each other, and the sound pressure threshold set by the salient rate spectrum and the sound pressure threshold set by the Fourier spectrum are also different from each other. . For example, the sound pressure threshold applicable to the prominence rate spectrum can be set to 6dB, and the sound pressure threshold applied to the Fourier spectrum can be set to 50dB, but the present invention is not limited thereto.

Step S13: Obtain a frequency range corresponding to the target frequency, the frequency range includes an upper limit frequency and a lower limit frequency. Specifically, the processor 20 sets the upper limit frequency and the lower limit frequency according to the target frequency, and sets the frequency range according to the upper limit frequency, the target frequency and the lower limit frequency. Yu Sheng In an embodiment where the audio spectrum is a Fourier transformed spectrum, the frequency range includes the target frequency. Furthermore, the processor 20 may use the target frequency as the center frequency, and take the frequency of plus or minus N Hz of the center frequency as the upper and lower limit frequencies, where N is, for example, between 10 and 20.

In an embodiment where the sound spectrum is a salient rate spectrum, the processor 20 first converts the target frequency to obtain a conversion frequency, and then obtains a frequency range according to the conversion frequency. Please refer to FIG. 3 further. FIG. 3 is a flow chart of the step of obtaining the frequency range in the sound processing method according to an embodiment of the present invention. As shown in FIG. 3 , the step of obtaining the frequency range may include steps S131 and S132. The operation of the sound processing device 1 shown in FIG. 1 is exemplarily described below as step S131 and step S132.

Step S131: Converting the converted frequency corresponding to the converted target frequency in the Fourier spectrum of the sound waveform. Specifically, the processor 20 converts the target frequency in the prominence frequency spectrum into a frequency in the Fourier spectrum as the conversion frequency. The psychoacoustic prominence rate is based on 1/12 octave band as the bandwidth analysis, so the corresponding frequency in the Fourier spectrum can be inversely deduced from the frequency in the prominence rate spectrum within the bandwidth, and the sound pressure can be automatically captured value.

Step S132: Obtain a frequency range according to the conversion frequency. Specifically, the processor 20 sets the upper limit frequency and the lower limit frequency according to the conversion frequency, the conversion frequency is between the upper limit frequency and the lower limit frequency, and the processor 20 sets the frequency range according to the upper limit frequency, the conversion frequency and the lower limit frequency. Furthermore, the processor 20 may convert the frequency into a center frequency, and take a frequency of plus or minus N Hz of the center frequency as the upper and lower limit frequencies, where N is, for example, between 10 and 20.

In particular, the sound pressure level of some noises is not high, and there will be no peaks exceeding the critical value of the sound pressure on the Fourier transform spectrum, but it will bring uncomfortable feelings to the user, because these noises contain There are many prominent pure tone components, and the prominence rate is a parameter used to evaluate whether these components are prominent. Therefore, through the above implementation of searching the target frequency from the prominence frequency spectrum, the effect of subsequent noise removal can be improved. In yet another implementation, the processor 20 may simultaneously search the target frequency from the Fourier transform spectrum and the salient rate sound spectrum, and perform subsequent steps on each target frequency value.

Step S14: judging whether the first sound pressure difference corresponding to the target frequency and the upper limit frequency and the second sound pressure difference corresponding to the target frequency and the lower limit frequency are greater than the sound pressure difference critical value. Specifically, in the embodiment where the sound spectrum is a Fourier transform spectrum, the processor 20 calculates the difference between the target sound pressure value corresponding to the target frequency and the sound pressure value corresponding to the upper limit frequency as the first sound pressure value difference, and calculate the difference between the target sound pressure value corresponding to the target frequency and the sound pressure value corresponding to the lower limit frequency as the second sound pressure value difference, and then judge the first sound pressure value difference and the second sound pressure value difference Whether they are all greater than the critical value of sound pressure difference. The above-mentioned sound pressure values corresponding to each frequency are sound pressure values corresponding to each frequency in the Fourier transform spectrum.

In the embodiment where the sound spectrum is the salient rate spectrum, the processor 20 calculates the difference between the sound pressure value corresponding to the conversion frequency converted from the target frequency and the sound pressure value corresponding to the upper limit frequency as the first sound pressure value difference, and calculate the difference between the sound pressure value corresponding to the conversion frequency and the sound pressure value corresponding to the lower limit frequency as the second sound pressure value difference, and then judge whether the first sound pressure value difference and the second sound pressure value difference Both are greater than the critical value of sound pressure difference. The above-mentioned sound pressure values corresponding to each frequency are sound pressure values corresponding to each frequency in the Fourier transform spectrum.

When the processor 20 judges that the first sound pressure difference and the second sound pressure difference are greater than the sound pressure difference critical value, continue to step S15; when the processor 20 judges that the first sound pressure difference or the second sound pressure difference is not If it is greater than the sound pressure difference threshold, go to step S17.

Step S15: Record the target frequency as the abnormal peak frequency. Specifically, when the processor 20 judges that both the first sound pressure difference and the second sound pressure difference are greater than the sound pressure difference critical value, the processor 20 judges that the target frequency corresponds to an abnormal peak, and records the target frequency as abnormal peak frequency.

Step S16: Filter the sound waveform according to the abnormal peak frequency and the preset bandwidth. Specifically, after obtaining the abnormal peak frequency, the processor 20 sets the upper boundary frequency and the lower boundary frequency of the filter according to the preset bandwidth, and uses the frequency range composed of the upper boundary frequency and the lower boundary frequency as the filtering range to filter the sound waveform . Wherein, the filtering performed by the processor 20 on the sound waveform may be band-stop filtering, high-pass filtering or low-pass filtering, especially band-stop filtering. The preset bandwidth can be pre-stored in the processor 20, and can be set as plus or minus N Hz of the abnormal peak frequency, where N is, for example, between 10 and 20. In one implementation, the processor 20 automatically filters the sound waveform after obtaining the filtering range. In another embodiment, the processor 20 presents the filtering range to the operator through a display,

Step S17: Do not act or record the target frequency as the normal peak frequency. Specifically, when the processor 20 judges that the first sound pressure difference or the second sound pressure difference is not greater than the sound pressure difference critical value, the processor 20 judges that the target frequency is a normal peak frequency, and may not act or record the target frequency. The frequency is the normal peak frequency.

In addition, in addition to the above steps, the sound processing method may further include: when it is determined that the target frequency is a multiple of the rotation frequency of the fan, it is determined that the fan is the source of the abnormal sound. This step can be performed after step S15. Specifically, the processor 20 calculates and judges whether the abnormal peak frequency is a multiple of the rotational frequency of the fan. When it is determined that the abnormal peak frequency is a multiple of the rotational frequency of the fan, the processor 20 determines that the abnormal peak frequency is related to the rotational frequency of the fan It is judged that the fan is a source of abnormal sound; when it is judged that the abnormal peak frequency is not a multiple of the rotational frequency of the fan, the processor 20 judges that the abnormal peak frequency has nothing to do with the rotational frequency of the fan and the abnormal peak frequency may be caused by other abnormal sound sources (such as motor vibration) . This step can assist the operator (such as an engineer) to clarify the source of the abnormal sound.

In another embodiment, the sound processing method may further include a step of defining a critical value of the sound pressure difference in addition to the steps described in the foregoing embodiments. Please refer to FIG. 1 and FIG. 4 . FIG. 4 is a flow chart of defining a sound pressure difference critical value in a sound processing method according to another embodiment of the present invention. As shown in Figure 4, the step of defining the critical value of the sound pressure difference may include Step S21~Step S23, and Step S21~Step S23 may be executed at any point in time before Step S14 shown in Figure 2, and Step S21 and Step S22 The execution sequence of the two steps can be reversed to that shown in FIG. 4 or executed simultaneously. The step of defining the critical value of the sound pressure difference shown in FIG. 4 is applicable to the sound processing device shown in FIG. 1 , but is not limited thereto. The operation of the sound processing device 1 shown in FIG. 1 is exemplarily described below as step S21 to step S23.

Step S21: Obtain multiple first sound pressure values corresponding to multiple first frequency points greater than the upper limit frequency. Specifically, the processor 20 calculates the estimated The upper limit frequency, and then obtain multiple first frequency points between the upper limit frequency and the estimated upper limit frequency and corresponding multiple first sound pressure values. For example, the upper limit frequency is 538 Hz, the frequency resolution is 3.125 Hz, the number of first frequency points is set to 10, the estimated upper limit frequency is 600.5 Hz, and the processor 20 obtains 10 first frequencies between 538 Hz and 600.5 Hz The 10 first sound pressure values corresponding to the points.

Step S22: Obtain multiple second sound pressure values corresponding to multiple second frequency points less than the lower limit frequency. Specifically, the processor 20 calculates the estimated lower limit frequency based on the lower limit frequency and the frequency resolution, and then obtains multiple second frequency points between the lower limit frequency and the estimated lower limit frequency and their corresponding multiple second sound pressures value. For example, the lower limit frequency is 518 Hz, the frequency resolution is 3.125 Hz, the number of second frequency points is set to 10, the estimated lower limit frequency is 455.5 Hz, and the processor 20 obtains 10 second frequencies between 455.5 Hz and 518 Hz The 10 second sound pressure values corresponding to the points.

Step S23: Use the average value of the first sound pressure value and the second sound pressure value as the sound pressure difference threshold. Specifically, the processor 20 averages the first sound pressure value and the second sound pressure value to obtain an average value, and uses the average value as the sound pressure difference threshold. For example, the processor 20 averages 10 first sound pressure values and 10 second sound pressure values to obtain an average value as the sound pressure difference threshold.

As mentioned above, in one embodiment, the sound processing device can provide a measurement interface and a sound filter playback program interface. Please refer to FIGS. 5-7 , wherein FIG. 5 is a measurement interface of a sound processing device according to an embodiment of the present invention, and FIGS. 6 and 7 are respectively according to an embodiment of the present invention before performing filtering. And the filtered sound playback program interface.

As shown in FIG. 5 , the measurement interface includes a plurality of sound-related parameter setting fields, a file path setting field, a record time field, a start recording button, and a recording stop button. The operator can first set the relevant sound parameters, and then enter the storage path and recording seconds, press the start recording button to start recording, and stop recording at any time by pressing the stop recording button.

Fig. 6 and Fig. 7 exemplary present the sound processing method described in preceding embodiment Spectrograms before and after processing. As shown in Figure 6, the Fourier transform spectrum shows the peaks P1 and P2 whose sound pressure is 50dB higher than the sound pressure critical value, corresponding to the target frequencies 528Hz and 1056Hz respectively, and the prominence rate spectrum shows the peaks whose sound pressure is 6dB higher than the sound pressure critical value P3 and P4 correspond to target frequencies of 485.766 Hz and 1029 Hz, respectively. Wherein, 528 Hz of the Fourier transform spectrum corresponds to 485.766 Hz of the prominence rate spectrum, and 1056 Hz of the Fourier transform spectrum corresponds to 1029 Hz of the prominence rate spectrum. With the aforementioned sound processing method using the Fourier transform spectrum as the sound spectrum, the sound processing device can determine that the sound pressure difference between the target frequency 528 Hz and 1056 Hz is greater than the sound pressure difference critical value, and it can be judged as an abnormal peak frequency. With the aforementioned sound processing method using the prominence frequency spectrum as the sound spectrum, the sound processing device can convert the target frequencies 485.766 Hz and 1029 Hz into 528 Hz and 1056 Hz respectively, and then judge them as abnormal peak frequencies.

As shown in Figure 7, the sound processing device can automatically fill in the frequency of the abnormal peak frequency plus or minus 10 Hz in the lower boundary frequency (Low Freq.) and upper boundary frequency (Up Freq.) of the filter as the filter bandwidth, and the filter setting for band resistance. From the Fourier transform spectrogram in Figure 7, it can be seen that the peak waves at 528Hz and 1056Hz have disappeared, and the sound pressure values corresponding to 485.766Hz and 1029Hz on the prominence rate spectrogram have also dropped to a value close to zero, indicating uncomfortable The sound has disappeared.

In summary, the sound processing method and sound processing device of the present invention use the sound pressure critical value and the sound pressure difference critical value to judge the abnormal peak frequency and filter the sound waveform according to the abnormal peak frequency and the preset bandwidth. This improves the accuracy of judging the abnormal peak frequency, thereby improving the effect of filtering, and can assist operators to clarify noise problems. Furthermore, the sound processing device of the present invention uses a recorder to record the subjectively listened sound, and uses a processor to analyze the abnormal peak frequency of the recorded sound, so that real-time recording and sound quality analysis can be achieved.

Although the present invention is disclosed above with the foregoing embodiments, they are not intended to limit the present invention. Without departing from the spirit and scope of the present invention, all changes and modifications are within the scope of patent protection of the present invention. For the scope of protection defined by the present invention, please refer to the appended scope of patent application.

S11~S17: Steps

Claims (10)

A sound processing method, comprising executing with a processor: obtaining a sound spectrum of a sound waveform; judging a target frequency in the sound spectrum corresponding to a target sound pressure value higher than a sound pressure critical value; obtaining a target frequency corresponding to the sound pressure threshold A frequency range of the target frequency, the frequency range includes an upper limit frequency and a lower limit frequency; judging a first sound pressure value difference corresponding to the target frequency and the upper limit frequency and a first sound pressure value corresponding to the target frequency and the lower limit frequency Whether the difference between the two sound pressure values is greater than the threshold value of the sound pressure difference; when it is judged that the difference between the first sound pressure value and the second sound pressure value difference is greater than the critical value of the sound pressure difference, record the target frequency as an abnormal peak frequency ; and filtering the sound waveform according to the abnormal peak frequency and a preset bandwidth. The sound processing method as described in Claim 1, wherein the sound spectrum is a salient rate spectrum, and obtaining the frequency range corresponding to the target frequency includes: converting the target frequency corresponding to a Fourier spectrum of the sound waveform switching frequency; and The frequency range is obtained according to the conversion frequency; wherein the first sound pressure value difference is the difference between the sound pressure value corresponding to the conversion frequency and the sound pressure value corresponding to the upper limit frequency, and the second sound pressure value difference is the difference between the sound pressure value corresponding to the conversion frequency and the sound pressure value of the lower limit frequency. The sound processing method as described in Claim 1 further includes: when it is judged that the target frequency is a multiple of a rotation frequency of a fan, it is judged that the fan is a source of abnormal sound. The sound processing method as claimed in claim 1, wherein the filtering performed on the sound waveform is band-stop filtering. The sound processing method as described in Claim 1, further comprising: obtaining a plurality of first sound pressure values corresponding to a plurality of first frequency points greater than the upper limit frequency; obtaining a plurality of second frequency points lower than the lower limit frequency a plurality of corresponding second sound pressure values; and using an average value of the first sound pressure values and the second sound pressure values as the sound pressure difference threshold. A sound processing device comprising: a recorder for recording a sound waveform; and A processor, connected to the recorder, and generates a sound spectrum according to the sound waveform, the processor performs the following steps: judging a target frequency corresponding to a target sound pressure value higher than the sound pressure critical value in the sound spectrum ; Obtain a frequency range corresponding to the target frequency, the frequency range includes an upper limit frequency and a lower limit frequency; judge a first sound pressure value difference corresponding to the target frequency and the upper limit frequency and the target frequency and the lower limit frequency Whether a corresponding second sound pressure value difference is greater than the sound pressure difference critical value; when it is judged that the first sound pressure value difference and the second sound pressure value difference are greater than the sound pressure difference critical value, record the target frequency is an abnormal peak frequency; and filtering the sound waveform according to the abnormal peak frequency and a preset bandwidth. The sound processing device as described in claim 6, wherein the sound spectrum is a salient rate spectrum, and the processor converts a conversion frequency corresponding to the target frequency in a Fourier spectrum of the sound waveform and obtains the conversion frequency according to the conversion frequency. frequency range, the first sound pressure value difference is the sound pressure value corresponding to the conversion frequency and the upper The second sound pressure difference is the difference between the sound pressure value corresponding to the conversion frequency and the sound pressure value corresponding to the lower limit frequency. The sound processing device as described in claim 6, wherein the processor further determines that the target frequency is a multiple of a rotation frequency of a fan, and determines that the fan is a source of abnormal sound. The audio processing device as claimed in claim 6, wherein the filtering performed by the processor on the audio waveform is band-stop filtering. The sound processing device as described in Claim 6, wherein the processor further obtains a plurality of first sound pressure values corresponding to a plurality of first frequency points greater than the upper limit frequency, and obtains a plurality of second sound pressure values lower than the lower limit frequency A plurality of second sound pressure values corresponding to the frequency points, and an average value of the first sound pressure values and the second sound pressure values is used as the sound pressure difference critical value.

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