JP2017076949A - Method of generating reverberation sound in consideration of air absorption of sound - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of generating a reverberation sound in consideration of a fact that the larger the concert hall is, the higher the frequency components of a reflected sound that are lost are and generating a reverberation sound that is reproduced by a sound field of the concert hall generated by a sum of sounds of main speakers L and R.SOLUTION: An output signal (left) and an output signal (right) obtained by approximately generating reverberation sound signals by reducing with a lowpass filter of 12 dB/oct high frequency components of difference components L-R and R-L between an audio signal L on the left and an audio signal R on the right of a stereo signal.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method of approximately generating a reverberant signal from a difference component between a left audio signal (L) and a right audio signal (R) of a stereo signal using a 12 dB / oct low-pass filter. is there.

  There are many surround systems, but sound air absorption is not considered.

  Actual application 2015-2153

  A component LR (RL) of the difference between the left audio signal (L) and the right audio signal (R) of the stereo signal is considered to be a normal reflected sound. At the same time, the difference component L-R (R-L) contains many direct sounds. However, since the sound is absorbed by the air as the component has a higher frequency and the distance increases, the higher the frequency component is lost in the reflected sound, the larger the performance venue. Therefore, it is necessary to moderately attenuate the high frequency component of LR (RL) according to the size of the performance hall.

  It is generally considered that sound is absorbed by air in proportion to the square of the frequency. Therefore, if the high band is attenuated with a 12 dB / oct low-pass filter that attenuates in proportion to the square of the frequency, it is considered that the sound is more like reverberation. In addition, by making the cut-off frequency of the low-pass filter variable, it is possible to realize high-frequency attenuation that matches the size of the performance venue. FIG. 1 shows a reverberation sound obtained by attenuating a high frequency range of a difference component LR (RL) between a left audio signal (L) and a right audio signal (R) of a stereo signal with a 12 dB / oct low-pass filter. 2 represents the present invention for approximately generating the following signal.

  As a result of repeated trial listening, the optimum cut-off frequency for generating a reverberation sound signal at a performance venue with a reverberation time T (seconds) is a time Tc required for the sound of that frequency to be attenuated by 3 dB by air absorption, Tc≈0. It can be determined to be 25T (seconds). At this time, the distance required for the sound having the same frequency as the cut-off frequency to be attenuated by 3 dB by air absorption is 340 Tc (m) when the sound speed is 340 (m / sec). The cut-off frequency can be determined by calculating back from the data (for example, JIS Z 8378). FIG. 3 shows the cutoff frequency obtained by this method when the reverberation time T (seconds) is 1.2 (seconds), 2.0 (seconds), and 3.6 (seconds).

The present invention has the following effects.
(1) The difference component LR (RL) between the L and R of the stereo signal can be made closer to reverberant sound by attenuating the high range using a 12 dB / oct low-pass filter.

(2) The time of Tc≈0.25 T (seconds) is the time required to reach about 97% of the steady state in the sound field growth process, and it is considered that a dispersed sound field is generated approximately. (The growth rate of the energy density of the sound field can be expressed as 1-exp (−13.8 t / T) using time t and reverberation time T, but 0.97 is obtained by substituting t = 0.25T. Can be obtained.)

(3) The sound field of the performance venue can be reproduced by the sum of the reverberant sound generated by the system using the present invention and the sounds of the main speakers L and R.

The figure showing the present invention using the generation of the difference component LR (RL) of the left audio signal (L) and the right audio signal (R) of the stereo signal and the low-pass filter of 12 dB / oct. is there. It is the data of air absorption in the case of temperature 20 degree | times and humidity 50% calculated from "calculation of the air absorption in the propagation of an outdoor sound (JIS Z 8378)". However, the values in parentheses are for a temperature of 25 degrees and a humidity of 60%. It is a table | surface which shows the magnitude | size and the reverberation time T (second) of a performance hall, and the cutoff frequency of 12 dB / oct low-pass filter corresponding to it. It is an example of the circuit which implements this invention using a headphone amplifier, a trans | transformer, and a low-pass filter. It is a table | surface which shows the value of L (mH) of the low-pass filter of Example 1, and C (micro F), the cutoff frequency corresponding to it, the magnitude | size of a performance hall, and reverberation time T (second). Compare the frequency characteristics of the low-pass filter when the cut-off frequency is about 3 kHz and the frequency characteristics of air absorption when the sound advances about 170 m (after about 0.5 seconds) when the temperature is 20 degrees and the humidity is 50%. FIG. It is an example of composition of a speaker using the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 4 is an example for easily realizing the present invention, which uses a circuit that generates a difference component LR (RL) from L and R of a stereo signal using a transformer, a coil, and a capacitor. This circuit generates a reverberant signal approximately from L and R of a stereo signal by combining a low-pass filter of 12 dB / oct. An output transformer (SANSUIST-32) having a primary impedance of 1200Ω and a secondary impedance of 8Ω is used as the transformer, and the output of the headphone amplifier is used in order to extract the performance. In this embodiment, the values of L (mH) and C (μF) can be switched between three levels. FIG. 5 shows three levels of L (mH) and C (μF) values used in this example and the cut-off frequencies at that time. Corresponds to time T). In addition, the resistance value of the low-pass filter is set to be a quality factor Q≈0.7 (Butterworth filter) for simplicity of design, but the air absorption characteristics are sufficiently approximated as shown in FIG. doing. FIG. 7 shows the configuration of the speaker used in this embodiment, but the LR (R−L) speaker is effective even when only the rear side or the front side is sounded.

L Audio signal to the left of the stereo signal.
R Audio signal to the right of the stereo signal.
T (seconds) Reverberation time at the performance venue.
Tc (seconds) Time required for a sound having the same frequency as the cutoff frequency to be attenuated by 3 dB due to air absorption.
L (mH) The inductance of the low-pass filter coil.
C (μF) Capacitance of the low-pass filter capacitor.

Claims (2)

  The difference component LR (RL) of the difference between the left audio signal (L) and the right audio signal (R) of the stereo signal is attenuated by a 12 dB / oct low-pass filter to approximate a reverberant signal. To create reverberant sound by generating it and playing it from a speaker through an amplifier.   2. The method according to claim 1, wherein when a reverberation sound signal of a performance venue with a reverberation time T (seconds) is generated, a sound having the same frequency as the cut-off frequency is absorbed by air when determining the cut-off frequency of the low-pass filter of 12 dB / oct. Therefore, when the sound speed is 340 (m / sec), the sound having the same frequency as the cut-off frequency is attenuated by 3 dB due to air absorption so that the time Tc required for 3 dB attenuation at Tc becomes 0.25 T (seconds). The cutoff frequency is determined by calculating backward from sound air absorption data (for example, JIS Z 8378) so that the distance required for this is 340 Tc (m).

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