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TECHNICAL FIELD The present invention relates to a sound field correction device, and more
particularly to a device that corrects a sound field at a predetermined listening point in a sound
field produced by a direct radiation sound from a speaker and its reflection sound.
BACKGROUND ART As an example of an acoustic device, there is one as shown in FIG. In the
figure, a reproduction signal from the magnetic tape reproduction head 1 is amplified by a
reproduction amplifier 2 and supplied to a frequency characteristic adjustment amplifier 3. The
output of the amplifier 3 amplifies the power by the speaker drive amplifier 4 and drives the
speaker 5. In such a configuration, the signal obtained by the reproducing head 1 is converted
into a signal having a normal frequency characteristic by the reproducing amplifier 2 and
amplified, and the frequency characteristic variable amplifier such as the graphic equalizer in the
next stage, the bus / treple control, etc. 3 to adjust the desired reproduction frequency
characteristic to operate as a speaker drive signal. In such an acoustic system, the path of the
sound emitted from the speaker 5 to reach a predetermined listening point 6 in the sound field
shown in FIG. 2 is, for example, a direct sound as indicated by a solid line 7 and a dotted line 8
There is a reflected sound from a wall or the like as shown. In this case, at the listening point 6, a
peak or dip occurs at a certain frequency due to mutual interference of sounds by the both, and
disturbance occurs in the frequency characteristic at the listening point 6. Therefore, the
disturbance of the frequency is corrected by the graphic equalizer 3 or the like. Here, the
principle number characteristics that cause peaks and dips related to frequency characteristics
will be briefly described below. In FIG. 2, assuming that the distance to the listening point 6 of
the direct pass 7 is l1, that of the reflected pass 8 is l2, and the sound speed is c, the listening
point sound SM is the direct sound SS and the reflected sound SR. It is expressed by the sum of
SM = SS + SR = (A / l1) ejω (t-l1 / c) + (KA / l2) ejω (t-l2 / c) (1) where K is the reflectivity of the
wall , A is the signal strength at the vibration plane of the speaker. The reflected sound SR is
considered to be fixed-end reflection in most cases, so the acoustic impedance is large, and it may
be considered that reflection occurs in the same phase (the acoustic impedance is small in the
case of the free-end reflection, SM = SS-SR). Here, the following equation is obtained by
modifying the equation (1). SM = (A / l1) {ej? (T-l1 / c) + Bej? (T-l2 /) (2) where B = Kl1 / l2. Now,
assuming that B = for simplicity, the frequency at which the peak occurs is f = nc / (12−11) (3),
and the frequency at which the dip occurs is f = (n + 1⁄2) C) (12-11) ... (4).
n is an integer. Although FIG. 3 shows the results of the equations (3) and (4), since B ≠ 1 in
practice, the sound pressure at the dip frequency can not be zero, and also at the peak frequency.
SM can not be 2 A / l1. The dotted lines in the figure indicate the case of free end reflection for
reference. From the above, it can be seen that the disturbance of the frequency characteristic due
to the interference of the reflected sound is caused by the difference in the distance between the
two sounds. Therefore, when the installation state of the device or the listening position changes,
the frequency of the peak or dip changes, so, for example, when trying to correct it with the
graphic equalizer 3, a large number of circuits are required and the adjustment is very difficult. It
will be difficult. Furthermore, it is impossible to compensate for peak and dip in an apparatus
having only a bass treble control function. SUMMARY OF THE INVENTION The present invention
has been made to eliminate the disadvantages of the prior art as described above, and the object
of the present invention is to characterize the original signal and the original signal in order to
eliminate the reflected sound in the sound field. An object of the present invention is to provide a
sound field correction device in which the frequency characteristics of the sound in the sound
field are made uniform by superimposing the changed signals on each other to make a speaker
signal. The sound field correction apparatus according to the present invention adjusts the level
and phase of the original signal and mixes it into the original signal to drive the speaker, thereby
eliminating the influence of the reflected sound at a predetermined listening point in the sound
field. It is a structure. Embodiments Embodiments of the present invention will be described
below with reference to the drawings. FIG. 4 is a block diagram of the embodiment of the present
invention. The original signal input is delayed in the delay unit 9 and applied to the level adjuster
10. The level-controlled signal is input to the inverter 11 and the buffer 12, and the output of the
inverter 11 and the buffer 12 is alternatively selected and derived at the switch 13. The output of
the switch 13 and the original signal are added by the adder 14 to become a speaker driving
signal output. Assuming that the reflector makes fixed-end reflection in the sound field shown in
FIG. 2, the switch 13 selects the output of the inverter 11. Now, assuming that the original signal
S1 is S1 = Aejωt, the delay time by the delay unit 9 is − (12−11) / c, and the adjustment level
by the level adjuster 10 is KAl1 / l2, the selection output SD by the switch 13 is SD =-(KAl1 / l2)
ejω (t- (l2-l1) / c) (5). Therefore, the output S0 of the adder 14 is expressed as S0-S1 + SD =
Aejωt-(KAl1 / l2) ejω (t-(l2-l1) / c) (6).
Since this signal S0 is applied to the speaker, the sound SM at the listening point 6 in FIG. 2 is
expressed by the following equation using equations (1) and (6). SM = (A / l1) ejω (t-l1 / c)-(KA /
l2) ejω (t- (l2-l1) / c-l1 / c) + (KA / l2) ejω (t-l2 / c) )-(K / l2) (KAl1 / l2) ejω (t- (l2-l1) / c-l2 / c)
= (A / l1) ejω (t-l1 / c)-(K2Al1 / l2) ejω () t- (2l2-l1) / c) (7) Here, since 0 ≦ K <1, 0 <l1 / l2 <1,
the second term of the equation (7) It is sufficiently smaller than the first term, and therefore SM
≒ (A / l1) ejω (t-l1 / c) (8), the reflected wave can be eliminated. In the case of free end
reflection, the output of the buffer 12 may be selected by the switch 13. The delay unit 9 is also
equivalent to a phase shifter. In this case, as shown in FIG. 5, a known example of a transistor, a
resistor and a capacitor can be used. In the above example, the delay time of the delay unit 9 is a
fixed type set to match l1 and L2 in FIG. 2, but an actual sound field space exists infinitely and l1
and l2 also exist infinitely. Therefore, to cope with this, the delay time and the level adjustment
are configured to be variable. Also, in the case of multipoint reflection occurring in a sound field
where the ratio of longitudinal, lateral, upper, and lower is almost the same as in a car interior
sound field, for example, in the case of single point reflection as shown in FIG. There is almost no
difference in the delay of the reflected sound, so it is sufficient to correct some delay time after
correcting for any one reflected sound. FIG. 6 shows the characteristics at the listening point of
the in-vehicle sound field in such a case, and (A) shows that all the path lengths ln of each sound
are almost equal, l1 ≒ l2 ≒ l3 ≒ l4 =. (B) shows the synthetic sound pressure characteristics of
the listening point by all the reflected sounds. In this case, for example, after the correction of the
reflected sound arriving at the path of l1 is performed according to the above embodiment, a
slight correction of the delay time may be performed. As described above, according to the
present invention, the signal obtained by changing the characteristic of the original signal so as
to cancel the reflected sound in the sound field is added to the original signal to form a speaker
signal, so that the frequency characteristic at the listening point However, the cost can be
reduced because the adjustment can be simplified and the configuration can be simplified.
Brief description of the drawings
Fig. 1 is a block diagram of an example of a conventional acoustic system, Fig. 2 is a view
showing an example of a direct sound and its reflected sound in a sound field, and Fig. 3 is a
frequency of sound pressure at a listening point. Fig. 4 is a block diagram showing the
characteristics, Fig. 4 is a block diagram of an embodiment of the present invention, Fig. 5 is a
circuit diagram of a specific example of Fig. 4 and Fig. 6 is a sound field where multipoint
reflection occurs in a vehicle interior etc. It is a figure which shows the frequency characteristic
of the sound pressure of a listening point.
Descriptions of symbols of main parts 9: delay 10: level adjuster 11: inverter 13: switch 14:
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