Noise masking system and method in image forming apparatus

A noise masking system in an image forming apparatus such as a laser beam printer or a copying machine having a drive mechanism acting as a noise generation source during operation, the noise masking system comprising a masking sound generator for generating a sound to mask the noise and masking sound control means which controls the masking sound generator to generate a masking sound of a frequency range including a main-component frequency of the noise. The masking sound thus generated is of a frequency range from a lower-limit frequency to an upper-limit frequency in a critical band of the main-component frequency of the noise. The noise masking system masks noise to eliminate a psychological unpleasant feeling caused by frequency fluctuation. It is small-sized and inexpensive.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a noise masking system and method in an 
image forming apparatus such as, for example, an office automation (OA) 
apparatus, a laser beam printer, or an electrophotographic copying 
machine, using drive motors as drive sources for operation. The noise 
masking system and method according to the present invention generates a 
masking sound for cancelling noises generated from the drive motors which 
noises cause an unpleasant feeling. 
2. Description of Related Art 
In a conventional image forming apparatus such as a laser beam or an 
electrophotographic copying machine there are used a plurality of 
mechanical drive motors, which are special drive motors developed along 
the recent tendency to digitization. 
For example, in a digitized image forming apparatus, the reading of an 
image is performed by scanning an image carrier (original) with a light 
source, e.g. light emitting diode (LED), and reading the image by a charge 
coupled device (CCD). For recording an image there is used an image 
recorder, which scans a recording medium with light beam emitted from a 
light source, e.g. laser diode, and modulated by an image signal or a 
character signal and then records the image(prepares an original). In this 
case, as an optical scanner for the light beam there is used an optical 
deflector. The optical reflector comprises a rotary polyhedron mirror 
having a plurality of reflective surfaces on the outer periphery thereof 
and a drive motor for rotating the said rotary polyhedron mirror. An 
example of the drive motor used in such an optical deflector will be 
described below. 
FIG. 26 is a perspective view explaining the construction of an optical 
deflector (optical scanner). In the same figure, numeral 51 denotes a 
drive motor, numeral 52 denotes a rotary polyhedron mirror, 53 a laser 
beam source, 54 a collimator lens, 55 a light condensing optical component 
(condenser lens), and 56 a recording member (photosensitive drum). 
The construction of such an optical deflector used in an image forming 
apparatus, as well as an image recording method, will now be described 
with reference to FIG. 26. In recording an image, the rotary polyhedron 
mirror 52 is rotated in the direction of arrow A by the drive motor 51. 
The laser beam source 53 is constituted by a laser such as a semiconductor 
laser or a gas laser. Light beam emitted from the laser light source 53 is 
modulated with an image signal by means of a modulator (not shown) and the 
thus-modulated light beam is incident on one reflecting mirror surface of 
the rotary polyhedron mirror 52 through the collimator lens 54. The light 
beam reflected by the reflecting mirror surface of the rotary polyhedron 
mirror 52 is projected on the recording member 56 through the light 
condensing optical component 55. In this case, with rotation of the rotary 
polyhedron mirror 52 in the direction of arrow A, the reflected light beam 
is deflected in the direction of arrow B and scans the recording member 56 
horizontally. Along with this horizontal scanning, the recording member 56 
is rotated in the direction of arrow C, whereby a vertical scanning is 
performed. In this way a two-dimensional image is written onto the 
recording member 56. 
The drive motor 51 used in such an optical deflector is of the type in 
which there is used a rotary bearing such as a dynamic pressure air 
bearing or ball bearing, using one of a sleeve and a shaft both fitted 
together as a rotating member and the other as a stationary member, and a 
rotational torque is generated by a magnetic circuit composed of a 
permanent magnet attached to the rotating member and an electromagnetic 
coil wound round an annular iron core mounted in the stationary member. 
Thus, the drive motor 51 has a magnetic circuit functioning also as a 
magnetic bearing which holds a rotor in the axial direction. 
Consequently, when the image recording described above is performed, there 
arises a noise upon operation of the drive motor 51. A description will 
now be given of noises which occur with change in the number of 
revolutions of the drive motor. As shown in FIG. 27, a noise occurs and 
changes as the number of revolutions of the drive motor in the optical 
scanner changes. The timing chart of FIG. 27 shows noises occurring in the 
process from when the power is turned ON until when a series of image 
forming operations are completed. This change in the noise level is almost 
the same as the change in the number of revolutions of the drive motor 
acting as a main component of the drive mechanism. It is FIG. 28 that 
explains the change in the number of revolutions of the drive motor 51 
alone. In FIG. 28, it is not the noise level (dB) but the number of 
revolutions, f, that is plotted along the axis of ordinate. The value of 
dB (loudness) itself changes little even with a change in the number of 
revolutions. A change in the frequency (timbre) of noise which occurs with 
a change in the number of revolutions is offensive to the ear. 
As shown in FIG. 28, upon turning ON of the power, the number of 
revolutions of the drive motor is increased up to a predetermined value. 
If a predetermined processing is not started after continuance of the 
predetermined number of revolutions, a stand-by mode starts, in which the 
number of revolutions is decreased and the motor assumes a rest state. 
Thereafter, when the start of the processing is instructed, the drive 
motor starts to rise, and when the number of revolutions of the drive 
motor has reached a predetermined value, the drive motor starts to operate 
for the predetermined processing. Then, upon termination of the operation, 
some fans stop rotation, and after continuance of rotation for a certain 
time for cooling, the drive motor starts to slow down. The drive motor 
slows down to the number of revolutions preset for the stand-by mode, 
which revolutions are then continued, that is, the drive motor continues 
to stand by. 
Thus, in the image forming apparatus, if no processing is performed for a 
while after turning ON of the power, a switching is made into the stand-by 
mode in several to several ten seconds. But this is for diminishing the 
power consumption during stand-by. Most drive mechanisms in the apparatus, 
except fans for heat radiation, come into a rest state. In the stand-by 
mode, the optical deflector is usually slowed down to the half or so of a 
predetermined number of revolutions. This is for shortening the time 
required from when the drive motor starts to rise until when its 
predetermined number of revolutions is reached, in preparation for the 
regular operation. According to a certain type of image forming apparatus 
developed recently, the number of revolutions is decreased to even zero in 
the stand-by mode for the purpose of further diminishing the power 
consumption in the same mode. 
For performing the image forming processing in the stand-by mode, the 
operator is required, for example, to depress a button on the control 
panel to input a processing start signal, whereupon the image forming 
apparatus goes into an operation mode and the drive motor of the optical 
reflector starts to rise. The revolution of the motor is increased until 
reaching the predetermined number of revolutions. At this time, the drive 
motor of the optical reflector is required to rotate at high speed in a 
short time from the standpoint of an image forming cycle for example. For 
this reason, the drive motor of the optical deflector is constituted so as 
to be used at a higher number of revolutions than that of the usual 
motors. Its number of revolutions is 5,000 or more, or even 10,000 or more 
as the case maybe. In this case, a large current flows in the drive motor 
at the leading edge of the motor to increase the number of revolutions 
rapidly, with the result that a very loud noise occurs. This noise is a 
fluctuating noise interlocked with the change in the number of 
revolutions, which is very offensive to the human ear and causes 
unpleasant feeling. 
After the drive motor has reached the predetermined number of revolutions, 
the image forming processing is started, and after completion of a series 
of operations, the various mechanisms of the apparatus come into a rest 
state. In the event the next processing is not performed even after the 
lapse of a certain time, a switching is made again into the stand-by mode 
and the drive motor slows down. The drive motor eventually stops rotation 
and assumes a complete stand-by state. 
The noise from the motor is a fluctuating noise interlocked with the number 
of revolutions, but a human becomes aware of the fluctuation because the 
human is sensitive to a change of sound. Analysis of a frequency spectrum 
of the fluctuating noise shows that a gentle distribution is present over 
a wide frequency band and that sharp peaks projecting from the base 
spectrum are present in several frequency bands. It is seen that the said 
sharp peaks fluctuate. Among the sharp peaks, a main-component frequency, 
which is high in sound pressure level, ranges from several hundred Hz to 
several kHz. The sound in this frequency band gives rise to a great 
unpleasant feeling because the human is auditorily sensitive to such 
sound. That is, this sound is a fluctuating noise of high frequency which 
causes shrillness. If one hears this fluctuating noise, he recognizes it 
as a very unpleasant noise because of shrillness. 
Heretofore there have been proposed techniques for suppressing this type of 
unpleasant noises caused by frequency fluctuation, such as, for example, 
those proposed in Japanese Published Unexamined Patent Application Nos. 
Sho 63-59797 and Hei 6-175443. According to the "step motor driving 
method" proposed in Japanese Published Unexamined Patent Application No. 
Sho 63-59797, the change with time of frequency at the leading edge of a 
drive motor is like plural curves to mitigate an abrupt change. In the 
"image forming apparatus" proposed in the Japanese Published Unexamined 
Patent Application No. Hei 6-175443, at the leading edge of a polygon 
mirror driving motor, another operation noise is caused to fall down 
forward to cover the operation noise of the motor, thereby making the 
motor noise difficult to hear (masking). 
According to the above conventional methods, a design is made so that the 
change of frequency with time describes plural curves in order to 
eliminate a psychological unpleasant feeling caused by noise at the 
leading edge of the drive motor in the optical deflector. This 
construction is somewhat effective in mitigating an abrupt change of 
sound, but cannot eliminate unpleasant feeling because the frequency 
fluctuation is almost recognized. 
According to the above conventional method in which the motor operation 
noise is covered with another operation noise to make it difficult to 
hear, the noise (sound volume) as a whole further increases and causes 
noisiness. Thus, it is impossible to eliminate unpleasant feeling. 
Besides, other portions of the image forming apparatus are separately 
operated continuously during the period from turning ON of the power or 
termination of the image forming processing until switching into the 
stand-by mode. This is undesirable from the standpoint of low power 
consumption. 
SUMMARY OF THE INVENTION 
The present invention has been accomplished for solving the various 
problems mentioned above, and it is an object of the invention to provide 
a noise masking system in an image forming apparatus of small size and low 
cost such as, for example, a laser printer or a copying machine, capable 
of masking noise so as to eliminate a psychological unpleasant feeling 
caused by frequency fluctuation. 
In order to achieve the above-mentioned object, in the first aspect of the 
present invention there is provided a noise masking system in an image 
forming apparatus having a drive mechanism which causes noise during 
operation, the noise masking system is characterized by including a 
masking sound generator for generating a sound to mask the noise and 
masking sound control means for controlling the said masking sound 
generator to generate a masking sound having a frequency of a range 
including a main-component frequency of the noise. 
According to the present invention, in the second aspect thereof, there is 
provided a noise masking system in combination with that in the first 
aspect, characterized in that the masking sound control means is sound 
control means which generates a masking sound having a frequency of the 
range from a lower-limit frequency to an upper-limit frequency of a 
critical band frequency in the main-component frequency of the noise. 
According to the present invention, in the third aspect thereof, there is 
provided a noise masking system in combination with that in the first or 
second aspect, characterized in that the masking sound generated by the 
masking sound control means or the sound control means is a noise type 
masking sound not having any outstanding sound pressure peak in a specific 
frequency. In the fourth aspect of the invention, the said masking sound 
is a pure tone type masking sound having an outstanding sound pressure 
peak in the specific frequency. 
According to the present invention, in the fifth aspect thereof, there is 
provided a noise masking system in an image forming apparatus in 
combination with that in the third aspect, characterized in that the 
frequency and sound pressure of the noise type masking sound are in an 
inverse proportion to each other. In the sixth aspect of the present 
invention, the sound pressure distribution of the pure tone type masking 
sound relative to the frequency is either a triangular distribution or a 
normal distribution. In the seventh aspect of the invention, the frequency 
distribution of the pure tone type masking sound is a symmetric 
distribution with the foregoing specific frequency as the center. 
According to the present invention, in the eighth aspect thereof, there is 
provided a noise masking method in an image forming apparatus having a 
drive mechanism which causes noise during operation, characterized in that 
a masking sound having a frequency of a range including a main-component 
frequency of the noise is generated from a masking sound generator. 
According to the present invention, in the ninth aspect thereof, there is 
provided a noise masking method in an image forming apparatus having a 
drive mechanism which causes noise during operation, characterized in that 
a masking sound having a frequency of the range from a lower-limit 
frequency to an upper-limit frequency of a critical band frequency in a 
main-component frequency of the noise is generated from a masking sound 
generator. 
According to the present invention, in the tenth aspect thereof, there is 
provided a noise masking method in combination with that in the eighth or 
ninth aspect, characterized in that the masking sound is a noise type 
masking sound not having any outstanding sound pressure peak in a specific 
frequency. In the eleventh aspect of the invention, the masking sound is a 
pure tone type masking sound having an outstanding sound pressure peak in 
the specific frequency. In the twelfth aspect of the invention, the 
frequency and sound pressure of the noise type masking sound are in 
inverse proportion to each other. 
According to the present invention, in the thirteenth aspect thereof, there 
is provided a noise masking method in an image forming apparatus in 
combination with that in the eleventh aspect, characterized in that the 
sound pressure distribution of the pure tone type masking sound relative 
to the frequency is either a triangular distribution or a normal 
distribution. In the fourteenth aspect of the invention, the frequency 
distribution of the pure tone type masking sound is a symmetric 
distribution with the foregoing specific frequency as the center. 
According to the present invention, in the fifteenth aspect thereof, there 
is provided a noise masking system in an image forming apparatus having a 
drive mechanism which causes noise during operation, characterized by 
including correlation-signal producing means for producing a correlation 
signal correlated with the noise, a masking sound generator for generating 
a noise type masking sound to mask the noise, and masking sound control 
means for controlling the masking sound generator to vary the noise type 
masking sound in response to a change of the correlation signal. 
According to the present invention, in the sixteenth aspect thereof, there 
is provided a noise masking method in an image for making apparatus having 
a drive mechanism which causes noise during operation, characterized in 
that a correlation signal correlated with the noise is produced and a 
masking sound generator for generating a noise type masking sound to mask 
the noise is controlled to vary the noise type masking sound in response 
to a change of the correlation signal. 
Thus, in the noise masking system in an image forming apparatus according 
to the present invention, which has such various features as mentioned 
above, there are used a masking sound generator for generating a masking 
sound to mask the noise and masking sound control means. Against a 
fluctuating noise of the noise generated from the drive mechanism during 
operation, the masking sound control means controls the masking sound 
generator to generate a masking sound having a frequency of a range 
including a main-component frequency of the noise, thereby masking the 
fluctuating noise to diminish unpleasant feeling caused by the noise. 
In this case, the masking sound control means uses sound control means to 
generate, for example, a masking sound having a frequency of the range 
from a lower-limit frequency to an upper-limit frequency of a critical 
band frequency in a main-component frequency of the noise. More 
specifically, a noise is produced which is band-limited so as to contain a 
main-component frequency of noise generated at the leading edge to a 
predetermined number of revolutions of the drive motor or at the trailing 
edge, and the band-limited noise is generated as a sound wave from a 
speaker to prevent fluctuation of the main-component frequency from being 
recognized as noise. 
According to one mode, the masking sound generated by the masking sound 
control means or the sound control means is a noise type masking sound not 
having any outstanding sound pressure peak in a specific frequency, while 
according to another mode it is a pure tone type masking sound having an 
outstanding sound pressure peak in the specific frequency. The use of a 
noise type masking sound is advantageous in that a fluctuating noise of 
the noise generated at the leading or trailing edge of the drive motor 
becomes difficult to be recognized. Further, by generating a pure tone 
type masking sound having an outstanding sound pressure peak in the 
specific frequency during generation of the fluctuating noise, the 
fluctuating noise is masked by such a masking sound and becomes difficult 
to be recognized in the auditory sense. 
The frequency and sound pressure of the noise type masking sound are made 
inversely proportional to each other, that is, the distribution of power 
on the frequency shaft of a band-limited noise is given a form in which it 
is inversely proportional to frequency, thereby preventing the added 
band-limited noise from being recognized. Moreover, the distribution of 
sound pressure of the noise type masking sound relative to frequency is a 
distribution selected from triangular distribution, trapezoidal 
distribution and normal distribution, whereby the auditory sensitivity to 
frequencies spaced apart from the main-component frequency is suppressed 
to render the band-limited noise less audible and prevent recognition of 
the noise based on the main-component frequency. 
In this case, the frequency distribution of the noise type masking sound 
may be a symmetric distribution with the foregoing specific frequency as 
the center. Even by so doing, the auditory sensitivity of frequencies 
spaced apart from the main-component frequency can be suppressed to render 
the band-limited noise less audible and it is possible to keep the sound 
based on the main-component frequency out of recognition. 
In the noise making system in an image forming apparatus according to the 
present invention, the noise as a frequency fluctuating noise at the 
leading or trailing edge of revolution of the drive motor becomes 
difficult to be recognized auditorily, whereby a psychological unpleasant 
feeling is suppressed. The noise created as a masking sound and 
band-limited can diminish unpleasant feeling without recognition of an 
increase of noise. 
Regarding to what degree the frequency fluctuation of the main-component 
frequency is to be recognized and to what degree the increase of noise is 
to be recognized by the addition of the band-limited noise, the operator 
can adjust them by making operations on the control panel. The 
fluctuating-noise of the main-component frequency in the drive motor can 
be set to the extent of not being recognized by the operator and people 
present thereabouts, thereby diminishing their unpleasant feeling. 
In the noise masking system according to the present invention, moreover, 
the amplitude of a sound which amplitude corresponds to an average 
amplitude of the band-limited noise added as a masking sound is maintained 
in a predetermined state under varying environments and conditions, 
whereby the suppression of unpleasant feeling can be effected stably. 
Alternatively, by making adjustment so that the distribution of the added 
band-limited noise is given an inversely proportion form to frequency, it 
is possible to diminish the degree of recognition of noise increase based 
on the addition of the band-limited noise. 
In the noise masking system according to the present invention, by changing 
the band and band width of noise following variation in the main-component 
frequency of the drive motor, it becomes possible to decrease the 
bandwidth of noise. Consequently, the degree of recognition of noise 
increase based on the addition of the band-limited noise can be further 
diminished. Moreover, by optimizing the band, amplitude and distribution 
of noise in such a manner that the frequency band of the band-limited 
noise to be added is equal to the critical band of the main-component 
frequency in the drive motor and that the noise energy is equal to the 
energy of the main-component frequency, it becomes possible to minimize 
the recognition degree for frequency fluctuation of the main-component 
frequency and the degree of recognition of noise increase based on the 
addition of the band-limited noise. 
Further, in the noise masking system according to the present invention, it 
is possible to make difficult the recognition of noise generated at the 
transition from fluctuation to steady state or from steady state to 
fluctuation of the main-component frequency in the drive motor, whereby 
the sense of incongruity induced by the shift of sound to an ON or OFF 
state of the motor can be eliminated. Besides, since the noise of the 
drive motor is not covered with an operation noise or a partial operation 
noise, it is not necessary to make the noise duration time long, nor is it 
necessary to provide electric power for operation. Additionally, in 
comparison with the reduction of noise at the source, using a complicated 
structure, and the use of the expensive silencer, it is possible to attain 
a system configuration of simple structure and low cost.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A plurality of embodiments of the present invention will be described 
concretely hereinunder with reference to the accompanying drawings. It is 
to be understood, however, that the present invention is not limited to 
those embodiments and that although the generation of an addition sound in 
the following embodiments will be at the leading edge of a drive motor, it 
is also the same at the trailing edge of the motor, both being different 
only in point of time. 
Reference will first be made to the first embodiment of the present 
invention. FIG. 1 is a block diagram explaining the construction of a 
noise masking system in an image forming apparatus according to the first 
embodiment. In the same figure, the numeral 1 denotes a drive motor, 
numeral 2 denotes a motor control circuit, 3 a band noise generating 
circuit, and 4 a speaker. 
In the block diagram of the noise masking system shown in FIG. 1, the motor 
control circuit 2, for controlling the number of revolutions of the drive 
motor 1, acquires a signal related to the number of revolutions from the 
drive motor. For example, if the drive motor 1 is of the type which uses a 
magnetic force created by a permanent magnet as a drive source, then the 
magnetic flux density around the permanent magnet is measured, the number 
of N-S pole switchings is detected from the number of zero points of the 
flux density, and the quotient obtained by dividing the number of N-S pole 
switchings by the number of poles of the permanent magnet is obtained as a 
revolution signal of the drive motor 1. 
An output signal from the motor control circuit 2 is fed to the band noise 
generating circuit 3, which passes the generated noise through a 
predetermined filter to obtain a band-limited noise as a band noise and 
provides the band noise to the speaker 4. With a change in the number of 
revolutions of the drive motor 1 as a trigger and using the band noise 
provided from the band noise generating circuit 3 as an additional sound 
(masking sound) against the generated noise, the speaker 4 generates a 
sound wave in response to the rising noise of the drive motor. 
Description is now directed to the flow of processing performed for masking 
the motor rising noise. FIG. 2 is a diagram explaining an additional sound 
generation timing against the noise generated from the motor. In the same 
figure, hatched portions indicate additional sound generation timings 
responsive to the generation of noise at operation timings of the motor. 
Upon turning ON of the power, first, the revolution of the drive motor 
rises and is increased up to a predetermined number of revolutions, as 
shown in FIG. 2. At this time, there occurs a noise based on frequency 
fluctuation which noise causes unpleasant feeling, and therefore a first 
additional sound is generated during this period. When the predetermined 
number of revolutions of the motor has been reached, the rotation of the 
motor is continued at this constant number of revolutions. If this state 
is not followed by any processing, a stand-by mode is started, in which 
mode the number of revolutions is decreased and the motor assumes a rest 
state. But also in the course of decreasing the number of revolutions 
there occurs a noise based on frequency fluctuation which noise causes 
unpleasant feeling, and therefore a second additional sound is generated 
at this time point. 
When the start of processing such as image recording is instructed after 
the stand-by mode which has been continued at a constant small number of 
revolutions, the drive motor starts to rise, and when its revolution has 
reached a predetermined number of revolutions, the operation of the motor 
for the said processing is started. Here again, in the course of 
increasing the number of revolutions there occurs a noise based on a large 
frequency fluctuation which noise causes unpleasant feeling, and therefore 
a third additional sound is generated during this period. When the 
processing such as image recording is completed and the operation of the 
drive motor is over, the rotation of some fans is stopped and the number 
of revolutions of the motor is decreased for the reduction of electric 
power, allowing rotation to be continued for a certain time. During this 
period, that is, in the course of decreasing the number of revolutions, 
there occurs little frequency fluctuation, so there is not generated any 
additional sound. After the rotation has been continued for a certain 
time, the drive motor starts to slow down and its revolution decreases to 
the stand-by revolution, at which the stand-by state is continued. During 
this period, that is, in the course of rapidly decreasing the number of 
revolutions, there occurs a noise based on frequency fluctuation which 
noise causes unpleasant feeling, and therefore a fourth additional sound 
is generated also during this period. 
Thus, at the time of change in the number of revolutions of the drive motor 
there occurs a fluctuating noise based on frequency fluctuation, which 
noise cause unpleasant feeling. Therefore, at every generation of such a 
noise there is produced an additional sound (the first to the fourth 
additional sound) to mask the fluctuating noise in response to frequency 
fluctuation in the number of revolutions of the motor. 
Reference will now be made to the relation between the change of an 
additional sound with time and frequency. Mainly three kinds of noises are 
usually generated from the drive motor 1, which are an electromagnetic 
noise generated from an electromagnetic coil or from an iron core at the 
time of switch-over of an electric current flowing in the drive motor, a 
wind striking noise created by the friction between a rotary polyhedron 
mirror and air, and a bearing noise created by a mechanical shaft-bearing 
contact. The electromagnetic noise is close to a pure tone having sharp 
peaks in a narrow frequency band. The wind striking noise is a hydraulic 
noise having gentle peaks in a wide frequency band. And the bearing noise 
is close to a pure tone having many sharp peaks according to the shape and 
size in the case of using a ball bearing, which noise is not generated in 
the use of an air bearing. It is known that the frequencies of these 
noises are in a proportional relation to the number of revolutions of the 
drive motor. 
FIG. 3 is a diagram showing a relation between the change of an additional 
sound with time and the motor rising noise. In the same figure, the curve 
expressed by a solid line represents the change of a main-component 
frequency of noise which occurs at the leading edge of rotation of the 
drive motor 1. The main-component frequency indicates a frequency of a 
high sound pressure level detected by frequency-analysis of the noise 
generated from the motor, which frequency is in corresponding relation to 
the number of revolutions of the drive motor. The rotation of the drive 
motor rises from a zero revolution or from a stand-by mode in which the 
revolution is not zero. Therefore, as shown in FIG. 3, the frequency of 
the motor rising noise increases with the change of time. This frequency 
fluctuating pure tone causes unpleasant feeling. 
On the other hand, as shown also in FIG. 3, the additional sound is a 
band-limited nose as indicated by hatching, whose frequency band is 
limited to the range from a leading-edge frequency f.sub.0 of the 
main-component frequency to a post-rise frequency f.sub.1. More 
specifically, the additional sound is a noise constituted by waves of a 
predetermined amplitude and of random frequencies and random phases 
falling under the range from a lower-limit frequency f'.sub.0 of a 
critical band frequency in leading-edge frequency f.sub.0 to an 
upper-limit frequency f'.sub.1 of the critical band frequency in the 
post-rise frequency f.sub.1. 
The thus band-limited noise has such a frequency distribution (probability 
distribution of frequency) as shown in FIG. 4(a) in which frequency is 
plotted along the axis of abscissa and the intensity of noise component 
plotted along the axis of ordinate. In FIG. 4(a), a main-component 
frequency generated at the leading edge of revolution of the motor, 
indicated with a solid line, fluctuates in frequency from f.sub.0 to 
f.sub.1. At this time, the main-component frequency becomes 
indistinguishable from the band-limited noise as an additional sound, so 
that fluctuation of the main-component frequency is difficult to be 
detected. Besides, since the added noise is band-limited, there is little 
increase of the noise volume. Consequently, the so-called "noisiness" is 
difficult to be detected in the auditory sense. 
Further, as shown in FIG. 4(b), the band-limited noise as an additional 
sound is given a distribution form in which the noise component intensity 
is in inverse proportion to frequency. As a result, by the effect of 
"(1/f) fluctuation," it becomes more difficult to detect an increase of 
noise auditorily. Consequently, it becomes audible pleasantly. It has been 
made clear through various studies that sounds having a distribution form 
in which fluctuating intensities are in inverse proportion to fluctuating 
frequencies, namely, the so-called "(1/f) fluctuation," are usually felt 
comfortable to human. The effect of the "(1/f) fluctuation" is here 
utilized. 
Generally, the motor control circuit 2 controls the drive motor 1 in such a 
manner that at the initial stage of rise a larger current than in the 
steady state is allowed to flow in the motor in order to shorten the rise 
time of the motor, while when the motor revolution approaches a 
predetermined number of revolutions, the electric current is adjusted 
small in order to diminish overshoot. This fluctuating noise of high 
frequency which varies with the lapse of time causes a psychological 
unpleasant feeling. In this connection, since the frequency generated at 
the leading edge of revolution of the drive motor 1 is a high frequency of 
a narrow band and fluctuates, it is recognized easily. 
In this case, therefore, a band-limited noise which contains a 
main-component frequency of the fluctuating noise is added and the 
main-component frequency is allowed to merge into the fluctuating noise, 
making the fluctuating noise of the main-component frequency itself 
difficult to be recognized to diminish the unpleasant feeling. However, if 
an increase of the noise volume is recognized as a result of addition of 
such additional noise, the unpleasant feeling may be rather enhanced. 
Therefore, it is necessary that the addition of the band-limited noise be 
kept to a minimum required level. If the amplitude distribution of the 
band-limited noise on the frequency axis is rendered inversely 
proportional to frequency, the foregoing "(1/f) fluctuation" takes effect, 
so that it becomes more difficult to detect the increase of noise and the 
noise becomes audible pleasantly. 
Moreover, as shown in FIG. 5, if the band-limited noise as an additional 
sound is added continuously from just before the rise of fluctuating noise 
generated at the leading edge of revolution of the drive motor to the 
subsequent steady sound, it becomes possible to make the noise recognition 
more difficult. 
FIG. 5 is a diagram showing a relation between the change of an additional 
sound with time, including its state up to steady operation, and a motor 
rising noise. In the case where a band-limited noise is added as an 
additional sound to a pure tone type rising noise, the fluctuation of a 
fluctuating main-component frequency is difficult to be recognized and the 
change in the way of feeling involves no sense of incongruity if it falls 
under the frequency band of the added noise even if the main-component 
frequency fluctuates with the lapse of time as mentioned previously. In 
this case, by continuing the addition of the band-limited noise from just 
before the rise of the fluctuating noise up to part of the subsequent 
steady state, as shown in FIG. 5, it is possible to make the frequency 
fluctuation further difficult to be recognized. 
In the case where a band-limited noise is added as an additional sound to 
the fluctuating noise generated at the leading edge of revolution of the 
drive motor to mask the motor rising noise as in this embodiment, the 
evaluation of the masking effect depends on the auditory sense of each 
individual person, so it is preferred that the loudness of the additional 
sound be adjustable. This is attained in the second embodiment of of the 
invention as will be described below. 
FIG. 6 is a block diagram explaining the construction of a noise masking 
system in an image forming apparatus according to the second embodiment of 
the present invention. In the same figure, the numeral 1 denotes a drive 
motor, numeral 2 denotes a motor control circuit, 3 a band noise 
generating circuit, and 4 a speaker. These components are the same as in 
the first embodiment (FIG. 1). Further, the numeral 6 denotes a control 
panel and numeral 7 denotes an amplitude changing circuit. 
The noise masking system in an image forming apparatus according to the 
second embodiment of the present invention will now be described with 
reference to the block diagram of FIG. 6. In the second embodiment, the 
control panel 6 and the amplitude changing circuit 7 are newly provided. 
The band noise generating circuit 3 provides a band-limited noise to the 
amplitude changing circuit 7, which in turn adjusts the amplitude of the 
band-limited noise as an additional sound in accordance with a command 
issued from the control panel 6. 
The control panel 6 sends a signal of designating a desired amplitude of 
the additional sound to the amplitude changing circuit 7. More 
particularly, the operator designates a desired amplitude on the control 
panel 6 and a related signal is transmitted from the same panel to the 
amplitude changing circuit 7. In accordance with this signal the circuit 7 
changes the amplitude distribution of the band noise generated by the band 
noise generating circuit 3 and causes the speaker 4 to generate an 
additional sound as a sound wave. The operator designates the amplitude to 
adjust the degree of noise recognition so that the noise of a 
main-component frequency is not recognized by the operator and people 
present around the operator. In this way it is possible to suppress 
unpleasant feeling for each individual person. 
In the case where a noise which has been band-limited to the frequency 
range of a fluctuating noise of the main-frequency component of a drive 
motor rising noise is added to the motor rising noise to mask it, the 
second embodiment adopts a method in which the loudness of the added noise 
can be adjusted according to an auditory desire of each individual person. 
However, this method is complicated because the adjustment must be made 
separately for each noise. In order to eliminate this complicatedness it 
is possible to constitute the noise masking system so as to make the 
adjustment in question automatically. That is, it becomes possible to 
adjust the amplitude of the additional sound as a noise masking sound. As 
to a more minute adjustment, this may be done according to the desire of 
each individual person. The following description is now provided about a 
noise masking system having such a construction as the third embodiment of 
the invention. 
FIG. 7 is a block diagram explaining the construction of a noise masking 
system in an image forming apparatus according to the third embodiment of 
the present invention. In the same figure, the numeral 1 denotes a drive 
motor, numeral 2 denotes a motor control circuit, 3 a band noise 
generating circuit, and 4 a speaker. These components are the same as in 
the first embodiment (FIG. 1). Further, the numeral 8 denotes an amplitude 
control circuit and numeral 9 denotes a sensor mike for sensing a motor 
rising noise. 
The noise masking system in an image forming apparatus according to the 
third embodiment of the invention will now be described with reference to 
the block diagram of FIG. 7. In third embodiment, the amplitude control 
circuit 8 and the sensor mike 9 are provided in addition of the components 
used in the first embodiment. The sensor mike 9 senses a motor rising 
noise and the amplitude control circuit 8 adjusts the amplitude of an 
additional sound automatically in accordance with a signal obtained by 
sensing the motor rising noise. More specifically, the noise masking 
system of this embodiment is constituted in such a manner that the 
amplitude of the addition sound, which is a band-limited noise, supplied 
to the amplitude control circuit 8 from the band noise generating circuit 
3 is adjusted automatically by the amplitude control circuit 8 in 
accordance with the detected output provided from the sensor mike 9. 
Thus, in the third embodiment illustrated in FIG. 7, the construction of 
the second embodiment illustrated in FIG. 6 is further developed and there 
is provided the sensor mike 9 for sensing the amplitude of a motor rising 
noise and also provided is the amplitude control circuit 8 corresponding 
to the amplitude changing circuit 7 used in the second embodiment. In this 
construction, the amplitude of a main-component frequency of a motor 
rising noise is detected by the sensor mike 9, which in turn sends the 
detected signal to the amplitude control circuit 8. The amplitude control 
circuit 8 controls an increase or decrease of amplitude actively as the 
main-component frequency fluctuates with the lapse of time. In this way it 
is possible to maintain the amplitude in a predetermined state and effect 
the suppression of unpleasant feeling stably in response to 
environment-induced changes of the main-component frequency and changes of 
various conditions. 
Thus, against the noise generated at the leading edge of revolution of the 
drive motor, a noise which has been band-limited to the frequency range of 
a fluctuating noise of the main-component frequency in the motor rising 
noise is added to mask the motor rising noise so as to decrease the degree 
of its recognition. In this case, if the suppression of unpleasant feeling 
can be done stably, it is preferred that the loudness of the added noise 
as a whole be as small as possible, whereby the degree of "noisiness" 
which a human feels can be decreased to a greater extent. For example, 
therefore, if the band component of the added noise is rendered 
corresponding to the fluctuation of the main-component frequency in the 
motor rising noise, it becomes possible to obtain a satisfactory masking 
effect even if the amplitude of the added noise is further diminished. A 
noise masking system having such a construction will be described below as 
the fourth embodiment of the invention. 
FIG. 8 is a block diagram explaining the construction of a noise masking 
system in an image forming apparatus according to the fourth embodiment of 
the invention. In the same figure, the numeral 1 denotes a drive motor, 
numeral 2 denotes a motor control circuit, 3 a band noise generating 
circuit, and 4 a speaker. These components are the same as in the first 
embodiment (FIG. 1). Further, the numeral 10 denotes a revolution 
detecting circuit, numeral 11 denotes a timing control circuit, and 
numeral 12 denotes an amplitude control circuit. 
The noise masking system of the fourth embodiment will now be described 
with reference to the block diagram of FIG. 8. In this fourth embodiment 
the revolution detecting circuit 10, timing control circuit 11 and 
amplitude control circuit 12 are newly provided. In accordance with a 
signal provided from the motor control circuit 2 the revolution detecting 
circuit 10 detects the number of revolutions of the drive motor and 
thereby detects fluctuation of a main-component frequency contained in the 
noise generated at the leading edge of revolution of the motor. The 
amplitude of a band-limited noise fed from the band noise generating 
circuit 3 is adjusted in the amplitude control circuit 12 and the 
so-adjusted noise is provided to the timing control circuit 11. With the 
signal detected by the revolution detecting circuit 10 as a trigger 
signal, the timing control circuit 11 transmits the noise whose amplitude 
has been adjusted by the amplitude control circuit 12 to the speaker 4, 
which in turn generates a sound wave as an additional sound. In this case, 
the signal from the revolution detecting circuit 10 is applied continually 
to the band noise generating circuit 3, which circuit 3 makes a band 
limitation in a successive manner so that a wide band of noise becomes 
equal in its band to the critical band of the main-component frequency in 
the motor rising noise. 
In this band limitation, with the main-component frequency, f, in the drive 
motor 1 as the center, a band width .DELTA.fc in question is expressed by 
the following equation 1: 
Equation 1! 
EQU .DELTA.fc=25.0+75.0{1.0+1.4(f/1000).sup.2 }.sup.0.69 
In the amplitude control circuit 12, the noise amplitude is controlled so 
that the power of the band noise (band-limited noise) becomes equal to the 
power of the main-component frequency in the drive motor 1 which is 
pre-stored. In this case, the power of the band noise is expressed by the 
following equation 2: 
Equation 2! 
EQU P=10 log.sub.10 .intg.10.sup.(B(f)/10) df 
where B(f) stands for an effective value of amplitude at the band noise 
frequency f. In this case, the motor rising noise and the band noise are 
in such a relation as shown in FIG. 9. 
As to the band noise distribution on the frequency axis, a distribution 
which becomes smaller as the main-component frequency goes away, tends to 
audible better. In view of this point, for example such triangular, 
trapezoidal and normal distributions as shown in FIGS. 10(a), 10(b) and 
10(c), respectively, are utilized as distributions on the band noise 
frequency axis. 
The band noise produced as a distribution having any of such distribution 
forms on the frequency axis is fed to the amplitude control circuit 12 
from the band noise generating circuit 3, in which the amplitude of the 
band noise is controlled. The band noise is then fed to the timing control 
circuit 11, which in turn causes a sound wave to be generated as an 
additional sound from the speaker 4 in synchronism with the change in the 
number of revolutions of the drive motor 1. In this case, it is known that 
in the critical band of the band-limited noise and in a minimum required 
region of the noise influential in the main-component frequency, the 
main-component frequency gets mixed with the band noise when the power of 
the band noise and that of the main-component frequency become equal to 
each other, resulting in the motor rising noise becoming no longer 
audible. At this time, the power of the added noise becomes minimum and 
the detection of noise increase is minimized. 
(Experiment 1) 
For checking the noise masking effect in the above embodiments there were 
made sincere evaluation tests. In the first sincere evaluation test the 
construction of the first embodiment was adopted and a noise which had 
been band-limited so as to contain a main-component frequency of noise 
generated at the leading edge of revolution of the drive motor 1 was added 
while changing its level in four stages (including the case where the band 
noise is not added). The added band noise had such a form as the amplitude 
of each component was in inverse proportion to frequency. A total of four 
types of sounds were provided to let eighteen panelists to hear and 
evaluate with respect to three items--"noisiness," "unpleasantness" and 
"shrillness"--. As the evaluation method there was used such a five-stage 
category evaluation method as shown in FIG. 11. The results obtained are 
as shown in FIG. 12. 
As to the effect obtained by the noise masking system of the first 
embodiment, it is seen from the graph of evaluation results of FIG. 12 
that the evaluation item "unpleasantness" shows a substantially constant 
tendency even with increase of the added noise level and that the degree 
of "noisiness" increases as the added noise level becomes higher. As to 
"shrillness," this evaluation item tends to be mitigated as the added 
noise level becomes higher, with a maximum of 31% mitigation recognized in 
comparison with the maximum original sound. From this result it turned out 
that "shrillness" could be diminished by adding the band-limited noise. 
In the second sincere evaluation test there was adopted the construction of 
the third embodiment and there was added a noise of a band corresponding 
to a critical band with a main-component frequency of a motor rising noise 
as the center, the motor rising noise being generated at the leading edge 
of revolution of the drive motor 1. The band noise amplitude was adjusted 
so that its energy became equal to the energy of the main-component 
frequency. Further, the power distribution of the band noise on the 
frequency axis was adjusted into a triangular distribution. Two types of 
sounds, one being the motor rising noise plus the band noise and the other 
being the motor rising noise alone, were provided to let eighteen 
panelists to hear and evaluate with respect to three items--"noisiness," 
"unpleasantness" and "shrillness"--. There was used the same evaluation 
method as in the first sincere evaluation test, namely, such a five-stage 
category evaluation method as shown in FIG. 11. The results of the test 
are as shown in FIG. 13. 
According to the noise masking system of the third embodiment, as is seen 
from the graph of evaluation results of FIG. 13, "noisiness" shows a 
substantially constant tendency, and "unpleasantness" and "shrillness" are 
mitigated about 17% and 21%, respectively, in the case of addition of the 
band noise. From this result it is seen that if a noise of a band 
corresponding to the critical band of a main-component frequency 
containing in the noise generated at the leading edge of revolution of the 
drive motor 1 is added to the motor rising noise, both "unpleasantness" 
and "shrillness" can be diminished without increase of "noisiness." 
According to the noise masking systems of the first to fourth embodiments, 
asset forth above, a main-component frequency of a noise generated at the 
leading edge or trailing edge of revolution of the drive motor to a 
predetermined number of revolutions is detected (the number of revolutions 
of the drive motor at the leading or trailing edge is detected), a noise 
band-limited to contain the main-component frequency is produced, and the 
noise thus produced is outputted as a sound wave. In this case, the 
amplitude of the band-limited noise is changed according to the operator's 
desire or is controlled according to the noise loudness detected by the 
sensor mike, then the band noise is outputted from the speaker as an 
additional sound to mask the motor rising or trailing noise. Therefore, it 
is possible to provide an image forming apparatus such as a laser printer 
or a copying machine of small size and low cost free of any psychological 
unpleasant feeling based on frequency fluctuation. Further, the number of 
revolutions of the drive motor at its leading or trailing edge is detected 
and in accordance with the detected revolutions the band of the noise to 
be added is limited so as to correspond to the critical band of the 
main-component frequency, whereby the psychological unpleasant feeling can 
be mitigated to a further extent. 
In the case where the number of revolutions at the leading or trailing edge 
of the drive motor is detected and the noise to be added is band-limited 
to the critical band of the main-component frequency in accordance with 
the detected revolutions, the said band limitation may be substituted by 
the addition of a pure tone in synchronism with a fluctuating noise of the 
main-component frequency at the leading or trailing edge of the drive 
motor. In this case, it is possible to suppress unpleasant feeling based 
on high-frequency noise although fluctuation is felt. Besides, a simple 
construction suffices because a pure tone frequency producing circuit can 
be used as an additional sound generating circuit. Thus, where a 
high-frequency noise is strong in the motor rising noise, the masking 
method just mentioned above can be adopted in combination with the masking 
method adopted in the foregoing embodiments, whereby the psychological 
unpleasant feeling can be further mitigated. This construction will be 
described below as the fifth embodiment. 
Reference will now be made to the noise masking system of the fifth 
embodiment. FIG. 14 is a block diagram explaining the construction of the 
noise masking system of the fifth embodiment. In the same figure, the 
numeral 1 denotes a drive motor, numeral 2 denotes a motor control 
circuit, 4 a speaker, 10 a revolution detecting circuit, 11 a timing 
control circuit, and 23 a frequency producing circuit. In the construction 
of this fifth embodiment, the band noise generating circuit 3 used in the 
previous embodiments (first to fourth embodiments) is substituted by a 
frequency producing circuit 23. 
In the noise masking system of the fifth embodiment illustrated in FIG. 14, 
the motor control circuit 2 acquires from the drive motor 1 a signal 
related to the motor revolution in order to control the number of 
revolutions of the drive motor 1. For example, if the drive motor 1 
utilizes a magnetic force induced by a permanent magnet as a drive source, 
a magnetic flux density around the permanent magnet is measured to detect 
the number of zero points in the flux density, and the number of N-S pole 
switchings is detected on the basis of the number of zero points. Then, 
the quotient obtained by dividing the number of N-S pole switchings by the 
number of poles of the permanent magnet is obtained as a revolution signal 
of the drive motor 1. 
The revolution detecting circuit 10 obtains the revolution signal from the 
signal obtained by the motor control circuit 2. The revolution signal thus 
obtained is transmitted from the revolution detecting circuit 10 to the 
timing control circuit 11 together with the revolution signal which 
follows one step later. In synchronism with this timing the timing control 
circuit 11 reads in a pre-stored frequency from the frequency producing 
circuit 23 and then compares the number of revolutions obtained in the 
revolution detecting circuit 10 with the number of revolutions during 
operation of the drive motor 1 which is stored in advance, to detect an 
operating state of the motor. Further, there is obtained a difference from 
the revolution signal which follows one step later, to detect the state of 
the motor at the leading edge. 
Then, in synchronism with the change in the number of revolutions 
continuing from the stand-by condition the timing control circuit 11 sends 
a signal to the speaker 4, causing the speaker to generate an additional 
sound (masking sound), allowing the additional sound to be added to the 
motor rising noise generated from the drive motor 1, to mask the motor 
rising noise with the additional sound. In this way the noise generated at 
the leading edge of revolution of the motor is masked. In this case, the 
additional sound adding timing is the same as that illustrated in FIG. 2. 
The following description is now provided about the relation between the 
change of the additional sound with time and frequency. Three types of 
noises are mainly generated from the drive motor 1, which are an 
electromagnetic noise generated from the electromagnetic coil or the iron 
core at the time of switch-over of an electric current flowing in the 
drive motor, a wind striking noise induced by friction between a rotary 
polyhedron mirror and air, and a bearing noise caused by a mechanical 
shaft-bearing contact. The electromagnetic noise is close to a pure tone 
having sharp peaks in a narrow frequency band, and the wind striking noise 
is a hydraulic noise having gentle peaks in a wide frequency band. The 
bearing noise is close to a pure tone having many sharp peaks based on the 
shape and size of a ball bearing if used. The bearing noise is not 
recognized in the case of an air bearing. It is known that the frequencies 
of these noises are in a proportional relation to the number of 
revolutions of the drive motor. 
FIG. 15 is a diagram showing a relation between the change of an additional 
sound with time and a motor rising noise. In the same figure, the curve 
indicated by a solid line represents fluctuation of a main-component 
frequency of a noise generated at the leading edge of revolution of the 
drive motor 1. The drive motor rises from the state of zero revolution or 
from a stand-by state where the revolution is not zero. On the other hand, 
the frequency of an additional sound indicated with a broken line is set 
higher or lower than the main-component frequency of the motor rising 
noise, which additional sound is added so as not to surpass the noise of 
the main-component frequency auditorily. Against the motor rising noise, 
as shown in FIG. 16, the additional sound takes the form of a pure tone or 
a form close to a pure tone for which the main-component frequency of the 
motor rising noise is high or low. 
In order to shorten the rise time of the drive motor 1, the motor control 
circuit 2 causes a larger current than in the steady state to flow in the 
drive motor at the initial stage of rise, while when the revolution of the 
motor approaches a predetermined number of revolutions, the motor control 
circuit adjusts the electric current small for diminishing overshoot. This 
fluctuating noise of high frequency which fluctuates with the lapse of 
time creates a psychologically unpleasant feeling. In the case of a 
complex sound of several sounds, the way of human feeling of timbre 
depends on the component sounds, i.e., sounds of added frequency 
components. In view of this point, to a main-component frequency of noise 
generated at the leading edge of revolution there is added a sound of a 
higher or lower frequency than the main-component frequency to widen the 
frequency band of the noise in excess of the main-component frequency, 
thereby making the noise difficult to be recognized. In this case, 
moreover, by adding the additional sound continuously to both motor rising 
noise and subsequent steady noise, as shown in FIG. 17, it is made 
possible to make the noise difficult to be recognized in excess of the 
main-component frequency in a steady state. 
In the case where a sound of a higher or lower frequency than the 
main-component frequency of the motor rising noise is added to the motor 
rising noise to mask the same noise as in this embodiment, the effect of 
the masking corresponds to an auditory evaluation of each individual 
person and therefore the loudness of the additional sound is made 
adjustable according to an auditory desire of each individual person. This 
is attained by the sixth embodiment of the present invention. 
FIG. 18 is a block diagram explaining the construction of a noise masking 
system according to the sixth embodiment of the present invention. In the 
same figure, the numeral 1 denotes a drive motor, numeral 2 denotes a 
motor control circuit, numeral 4 denotes a speaker, 6a control panel, 7 an 
amplitude changing circuit, 10 a revolution detecting circuit, 11 a timing 
control circuit, and 23 a frequency producing circuit. These reference 
numerals are common to those used in the previous embodiments and indicate 
the same components as in the previous embodiments. 
The noise masking system of the sixth embodiment will now be described with 
reference to FIG. 18. In this sixth embodiment the control panel 6 and the 
amplitude changing circuit 7 are provided, and the amplitude of an 
additional sound to be supplied from the frequency producing circuit 23 to 
the timing control circuit 11 is adjusted by the amplitude changing 
circuit 7 in accordance with a command provided from the control panel 6. 
More specifically, in accordance with a control signal provided from the 
motor control circuit 2 the revolution detecting circuit 10 detects a 
change in the number of revolutions at the leading or trailing edge of the 
drive motor 1 and transmits the detected signal to the timing control 
circuit 11. With the detected signal as a trigger signal, the timing 
control circuit 11 generates an additional sound in such a manner that a 
frequency which has been produced beforehand by the frequency producing 
circuit 23 is added to a main-component frequency of the noise generated 
from the motor. In this case, the timing control circuit 11 drives the 
speaker 4 to output a sound wave as the additional sound while 
synchronizing the additional sound with time-dependent fluctuations of the 
main-component frequency. The operator operates the control panel 6 so 
that a signal for setting a desired amplitude of the additional sound is 
provided from the control panel to the amplitude changing circuit 7. 
The loudness of the additional sound outputted from the speaker 4 is 
adjusted on the control panel 6 by the operator so as to suit the 
operator's desire and mitigate the unpleasant feeling of noise generated 
from the image forming apparatus. An appropriate signal is provided from 
the control panel 6 to the amplitude changing circuit 7, which in turn 
changes the amplitude of the sound to be used as the additional sound in 
accordance with the received signal and causes the sound to be outputted 
as a sound wave from the speaker 4. This sound wave is added to the 
main-component frequency. Thus, on the control panel 6 the operator can 
adjust the recognition degree of noise to the extent that the noise of the 
main-component frequency is not recognized by the operator and people 
present around the operator. Moreover, it is possible to suppress 
unpleasant feeling according to the desire of each individual person. 
In the sixth embodiment described above, in the case of adding an 
additional sound to the motor rising noise to mask the noise, the loudness 
of the additional sound can be adjusted according to an auditory desire of 
each individual person. However, this is complicated because the 
adjustment must be made separately for each noise. For eliminating this 
complicatedness it is possible to constitute the noise masking system so 
as to permit this adjustment to be done automatically, whereby the 
amplitude of the additional sound for masking the noise can be adjusted 
automatically. Further, minute adjustments may be made according to the 
desire of each individual personas in the sixth embodiment. A noise 
masking system having such a construction will be described below as the 
seventh embodiment. 
FIG. 19 is a block diagram explaining the construction of a noise masking 
system according to the seventh embodiment of the present invention. In 
the same figure, the numeral 1 denotes a drive motor, numeral 2 denotes a 
motor control circuit, numeral 4 denotes a speaker, 10 a revolution 
detecting circuit, 11 a timing control circuit, and 23 a frequency 
producing circuit. These components are the same as in the fifth 
embodiment (FIG. 14). Further, the numeral 9 denotes a sensor mike for 
sensing a motor rising noise generated from the drive motor 1 and numeral 
12 denotes an amplitude control circuit. 
The noise masking system of the seventh embodiment will now be described 
with reference to the block diagram of FIG. 19. In the seventh embodiment 
the amplitude control circuit 12 and the sensor mike 9 are provided in 
addition to the components of the fifth embodiment (FIG. 14). The sensor 
mike 9 detects the amplitude of a main-component frequency of a motor 
rising noise, and in accordance with this detected signal the amplitude 
control circuit 12 adjusts the amplitude of an additional signal 
automatically. More specifically, the amplitude of an additional sound 
having plural frequencies, which is provided from the frequency producing 
circuit 23 to the timing control circuit 11, is adjusted automatically by 
the amplitude control circuit 12 in accordance with the detected output 
provided from the sensor mike 9. 
Thus, in the seventh embodiment illustrated in FIG. 19, the construction of 
the sixth embodiment illustrated in FIG. 18 is further developed and there 
is provided the sensor mike 9 for sensing the motor rising noise. Also 
provided is the amplitude control circuit 12 which corresponds to the 
amplitude changing circuit 7 used in the sixth embodiment. In this 
construction, the amplitude of a main-component frequency contained in the 
motor rising noise is detected continually by the sensor mike 9 and the 
detected signal is transmitted to the amplitude control circuit 12, which 
in turn controls actively an increase or decrease in amplitude of the 
additional sound against time-dependent fluctuations of the main-component 
frequency. In this way the amplitude is maintained in a predetermined 
state in response to environmental changes of the main-component frequency 
and changes of various conditions, whereby the suppression of unpleasant 
feeling can be done stably. 
(Experiment 2) 
In order to check the noise masking effect of the fifth to seventh 
embodiments described above there was made evaluation in terms of sincere 
evaluation tests. More specifically, using a drive motor with a 
main-frequency component rising from 800 Hz to 3200 Hz in five seconds, 
there was produced a fluctuating complex sound with a pure tone added 
synchronously, the pure tone being lower in frequency by 100 to 200 Hz 
than the main-component frequency. Eighteen panelists heard the 
fluctuating complex sound and evaluated with respect to three 
items--"noisiness," "unpleasantness" and "shrillness"--. As the evaluation 
method there was used such a seven-stage category evaluation method as 
shown in FIG. 20. The results obtained are as shown in FIG. 21. 
According to the noise masking system of these embodiments, as shown in 
FIG. 21, "unpleasantness" was diminished a maximum of 25% in comparison 
with the original sound, and "shrillness" was diminished a maximum of 32% 
in comparison with the original sound. As to "noisiness" there was little 
change. According to the panelists, they felt noise fluctuation, but the 
noise itself was scarcely unpleasant. 
Thus, from the results of this experiment it turned out that by adding the 
pure tone to the main-component frequency of noise generated at the 
leading edge of revolution of the drive motor both "shrillness" and 
"unpleasantness" could be diminished without increase of "noisiness." 
Thus, according to the noise masking systems of the fifth to seventh 
embodiments, a main-component frequency of noise generated at the time of 
rise to a predetermined number of revolutions of the drive motor is 
detected, then a leading or trailing number of revolutions of the drive 
motor is detected, a frequency higher or lower than the main-component 
frequency is produced, the sound of the frequency thus produced is 
outputted from the speaker while controlling its timing so as to be 
synchronized with fluctuations of the main-component frequency with the 
lapse of time, and this sound from the speaker is added to the motor 
rising noise to mask the noise. In this case, the amplitude of the 
additional sound is changed as desired or is controlled according to the 
loudness of the noise detected by the sensor mike. In this way, the sound 
of a higher or lower frequency than the main-component frequency is 
outputted as a noise masking additional sound from the speaker in 
synchronism with time-dependent fluctuations of the main-component 
frequency, whereby it is possible to provide an image forming apparatus 
such as a laser printer or a copying machine of small size and low cost, 
not giving rise to any psychological unpleasant feeling based on a 
high-frequency fluctuating noise. 
In the fifth to seventh embodiments the number of revolutions at the 
leading or trailing edge of revolution of the drive motor is detected and 
a higher or lower frequency than the main-component frequency is generated 
according to the detected number of revolutions and is added as a masking 
sound to the motor rising noise in synchronism with time-dependent 
fluctuations of the main-component frequency. In this case, even if the 
sound of a higher or lower frequency than the main-component frequency is 
not changed according to the main-component frequency, if it is a sound of 
a certain frequency falling under the fluctuation range of the 
main-component frequency, it is possible to make the fluctuating noise at 
the leading or trailing edge of motor revolution difficult to hear to a 
satisfactory extent and thereby suppress the psychological unpleasant 
feeling. 
In this case, by eliminating time-dependent fluctuations of frequency, the 
secondary sound generated as a masking sound permits a recognizable noise 
to be heard as a steady sound, whereby the fluctuating frequency noise of 
the drive motor can be rendered difficult to hear and hence it is possible 
to suppress the psychological unpleasant feeling. Besides, by making the 
frequency of the masking sound to be added to the motor rising noise equal 
to the main-component frequency in steady operation, it is possible to 
stop the generation of the added masking sound without causing any 
incongruity sense during processing operation. Further, it is possible to 
suppress power consumption during the operation. Description will be 
directed below to an embodiment having such a construction. 
FIG. 22 is a block diagram explaining the construction of a noise masking 
system according to an eighth embodiment of the present invention. In the 
same figure, the numeral 1 denotes a drive motor, numeral 2 denotes a 
motor control circuit, 4 a speaker, 10 a revolution detecting circuit, and 
24 a secondary sound source control circuit. In the construction of this 
eighth embodiment, the secondary sound source control circuit 24 is 
provided in place of the timing control circuit 11 and the frequency 
producing circuit 23 both used in the fifth embodiment. 
In the noise masking system of the eighth embodiment illustrated in FIG. 
22, the motor control circuit 2, for controlling the number of revolutions 
of the drive motor 1, acquires a signal related to the number of 
revolutions from the drive motor 1. For example, if the drive motor 1 
utilizes a magnetic force created by a permanent magnet as a drive source, 
a magnetic flux density around the permanent magnet is measured to detect 
the number of zero points of the flux density, thereby detecting the 
number of N-S pole switchings, and the quotient obtained by dividing the 
number of N-S pole switchings by the number of poles of the permanent 
magnet is obtained as a motor revolution signal. 
Thus, the revolution detecting circuit 10 obtains the revolution signal 
from the signal obtained by the motor control circuit 2. The revolution 
signal is then fed from the revolution detecting circuit 10 to the 
secondary sound source control circuit 24 together with the revolution 
signal which follows one step later. The secondary sound source control 
circuit 24 compares the number of revolutions detected by the revolution 
detecting circuit 10 with a pre-stored number of revolutions of the drive 
motor 1 during operation, and thereby detects an operating condition. 
Further, by taking a difference from the revolution signal which follows 
one step later, the secondary sound source control circuit becomes aware 
of the motor rising state. 
When there is a change in the number of revolutions from the stand-by 
state, the secondary sound source control circuit 24 sends a signal to the 
speaker 4, causing the speaker to output an additional sound (masking 
sound), which additional sound is added to the motor rising noise 
generated from the drive motor 1. In this way the motor rising noise is 
masked by the additional sound. The additional sound adding timing is the 
same as that shown in FIG. 2. 
Reference will now be made to the relation between the change with time of 
the additional sound and frequency. Three types of noises are mainly 
generated from the drive motor 1, which are an electromagnetic noise 
generated from the electromagnetic coil or iron core at the time of 
switch-over of an electric current flowing in the drive motor, a wind 
striking noise generated by friction between a rotary polyhedron mirror 
and air, and a bearing noise generated by a mechanical shaft-bearing 
contact. The electromagnetic noise is close to a pure tone having sharp 
peaks in a narrow frequency band, and the wind striking noise is a 
hydraulic noise having gentle peaks in a wide frequency band. The bearing 
noise is close to a pure tone having many peaks according to the shape and 
size of a ball bearing if used. In the case of an air bearing, noise is 
not recognized. It is known that the frequencies of these noises are in a 
proportional relation to the number of revolutions of the drive motor. 
FIG. 23 is a diagram showing a relation between the change with time of an 
additional sound added as a masking sound and a motor rising noise. In the 
same figure, the curve indicated with a broken line represents the change 
of a main-component frequency of a noise generated when the revolution of 
the drive motor 1 rises. The revolution of the drive motor rises from zero 
or from a stand-by mode wherein the number of revolution is not zero. On 
the other hand, as to the frequency of a masking sound (secondary sound), 
both higher and lower frequencies than the main-component frequency of the 
motor rising noise can be utilized as indicated with solid lines. The 
masking sound is a steady sound which does not fluctuate with the lapse of 
time. Against the motor rising noise, as shown in FIG. 24, the sound 
pressure of the secondary sound for masking is set so that the higher the 
frequency, the lower the sound pressure. 
As mentioned previously, in order to shorten the rise time of the drive 
motor 1, the motor control circuit 2 makes control in such a manner that a 
larger electric current than in the steady state of the motor is allowed 
to flow at the initial stage of the motor revolution and that as the motor 
revolution approaches a predetermined number of revolutions, the electric 
current is adjusted to a small current for diminishing overshoot. 
Therefore, such a fluctuating noise of high frequency which fluctuates 
with the lapse of time gives rise to a psychological unpleasant feeling. 
In the case of a complex sound of several sounds, the timbre audible to a 
human is influenced by both minimum and maximum frequency components and 
hence the fluctuating noise off frequency is made difficult to hear by 
adding a secondary sound of a higher or lower frequency than the 
main-component frequency of the motor rising noise. 
In this eighth embodiment it is assumed that the revolution of the drive 
motor rises from the stand-by mode wherein the number of revolution is not 
zero. However, in the case where the drive motor is OFF in the stand-by 
mode, it is impossible to establish a lower frequency of a secondary sound 
relative to the motor rising noise. In this case, a lower frequency than 
the main-component frequency in several seconds after the rise of motor 
revolution should be established, whereby there is obtained an effect 
almost equal to that obtained above except the period just after the rise 
of motor revolution. 
In this case, moreover, by making the secondary sound into a stationary 
sound whose frequency does not fluctuate with the lapse of time, it 
becomes possible to render the frequency fluctuating noise more difficult 
to hear and thus possible to enhance the noise masking effect. In the 
eighth embodiment a stationary sound whose frequency does not fluctuate 
with the lapse of time is used as the additional sound. For example, 
however, even by the addition of a sound small in the rate of change to 
the motor rising noise, it is possible to expect a certain degree of 
effect. 
During steady operation of the drive motor it is not necessary to generate 
the secondary sound because a fluctuating noise of frequency does not 
occur. Therefore, by making the frequency of the secondary sound equal to 
the number of revolutions of the drive motor in steady operation 
(processing operation), it becomes possible to stop the generation of the 
secondary sound without causing any incongruity sense. Consequently, it is 
possible to prevent increase in power consumption during the processing 
operation. Further, against the main-component frequency of the motor 
rising noise, if the sound pressure of the secondary sound as a masking 
sound is settled so that the lower the frequency, the lower the sound 
pressure, that is, if frequency and reciprocal sound pressure are rendered 
proportional to each other, it is no longer possible that only a specific 
frequency will be conspicuous in the auditory sense, and therefore the 
psychological unpleasant feeling can be suppressed to a further extent. 
Now, a description will be given of a modification of the eighth 
embodiment. Although in the noise masking system of the eighth embodiment 
the revolution detecting circuit 10 acquires a revolution signal from a 
signal provided from the motor control circuit 2, a modification may be 
made so as to obtain the former signal directly from a signal provided 
from the drive motor 1. A noise masking system according to this 
modification is illustrated in FIG. 25. In the same figure, the numeral 1 
denotes a drive motor, numeral 2 denotes a motor control circuit, 4 a 
speaker, 24 a secondary sound source control circuit, and 25 a modified 
revolution detecting circuit. 
In this modification, as a ninth embodiment of the present invention, the 
revolution detecting circuit 25 directly uses a signal provided from the 
drive motor 1 and does not use a signal provided from the motor control 
circuit 2, as shown in FIG. 25. For example, since the drive motor 1 is a 
part of an optical deflector (optical scanner), as shown in FIG. 26, by 
providing a part or the outside of the recording member 56 with a light 
beam sensor such as a photosensor, it becomes possible for the revolution 
detecting circuit 25 to acquire revolution information directly. This 
ninth embodiment is characteristic in that a second sound generating 
mechanism can be constituted separately from the drive motor 1. This is 
advantageous in that the maintainability is improved and the freedom of 
design is enhanced. 
In the noise masking system according to the present invention, asset forth 
hereinabove, the number of revolutions the leading or trailing edge of 
revolution of the drive motor is detected correspondingly to a 
main-component frequency of noise generated at the time of rise to a 
predetermined number of revolutions of the drive motor or at the time of 
fall thereof, then on the basis of the detected number of revolutions 
there is produced, for example, a band-limited noise as a masking sound 
having a frequency range including the main-component frequency, and the 
band-limited noise is outputted from the speaker. Consequently, it is 
possible to provide an image forming apparatus such as a laser beam 
printer or a copying machine of a small size and low cost, not giving rise 
to any psychological unpleasant feeling based on frequency fluctuation.