GB2050095A - Sound volume control in an automobile - Google Patents

Abstract

A microphone 1 detects ambient sound and controls the volume of a radio or the like on the basis of such sound, a filter 31 ensuring that only certain (low frequency) components of the ambient sound spectrum are utilized in the control. A rectifier 32 and smoothing circuit 33 supply the control signal to the volume control stage 4. <IMAGE>

Description

SPECIFICATION A sound volume control device for an acoustic apparatus carried in an automobile This invention relates to sound volume controlling devices for controlling the sound volume of an acoustic apparatus for a vehicle depending on the levels of varying ambient noises.

In listening to music or news provided from a stereo system or a radio of a car, masking of such meaningful sounds as music and news for listeners (which will be hereinafter refered to as listening sounds) by ambient noises is a trouble. Ambient noises are seldom problematic in receiving listening sounds in a quiet room and masking by noises may become puzzling only in ordinary rooms. However, in a car, e.g. a sedan, the level of a noise varies greatly with vehicle speed as shown in Fig. 1, the level reaching, and at times exceeding, the level (approximately 60-65 dB (A)) of an ordinary listening sound in a calm room. The audible level must be varied more than 10 dB depending on the change in noise level, as shown in Fig. 2.

As shown in Fig. 2 there is a relationship between the audible level of a listening sound received in a noise and the noise level. In order for a listener to perceive a listening sound, the listener must adjust a level adjusting device to obtain an appropriate level of the listening sound in response to changes of the noise level. Otherwise the listening sound becomes too weak in some cases and too loud in other cases. In such cases, however, most drivers hesitate to adjust the sound level from a point of safety.

Hence it would be desirable to provide a device that enables automatic control of a listening sound level in response to the varying a noise level so that the sound can be always maintained at a desirable level. Conventional approaches to such automatic control of a sound level are grouped into two categories, one involving detection of either the vehicle speed or the rotational frequency of the engine of a car to obtain a control signal proportional to the rotational frequency, in which the signal controls the level of the sound level, and another involving direct detection of noises by means of a microphone to obtain a control signal proportional to the level of the noises, in which the signal controls the sound level.

The former approach has an advantage that its control signal does not contain any components of the listening sound, hence that it requires only a simple circuit to realize desired control easily. On the other hand, however, it requires a sensor to be installed on the car for detecting the vehicle speed or the rotational frequency of the car. Installation of such a sensor and wiring therefor, however, require much time. Moreover, the signal does not always represent the noise level.

On the other hand, the latter approach has an advantage that it can provide a control signal corresponding to the noise level since the noise is detected by a microphone. Any additional work for the installation will be needed for a volume control device of this type if its microphone is assembled in the body of an associated acoustic-apparatus (e.g.

automobile stereos). If, however, the microphone is installed inside the passenger room of a car, it detects noises and the listening sound simultaneously to producs a signal which contains a component due to listening sound, thereby causing hauling. The reason for this will be described later.

One way of preventing this hauling is to give the control signal a long time constant, which results in abnormally slow adjustment of the listening sound level, which is unacceptable for practical purposes.

If the microphone is mounted outside the room, it does not introduce hauling. However, the microphone must be then mounted within the engine room or on an external surface of a car, and hence requires some protection against ducts, oils, water, heat, etc. Mounting work is also required for the microphone.

In spite of the above advantage, sound volume control devices using microphones have not been in wide use due to the greater disadvantages as mentioned.

An object of this invention is to provide microphone-equipped sound volume control devices which can automatically adjust the sound volume, the tone, or both of them of a listening sound depending on the level of ambient noises, the devices having no such conventional disadvantages that microphones placed inside the room are likely to introduce hauling, and that mounting and wiring work is required if a microphone is mounted at places where it is little affected by a listening sound.

The invention utilizes the fact that noises in a passenger room and a listening sound such as music and news have different spectra. The invention is characterized in that both signals are received by a microphone and passed to a low-pass filter or a band-pass filter, and that only specific frequency components of the noises are separated and used as control signals.

According to the invention the microphone can be placed inside the room of a car without causing hauling, and therefore favorable listening sounds can be attained. Furthermore, the microphone may be built in inside the acoustic apparatus, e.g. automobile stereos, so that otherwise necessary wiring and mounting work can be eliminated. Thus, even general users can mount such devices. In addition, since the microphone is not placed outside the room or in the engine room of the car, no means are necessary to prevent water, dusts, or oils. Moreover, since control signals are formed mainly from the noise detected inside the room, volume and tone, control of the listening sound is performed naturally and smoothly.Since the control signal does not involve signals representing the rotational frequency or the vehicle speed, the invention can be used independently of other apparatus for the latter signals. According to the invention a circuit required therefor to extract specific frequency components of a noise is simple in structure.

These and other objects of the invention will be seen by reference to the description, taken in conjunction with the accompanying drawings, in which: Figure 1 shows a relationship between the vehicle speed and the noise level inside the passenger room of a car; Figure 2 illustrates a typical relationship between the noise level and level of a listening sound heard in the noise; Figure 3 is a 1 /3-octave band spectrum of a noise inside the room of a running car; Figure 4 shows spectra of a noise and a listerning sound inside the room of a car; Figure 5 is a block diagram of an embodiment of this invention; Figure 6 is an example of the circuit for an automatic volume control device according to the invention.

Further details of the invention will be described below. Prior to the invention the present inventor performed measurements concerning the characteristics shown in Figs. 1 and 2, and, following confirmation of several facts as mentioned above, analyzed noises involved. The analyses are 1 /3-octave band analyses. The reason for the use of them is that noises inside the room of a car have continuous spectra with little sharp line spectra. Fig. 3 shows an example of a 1 /3-octave band measurement of noises inside the room of a car.

The following can be said from the analyses. The energy of the noise inside the closed room of a running car is contained mostly in a low-frequency region thereof and energy density decreases with the frequency. This tendency is generally true for all makes of cars and for all vehicle speeds.

A relationship between a noise and an audible listening sound is also examined. Fig. 4 schematically illustrates frequency characteristics of a noise and a listening sound inside the room of a car. In this Figure, A is the spectrum of a noise under a normal running condition, B, is the spectrum of a listening sound under the same normal running condition in which noise condition is A (in general, the level of the listening sound is variable with time when a sound volume control knob of an acoustic equipment is maintained at constant, and thus B1 is considered to be a mean value of its variance), and B2 is the spectrum of the listening sound under the condition that the varying listening level is at the maximum. Although the levels of such a noise and a listening sound are varying in time, the maximum intensity of a noise spectrum normally exceeds that of a listening sound.In this case the output of the microphone is mainly due to the noise present.

Hence the control signal obtained therefrom corresponds to the noise level and can control the sound volume of the listening sound.

However, the level of the listening sound itself sometimes changes greatly with time. For example, in the case of music, the level might exceeds the maximum noise level as shown by a spectrum B2 in Fig. 4. This often happens when the noise level is low and the level of the sound is rather high. In this case the source of the output of the microphone is mainly due to the listening sound, in which case hauling will take place.

In addition to the noise under consideration, there are many other possible sources of noises having noise levels greater than that of the former noise and causing hauling. They are, for example, conversation of people in the car, touching a portion of the car in the neighbourhood of the microphone, and children's cries.

The invention uses, as the source of control signals, a portion of a noise spectrum having frequency less than X where X is the frequency at which the noise spectrum crosses the spectrum of the listening sound B2, as shown in Fig. 4. Note that the noise level always exceeds that of a listening sound in this region. According to the experiments done, components of a listening sound with frequencies less than 90 Hz are far weaker than those of the noise. Hence, a signal obtained by passing the output signal of the microphone through a low-pass filter having a cutoff frequency of about 90 Hz, is mainly due to the noise and is little influenced by the listening sound or by conversation. The values of X are to be properly determined individually according to, for example, the makes and the expected running conditions of cars. From the results of experiments performed with cutoff frequency fc of the low-pass filter being 75, 106, and 1 50 Hz and a characteristic cutoff value of the filter being approximately - 60 dB/Oct., it was found that control signals may be affected by a listening sound when fc > 150 Hz. Thus, the cutoff frequency should not be greater than 1 50 Hz, but need not be smaller than 90 Hz.

The invention will be described further, with reference to an embodiment.

Fig. 5 is a schematic diagram showing the structure of an embodiment of the invention.

Fig. 6 shows details of the circuit for the above embodiment.

In Fig. 6 a microphone, e.g. an electret microphone, receives a noise, which is ampli fied by an amplifier 2. The amplified signal from the amplifier 2 is delivered to a circuit means 3 to obtain a volume control signal for controlling the volume of sound reproduced by an acoustic apparatus. The circuit means 3 comprises a filter 31 which separates noise components from the output of the amplifier 2, a rectifier 32, e.g. a bridge rectifier, to rectify the noise components given by the filter 31, and a smoothing-and-time-constant circuit 33 which smoothes the output of the rectifier 31 to generate a DC control signal, modifies the signal generated to one having a certain time constant and then transmit the resultant signal to a sound reproducing circuit 4.The filter 31 consists of, for example, a band-pass filter, which comprises a CR type high pass filter and an active-type low-pass filter as seen in Fig. 6. The CR type high pass filter blocks oscillatory components having frequencies less than about 20 Hz and passes other components having frequencies greater than the above. The active-type low-pass filter passes those components having frequencies less than the above mentioned cross frequency X. Since the structures of such CR type high pass filter and active type low-pass filters are well known to those skilled in the art, detailed description thereof will not be given here. The filter 31 in this embodiment is a band-pass filter, which may be substituted for by a low-pass filter, as already mentioned above.The time-constant circuit 33 determines quickness of the response of the apparatus following a noise which is varying with time and comprises resistors R33, and R332 and a capacitor C331. The time constant T of the time-constant circuit 33 may be arbitrarily chosen in a range which is normally 1-5 seconds. The time constant in this region is appropriate to reproduce sounds such that the sounds are felt natural and favorable to listeners. This control signal from the time-constant circuit 33 varies in response to noise signals passing the filter 31. The level of such a noise signal is usually in very good correspondence with the noise level. The control signals are applied to the gate of a field effect transistor (FET) Tr42, which is connected with the emitter of an amplifier transistor Tr4, in the sound reproducing circuit 4 and serves as a volume control means.The drain of the FET Tr42 is connected with the emitter of the transistor Tr4, via a capasitor C41. The source of the FET is grounded via a capacitor C42 and a variable resistor VR, and is also connected with a movable terminal of another variable resistor VR2. The variable resistor VR, is for adjusting amplification of a sound volume, while the variable resistor VR2 is for setting a pinch-off voltage. The FET Tr42 gives rise to varying resistance across the source and the drain thereof in response to the level of the signal applied thereto, so that the amplification by the amplifier transistor Tr4, varies with this resistance, thereby fulfilling volume control depending upon the level of a noise. In a performance test using an actual sedan in which use in made of a filter 31 of the volume control device comprising a CR type high pass filter with its cutoff characteristic value being - 6 dB/Oct. and f, = 20 Hz and quinted active type low-pass filters with their cutoff characteristic values being - 30 dB/Oct. and fc = 90 Hz, observers felt the volume of a listening sound almost constant and no manual adjustment was needed. No hauling was observed, either. Thus the practical utility of the device was proved. The function of the device will be made certain if the passing-band freqency range of the filter 3 is chosen to be 22 Hz-80 Hz.

Claims (5)

1. A sound volume control device for controlling the sound volume of an acoustic apparatus carried in an automobile, comprising: a microphone for receiving ambient sounds to generate electric signals with the levels corresponding to the level of said ambient sounds; an amplifier for amplifying said electric signals; a rectifier circuit for rectifying said electric signals thus amplified to generate sound volume control signals therefrom; a circuit means to transmit said control signals to a sound volume control portion of a sound reproducing circuit system; wherein said sound volume control device includes a filter to be connected in front of said rectifier circuit, for cutting off at least those spectral components of a signal that have frequencies higher than the frequency at which the spectrum of the listening sound having a maximum sound pressure level crosses that of an ambient noise.

2. A sound volume control device according to Claim 1, wherein said filter is a lowpass filter for passing noise components having frequencies lower than the frequency at which the spectrum of a listening sound having a maximum sound pressure crosses that of an ambient noise.

3. A sound volume control device according to Claim 2, wherein said filter can further block low-frequency components of noises due to running engines.

4. A sound volume control device according to Claim 1, wherein said filter is fed as an input, with the output of said microphone receiving ambient sounds inside the room of an automobile.

5. A sound volume control device substantially as hereinbefore described with reference to and as shown by the accompanying drawings.