So-called ear fitting type noise reducing apparatus which is used in working in an environment in which external noises are very loud has been widely known. This ear fitting type noise reducing apparatus is worn in such a manner that the headphone cap covers the ears for reducing the external noises. The headphone cap is pressed upon the temple of the head so that external noises from the environment will not pass through a gap between the temple of the head and the headphone cap. The noise reducing apparatus has been used in such a manner.
However, it is necessary to strongly press the headphone cap of the above mentioned prior art noise reducing apparatus upon the temple of the head in order to prevent external noises from the environment from entering the gap between the headphone cap and the temple of the head. Accordingly, an oppressive sensation is felt on the temple of the head and the noise reducing apparatus per se is large in size and heavy. Therefore, use of this apparatus for a long period of time is unbearable. Also a headphone cap cannot shield noise having low frequencies (several hundred Hz to not higher than 1 KHz). If noises are reduced by the above mentioned method and reproduced sounds are attempted to be listened to, an effect of so-called localization in head occurs due to reflections of the reproduced sound in the headphone cap so that an adverse influence is given to communication, etc. The effect of localization in head occurs as follows: Reflections occur between the headcap and the entrance of the external auditory canal since the acoustic impedance in the entrance of the external auditory canal is different from the impedance in the external auditory canal. The reflected wave returns to the ear drum, resulting in that a sound image is localized in the head. An uncomfortable feeling as if the ear were plugged is felt.
So-called active type earphone devices have been known as earphone devices used in working in loud noise places. Such type earphone devices reduce the noises generated near the headphone unit by converting the external noises into electrical signals by a microphone unit and by negatively feeding back the converted electrical signal in an opposite phase via a negative feed back loop.
Such active type earphone device has a high sound shielding ability and is thus capable of almost shielding external sounds. Accordingly, it is very inconvenient for the user to remove the earphone device from the ear each time when the user desires to listen to external sounds such as emergency information or human voices.
The present invention was made under such circumstance.
It is an object of the present invention to provide an earphone device which will not cause an effect of localization in head and can be used for a long period of time without pressing the head and is capable of effectively reducing external noises.
It is another object of the present invention to provide an earphone device within which external sound can be listened to even if the earphone device which has been worn is not removed.