A headphone having a noise-cancelling function (noise-cancelling headphone) includes a microphone and a noise-cancelling circuit (hereinafter referred to as “NC circuit”). The microphone collects external sounds (hereinafter referred to as “noises”) around the headphone. The NC circuit generates cancelling signals corresponding to the noises collected by the microphone. The headphone combines sound waves corresponding to the cancelling signals generated by the NC circuit and sound waves corresponding to the reproduced signals from a sound source such as a music player connected to the headphone, and then outputs the combined sound waves from a driver unit. That is, the headphone outputs musical sounds (hereinafter referred to as “reproduced sounds”) derived from the reproduced signals from the sound source with cancelling (muting) of the noises.
Examples of the scheme for generating cancelling signals include a feedback scheme (hereinafter referred to as FB) and a feedforward scheme (hereinafter referred to as FF).
A built-in microphone of the FB noise-cancelling headphone is disposed in the interior of a housing unit (ear piece) of the headphone and near the ear of the user. The NC circuit analyses in real-time the signals of the noises collected by the microphone and generates cancelling signals minimizing the noises at the position of the eardrum of the user. The FB noise-cancelling headphone collects the noises at the position near the ear of the user. Thus, the noise-cancelling effect of the FB noise-cancelling headphone is higher than that of the FF noise-cancelling headphone. In addition, the response of the FB noise-cancelling headphone to the variation of noise components is good.
However, when the built-in microphone of the FB noise-cancelling headphone collects the reproduced sounds in addition to the noises, and then the NC circuit generates cancelling signals, the sound quality of the reproduced sounds output from the headphone are degraded. In addition, the ear piece of the FB noise-cancelling headphone should be sealed in the state which the headphone is worn on the head of the user, to enhance the noise-cancelling effect. When the ear piece is sealed, the reproduced sounds output from the headphone may be muffled. Thus, the sound quality of the reproduced sounds are degraded. To address the problem, a FB noise-cancelling headphone generally includes a filter to correct the sound quality of the reproduced sounds.
On the other hand, a built-in microphone of the FF noise-cancelling headphone is disposed at the exterior of the housing unit of the headphone. The NC circuit analyses the signals of the noises collected by the microphone and predicts the variation in the noises that will reach the eardrum of the user wearing the headphone. The NC circuit generates cancelling signals based on the result of the prediction. The FF noise-cancelling headphone does not need the placement of the microphone on a limited space near the ear of the user. In addition, the built-in microphone of the FF noise-cancelling headphone is disposed at a position remote from the driver unit. Thus, the FF noise-cancelling headphone is less likely to collect the reproduced sound output from the driver unit and to generate cancelling signals from the reproduced sounds than the FB noise-cancelling headphone. That is, the sound quality of the reproduced sound output from the FF noise-cancelling headphone are less susceptible to the cancelling signals generated from the reproduced sounds than the sound quality of the reproduced sound output from the FB noise-cancelling headphone.
Unfortunately, the FF noise-cancelling headphone often indicates directionality of the noise-cancelling effect depending on the position of the built-in microphone. Unlike the built-in microphone of the FB noise-cancelling headphone, the built-in microphone of the FF noise-cancelling headphone is disposed at the exterior of the housing unit. Thus, the FF noise-cancelling headphone generates noises caused by wind pressure of the blowing of the wind, for example. In other words, the FF noise-cancelling headphone is susceptible to the influence of the wind around the headphone. As a result, the FF noise-cancelling headphone can cause a feeling of strangeness or discomfort to the user, when the noise-cancelling function is activated.
Schemes have been proposed to dispose a microphone in the interior of the housing unit of the FF noise-cancelling headphone to avoid the influence of the wind described above (for example, refer to Japanese Unexamined Patent Application No. 2010-109799).
The FF noise-cancelling headphone disclosed in Japanese Unexamined Patent Application No. 2010-109799 includes a driver unit, a baffle plate, and a microphone. The microphone is disposed behind the baffle plate to which the driver unit is attached (that is, in the interior of a rear air chamber). The baffle plate includes a flange portion and a groove. The flange portion is disposed on the forward portion and the rear portion in the thickness direction of the baffle plate. The groove is formed between the flange portion and along the outer circumference of the baffle plate. The flange portion that is disposed on the rear portion of the baffle plate has a sound collecting hole penetrating the flange portion in thickness direction. The sound collecting hole is in communication with the groove. The external noises are collected by the microphone through the groove and the sound collecting hole.