It is known to provide earphones which may be inserted into a user's ear cavity or headphones comprising a small loudspeaker mounted on a headband and arranged to be placed against or over the user's ear. Such sound sources transmit sound to a user's inner ear via the ear drum using air pressure waves passing along the ear canal.
A typical conventional earphone uses a moving coil type transducer mounted in a plastic housing. The moving coil is connected to a light diaphragm which is designed to fit into the entrance of the ear canal. The moving coil and diaphragm are light and are coupled intimately to the eardrum at the other end of the ear canal. The acoustic impedance of the eardrum and ear canal seen by the moving coil transducer is relatively small. This small impedance in conjunction with the intimate coupling means that the motion requirements of the moving coil transducer are relatively low.
A moving coil transducer requires a magnetic circuit, which typically contain metal parts, e.g. steel or iron pole pieces, to generate magnetic field lines for the coil to move. These parts provide a relatively large inertial mass which combined with the low motion requirement means that relatively little vibration enters the housing.
There are disadvantages associated with both headphones and earphones. For example, they may obstruct normal auditory process such as conversation or may prevent a user from hearing useful or important external audio information, e.g. a warning. Furthermore, they are generally uncomfortable and if the volume of the sound being transmitted is too high they may cause auditory overload and damage.
An alternative method of supplying sound to a user's inner ear is to use bone conduction as for example in some types of hearing aids. In this case, a transducer is fixed to a user's mastoid bone to be mechanically coupled to the user's skull. Sound is then transmitted from the transducer through the skull and directly to the cochlea or inner ear. The eardrum is not involved in this sound transmission route. Locating the transducer behind the ear provides good mechanical coupling.
One disadvantage is that the mechanical impedance of the skull at the location of the transducer is a complex function of frequency. Thus, the design of the transducer and the necessary electrical equalisation may be expensive and difficult.
Alternative solutions are proposed in JP56-089200 (Matsushita Electric Ind Co Ltd), WO 01/87007 (Temco Japan Co, Ltd) and WO 02/30151 to the present applicant. In each publication, a transducer is coupled direct to a user's pinna, in particular behind a user's earlobe, to excite vibration therein whereby an acoustic signal is transmitted to the user's inner ear.
As set out in WO 02/30151, the transducer may be piezoelectric. Like the moving coil type transducer in a conventional earphone, the piezoelectric transducer requires protection from mechanical damage. Furthermore, the piezoelectric transducer must be mechanically coupled to the pinna and this coupling must be protected. Accordingly, the transducer may be mounted in a protective housing.
The piezoelectric transducer is not in intimate coupling with the eardrum and drives through the relatively high impedance of the pinna. Furthermore, sound is transmitted to the eardrum through a mechanical coupling rather than an audio coupling. Accordingly, a relatively high level of vibration energy is required to maintain the same level at the eardrum as a conventional earphone.
Unlike in a moving coil type transducer, a piezoelectric transducer does not have a high inertial mass to which the vibrations may be referenced. Accordingly, the housing may vibrate to produce unwanted external sound radiation. Such leakage of sound radiation may annoy nearby listeners and may reduce the privacy for the wearer and is detrimental to the performance of the audio apparatus. Accordingly, an object of the invention is to provide an improved design of housing.