AREA OF THE INVENTION Hearing aid with a casing accommodating at least one sound to electric converting transducer converting sounds in the environment into electrical signals and a signal processing unit for processing the electrical signal according to the needs of the user and an electrical to sound transducer for receiving the amplified electrical signal and delivering a sound signal to the ear.

BACKGROUND OF THE INVENTION In hearing aids it is a known problem, that sounds either generated inside the casing or entering the casing from the environment may enter the microphone and disturb the function of the hearing aid. A major sound source within the casing is the receiver, and when sounds from the receiver reaches the microphone it may give rise to disturbing feedback signals. Also it is known that noise transmitted through the casing structure and originating from handling or operation of the hearing aid may enter the microphone and cause uncomfortably loud sounds.

SUMMARY OF THE INVENTION The object of the invention is to provide a hearing aid wherein the above mentioned problems are solved. This is achieved by a hearing aid of the above mentioned type, wherein at least one further sound to electric converting transducer is provided inside the casing, whereby a sound passage leads from the interior of the casing to one of the at least one further transducers and/or whereby a sound passage is provided between the casing and one of the at least one further transducers. The signal from the further transducers or microphones is fed to the DSP unit, which performs the signal processing.

Here signal processing is carried out in order to perform operations representing a suppression of unwanted signals from the signal of the main microphone, such that sound from within the casing and/or sounds transmitted through the casing and which have entered the main transducer may be compensated for. This suppression of unwanted

signals is facilitated by the signals from the further microphones. This has the advantage that the demands for sound isolation of the receiver can be reduced, and also the demand that the main transducer or microphone be isolated from the casing, are not quite as strict. This gives room for more compact designs of these items, and thereby the possibility of smaller casings. Only one microphone may be used for picking up unwanted signals and in this case it can be dedicated to either internal noise or noise transmitted through the casing walls. It is also possible that the one further microphone is arranged to receive both un-wanted signals, however, this may cause some problems at the signal processor as the two un-wanted signals may not have to be treated in exactly the same way. Otherwise two microphones are provided and one of these is dedicated to internal noise and the other to noise transmitted through the casing walls.

Especially by the use of the very small MEMS transducers as mentioned in claim 2 the advantages becomes noticeable, as microphones produced in this technology are very small. Also several microphones may be produces at the came substrate and at advantageous prices.

In a preferred embodiment the microphones are produced on a semiconductor chip. In this way the microphones becomes completely integrated with the signal processing unit of the hearing aid, and this saves both space and wiring in the hearing aid.

In an embodiment of the invention a first further transducer is provided with a sound passage leading to the interior of the casing and a second further transducer is provided which is acoustically connected to the casing. This is advantageous as the noise signals from within the casing and the noise signal from the casing as such may have to be treated differently.

In a further embodiment the at least one further transducer has a sound path leading to the interior of the casing and is acoustically connected to the casing. Thereby one and the same transducer is used for picking up both the sound signal from within the casing and the sound signal in the casing wall. This may cause a somewhat more complicated DSP processing but on the other hand only one transducer is necessary.

In an embodiment the at least one further transducer is arranged adjacent to the receiver.

The receiver or loudspeaker is often the main source of sound within the casing, and by placing the further microphone near this source a good representation of the signal may be obtained. This facilitates the compensation for the possible feed back effect caused by this signal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic representation of a hearing aid according to prior art, and FIG. 2 shows a schematic representation of a hearing aid according to the present invention.

DESCRIPTION OF A PREFERRED EMBODIMEN The hearing aid 1 shown in fig. 1 has a casing 5. The casing 5 may be the shell around the components of an ITE (In the ear) hearing aid or the casing of a BTE (Behind the Ear) hearing aid. The hearing aid 1 has a sound to electrical signal converting transducer or microphone 2 for picking up sounds from the environment and a signal processing unit 4, wherein the sound signal from the microphone undergoes processing. The signal processing unit outputs an electrical signal to the receiver 3, and the receiver outputs a sound signal to the ear. The microphone 2 is shown as a single microphone, but several microphones may be incorporated into the hearing aid in order to capture sounds from the environment so as to produce various effects such as directionality. The expression "main microphone"is used for the microphone 2 or group of microphones 2, which captures the sound signal from the environment. Also the apparatus contains some sort of energy supply system like a battery. This all is evident for the skilled person and is not further described.

A hearing aid according to the invention is shown schematically in fig. 2. In this hearing aid two supplemental microphones 6,7 are arranged within the casing 5.

The microphone 6 is arranged to have a good sound-conducting path to the material of the casing 5. Thereby this microphone will pick up sounds, which are transmitted through the casing material. In this way the further microphone 6 picks up un-wanted

sounds coming from handling the apparatus or operation of the apparatus. Also signals from the receiver may leek to the casing wall causing the material of the casing to function as a feed back path. The signal from this microphone is used within the signal processing unit in order to compensate for the same signal picked up be the primary microphone. Signal processing known in the art, using the signals from the further microphone 6 and from the primary microphone 2 could do this.

The further microphone 7 is placed within the apparatus casing 5 and will pick up sound within the apparatus. This could be sound coming from the receiver 3. The receiver 3 is arranged so as to radiate as little sound energy as possible to the inside of the casing, but some sound is bound to radiate to the inside of the casing, and this sound may reach the primary microphone 2 and cause disturbing feed back problems. By picking up the unwanted signal with the further microphone 7 and processing this signal along with the signal received by the primary microphone 2 this source of feed back may be considerably reduced.

In fig. 2 the two microphones 6 and 7 are shown as two separate microphones, but they could be integrated in one and the same unit. Such a microphone-unit must have a good sound conducting connection with the material of the casing and at the same time an opening to the interior of the casing.

The operation of the signal processing unit in order to compensate for the unwanted signals which has entered the primary transducer and which are also captured by the one or more further transducers is well known in the art. Examples of such signal processing are given in US 5201006 and US 5033090.