GB2267411A

GB2267411A - Noise reducing earphone with combined microphone/loudspeaker

Info

Publication number: GB2267411A
Application number: GB9211136A
Authority: GB
Inventors: Mark John Snee
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-05-26
Filing date: 1992-05-26
Publication date: 1993-12-01
Also published as: GB9211136D0

Abstract

A noise reducing earphone has a single diaphragm 3, a driving coil 4 and a moving observer coil 6. The diaphragm 3 both transmits and receives sound. Two coils are necessary to distinguish between transmitted and received sound. It works on the principle that the total power of the driving coil 4 minus the applied electrical power equals the power of the received sound. <IMAGE>

Description

NOISE REDUCING EARPHONE This invention relates to a noise reducing earphone.

Several earphones have been developed to reduce the level of environmental noise entering the ear. They generally fall into two categories. One type prevents noise from entering the ear with a sealed earphone to outer-ear interface. The other type eliminates noise by destructive interference of sound waves.

This latter type has a microphone and speaker which are positioned close together. The microphone picks up noise from the surroundings and the signal is processed using feedback control to produce the desired canacelling response at the speaker.

According to the present invention there is provided a noise reducing earphone comprising a moving diaphragm, a permanent magnetic core, a driving coil, a moving observer coil, a casing, means for deriving an incoming sound signal from the signals produced at each coil.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Fig.1 shows the earphone in cross-section.

Referring to the drawing, the earphone 1 comprises a casing 2, a single diaphragm 3, a driving coil 4, a permanent magnetic core 5, and also a moving observer coil 6. It works on the principle that the total power of the driving coil 4 minus the applied electrical power equals the power of the recieved sound. It deals with instantaneous power not r.m.s. power.

The reason for considering power is that it is not possible to obtain the recieved sound signal by simply subtracting the sound producing signal from the signal at the driving coil 4. This is because the relationship between the applied voltage and the corresponding voltage across the driving coil 4 is noo-linear. When the driving coil 4 moves, it moves the diaphragm 3 and also air molecules which have inertia and damping properties. This movement induces a voltage in the driving coil 4 because it is moving through the permanent magnetic field. Also, the self inductance of the driving coil 4 produces an opposing voltage whenever it's magnetic field is increasing or decreasing. It is these changing magnetic fields which cause the non-linearity between the applied signal and the voltage across the driving coil 4.

There are additional problems. When a sound producing signal is applied to the driving coil 4, the force between the permanent magnetic field and the driving coil's magnetic field varies. When incoming sound strikes the diaphragm 3, the diaphragm 3 is deflected against this varying force.

There is also the Doppler effect. The velocity of the diaphragm 3 affects the apparent frequency of the incoming sound.

This invention utilises the fact that voltages are induced in a coil whenever it is subjected to a changing magnetic field. The magnetic field may be changing in magnitude or it may be moving. For simplicity, the two coils are given equal numbers of turns. The voltages across them are as follows: VD= V -2BNUr(UT + + - LdI dt V0 = -2BN#r(UT # UR) - LdI dt where VD= voltage across driving coil VO= voltage across observer coil VA= applied voltage B = field strength of permanent magnet N = number of turns of each coil L = inductance of each coil r = radius of each coil T = velocity of coil for transmitted sound UR= velocity of coil for recieved sound I = the current which causes a change in the overall magnetic field strength The minus signs denote the fact that the induced voltages tend to oppose the applied voltage. The moving observer coil 6 should have a large resistor in series to minimise the current through it. A large current would affect the performance of the driving coil 4 adversely.

When VO is subtracted from VD, the remaining voltage is equal to the applied voltage. The applied power is then given by the equation: PA R where R is the circuit resistance.

The total power of the driving coil 4 is. given by the equation: PT = IDVD + 2BtNr(U + UR )Io + LIDdI dt where ID = current through the driving coil 4.

is is subtracted from PT to give the power of the recieved sound. The recieved sound voltage is then given by the equation:

where VR = recieved sound voltage PR = recieved sound power R = circuit resistance This gives a magnitude only. The sign can be obtained in the following way: Before ID is used to obtain PT, it is given the same sign as that of the applied voltage. The sign of PR is stored, and the magnitude only of is used in the equation for VR. If the sign of P is negative, then VR used in the equation for V . If t R is given the opposite sign of VA If it is positive, then VR is given the same sign as VA.

The steps mentioned so far adequately overcome most of the problems of obtaining a recieved sound signal. However, they do not deal with the Doppler effect. If the sound is to be 'listened' to by a microprocessor, then the Doppler effect must be considered. The movement of the diaphragm 3 causes a distortion of the observed,incoming sound frequency.To calculate this distortion, the transmitting velocity of the diaphragm 3 must be known. This may be approximated by the applied voltage. An exact value for the transmitting velocity can be obtained with a stationary observer coil but this makes things even more complicated.

If the purpose of the earphone is just simply to eliminate all noise, then the Doppler effect can be ignored because the earphone is not being selective. Any frequency distortion is irrelevant because the diaphragm 3 will just eliminate the sound as it encounters it. The degree of elimination can be controlled by a type of volume control.

Because the incoming sound is travelling across the surface of the diaphragm 3, the diaphragm 3 is less responsive to sound in the higher frequency range. This sound has a shorter wavelength and as it moves across the diaphragm 3 it has high pressure and low pressure components causing a deflection. These opposing components tend to cancel and reduce the overall deflection of the diaphragm 3. The feedback control circuit, controlling the diaphragm 3 response, should therefore be made to be most sensitive to these higher frequecies.

Claims

1 A noise reducing earphone comprising a moving diaphragm, a permanent magnetic core, a driving coil,a moving observer coil,a casing, means for deriving an incoming sound signal from the signals produced at each of the forementioned coils.

2 A noise reducing earphone as claimed in Claim 1 wherein the observer coil is attached to the diaphragm adjacent to the driving coil.

3 A noise reducing earphone as claimed in Claim 2 wherein the power of the incoming sound signal is differentiated from the power of the transmitting sound signal.

4 A noise reducing earphone as claimed in Claim 3 wherein an incoming sound signal is derived from the power of the incoming sound signal.

5 A noise reducing earphone as claimed in Claim 4 wherein the incoming sound signal is processed using feedback control to produce a response which cancels the undesirable components of the incoming signal.

6 A noise reducing earphone as claimed in Claimed 5 wherein the response is mixed with the sound producing signal to give the overall transmitting sound signal which drives the diaphragm.

7 A noise reducing earphone substantially as described herein with reference to fig.1 of the accompanying drawing.