GB2261343A - Directional single-ear earphone, hearing aid - Google Patents

Abstract

Localisation of sound through a single ear is provided by introducing into the ear canal a plurality of sound sources each at a different location using tubes 19a, 19b. Each sound sources derives its input from a different transducer 16a, 16b. The transducers are arranged such that they are each most responsive to sound originating from a different direction. In this form the invention provides a directional hearing aid. A further embodiment (Fig. 11) provides a single-ear earphone for a personal stereo cassette player. There is extensive discussion of a mechanism by which a single ear may be used to locate the direction of a sound source. <IMAGE>

Description

Improvements In Sound Reproducinq Devices This invention relates to sound reproduction and more particularly to a method and device which enables localisation of sound through a single ear. The invention is of particular relevance to two devices that can reproduce sound into one ear, a hearing air and the earpiece for a personal cassette player.

According to one aspect of the invention there is provided a method of providing localisation of sound through a single ear, characterised in the steps of introducing into the ear canal a plurality of sound sources, each at a different location and each derived from a different transducer, and arranging that the transducers are each most responsive to sounds originating from a different direction.

According to a second aspect of the invention there is provided a device for introducing two or more separate sources of sound into a single ear.

The invention also provides a device for carrying out the method characterised in means for introducing into the ear canal of a single ear a plurality of sound sources each at a different location in the canal.

Directional hearing is maintained in the case of the hearing aid, and stereo hearing is obtained from one earpiece in the case of the personal cassette player.

In order that the invention and its various other preferred features may be understood more easily, some experimental investigations and a method and construction in accordance with the invention will now be described, by way of example only, with reference to the schematic drawings in which: Figure 1 illustrates an arrangement for a so called "two tubes experiment"; Figure 2 illustrates parts of the inner ear; Figures 3a and 3b illustrate the function of the parts illustrated in Figure 2; Figures 4a and 4b illustrates the principle of experimental work relating to the ear by analogy to the "pecking duck principle"; Figures 5a to 5c illustrate the action of the outer hair cell with; Figure 5a showing a cross section of the OHO region; Figure 5b showing a plan view of the OHO region indicating the "bow of ship action"; and Figure 5c showing a plan view of the OHC region indicating the "bucket action"; Figures 6a to 6c illustrates the differential innovation of the OHC with; Figure 6a illustrating the normal condition with all three equally innovated; Figure 6b illustrating the inner innovated; Figure 6c, indicating the outer innovated; Figure 7 illustrates a typical hearing aid; Figure 8 illustrates the principle of the present invention; Figure 9 illustrates a behind the ear hearing air constructed in accordance with the present invention; Figure 10 illustrates a conventional ear piece for a personal cassette player; and Figure 11 illustrates an ear piece for a personal cassette player constructed in accordance with the present invention.

Localisation of hearing obtained by one ear involves both direction and distance. Our investigation looks at direction only.

There is already evidence for directional hearing in one ear. Clinical evidence shows that in free field audiological testing one ear can act as two, as one cannot determine monaural hearing loss in such investigations (I.

Tucker & M. Nolan "Educational Audiology" Croom Helm 1984 p29).

Scientific investigations have been controversial where a theory of binaural directional hearing is the accepted view. However many have provided evidence for monaural directional hearing, e.g. J Blauert 1969 (J.

Blauert "Sound Localisation In The Median Plane" Acoustica Vol.22 1969/70 p205-213), D W Batteau 1966 (D.W. Batteau "The Role of The Human Pinna In Human Localisation" 1966 p158-180) and more recently Gulick et al 1989 (W. Lawrence Gulick, G.A. Gescheider, R.D. Frisna "Hearing: Physiological Acoustics, Neural Coding, Pschoacoustics" Oxford University Press 1989 p317-339). Thus our suggestion is not a new one.

The assumption of localisation by one ear overcomes problems with the current binaural theory for directional hearing covered in the Gulick et al reference. However there are problems with the physics for directional hearing by one ear. The size of the ear drum is very small and the directional response should only occur at high frequencies (well above the oral range). It is probable that one ear localisation has been dismissed by past theorists for this reason.

To clarify the situation the inventors devised an experiment which demonstrated that directional hearing was achieved by one ear, and more specifically from the ear canal inwards. To explain how this could occur, a new theory was produced which gives an explanation of the hearing mechanisms. This experiment was called the two tubes experiment.

Referring now to Figure 1, two tubes 1,2 driven from small speakers 3,4 were introduced into the ear canal 5.

The tubes ends are maintained in fixed location to one another by only a small attachment 6 at their ends (in order to minimise the cross coupling of sound). The direct route for the sound from the speakers to the ear was reduced by surrounding the speakers with a sound absorbing material 7.

Changing the relative amplitude of the same sound in the two tubes altered the apparent localisation to the listener. This localisation was always in the plane of the two tubes. If this was horizontal, the perceived sound direction could be moved from front to back and visa versa.

If it was vertical, perception changed to up and down (and visa versa).

It was postulated that the relative amplitude between the two tubes changed the angle of the wave front that progressed to the ear drum. The location effect was lost if the ends of the tube were displaced so that vector combination of the emerging sounds was lost. Thus it was established that the eardrum and mechanisms beyond respond to the direction of sound as well as vibration.

The Outer Ear.

We begin our explanation at the outer ear, which consists of a cartilageneous outer one third and the remaining two thirds is a bony cylinder. This means the later part is very regular so that the propagation of sound, particularly a sharp wavefront will remain consistent for arrivals from different directions.

The Middle Ear.

For approximately 80% of its area the ear drum consists of a rigid cone, which is sealed to the annulus of the meatus by the flexible pars flaccidae. The ear drum can therefore undergo quite large displacements, so impulse sounds can displace the ear drum according to direction. It is suggested that it is the displacement of the ear drum that transduces sound direction and it is vibration that transduces the sound.

The Ossicles.

This complicated 3 bone structure now becomes clear if it is assumed they act to convey displacement of the ear drum as well as vibration. (G.v Bekesy "Experiments In Hearing" McGraw-Hill 1960 p202) has shown in his work that the two axis for displacement of the ear drum result in rotation of the stapes.

How this occurs is shown in Figure 2. The centre of rotation of this system lies in the bulbous head of the malleus 8 to which the pars tensa 9 is rigidly fixed.

Displacement of the pars tensa results in rotation of the malleus about this centre 10. The head of the incus 11 is a "socket" around the "ball" head of the malleus 8, such that it rotates about the same centre.

In short the vertical displacement of the pars tensa, causes rotation of the stapes 12 about its long axis. This is illustrated schematically in Figures 3a and 3b. For horizontal displacement, the stapes 12 rotates about its short axis. The system of pars tensa 9 and 3 bone ossicles is thus capable of conveying information on the position of the tympanum relative to the annulus of the meatus. (W.

Lawrence Gulick et al op. cit. p79 fig 4.3) Hence it is suggested that the stapes conveys directional information to the cochlea. This would also explain the close location of the vestibular system to the stapes.

The Cochlea.

In 1960 Bekesy noted "the paradoxical wave motion" phenomenon in the cochlea. So that for continuous waves (ie pure tones) there is no difference in response of the basilar membrane (BM) even if the location of the stapes and oval window are swapped with each other.

Since the meatus is regular in shape a continuous wave will resonate like an organ pipe. Thus we suggest pure tones are not represented in direction, but pulse signals are. Fast rising impulse sounds will have a high frequency spectrum. It is interesting to note that there is a long length of the BM devoted to high frequency resolution, which is not compatible with the sensitivity of hearing and therefore the assumed importance of frequency content to human understanding. It is suggested that this is explained by its role in analysing impulse sounds and hence directional hearing. We also note the stapes is in the optimum position for pulse signals to reach the region of high frequency (thus overcoming the problem of paradoxical wave motion).

An additional support to this hypothesis is presbycusis, which commonly occurs with age. This involves loss of high frequency sensitivity (1000 Hz +), yet the major symptom is the inability to discriminate within environmental noise.

However, this hypothesis needs another variable other than place of displacement of the BM in order to obtain direction. It is suggested that this is given by the motion of fluid along the BM, i.e. at right angles to its normal displacement due to frequency. Such variations in the motions of the fluid in the BM were observed and described in (G.v Bekesy op.cit p510-524, 532-533). To develop our theory further, first requires discussion on the role of the outer hair cells (OHC'S).

The Outer Hair Cells.

The results of experimental work and clinical observations show that OHC'S do not effect frequency resolution, as is shown in (Nebury & Clark 1978 as quoted in J.F. Willot "The Auditory Psychobiology of The Mouse" Charles C Thomas 1983 p194), (Evans & Wilson "Psychophysics & Psychophysiology of Hearing" p256.). But evidence suggests they do affect threshold as shown in (J.F. Willot "The Auditory Psychobiology of The Mouse" CHarles C Thomas 1983 p155) and the ability to hear a signal in a noise background is described in (Ehert 1979b quoted in above p194). A new theory of the role of the OHC'S compatible with these findings is now given.

As the BM vibrates the tectorial membrane and recticular laminate of the organ of corti are displaced medially into the endolymph, because they are stood off from the BM by the pillars of corti, see (G.v Bekesy op.cit.

p488).

We illustrate our hypothesis first by analogy with reference to Figures 4a and 4b. Viewed in cross section the internal sulcus can be viewed as the head 13 of a "duck" whose open beak 14 is formed by the tectorial membrane and the recticular laminate. The "duck's" neck is formed by the pillars of corti and as the BM vibrates the open mouthed "duck" pecks the fluid from the endolymph (Figure 4a).

It is suggested that the OHC'S are critical in passing the fluid from the scala media to the inner sulcus.

Referring now to Figures 5a, 5b and Sc the structures of the OHC'S 15 are such that they collect fluid as they move and pass it back to the inner hair cells. They can be viewed as tiny "open buckets". As they push forward the "V" shape of the OHC'S acts like the bow of a ship and fluid passes by the "V" in a streamlined fashion (Figure 5b). On the return the fluid is restricted in flow by the inverse "V" (Figure sic).

The overall action is like that of a diode in the detector circuit of a radio. The sinusoidal motion of the organ of corti is converted into one way motion of fluid from the scala media to the internal sulcus. The action is full wave rectification.

For a transient impulse signal, which is of interest for directional hearing, the organ of corti is jerked in motion and the resulting backward flow to the inner hair cells may occur as only one or two pulses. The action described requires a stiff structure to the OHC'S themselves in order to withstand the dynamic fluid pressures. Such a possibility is shown in scanning electron images of the cilia in t13].

If the height of the OHC'S can be varied, then there is a way of affecting the magnitude of flow over the inner hair cells and hence their audio response at that location.

With this ability over the length of the Organ of Corti, it follows that the OHC'S are capable of forming a "matched filter" to a given signal. For example, the OHC'S could be erected to select the sound of a violin from a piece of music. This "matched filter" role for the OHC'S explains their importance in hearing a signal in a noise background.

Investigations indicate the cilia do have retractable action by the contraction of the inner fibres see (as above p182-185), A similar action and structure has been described in detail for the microvilli of intestinal epithelial cells by (M.S. Mooseker & L.G. Tilney "Organisation of Actin Filament-Membrane Complex: Filament Polarity & Membrane Attachment In The Microvilli of Intestinal Epithelial Cells" The Journal Of Cell Biology Vol 67 1975 p725-743).

Their "tripod" structure of different "leg" lengths see (A.F. Jahn & J. Santos-Sacchi "Physiology of the Ear" Raven Press 1988 p184) allows more than variation of overall height. It allows differential innervation of inner, mid and outer of the 3 cilia, which thus allows the OHC'S collectively to lean and operate like flaps. This is illustrated schematically in Figures 6a, 6b and 6c.

Although separate allia, they are attached to each other at the top by small fibres, as illustrated.

This structure and its action, may explain the shallow "W" form of what has previously been described as the "V" formation of the OHC'S. This shape will allow each side to be folded independently thus the OHC'S may be viewed as two separate flaps, each capable of being raised or lowered independently.

With one side raised and the other adjacent side partly lowered the OHC'S group can now be directional in selecting flow, for example with both sides the same height the direction of flow pushed back to the inner hair cells passes through the apex of the "V" and at an equal angle to both sides. If a side is lower, the OHC'S have vector control of direction of flow to inner hair cells.

It is suggested that for impulse signals, where the stapes is angled at entry to the Cochlea, a directional flow is induced in the Cochlea. (NB: it must be remembered that this is of short duration in time) By the mechanism of varying the height of the sides of the OHC'S, they can select a direction to maximise the flow to the inner hair cells and hence the sound perceived.

A review of the OHC'S and their role according to the suggestions made is now given. Hearing in the cochlea is governed by two sets of hair cells. The inner cells perceive sound by the flow of the endolymph past them. The amount of this is governed by the OHC'S and the amplitude of the vibration of the organ of corti in and out of the endolymph, for a given location.

The OHC'S have combined functions of "scoop" and "gate", "scoop" refers to the way they pass endolymph back to the inner hair cells, "gate" refers to the way they can be altered to favour a particular direction along the recticular laminate.

Conclusion.

This new theory has several important implications.

Firstly it presents a need to review current theory of the physiology of the ear. Our theory presents a plausible explanation for the detail of the middle ear, its strange ossicular structure and the complex way it conducts sound.

It also explains the intricate structure of the cochlea. It provides the reason why such a long length of the BM is devoted to high frequencies sounds. A new role for the organ of corti and the OHC's is also given.

Secondly, if we now assume from our evidence that one ear can hear directionally, we imply a new approach to the psychological elements of hearing, i.e. the way humans perceive sound. It implies that one good ear is enough to perceive sound correctly and explains why people with only one normal are never require hearing aids or therapy.

Thirdly, it also gives an explanation of why "lateralisation" occurs with stereo headphones. Normally the only way each ear can receive the same direction and similar intensity of sound is from those that are within the head.

Finally this new theory implies a review of current auditory examination. We stress a need to test hearing, for whatever purpose using the sounds which test it best, i.e.

pulse signals. In particular the currently favoured pure tones and use of headphones are not a satisfactory test of the directional function of hearing.

The principle illustrated by the "Two Tubes Experiment" in which directional hearing is obtained by introducing tubes carrying sound into the ear canal can be used to improve a hearing aid. A typical hearing aid is shown in Figure 7.

A microphone 16 receives the sound at its location and transduces it into an electrical signal. This passes to electronics circuitry 17, where the signal is amplified.

The amplified signal passes to a transducer 18 (i.e.

piezoelectric crystal or electromagnetic driven diaphragm), where it is changed back to sound. The transducer drives a tube 19, which is small enough to be introduced into ear canal 20.

The physical realisation of this arrangement can vary from a large microphone and electronics (body worn aid) to a miniaturised version which can be worn behind the ear and an even smaller version which can be worn in the ear.

An arrangement constructed in accordance with the present invention is shown in Figure 8 and uses two separate channels (labelled a and b) of microphone 16a, 16b, electronics circuitry 17a, 17b, transducer 18a, 18b and tube 19a, 19b as already described for Figure 7. The size of the tubes are such that both can be introduced into the ear canal alongside each other. Microphone 16a is arranged to face forward or otherwise constructed to favour transducing sounds, i.e. given a larger electrical output, for sounds coming from a forward direction. Microphone 16b is arranged to face backward or otherwise constructed to favour sounds, i.e. given an larger electrical output, for sounds coming from a backward direction.Thus if there is a sound that is forward of the hearing air it will be heard louder in tube 19a than in tube 19b, and conversely a sound that is backward of the hearing aid will be heard louder in tube l9b than in 19a. The tubes 19a and 19b are arranged to be alongside each other when introduced into the ear canal.

Tube 19a is positioned forward of tube 19b, i.e. closer to the face of the wearer, thus corresponding to the operation of the microphone 16a, thus the wearer of the aid will perceive a louder sound in 19a than in 19b to be coming from a direction from the front. Conversely tube 19b is positioned towards the back of the head, corresponding to the operation of the microphone 16b, thus the wearer will perceive a sound louder in tube 19b than in 19a to be coming from a direction the rear or behind them.

Obviously the standard hearing aid of Figure 7 cannot give a locational awareness of different sounds coming from different directions. In practice it has the opposite effect in that it amplifies the sounds from all locations and these are heard all mixed in together by the wearer of the aid. The effect of this is the same as recording conversation in a room from one microphone placed somewhere in the middle. On play-back of such recordings one hears a cacophony of voices and other noises like coughing, and clinking of glasses or teacups, such that it is virtually impossible to make out an individual conversation from the background noise. The drawback described is frequently the reason why the conventional hearing aid is either not worn or turned off by the elderly. The new hearing aid will overcome this problem by providing directional information.

The arrangement of microphones and tubes in the ear canal previously discussed and illustrated in Figure 8, have been concerned with discriminating sounds in the front to back plane. Obviously, if the same arrangement is rotated through 90 degrees so that the microphone 16a faces upwards or favours upward sounds, and microphone 16b faces downwards or favours downward sounds, then the hearing aid will discriminate sounds in the vertical plane. Thus if the number of microphones and the associated electronics, transducer and ear tube is increased to four, a hearing aid can be made which gives discrimination of sound both front to back and up and down. However, in practice the requirement is for discriminating sounds in the front to back plane. Moreover, the four channel system would be more expensive and fitting four tubes into the ear canal more difficult.

The physical arrangement for the hearing air of Figure 8, can vary, like the conventional aid, from large components, which are body worn to miniature components, which are worn behind or in the ear. A physical realisation for an aid worn behind the ear is illustrated in Figure 9.

In Figure 9 a body 21 of the device is shaped and of a size such that it will fit behind the outer ear without being obvious. The other advantage of this arrangement is that a microphone 22 is in a location that naturally favours the reception of sound in a forward direction. Similarly, a microphone 23 naturally favours the reception of sound behind or to the rear of the outer ear. Tubes 24 and 25 carry the amplified sounds obtained from microphones 22 and 23 respectively by using electronics circuitry and transducers as already described for Figure 8, but with the components miniaturised and concealed within the body 21.

In use the tubes 24 and 25 curve under the base of the outer ear and enter into the ear canal in the manner of existing signal channel behind-the-ear aids. Tube 24 is located towards the front and tube 25 towards the rear in the same way as has already been explained for tubes 19a and 19b of Figure 8.

Personal Cassette Player Shown in Figure 10 is a typical known arrangement of an earpiece for reproducing sound into one ear. Two such earpieces are normally worn, one in each ear either independently or as a part of a headset. In the case of the personal cassette player, the separate earpiece in each ear carries the separate left and right channels for the purpose of reproducing stereo sound to the listener. The electrical input to the earpiece enters via wires 26. These drive an electrical to acoustic transducer 27, i.e. piezo-electric crystal or electromagnetic driven diaphragm. The transducer is held within a body 28 so that the sound generated within the body cavity is communicated into the ear via a hole 29.

The earpiece plugs into the entrance to the ear canal at 30, the overall size of the earpiece is quite small such that it fits into the auricle.

Shown in Figure 11 is an earpiece constructed in accordance with the invention which is similar in size and application to that already described for Figure 10.

However, the Figure 11 earpiece allows the reproduction of stereo sound in one ear only using the principles already described for the hearing aid. A body 30 of the earpiece has an ear plug 30a is divided into two cavities by a region of sound isolating material 31, which can operate by absorbing or reflecting sound. An electrical to acoustic transducer 32 is driven by an electrical input through wires 33 and the sound it generates is communicated through an earplug 34 via a hole 35. The same process occurs for the parts correspondingly labelled 32b, 35b, 33b. In use the electrical left channel output from the personal cassette player, or other stereo reproducing music system is connected to 33a and the right channel output is connected to input 33b, or of course visa versa. A stereo reproduction of sound/music will then be heard in one ear.

This will have the distinct advantage in the case of personal cassette players of leaving the other ear to hear normal environmental sounds, i.e. increase the awareness and therefore safety for wearers.

Claims (12)

1. A method of providing localisation of sound through a single ear, characterised in the steps of introducing into the ear canal a plurality of sound sources, each at a different location and each derived from a different transducer, and arranging that the transducers are each most responsive to sounds originating from a different direction.

2. A device for introducing two or more separate sources of sound into a single ear.

3. A device for carrying out the method of claim 1, characterised in means for introducing into the ear canal of a single ear a plurality of sound sources each at a different location in the canal.

4. A device as claimed in claim 2 or 3, characterised in that the separate sound sources (a,b) are introduced into the ear canal of a single ear by tubes (19a, 19b).

5. A device as claimed in claim 4, characterised in that the sound in each tube (19a, 19b) is generated from separate electrical to acoustic transducers (18a, 18b) and the electrical signals to the transducers are obtained from separate microphones (16a, 16b).

6. A device as claimed in claim 5, characterised in that each microphone (16a, 16b) gives increased response to the sound from a different direction so that the overall sound perceived by the listener, through the tubes, enables separation of sounds from different directions.

7. A device as claimed in claim 6, characterised in that the microphones (16a, 16b) point in different directions.

8. A device as claimed in claim 2 or 3, characterised in an earpiece (30) having an earplug (30a) provided with a plurality of holes (35a, 35b) arranged such that where the sound is introduced into the ear by the earplug each hole communicates a different one of the sound sources from a separate electrical to acoustic transducer (32a, 32b)

9. A stereo personal cassette player, characterised in the provision of a device as claimed in any one of claims 2 to 8 in which the two output channels for sound from the player provide said separate sources of sound in the device.

10. A hearing aid, characterised in the provision of a device as claimed in any one of claims 2 to 8.

11. A method of providing localisation of sound through a single ear substantially as described herein with reference to the drawings.

12. A device for introducing two or more separate sources of sound into a single ear substantially as described with reference to the drawings.