1995-02-13
1996-08-20
Hindenburg, Max
Surgery
Truss
Pad
A61B 500
Patent
active
055469566
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to apparatus for, and a method of, testing hearing.
BACKGROUND TO THE INVENTION
EP 0015258 discloses hearing faculty testing apparatus comprising an aural probe for insertion into a subject's external ear canal. The probe includes an electroacoustic transducer for projecting sound into the ear canal and for responding to sound waves returned from the inner ear in response to the projected sound. The transducer is energised by a pulse generator which is said to have a maximum useful frequency of about 50 Hz, because at higher frequencies the echoes overlap with succeeding pulses. Throughout, this prior patent specification refers to extracting echoes by electronic time gating. The need to avoid overlapping and to extract the echoes by time gating represents a disadvantage which the present invention aims to overcome.
DISCLOSURE OF THE INVENTION
According to one aspect of the invention apparatus for testing hearing comprises a sound source for applying a sound to a subject's ear and a sound detector for detecting the sound returned from the subject's inner ear in response to said sound, characterised in that said sound source produces a sequence of sounds constituting a maximum length sequence (MLS), a variant of such a sequence or a variant of a similar sequence. Possible variants are Legendre sequences, M-pulse sequences and De Bruijn sequences.
According to another aspect of the invention there is provided a method of testing hearing, wherein sound is applied to a subject's ear and the sound reflected from the subject's inner ear is detected, characterised in that the sound sequence constitutes a maximum length sequence (MLS), a variant of such a sequence or a variant of a similar sequence. The invention thus resides in the use of a maximum length sequence (or similar sequence) to record Evoked Otoacoustic Emissions (EOAEs). The invention uses stimulation at rates higher than 50 Hz.
Two sequences may be used simultaneously and two ears may be tested simultaneously.
EOAEs are acoustic signals produced by the motile cochlear hair cells in response to the input stimulus and transmitted through the middle ear space to the external acoustic meatus where they can be recorded using a microphone. EOAEs are like fingerprints and vary markedly between ears and individuals but are remarkably stable within an individual.
A maximum length sequence (MLS) is a quasi-random binary sequence with strictly defined mathematical properties. The MLS is a pseudorandom binary (or multi level sequence) x(n) periodic with period p, having the property that ##EQU1## where a and b are constants. In other words, the sequence's autocorrelation function is two valued. Various techniques, such as shift registers, can be employed to produce such sequences which include maximum length sequences (also called PN sequences and m-sequences) and Legendre sequences.
The original work on these sequences was carried out in the field of radar but, since then, they have been applied to the recording of electrical evoked potentials (Eysholdt, U and Schreiner, Chr., 1982).
The invention enables this technique to be used to record otoacoustic. evoked emissions.
The property of MLSs that makes them of interest and applicability is the fact that in order to record a response of say 20 ms duration, the time between the stimuli used to obtain this response can be considerably less than the response itself. Thus by recourse to the invention it is possible to stimulate at a rate of 500 or more clicks a second, instead of stimulating at less than 50 clicks a second. In this way, a sufficient number of responses is obtained that will produce a clear average waveform in very much less than the normal test time. The responses overlap each other and, if normal stimulation were applied at such a rate, the waveform would be the product of the overlapped responses and would be worthless. However, if a MLS is used the original response can be recovered from the overlapped, averaged waveform.
This technique is part
REFERENCES:
Burkard et al. "A Comparison of Max. Length & Legendre Sequences . . . " J. Acoust Soc. Am. 87(4) Apr. 1990 pp. 1656-1664.
Shi et al., "The use of M pulse sequences . . . " IEEE Engin. Con. Nov. 1989.
Avan et al., "Quantitative assessment . . . emissios" Hearing Research 52 1991 pp. 99-112.
Eysholdt et al., "Maximum Length Sequence . . . Responses", Audiology 21 1982 pp. 242-250.
Hindenburg Max
Medical Research Council
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