Transcript Audiometer
AUDIOMETRY An Audiometer is a machine, which is used to determine the hearing loss in an individual . Classification Audiometers Pure tone audiometer Air conduction Bone conduction Speech audiometer Speech thresholds Audiometer • Generally employed transducers in audiometer are the following: • Earphone • Microphone • Bone-vibrator • Loud speakers Earphones: • Earphones are usually of the moving coil type and gives reasonably flat frequency response upto 6 KHz after which their sensitivity decreases rapidly. • They are specially designed for audiometric applications rather than for communication purposes . Microphones: • These are used to translate wave motion in space into electrical signal. • Two types which are carbon button changes resistance with the pressure. • The second one is the electrodynamic type in which the voltage is induced in a coil by its motion relative to a magnet Simple audiometer • An audiometer will essentially have an oscillator along with a pair of head phones. • Pure tone audiometers and speech audiometers are two main groups of audiometers and are grouped according to the basis of the stimulus they provide to elicit audio response . • The intensity range of most audiometers starts from approximately 15 dB above normal to 95 below normal over a frequency range from approximately 500 to 4000 Hz. Pure tone audiometer • Pure Tone Audiometer works on the principle of presenting specific pure tone signals to the subject and determining the intensity at which they can barely hear these signals . • They are calibrated in terms of frequency and output • A pure tone is the simplest type of auditory stimulus . • generate test tones in octave steps from 125 to 80000Hz, the signal intensity ranging from – 10 dB to + 100 dB. • frequency range of 300-3000Hz . • Changes in threshold sensitivity associated with various middle ear surgical procedures can be monitored more accurately with pure tone than speech tests . AIR & BONE CONDUCTION: • Air conduction is the transmission of sound through the external & middle ear to the internal ear. • Bone conduction refers to the transmission of sound to the internal ear mediated by mechanical vibration of the cranial bones & soft tissues. Speech audiometer • To carry out tests with spoken voices . • These tests are particularly important before prescribing hearing-aids. • CONSTRUCTION: • A double band tape recorder is preferred to interface the two channel audiometer units. • Masking noise is supplied by the noise generator. • The two channels supply the two head-phones or the two loud speakers of 25 W each. Bekesy Audiometer System • OPERATION: • The instrument generates a pure-tone signal which is presented to him through an air-conduction earphone. • The subject is asked to press a switch when the tone is heard and to release the switch when it is not heard. • A pen connected to the attenuator traces a continuous record of the patient’s intensity adjustments on an audiogram chart, producing a graphic representation . Bekesy Audiometer System Bekesy Audiometer System • ELECTRICAL SECTION • Oscillator circuit • Modulator circuit. • Automatic attenuator. • Control circuits • Master clock generator • MECHANICAL SECTION • Carriage device • Writing system Oscillator circuits • This oscillator generates test signals with frequencies of 125, 250, 500, 1000, 1500, 2000, 3000, 4000, 6000, and 8000Hz. • This sequence is first presented to the left ear automatically, each tone for 30s, and then to the right ear, the shift between the frequencies being noiseless. • After both ears have been tested, a 1 kHz tone is presented to the right ear to provide a useful indication of the test reliability. Modulators • The models of modulators are available “Pulse” or “Cont”. • In the ‘Pulse’ mode the test signal is modulated giving a signal which is easily recognized by the patient. • In the ‘Cont’ mode no modulation is applied, giving a signal suitable for use, while calibrating the audiometer . Attenuator • The attenuation range is 100dB, thereby covering the range of hearing levels from – 10 to + 90 dB. • When the test is initiated, the attenuator starts at its top position of –dB and then increases the level with a rate of 5 dB/s. • The pen drive is controlled by means of the hand switch operated by the patient. • Pressing the switch decreases the output from the potentiometer and thereby the level in the ear phones . • Mechanical section: • Mechanical carriage with the writing system is driven by a stepping motor via a toothed belt. • The speed and direction of rotation of the motor are automatically controlled via the logic control system. • When the test is initiated and the patient indicates that he hears the signal by pressing hand switch, the carriage moves along the X-axis (Frequency axis) of the audiogram in tune with the frequency of the test signal. • When the complete test is finished the carriage and writing system returns to the start position. Writing System • Operation: • operated by the pen drive, which is driven by a stepping motor. • The pen drive moves the pen, and with it the wiper of the automatic attenuator, along the Y-axis (hearing level axis) with a constant speed corresponding to the change in attenuation of 5dB/s. • The direction of movement of the pen is determined by the position of the hands witch operated by the patient. • Limit switches are also included with the pen drive. Audiogram Chart The audiogram is printed in standard 5 format (148*210mm). MASKING IN AUDIOMETRY • Need for masking: • In case of monaural & asymmetrical binaural hearing losses, there is a serious difficulty in obtaining accurate measures of hearing for the poorer ear. • This problem can be overcome by eliminating responses from the better ear by masking in order, to shift the threshold to high level, permitting greater intensities to be presented to the poorer ear without any danger of cross-over. MASKING IN AUDIOMETRY • Efficiency: • Masking efficiency depends upon the nature of masking sound as well as intensity. • A pure tone can be used to mask other pure tones. Noise • White noise: • White noise is a noise containing all frequencies in the audible spectrum at approximately equal intensities . • Saw tooth noise: • Saw tooth noise is a noise in which the basic repetition rate is usually that of the mains voltage & contains only those frequencies that are multiplies of the fundamental .