Josefina Gutierrez

*National Rehabilitation Institute Mexico* 

### **1. Introduction**

The hearing system is composed of several components that, by means of a physiological process, break down the broad spectrum of frequencies and intensities of sounds from the environment (speech, music, signals and noise) into frequency components and temporal patterns. These acoustic signals are transmitted to the temporal lobes of the Central Nervous System by electric stimuli to generate the neural message (Salesa et al., 2005). The hearing process utilizes acoustic, mechanical and electrical principles, and in addition analyzes sound waves, removes noise and compares these with signals that have been previously registered in the memory of the subject. In this manner, we are able to know when a person is speaking, when we are hearing a musical note from a violin or a flute, or when a bird is singing.

The auditory system possesses a wide dynamic range for perceiving sounds; humans with normal hearing detect tonal frequencies from 20 Hz to 20 kHz. Acoustic intensity is proportional to sound vibration amplitude. Commonly, sound intensity is measured in terms of decibels as dB = 10 log(*I*/*I*0), where *I*0 is the reference intensity, or equivalently for acoustic pressure, dB = 20 log(*P*/*P*0), where *P*0 is the reference pressure (in Pa). Tonality represents the number of vibrations in time and is measured in cycles/sec or Hz. Timbre characterizes all of the harmonics overheard in a clear sound, allowing differentiation between two sounds with the same tonality and intensity.

The dynamic range at 2-4 kHz*,* the span between threshold and pain, is approximately 120 dB. The minimum threshold for sound occurs between 2 and 5 kHz and is approximately 20μPa. At the low end of the auditory spectrum, the threshold is 80 dB higher, while at the high end, it is 70 dB higher. Intensity differences of 1 dB can be detected, while frequency differences of 2-3 Hz can be detected at frequencies below 3 kHz (Aitkin, 1990).

Sounds are normally transmitted both by Air conduction (AC), and by Bone conduction (BC). Conduction of sound starts in the pinna or auricle and passes through the external auditory canal on its way to the eardrum, where sounds are amplified to frequencies ranging from 5,000-6,000 Hz at 20 dB. Later, the pressure and strength of the sound wave vibration that reaches the tympanic membrane, particularly of low frequencies of up to 1,500 Hz, are amplified in the middle ear. The acoustic vibration of the sound is thus transformed into a mechanical vibration to be transmitted to the organ of Corti, located in the cochlear duct. The cochlea is a hydromechanical frequency analyzer whose major role is to turn the acoustic signal into a frequency map through which each frequency is assigned to certain

Technology for Hearing Evaluation 5

The Schwabach test determines BC duration. The instrument is placed on the anterosuperior region of the mastoid. Normal duration is 20 sec. The Schwabach test makes a comparison between bone conduction of the patient and the examiner. The diapason is placed between the patient and examiner and a sound is created. If the patient can no longer hear the sound even though the examiner can hear it, it is indicative of sensory neural loss; if the examiner stops hearing the sound and the patient continues to hear it, it suggests conductive loss. However, this test is completely dependent on the hearing powers of the examiner. In conduction hypoacusis – the duration is prolonged, while in sensory neural

The Weber test utilizes the low frequency of the instrument (126 Hz), which is placed on vertex. Normally the vibration is heard equally in both ears. This is a common medical test conducted in the physician´s office in which a comparison is made between two hearings. In the first, the base of the tuning fork is applied to the bone and hearing is elicited after this. When the sound is completely inaudible, the vibrating top is brought near the external ear canal. If there is a positive value, it is considered that there is an air-bone gap due to which

The Rinne test employs the diapason as initially placed near the ear to explore aerial conduction and after that, on the mastoid to explore bone conduction. Normally, air conduction is better than bone conduction. With BC/AC=½, the Rinne test is normally positive. In this test, a 512 Hz tuning fork is placed on the forehead of the patient. An ipsilateral conductive hearing loss is occurred if the sound appears to be louder on one side than on the other. A contralateral sensorineural hearing loss may also be determined by use

This test uses words that the subject hears through headphones and he/she is required to identify them. This test depends on the knowledge of language; it must be adjusted to both

hypoacusis, the duration is shortened.

there is a conductive loss.

Fig. 1. Set of tuning fork

**2.1.2 Speech perception test** 

of this test.

groups of receptor cells and their nerve fibers. The cochlear nerve innervates the organ of Corti; it possesses myelinized efferent fibers that carry spatial orientation information from the cochlea to the brain. Efferent signals are amplified and transmitted, frequency discrimination is increased, acoustic otoemissions originate, and mechanical changes produced in the liquid medium of the sound waves are detected (Steele et al., 2000).

Hearing loss can occur at any age. Otorhinolaryngologists, surgeons, and audiologists with assistance technology are readily able to identify different degrees of hearing loss. Children and adults experiencing significant hearing loss suffer specific problems and may need special assistance. Early identification of hearing loss and its treatment is essential for successful speech development, the child's intellectual growth and the adult's social integration.

Hearing impairment is caused by either loss in sensitivity (loss in perceived loudness), loss in the ability to discriminate different speech sounds, or both. Loss of loudness may be due to either increased mechanical impedance between the outer and inner ear or reduced sensitivity of the sensory hearing organ. Loss of the discrimination ability is basically associated with damage to the sensory organ, although other neural structures at higher levels may also be involved.
