What Does Human Ear Consists of?
Our human ear consists of external ear, middle ear and inner ear. The human audibility range is between 20 to 20000 Hz. When the sound waves enter through outer ear, they are conducted through ear canal and reaches the ear drum. We also have tympanic membrane that separates ear canal and middle ear cavity. Depending on the loudness level, ear drum vibrates. From the middle ear, sound waves are transmitted to receptor cells. The inner ear is also called as cochlea consists of coiled passage filled with fluid. The nerve impulses produced are propagated through the nerve cells and reaches the brain at a speed of 100 m/s.
Few disorders in ear are in the outer ear, when there is any swelling or accumulation due to foreign particles, the acoustic pathway gets blocked. Due to this the hearing capacity is reduced. In the middle ear, hearing is affected due to problems in ear drum and cochlea. This will result in mismatch between air and bone conduction impedance. In the inner ear, when a person is exposed to noise for long hours, they get acoustic injury. At this stage, the electrical signals are not transmitted to the brain.
The methodology used for getting diagnostic information about the hearing ability is called Audiometry. The devices used for it is Audiometer. Audiometers evaluate the effectiveness of ear protectors. In industries audiometers are used to measure the noise level. Audiometer is an electronic device, to measure the human hearing level. The hearing loss is measured between -10dB to 100dB, which is the audible range of frequency for a human. There are three types of audiometers.
Pure Tone Audiometers
They measure air conduction and bone conduction of hearing. They identify the amount of hearing loss.
They determine the threshold level of speech reception
These are utilised in industries to measure the threshold of hearing with respect to noise level. They are also used in military applications for soldiers.
Block Diagram of a Basic Audiometer
A single frequency sound wave is called pure tone signal. Pure tone audiometers measure the hearing loss of a person. Generally, they produce the tone with a frequency range of 125 to 8000 Hz. With a signal intensity value varying between -10dB to 100dB. The block tone generator consists of LC coupled oscillator, in which inductance and capacitance can be varied. The coupled system is given to a power amplifier. Through a head phone or ear phone, the signals are sent to the ear. When this audiometer acts as Speech audiometer, the clinical tests are done with different recorded voices using a tape recorder. Whereas the screen audiometer measures the threshold level in presence of noise. So doctors does the clinical testing in presence of noise to check the problems in ear and then suggests the medication or surgery.
Block Diagram of a Basic Hearing Aid
A microphone is connected with a FET preamplifier. It is then connected to automatic gain control. There is a filter bank consisting of low pass and high pass filter. Filter bank filters the unwanted noise and filtered signal is sent to the mixer circuit. Filter bank provides low frequency attenuation signal. Digital registers store control words that a programmer provides. They also control other circuits in the block diagram.