Audiometry is a fundamental diagnostic tool used by biomedical engineers to assess hearing function and detect auditory impairments. Proper understanding of audiometry principles and sound properties is essential for maintaining audiometric equipment to ensure accurate and reliable hearing assessments. In Kenya, where audiology services are expanding in hospitals and clinics, biomedical engineers play a crucial role in supporting audiologists by ensuring that audiometers function optimally. This chapter explores the principles underlying audiometry and the physical properties of sound that influence hearing tests and equipment performance.

7.1 Principle of Audiometry

Audiometry is the scientific method to measure a person's hearing acuity by evaluating their response to sound stimuli of varying frequencies and intensities. Biomedical engineers working in healthcare facilities such as county hospitals or university health centers must grasp audiometry principles to maintain and calibrate audiometers accurately. This ensures that hearing loss diagnoses and interventions are based on precise data.

Measurement of Hearing Thresholds

The core principle of audiometry involves determining the softest sound a person can hear at different frequencies, known as the hearing threshold. This threshold is measured using pure tones delivered via air conduction (headphones) or bone conduction (bone vibrator). The audiometer presents these tones in a controlled manner, gradually adjusting intensity until the patient signals detection.

• Threshold determination: The hearing threshold is identified when the patient consistently detects the tone at the lowest decibel level.
• Frequency range: Audiometers typically test frequencies from 250 Hz to 8000 Hz, covering the speech frequency range critical for communication.
• Air vs. bone conduction: Air conduction evaluates the entire auditory pathway, while bone conduction bypasses the outer and middle ear to assess cochlear function directly.
• Masking: To isolate the test ear, masking noise is introduced to the non-test ear when cross-hearing might occur.
• Threshold variability: Patient factors such as attention and environmental noise can affect threshold measurement, emphasizing the need for precise equipment calibration.

Types of Audiometry Tests

Audiometry encompasses various test types to assess different aspects of hearing. Biomedical engineers should be familiar with these to support audiologists in equipment selection and maintenance.

• Pure tone audiometry: The standard test measuring hearing thresholds across frequencies using pure tones.
• Speech audiometry: Evaluates the ability to detect and recognize speech sounds, aiding in functional hearing assessment.
• Impedance audiometry: Measures middle ear function by assessing eardrum compliance and acoustic reflexes.
• Otoacoustic emissions: Tests cochlear outer hair cell function through sounds generated by the ear itself.
• Auditory brainstem response (ABR): Assesses neural pathways by recording electrical activity in response to sound stimuli.

Calibration and Accuracy in Audiometry

Maintaining audiometer accuracy is critical for valid hearing assessments. Biomedical engineers in Kenyan health facilities must ensure routine calibration and adherence to standards.

• Calibration standards: Audiometers should comply with ISO 389 and ANSI S3.6 standards, defining reference hearing levels and test procedures.
• Frequency accuracy: The tones produced must be within ±1% of the nominal frequency to ensure correct stimulus delivery.
• Intensity accuracy: Sound intensity levels should be accurate within ±3 dB across frequencies.
• Environmental conditions: Testing rooms must have controlled ambient noise levels to avoid threshold distortion.
• Routine checks: Daily biological calibration using a normal-hearing individual or a sound level meter helps detect equipment drift.

Role of Biomedical Engineers in Audiometry Maintenance

Biomedical engineers ensure audiometer reliability through preventive and corrective maintenance, calibration, and troubleshooting.

• Preventive maintenance: Includes cleaning transducers, checking cables, and verifying controls to prevent malfunction.
• Calibration verification: Conducting periodic calibration checks to confirm accuracy against reference devices.
• Repair and replacement: Identifying faulty components such as headphones or bone vibrators and replacing them promptly.
• Documentation: Keeping detailed maintenance and calibration records for regulatory compliance and quality assurance.
• User training: Supporting audiologists and technicians by providing guidance on proper equipment use and care.

Chapter Summary

This chapter explored the principle of audiometry, explaining how sound is measured to assess hearing ability. It detailed the properties of sound, including frequency, intensity, and duration, which are fundamental to understanding audiometric testing. The chapter described the various parts of an audiometer, such as the oscillator, amplifier, attenuator, earphones, mask, and filters, highlighting their specific roles in producing and controlling test signals. Different types of audiometers were examined, illustrating the range of devices available for hearing assessment. The functions of audiometers were outlined to show how they help detect hearing loss and evaluate auditory function. Maintenance procedures were discussed to ensure the equipment remains reliable and accurate over time. The chapter also covered calibration methods that guarantee the audiometer produces precise and consistent measurements. Finally, it introduced hearing aid implants, emphasizing their specifications and the recommended quantities for effective hearing rehabilitation.