ABA Electrical Systems & Wiring 3 — Questions and Answers
Question 1: In an audiometer circuit, the attenuator is typically placed:
- After the power amplifier output stage
- Before the signal generator to limit input noise
- Between the signal generator and the power amplifier (Correct answer)
- After the transducer to control patient feedback
Correct answer: Between the signal generator and the power amplifier
Placing the attenuator between the oscillator and amplifier ensures that noise from the amplifier is not attenuated, but signal level control is achieved before amplification.
Question 2: Impedance matching between an audiometer and an earphone transducer is important because:
- It maximizes voltage standing wave ratio
- It ensures maximum power transfer and prevents signal reflections (Correct answer)
- It eliminates the need for a calibration microphone
- It increases the transducer resonant frequency
Correct answer: It ensures maximum power transfer and prevents signal reflections
Maximum power transfer occurs when source and load impedances are matched, ensuring accurate and efficient delivery of the test signal to the transducer.
Question 3: A capacitor in an audiometer circuit is used to block DC while passing AC signals primarily because:
- Capacitors store magnetic energy proportional to frequency
- Capacitive reactance decreases with increasing frequency allowing AC but blocking DC (Correct answer)
- Capacitors amplify signals above their resonant frequency
- DC current charges the capacitor and destroys it immediately
Correct answer: Capacitive reactance decreases with increasing frequency allowing AC but blocking DC
Capacitive reactance (Xc = 1/2πfC) approaches infinity as frequency approaches zero (DC), effectively blocking DC while allowing AC signals to pass.
Question 4: Which circuit element in a hearing aid output stage limits maximum acoustic output to protect residual hearing?
- A low-pass filter capacitor
- An automatic gain control (AGC) or output limiting circuit (Correct answer)
- A series resistor voltage divider
- A transformer coupled output stage
Correct answer: An automatic gain control (AGC) or output limiting circuit
AGC circuits and output limiters are specifically designed to prevent the hearing aid from exceeding prescribed maximum output levels (OSPL90).
Question 5: In an electroacoustic analysis setup, a 2-cm³ coupler has a known acoustic impedance because it:
- Mimics the exact geometry of the average human ear canal
- Provides a standardized reproducible cavity for consistent SPL measurements (Correct answer)
- Contains a calibrated reference microphone matched to IEC standards
- Uses piezoelectric material to generate reference tones
Correct answer: Provides a standardized reproducible cavity for consistent SPL measurements
The standardized volume of the 2-cm³ coupler provides a reproducible acoustic load, allowing consistent and comparable hearing aid output measurements across facilities.
Question 6: When an audiometer's power supply filter capacitors degrade, the MOST likely symptom is:
- Loss of high-frequency tone generation
- Increased ripple noise superimposed on the test signal (Correct answer)
- Failure of the bone conduction oscillator circuit
- Random audiometer calibration drift above 4000 Hz
Correct answer: Increased ripple noise superimposed on the test signal
Filter capacitors smooth the rectified AC supply; when they degrade, residual AC ripple passes through to the signal chain, adding noise to the output.
Question 7: Ohm's law applied to the electrical equivalent circuit of the middle ear predicts that increasing the stiffness of the ossicular chain will:
- Decrease the electrical resistance analog
- Increase the capacitive reactance analog, reducing low-frequency energy transfer (Correct answer)
- Increase the inductive reactance analog, reducing high-frequency energy transfer
- Have no effect on the acoustic impedance at any frequency
Correct answer: Increase the capacitive reactance analog, reducing low-frequency energy transfer
In the electrical analogy, stiffness maps to capacitance; increased stiffness reduces compliance, raising capacitive reactance and impeding low-frequency sound transmission.
ABA Electrical Systems & Wiring 3
In an audiometer circuit, the attenuator is typically placed: