How to Assess Hearing Aid Function

How to assess Hearing Aid function

Key Points

Simulate ear coupling or couple to the ear
Present test signals
Interpret Hearing Aid Output

Couplers and ear simulators

a coupler is a cavity
- on one end you connect a hearing aid
- on the other end is a microphone that picks up the output of a hearing aid
- allows for repeatable measurements
- not the same as REAL ear
  - especially if you are a child

We order or validate a hearing aid based on coupler data

TDH-50 in HL; 6cc, HL
Insert in HL; 2cc, HL
Insert in SPL; 2cc, SPL (but it's still not perfect)

2cc Coupler

2cc coupler because it is close to the residual ear canal volume after aid insertion
Provides a means of quality control
compare output of one unit of a Hearing Aid model with that of another unit of the same model
Exchange data between laboratories
volume of a typical adult ear between the medial end of an ear mold and the tympanic membrane is about 1.2cc not 2cc
2cc coupler curve on manufacturers specification sheets are not intended to reflect the response of the instrument in a user's ear

Coupler Types: Standard

HA-2; 2cc
- includes earmold simulator (extension on coupler)
- it is connected to the BTE via the tubing
- it can also be used for body level aids
  - Velcro helps keep the unit in place during measurement
  - Center the BTE microphone over the loudspeaker
HA-1; 2cc
- tests ITE (in the ear) and thin tubes and RIC (receiver in the canal) devices with no ear mold
  - Be sure "leaks" in the putty are sealed, or will have excessive 500 Hz gain!
- no ear mold simulator
- the aid is coupled with putty to the HA1 coupler

Special Couplers: Not Standard

CIC; 0.5cc
Open; 2cc
- the open fit coupler provides a more realistic frequency response than a 2-cc coupler
- it is easier to attach to the hearing aid

Artificial ear or ear simulator

an artificial ear or ear simulator is far more sophisticated device for the calibration of an earphone

Zwislocki Coupler

alternate coupler to overcome the problem faced with 2cc coupler
reproduces eardrum impedances of a typical adult human ear
it's volume is close to estimates of the volume remaining when the meatus is occluded with an ear mold; 1.2cc
coupler has four side branch resonators that synthesize the acoustic impedance variations in real ears
- TM is a one half inch condenser microphone

KEMAR

Utilizes the Zwislocki coupler to simulate the acoustic response of a human ear in a free field
consists of a head and torso and has the dimensions of an average human adult, including pinnae and ear canals

Reference Test Microphone

The reference test microphone is placed next to the hearing aid microphone
- monitors the SPL reaching the hearing aid from the loudspeaker
the control microphone circuit adjusts the output from the loudspeaker to achieve the desired SPL in the field
PRESSURE METHOD: Microphone is in the test box during test. It corrects the SPL in the field during every measurement
SUBSTITUTION METHOD: Microphone is placed in position prior to the test measurement. The control mic stores information about the discrepancy between the actual and desired SPL in the field. During actual measurement the test box adjusts the SPL based on this information.

Testing Omnidirectional vs. Directional microphones

Omnidirectional microphones using the pressure method
- the control mic and hearing aid mic need to be close to each other and the same distance from the loudspeaker
Directional Mics
- it is important that the sound from the loudspeaker sound is directed toward the hearing aid at the same angle it would when the hearing aid is on the ear

Signal: Pure Tones

pure tone sweeps: pure-tones with continuously changing frequencies presented at the same fixed level over the range of audible frequencies in a fixed amount of time
pure tone and pure tone sweeps are simple because frequency and intensity levels can be precisely controlled and no special frequency analyzers are needed for their measurement
The signals are used to estimate
- frequency gain/output characteristics of hearing aids
- input output characteristics
- saturation distortion
- attack/release times
- current drain of hearing aids
One frequency is presented at a time
- interactions across frequencies that may occur in nonlinear signal processing cannot be examined
performance of nonlinear hearing aids cannot be generalized to complex sounds using results obtained from sinusoids
not appropriate if you want to measure performance of the hearing aids to real-life complex signals

Signal: Composite signals

artificial signals with same long term spectral characteristics of speech
- speech spectrum shaped composite signals
advantage: many frequencies can be tested in a short time
can be synthesized with controlled characteristics so that specific processing algorithms on a hearing aid may be optimally evaluated
results obtained from the composite signal better predict the performance of the hearing aids to real life complex signals
many different speech spectrum shaped composite signals each synthesized with different criteria
- similar but not identical to "real" speech
commercial speech spectrum composite signals
- 100Hz - 8000Hz spaced in 100Hz interval
- slope of -6dB/octave
ICRA signal
- designed to approximate the ideal long-term far-field speech spectrum
- slope of -9dB/octave
Dynamic stimulus of sweeping sinusoids over the speech range
- modulated to resemble the dynamic range of typical speech
- useful for analyzing the dynamic response of a hearing aid to speech-like signals

Signal: Speech Signals

Speech is a complex broadband signal that has varying spectral and intensity content over time
good stimulus for testing hearing aids because it is the signal that most hearing aid wearers would want to hear in their daily environments
complex stimuli: interactions among frequencies may be revealed by this stimulus
Challenges using speech as a test signal
- real speech as a test signal must be standardized
- speech spectrum varies substantially across talkers, gender, age, etc
- ISTS was developed to address these issues
  - based on natural recordings of speech
  - non-intelligible due to remixing and segmentation
  - ISTS is shaped according to the LTASS
    - Long Term Average Speech Spectrum

Gain Frequency and OSPL 90 response

OSPL 90 defined by IEC and ANSI the maximum output to be measured with a 90dB input signal
Level is high enough to cause most hearing aids to reach their maximum output level at each frequency
when the hearing aid has reached its limit for any input signal, it is said to be saturated
different when measured with a narrow band signal vs broadband (saturation in a single band vs all bands)

Full On vs Reference Test Gain

amount of gain measured depends on the volume setting of the instrument
- if the hearing aid is set to max it is called full-on gain
- if the volume is set at reference set gain the curve is called basic frequency response or the frequency response curve
tells you about how the aid when it is not saturating
the reference test position is 17dB down from the High Frequency Average (HFA) output measured with a 60dB input
OSPL90 is measured with a 90dB input

Harmonic Distortion

measured using a pure tone signal and analyzing the output waveform to measure the distortion components relative to the total power of the signal
can be expressed in dB or percentage
when power in all harmonics is summed the final number is the total harmonic distortion
THD is measured with medium level signal set to reference test position
for broadband signals, coherence between input and output signal is used as a measure of distortion
- coherence quantifies the proportion of output signal at each frequency that is linearly related to the input signal at the same frequency
- ranges from 1 when there is no distortion to 0 for maximum distortion
Why measure distortion?
- does the hearing aid meet technical specifications?
- comparing the fidelity of the two instruments
- to determine if there is peak clipping in the circuit
  - compression aids should have THD below 10%
- determine the highest input level that can be passed through the hearing aid without unacceptable distortion

Internal Noise

Generated by microphone and amplifier
quantified as the Equivalent Input Noise (EIN)
- EIN is the amount of noise that would have to be applied to the input of a noiseless hearing aid with the same response, to have equivalent noise from the output
- SPL of an external noise source at the input that would result in the same coupler SPL as that caused by all the internal noise sources in the hearing aid
- EIN is measured as input for three reasons
  - it will not be affected by volume setting
  - it is thought to be primarily from microphone and input amplifier
  - it does not bias high gain instruments
- EIN is calculated by measuring the magnitude of the noise at the output of the hearing aid and subtracting the gain of the hearing aid for soft sounds from it
  - (Coupler SPL with no input) - (HFA gain with a 50 dB input SPL)