Hearing aid specifications DrEbtessam Nada Hearing aids specifications

Hearing aid specifications Dr-Ebtessam Nada

Hearing aids’ specifications A specification is the detailed description of a hearing aid which is normally presented as a specification or performance sheet. 2

Hearing aids’ specifications Definition: A number of different H. A. electroacoustic characteristics, which includes: – – – Gain and maximum output, Frequency response, Distortion, Internal noise and AGC circuit characteristics 3

Hearing aids’ specifications (1) MAXIMUM OUTPUT 4

MPO All hearing aids have a maximum output limit, which is the most the hearing aid can handle, due mainly to limitations imposed by the output stage of the hearing aid (the receiver). The maximum output may be referred to in a number of different ways, all of which have very similar meanings. These include: 5

MPO • Saturation sound pressure level (SSPL); • Saturation sound pressure level with an input of 90 d. BSPL (SSPL 90); • Output sound pressure level (OSPL); • Output sound pressure level with an input of 90 d. BSPL (OSPL 90); • Maximum power output (MPO); • Maximum output. 6

Maximum power output (MPO( Maxim um SSPL • SSPL 90 is a practical measure of SSPL, which is the SPL developed in a 2 -cc coupler with an input of 90 d. B SPL and gain control is in the full-on position. 7

Maximum power output (MPO( Saturation: • Is the condition in the circuit when an increase in the input signal produces no further increase in the output signal and it define the level at which H. A. limits output. • This input signal is always 90 d. BSPL. • Related directly to UCL when measured with insert earphone and expressed in d. B SPL 8

Hearing aids’ specifications (2) ACOUSTIC GAIN 9

Acoustic Gain • This is the amount of amplification provided by the aid, which is the difference between the input and the output SPL measured under specified operating condition and at a specified frequency ( 1 k. Hz or 1. 6 k. Hz ) or as an average of several frequencies (e. g. high-frequency average of 1000, 1600 and 2500 -Hz values). – Input = 60 d. BSPL – Output = 80 d. BSPL – Gain = 20 d. B 10

The types of acoustic gain usually presented on a specification sheet include Full on gain Maximum gain Reference gain 11

Full-on Gain (FOG) • This is the gain with the volume control set to its maximum position. 12

Maximum Gain • This is the highest possible gain from the H. A. measured with an input that will not drive the aid into saturation, usually 60 d. B, and volume control turned fully on and with all trimmers set to give maximum output. 13

Reference Test Gain • Represents more of a 'user volume. • This is the gain measured with an input of 60 d. B SPL and the volume control set so that the output is 17 d. B below the HFASSPL 90. 14

Hearing aids’ specifications (3) FREQUENCY RESPONSE 15

Frequency Response It is the SPL developed in a test box or in patient’s ear by the H. A. expressed as a function of frequency obtained with an input signal of 60 d. B SPL and H. A. at the reference test gain setting. 16

Frequency response The frequency response of a hearing aid is presented as a line on a graph that shows how the aid performs over the frequency range. It show the gain, or the output, varies with frequency 17

Frequency Range (bandwidth) The range of frequencies over which H. A. is considered effective can be determined based on the frequency response curve. Extracted from Frequency Response Curve by calculating the Average of 1000, 1600, and 2500 Hz, and subtracts 20 d. B then Draw horizontal line. Where horizontal line crosses response curve at left is f 1 (lowest frequency) and on right is f 2 (highest frequency). 18

(4) Distortion HEARING AIDS’ SPECIFICATIONS 19

Amplitude non-linearity (Distortion) • Definition: failure of the system to transmit or reproduce a perceived waveform with exactness. • Value: hearing aid distortion has a bad effect on the headroom which is the most common cause of hearing aid rejection. 20

Amplitude non-linearity (Distortion) • An ideal hearing aid would produce an output that was identical to the input signal, only amplified. However, a hearing aid is not capable of doing this and when the output does not reproduce exactly the input, it is said to be distorted. 21

Amplitude non-linearity (Distortion) • Headroom can be defined as the “residual dynamic range of a hearing aid, expressed as the difference in d. B SPL between a given output and the level of saturation of the device • Limited headroom’ problems really refer to the hearing aid’s inability to transduce intense signals such as music. 22

Harmonic distortion • Harmonics frequency; – – are multiples of the original The fundamental frequency = 2 k. Hz The first harmonic = 1 x 2 k. Hz = 2 k. Hz The second harmonic = 2 x 2 k. Hz = 4 k. Hz The third harmonic = 3 x 2 k. Hz = 6 k. Hz • The harmonic content decreases with increasing frequency and measurements usually only consider the second and third harmonics. 23

Harmonic distortion • It is measured using an input of 70 d. B SPL at 500 and 800 Hz and 65 d. B at 1600 Hz to simulate a louder than average input signal and with the H. A. at reference test gain position. 24

Harmonic distortion • The total harmonic distortion is The test for percent total harmonic distortion (%THD) measures the power produced in the output of a hearing aid at frequencies equal to the second and higher harmonics of the input signal as compared to the power produced at the actual frequency of the input signal. 25

Harmonic distortion The degree of distortion is expressed in one of two ways • Distortion ratio, where the distortion product is given as a number of d. B below the total output signal. • Distortion factor, where the distortion product is given as a percentage of the total output signal. 26

Harmonic distortion • A distortion ratio of 20 d. B or a distortion factor of 3 -10% is considered the maximum acceptable. • Harmonic distortion increases as the hearing aid nears saturation point and it is therefore normally measured below this point, at the reference test position, or, in the clinic, it may be measured at the wearer's volume setting. 27

Other types of distortion Intermodulation distortion • This occurs when there are two or more input signals and this results in additional frequencies, which are the arithmetic sum or difference of the input frequencies. • Intermodulation products, unlike harmonics, occur not only above the frequency of the fundamental but also below. 28

Other types of distortion Transient distortion • Sometimes known as 'ringing', occurs when there is a rapid change in the signal such that the hearing aid cannot reproduce the sharp rise and fall times. Frequency distortion • the sharp peaks in the frequency response, often caused by resonance in the receiver. 29

(5) Equivalent input noise level HEARING AIDS’ SPECIFICATIONS 30

Equivalent Input Noise Level (EIN) • Internal random noise generated by the hearing aid electronics. • It is bothersome for those with minimal hearing loss in the lower frequencies especially when H. A. is worn in quiet environment. • For measurement, the H. A. is placed in the test box at the user gain setting, at 50 db input. • The sound source is turned off and the output is recorded. 31

d. B SPL Eardrum reference Sounds get louder as you go UP the scale Understanding an SPLogram The Unaided SPLogram Maximum output targets Loud speech Avg. speech Soft speech Threshold (d. B SPL TM) Normal hearing

Hearing aids’ specifications “data sheet” • The main function of the data sheet is to provide information in the form of tables and graphs illustrating the results of performance tests carried out to comply with the requirements of relevant standards. 33

Hearing aids’ specifications “data sheet” • The standards aim to achieve a uniformity of hearing aid performance description that facilitates valid comparison between different models. 34

Purpose • • • Technical comparison. Evaluation. Production control. Delivery tolerances. Selection. Training and teaching. 35

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