Description of electro -acoustic characteristics of hearing

instruments and techniques for clinical fitting.

Selection criteria, Types, Styles and Technology of Amplification devices

Dr. Ghulam SaqulainM.B.B.S., D.L.O., F.C.P.S

Head of Department of ENTCapital Hospital, Islamabad

ELECTROACOUSTIC CHARACTERISTICS OF HEARING AIDS

• The fundamental purpose of a hearing aid is to provide sufficient acoustic information to allow the hearing-impaired person to maximize his or her communication skills.

• Several electroacoustic parameters are used to describe the performance of hearing aids.

ANSI• The American National Standards Institute (ANSI) has developed standards so that different hearing aids can be compared across clinics.

• The ANSI standard requires that these electroacoustic properties be measured in either an anechoic chamber or a specially designed test box containing absorbent material sufficient to reduce background noises.

• Electroacoustic measurements are accomplished by directing the output of a hearing aid into a 2-cm3 coupler (a hard-walled cavity with a volume of 2 cm3).

• The three most important characteristics associated with the ANSI standard continue to be gain, output sound pressure level with a 90 dB input and frequency response.

• The most recent standard, updated in 1996, added methods for measuring gain in compression instruments, tests for induction coil sensitivity, equivalent input noise tests, as well as changes in some terminology.

Gain of Hearing Aid• The gain of a hearing aid reflects the difference in the output

of the instrument relative to its input. • This measurement is obtained by presenting frequency-

specific input signals of fixed (usually 50 or 60 dB) SPL to the microphone, then measuring the resultant output SPL. For example, assume that a 2,000-Hz tone is presented at an input level of 60 dB and the measured output is 110 dB SPL. The gain of the hearing aid at this frequency is 50 dB. • By definition, gain will vary as a function of input intensity

level in nonlinear hearing instruments (those using compression or expansion). As a result, gain is measured using several different input levels during evaluation of these devices.

Gain can be represented in different ways: • Full-on gain reflects the amount of amplification achieved

when the volume control is adjusted to its maximum position.

• ANSI recommends that the hearing aid gain be measured across the frequencies 1,000, 1,600, and 2,500 Hz, and refers to this measurement as the high-frequency average or the high-frequency full-on gain.

• Reference test gain describes the amount of amplification obtained when the volume control is adjusted such that the average gain at 1,000, 1,600, and 2,500 Hz is 17 dB below the OSPL90 or full on if the hearing aid has mild gain.

• Another gain measurement is referred to as use gain or as-worn gain. In this instance, the gain is measured with the volume control adjusted to its normal use position. This output gives a more realistic indication of the amount of gain the aid provides for the patient

OSPL90• The OSPL90 of the hearing aid yields the maximum

amount of amplification provided by the instrument. • As the input level to a hearing aid increases, the

output level also increases up to a certain point, above which further increases in input do not affect the change in output. When this occurs, the hearing aid is said to have been driven into saturation. • The OSPL90 of a hearing aid is obtained by delivering

a 90-dB input signal from the loudspeaker in the test chamber to the microphone input of the hearing aid and measuring the overall output of the instrument across test frequencies.

Frequency Response of a Hearing Aid

• The frequency response of a hearing aid describes the gain of a hearing aid across a range of frequencies. • The range of frequencies for which a hearing aid offers

amplification is limited. • The aid's frequency response is determined by measuring its

reference test gain. From that average, 20 dB is subtracted and a line is drawn parallel to the abscissa until it intersects the low-frequency end of the curve and the high-frequency end. These two cutoff points then represent the aid's frequency response.

TECHNIQUE OF CLINICAL FITTINGThe methods for working with hearing instruments and hearing-

impaired individuals rely largely on: • Asking the right questions, • Caring for the patient, • And an imagination that draws on solid audiological knowledge.

There is not likely to be only one appropriate approach for any given patient. Likewise, any strict formula for hearing aid selection without the questioning, caring, and imagination of the practitioner would be probably be too restrictive for real-world dispensing. Only time and experience will tell which of these (or other) methods will work for you and your patients. And the methodology will change as new technology becomes available and new information emerges relative to amplification and the people-machine interface that dispensing professionals strive to bridge in their practices each day.

The Basics of Hearing Aid Selection

• Hearing aid selection is a complex part of hearing rehabilitation. • The selection process follows the clinician’s assessment of a

patient’s candidacy for amplification, and precedes the hearing aid fitting, verification, and validation processes. • The clinical challenge is to weigh the many factors in the

selection process to achieve an optimum fitting. • It is a fact that there is not just one possible hearing aid fitting

per patient. Therefore, patients and clinicians usually have many choices in the selection of treatment. • Essentially, the patient’s goals, the clinician’s assessments, and

all of the potential fittings somehow must merge during the fitting process to arrive at the most successful rehabilitation outcome.

Selection ProcessThe selection process is divided into three parts:

• Part 1: • Selection by the Physical Factors of Cosmetic, Anatomical Issues, and • Selection by Needs Assessment;

• Part 2: • Selection by Measurement, Circuitry and Components, and Expert

Fitting Techniques, and • Part 3: • Selection by Perception and Paired Comparisons.

(Note: At present all dispensing professionals favors usage of most of these three parts in order to yield the best outcomes for the applied amplification of hearing-impaired patients)

An everyday three-part categorization may provide a practical framework for the selection process used in most practices:1. Clinical Considerations    a. Type of hearing loss (sensorineural, conductive, or mixed);    b. Degree of hearing loss (mild, moderate, severe, or profound);    c. Sensitivity to sounds, tolerance/recruitment problems, and dynamic range;    d. Psychological attitude toward correction (eg, motivation and the primary motivator);    e. Contraindications for correction.

2. Physical Conditions of the Patient    a. Shape and size of ears and ear canal;    b. Manual dexterity and finger sensitivity;    c. Mental acuity.

3. Patient Wishes/Preferences    a. Cosmetic;    b. Needs assessment;    c. Appropriate circuit choices (digital, programmable, etc);    d. Appropriate controls (eg, AGC, VC, remote control, directional microphones, etc).

Selection by Physiological, Anatomical, and Dexterity Factors: • The anatomical parameter of the hearing aid selection process includes otoscopy

and a general examination of the ear canal and external ear. • The physical status of the entire external ear and eardrum is important in the

hearing aid selection process. It should be noted that all external ears differ as much as fingerprints differ from person to person. • The right and left ear of the same individual can differ substantially, as well.• Otoscopy in the hearing aid selection process reveals diseased or non-diseased

ears, the condition (or even absence) of the eardrum, and the size, shape, and configurations of the canal and concha. All of these observations can affect the selection process. • Deformed or malformed ears will require an altered selection of hearing aids,

since these problems will affect retention and feedback during hearing aid usage.• An extreme condition, such as the absence of the pinna, will alter the selection

process by the dispensing professional since there may be no concha, canal, or both.

• Another anatomical situation that can occur is the presence of a surgically treated ear that has a large irregular ear canal that may open wider once past the canal opening. Findings of this anatomical situation will require careful selection to avoid fitting complications. • In recent years, completely in the canal (CIC) style hearing

aids emphasize the need for observing canal configurations. • The canal inspection may reveal tenderness, abnormalities

(eg, stenosis), surgical scars, dermatological conditions, fat or thin canals, prolapsed canals, unusual bends upward or at peculiar angles, etc. These visual findings may suggest contraindications for such choices as CIC type instruments and other special selections by the clinician.

The question of dexterity and mobility are additional physical factors in selecting hearing aids. • The insertion, removal, and operation of various hearing aid

styles must be evaluated carefully by the clinician. • Stroke patients, in particular, may have inhibited manual capabilities. • Some patients may have reduced sensitivity of touch in their

fingertips, and medications can also affect the above.• Likewise, hand size and the presence of tremors (eg, Parkinson

disease) are in this group of factors in selecting an appropriate hearing aid for the individual.

• A very easy protocol to incorporate into the selection process is to have the patient operate a sample hearing aid. In this way, one can observe in advance their use of a remote control, volume control, etc, prior to the hearing aid being selected. The process should also include the patient’s use of other controls and battery insertion and removal.

Circuit Selection and Needs Assessment: • Initially circuit choice was not an option. The patients elected to

use amplification or they didn’t. • The present situation has literally hundreds of hearing aid product

lines, models, and styles from which to choose. • A simplified breakdown of current circuit options : • Analog linear (traditional linear aid) • Standard Compression (automatic gain control at input/output )• Advanced Compression (automatic signal processing, eg, BILL, TILL) • Advanced Non-Digital Circuits (analog programmable via trimpots) • Digitally Controlled Analog (analog programmable via computers or

proprietary programming device) • Digital Signal Processing (DSP), including those that employ special circuits

(loudness mapping, electronic shaping, noise reduction algorithms, feedback and occlusion reduction, etc)

Other Important Circuit Options • Multiple channels/bands• Multiple memories (separate programs/memories (or automatically

switching memories)• Directional microphone systems (that can substantially alter hearing aid

performance and user satisfaction)• Manual volume control/override or remote control The actual programming of the aid is critical in defining what the device actually does (eg, it’s technically possible to program a digital aid to function as if it were a traditional analog linear aid).

In the future, the above distinctions may become even more confusing as new software-controlled open-system and binaurally programmable hearing aids emerge.

It’s even possible that digital instruments will end up replacing virtually all of the above circuit choices in the next decade