A Review of the Basics: Acoustics and Basic Audiometry

HIS 230: Advanced Audiometry

What is sound? In the physical sense

◦it is a disturbance of molecules that is propagated through some elastic medium, like the air

In the psychological sense◦sound is the act of hearing

something

Simple Harmonic MotionThe most basic form of soundRepresented by a sine wave, a

series of repeated compressions and rarefactions

Valente, page 8

Acoustics vs. PsychoacousticsAcoustics are physical

properties of a sound that can be measured◦Frequency, intensity, wavelength

Psychoacoustics refers to our human perceptions of sound◦pitch and loudness

FrequencyThe number of cycles per second

◦ One cycle consists of one compression and one rarefaction of the molecules in the medium (air)

Unit of measure=Hertz (Hz)◦ Human ear bandwidth=20 to 20,000 Hz

1000 Hertz = 1 kiloHertz (kHz)

Pure Tone vs. Complex SoundA pure tone has almost all its

energy located at one frequency (sine wave)◦We use pure tones during

audiometric testingA complex sound is composed of

multiple frequencies◦Most of the sounds in our day-to-day

lives are complex in nature speech

Frequency and PitchRecall that frequency is an

acoustic property of sound that can be physically measured◦Unit of measure is the hertz (Hz)

Pitch is a psychoacoustic property of sound, in that it represents our human perception of how “high” or “low” sounds of various frequencies are.

Frequency & WavelengthWavelength=the distance a

soundwave travels during one cycle of vibration◦ Low frequency sounds have a

longer wavelength◦ High frequency sounds have a

shorter wavelengthIf the waves at left occurred

over 1 second, what would the frequency of each be?◦ 3 Hz◦ 10 Hz

Image from: blog.leeburrows.com

Fundamental FrequencyThe fundamental

frequency is the frequency in a complex sound that has the lowest frequency and the most energy (100 Hz)

Harmonics are multiples of the fundamental frequency and contain less energy than the fundamental frequency or the preceding harmonic (300, 500, 700, 900 Hz)

Image from: beausievers.com

Resonant FrequencyThe resonant frequency is the frequency at which

a system will be most easily set into vibration◦ It is the point where mass and stiffness are equal and

there is no opposition to the flow of energyIn a tube, with one open end and one closed end:

◦ The longer the tube, the lower the resonant frequency◦ The shorter the tube, the higher the resonant frequency

The average resonant frequency of the ear canal is approximately 3000 Hz, but will vary depending on the patient’s ear canal size and shape!

Note: this will be very important in the fitting of hearing aids and is the reason why individual REAL-EAR VERIFICATION is so important

Intensity and LoudnessRecall that intensity is an

acoustic property of sound that can be physically measured◦Unit of measure is the decibel (dB)

Loudness is a psychoacoustic property of sound, in that it represents our human perception of how “loud” or “soft” sounds of various intensities are.

IntensityUnit of measure=decibel (dB)

◦ Human ear dynamic range=0-140 dB SPLdB scale is a logarithmic scale

◦ This allows for the huge range of sound pressures that our ears can detect to be stated in simpler terms

Several dB scales exist, each with a different frame of reference. We are most interested in the following:◦ dB SPL (sound pressure level)◦ dB HL (hearing level)◦ dB SL (sensation level)

dB SPLSPL=sound pressure level

◦A pressure measurement Commonly used to express noise

measurements with a sound level meter The most common dB scale used in

hearing aid testing (electroacoustic/test box and real-ear measurements)

dB HLHL=hearing level; the dB level on the

audiometer when performing a hearing test. Ours ears do NOT have a flat frequency

response ◦ They are more sensitive in the mid-frequencies (500-

5000 Hz) than at other frequencies the average normal hearing person has a threshold of 7

dBSPL at 1000 Hz, but it requires almost 50 dBSPL to hear a sound at 125 Hz.

Each of these levels for normal hearing in dB SPL is equal to 0 dB HL (audiometric zero).

Without dB HL, a normal audiogram would form an arched shape rather than a straight line, which would be difficult to explain to patients!

dB SLSL=sensation level; the amount

of dB over a patient’s threshold◦Commonly used to refer to the level

by which one should perform speech testing

◦Also used in the assessment of tinnitus and tinnitus masking

AUDIOLOGIC ASSESSMENT

You can’t fit a hearing aid until you can perform/interpret an audiogram!

Being able to perform an accurate hearing test and interpret the obtained results are imperative to:◦Making appropriate

recommendations Is a medical referral required? Amplification options Hearing aids versus cochlear implant?

◦Fitting hearing aids appropriately!

Before the audiogram…The test environment should meet ANSI

standards:◦Calibrated equipment

The calibration of testing equipment is generally required annually by each state as part of professional licensure.

Each audiometer is calibrated with its specific headphones/inserts coupled to it and in the test booth that it corresponds to. You cannot move headphones or audiometers around at will.

Daily listening checks are important (See Table 3-3)

◦Preferably, a sound isolation booth To reduce the likelihood of artifacts created by

other noises in the environment

Taking a Case History

Any case history form must provide space for a statement of the perceived problem—including why professional services are being sought. ◦It is helpful to know the patient/client’s

own attitude about the appointment.◦Knowing the reason for the visit can

provide powerful insights before the rest of the history has been completed or the first test has been administered.

Information to be collected should include:Hearing loss query (one or both ears, duration)Family history of ear diseaseFamily history of hearing impairmentHistory of noise exposureHistory of head or ear traumaPast related ear surgery, childhood

illness/diseaseReports of vertigo and/or tinnitusHistory of hearing instrument use

Taking a Case History

Before the audiogram…Seating the patient at a right angle to

the tester allows for the observation of facial expressions and ease of communication, without allowing the patient to watch the tester working, turning dials, etc.

Otoscopy should always occur before your testing begins

Clear and concise instructions to the patient help to avoid errors in your testing

Basic Hearing EvaluationAudiogram

◦ Puretone audiometry Air-conduction (AC) and bone-conduction (BC) testing

◦ Speech audiometry Speech reception threshold (SRT)

This may also be called speech recognition threshold Word recognition score (WRS)

This may also be called speech discrimination score

◦ The goal is to determine How well you hear How clearly you hear speech If there is a medical reason for hearing loss If there is a need for some sort of intervention

Puretone AudiometryRecall that human ears have an audible

bandwidth (frequency range) of 20 to 20,000 Hz

In audiometric testing, the stimuli are puretones from 250 to 8000 Hz. ◦Why?

because human speech falls within these frequencies

◦Most commonly octave bands at 250, 500, 1000, 2000, 4000, and 8000 Hz are tested

◦The interoctave frequencies of 3000 and 6000 Hz are also commonly tested in adults

Puretone AudiometryAir-conduction testing

◦Performed with headphones or insert earphones

◦Takes into account the entire auditory pathway Outer ear, middle ear, inner ear, nerve, brain

Bone-conduction testing◦Performed with a bone-conduction oscillator

placed on either mastoid bone (most common) or the forehead

◦Directly stimulates the inner ear and nerve Bypasses the outer and middle ear

Puretone AudiometryThreshold is the softest sound that a listener can

hear 50% of the timeA bracketing technique is used to establish

threshold. ◦ Remember…down 10, up 5.◦ If the patient responds, reduce the stimulus intensity by

10 dB. If no response, increase intensity using 5 dB steps. Repeat this procedure until you find threshold.

The patient’s thresholds are recorded on the audiogram.◦ I prefer to start testing at 1000 Hz at 50 dB HL. ◦ I always use a pulsed puretone, as it is preferred for

patient’s with tinnitus. In addition, it is important to vary the timing of your presentation of the tones so the patient isn’t guessing at a pattern

Puretone Threshold Symbols

Note: Bone conduction thresholds obtained on the

forehead are indicated by a ^ symbol.

Normal Hearing

Normal AC and BC thresholds

Many different scales exist regarding degree of HL

For the purposes of this class, we will use the scale on the next slide.

Degree of Hearing Loss This is the exact scale

that I use in interpreting audiograms◦ Some clinics are more

liberal and consider normal hearing to be any threshold up to 25 dBHL

In determining the degree of loss, the textbook approach would be to calculate the puretone average (PTA=average dB of AC thresholds at .5, 1, 2 kHz) and compare the PTA to the scale at right. From: Northern, J. Hearing Disorders (3rd

ed)