Improved Hearing Assessment in Noisy Environments

Michael Fisher1,2, Ben Rudzyn1,2, Gordon Jarvis2 and Harvey Dillon1,2

1. The HEARing CRC, 2. National Acoustic Laboratories

XX Audiology Australia National Conference 2012 Adelaide

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Introduction

The accuracy of pure tone audiometry is dependent on the amount of ambient noise reaching the cochlea.

Environmental noise reaches the cochlea by several paths

The two main paths being: 1. Air Conduction2. Bone Conduction

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Introduction

• Environmental noise may mask lower level test tones

• Elevated hearing thresholds may result for people who would normally hear these lower level test tones in a quieter environment

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Introduction

The conventional solution to this problem is to use a soundproof booth to attenuate the environmental noise.

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There are many situations in which a clinician may wish to conduct a hearing assessment but it is impractical to use a soundproof booth, such as testing in a client’s

School

Home

Workplace

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Introduction

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Introduction

For these situations there are several commercially available devices that reduce the environmental noise such as:

• TDH 39 supra-aural headphones with circum-aural enclosures i.e. Audiocups,

• ER 3A & ER 5 insert earphones worn with/without earmuffs.

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Investigation Aim

To find the best practical solution to achieve accurate

audiometric hearing assessment without employing a

soundproof booth.

Provide advice on devices to use

SLM

Provide advice on the Maximum Permissible Ambient Noise Levels

Provide advice on correction factors for bone conduction measurements

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Investigation – Detailed Plan

Part 1. Preliminary investigation:

Objective testing of external noise attenuation using an

acoustic mannequin for:

I. Single attenuators – headphones or insert earphones

II. Dual attenuators – combinations earmuffs & insert

earphones

III. Effect of insert earphone delivery signal wire / tube on

earmuffs

Objective testing (10 subjects) of external noise attenuation

using the Microphone in Real Ear (MIRE) technique for single and

dual protection

Part 2. Subjective evaluation of selected devices (ER-3A and

MSA earmuffs):

Subjective testing (24 subjects) of the selected devices in terms

of:

I. external noise attenuation using the Real Ear Attenuation

at Threshold (REAT) technique for single and dual protection

II. bone and air conduction thresholds for single and dual

protection

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Part 2 – Subjective evaluation

EARTONE 3A or ER 3A insert earphones

MSA 766243 left / RIGHT, High, Yellow, Headband earmuff

Selected Earphone

Selected Earmuff

Subjective assessment of the selected devices (as

determined by objective measurement in Part 1)

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The tips may be changed without removing the piece of Libby Horn.

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Part 2 – Subjective evaluation

Piece of Libby Horn

The piece of Libby Horn is used to prevent buckling of the insert-tip’s tube when the earmuff is placed over the insert earphone.

A piece of Libby Horn fits firmly over the surround of the ER-3A nipple and extends to almost the length of the exposed insert-tip’s tube.

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Part 2 – Subjective evaluation

The ER-3A tube should have a gentle curve within the earmuff and should run down the neck.

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Method – Attenuation evaluation

Subjective testing (REAT Method)

Subjects: 24 normal hearers

Test: Békésy type automatic threshold determination

Testing signal: third-octave narrow-band noise at audiometric frequencies presented in a diffuse environment

Attenuation (Att) equals the difference in threshold (Th)

Diffuse sound field

Open Ear

Insert earphonesfitted

Insert earphonesand earmuffsfitted

Attearphone+earmuff = Thearphone+earmuff – Thopen

Attearphone = Thearphone – Thopen

Obtain Thopen

Obtain Thearphone

Obtain Thearphone+earmuff

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Results – Attenuation evaluation

Results show the average attenuation of the insert earphones (ER-3A) alone and insert earphones in combination with the earmuff (MSA 766 243 left/RIGHT)

*Also shown for comparative purposes is the average attenuation of the TDH 50 headphones with MX-41 cushions and the Audiocups devices [Berger 89].

Average attenuation (dB) at one octave centre frequencies (Hz)125 250 500 1 k 2 k 4 k 8 k

Insert Earphones 31 30 32 33 33 41 42Insert Earphones & Muffs 35 41 49 44 37 49 45

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Results - Maximum Permissible Ambient Noise Levels (MPANL’s)

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Air conduction thresholds are assessed with and without the earmuffs to determine if the addition of earmuffs has any effect on thresholds. Bone conduction thresholds are assessed to determine the effect of the addition of insert earphones alone and insert earphones in combination with earmuffs.

Method – Evaluating the effect on audiometric thresholds

Bone conducted vibrationmoves ear canal walls as

well as cochlear membranes and middle

ear ossicles etc

BoneConductor(Centre of forehead)

Extra vibration of tympanic membrane results from increased sound pressure in the ear canal.

This extra sound pressure is due to occlusion of the ear canal by insert

earphone and earmuff

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Method – Evaluating the effect on audiometric thresholds

Subjective testing

Subjects: 24 normal hearers

Test: manually determine • air conduction thresholds • bone conduction (unmasked) thresholds

Frequencies tested: 125 Hz (air only),250, 500, 1,000, 2,000, 4,000 & 8,000 Hz

Step size:• air conduction: 5 dB • bone conduction: 1 dB

The correction equals the average difference in thresholds with and without the devices fitted

Open Ear

Insert earphonesfitted

Insert earphonesand earmuffsfitted

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There was no effect on air conduction thresholds from placing earmuffs over the insert earphones.

There was a considerable effect on bone conduction thresholds from placing insert earphones in the ear canals with/without earmuffs.

The change in bone conduction thresholds can be corrected for (assuming no conductive loss) with the following correction factors

Note if the bone conductor is calibrated for mastoid placement then a further correction to the measured thresholds values is required, ISO 389-3:1994 E Annex C provides the following (informative) correction factors.

Results – effect on audiometric thresholds

Correction factors for bone conduction thresholds (for normal hearers with bone conductor on the forehead) (dB)

Test Tone Frequency (Hz) 250 500 1000 2000 4000 8000Nearest 1 dB 22 16 10 3 -1 1Nearest 5 dB 20 15 10 5 0 0

Mastoid to Forehead Correction Factor (dB)Test Tone Frequency (Hz)

250 500 1000 2000 4000 8000Nearest 1 dB -12 -14 -9 -12 -8 -10

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Conclusions

1. Double protection provides significantly better attenuation than single

protection using achievable insertion depths

2. The commonly used ER-3A insert earphones perform as well if not

better than other insert earphones when used in combination with

good earmuffs

3. The combination of the ER-3A insert earphone and a MSA left/RIGHT

“High” earmuff enables threshold testing to 0 dB HL in high

background noise levels, MPANL’s (minimum one-third octave noise

level: Lmax 41 dB SPL at 2 kHz)

4. There is no change in air conduction thresholds as a result of using

earmuffs over insert earphones.

5. The change in the bone conduction thresholds due to insert

earphones and earmuffs being worn by normal hearers can be

compensated for.

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In memory of the late

Ben Rudzyn

Summary & Acknowledgements

This research was financially supported by the HEARing CRC established and supported under the Australian Government’s Cooperative Research Centres Program

Advice on which insert earphone to use

Advice on which earmuff to use Advice on the Maximum Permissible

Ambient Noise Levels for a minimum achievable threshold

Advice on correction factors for bone conduction measurements

MSA 766243 left / RIGHT, High, Yellow, Headband earmuff

EARTONE 3A or ER 3A insert earphones

Correction factors for bone conduction thresholds (for normal hearers with bone conductor on forehead)

dBTest Tone Frequency Hz

250 500 1000 2000 4000 8000Nearest 1 dB 22 16 10 3 -1 1Nearest 5 dB 20 15 10 5 0 0

SLM

Special thanks to Lyndal Carter