variances in the audiogram data of individuals undertaking...

Variances in the Audiogram Data of Individuals

Undertaking Audiometry on Different

Calibrated Audiometers

August 7, 2013

L G Davison*, BScC Barlow, PhD, MIOA

M Ashmore, RHAD, MIOAR Weinstein, RHAD

Southampton Solent UniversityFaculty of Maritime and Technology

*Point of contact: [email protected]

mailto:[email protected]

Audiometer variances: Subjective measurement

Abstract

This test was performed on a cohort of university employees and students, to measurethe variation in audiogram data that can be experienced when using different traceablycalibrated audiometer systems, over a period of 3 weeks.

The test group was: a Maico MA51, Siemens Unity 2, and a Kamplex KLD23, all ofwhich were traceably calibrated up to 3 months before the tests took place.

The results showed a reasonably large spread of data points, with a much higherlevel of variation in the high frequency area. Mean and standard deviation resultsare provided to show the variability on each test subject, with the peak standarddeviation of audiometer variance at 20.21 dB.

1. Introduction

In the field of audiology, hearing assessmentusing a manually operated audiometer isconsidered the gold standard for accuracy.It is for this reason that it is vitally impor-tant to make efforts to continually assessthe accuracy and practicality of the prac-tice, in order to see that it meets modernrequirements.

This study is designed to assess a cohortof individuals to determine their hearingthresholds, and then compare this to assess-ments made with different audiological de-vices , over subsequent weeks. The aim is todetermine if the practice properly meets re-quirements of patients, and identify sourcesof error that could contribute to an incorrectassessment, and therefore diminish patientcare.

2. Methods

2.1. Procedure

For this test, a cohort of people who wereconcerned about their hearing were assem-bled, using a promotion in a University-wideemail. The people therefore were not se-lected with any gender or age bias, other

than those imposed by the demographic ofthe University staff and students.

The test schedule was designed over 3 weeks,with the subjects taking the test at the sametime of day each week. The reason for thisis to negate learning effects as much as pos-sible, wherein people become ”better” atthe tests over time. The tests were at aconstant time of the day in order to controlfor varying levels of concentration experi-enced during the day, which might effectthe test. This was kept to as much as pos-sible, however due to the nature of testingpeople it was sometime necessary to be flex-ible. The dates and times were noted onthe audiogram sheets.

Each of the tests were performed to BSA(British Society of Audiology) proceduralguidelines [5], which are based on, and standalongside the British Standard procedure[3]. A precursor to the audiometric examin these guidelines is otoscopy, which wasperformed on all patients before each test,and any otological abnormalities noted. Ac-cording to guidelines the subjects were alsoasked relevant questions to assess their hear-ing state at the time, including enquiringabout any recent loud noise exposure, orotorhinolaryngological issues that were on-going. The answers to these questions wererecorded in the subject notes.

Subjects were measured over normal au-

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diometric frequencies, 500 Hz, 1 kHz, 2 kHz,3 kHz, 4 kHz, 6 kHz, and 8 kHz

2.2. Equipment

The test took place in a Hemi-anechoicchamber with a noise floor of 16 dBA, inorder for noise to not be a factor in the

test.

The audiometer test group is as follows:

• Maico MA51

• Siemens Unity 2

• Kamplex KLD23

The only other specialist equipment usedwas an otoscope.

3. Results

The full table of results can be found in the appendix.

Figure 1: The average Standard Deviations across audiometric bands

4. Discussion and Conclusions

Due to the fact that all of the audiometerson test use the same headphone type (TDH-

39p), they are all calibrated using the samecalibration parameters, as defined in BS ENISO 389-1 [1]. Due to this, the audiometersall performed similarly in assessing the hear-

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ing acuity of the subjects on test. There aresome sources of error in the measurementprocess, however that are worthy of note.

Primarily the ambiguity of the calibrationprocess defined in BE EN ISO 8235-1 [3]leads to a potential source of error. There isno mandatory accreditation body or schemethat operates in relation to calibration ofaudiometric devices, the standard simplystates that the calibration centre must be a“Suitable Laboratory”[3], which can be inter-preted in a variety of ways. The RETSPLs(Reference Equivalent Sound Pressure Lev-els) that are defined for the TDH-39 head-phones are provided twice, once for a refer-ence coupler designed for the headphones,and again for the more modern IEC artifi-cial ear [2] design that is proposed as thenew calibration standard [1]. Again, this isnot ideal, providing two sets of calibrationspecifications can lead to confusion and in-accuracy. It is therefore agreed with thefindings of the National Physical Labora-tory in their recommendation[6] of revisionsto BS EN ISO 60318 to only include a spec-ification for calibration on the IEC artificialear.

Another source of error is the use of differ-ent headband designs in the use of TDH-39headphones, which supply varying tensionto the subject’s ears. A further experimentcould be proposed to measure the same cali-brated headphone cups on individuals usingdifferent band designs and conditions, in asimilar way to this experiment. As it stands,

it is reasonable to assume that headphonetension differences attribute some of thevariability seen in these data.

It is worth noting that variance is very sub-ject dependant. The highest standard devi-ation in results across the three tests wassubject 2, with 20.21, which represents alarge variability. Looking at the raw data,it can be seen that this was caused by awayward result on the first test in the leftear at 6 kHz. It can be hypothesized thathigher levels of variability could be due todifferent levels of patient noise exposure, oreven simply down to their mood or concen-tration.

Subject variability is more pronounced inthe high frequency areas of the audiogram,as evidenced by the graph on page 3. Thereare two main reasons that could attributethis error. One is that directionality ef-fects in the TDH-39 headphone design cancause headphone placement to be a factor,with some audiologists potentially placingthe headphones onto the patient’s ears in amore on-axis orientation, causing a louderresponse than other tests. The other mainfactor could be temporary threshold shiftcaused by exposure to loud noise [4]. Hu-man ears are more efficient in the 6 kHzrange [7], and so this frequency area is morelikely to be effected by threshold changesdue to loud noise.

Overall there is a 4.09 average standard de-viation across the data, which is a significantvariation in a gold standard test.

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5. Appendices

A. Subject Audiograms

Figure 2: A graph of all three audiometer outputs for subject 1

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B. Mean and Standard Deviation Graphs

Figure 14: A graph of the mean and standard deviation error bars for subject 1

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References

[1] Acoustics - Reference zero for the calibration of audiometric equipment. Part 1:Reference equivalent thresholds for pure tones and supra aural headphones , 2000.

[2] Electroacoustics - simulators of human head and ear. Part 1: Ear simulator for themeasurement of supra-aural and circum-aural headphones, 2009.

[3] Acoustics - Audiometric test methods. Part 1: Pure tone and bone conductionautiometry (ISO 8253-1:2010), 2010.

[4] American Hearing Foundation. Noise induced hearing loss [http://american-hearing.org/disorders/noise-induced-hearing-loss].

[5] British Society of Audiology. Recommended procedure - pure-tone air-conductionand bone conduction threshold audiometry with ans without masking. British Societyof Audiology Website, 2011.

[6] Timothy R Sherwood and Richard George Barham. A Review of the Relative Technicaland Practical Merits of the IEC Reference Coupler and Artificial Ear and Their FutureRole in Pure-tone Audiometry. Citeseer, 2004.

[7] H Takeshima Y Suzuki. Equal loudness level countours for pure tones. Journal of theAcoustical Society of America, 2004.

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