In brief
An audiogram is a graph showing, in decibels, the faintest sound that can be heard at each frequency; the lower the line, the poorer the hearing. Noise-induced hearing loss typically starts in both ears with a V-shaped notch around 4000 Hz and spreads over time to the speech frequencies. The real information comes not from a single test but from comparing the pre-employment test with periodic tests carried out under the same conditions.
What is an audiometry test, and who needs a hearing test?
An audiometry test finds a person's hearing threshold, that is the faintest sound they can hear at each frequency, by playing sounds at different frequencies and intensities through headphones. The result is shown on a graph called an audiogram. The basic test used in workplace health surveillance is pure-tone audiometry.
Who must have a hearing test is set by Art. 13 of the Noise Regulation:
- Workers whose daily noise exposure exceeds the upper exposure action value of 85 dB(A) must have a hearing test arranged by the employer.
- Workers exceeding the lower exposure action value of 80 dB(A), where the risk assessment and measurement results indicate a health risk, may have a hearing test.
- Beyond these, the workplace physician determines the frequency and scope of health surveillance.
For a worker starting a noisy job, the pre-employment test is particularly important: all later tests are compared with this baseline. In hazardous and very hazardous work, a worker cannot start without a fitness-for-work report; we explained where the report is obtained in our article on the pre-employment health report. For details of the values, see our article on the Noise Regulation: 80, 85 and 87 dB.
Reading the audiogram: axes and symbols
Audiogram interpretation starts with knowing how the graph is drawn. The point most often confused at first glance is that the vertical axis is "upside down": the top shows good hearing, the bottom shows loss.
| Element | What does it show? | How is it read? |
|---|---|---|
| Horizontal axis | Frequency (Hz), usually from 250 to 8000 Hz | Low to high pitch from left to right |
| Vertical axis | Hearing threshold (dB HL) | 0 at the top; the lower, the greater the loss |
| O symbol (usually red) | Right ear, air conduction | Threshold measured through headphones |
| X symbol (usually blue) | Left ear, air conduction | Threshold measured through headphones |
| < and > symbols | Right and left ear, bone conduction | Threshold measured with a vibrator placed behind the ear |
| Gap between air and bone conduction | Sign of conductive loss | Suggests an outer or middle ear problem |
In most classifications, thresholds up to 20–25 dB HL are considered normal. If air and bone conduction thresholds are both low, the loss originates in the inner ear (sensorineural); noise-induced hearing loss is of this type. If air conduction is low but bone conduction is normal, that is if there is a gap between them, a cause unrelated to noise such as an earwax plug or a middle ear problem is considered, and the worker is referred for an ear, nose and throat assessment.
The 4000 Hz notch: the signature of noise-induced loss
The list of occupational diseases clearly describes how noise-induced hearing loss appears on the audiogram (Regulation on Procedures for Determining Loss of Working Capacity and Loss of Earning Capacity in the Occupation, Annex 2, E-3): initially a V-shaped dip at 4000 Hz is seen; the loss then spreads to the 500–2000 Hz frequency range. At this second stage, the worker starts to have difficulty understanding speech.
You can recognise the typical picture by these features:
- Bilateral and symmetrical: A notch of roughly the same depth in both ears.
- Deepest point in the 3000–6000 Hz range: Usually 4000 Hz, sometimes 6000 Hz.
- Recovery at 8000 Hz: The line rises again after the notch. In age-related loss, by contrast, the line falls steadily towards the high frequencies and there is no recovery at 8000 Hz.
- Sensorineural type: Air and bone conduction fall together.
A unilateral loss or one with a marked difference between the ears, a sudden-onset loss or a conductive loss does not fit this picture. In these cases the physician refers the worker for further assessment to investigate other causes.
Comparing the pre-employment and periodic tests
A hearing test result should not be read on its own as "normal" or "abnormal". The real question is: has this worker's hearing changed since the day they started? For the comparison to be meaningful, the tests must be carried out under the same conditions:
- Same frequencies: At least 500, 1000, 2000, 3000, 4000, 6000 and 8000 Hz.
- Ears rested from noise: A test at the end of a shift can make temporary threshold shift look like permanent loss. Schedule the test at the start of the shift or after a rest period.
- Quiet environment and calibrated equipment: The booth or room where the test is carried out must be quiet enough, and the audiometer's calibration must be valid.
- Ear examination: An earwax plug, a cold or an ear infection affects the result; it is checked before the test.
- Same recording format: Results are kept with their numerical values; a report that only says "normal" does not allow comparison.
Take the following right ear record as an example:
| Frequency (Hz) | 500 | 1000 | 2000 | 3000 | 4000 | 6000 | 8000 |
|---|---|---|---|---|---|---|---|
| Pre-employment (age 38), dB HL | 10 | 10 | 10 | 15 | 15 | 15 | 10 |
| Periodic test (age 44), dB HL | 10 | 10 | 15 | 30 | 45 | 35 | 20 |
| Difference, dB | 0 | 0 | 5 | 15 | 30 | 20 | 10 |
Reading: the low frequencies have not changed, the largest change is at 4000 Hz, and at 8000 Hz the line rises again. This picture fits the early stage of noise-induced loss. To separate out the effect of age, the explanatory note to the list requires a decline of half a decibel for each year of age after 40 to be counted as physiological decline in the occupational disease assessment. In the example, 2 dB is deducted for the 4 years after age 40; it is clear that the 30 dB change at 4000 Hz cannot be explained by age. If there is a similar picture in the left ear, the physician has the test repeated to confirm the finding and starts the workplace process.
What to do at the workplace when a loss is found
Art. 13/2 of the Noise Regulation requires the workplace physician to inform the worker of the result when noise-induced hearing loss is detected. The employer reviews the risk assessment and the measures taken, implements measures such as assigning the worker to another job without noise exposure, and arranges for the health of other workers with similar exposure to be reviewed.
The audiogram also guides this review. If a similar notch appears in several workers from the same department, the problem is not individual but the department's noise level or hearing protection practice. Measurements in that department should be repeated, and the type of hearing protection and its rate of use should be checked on site. For this, read test results not only person by person but also department by department: putting results side by side for groups such as the weaving hall, preparation, maintenance and warehouse shows in which area prevention should be prioritised. Confidentiality of health information is maintained in this comparison; the employer receives not individual results but a department-level assessment and the recommended measures. For early signs, informing the worker and the occupational disease process, see our article on noise-induced hearing loss.
Common mistakes
- Carrying out the test at the end of a shift, after a noisy day.
- Skipping the pre-employment test for a worker starting a noisy job.
- Writing the result only as "normal" or "mild loss" and not keeping the numerical values.
- Testing at different frequencies or under different conditions each year.
- Closing the file by attributing the loss to age without making a comparison.
- Not reflecting a recurring finding in the same department in measurements and the risk assessment.
We plan hearing tests with authorised healthcare facilities and assess the results together with noise measurements and previous tests. Bursa Sistemer OSGB's health surveillance service monitors hearing tests in weaving, spinning and metalworking businesses in Bursa and neighbouring provinces by comparing them with the pre-employment record. For risks specific to weaving halls, see our article on OHS in weaving businesses and our textile page, and for examination intervals, our article on periodic examinations and tests. You can track test dates with the OHS calendar tool.
Frequently Asked Questions
How many decibels is normal in a hearing test result?
In most classifications, thresholds up to 20–25 dB HL at each frequency are considered normal. From the workplace's point of view, however, what matters is not a single value but the change compared with the pre-employment test.
What does a 4000 Hz notch mean?
A V-shaped dip around 4000 Hz on the audiogram with recovery at 8000 Hz is the typical early finding of noise-induced hearing loss. Seeing it similarly in both ears strengthens this interpretation.
How often should a hearing test be carried out?
The Noise Regulation does not give a fixed interval; the workplace physician determines it. The test is mandatory for exposure above 85 dB(A), and may be carried out where exposure exceeds 80 dB(A) and there is a risk.
What should a worker with a poor test result do?
First, the test is repeated with ears rested from noise. If the loss is confirmed, the workplace physician informs the worker and the employer reviews the measures; if necessary, the worker is referred for further assessment.
An exposure-based test plan, pre-employment and periodic examinations, confidential health files and an examination schedule tracked per person.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.