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Audiometer: Correction factor for atmospheric pressure
Zemar SOARES1; Davi A. BRASIL2; Viviane FONTES3
1
2
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Electroacoustics Lab. - Inmetro, Brazil
Dimensional Metrology Lab. - Inmetro, Brazil
Phonoaudiologist of SMS/SUBVISA/NUSAT, Brazil
ABSTRACT
Audiometers are electroacoustic equipment used by audiology professionals to measure the auditory acuity.
The general and specific requirements that characterizes how it should be an audiometer is described in IEC
60645-1 (2012), including its calibration. However, this technical document does not allow the use of the
audiometer at range of atmospheric pressure out of 98 kPa to 104 kPa. This means that approximately in
cities higher than 290 meters altitude the audiology professionals may not use the audiometer. This paper
presents correction factors for audiometric earphones coupled both with 6cc couplers (IEC 60318-3) as in
artificial ears (IEC 60318-1). Measurements in a vacuum/pressure chamber were taken from sea level to
equivalent atmospheric pressure at altitudes of 1600 meters. The different values at each altitude it enabled to
determine the correction factor that lets audiology professionals can use the audiometer at different altitudes
without the loss of quality of the results.
Keywords: Audiometer, atmospheric pressure, correction factor, earphones, audiometry
1. INTRODUCTION
The audiometers are measuring instruments applied to health (specifically audiology) widely used in a
developed society. Brazil has 38.753 Audiology professionals (Federal Council of Phonoaudiology of
Brazil - Set 2015). The estimated number of audiometers in the country is around 5000. Taking these
numbers it is noticed a significant number of audiometers that need to be evaluated by mean of periodic
checks.
This number of audiometers in Brazil shows a potential deal for calibration laboratories, however, IEC
60645-1 2012 [1] limits the calibration of the audiometer to an atmospheric pressure range of 104 kPa to 98
kPa. The value of 98 kPa is related to altitude of approximately 290 meters. For European countries, this
altitude level may not be very significant, but for America Latin countries this altitude level significantly
limit calibration of audiometers, consequently the application of these in clinical diagnostics.
A significant number of Brazilian cities is above this level altitude of 290 meters, implying still a
number around 3000 audiometer that should be calibrated but the IEC 60645-1 does not allow. However, it
is known that this is not the procedure that has been used. The audiometer are calibrated even being above
the level of 290 meters. Therefore, bring systematic errors in the calibration process. The magnitude of this
systematic error is a function of altitude level (or atmospheric pressure) where the audiometer was
calibrated. The sensitivity of earphones as a function of frequency changes directly with the change of the
local atmospheric pressure.
This article aims to measure these systematic errors (changes sensitivity) caused by different
atmospheric pressures due to different levels of altitude. The result of this investigation shows the
correction factor of atmospheric pressure as a function of frequency.
2. THEORETICAL CONSIDERATIONS
Considering the earphone as a source of the volume velocity u and the coupler as a cavity of volume V,
then the relationship between the alternating pressure p (detected by microphone inside the coupler) and the
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volume velocity u is [2]
(1)
where Ps is the atmospheric pressure,
is the specific heat ratio of air, f is the frequency of the driving
sinusoidal signal and j = (-1)1/2 is the imaginary number.
Taking the equation (1) is possible to estimate the theoretical variation of p with the decreasing of
Ps shown in the equation (2).
(2)
3. METHODOLOGY
The measuring reference of this investigation is the sound pressure level (SPL) emitted by the headset
into the coupler (artificial ear and 6cc). To obtain the reference SPL, it was necessary to use an excitation
signal (Swept Sine) with constant envelope [3] (100 Hz to 10 kHz) which was directed at the headphone
under test. At sea level (101,325 kPa) the SPL at 1 kHz emitted by the earphone was adjusted (voltage) to
approximately 90 dB. The audio analyzer used to perform this measurement of SPL was "CMF22 +
Monkey Forest."
For the simulation of different atmospheric pressures and consequently different altitude levels, a
Vacuum/Pressure chamber model 8700 of the Theodor Friedrichs was used. A
BaroThermoHygrometer PTU300 Vaisala model was used to monitor the environmental conditions
within the Vacuum/Pressure chamber. Figure 1 shows a measurement system used in this work.
Figure 1 - measuring system consists of signal analyzer, vacuum/pressure chamber, barothermohygrometer,
coupler (artificial ear and 6cc) and earphone under test.
The earphones under test used in this work were the TDH 39 (Telephonics) and DD 45 (Radioear). For
coupling the earphones to the measurement microphone were used the artificial ear (B&K4153) and the 6cc
coupler (B&K 4152).
The static pressure applied in the chamber to simulate different atmospheric pressures were 103,325 kPa,
98 kPa, 95 kPa, 92 kPa, 89 kPa, 86 kPa and 83 kPa. These simulations of atmospheric pressures correspond
from the altitude of the sea level to an altitude level of approximately 1700 meters. They were purposely
chosen to cover the altitudes of large Brazilian cities. For example, Campinas (~ 94 kPa), São Paulo (~ 93
kPa), Belo Horizonte (~ 92 kPa), Curitiba (~ 91 kPa), Brasilia (~ 89 kPa). Also, include medium and small
Brazilian cities that have atmospheric pressures close to 86 kPa and 83 kPa.
Before starting any measurement, the sound level calibrator was coupled to the microphone of artificial
ear to adjust the gain of the measurement system input. Then the earphone under test was coupled to the
artificial ear following the standard recommendation of force applied under it. The vacuum/pressure
chamber was adjusted to a pressure of 101,325 kPa. The excitation signal was directed to the earphone
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under test. The SPL emitted by the earphone was recorded by the audio analyzer as a function of frequency
(100 Hz to 10 kHz). This SPL recorded is assumed as the reference value for all other SPL recorded at
different pressures inside the vacuum/pressure chamber.
For the sequence of pressures inside the vacuum/pressure chamber, the pressure value was adjusted and
time waiting for at least 3 minutes for the internal equalization of the coupler before of start the measures of
SPL. Then after this time waiting, the excitation of the earphones was started and the SPL as a function of
frequency was recorded.
The test was repeated 3 times so that it could have an estimate of the repeatability of the measurement
result.
The correction factor proposed in this paper is the difference between the SPL measured, for example,
98 kPa for the SPL measured in 103,325 kPa. So many deviations were determined for each pressure
relative to the pressure at sea level. The correction factor allows to the laboratories correct the SPL
measured during calibration of audiometer at level of altitude higher than sea level.
4. RESULTS OF MEASUREMENT
Figure 2 shows the difference between the SPL measured at high level of altitude and SPL measured at
sea level. The result of this difference is presented as mean deviation between 4 earphones TDH 39 coupled
to the artificial ear. Figure 3 shows the standard deviation of the deviations calculated between 4 earphones
TDH 39 coupled to the artificial ear.
Noting that two earphones means: a left and a right earphone, for example of one headset TDH 39.
Figure 2 - Difference between the SPL measured at the high level of altitude and the SPL measured
at sea level. Results expressed as average of deviation between 4 earphones TDH 39 coupled to the
artificial ear
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Figure 3 - Standard deviation calculated from the deviations determined between 4 earphones TDH
39 coupled to the artificial ear
In the same way, measurements were taken using 4 earphones TDH 39 coupled to the 6cc coupler.
Figure 4 and 5 shows respectively the results of differences of the SPL measured (deviation of the
SPL of sea level) and standard deviation.
Figure 4 - Difference between the SPL measured at the high level of altitude and the SPL measured
at sea level. Results expressed as average of deviation between 4 earphones TDH 39 coupled to the 6cc
coupler
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Figure 5 – Standard deviation calculated from the deviations determined between 4 earphones TDH 39
coupled to the 6cc coupler
For the results with the earphone DD 45, it was used two earphones (left and right of the DD 45).
The results of Figures 6 represents the average value of the deviations found with two earphones.
Figure 6 - Difference between the SPL measured at the high level of altitude and the SPL measured
at sea level. Results expressed as average of deviation between 2 earphones DD 45 coupled to the
artificial ear
Also, measurements were taken using 2 earphones DD 45 coupled to the 6cc coupler. Figure 7
shows the results of differences of the SPL measured (deviation of the SPL of sea level).
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Figure 7 - Difference between the SPL measured at the high level of altitude and the SPL measured at sea
level. Results expressed as average of deviation between 2 earphones DD 45 coupled to the 6cc Coupler
Freq. Resp. TDH39+6cc
Figure 8 - frequency response curves for different atmospheric pressures. Red (101,325 kPa), Gray (98
kPa), Yellow (95 kPa), Light Blue (92 kPa), Green (89 kPa), Blue (86 kPa) and Brown (83 kPa)
In Figure 8, it is possible to note that the frequency response curves tend to shift to the left with the
decrement of the simulated atmospheric pressure inside the vacuum/pressure chamber. This left shift does
not occur at frequencies close to 6 kHz, where the resonance (1th mode) of the 6cc coupler does not seem to
change with the variation of pressure. In the frequency range where the shift to the left (from 2.5 kHz to 3
kHz) shows that the earphone resonance frequency decreases with decreasing atmospheric pressure.
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Further noting Figure 8 can be justified the reason of the correction factor around 2kHz be positive or
zero, would be expected to have negative correction values for the entire frequency range. As the resonance
frequency of the earphone decreases, so when comparing the curve corresponding to lower atmospheric
pressure against the reference curve (101,325 kPa) it is possible note that the value of the SPL measured at
frequency of the resonance peak (curve 101,325 kPa) increases. This value SPL increases because there
was a shift of the resonance to the left. However, there are frequencies that cause the shift of the resonance
to the left, leading to the computed differences between the measured SPL to near to the zero.
Another important point to note is that below 200 Hz coupling between the earphone and the 6cc
coupler does not seem to be enough. A leak seems to lead the internal volume of 6cc coupler to increase it
until to the large external volume. This causes the correction factor come close to zero because the new
propagation model (different from equation (1)) does the SPL to be more insensible to the variations of
atmospheric pressure.
Taking equation (2) and comparing it with the results shown in Figures 2, 4, 6 and 7 can only agree
in the frequency range of 3,15 kHz to 6,3 kHz. The arguments that justify this are already described in
the previous three paragraphs.
5. CONCLUSION
This work presented a measurement procedure for determining the differences in sensitivities of
headphones TDH 39 and DD 45 when coupled to the artificial ear and 6cc coupler.
The results show that it is possible to measure the change of the sensitivity of earphones TDH 39 and
DD 45 in the form of deviations from the sensitivity to sea level. Through these deviations are possible
establish a correction factor for these earphones when coupled to the artificial ear and 6cc coupler.
Based on the determined correction factor in this work it is possible to correct the measured SPL at
different altitudes from sea level. In each measured frequency, simply add the correction factor related to
altitude where the measurement was carried out. With this procedure the SPL measurement result is equal
to the SPL measured at sea level.
Even with few samples tested of earphones, 4 for TDH 39 and 2 for the DD 45, it is possible have a
quantification of this sensitivity of deviations related to the sea level.
This work is the beginning of an investigation that is ongoing and main objective is the search
results with at least 20 headphones TDH 39 and 20 earphones DD 45. With this sample quantity is
possible to measure dispersions of the results related to the production line of these earphones,
ensuring a good estimate of uncertainty of the correction factor proposed in this project.
ACKNOWLEDGEMENTS
Thanks to the Ministry of Health of Brazil to finance part of this work.
REFERENCES
1. IEC 60645 Electroacoustics - Audiometric equipment – Part 1: Equipment for pure-tone audiometry,
2012;
2. AIP Handbook of Condenser Microphones Theory, Calibration, and Measurements, George S. K Wong
and Tony F. W. Embleton, AIP Press, Chapter 4, ISBN 1-56396-284-5, 1995;
3. Müller, S.; Massarani, P.: Transfer-Function Measurement with Sweeps, Journal of Audio Engineering
Society, 80, 2001.
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