Audiometer: Correction factor for atmospheric pressure

Transcription

1 Audiometer: Correction factor for atmospheric pressure Zemar SOARES 1 ; Davi A. BRASIL 2 ; Viviane FONTES 3 1 Electroacoustics Lab. - Inmetro, Brazil 2 Dimensional Metrology Lab. - Inmetro, Brazil 3 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 (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 ) as in artificial ears (IEC ). 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 Audiology professionals (Federal Council of Phonoaudiology of Brazil - Set 2015). The estimated number of audiometers in the country is around 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 [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 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 1 zmsoares@inmetro.gov.br 2 dabrasil@inmetro.gov.br 3 viviane_fontes@hotmail.com 695

2 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 696

3 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 697

4 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 698

5 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). 699

6 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 (1 th 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. 700

7 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 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 , 1995; 3. Müller, S.; Massarani, P.: Transfer-Function Measurement with Sweeps, Journal of Audio Engineering Society, 80,