General about Calibration and Service on Audiometers and Impedance Instruments

Transcription

1 General about Calibration and Service on Audiometers and Impedance Instruments Calibration and Service of Audiometers - page 1 of 14 Rev /JF

2 General about Calibration and Service on Audiometers Purpose General about service General description of an audiometer The intention with this paper is to give new technicians or others who are interested, a brief explanation about the methods and necessary equipment used by the calibration and service of audiometers. An audiometer, together with accessories, is a delicate instrument, and should be treated with great care. Otherwise you cannot ensure that the measured results are correct and comparable to earlier tests and investigations made with other instruments. Anyhow, the normal use will often cause a slight change of the mechanical and electrical properties, mostly caused by normal wear of the instruments. It is therefore necessary to make periodic service on the audiometers, where the electrical and mechanical properties are checked by a skilled technician, who also makes judgements concerning eventual exchange of vital parts as cords and headsets in order to avoid defects in the future. The recommended interval between periodic service will normally be one year or less, for instruments in daily use. The basic audiometer produces pure sine tones of fixed frequencies, which are presented to the patient through a set of specially approved transducers. The output of the transducers can be controlled by a calibrated attenuator in the audiometer and the tone can be interrupted or presented without introducing any switch noise. The international standards IEC 645-1, IEC 645-2, IEC and IEC specifies the minimum demands for different kinds of audiometers. Air Conduction Earphones Measuring equipment for calibration - general Artificial ear For audiometry purposes, only a few types of earphones are approved. This is due to very strict demands to the mechanical and electrical properties. It is very important that the cushion has the correct shape and that the fitting to the earphone is good. Bad fitting of the cushion to the earphone can cause errors in the measurements. To be able to make a comparable and repeatable calibration of the output sound level of the earphones, it will be necessary for the technician to have a standardised set of reliable measuring equipment. The most important part of equipment is a standardised coupler, which is a sort of artificial ear, where the earphone can be placed in order to measure the acoustical output. In this way it will be possible to use the internationally standardised calibration values and to compare the measured results made by different laboraties. Calibration and Service of Audiometers - page 2 of 14 Rev /JF

3 Microphone Measuring amplifier Acoustic calibrator Calibration of pure tone air conduction The earphones for audiometers used by Interacoustics, are Telephonics type TDH39, and they are checked and calibrated by means of a coupler made in accordance to IEC 303. This coupler has a volume of 6cc but can be converted to a 2cc coupler using a supplied adapter. During the measurements the earphone is placed on top of the coupler, carefully centred over the opening, with the recommended total weight of 4,5N (459g). This static force can be supplied by a special spring arrangement or a fixed weight. It is important that the earphone is applied to the coupler without acoustical leakage. The coupler is supplied with a 1 inch condenser microphone of the pressure type. This is a very delicate device, and should be treated with great care by disassembling and assembling. The microphone has a very high output impedance and requires a polarisation voltage of typically 200V in order to function. To avoid problems with noise, a preamplifier is normally integrated on the coupler and this is again connected to a special input on the connected sound level meter, having the facilities of supplying the appropriate supply voltages. The coupler is connected to a sound level meter. This is a measuring amplifier with selectable amplification levels and an indicating display to show the measured sound pressure level. Sound pressure level (SPL), is expressed in db above a reference sound level of 20 µpa, which is defined as the minimum audible sound level. It is an advantage to have a measuring amplifier with an output for monitor, where an oscilloscope and frequency counter can be connected. During all calibration, it is important that the signals are undisturbed, and this can be monitored by using the oscilloscope, while the frequency counter can be used for the check of the exact frequency of the tones from the audiometer. In order to ensure that the indicator of the measuring amplifier is showing the correct values, it is advisable to have an acoustical calibrator. This device produces a calibrated sound pressure level and fits directly over the 1 inch microphone. The indicator on the measuring amplifier should, with the calibrator connected, show the level printed on the label of calibrator. If this is not the case, the sensitivity of the measuring amplifier should be adjusted, or the deviation should be taken in account by the measurements. The output level of an audiometer is normally expressed in Hearing Level (db HL), and can at least be changed in steps of 5 db, typically ranging from -10dB to 100 db. The HL indicates, that the instrument is calibrated to have output levels other than SPL (Sound Pressure Level). The international standard called ISO is providing the lowest sound pressure levels in db re. 20 µpa (SPL) for each audiometer Calibration and Service of Audiometers - page 3 of 14 Rev /JF

4 Microphone corrections frequency, which just can be heard by a normal hearing person. These values corresponds to a HL-setting of 0 db on the audiometer. Unfortunately this level is so low, that normal measuring equipment will be unable to give any readable results due to noise. During the check or calibration of the audiometer, the attenuator therefore has to be increased in order to make the earphone produce a usable level (normally 70 or 80 db HL). This means that the reading on the indicator of the measuring amplifier will be the sum of ISO levels and the selected HL-value. Before the calibration can be carried out, it is necessary to add some corrections, due to the fact that the 1 inch microphone is not linear through the whole frequency range. When you buy a microphone, you also get a chart showing its frequency characteristic. This is used to find, for each audiometer frequency, the deviation from its 0 db-level. A positive deviation indicates that the output of the microphone is higher than expected with a constant reference level, while a negative shows that the output is lower. Typically, the output will be flat at lower frequencies and fall off at higher frequencies. Positive deviations are added, while negative are subtracted in order to obtain the correct indication on the measuring amplifier. The correct indication for each frequency will then be: db HL+ ISO Δmic. Calibration of narrow-band masking noise Masking noise is calibrated in the same manner as pure tone. Only the reference values are different. Narrow band masking is calibrated using the values from ISO plus the values from ISO taking the figures valid for one third octave bandwidth. Also here the correction values for the microphone should be taken in account. Bone conduction Bone conductors Artificial mastoid The bone conductor is intended to produce vibration instead of sound. ISO gives standard reference threshold vibration values for bone conductors with a specified circular tip of an area of 175mm² and a static pressure of 5,4N (551g) on the human mastoid bone. The Threshold values in ISO are expressed in db above 1 µn and as for the pure tone, they are valid for an attenuator-setting of 0 db HL on the audiometer. A bone conductor is a small and compact device and is very sensitive to shock, which can have bad influence on its properties. It is therefore very important that it is not dropped on a hard surface. For the calibration purpose, there must be used another type of coupler than for pure tone. This device is called an artificial mastoid and it Calibration and Service of Audiometers - page 4 of 14 Rev /JF

5 Artificial mastoid, correction values Measuring amplifier Bone conductor calibration has a surface with mechanical properties, comparable to the human mastoid bone. IEC 373 describes such a mechanical coupler and it has a built-in accelerometer which delivers a calibrated signal to be used for the calibration of the bone conductors. As for the microphone, the artificial mastoid is supplied with a frequency chart which should be used for the determination of the correction factors. 3 different charts will normally be delivered, but use the chart for force sensitivity for this purpose. The chart also has a field with information about the force sensitivity at 1000Hz. This value is expressed in mv/n, but has to be converted to db re. V/N before further calculations are made. Having done that, it is possible to calculate the output of the artificial mastoid at the desired frequencies, simply by adding the ISO threshold values, the 1000Hz force level, the correction factors and the HL setting for each frequency - all expressed in db. If desired, the calculated db - values can easily be converted to µv, as the values are expressed in db re. 1µV. The correct reading on the measuring amplifier for each frequency will then be: db HL + ISO Force Sensitivity + Δmast. The artificial mastoid uses no preamplifier and can therefore be connected directly to the voltage input of the measuring amplifier. Due to the high impedance of the artificial mastoid, it is important that it is connected to a measuring amplifier with a high impedance input, typical above 1 MΩ, otherwise there will be errors in the measured values. During the measurements, it is important that the bone conductor under test is correctly placed on the middle of the surface of the artificial mastoid, and that the static pressure is correctly adjusted. Due to temperature sensitivity of the artificial mastoid, it is important that the ambient temperature is remained constant. The most exact calibration values are obtained at an ambient temperature of 23 C. Service on Audiometers and impedance instruments General Although the instruments from Interacoustics are made to be very reliable devices, it will be necessary to carry out periodic controls by a specialist in order to ensure that the performances are kept. Calibration and Service of Audiometers - page 5 of 14 Rev /JF

6 Checkpoints Cleaning Periodic replacements This should be done at least once a year by a skilled person with the necessary equipment. The equipment needed is shown in the enclosed Set-up for Calibration of Audiometers and Impedance Instruments. By the control it is checked that the transducers are delivering the correct signals, levels and frequencies, as well as the different function modes are tested. Special attention should be made to proper functioning of parts which is exposed to wear due to normal handling. This concerns the following: control devices, e.g. knobs and push buttons, indicating devices, e.g. lamps and displays, cords, plugs, headbands, earphone cushions, hoses and tubes for impedance probes. The rubber of the earphone cushion is often very contaminated and should therefore be cleaned. If it is inflexible or damaged, it will be necessary to replace it. Probes for impedance instruments will very often be contaminated with earwax, both in the tip of the probe and in the tiny tubes. These has to be cleaned in water and by means of a cleaning wire. Note that only the detachable mechanical parts of the probe are allowed to be exposed to water. On impedance instruments, it is recommended, by each periodic control, to replace the short hoses, connecting the tubes to the transducers in the probe, as they very easily can be damaged and cause leaks. Use the same type of hose, cut in the same length as the original ones. Additional Information Calibration chart Related standards IEC IEC It is advisable that the technician makes a chart showing all audiometer frequencies together with the calculated calibration values which corresponds to the set of equipment he uses. Enclosed you will find 2 examples of records, showing how calibration charts can be designed. One is intended for the acoustical coupler, while the other is for the artificial mastoid. The records are included in the Interacoustics Quality Manual: Test Procedures. Audiometers part 1: Pure tone audiometers Audiometers part 2: Equipment for speech audiometry Calibration and Service of Audiometers - page 6 of 14 Rev /JF

7 IEC IEC IEC ISO ISO ISO ISO ISO ISO ISO ISO 6189 ISO ISO ISO Audiometers part 3: Auditory test signals of short duration for audiometric and neuro-otological purposes Audiometers part 4: Equipment for extended high-frequency audiometry Instruments for the measurement of aural acoustic impedance/admittance. (Former ISO 389) Acoustics - Reference zero for the calibration of audiometric equipment Part 1: Reference equivalent threshold sound pressure levels for pure tones and supra-aural earphones. Acoustics - Reference zero for the calibration of audiometric equipment Part 2: Reference equivalent threshold sound pressure levels for pure tones and insert earphones. (Former ISO 7566) Acoustics - Reference zero for the calibration of audiometric equipment Part 3: Reference equivalent threshold force levels for pure tones and bone vibrators. (Former ISO 8798) Acoustics - Reference zero for the calibration of audiometric equipment Part 4: Reference levels for narrow - band masking noise. Acoustics - Reference zero for the calibration of audiometric equipment Part 5: Reference equivalent threshold sound pressure levels for pure tones in the frequency range 8 khz to 16 khz. (Not yet issued) Acoustics - Reference zero for the calibration of audiometric equipment Part 6: Reference equivalent threshold sound pressure levels for acoustic test signals of short duration. Acoustics - Reference zero for the calibration of audiometric equipment Part 7: Reference threshold of hearing under free-field and diffuse field listening conditions. Acoustics Pure tone air conduction threshold audiometry for hearing conservation purposes. Acoustics Audiometric test methods Part 1: Basic pure tone air conduction threshold audiometry. Acoustics Audiometric test methods Part 2: Sound field audiometry with pure tone and narrow-band test signals. Acoustics Audiometric test methods Part 3: Speech audiometry. Enclosed Pure Tone Air conduction Calibration Instruction chart. Bone Conductor Calibration Instruction chart Set-up for Calibration of Audiometers and Impedance instruments. Survey of reference values for audiometer calibration at standard frequencies. Survey of speech reference values. Calibration and Service of Audiometers - page 7 of 14 Rev /JF

8 Calibration and Service of Audiometers - page 8 of 14 Rev /JF

9 Pure Tone Air conduction Calibration Instruction Calibration Stand no.: Sign.: Measuring Amplifier type: sn.: Artificial Ear type: sn.: Microphone type: sn.: Cathode Follower type: sn.: Audiometer settings ISO for TDH 39 Mic. correction values Calibration values Max. allowed deviation [Hz] [db HTL] [db SPL] [db] [db SPL] [± db] The microphone correction values are found by reading the B&K calibration chart for the respective microphone. Read the output level of the microphone on the Y-axis of the calibration chart, at the frequency of the calibrator.(1000 Hz). Read the deviation from this level at each audiometer frequency and write it in the column for "mic. correction values". When the calibration curve is above this level, the correction value is positive, while negative when below. The values in the column for "calibration value" are found as an addition of attenuator, threshold and microphone correction levels. Calibration and Service of Audiometers - page 9 of 14 Rev /JF

10 Bone Conductor Calibration Instruction The thresholds for bone conductor are given in force values. These values has to be converted into voltage output for a given artificial mastoid. Output of artificial mastoid = T F + S FDV + C F + Att. setting T F S FDV C F : Threshold force (BC standard) [db re. 1µN] : Force sensitivity of the artificial mastoid at 1000 Hz. This value is given in mv/n on the calibration chart and has to be converted to db re. 1V/N. (= 1µV/µN) = 20 log (Force Sensitivity [V]) ex.: 20 log 0,16 V = -15,9 db : Correction factor for each frequency, read on the "constant dynamic force" curve of the artificial mastoid, giving the deviations from the 0 db line. Artificial mastoid no. Force sensitivity : mv/n S FDV = 20 log V = db Audiometer settings Art. mast. values ISO Freq. Att. S FDV C F T F Req. reading [Hz] [db] [db] [db] [db] [dbv] [mv] ,0 58,0 48, ,5 36,5 31,0 30,0 35,5 40,0 40,0 Calibration and Service of Audiometers - page 10 of 14 Rev /JF

11 Measuring Equipment Recommended set-up for Calibration of Audiometers and Impedance Instruments Oscilloscope Oscillator Multimeter Measuring amplifier Frequency counter Artificial mastoid 1 inch mic. with pre-amp Art. Ear 4152 as IEC 303 coupler (6cc) Art. Ear 4152 as IEC 126 coupler (2cc) Manometer MHC1 Bone conductor Loudspeaker Air conduction headset Insert Earphone Contra Earphone Ipsi Pressure Audiometer Impedance Audiometer The drawing shows the different types of equipment needed to make a complete calibration of Clinical Audiometers and Impedance Instruments. The measuring amplifier and Microphone, Artificial Ear and Artificial mastoid, are normally supplied from Brüel & Kjär although there will be other trade marks with similar equipment. The Brüel & Kjär equipment can be purchased through Interacoustics. A portable sound level meter can be used instead of the stationary measuring amplifier, but will have some disadvantages as it normally lacks the monitor output for oscilloscope and frequency counter as well as direct signal voltage measurement facilities. Calibration and Service of Audiometers - page 11 of 14 Rev /JF

12 The oscilloscope, frequency counter, tone generator and multimeter are common equipment which can be bought in a wide range of quality and prices. Calibration and Service of Audiometers - page 12 of 14 Rev /JF

13 Survey of reference values for audiometer calibration at standard frequencies Freq. ISO ANSI S3.6 (TDH39) ISO 389-4/ ANSI S3.6 (NB) ISO ANSI S3.6 (BC) ISO 389-2/ ANSI S3.6 (Insert) ISO Tone (FF) ISO Diffuse field (FF) (TDH39) (BC) [Hz] [db re. 20 µpa] [db re. 20 µpa] [db re. 20 µpa] [db re. 1µN] [db re. 1µN] [db re. 20 µpa] [db re. 20 µpa] [db re. 20 µpa] , ,0 22, , ,0 18, , ,5 14, ,5 25, ,0 11, ,5 8, ,0 5, ,5 11, ,5 4,0 3, ,5 5 52,5 52,5 4 2,5 1, , ,5 48,5 2 2,0 1, ,5 2,0 1, ,5 42,5 0 2,0 0, , ,5 0, ,5 6,5 6 36,5 36,5 2 0,5-1, ,5 35,5 2 0,0-1, ,5-1, ,5 6 29,5 29,5 5-4,0-3, ,5-6,0-4, ,5-4, ,5 9,5 5 35,5 35,5 5,5-6,5-5, ,0-3, ,5 15, ,5-0, ,0 0, ,5 5,5 TDH39 is calibrated using a 6cc acoustic coupler made in accordance to IEC 303 BC is calibrated using a mechanical coupler made in accordance to IEC 373 (1990-1) Insert phones are calibrated using a 2cc acoustic coupler made in accordance to IEC 126 Reference values for calibration of high frequency Freq. Interacoustics Standard (Koss/pro) ISO/TR HDA200 [khz] [db re. 20 µpa] [db re. 20 µpa] , , Koss/pro is calibrated using an acoustic coupler made in accordance to IEC 318 Calibration and Service of Audiometers - page 13 of 14 Rev /JF

14 Survey of speech reference values IEC 645-2, type A or B IEC 645-2, type A-E or B-E ANSI S3.6 type A, B or C ANSI S3.6 type A-E, B-E or C-E [db] [db] [db] [db] Earphones Calibration signal 20 0 (3,8)* 19,5 19,5 Speech weighted masking 20 0 (3,8)* 19,5 19,5 Bone conductor Calibration signal 55 NS(55) 55 NS(55) Speech weighted masking 55 NS(55) 55 NS(55) Loudspeaker Calibration signal ,5 14,5 Speech weighted masking ,5 14,5 Insert phones Calibration signal 20 NS(20) 12,5 NS(12,5) Speech weighted masking 20 NS(20) 12,5 NS(12,5) Insert masking phones Speech weighted masking 20 NS(20) 12,5 NS(12,5) NS = Not Standardised yet. Values in brackets are used by factory calibration. * Values in brackets are valid for Germany Calibration and Service of Audiometers - page 14 of 14 Rev /JF