Detail of GB/T 42553-2023

Introduction of GB/T 42553-2023

Electroacoustics - Methods to determine corrections to obtain the free-field response of a sound level meter 1 Scope This document provides information on the corrections required over a range of frequencies in order for a periodic test of a sound level meter to be performed according to IEC 61672-3. These corrections include: ——corrections for the typical effects of reflections from the case of the sound level meter and diffraction of sound around the microphone; ——corrections for the deviation of the typical microphone frequency response from a uniform frequency response, where the actual microphone response cannot be measured; ——corrections for the influence on the frequency response of a typical microphone of a specified windscreen and any other accessory that is part of the configuration for normal use of the particular sound level meter submitted for testing. This document includes discussion about uncertainties of measurement of the required corrections. In some instances a maximum permitted expanded uncertainty for the manufacturer or testing laboratory is given. This maximum permitted expanded uncertainty excludes any component due to the variability of different samples of artefact (for example, microphone or windscreen). It should be noted that if large uncertainties of measurement are quoted for each of the individual corrections, when they are combined to account for the configuration of sound level meter under test, the large individual uncertainties may result in a failure to conform to the maximum permitted expanded uncertainties of measurement given in Table A.1 of GB/T 3785.1-2023 and hence a failure of the sound level meter to conform to GB/T 3785.1. In addition, this document describes methods for determining these corrections, over the frequency range of interest, and explains the adjustment value at the calibration check frequency to be quoted by the manufacturer of the sound level meter (also required by IEC 61672-3). When the sound level meter manufacturer recommends the use of a sound calibrator, comparison coupler, or an electrostatic actuator for periodic testing of the acoustical response of a sound level meter at various frequencies, this document describes methods of measurement of the corrections required to adjust the indication on the sound level meter to an equivalent free-field level, over the frequency range of interest. These corrections relate to a specific model of sound calibrator, comparison coupler or electrostatic actuator, microphone and sound level meter (also required by IEC 61672-3). The aim of this document is to ensure that the adjustment value at the calibration check frequency and all corrections are determined using consistent and appropriate methods. It is intended that this document will be used by manufacturers to determine adjustment values and corrections, by laboratories performing pattern evaluation tests according to IEC 61672-2, and by laboratories performing periodic tests according to IEC 61672-3. Laboratories performing periodic tests according to IEC 61672-3 will also need to consult this document to ensure that the expanded uncertainties of measurement for the corrections quoted by the manufacturer do not exceed the maximum permitted values. The corrections obtained by use of the methods given in this document are the result of measurements made using samples of the devices. It is possible that these corrections may not be totally representative either for all batches produced or over time. Repeating the measurements at regular intervals is recommended to ensure that no changes are required to the corrections stated in the instruction manual. This document does not specifically cover the case where the sound level meter is fitted with a microphone intended for use in random-incidence sound fields, as information is given in IEC 61183. 2 Normative references The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. GB/T 3785.1-2023 Electroacoustics - Sound level meters - Part 1: Specifications ISO/IEC Guide 98-3 Uncertainty of measurement - Part 3: Guide to the expression of uncertainty in measurement (GUM:1995) ISO/IEC Guide 99 International vocabulary of metrology - Basic and general concepts and associated terms (VIM) IEC 60942 Electroacoustics - Sound calibrators IEC 61094-1 Measurement microphones - Part 1: Specifications for laboratory standard microphones IEC 61094-5 Measurement microphones - Part 5: Methods for pressure calibration of working standard microphones by comparison IEC 61094-6 Measurement microphones - Part 6: Electrostatic actuators for determination of frequency response IEC 61183 Electroacoustics - Random-incidence and diffuse-field calibration of sound level meters IEC 61672-2 Electroacoustics - Sound level meters - Part 2: Pattern evaluation tests IEC 61672-3 Electroacoustics - Sound level meters - Part 3: Periodic tests 3 Terms and definitions For the purposes of this document, the terms and definitions given in the ISO/IEC Guide 99, as well as the following apply. 3.1 adjustment value at the calibration check frequency value, at the calibration check frequency, for adjusting the sensitivity of the sound level meter, as specified by the manufacturer to be added to the displayed value of the sound level meter to obtain the stated sound pressure level from the calibrator while the sound level meter is exposed to the sound pressure in the calibrator, in order to optimise performance of the sound level meter over the complete frequency range Note: The adjustment value is expressed in decibels (dB). 3.2 microphone microphone cartridge electroacoustic transducer by which electrical signals are obtained from acoustic oscillations 3.3 microphone reference point point specified on, or close to, the microphone to describe the position of the microphone Note: The microphone reference point will usually be at the centre of the diaphragm of the microphone. 3.4 free-field correction for sound level meter value, at a particular frequency, that, during calibration or verification, is added to the level displayed by the sound level meter to obtain its response to a progressive sound wave in the free field of sound pressure level equal to that produced or simulated by the sound source, dependent on the specific models and configurations of sound level meter and sound source Note 1: The IEC 61672 series permits the use of a sound calibrator, a comparison coupler or an electrostatic actuator as the sound source. The values of the corrections for these sound sources are not necessarily equal. Note 2: The value of this correction depends on the direction of incidence of the progressive sound wave. Note 3: When the free-field correction is stated, the reference direction is assumed unless an alternative direction is given. Note 4: A measurement with an electrostatic actuator is not an absolute measurement, but is referenced to a sensitivity at a particular frequency, for example the calibration check frequency. Note 5: Corrections are expressed in decibels (dB). 4 Reference environmental conditions Reference environmental conditions for specifying the adjustment value and corrections are: ——temperature: 23 °C ——static air pressure: 101.325 kPa ——relative humidity: 50 %

Contents of GB/T 42553-2023