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COMPARATIVE ANALYSIS OF TECHNIQUES OF ATTENUATION THE ACOUSTIC REFLECTIONS EFFECT AT FREE-FIELD CALIBRATION OF AIRBORNE SOUND RECEIVERS
DOI 10.24411/2072-8735-2018-10122
Roman O. Lavrov,
Alexander Mozhaysky Military Space Academy, Saint Petersburg, Russia, 9432923@mail.ru
Yury A. Kuvykin,
Alexander Mozhaysky Military Space Academy, Saint Petersburg, Russia, original.rus@mail.ru
Keywords: frequency response, time-selective post-processing, calibration, airborne, free field, time delay spectrometry, moving complex weighted average.
In article the comparative analysis of significant attenuation techniques of the acoustic reflections effect at free-field measurement of sound receivers sensitivity (calibration) is carried out. The modern methods allowing to approach to the free field conditions due to the special signal shaping and processing techniques using such tools as time averaging, different types of broadband signal as a source, modeling and extrapolation of the reflection-free part of the signal, reflections elimination using mathematical signal processing have been considered. Time-selective post-processing currently used to attenuate the acoustic reflections effect at free-field calibration of airborne sound receivers by an absolute reciprocity technique has been considered. It is suggested to extend it for the calibration of airborne sound receivers by relative methods (comparison or collation). The borrowing of techniques used in hydroacoustics to attenuate the acoustic reflections effect at calibration of measuring transducers in water, namely, the time delay spectrometry and the moving complex weighted average of the initial frequency dependence, as well as their use as an alternative to the selective post-processing time is recommended. Significant limitations of each method are identified, and the main advantages of their use during attenuation of the airborne acoustic reflections effect are determined.
The comparative analysis results suggest that in the next phase of research the software and hardware implementation of the installation for the free-field calibration of airborne sound receivers should be done and the comparative practical field-testing of the methods considered in the work may be carried out.
Information about authors:
Roman O. Lavrov, Candidate of Engineering Sciences, Professor, Deputy Head of the Department of metrological support of armament, military and special equipment of the Alexander Mozhaysky Military Space Academy, Saint Petersburg, Russia
Yury A. Kuvykin, Junior Scientific Assistant of the Department of metrological support of armament, military and special equipment of the Alexander Mozhaysky Military Space Academy, Saint Petersburg, Russia
Для цитирования:
Лавров Р.О., Кувыкин Ю.А. Сравнительный анализ методов ослабления влияния отражений звуковой волны при градуировке приемников звука в воздушной среде по свободному полю // T-Comm: Телекоммуникации и транспорт. 2018. Том 12. №7. С. 59-63.
For citation:
Lavrov R.O., Kuvyki Yu.A. (2018). Comparative analysis of techniques of attenuation the acoustic reflections effect at free-field calibration of airborne sound receivers. T-Comm, vol. 12, no.7, pр. 59-63.
T-Comm Vol.12. #7-2018
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Introduction
Determination of the frequency response within the working frequency range of the airborne sound pressure measuring transducers, depending on the application, is carried out in the free-field (field), in the pressure field (pressure) or in the diffuse field. This operation is commonly referred to as the measurement microphone calibration [ 1 ].
An anechoic chamber (AC) supporting free-field conditions is used for the free-field calibration of airborne sound receivers (measurement microphones). It is believed that in such a chamber there is only a direct wave of the emitter. However, it is difficult to put the AC into practice - the efficiency of damping coatings used to attenuate reflections, in case of broadband measurements is not sufficient for all tasks. Calibration accuracy improvement in the anechoic chamber requires a more significant sound wave attenuation reflected by the chamber walls, the influence of which was previously neglected.
The free field conditions can be approached due to the special signal shaping and processing techniques using time averaging, different types of broadband signal as a source, modeling and extrapolation of the re flection-free part of the signal, reflections elimination using mathematical signal processing.
The application of special signal shaping and processing techniques allows to:
- extend a working frequency range down and reduce the calibration error in the existing ACs;
- not to build the large ACs;
- bring the results of measurements in empty chambers to the same results in the ACs.
The article briefly describes the principles and presents a preliminary analysis of modern methods of reducing the reflection effects on the results of measurement microphone calibration. Special signal shaping and processing techniques at calibration of sound pressure receivers are now widely used in water, as it is much easier to build the ACs for airborne measurements than to dampen the walls of a hydroacoustic tank.
Currently, the time-selective post-processing (VSP) technique is used for calibration of microphones by the field [2].
The time-selective post-processing technique is used for calibration of microphones by the absolute technique (the reciprocity technique according to the state standard GO ST R IEC 61094-2-2001. State system for ensuring the uniformity of measurements. Measurement microphones [3]. Primary method for pressure calibration of laboratory standard microphones by the reciprocity technique), which is used only for realization of the sound pressure unit by the state primary standard GET 19-2010.
The aim of the paper is to consider the following options:
1. The time-selective post-processing technique's implementation for calibration of microphones by the relative technique at lower stages of the state verification schedule (GOST P. 8.7652011. State system for ensuring the uniformity of measurements. State verification schedule for means of measuring the airborne sound pressure in frequency range from 2 Hz to 100 kHz).
2. Borrowing of techniques used in hydro acoustics for the calibration of primary measuring transducers in water and their application as an alternative to the time-selective post-processing technique.
Time-selective post-processing
The time-selective post-processing technique is based on the use of time windows when using a pulsed mode of radiation, as shown in figure [4].
The detailed complex frequency dependence measured in the presence of minor reflections is converted into a time dependence by the inverse Fourier transform. The singularity of the time dependence obtained in such conditions is that it is a free-field impulse response and the large-scale copies of this response, shown in figure I by "reflections", delayed at the time of arrival of reflections. If the reflection delay is large enough, the impulse response and its delayed copies are well separated in time. In this case, the reflections effect can be eliminated by applying to the time dependence the selective time window, conditionally shown in figure I. The final operation is the forward Fourier transform, which gives a frequency response free from the reflections effect.
Bper.iH te.r.ekthbhoe okho
OTpüKcHHfl 3B>'KOBOli BOWtl
Figure 1. Time-selective window
The use of time-selective post-processing is due to the fact that the physical emitter and receiver have frequency response that at low and high frequencies tend to zero. As a result, the continuity requirement will be ensured only if there is full frequency dependence. Actual measurements can be performed only in a limited frequency range.
One of the reasons for this is the final signal-to-noise ratio, which does not allow to measure the frequency dependence in the areas of its strong attenuation, which necessarily include very low and very high frequencies [5]. Therefore, in order to ensure the required accuracy of the transformations, there is a need to fill the missing parts of the frequency dependence, using a priori information [4].
Time-selective post-processing requires the frequency dependence "from zero to zero" In addition, the required impulse response and reflections will always be lengthy (different from the Delta function). The need to measure signals of small levels forces to use ACs.
However, the reflections effect is eliminated completely, since only those windows that zero the time dependence are used, starting from a certain point in time preceding or equal to the time of first early reflection arrival (figure 1).
If the impulse response length exceeds the early reflection arrival time, the time-selective window can cut off a significant part of the impulse response. In addition, the desired frequency response distortions by smoothing may be unacceptably large. This makes it very critical towards choosing the window [5].
Time delay spectrometry
A technique of time-delay spectrometry (TDS) is based on the frequency selection of the direct signal.
The technique is based on the emission of a linear frequency modulated signals (LFM) and the heterodyning of the response received by the receiver using as a reference LFM signal identical to the emitted one. The output signal has almost no noise components. Time response characteristics of the signal are displayed in the frequency domain, which makes it possible to use standard spectral processing procedures for their analysis and radically reduce the requirements lor the complexity, accuracy and speed of the equipment used [6J, [7], [81.
Frequency selection leads to the averaging (smoothing) by frequency dependence of the emitter-receiver system in the chamber with reflections. The frequency-averaging element in the TDS technique is a narrow-band moving filter that emits a direct LFM wave. If the filter bandwidth is set, the LFM signal sweep rate must be selected to achieve sufficient elimination of reflections. By contrast, in case of a given LFM signal sweep rate, it is necessary to select the filter bandwidth corresponding to the reflection elimination.
The TDS and LFM techniques have one common significant drawback: the working frequency range in which they can be implemented is limited by the geometrical dimensions of the room. In both cases (TDS and LFM), the recording of at least two periods of the sound wave is necessary for measurements. Conversely, the sound wavelength at a frequency of 20 Hz is about 17 meters. For free-field measurements at this frequency, the distance from the emitter to the receiver must be at least 34 meters. Thus, to determine the microphone frequency response in the low-frequency region, it is additionally necessary to apply an actuator or a small volume chamber [5].
Moving complex weighted average technique
The moving complex weighted average technique of the initial frequency dependence (MCWA) is developed at the All-Russian Scientific Research Institute of Physical-Technical and Radiotechnical Measurements [9]. The initial data for the MCWA technique are obtained by measuring the frequency dependence of the transient impedance of the "emitter - non-anechoic receiver-chamber" system and time delays of the first significant reflections. Using the MCWA technique, the frequency response of the free-field emitter-receiver system is restored by a moving weighted average of the complex frequency dependence of the emitter-receiver system measured in a chamber with reflections. The weighing function is determined by the time delay at the reception point of significant reflected signals related to the direct emitter signal.
To reduce the error caused by the unevenness of the desired frequency response, the editing - balancing of the desired frequency response is used. One of the methods of balancing is the step-by-step MCWA application. The essence is that the frequency dependence obtained by the first MCWA application is approximated by a complex function. The initial frequency de-
pendence is balanced by multiplying it by a function that inverses the approximating function. In this way, it is possible to significantly reduce the unevenness of the desired frequency response. In step 2, the same MCWA is applied to the balanced initial frequency dependence.
The dependence obtained by repeated application of MCWA is multiplied by the approximating function. The result is considered as an improved estimate of the desired frequency response of the free-field emitter-receiver system. The steps can be repeated, if necessary [11],$2].
If you look ai this operation differently, the desired impulse response and its copies (reflections) are compressed in time, approaching the form of the Delta function, the more evened out the desired frequency response. This ensures a good division of the impulse response and reflections, which facilitates the reflections elimination and, accordingly, the error reduction. This is shown in the diagrams in figure 2.
It must be stressed that the error reduction in this case is due to the involvement of additional a priori information on the form of the desired frequency response.
Частотная зависимость, измеренная в поле отражающей камеры
IFFT
Время-селективное окно
S Отражения
Частота
Время
Figure 2. The moving complex weighted average technique of the initial frequency dependence
The spectra of time-selective windows can have a large number of significant side lobes, so that the remote areas of the averaged frequency dependence will contribute to the result of the weighted average. The side lobes effect of the time-selective window spectrum will be evident on the final frequency response in the form of minor oscillations even if the desired frequency response is smooth. Similar residual oscillations will be observed on the final frequency response obtained by the MCWA technique. however, the nature of these oscillations will be different.
In contrast to time-selective post-processing, the MCWA technique does not completely eliminate reflections, but only turns them to zero at the points of intersection by a time window of the time axis coinciding with the reflections arrival time. At the same lime, the reflections around these points are strongly eliminated.
T-Comm Vol.12. #7-2018
Refere ii ees
The result of filling (balancing) the complex frequency dependence (time-selective post-processing) is an impulse response close to the real one. This seemingly obvious advantage becomes a disadvantage in the case when the impulse response length exceeds the arrival time of the first reflection.
The measurements accuracy can be improved by attracting a priori information. In that event, the unknown frequency response is not determined, but the frequency response model based on the available a priori information is specified [13], [14], [15].
Conclusion
The study concluded following:
1. The TDS and LFM techniques do not allow for taking measurements in the low-frequency region of the working frequencies range, which makes it necessary to apply an actuator or a small volume chamber to determine the microphone frequency response.
2. The MCWA technique appears to be the most promising for the calibration of primary measuring transducers of airborne sound pressure under free-field conditions, because it has the lowest average band and, accordingly, the lowest low-frequency cutoff, which eliminates the need for additional measuring equipment.
3. The inherent disadvantage of MCWA, which is the elimination of reflections finite number and the reduction of all other reflections when used airborne, can be easily overcome by facing the reflecting surfaces of the room w ith materials and structures, sound absorption of which does not allow them to be used for anechoic chambers due to insufficient elimination of the first reflection. but enough to attenuate repeated reflections, while it is possible to use modern tools of digital signal processing to determine the reflections delay.
The next phase of study is the software and hardware implementation of the installation for free-field calibration of measurement microphones and comparative practical testing of the techniques considered in the work.
1. Kuvykin Y.A., Doroshenko E.Y. (2015), Pressure calibration of measurement condenser microphones in the small volume chamber. Proceedings of the XXXX science and technology conference of young scientists - military metrologists, Mytischl, pp. 248-257.
2. Richard Barliam, Salvador Barrera-Figueroa and Janine E M Avison. (2014). Secondary pressure calibration of measurement microphones. Metrología, Vol. 51, No. 3, pp. 129-138.
3. L. Beranek. (1952). Acoustic measurements. Publishing house of foreign literature, Moscow.
4. Randall R.B. (1989). Frequency analysis. Denmark. Brtilil and Cier. 389 p.
5. Nikolaenko A.S., Kolesov S.Y. (2013). Comparative analysis of modem methods of microphone field calibration. Proceedings of scientific and practical conference of young scientists, postgraduates and specialists, Mendcleevo.
6. Naumov S.S., Knyazeya N.I., Burenkov S.V. (1995). Multifunctional acoustic measurements based oil modified time delay spectrometry technique. Acoustic measurements. Techniques and measures. IV Session pf the Russian Acoustical Society. pp. 9-14.
7. tleyser R. (1989). Acoustical measurements by time-delay spectrometry. J Audio Eng. Soc. No. 15, p, 370.
8. Greiner R. Waniá J. and Noedjovich G. (1989). A digital approach to time-delay spectrometry. J Audio Eng. Soc. No. 37, p. 593.
9. Isaev A.E. (2008), Accurate calibration of the sound pressure receivers in water under free-field conditions. Monograph, Mendeleevo, FSUE VNIIFTRl,
10. Isaev A.E., Matveev A.N. (2008). Two approaches to field calibrating hydrophones with continuous radiation in anechoic tank. Measurement technique. No. 12, pp. 47-51.
11. Isaev A.E., Nikolaenko, A.S., Chernikov V.I. (2015). Acoustic free-field measurements in reverberation sound field using transfer function of the tank. Proceedings of the 6th All-Russian science and technical conference "Technical Problems of tVorld Ocean Exploration", Marine Technology Institute of Far Eastern Branch of the Russian Academy of Sciences, Vladivostok.
12. ¡saev A.E., Matveev A.N. (2010). Application of the moving complex weighted average technique to restore the uneven frequency response of the receiver. Acoustic journal. Vol. 56, No. 5, pp. 651-654.
13. Isaev A,E„ Matveev A.N, (2010), increasing the frequency resolution in the processing of acoustic signals by moving complex weighted average technique. Acoustic journal. Vol. 56, No. 2, pp. 277-283.
14. Isaev A.E., Matveev A.N. (2012). Reduction of field calibration error of hydrophones in the reflecting tank. Measurement technique. Edition No. 12, pp. 61-63.
15. fsaev A.E., Matveev A.N. (2009). Field calibration of hydrophones at continuous radiation in the reverberation tank. Vol. 55, pp. 727-736.
СРАВНИТЕЛЬНЫЙ АНАЛИЗ МЕТОДОВ ОСЛАБЛЕНИЯ ВЛИЯНИЯ ОТРАЖЕНИЙ ЗВУКОВОЙ ВОЛНЫ ПРИ ГРАДУИРОВКЕ ПРИЕМНИКОВ ЗВУКА В ВОЗДУШНОЙ СРЕДЕ
ПО СВОБОДНОМУ ПОЛЮ
Лавров Роман Олегович, Военно-космическая академия имени А.Ф.Можайского, г. Санкт-Петербург, Россия,
9432923@mail.ru
Кувыкин Юрий Александрович, Военно-космическая академия имени А.Ф.Можайского, г. Санкт-Петербург, Россия,
original.rus@mail.ru
Аннотация
Проведён сравнительный анализ методов значительного ослабления влияния отражений звуковой волны при измерении чувствительности приемников звука (градуировке) по свободному полю. Рассмотрены современные методы, позволяющие приблизиться к условиям свободного поля за счёт применения специальных способов формирования и обработки сигналов, использующих такие инструменты как временное усреднение, различные типы широкополосного сигнала в качестве источника, моделирование и экстраполяцию свободной от отражений части сигнала, подавление отражений с помощью математической обработки сигнала. Рассмотрена время селективная постобработка, применяемая в настоящее время для ослабления влияния отражений звуковой волны при градуировке приемников звука в воздушной среде по свободному полю абсолютным методом взаимности. Предлагается ее распространить для градуировки приемников звука в воздушной среде относительными методами (сравнения или сличения). Предлагается заимствование используемых в гидроакустике методов ослабления влияния отражений звуковой волны при градуировке первичных измерительных преобразователей в водной среде, а именно спектрометрии временных задержек и скользящего комплексного взвешенного усреднения исходной частотной зависимости, а также их применение в качестве альтернативы время селективной постобработке. Выявлены существенные недостатки каждого метода, а также определены основные преимущества при использовании их при ослаблении влияния отражений звуковой волны в воздушной среде. Предлагается с учетом результатов проведенного сравнительного анализа в следующем этапе исследований осуществить программно-аппаратную реализацию установки для градуировки приемников звука в воздушной среде по полю и провести сравнительную практическую апробацию рассмотренных в работе методов.
Ключевые слова: амплитудно-частотная характеристика; время селективная постобработка; градуировка; воздушная среда; свободное поле; спектрометрия временных задержек; скользящее комплексное взвешенное усреднение.
Литература
1. Кувыкин Ю.А., Дорошенко Е.Ю. Градуировка по давлению микрофонов измерительных конденсаторных в камере малого объема. Материалы XXXX научно-технической конференции молодых ученых - военных метрологов, г. Мытищи, 2015. С. 248-257.
2. Richard Barham, Salvador Barrera-Figueroa and Janine E M Avison. Secondary pressure calibration of measurement microphones. Metrologia, Vol. 51, No. 3, 2014, рр. 129-138.
3. Л. Беранек. Акустические измерения. Перевод с английского под редакцией Н.Н. Андреева. Издательство иностранной литературы. Москва. 1952.
4. Рандалл Р.Б. Частотный анализ. Дания: Брюль и Къер. 1989. 389 с.
5. Николаенко АС., Колесов С.Ю. Сравнительный анализ современных методов калибровки микрофонов по полю. Доклады научно-практической конференции молодых учёных, аспирантов и специалистов, Менделеево, 2013.
6. Наумов С.С., Князева Н.И., Буренков С.В. Многофункциональные акустические измерения на основе модифицированного метода спектрометрии временных задержек. Акустические измерения. Методы и средства. IV сессия Российского акустического общества. 1995. С. 9-14.
7. Heyser R. Acoustical measurements by time-delay spectrometry. J Audio Eng. Soc. 1989. No. 15. p. 370.
8. Greiner R. Wania J. and Noedjovich G. A digital approach to time-delay spectrometry. J Audio Eng. Soc. 1989. No. 37. p. 593.
9. Исаев А.Е. Точная градуировка приёмников звукового давления в водной среде в условиях свободного поля. Монография. Менделеево: ФГУП "ВНИИФТРИ", 2008.
10. Исаев А.Е., Матвеев А.Н. Два подхода к градуировке гидрофонов по полю при непрерывном излучении в заглушенном бассейне. Измерительная. техника, 2008. № 12. С. 47-51.
11. Исаев А.Е., Николаенко АС., Черников И.В. Акустические измерения по свободному полю в реверберационном звуковом поле с использованием передаточной функции бассейна / Труды 6-й Всероссийской научно-технической конференции "Технические проблемы освоения Мирового океана", ИПМТ ДВО РАН, г. Владивосток, 2015.
12. Исаев А.Е., Матвеев А.Н. Применение метода скользящего комплексного взвешенного усреднения для восстановления неравномерной частотной характеристики приемника. Акустический журнал. 2010 г. Том 56. № 5. С. 651-654.
13. Исаев А.Е., Матвеев А.Н. Повышение частотного разрешения при обработке акустических сигналов методом скользящего комплексного взвешенного усреднения. Акустический журнал. 2010 г. Том 56, № 2. С. 277-283.
14. Исаев А.Е., Матвеев А.Н. Уменьшение погрешности калибровки гидрофонов по полю в отражающем бассейне. Измерительная техника. 2012. Выпуск № 12. С. 61-63.
15. Исаев А.Е., Матвеев А.Н. Градуировка гидрофонов по полю при непрерывном излучении в реверберирующем бассейне. 2009. Том 55. С. 727-736.
Информация об авторах:
Лавров Роман Олегович, к.т.н., доцент, Заместитель начальника кафедры метрологического обеспечения вооружения, военной и специальной техники, Военно-космической академии имени А.Ф.Можайского, г. Санкт-Петербург, Россия
Кувыкин Юрий Александрович, Адъюнкт кафедры метрологического обеспечения вооружения, военной и специальной техники Военно-космической академии имени А.Ф.Можайского, г. Санкт-Петербург, Россия
