1 Introduction In people's daily activities, the reliability of products is often involved. However, the birth of the reliability discipline is not long. China has established a reliability test base since the mid-1910s, which has promoted the reliability process of domestic components and laid a good foundation for the widespread entry of domestic components into the world market. .
The country has become a big speaker manufacturing country, but has not yet become a speaker manufacturing power. With the rapid increase in production, quality and reliability still need to be improved. In fact, the reliability of the speaker is an important feature of the high-end products. For this, some speaker manufacturers are not enough to understand, and the concept is also blurred. Therefore, we must reverse the emphasis on speaker performance index design, ignore the tendency of reliability design, and fully understand the connotation and significance of reliability.
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2 speaker reliability
2.1 Definition and scope of reliability The definition of speaker reliability is: “The ability of the speaker product to perform the specified function under specified conditions and within the specified time.†It is the time quality indicator of the speaker product after leaving the factory. To describe whether the speaker is vulnerable and reliable during use. As user requirements increase, speaker structures become more complex (such as car speakers), output power is getting larger (such as PA speakers), and the environment is getting more and more harsh (such as outdoor speakers), which will lead to reliable speaker products. The level of sex has declined. At the same time, if new materials, new processes or new technologies are used, the factors that make the speakers unreliable increase.
The reliability of the loudspeaker can also be defined as: "The number of faults allowed for the loudspeaker product under specified conditions and within the specified time." The mathematical expression is the mean time between failures (MTBF). Random faults can be considered inevitable and acceptable, resulting in failures due to design reasons or manufacturing processes, as long as they are within the allowable number, no further traceability is often made. To this end, as early as 1995, the international concept of reliability and the old concept that random failure cannot be avoided were questioned. At the same time, the failure physical method was introduced in reliability engineering. In Europe, the original MTBF was replaced by the maintenance-free use period (MFOP), and the distribution law of the failure rate bathtub curve was broken [21]. Therefore, combining the failure physics method and the failure analysis method _3_, designing a speaker product without random failure may not be a delusion. A considerable number of foreign companies have already carried out productive work in this regard.
The “prescribed conditions†in the definition of reliability determine the wide range of reliability, and the reliability of the product has a great relationship with the working state of the product, the conditions of use, and the environmental conditions for storage and transportation. Conditions can be divided into two categories of conditions of use and environmental conditions. The conditions of use refer to the stress conditions that work inside the product, including various electrical stresses, chemical stresses, and physical stresses. Ambient environmental conditions include various environmental stress conditions such as temperature, humidity, air pressure, harmful gases, mold, salt spray, impact, vibration and radiation. In this sense, environmental testing also falls under the scope of reliability testing. These stress conditions can be applied separately or in combination, and the effect of the integrated application on the reliability of the speaker product will be more significant.
2.2 Reliability Technology
2.2.1 Derating Design The purpose of the derating design is to reduce the basic failure rate of the key components that have a high impact on the reliability of the loudspeaker when the loudspeaker is working. In the design of speaker systems, derating design is more common. In the design of the speaker unit, the idea of ​​derating design is reflected by the use of a larger area of ​​the positioning piece, a larger diameter voice coil, the lead and the shaping design of the lead.
2.2.2 Redundant Design The idea of ​​redundant design is not adequately reflected in the design of the speaker or speaker system due to cost. The application of multi-strand braided wire or the use of dual positioning segments may reflect some of the ideas of redundant design.
2.2.3 Thermal design The failure rate of the speaker will increase as the operating temperature increases. To reduce the failure rate, the operating temperature must be lowered. Henricksen C. A. Theoretically, the heat transfer mechanism of the speaker is discussed [41. The main reason for the fever of the speaker is the heat of the sound, so lowering the operating temperature of the speaker can start by reducing the heat of the voice coil and improving the heat dissipation of the voice coil and the magnetic circuit. The heat meter of the speaker is: (1) The heat dissipation channel is formed by designing the overnight L on the pole core, the positioning piece, the basin frame, the root of the paper cone, and the voice coil skeleton. At the same time, care must be taken to prevent airflow noise caused by poor design. (2) Use magnetic fluid to improve the heat dissipation of the voice coil. This method sometimes cannot be used in combination with method (1). (3) Use black devices to improve the heat radiation and heat transfer capability of the device. (4) Increase the heat sink for heat dissipation if cost permits. (5) For extremely high power and continuous use speakers, oil cooling or water cooling technology may be required to improve reliability. (6) Improve the heat resistance of materials and adhesives. Many speaker designers use this method instinctively when they encounter problems, but simply improving the heat resistance of materials and adhesives is not a good reliability solution. Because the stability of adhesives and materials may become an uncertain factor in the reliability of the speaker. Of course, when using rubber or plastic parts, special attention needs to be paid to the resistance of these parts to temperature or temperature shocks.
2.2.4 Reliability Estimation Reliability prediction has been controversial. It has been suggested that the term reliability estimation should be used because it is found in practice that the reliability prediction has little correlation with the actual reliability of the product, lack of experience or improper application. Can produce poor design. In the GJB/Z299B-98 "Electronic Equipment Reliability Prediction Manual" circle, the mathematical model of the speaker failure rate is not given like other components, but the operating failure rate of the speaker is directly given p=O. 13xl0~/h.
3 Loudspeaker reliability test Because the speaker user range is very wide, the test method of speaker reliability is similar, but there are some differences in test stress and failure criterion. Therefore, the author mainly makes the test purpose, test stress and failure criterion. discuss.
3.1 Reliability test classification Reliability test is usually divided into environmental test, life test, screening test, field test and identification test. Loudspeaker reliability tests are often exposed to environmental tests, life tests, and field tests. The maximum noise power test of the speaker can reflect the state of the speaker reliability to some extent, but it is not a reliability test in the strict sense. However, because of its test methods, test stresses, and failure criteria, it is highly correlated with other reliability test items. Some of the controversies also stem from the understanding of the test and some discussion is needed. The program source filters of this test project are commonly used in GB, IEC, DIN, JIS, EIA and IHF. The first four items are equivalent, and some users may not understand it well and need to explain it. A considerable number of foreign users, especially large users, are required to test in the range of 20 Hz to 20 kHz, which is different from the GB standard, and the test results show an increase in failure rate. The GB factory r9397-1996 "General Specification for Direct Radiant Electric Speakers" describes the failure criterion as "no thermal damage and mechanical damage after the test, and meets the requirements of 4.2". The 4.2 requirement in the general specification is the determination of the utterance anomaly. The utterance anomaly refers to silence, ringing, obvious garbage, severe abnormal sound and mechanical sound. Therefore, the test failure criteria should be divided into two categories, the first is thermal damage and mechanical damage, and the second is abnormal sound. The actual situation is that when the second type of failure (anechoic abnormality) occurs, the first type of failure (thermal damage and mechanical damage) must occur. The first type of failure (thermal damage and mechanical damage) does not necessarily make a clear sense of the second type of failure (anechoic abnormality). For example, the multi-strand braided wire of the speaker has broken several strands, and sparking has occurred. It is dangerous to use in the environment, but the pure tone can still meet the requirements of 4.2 in the general specification. For example, the use of plastic parts of the speaker, plastic knee parts deformation or hot melt phenomenon, pure tone detection in laboratory conditions can also meet the requirements of 4.2 of the general specification, but due to actual use conditions or installation conditions, it may This type of deformation or hot melt of the plastic parts of the speaker tends to be severe and causes failure. Therefore, it is one-sided to use the second type of failure as the failure criterion for the test, which is the main reason for the controversy of the test results. Some users will require changes in the amplitude of the speaker indicators before and after the test, such as the resonant frequency, impedance and sensitivity. The magnitude of the change is small, and it is quite difficult for the middle and low-end speakers. After the test, the pure tone detection power, a considerable number of users require listening at rated power.
3.2 Life test The life test of the speaker is an important part of the reliability test of the loudspeaker. Through this test, the reliability characteristics such as the failure law, failure rate and average life of the product can be known. In order to resolve the contradiction between the number of test samples and the test period, an accelerated life test can be used. The accelerated life test is divided into a constant stress accelerated life test, a step stress accelerated life test, and a sequential stress accelerated life test. SJ/T10601-94t71 ((Home Speaker Reliability Requirements and Test Methods) and EIA-426B-1998f8 (Loudspeakers, Optimum Amplifier Power) provide life test and constant stress accelerated life test methods for loudspeakers. AES2-1984 (r2003) T91 (AES Recommended Practice Specification. of Loudspeaker Components Used in Professional Audio and Sound Reinforcement) can be used as a reference for the stepping stress accelerated life test of the speaker. The step stress accelerated life test can be used to determine the limit level of the speaker to withstand safe electrical stress. When comparing different batches or different manufacturers' loudspeakers, it is easy to ascertain the influence of process changes or design changes on product performance. It should be noted that the speaker step stress accelerated life test should only be used as the speaker constant stress accelerated life test. Supplementary test for preliminary test or constant stress accelerated life test of loudspeaker. When performing the life test of the loudspeaker, the monitoring of the test sample ensures the accuracy of the test result. Due to the particularity of the loudspeaker, especially the high-powered loudspeaker, the monitoring is usually difficult.
3.3 Mechanical test
GB/T9397—1996 “General Specification for Direct Radiant Electric Speakers†specifies the basic mechanical test contents of the loudspeakers, which can be referred to for implementation. But some users, especially large car manufacturers, have higher requirements than this standard. For example, the drop test has increased the unpackaged drop project. In the case of packaging, the direction of the drop is one corner, three sides and six sides; the sliding impact test, the angle A is from 6O. Increase to 75. The linear distance of the sliding increases from 600 mm to 1000 mm; the collision test increases the door impact test (field test), the waveform is approximately half sine wave, the acceleration is 300 m/s, the duration is 6 ms, and the number of impacts is 100 000 times. The speakers are mounted vertically.
3.4 Climate Environment Test The climate environment test mainly refers to the temperature and humidity test. The test temperature range for automotive or outdoor speakers may be extended by 31J-4o-85. C.
3.5 Temperature Cycling Impact Test The temperature cycling impact test of the loudspeaker is one of the ultimate stress tests of the loudspeaker. The purpose of the test was to evaluate the ability of the loudspeaker to withstand extreme high temperatures and extreme low temperatures, as well as the effects of alternating extreme and high temperatures on the loudspeaker. Due to speaker design or process reasons, many speakers will fail on this project. For example, the folding ring may be deformed or the like. The stress and stress application cycles for this test will vary for different customers. Different stress and stress application cycles yield different results, and mutual questions are not comparable.
3.6 Salt spray test and alternating salt spray test The salt spray test of the loudspeaker (4) has a misunderstanding of the purpose of the test. Contrary to the widely held view, the test is not to simulate the influence of the ocean atmosphere, but mainly to evaluate the protection. The quality and uniformity of the coating. The test to simulate the effects of oceanic atmosphere is the alternating salt spray test (11). The severity of the alternating salt spray test is divided into 6 levels. Grades 3-6 apply to products that are frequently used interchangeably in salty and dry atmospheres, such as automotive and automotive parts. At present, there is no unified understanding of the failure criteria for such tests in loudspeakers, which is quite troublesome.
3.7 Solar Radiation Test The solar radiation test port of the loudspeaker is to evaluate the thermal or photochemical effects of solar radiation on outdoor or car speakers. The test temperature is low. C, high temperature 40. c or 55. C. The main failures are appearance and mechanical failure, especially the plastic parts of the speakers and the coated plastic parts. The premise of this test is that the speaker product is exposed to solar radiation during use. So there are not many customers who require loudspeakers to do solar radiation testing.
3.8 Rain test Mainly used to observe the water permeability and sealing of cars or outdoor speakers. Usually, the speaker factory does not have the main test equipment raining room for the rain test, so the test is generally completed by the OEM. According to the test results, the OEM divides the loudspeaker into several grades that cannot be exposed to water, water permeability, water permeability and water resistance, and then selects different grades of speakers according to the different parts of the speaker used in the car.
3.9 Other test air pressure, noxious gases, mold and dust tests were not substantially implemented on the loudspeakers. Due to space limitations, we will not repeat them.
4 Loudspeaker reliability test design In order to evaluate the reliability of the product, a reasonable test plan must be adopted. Different test plans will produce different results, and mutual questioning is not comparable. Sometimes in order to find out the problem as soon as possible, it is necessary to carry out accelerated test or ultimate stress test. When doing these tests, it is necessary to pay attention to the increase of stress and not to destroy the original failure distribution law, otherwise the result of accelerated test or ultimate stress test is suspicious. The reliability test design shall include the provisions of the test sample, the determination of the test conditions, the determination of the test cut-off time, the determination of the measurement cycle and the determination of the failure criteria.
5 Conclusion Reliability discipline and reliability physics are mature subjects, and there is still a lot of research space in terms of reliability. Loudspeaker reliability technology should be used throughout the life of the loudspeaker, and reliability work during the design and manufacturing phases of the product is important to improve the inherent reliability of the product. Before conducting reliability testing, the speaker manufacturer may need to communicate fully with the user to facilitate the smooth implementation of reliability work.
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