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1. At first it was accepted that electricity was some kind of fluid, there being no experimental evidence to support the theory.

2. Some liquids proved to be good conductors of electricity.

3. Faraday is known to have measured the electric current for the first time in the world.

4. We know electrical charges to be positive and negative.

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Automobile Acoustic Engineering

By Snith

Sound reproduction inside the moving car. The quality of sounding inside moving car mainly depends on the level of noises. In the car of the middle class moving 90km/h the acoustic noise reaches the same level as the speech or music inside the car that doesn’t move. The noise level inside the car depends upon many reasons such as the quality of the road covering, automobile class and the way of inside car sound damper. It can reach 85db (when 85db is the level of a loud speech in premises). It’s known that noise influences greatly the low frequency part of sound signal spectrum. That’s why the noises inside the car impact the quality of reproduction of the low frequency at first. Spectrum distribution of the noise keeps permanent when the moving speed is changed, but its total level increases with the raise of the speed.

The influence of the inside acoustic on the quality of sound reproduction. It’s difficult to reach the high quality sounding for a small cabin and effect of sound image distortion on the stereophonic reproduction for non-symmetrical location of the hearers. The scientists have come to the conclusion: it’s unnecessary to long for the sharp reproduction of stereo-image, but to provide even saturation of the sound field by multiple reflections (diffusitivity). This can be reached by signal processing with the help of special processors that eliminate cophased signal elements of right and left channels in the field of middle and high frequency and add reverberation. Besides, sound processors can modulate acoustics of big halls, stadiums. The largest diffusitivity of the sound field can be reached by the placement of loudspeakers on the back shelf and in the front doors. Their location under the instrument board in the direction to the hearer is not desirable as the “direct” sound from the source of the signal prevails. The characteristic of the direction and acoustic power of automobile acoustic systems are also of great importance. It is known that more even characteristics of these parameters condition more even structure of reflections, i.e. much diffusive field. One shouldn’t forget about own resonances inside the car that are mostly expressed on the frequencies of 100-20db. As it’s difficult to damp these resonances by acoustic methods like inside car damping and loudspeakers location they use equalizers and correctors.

The ways of inside car sounding. The minimum set of automobile audio equipment consists of a tape recorder (combined with tuner and amplifier) and two acoustic systems. The simplest acoustic systems are produced on the base of wide strip head. Two- or three-stripped with SF/HF loudspeakers are placed on the diffuser-fixer of low frequency loudspeaker. Electrical power of such set, as a rule, is not high – 15-30W, that is defined by the maximum power of inside amplifiers. Maximum sound tension is not higher than 105 db. More perfect set consists of combined tape recorder/receiver, acoustic systems to which the signals are sent from the inside amplifier or from the separate power amplifier. Acoustic systems are placed under the instrument board or in front doors. The second pair of acoustic systems are placed as a rule on the back shelf. The advantage of the second set lies in the possibility to reach the most dynamic scope with the help of application of separate power amplifiers (40-100W per channel). The use of the second pair of AS makes deffusive field more even perfecting the conditions of listening for all passengers. The fullest set includes the third source – CD-player. Combined recorder/tuner has, as a rule, only preliminary amplifier, the signal of which is sent through equalizer to the set of active filters-crossovers. The filters give off low frequency and middle frequency signal elements and send them into separate amplifiers. LF loudspeakers are placed on the back shelf and use the inside volume of the boot. One of two pairs of multi-stripped AS (usually with passive separating filters) is placed in front doors, the second – in the back doors on the back shelf. The advantages of the last variant are the widening of the frequency scope to the low frequencies, increasing of dynamic range and reducing of distortions. The active filters-crossovers can be used for separation of signal in amplifiers. For low frequency loudspeaker besides crossover one can use active amplitude corrector that broaden frequency scope AS to the low frequencies. LF loudspeakers can be built in back shelf and have their own body and be placed over the back seat. Crossovers can have controlled frequencies of separation for optimum adjustment of system characteristics for automobile acoustics.

Automobile combined radio and tape receiver. There are many kinds of automobile radio and tape receivers. The most widespread ones are tape receivers because the tape is the most habitual and convenient audio medium. CD and MD changer can be connected to them that give the possibility for listening not only tapes but CD and MD. Tape receivers have mechanical control by rewind and tape ejection or full logic control (by pressing of the sensor keys). The main parameters that characterize the quality of the tape receiver reproduction are signal/noise relation, frequency reduction stripe and noise damping Dolby, B, C, S. Automobile CD receivers can have one or some discs. Their main parameter is qualitative disc reproduction when in pushing. To reach it many firms use high quality mechanism, others apply systems of countercoup memory. Automobile MD receivers are very convenient and practical in use for their ability to write blocks of songs on one’s own. But these models are less frequent for their high price. To make a good choice of automobile radio and tape receivers one should know the following:

Sound reproduction. Most models have two or four channels amplifiers. Their main feature is their power. The more power, the more powerful and qualitative sound is. To rich optimum sounding, it is necessary to connect auxiliary amplifiers by the linear exit. Approximately all receivers have balance and controls of high and low frequencies. Expensive models have graphic equalizers and digital processors of sound.

RDS. For reception of word information sent by radio stations RDS is used. The most popular functions of RDS are PS (Program Service) that allows to receive the name of radio station; TA (Traffic Announcement), that is used for switching from tape reproduction or disc to the reception of radio station, sending information for drivers at a given moment; AF (Alternating Frequencies)with the help of which receiver automatically switches to the most powerful signal of receiving radio station; PTY (Program Type) for detection of the type of the program; Clock Time for setting exact time and Radio Text for sending traveling line into the screen. (5794 p.s.)

Комментарий:

acoustic engineering – звукотехника; sound damper – звукозаглушение; db – децибел; premises – помещение; sound signal spectrum – спектр звуковых сигналов; to impact – влиять; distribution – распределение; permanent – постоянный; sounding – звучание; cabin – салон; sound image distortion – искажение звукового образа; location – размещение; hearer – слушатель; to long for – стремиться к; saturation – насыщение; diffusitivity – диффузность; to eliminate – устранять; cophased elements – синфазные составляющие; reverberation – реверберация; to modulate – моделировать; loudspeaker – громкоговоритель; back shelf – задняя полка; front doors - передние двери; instrument board – приборная доска; desirable – желательный; to prevail – преобладать; wide strip head – широкополосная головка; SF/HF – СЧ/ВЧ; set – комплекс; W – Вт; tension – давление; perfect – совершенный; scope – диапазон; AS – АС; filter-crossover – фильтр-кроссовер; to give off – выделять; boot – багажник; to broaden – расширение; widespread – распространённый; habitual – привычный; medium – носитель; Control – управление; rewind – перемотка; tape ejection – выброс кассеты; push – толчок; countercoup – противоударный; linear exit – линейный выход.