KIRCHNER ATB 805 Handbook
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Contents
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19.3 NEARFIELD MEASUREMENTHow to carry out acoustic measurements with loudspeakers and rooms are described in thefollowing chapters.By the near field measurement, the physical property of sound getting weaker over the distanceis made of use. A measurement microphone is placed 10cm away from the woofer. This way thedirect sound is many times greater than reflected sound, so that only the direct sound ismeasured. The measurement is only carried out up to 300Hz and extended by anothermeasurement for the frequency range above 300Hz. The second measurement is made at adistance of 1m to measure both mid and high tone speakers at the same time.Fig. 19.1 the menu Combine for the addition of low and high tone measurementsBy the near field measurement, all low frequency sources must be measured. These sourcesinclude low- middle tone speakers when the crossover frequency is lower than 300Hz. For themeasurement with ATB, the measurement constant measurement with averaging is chosen.During the measurement, the microphone is moved slowly from source to source. This way allsound sources are accounted. The path goes from the middle of the middle tone speaker to themiddle of the woofer. By bass reflex, the relationship of the woofer membrane radius to openingsize of the reflex tube is to be considered. According to the equation:Pressure ~ Power x AreaA small opening will cause more pressure than the larger surface area of the woofer membrane.That is why the opening is only measured at the rim. How far from the rim the microphone mustbe, but cannot be easily, calculated. Through the following description correct measurementresults can be achieved.In a small measurement room, a measurement is carried out with the boarder area microphone.The microphone is placed on the floor. As only the low frequency range is to be evaluated, thenormal positioning is chosen. To suppress the first floor reflexion the boarder area measurementis used. This avoids the disturbing depletion above 100Hz. The first reflexion can also not be55 |
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