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11.12.01
08:50
Seite 2
259
Sound measuring engineering Calibration
Sound µ Pa 200,000,000
Ideally the instrument should be calibrated before and after measurement. If measuring frequently, calibration should be carried out at least once a week.
dB 140 Pain threshold 130
20,000,000
120 Jets starting
Pneumatic hammer
110
2,000,000
100 90
200,000
80
Measuring tips Truck
70 Telephone 20,000
60 50
2,000
40 30 Library
200
20 10
Forest 20
0 Limit of audibility
Sound waves are fluctuations in the air pressure. If they are audible to the human ear we talk about audible sound. The fluctuations in
pressure occurring with audible sound are extremely low. At a normal pressure of 1013 mbar even changes in the µPa range can
The microphone in the 815 sound level meter is detachable
Handling
Unfortunately sound is reflected off walls, ceilings and off the user´s This has the advantage that the user body. Measurement errors can can move out of the sound field by occur if incorrectly handled. In order to avoid measurement errors holding the microphone with his arm streched out in the direction of when using sound level meters the sound source (See Figure). This watch out for the following: According to DIN/IEC guidelines a is the best way to avoid reflection sound level meter must be posierrors. tioned at a 0° angle to the noise source (See Fig.). This means that the microphone points directly in the direction of the noise source. The instrument housing and the min. 30 cm presence of the user may not only hinder the sound coming from a certain direction but they
stimulate the human ear. A suitable pressure sensor with the appropriate sensitivity is the microphone.
Measuring conditions are ideal if there are absolutely no interfering objects in the sound field. This would be the case, for example, on top of a mountain. Because there are no walls or ceilings on which sound can be reflected, free dispersion is guaranteed. (Free field) In a closed room there is usually a wall opposite the noise source. This can cause reflections which distort the measured result. (Reverberant field)
Example: - Office with carpet, curtains and partitions = Free field ! - Cellar with concrete walls, without furniture, highly reverberant = Reverberant field!
may even produce reflections causing many measurement errors. We recommend that you hold the instrument at least 30 cm – even better 50 cm – from your body.
Sound level meters have been designed for measurements in the free field. There is also a free field if the level falls by 6 dB per duplication of the distance from the source. This is the case in most rooms.
Send for the Sound Technical Manual.