Umwelt- Messtechnik Bayreuth

Basics of noise and sound pressure level measurement Definitions of acoustics to a hundred-fold, and an increase of 30 dB of Noise is caused by vibrations of a sound source, a thousand-fold increase in the sound intensity. such as cords or the loudspeaker diaphragm, and An increase or decrease the sound level by 6 dB requires for its propagation, a medium, such as represents a doubling or halving of the sound in- air, water or solid bodies. The vibrations of the tensity. noise source are transmitted to the medium. The- se vibrations and pressure fluctuations in the air Evaluation of the sound level to A or C can perceive our ears as sound. The fullness of As mentioned above, the human ear is not what a person can hear is primarily determined equally sensitive to all pitches. The greatest hea- by the volume and pitch. The greater the volati- ring sensitivity is between 1,000 and 4,000 Hz, lity, the louder the sound is perceived. Compared that is, deep sounds below 1,000 Hz and above with atmospheric air pressure to the underlying 4,000 Hz high notes you take relatively quiet as sound pressure variations are extremely small: In true tones medium frequency. This frequency-de- normal speech, for example, they are less than a pendent sensitivity of the human ear is in sound millionth of the air pressure! measurements simulated with evaluation curves, which are identified by letters. Especially interna- Frequency in Hertz tionally, the curve „A“ is used. Level values, which The pitch of the sound depends on the frequency are valued according to the curve „A“, expressed of the pressure fluctuations. The number of os- in dB (A). cillations of a sound wave in the Very quiet noise from 0 to 20 second is called frequency and dB (A), you can practically hear specified in the unit „Hertz“ (Hz). only under laboratory conditions, You feel a tone is higher, the grea- since the usual ambient sounds ter the frequency of the sound is. are much louder already. Far from A young person can hear sounds cities and roads be the ambient in the frequency range of about 16 noise 20 to 30 dB (A) if there is no to 20 000 Hz, but the person loses strong wind and no natural sound with age, the ability to detect very sources, such as waterfalls are ne- high frequency sounds. In our en- arby. In normal conversation, the vironment, pure tones come very sound levels are approximately 55 rarely. Rather, it has mostly to do dB (A), with very strong city traf- with a mixture of soft and loud, fic at 80 dB (A); jackhammer and high and low frequencies. This discotheques, reach up to 100 mixture is called noise. The human dB (A), the engine of a jet plane ear has between hearing threshold in 100 meters up to 130 dB (A). and pain threshold over a surprisingly large range Low-frequency sound of perception of sound pressure. Is the highest sound intensity in the frequen- cy range below 90 Hz, it is called low-frequency Indication of the noise level in decibels sound. This is customarily the case, when the dif- To avoid dealing with such huge numbers, it was ference between LCeq and LAeq exceeds 20 Db. agreed in the acoustic decades ago out SPL stated Low frequency noise with prominent tonal compo- in decibels (dB). The decibel scale is logarithmic, nents are measured according to DIN 45680 Ad- thus it is possible to record the sound intensity on dendum 1:1997-3. the relative value of 1 (threshold) to the value 1 trillion (pain threshold) in values from 0 to 130 Infrasound dB. The decibel scale apply so well as special cal- This category noise is assigned below a frequen- culation rules: If you increase a sound level of 10 cy of 20 Hz, for the people are not normally heard. dB, so that corresponds to a tenfold increase in The sound levels are perceived mainly as pulsa- sound intensity. An increase of 20 dB corresponds tions and vibrations.

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