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Sound effects

Analyzing noise to provide figures relating to physics and people.

A product’s quality in terms of its sound emis­sion is usually deter­mined by its dB(A) value, which refers to the sound pres­sure level gener­ated by an acoustic source at a certain distance. Depending on the level of the alter­nating pres­sures from which the sound pres­sure level is deter­mined, expo­sure to noise can cause effects ranging from slight reduc­tions in mental perfor­mance to pain and even uncon­scious­ness.


Fig. 1: The curves show how much sound pres­sure is needed so that humans perceive sound uniformly.

The concept of dB(A) also includes a means of assessing acoustic measure­ments to help answer the ques­tion, “How does noise affect people?” This ques­tion has kept scien­tists busy for decades and is now of increasing interest to engi­neers. For example, the following find­ings have been compiled in psychoa­coustic studies: For phys­i­o­log­ical reasons, humans do not perceive every frequency equally. Figure 1 shows the results of hearing exper­i­ments with tones of different frequen­cies. The curves show, depending on frequency, what sound pres­sure level is needed in order to be perceived uniformly by humans. This is called loud­ness.


Fig. 2: Curves of equal loud­ness level — isophones (ISO 226:2003)

This diagram provides a number of insights. For one thing, it can be seen that the human ear is most sensi­tive in the range between two and four kilo­hertz. Much higher or lower frequen­cies are perceived as quieter, even at the same sound pres­sure level. It can also be seen that this char­ac­ter­istic of hearing is not only depen­dent on loud­ness. The isophone at 40 phon was used for the dB(A) weighting (Figure 2). In addi­tion to dB(A), there are other frequency weighting methods. For example, dB(C) has been proposed as a better alter­na­tive for high sound pres­sure levels. The dB(D) weighting has become estab­lished for aircraft noise.

The dB(A) value and its reduc­tion from one product gener­a­tion to the next are sales argu­ments for indus­trial prod­ucts. This is insuf­fi­cient when the effect of noise reduc­tion on people is deter­mined by a signif­i­cantly different percep­tion than that of pure loud­ness. For example, other research has shown that a noise reduc­tion of approx­i­mately ten dB(A) is perceived as a halving of the loud­ness.

For the continued improve­ment of our prod­ucts, these issues cannot be neglected. This was one of the reasons for building the “Kombikanal,” a test stand at our St. Georgen loca­tion for combining air perfor­mance and acoustic measure­ments for fans to enable acoustic and psychoa­coustic analyses of the prod­ucts.


Percep­tion of noise levels

The human ear is capable of perceiving acoustic waves of very different ampli­tudes. At a frequency of 1 kHz, the hearing threshold is about 20 µPa; the pain threshold is approx. 64 Pa.

The dB scale was designed as follows to allow visu­al­iza­tion of this very high dynamic range:

where p0 = 20 µPa and p2 repre­sent the energy of the acoustic signal.

Exam­ples of noises, in dB(A)

160 Rifle shot near muzzle
130 Jet fighter at distance of 7 m
120 Airliner at distance of 7 m
110 Private aircraft at distance of 7 m
100 Circular saw
90 Car at 100 km/h at distance of 1 m
80 Car at 50 km/h at distance of 1 m
70 Lawn mower
60 Normal conver­sa­tion, car at distance of 15 m
50 Quiet radio music
40 Humming refrig­er­ator
30 Whis­pering
20 Drip­ping water faucet
10 Rustling leaves in forest
0 Defined threshold of hearing

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