3.5.2. Preliminary Research

AE sensors GT200 was used with a working frequency range of 130–200 kHz. The signal from the sensor was fed to the preamplifier with a gain of 26 dB and a frequency filter of 30–500 kHz. As a data collection system, a 4-channel external USB I/O module "E20-10," manufactured by "L-Card" company (Moscow, Russia), was used. The output data of this module was long time waveforms of the AE signal with a sampling frequency of 2.5 MHz.

#### 3.5.3. Noise Investigation

In the case of roller bearings of a rotary kiln in the operating mode, the noise level in the audible range is quite high. However, in the range above 30 kHz, due to the point dry contact between the shroud of the kiln and the roller, the acoustic noise, as a rule, was no more than 32 dBAE at the most unfavorable point of the structure: at the feed point of the batch. The only significant external source of acoustic noise is scoring and roughness on the surface of the roller or kiln bandage. When it touches the place of scuffing, an AE signal with amplitude of up to 100 dBAE arises. However, such noise signals can easily be excluded from further analysis, since their registration occurs with a certain period equal to the period of rotation of the roller support, or the kiln (Figure 17).

**Figure 17.** AE signals with periods (**a**) 12.5 s and (**b**) 50 s associated with scoring and roughness on the roller surface or a kiln bandage.
