For a low-frequency noise control in ducts, active sound absorbers represent a possible alternative to porous sound absorbers that work particularly efficiently at medium and high frequencies. Industrial applications in the field of heating and air conditioning technology are considered to be especially promising. However, the potential of this technology is limited by unfavorable ambient conditions. High temperatures, extreme sound levels, static and dynamic pressure loads and humid and aggressive media, for example, can impede the use of active components. In order to ensure the efficiency of active sound absorbers also in these cases, either resistant yet expensive components have to be used, or the sensitive parts of the active sound absorber have to be protected adequately against unfavourable ambient conditions. One possible solution is to spatially separate loudspeaker, microphone and electronics from the sound channel in order to avoid direct contact with the medium. The practical solution is based on a λ/4 resonator with known transmission properties, which is mounted at the side of the sound channel and connected to it by an opening. The insertion loss of such a branch with acoustically rigid termination can be changed and increased considerably by influencing the sound field in the resonator chamber with an active sound absorber cassette (ASDK) installed at its end.
Construction of the Active Exhaust Silencer (AES)
The active silencer, shown in figure 1, forms itself around the circular main duct and consists of two parts. At the bottom a porous absorber layer surrounds the main duct half. At the top side of the main duct there is a tube attached via a small opening. To protect the side branch mechanically and to avoid an exchange of gas from the main duct with that of the side branch the opening is covered by perforated sheet metal and fibre fabric. Additionally, the opening is sealed hermetically by heat-resistant foil.
In the side, there is attached over the whole length a porous absorber layer which serves at one hand as heat insulation and at the other hand as an acoustic absorber for the interior of the resonator. The end of the side branch is terminated by a small box containing a standard loudspeaker, an electret microphone and an analogue controller which forms an active silencer cassette. With these active components an electro-acoustic feedback loop is established. The feedback gain can be manually or automatically adjusted at the controller and determines within the stability limits the acoustic wall impedance of the ASC.
Figure 2 shows the results of the insertion loss measurements of the silencer in figure 1. The black curve depicts the case where a plate of sheet metal rigidly terminates the end of the silencer branch as in conventional passive side resonators. This configuration forms a combination of Helmholtz- and Quarter-Wavelength-Resonator with a first IL-maximum of 17 dB at about 160 Hz. Additionally, the IL increases with rising frequency due to the porous layer at the bottom half-pipe.
The measured IL changes little if an active cassette without feedback is assembled at the end of the side branch instead of the rigid termination (red curve). However, if the feedback loop is closed with the highest possible feedback gain the IL-maximum becomes broader and is shifted to about 50 Hz (green curve). With a feedback gain between the maximum and zero the low-frequency-maximum in the IL will be located somewhere between 160 Hz and 50 Hz. Thus, it is possible to tune the IL-maximum from about 160 Hz to about 50 Hz simply by adjusting the feedback gain. This opens up the opportunity for a manual or an automatic control of the IL depending on SPL-related operational states of a noise emitting device as e.g. the rotational speed of an internal combustion engine or the temperature of a burners exhaust gas.