Michael Möser

Active control of double-glazed windows. Part II: Feedback control

Abstract This paper describes an experimental investigation of an actively controlled double-glazed window. It is the second of two companion papers of which the first treated results obtained employing adaptive feedforward control. Herein, the outcome using adaptive feedback control is presented. This adaptive feedback controller has been tested in different configurations, i.e. fully and partially connected controllers. The differences between fully connected controllers with few filter coefficients and partially connected controllers with many filter coefficients are discussed. Additionally, tests with different traffic noise examples have been performed showing the ability of the actively controlled window to enhance protection against traffic noise.

Active control of double-glazed windowsPart I: Feedforward control

Abstract This paper describes an experimental investigation of an actively controlled double-glazed window. It is the first part of two companion papers, and it shows the results with a feedforward controller. In the second part about results obtained with adaptive feedback control will be reported. A laboratory version of the window was equipped with loudspeakers and microphones inside the cavity. Various positions of loudspeakers and microphones were tested with band-limited white noise excitation. Different combinations of distributed loudspeakers were realized driving them in parallel by single channels of the controller. Similarly the signals of microphones were summed to realize simple modal filters. Plane mode control as well as control of higher cavity modes were performed with a feedforward controller and the reference signal taken directly from the primary signal. Additionally tests with different traffic noise examples were performed showing the ability of the actively controlled window to enhance protection against traffic noise.

Improvement of sound barriers using headpieces with finite acoustic impedance

The paper deals with the reduction of the sound energy in the shadow region behind barriers by means of an attached body at the edge of the screen. The reflecting attributes of the barrier’s headpiece are described by a locally reacting impedance. Diffraction at ideal soft and hard bodies demonstrates the basic principle of tangential power transport parallel to their surface: the impedance must be chosen so that the tangential intensity near the edge is lowered, turning the incoming power in harmless directions. The differences due to finite impedances are then discussed. The physical principles are demonstrated in frozen pictures of the sound field for the different cases. Theoretical computations show considerably improved levels in the shadow zone for larger angles of diffraction. These are compared with empirical results, and practical applications are discussed.

Perception of sound

The perception of sound incidents requires the presence of some simple physical effects. A sound source oscillates and puts the surrounding air into motion. These oscillations are transmitted to the ear of the listener due to the compressibility and the mass of the air.

Active control of the blade passage frequency noise level of an axial fan with aeroacoustic sound sources

This paper presents results of a research project on active control of the blade passage frequency tone of an axial fan. The secondary sound field is generated by aeroacoustic sources, which are produced by actively controlling the flow around the impeller blade tips. Both amplitude and phase can be controlled in such a way that a destructive superposition with the primary sound field is possible. The flow distortions can be achieved by using different actuators; results using steady and unsteady jets of compressed air and piezo-electric actuators are presented.

Active noise control in axial turbomachines by flow induced secondary sources.

In conventional active noise control experiments, loudspeakers are used to generate the secondary antiphase sound field to be superimposed with the sound waves radiated from the primary source. In the present study, aerodynamic sound sources are used instead for that purpose. This is achieved by disturbing the flow field around the blade tips in such a way that additional periodic forces are set up which in turn form the secondary sound sources. To disturb the flow, either air is blown into the blade tip region through the casing wall, or piezo-electric actuators are installed in the fan casing wall to influence the flow conditions near the blade tips. The resulting aerodynamic sound sources are adjustable in both amplitude and phase. In this way active flow control is used to reduce the tonal noise components of the axial fan. Experimental results are presented for steady and unsteady air jets injected into the main flow at various axial positions relative to the rotor blades. It is shown that the method is successful for plane wave as well as for higher-order mode sound fields. The sound pressure level at the blade passing frequency was reduced by up to 20.5 dB.

Active Blade Tone Control in Axial Turbomachines by Flow Induced Secondary Sources in the Blade Tip Regime

To reduce the tonal noise of axial fans loudspeakers are typically used to generate the required anti-phase sound field. The space requirement and the weight of the loudspeakers inhibit practical application of this method. In the present study the secondary sound field is generated by injecting high speed air jets into the rotor blade tip regime. The air jets set up additional periodic forces on the blade tips which in turn form the required secondary acoustic sources. The jets are driven by a compressed air supply through small nozzles mounted flush with the inner casing wall. It has been shown, that this approach is very effective in controlling higher order mode sound fields at blade passage frequency and its harmonics. The main goal of the present work is to introduce fast feedback control to this application.

Reduction of axial turbomachinery tonal noise by means of flow induced secondary sources using an extremum-seeking control technique

In conventional active noise control experiments, loudspeakers are used to generate the secondary anti-phase sound field to be superimposed destructively with the sound waves radiated from the primary source. In the present study, aerodynamic sound sources are used to actively control the tonal noise of an axial fan. This is achieved by disturbing the flow field around the blade tips in such a way that additional periodic forces are set up which in turn form the secondary sound sources. To disturb the flow, air is blown into the blade tip region through the casing wall. The resulting aerodynamic sound sources are adjustable in both amplitude and phase. Results for closed loop control of the blade passing frequency using an extremum-seeking control technique are presented.

Active flow control to reduce the tonal noise components of axial turbomachinery.

Conventional active noise control experiments use loudspeakers to generate the secondary anti-phase sound field to be superimposed with the primary sound waves. In this study, flow induced aerodynamic sound sources are used for that purpose. This is achieved by disturbing the flow field in the blade tip region using air jets blowing through the fans casing wall. Hereby additional periodic forces are set up which in turn form the flow induced aerodynamic secondary sound sources, which are adjustable in both amplitude and phase. In this way active flow control reduces the tonal noise components of the axial fan. Experimental results are presented for steady and unsteady jets injected into the main flow at various axial positions relative to the rotor blades. The method is successful for plane wave as well as for higher-order mode sound fields. The sound pressure level at the blade passing frequency was reduced by up to 20.5 dB. To obtain a better understanding of the physical mechanisms involved in the interaction of the air jets and the rotor blades, flow visualisation experiments were carried out with a stationary two-dimensional blade cascade. The investigation revealed that the air injection produces vortex generator jets causing additional unsteady longitudinal structures in the flow. For the ANC experiments this can be interpreted as the generating mechanism of the additional aeroacoustic sources with dipole characteristics.

Analytical Modelling of Special Acoustic Absorbers

Main objective of the present work is the development of a three-dimensional model of Special Acoustic Absorbers (SAAs) and its application to SAA acoustic performance prediction. These absorbers consist of a circular volume with an inserted horn of conical or hyperbolic geometry. The model is based on a decomposition of the absorber volume into separate domains in which the equations of motion are solved separately. The overall behaviour is obtained by coupling the separate solutions via boundary conditions. Various horn geometries are accounted for through an approximation of the horn and annular domains by series of cylinders resp. annular cylinders. Viscous losses are included through application of a variational method. Predicted impedance and absorption spectra are compared with experimental results and good agreement is observed.