Active Noise and Vibration Control

Thomas Lagö started working with ANVC already in 1980. He was also involved in starting the active noise and vibration control research group at the Blekinge University, and also founded the company Active Control AB. In this work, that was initiated by the participation the European Community research project ASANCA (Advanced Studies on Active Noise Control in Aircraft). Thomas Lagö developed a research group that expanded greatly during his period, at the Blekinge University. Some of the projects were (Lagö was the manager for all of them):

ASANCA-II, a large European Community research project (Advanced Studies on Active Noise Control in Aircraft).

Pipe noise attenuation using stacked piezo-actuators. This project was performed in collaboration with the shipyard in Karlskrona (Karlskronavarvet)

Hearing conversation program utilizing active damping in ear muffs, speech enhancement using spectral subtraction like algorithms and virtual sound to decrease hearing loss for Super Puma pilots at the air force. The work was performed with several industrial partners.

AVIIS, Active Vibration Isolation In Ships. A collaborative project with Volvo Penta AB and Trelleborg AB aiming at decreasing the low frequency noise in high-performance boats. Classical passive methods have difficulties damping the noise due to weight restrictions on boats. Combined passive and active methods are then feasible.

CG13, active acoustic suppression in ships using loudspeakers. The shipyard, Karlskronavarvet, had an experimental boat, CG13, that was a very fast but noisy boat. A project was formed to investigate the possibility of quieting sections in the boat using active acoustic control. Per Sjösten was mainly in charge of this project.

Active Vibration Isolation in High-Speed Trains. The weight in X2000 had to be decrease by about 40%, resulting in a possible sound increase due to a sleeper frequency excitation of the car via the bogie. A consortium was initiated with partners from FFA (The Aeronautical Institute), ABB, KTH and others. The first aimed at investigating the bogie excitation and possible passive and/or active feasibility. High speed data was acquired and an inertial mass concept was proposed by Lagö. Full scale tests and implementations with control systems, handling both structural data as well as an Active Structural Acoustic Control (ASAC) control approach was implemented. The work has led to numerous papers at international conferences and Lagö was awarded a Best Poster Paper Award at the 16th IAVSD Symposium on Dynamics of Vehicles on Roads and Tracks, International Association for Vehicle System Dynamics, affiliated with the International Union for Mathematical and Applied Mechanics (IUTAM).

Active Vibration Suppression in Cutting Applications Using Embedded Actuators and Sensors. This project began as an extension of a project at Lund University, handling vibration suppression using magneto-strictive actuators. The principle was demonstrated, but Lagö proposed a completely different approach to the actuator design, using an embedded sensor and actuator approach, instead of the large and bulky sensors and actuators used. The latter, large sensors and actuators could never become a commercial product since the integration in a machine would have been impossible. The new slim design using an embedded concept makes a huge difference. This project has progressed and a complete system has been developed making it possible to demonstrate the accomplishment of the large vibration suppression. Several industry partners are involved, including Staffansboda Compagnie and Active Control AB. Sandvik Coromant, SECO Tools, NASA, Rolls Royce and other companies are very interested in this new technology.

Granö — a water poweer station with vibration challenges. In Sweden, it’s common with power plants along water courses. Today, Sweden has more than 1,800 hydroelectric power stations in various sizes throughout the country. Together, they stand for approximately half of the Swedish electrical power capacity. Many of them are old and are being renovated and upgraded as time goes by, partly to prevent failure but foremost to increase efficiency and power output. However, when optimizing a water turbine application for high efficiency, it is not uncommon that disturbing vibrations can occur. This may happen even if maximum power is not withdrawn from the generator. Previous, such a case was under study, a hydroelectric power plant was renovated and for some operating conditions like “high power output,” unacceptable vibrations were experienced. Multiple expert teams on turbine vibrations had been to the power plant but could not find the root cause of the problem. Hence, a strategy in finding the root cause for the high vibration levels as such, was created and successful. After the identification of the problem multiple solutions were proposed for mitigation. Structural modification was too unprofitable or basically impractical and was thus not considered. Active control was implemented and did successfully attenuate the harmful vibrations down to acceptable levels.

Gamla Ullevi: Below, some text in regards to this project can be found.

The Gamla Ullevi stadium is a new stadium, despite its name, replacing the old Ullevi stadium, hence the name. In the mid 80’s, vibration problems at Ullevi was a challenge due to the clay soil that is common in Gothenburg. The first time it really became a challenge was when Bruce Springsteen made the audience jump and cheer together with the music. Excitation frequencies of 1.8-2.3 Hz created severe vibrations and the building started to behave like during an earth quake. Even outside the stadium, people reported that the roads had earth quake like vibrations.

“Gothenburg is where in 1985 Bruce Springsteen nearly brought the house down – literally – after 120,000 fans crammed into the Ullevi stadium for his concert. The vibrations from all those dancing Swedes nearly caused the roof to collapse.” The Daily Mail reports in an article, [1].

In the Abstract of Erlingsson’s thesis, he states: The paper describes a case history where during two rock music concerts the audience at the large outdoor stadium Nya Ullevi, Gothenburg, Sweden, started to jump in time to the music. Violent vibration levels occurred in the ground and a part of the audience observed considerable motion in the structure. Based on finite element analysis, it is concluded that the underlying soft clay of the structure was excited with the same frequency as the beat of the music. The whole clay deposit under the stadium started, therefore, to vibrate with great variations in amplitudes over the field due to the complex geometry. Through interaction with the basement of the structure as well as its deep foundation the waves were transmitted to the superstructure. Parts of the structure framework have natural frequencies close to those of the beat. Consequently they started to vibrate violently. [2]

Today’s vibration challenges are different than the ones from mid 80’s. However, the clay was a major factor then and is a major factor now. This paper will discuss these challenges and the proposed system approach for effective mitigation of the vibrations.

Forced excitation via Jumping Soccer Supporters

The new stadium Gamla Ullevi (next to the Nya Ullevi stadium) exhibits vibration challenges and when the soccer supporters are jumping, two housing areas are excited beyond the comfort level. This prestigious project, that teams from all over the world are currently watching, is handled by an expert team in Sweden. Media is showing great attention to the project and the exposure has increased over the next last few months. Today, more than 600 pages (Google “hit” pages) can be found on Internet in regards to this project. Not all of them are relevant, but it shows the vast international interest. In the reference list, a series of web pages has been added for information purposes.

As depicted in Figure 2.1, the supporters are jumping on the upper part of the gallery. If they are in sync with each other, forced vibrations are travelling down the pillars, and further into the ground. Since the ground pillars are not going all the way into the rock, the stadium is in principle “floating” on the clay bed and the pillars in the ground are holding the stadium in place. However, from a vibration point of view, the complete structure can vibrate. The pillars are being pushed down into the ground about 60 meters. The depth depends on the location.

The pillars in the ground are being excited by the forced vibrations from the soccer supporters’ jumping. These vibrations are then transferred via the structure further down into the clay soil and then to the nearby housing areas.