Samenvatting

Over the last four months I have been working at Mitsubishi Turbocharger and Engine Europe
B.V. (MTEE) in Almere, Netherlands. As part of my Bachelor’s thesis assignment, I was given the task to
investigate the working principle of Helmholtz resonators and evaluate the factors influencing the sound
attenuation. The task is related to the development of a new turbocharger for BMW B58 engines. The
manufacturer aims to reduce noise pollution from the engine bay area, requiring the MTEE turbo
engineering design team to implement a silencer connected to the compressor side of the turbocharger.
In order to understand how the Helmholtz resonator influences the noise level in a system, the
thesis is divided into a theoretical model and the execution of physical testing. The results are
comparison between the theoretical calculations and test outcomes. The main metric that is used for
the outcome is transmission loss of the system. It is the difference between the noise level going into
the system and the noise level going out of the system and it is expressed in [dB].
The main question to answer is
“How does the cavity volume and neck size of Helmholtz resonator affect the transmission
loss and attenuation bandwidth of sound propagating in a tube?”
Prior to designing the test sample, a theoretical approach is taken. The resonator is represented as
mechanical system, consisting of mass, spring and damper. This allows the calculation of natural
frequency for different silencer dimensions. Changing the volume of air in the cavity acts as a change in
the spring/damper which alters the resonant frequency. The neck dimensions, on the other hand, alter
the mass of air thus also changing the excitation range. By approximate calculations of the impedance of
the system, some design guidelines are obtained.
The test sample design is created with possible variability in the neck width and cavity width. The
cavity of the resonator is concentric around the waveguide (pipe), which is common for turbocharger
applications. The designed parts are built at the machine shop of the company. Four pressure sensors
are used to measure noise fluctuations, two before the resonator and two after the resonator. In total
35 tests are executed, with varying resonator’s dimensions. The pressure data is processed with Python
scripts to derive the transmission loss for each test.
In conclusion, increasing the cavity volume in Helmholtz resonators generally decreases the peak
transmission loss and broadens the attenuation bandwidth, except for a neck width of 2mm where no
specific pattern is observed. When the neck width is varied from 2mm to 5mm while keeping the cavity
width constant, the peak transmission loss increases, but this does not align with theoretical predictions.
Additionally, the attenuation bandwidth decreases as the neck width increases, with this effect being
less pronounced for smaller cavity widths. Specific observations show that a neck width of 1mm does
not produce clear transmission loss peaks, while most measurements indicate a 50% peak bandwidth in
the 200-300Hz range and a peak transmission loss between 10-20dB. Although theoretical models
provide a basic framework, the experimental results reveal complexities that can inform further
research and the design of Helmholtz resonators for turbocharger applications.