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Helmholtz Resonator - Noise Control - Lab Manual, Study notes of Noise Control

Some of topics included in this course are: Fundamentals of Acoustics, Levels and Decibels, Divergence and Directivity, Hearing, Human Response to Noise, Frequency Analysis, Sound Sources and Fields, Room Acoustics, Sound Power, Noise Barriers, Outdoor Sound Propagation, Helmholtz Resonator and Vibration Control. Key points of this lab manual are: Helmholtz Resonator, Resonant Frequency, Acoustic System, Automobile Mufflers, Rigid Cavity, Resonator Cavity, Theoretical Calculation, Frequency Rang

Typology: Study notes

2012/2013

Uploaded on 10/02/2013

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ME458 Lab 9 08/17/00 page 1
ME458 Noise Control
Laboratory #9
Helmholtz Resonator
Objectives:
1. Investigate the behavior of a lumped acoustic element - a Helmholtz
resonator.
2. Compare theory with measured results.
3. Determine the resonant frequency of an acoustic system using a swept sine
input.
Background: In this experiment, you will study the dynamics of a Helmholtz
resonator. This is an example of a lumped acoustic element (1-D), commonly
used in automobile mufflers. They are excellent band-stop acoustic filters. You
have probably played with this device many times. Whenever you blow across
the top of an open bottle, you are demonstrating a Helmholtz resonator. The
resonator consists of a rigid cavity of volume V, neck of area A and length L as
shown in the figure below.
The resonance frequency of a Helmholtz resonator (assuming that the physical
dimensions are smaller than a wavelength) can be calculated by:
fcS
L
V
n=2
π
' L’ = effective length of the neck
Procedure:
Use a loudspeaker driven by a sine wave generator and audio amplifier as the
signal source. The source is placed a measured distance from the resonator. A
microphone (or sound level meter) is placed in the resonator cavity to measure
pf2

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ME458 Lab 9 08/17/00 page 1

ME458 Noise Control

Laboratory

Helmholtz Resonator

Objectives:

  1. Investigate the behavior of a lumped acoustic element - a Helmholtz resonator.
  2. Compare theory with measured results.
  3. Determine the resonant frequency of an acoustic system using a swept sine input.

Background: In this experiment, you will study the dynamics of a Helmholtz resonator. This is an example of a lumped acoustic element (1-D), commonly used in automobile mufflers. They are excellent band-stop acoustic filters. You have probably played with this device many times. Whenever you blow across the top of an open bottle, you are demonstrating a Helmholtz resonator. The resonator consists of a rigid cavity of volume V , neck of area A and length L as shown in the figure below.

The resonance frequency of a Helmholtz resonator (assuming that the physical dimensions are smaller than a wavelength) can be calculated by:

f

c S

L V

n =^

L’ = effective length of the neck

Procedure: Use a loudspeaker driven by a sine wave generator and audio amplifier as the signal source. The source is placed a measured distance from the resonator. A microphone (or sound level meter) is placed in the resonator cavity to measure

ME458 Lab 9 08/17/00 page 2

the response. For four different resonator combinations of neck length and volume, do the following:

  1. Set the length of the resonator neck (by choosing a tube) and set the position of the movable plug (set the volume). Carefully measure the dimensions of the resonator.
  2. Find the resonant frequency of the resonator by adjusting the sine wave frequency until the maximum response amplitude is found.
  3. Measure the SPL as the frequency of the sine wave is slowly varied in steps around resonance. Record the frequency and SPL at several frequencies.
  4. Plot the SPL as a function of frequency. Compare to the theoretical graph.
  5. Compare the measured resonant frequencies to the theoretical calculation

Reporting Requirements: Thoroughly document your test procedure, instrumentation, calculations, results, observations and conclusions in a full- length, professional quality report.

In your report:

  • = Thoroughly document the test procedure, instrumentation, results and your observations.
  • = List all sources of error in your experiment.
  • = Comment on the validity of the theoretical model of the Helmholtz resonator. Did your results agree with theory? Why or why not?
  • = What limits the frequency range of application of the model?