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
