Testing Overview: Absorption Muffler Systems

Planned Testing: 5 geometry configurations with 3 materials at 3 densities

·  1st iteration – Use standard muffler to determine ideal densities for each material

·  2nd iteration – Optimal material and density to compare the geometries and determine optimal

Recorded Data:

·  Sound Map

·  Waveform file

·  EGT (Exhaust Gas Temperature)

·  Ambient Temperature

·  Muffler configuration (Material, Density, Configuration)

·  Back pressure at exhaust port

Theory:

An absorption muffler works on the principle of energy in a pressure wave dissipating via a transfer to mechanical energy and dampening through absorption medium. As the pressure wave passes through the absorption medium, it acts on the medium and causes minute deflection and vibrations within. The damping effects of the absorption material transfer some of this mechanical energy into heat, removing it from the acoustic system. To maximize this effect it is ideal to keep the pressure wave within the absorption material for as long as possible, and to have as much of the wave pass through the absorption material as possible. The different geometric configurations of the absorption mufflers tested have been designed with this in mind.

Expected Results:

It is expected that for a normalized amount of absorption material per cross section, mufflers with longer path length will exhibit superior acoustical dampening.

It is likewise expected that the cavity configuration will, for a normalized cross sectional thickness and comparable length, outperform the standard muffler. This is based on the pressure wave remaining in the expansion chamber longer then it would in straight flow path.

Lastly, it is expected that the sine wave, or non-line of sight path, configuration to have superior noise dampening effects. This is based off of the entire pressure wave having to pass through the absorption material.

Back pressure effects are expected to be lowest for the straight flow path section, higher for the expansion tube section, and highest for the non-line of sight section. It is not yet clear if these differences in pressure will be significant.

Fidelity:

It is believe that high level of confidence in the test results can be achieved. All muffler systems will be tested on the same engine set up, under similar if not identical conditions. Major contributors of error such as flow temperature and ambient conditions will be recorded for corrections if necessary. The most serious risk to test fidelity is non-constant engine rpm, which should be manageable.

Testing Overview: Resonator Systems

Planned Testing: 2 different resonators (Concentric Tube and Helmholtz)

·  1st iteration – Large Concentric Tube Resonator using a pure tone source

o  Fixed Frequency & Fixed Length tests

·  2nd iteration – Small Concentric tube Resonator using a pure tone source

o  Fixed Frequency & Fixed Length tests

·  3rd iteration – Helmholtz resonator using a pure tone source

o  Fixed Length test

·  4th iteration – Large Concentric Tube & Helmholtz using waveform of SAE Engine

Recorded Data:

·  Ambient Noise Level

·  Location

·  Cavity Length

·  Frequency

·  Back Pressure at inlet

Theory:

Concentric Tube resonators work on the principle of self-cancelation of a pressure wave. In theory, if a wave can be reflected back upon itself at the proper point in the wave, then the crest and trough will overlap and the wave will be effectively canceled. This is done by optimizing the length of the expansion chamber in a Concentric Tube resonator to either ½ or ¼ the wavelength.

A Helmholtz resonator uses a perpendicular offshoot and expansion chamber, which simulates a mass-spring damper system, generating its own harmonic frequency. If the geometry is intentionally matched to produce resonance at the same frequency as the pressure wave through the perpendicular flow path, it should in theory produce a 180 degree out of phase waveform. This wave would then cancel a portion of the sound wave.

Expected Results:

It is believed that all three resonator configurations will have considerable noise reduction in pure tone at their targeted lengths. It is believed these resonators will have a significantly less effect on broadband noise from the non-idealized source, but may be useful in targeting the dominant harmonics so that a significant noise reduction can still be achieved.

Back pressure is believed to be highest with longer and higher diameter expansion chambers for Concentric Tube resonators. It is not believed that the expansion chamber size will be a significant contributor to back pressure for the Helmholtz resonator.

Fidelity:

It is believed that a high level of confidence can be achieved in the test results. These resonators will all be tested in a way that gives us a high level of control. All noise inputs will be coming from an electronic source, eliminating variation of the source noise as a contributor to error.

Testing Overview: ANC-Engine System

Planned Testing: ANC system from 11227 coupled to B&S engine from 11221

·  1st iteration – ANC system on recorded engine waveform

·  2nd iteration – ANC system on actual Briggs & Stratton lawnmower engine

Recorded Data:

·  Sound Map

·  Waveform

·  Ambient Temp

·  EGT (when applicable)

·  Back Pressure at exhaust port

Theory:

The ANC system will have a parallel pipe setup exiting to a dipole box with the exhaust stream. Readings taken here will be used by the Digital Signal Processor and 11227 algorithms to generate an inverted waveform that will combine with the true waveform at the dipole box, theoretically canceling. The system will need to be highly dynamic and responsive.

Expected Results:

Anticipated results are limited by knowledge of the 11227 setup. Noticeable noise cancelation is expected with the electronic generated noise source.

Fidelity:

A high level of fidelity is expected. The standard recording procedure will be used to take measurements for this system. It should be possible to see what the ANC system is doing by analyzing the waveform if unexpected results occur.