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- May 1, 2021
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- Introduction -
Erin (of Erin's Audio Corner) has expressed an interest in knowing the influence of screws in waveguides; if they protude from the waveguide, can the sound be affected? I have investigated this issue using a numerical simulation software. This is not an exhaustive analysis, but more of a fun weekend project.
- Simulation setup -
Software: COMSOL Multiphysics
Method: Finite Element
Geometry: Realistic 1" tweeter geometry, arbitrary waveguide (5 cm radius, 2 cm depth), 4 screws (0.67 cm diameter, protuding 1.2 mm), in baffle
Mesh: Quadratic Lagrangian elements (6 elements per wavelength @20 kHz)
DOFs: ≈ 262,000
Other: The exact same mesh was used for both cases; with and without screws. Calculation time on laptop around 1 hour per case. This is pure acoustics case, so there is no explicit solid mechanics; instead an acceleration is put on the tweeter surface. Exploited quarter symmetry.
- Observations -
The sound pressure level was evaluated with and without the screws, both in a 1 m distance and at 5 cm. The difference is neglible for both distances, and at the most relevant distance of the two (1 m) the difference is less than 0.5 dB. Even in the near field right around a screw the field is largely unaffected, but the overall influence of screws is larger than what I would have initially guessed...
SPL at 5 cm and at 1 m: (Yes, yes, this is not a good design, but it does not matter for the objective at hand, other than that relative difference typically show up more around dips and peaks as is also the case here; move a resonance slightly and you can get large differences.)
SPL difference, 1 m ('Screws' minus 'No screws'):
Sound pressure difference ('Screws' minus 'No screws') at 20 kHz visualized:
- Conclusions -
For this particular case the influence of the screws in fairly minimal, yet it is there. The wavelength even at 20 kHz is 1.7 cm, which is larger than the screws themselves, and so it is to be expected that the sound field will not be affected much. It should be noted that such an analysis should be carried out on a case by case basis, as waveguides (and screws) differ from product to product, and if we get near 1 dB difference it could be relevant. It is a computationally cheap single-physics analysis, and so companies with simulation capabilities should at least do this investigation internally, and provide results to those who request it.
/René
- About me -
BSEE, MSc (Physics), PhD (Microacoustics), FEM and BEM simulations specialist in/for loudspeaker, hearing aid, and consultancy companies. Own company Acculution, blog at acculution.com/blog
Erin (of Erin's Audio Corner) has expressed an interest in knowing the influence of screws in waveguides; if they protude from the waveguide, can the sound be affected? I have investigated this issue using a numerical simulation software. This is not an exhaustive analysis, but more of a fun weekend project.
- Simulation setup -
Software: COMSOL Multiphysics
Method: Finite Element
Geometry: Realistic 1" tweeter geometry, arbitrary waveguide (5 cm radius, 2 cm depth), 4 screws (0.67 cm diameter, protuding 1.2 mm), in baffle
Mesh: Quadratic Lagrangian elements (6 elements per wavelength @20 kHz)
DOFs: ≈ 262,000
Other: The exact same mesh was used for both cases; with and without screws. Calculation time on laptop around 1 hour per case. This is pure acoustics case, so there is no explicit solid mechanics; instead an acceleration is put on the tweeter surface. Exploited quarter symmetry.
- Observations -
The sound pressure level was evaluated with and without the screws, both in a 1 m distance and at 5 cm. The difference is neglible for both distances, and at the most relevant distance of the two (1 m) the difference is less than 0.5 dB. Even in the near field right around a screw the field is largely unaffected, but the overall influence of screws is larger than what I would have initially guessed...
SPL at 5 cm and at 1 m: (Yes, yes, this is not a good design, but it does not matter for the objective at hand, other than that relative difference typically show up more around dips and peaks as is also the case here; move a resonance slightly and you can get large differences.)
SPL difference, 1 m ('Screws' minus 'No screws'):
Sound pressure difference ('Screws' minus 'No screws') at 20 kHz visualized:
- Conclusions -
For this particular case the influence of the screws in fairly minimal, yet it is there. The wavelength even at 20 kHz is 1.7 cm, which is larger than the screws themselves, and so it is to be expected that the sound field will not be affected much. It should be noted that such an analysis should be carried out on a case by case basis, as waveguides (and screws) differ from product to product, and if we get near 1 dB difference it could be relevant. It is a computationally cheap single-physics analysis, and so companies with simulation capabilities should at least do this investigation internally, and provide results to those who request it.
/René
- About me -
BSEE, MSc (Physics), PhD (Microacoustics), FEM and BEM simulations specialist in/for loudspeaker, hearing aid, and consultancy companies. Own company Acculution, blog at acculution.com/blog