Simulation Analysis - Screws in Waveguide

[René - Acculution.com]

- 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

 

[MarkS]

Very interesting, thanks!

If a tiny little screw can cause a 0.5dB change, what does a grill do???

(My pet peeve is grill design as an afterthought for way too many speakers.)

 

[René - Acculution.com]

Very interesting, thanks!

If a tiny little screw can cause a 0.5dB change, what does a grill do???

(My pet peeve is grill design as an afterthought for way too many speakers.)

Well, I was surprised too but everything seems to check out simulation wise, albeit this was done fairly quickly. Regarding speaker grills I could simulate that too. The easiest case would be perforated metal and if someone has a particular geometry or loudspeaker model to be inspired by, I might find time to do that too. I sometimes hear manufacturers say that the grill has been designed so that it should be kept on for the best sound but I am not sure if that is actually the case or not.

 

[MarkS]

I am thinking of cloth grills stretched over a frame that is a few mm thick that is attached to the speaker front (usually with magnets these days). Revel, for example, designs these fancy tweeter waveguides, then slaps on a frame grill.

 

[René - Acculution.com]

The cloth is probably/hopefully very transparent but the frame could definitely be an issue. With simulations this can be quickly revealed, as can many other aspects of loudspeakers that are otherwise up for discussion for years among the engineers. I plan to make some more posts to address some of these, and my own blog also describes some.

 

[NTK]

Sometimes seemingly minor geometric mismatches can have quite significant effects. The alignment of the shadow flare around the KEF coax drivers is one example. See Erin's very well done detailed investigation and analysis.
https://www.audiosciencereview.com/...helf-speaker-review-erins-audio-corner.21349/

I think the presence/absence of a grill can have more significant effects.

 

[René - Acculution.com]

Sometimes seemingly minor geometric mismatches can have quite significant effects. The alignment of the shadow flare around the KEF coax drivers is one example. See Erin's very well done detailed investigation and analysis.
https://www.audiosciencereview.com/...helf-speaker-review-erins-audio-corner.21349/

I think the presence/absence of a grill can have more significant effects.

Thanks, that is a very good example. At least KEF had the best intentions, and there is a fix, but I am not sure other companies necessarily are as diligent when it comes to grills and other obstructions. Some don’t do measurements at all and fewer probably do simulation work.

 

[MarkS]

Revel does both, and still produces crappy sound-altering grills.

It's very frustrating. In my household, the visual esthetics of covered drivers are strongly preferred, and so I want speakers designed to be listened to with grills on. The number of these is very small.

 

[René - Acculution.com]

It is tricky to design it for both situations and some prefer the look of the drivers. With shape and or topology optimization there might be a way though.

 

[amirm]

Excellent work! Promoted to home page.

 

[amirm]

If a tiny little screw can cause a 0.5dB change, what does a grill do???

I did a quick series of test of Revel Grill versus a couple of projection screen material that were supposed to be "transparent." Here are the in-room measurements:

Revel grill in black did the best.

 

[René - Acculution.com]

Excellent work! Promoted to home page.

Thanks Amir. I just started watching your YouTube videos, I enjoy your approach and look forward to more. If anyone has things they would like to see simulated, let me know.

 

[MarkS]

It's not tricky at all, it just adds to the cost. For example, here is the old Thiel CS 2.3:

 

[amirm]

Thanks Amir. I just started watching your YouTube videos, I enjoy your approach and look forward to more. If anyone has things they would like to see simulated, let me know.

You are very kind. Love to see more simulations. One I like to see are diffractions around the edges of a rectangular speaker.

 

[MarkS]

I did a quick series of test of Revel Grill versus a couple of projection screen material that were supposed to be "transparent."

I'd like to see full spinoramas of a Revel with grill on and grill off.

 

[MZKM]

I am thinking of cloth grills stretched over a frame that is a few mm thick that is attached to the speaker front (usually with magnets these days). Revel, for example, designs these fancy tweeter waveguides, then slaps on a frame grill.

Audioholics has measurements of different grilles:
https://www.audioholics.com/loudspeaker-design/grilles

Some are definitely worse than others. I bought the Infinity R-series towers and center and while initially thought the grilles were very cheap (plastic, not wood) I came to think they did so to minimize interference (well, and shipping cost).

 

[MZKM]

I'd like to see full spinoramas of a Revel with grill on and grill off.

I recall one speaker Amir measured with & without the tweeter grille, think it was a Wharfedale.

 

[voodooless]

Very nicely done! Not really a surprising result. Tom Danley makes holes in his waveguides all the time for his synergy horns, and they perform very well regardless

BTW, if you want to make some decently performing horns check out Ath4.

 

[René - Acculution.com]

You are very kind. Love to see more simulations. One I like to see are diffractions around the edges of a rectangular speaker.

Ah, that is actually on my list and I have made the simulation setup but need some time to run it. This is a topic where people have some intuition that might not be quite right. Another one is the influence of cabinet vibrations on the total sound.

 

[René - Acculution.com]

Very nicely done! Not really a surprising result. Tom Danley makes holes in his waveguides all the time for his synergy horns, and they perform very well regardless

BTW, if you want to make some decently performing horns check out Ath4.

Thanks. I think Geddes mentioned this thread in an interview I saw recently but not sure.