Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Room Acoustics & Mods A place to discuss measuring and modifying room Acoustics for better sound

Helmholtz for a dummy
Helmholtz for a dummy
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 1st December 2018, 09:06 PM   #1
Bicicletta is offline Bicicletta
diyAudio Member
 
Join Date: Nov 2018
Location: Genova (Italy)
Default Helmholtz for a dummy

I would like to use the elements of a skyline diffuser i'm building as Helmholtz's absorbers to kick off the modal resonances of a little listening room.

Both have a narrow bandwidth and it is not possible to couple them by models (IMMO).

So I have to misure.

How?

Hitting the cavity by tapping? ... blowing in the hole ? ... noise?

Click the image to open in full size.

For the larger element, the original formula predicts the resonance at 140 Hz, and I see (empty cavity, mic in the cavity),

...
  Reply With Quote
Old 1st December 2018, 10:34 PM   #2
Bicicletta is offline Bicicletta
diyAudio Member
 
Join Date: Nov 2018
Location: Genova (Italy)
... my first mistake: for the larger element, the original formula predicts the resonance at 80 Hz.
I see, tapping on the element:

Click the image to open in full size.




then, blowing in the hole:


Click the image to open in full size.


but I have not enough power here to use noise.


...
  Reply With Quote
Old 1st December 2018, 11:18 PM   #3
Bicicletta is offline Bicicletta
diyAudio Member
 
Join Date: Nov 2018
Location: Genova (Italy)
For the smaller resonator the formula predicts, 205 Hz and I see, tapping, blowing and using pink noise...
But now I think that the predictions of the formula are more accurate than my measurement and more precise than the holes I have drilled.
Attached Images
File Type: jpg taplat7.jpg (50.9 KB, 242 views)
File Type: jpg blow7.jpg (51.0 KB, 241 views)
File Type: jpg 7pink.jpg (32.1 KB, 17 views)
  Reply With Quote
Old 2nd December 2018, 09:06 PM   #4
Bicicletta is offline Bicicletta
diyAudio Member
 
Join Date: Nov 2018
Location: Genova (Italy)
Default Damping

The prediction says 132 Hz for the element 15cm long.

My measurements say 147 Hz by tapping, blowing, or noising the empty resonator.


Maybe not rigid walls? Not sealed cavity? Dimensional error of the hole?

With the cavity losely filled with acrilic adsorber, I see 136Hz, and a larger chart.


They say that an Helmholtz resonator draws energy from the sound field, and partly dissipates it. Good

Partially it returns energy into the room, with a different phase. Good.

For a period of time that may be too long, damaging the decay. Not good.

Is there an empirical rule to handle this?
Attached Images
File Type: jpg 15e.jpg (77.1 KB, 20 views)
File Type: jpg 15f.jpg (75.2 KB, 14 views)
  Reply With Quote
Old 8th December 2018, 09:52 AM   #5
Bicicletta is offline Bicicletta
diyAudio Member
 
Join Date: Nov 2018
Location: Genova (Italy)
Default Saturdiy (yeah)

No rules arrived.
So, ignorant mode on: more is better than less.
The cavities will be completely filled.

Youtube
Attached Images
File Type: jpg material.jpg (100.9 KB, 58 views)
  Reply With Quote
Old 8th December 2018, 12:02 PM   #6
Bicicletta is offline Bicicletta
diyAudio Member
 
Join Date: Nov 2018
Location: Genova (Italy)
... it's time for lunch ...
Attached Images
File Type: jpg spaghetime.jpg (282.8 KB, 81 views)
  Reply With Quote
Old 8th December 2018, 04:38 PM   #7
Bicicletta is offline Bicicletta
diyAudio Member
 
Join Date: Nov 2018
Location: Genova (Italy)
... cut, capped, filled ...
Attached Images
File Type: jpg tubi.jpg (154.3 KB, 101 views)
  Reply With Quote
Old 10th December 2018, 10:40 AM   #8
hajj is offline hajj  Lebanon
diyAudio Member
 
hajj's Avatar
 
Join Date: Nov 2009
Nice experimentation!
As you have already noticed, stuffing the tubes with an absorbing material lower the tuning frequency but also gives a lower Q, meaning a lower peak in absorption, but more spread, which in some cases is a better option.
Try painting the tubes with some hardening resin, it should help you better match the predictions from the simulation.
__________________
Nick
  Reply With Quote
Old 10th December 2018, 12:45 PM   #9
Bicicletta is offline Bicicletta
diyAudio Member
 
Join Date: Nov 2018
Location: Genova (Italy)
Thank you so much for your encouragement.

Epoxy would be an optimal treatment for rigidity, sealing and bonding. But it is expensive for an artifact with an uncertain result. I hate polyester resin.

I think I can use an immersion in vinyl glue.

Any suggestion?

Can't wait for the next diy session.
  Reply With Quote
Old 10th December 2018, 02:38 PM   #10
mushroommunk is offline mushroommunk  United States
diyAudio Member
 
Join Date: Sep 2015
Location: Auburn Hills, Michigan
It looks like the tubes are made of cardboard? You could try brushing them with a 50/50 mixture of Titebond and water. I use that to seal my mdf enclosures sometimes. It soaks in some and leaves a decently smooth exterior. Trial it with one. Also could maybe coat them with Shellac or polyurethane or some other hardwood floor finish?
  Reply With Quote

Reply


Helmholtz for a dummyHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
ROAR with Helmholtz front resonator Circlomanen Subwoofers 49 11th January 2019 02:38 PM
formula for Q of Helmholtz resonator PeteMcK Everything Else 9 18th September 2012 03:12 PM
Helmholtz Resonance (again) chairforce Multi-Way 6 26th June 2010 05:31 AM
Cabinet brace act as helmholtz resonator? RyanW Multi-Way 17 3rd October 2007 12:08 AM
Dont want mr. Helmholtz in my sealed sub ! Coolin Subwoofers 18 14th October 2005 10:13 PM


New To Site? Need Help?

All times are GMT. The time now is 08:08 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki