The No.1 Website for Pro Audio
 Search This Thread  Search This Forum  Search Reviews  Search Gear Database  Search Gear for sale  Search Gearslutz Go Advanced
DIY helmholtz resonator made easy
Old 4 weeks ago
  #91
Here for the gear
 

this is my first experiment.... I hope to treat with HR or panel perforated the 20hz zone and the 40hz zone, or at least, to get some improvement, than ....with parametric eq finishing

Quote:
Originally Posted by Soundman2020 View Post
For a room with the dimensions you gave, there are no modes at 26 Hz. The next highest one is at 38.3 Hz. If you have a large dip at 26 Hz, it could be SBIR. That would explain why you didn't get any resuls with a Helmholtz resonator tuned to that frequency. SBIR is phase cancellation from a reflection, not resonance, so tuned traps cannot fix it.

- Stuart -
As my speakers are 1.1m far from the wall behind ... using the rule of 1/4wavelenght, I'd have a cancellation approximately at 77hz .. not 26
Old 4 weeks ago
  #92
Lives for gear
 

Quote:
Originally Posted by Metz71 View Post
I know it very well... the room mode calculators gave me 23hz as mode... but here attached my starting situation with no resonator in room, only rockwool panel from floor to ceiling in the corners
Very strange! I'm wondering if that might be a structural resonance, rather than a mode. The "speed of sound" effect from a few inches of mineral wool is not enough to account for the difference.

- Stuart -
Old 4 weeks ago
  #93
Lives for gear
 

Quote:
Originally Posted by Metz71 View Post
As my speakers are 1.1m far from the wall behind ... using the rule of 1/4wavelenght, I'd have a cancellation approximately at 77hz .. not 26
Back wall. Not front wall... You can get SBIR from any wall, and the rear wall is often guilty...

- Stuart -
Old 4 weeks ago
  #94
Quote:
Originally Posted by Soundman2020 View Post
Here's a fun "toybox" to play with, and help you figure it all out... http://www.acousticmodelling.com/ The best one in there is the multi-layer one.

Have fun!

- Stuart -
thanks stuart, I came up with this for 53Hz 3mm panel, 3mm wide holes and centre of the holes at 125mm with a 106mm air gap apparently is bang on 53Hz.

Do you know if the back of the HR matters? I plan on using another 3mm hardboard panel for the back just to seal it, and there will only be air inside no rockwool inside.

My other concern is that you can't get dead on 3mm size with hardboard in the UK, it's actually more like 2.8mm sometimes 2.6, presuming this would change the calculations.
Old 4 weeks ago
  #95
Here for the gear
 

Quote:
Originally Posted by Soundman2020 View Post
Back wall. Not front wall... You can get SBIR from any wall, and the rear wall is often guilty...

- Stuart -
you are right... i will study better this issue
Old 4 weeks ago
  #96
Lives for gear
 

Quote:
Originally Posted by pencilextremist View Post
Do you know if the back of the HR matters? I plan on using another 3mm hardboard panel for the back just to seal it, and there will only be air inside no rockwool inside.
The back needs to be very rigid, so that the depth inside does not change due to changing air pressure, vibration, etc. I normally use 3/4" plywood or MDF at least, and attach that firmly to the wall surface. You also need to take care that the cavity is sealed air-tight.

Without insulation inside, your device is going to be tuned very sharply (high Q), so you will need to be extra accurate with tuning it. Is there a reason why you don't want to have insulation in there? It lowers the Q and damps the resonance....

Quote:
My other concern is that you can't get dead on 3mm size with hardboard in the UK, it's actually more like 2.8mm sometimes 2.6, presuming this would change the calculations.
Yup, that will change the tuning. Try putting those numbers into the calculator, and you'll see that the frequency does change. I would suggest that after you buy the hardboard, you measure the real thickness carefully with a micrometer, put that number into the calculator, and adjust your hole spacing as necessary to get the frequency back where you want it.

Also, with such high Q, you should probably take into account the way you are going to lay out the holes, as that changes the calculations too. https://www.accurateperforating.com/...cent-formulas/ I'm not sure what layout the calculator assumes, buy my guess is just straight spacing.

I would strongly suggest you make your device tunable, so you can test it and change the tuning if it isn't right! With high Q and so many variables, it's unlikely that you'll hit it spot on.

Just so you are aware, there are other options for low frequency trapping: Tim's Limp Mass Bass Absorbers for example.


- Stuart -
Old 4 weeks ago
  #97
Lives for gear
 
mirochandler's Avatar
Quote:
Originally Posted by Metz71 View Post
My room is 7,5m long 4,5m large and 2,4m high (24ft7in x 14ft5in 7ft10in)

The HR at 20hz are box 120cm*60cm*20cm with a neck of 16cm

Tonight I will post some measurement
I find that size a lot too small for such a low freq.
But you built two of them, this helps a little bit. Generally the effect is not enough with such a small enclosure.

Your box has 144 liters, but for 20Hz I would build a box with ~400 Liters. A correction of 5-7dB will happen then.

But for the next Mode 38,2Hz the 120 liters would be fine.
this one has to be placed on the sidewalls (ceiling) - mostly.
Old 4 weeks ago
  #98
Lives for gear
 
mirochandler's Avatar
Quote:
Originally Posted by pencilextremist View Post
I plan on using another 3mm hardboard panel for the back just to seal it, and there will only be air inside no rockwool inside.

My other concern is that you can't get dead on 3mm size with hardboard in the UK, it's actually more like 2.8mm sometimes 2.6, presuming this would change the calculations.
A rigid enclosure is one of the most important points with HR´s. Even a 3/4" board is often not enough and needs some ribs inside.

Forget it to tune a HR´s only on the paper. In reality and also on the installing place it is always a few hz difference.
Simply drill more holes to increase the freq. or close some to lower the freq.

And please give some absorption material inside! (will lower the freq. a little bit)
Old 4 weeks ago
  #99
Here for the gear
 

Quote:
Originally Posted by mirochandler View Post
I find that size a lot too small for such a low freq.
But you built two of them, this helps a little bit. Generally the effect is not enough with such a small enclosure.

Your box has 144 liters, but for 20Hz I would build a box with ~400 Liters. A correction of 5-7dB will happen then.

But for the next Mode 38,2Hz the 120 liters would be fine.
this one has to be placed on the sidewalls (ceiling) - mostly.
thanks.... 400liters is a monster... have you seen my waterfall ?
Old 4 weeks ago
  #100
Lives for gear
 
mirochandler's Avatar
Quote:
Originally Posted by Metz71 View Post
thanks.... 400liters is a monster... have you seen my waterfall ?
yes, I know, but mounted on the ceiling (over the studio entry door) is often trouble-free and needs no space on the floor.
But you room does have it´s length and for me there is no other useful solution about the deepest modes.
The thing is: if you start to work on higher freq., then maybe you do not have enough space for such enclosures.
And the much you control the frequ. above the 20Hz the more it will be hearable. Therefore begin at the lowest modes.
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
Thread Tools
Search this Thread
Search this Thread:

Advanced Search
Forum Jump
Forum Jump