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DIY helmholtz resonator made easy Modular Synthesizers
Old 5th January 2017
  #61
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Quote:
Originally Posted by karumba View Post
in my understanding the result should be roughly the same & the shape doesn't really matter. of course, the amount/diameter of the wholes (=tuning) will be different.
The most important is the surface covered : the number of Helmotz so 10 15 may be 20. Of course an Helmotz with a surface of 20 m² is also a good idea.
Old 9th January 2018
  #62
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Quote:
Originally Posted by RedSkyRoad View Post
I'm trying to fix this null, so I'm gonna give this HR thing a go... How do we decide how big the boxes must be and what are the differences of the different calculations in the posted Excel sheet?
PS: How do you make it resonate in order to measure the frequency?
What are your results? Did HR tackle any of your problems?
I find my measurement being very similar to yours, serious bass problems .. dip in low end and peak at 130Hz plus phasing..
Thanks!)
Old 19th September 2018
  #63
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karumba's Avatar
i've moved the pictures to my own server, since dropbox removed the feature to embed them...
Old 2 weeks ago
  #64
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Hi guys, if you are still tuned into this thread, I'd like to thank
karumba for sharing his experience... now i'm able to build an HR and tune right...
Just a question: is the HR effective also with "null" ???

PS: sorry for my bad english
Old 2 weeks ago
  #65
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.... but if this is true "The sound energy is absorbed by the resonator. There isn't a re-emission of the sound with inverted phase to cancel out the incident sound" I think the answer is NO...

so ... how solve a null problem ?
Old 2 weeks ago
  #66
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mirochandler's Avatar
For sure, a null is also a summing of direct sound and reflection.
If you cancel the reflection – with an Helmholtz for example - then also the null is gone.
Old 2 weeks ago
  #67
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Quote:
Originally Posted by mirochandler View Post
For sure, a null is also a summing of direct sound and reflection.
If you cancel the reflection – with an Helmholtz for example - then also the null is gone.
thanks.. just to understand... you suggest to build an Helmotz at the freq in wicth I have a null ?
Old 2 weeks ago
  #68
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mirochandler's Avatar
yes.
There is one (or more) points (mostly in corners) where this frequ. is relatively loud.
There you have to place the helmholtz
Old 2 weeks ago
  #69
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Quote:
Originally Posted by mirochandler View Post
yes.
There is one (or more) points (mostly in corners) where this frequ. is relatively loud.
There you have to place the helmholtz
ok... now to discover the right point (or more) with REW I generate a sine wave at the freq (for ex. 70hz) with the mic in the listening position (for ex. 75db) and then (like a ghostbuster..) I move the mic in various position along the walls: where I register the greater differences (for ex. 90bd = 75+15 or 50db=75-15) these are "the" positions to place an Helmotz ?
Is it correct ?
Old 2 weeks ago
  #70
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Moving the mic around is one way to do it, yes. Another is to just use your head! At those low frequencies, your ears will be able to hear the modal peaks and nulls very well, so you can just walk around the room, stand on a chair, kneel down on the floor, etc. until you find out where the peak pressure is. It will be against one of your walls, for sure: modal pressures always peak at the wall surface.

- Stuart -
Old 2 weeks ago
  #71
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Ok.... so the right positions are where i've found a +15db but even if there's a -15db that means a great pressure just in a opposite phase ????
Old 2 weeks ago
  #72
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mirochandler's Avatar
Quote:
Originally Posted by Metz71 View Post
Ok.... so the right positions are where i've found a +15db but even if there's a -15db that means a great pressure just in a opposite phase ????
Yes, as soundman correctly wrote, search for a + on the walls, even if this frequ. is a null at your listening position.
Old 2 weeks ago
  #73
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Sorry... but my bad english is a problem... retry to explain:

Example
-In the listening position the 70hz is at 70db against an average of 85db in this position for the other freq
Along the wall there is this situation always at 70hz
Postion a) 90db
Position b) 75db
Position c) 50 db

So.... I have to put an HR only in the position a) or also in the position c) ????

Tks very much
Old 2 weeks ago
  #74
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mirochandler's Avatar
Quote:
Originally Posted by Metz71 View Post

So.... I have to put an HR only in the position a) ????
Yes, only on the place with the highest loudness.
Old 2 weeks ago
  #75
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Ooooook..... thank you very much
Old 2 weeks ago
  #76
I have a massive null at 106Hz in my room, looking into making a helmholtz, with MDF or chipboard, and maybe make the final one out of nicer veneered plywood so it looks good, there's also a big peak at 53 Hz, I was thinking of making one without dampening inside just as an experiment to start with, I am just using my ears and sine waves but don't have a proper measurement microphone, although I do have a neumann km84.. it's pretty flat, has anybody done this with just their hearing alone? I have tested the frequencies lots of times and it's always the same exact frequency to within 1 Hz where the peaks and nulls are, would appreciate any advice on this, and what a great thread.
Old 2 weeks ago
  #77
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Quote:
Originally Posted by pencilextremist View Post
I have a massive null at 106Hz in my room, looking into making a helmholtz, with MDF or chipboard, and maybe make the final one out of nicer veneered plywood so it looks good, there's also a big peak at 53 Hz, I was thinking of making one without dampening inside just as an experiment to start with, I am just using my ears and sine waves but don't have a proper measurement microphone, although I do have a neumann km84.. it's pretty flat, has anybody done this with just their hearing alone? I have tested the frequencies lots of times and it's always the same exact frequency to within 1 Hz where the peaks and nulls are, would appreciate any advice on this, and what a great thread.
as the 106 is exactly double of the 53, it's very probable that if you treat the 53, the 106 will be gone as well
Old 2 weeks ago
  #78
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Mantik's Avatar
Quote:
Originally Posted by karumba View Post
from my experience, the dimensions matter (or are important) for panel resonators. but for HRs, only the volume is of interest. it shouldn't be too low (maybe my version is a kind of lower bound). i have to say i'm experienced (i'm interested in acoustics since 10+ years), but i'm not an expert, so maybe someone else would like to clarify.
Its quite simple. An HR has only one purpose, fighting standing modes or „standing waves“. The volume of the HR has to match the air volume moved of room mode wave frequency you seek to stop with the HR.

The air of the ringing wave gets into HR, through the holes, and because it matches the volume, it starts „resonating“ in the same frequency, as a result nulling it out. Makes sense right?

Thats why the volume of an HR has to be calculated individually for each room mode. Per se formulars do not work, or by not exact coincidence. So you are right the volume matters, the actual size not as long you keep the calculated value of m^3 in the HR constant.

Friend of mine is a building engineer specialised in emission prevention, which includes audio. I do not have the formular at hand, but I guess some
research would bring it up easily.
Old 2 weeks ago
  #79
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Equation for Helmholtz slat resonators (also known as "slot walls" sometimes):

fo = 2160*sqrt(r/((d*1.2*D)*(r+w)))

r = slot width
d = slat thickness
1.2 = mouth correction
D = cavity depth,
w = slat width
2160 = c/(2*PI) but rounded
c = speed of sound in inch/sec.

As you can see, it is the depth of the cavity that matters, not the volume. This is because the volume is always proportional to the depth: If you make the device twice as wide, then it has twice the volume. If you make it 6 times wider, then it has 6 times the volume. So in the equation, you ONLY need to know the depth of the cavity: that's the variable that actually tunes the device (along with panel thickness and slot/slat dimensions, of course).

The perf panel equation is similar: It only needs depth, not volume.

- Stuart -
Old 1 week ago
  #80
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mirochandler's Avatar
I think it is needed to distinguish between a definite volume with a definite hole in it - and a kind of “wall-helmholtz” where the slits and depth are defined.

Two total different things in practice.
In my experience the volume works better with deep freq. and a wall better above ~80Hz.
Old 1 week ago
  #81
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Quote:
I think it is needed to distinguish between a definite volume with a definite hole in it - and a kind of “wall-helmholtz” where the slits and depth are defined. Two total different things in practice.
It's the exact same principle, actually. The same equations apply to all forms of Helmholtz Resonator: they are all derived from the same underlying basis. It's just a simple mass-spring system, where the "mass" is the air trapped in the neck of the device, along with the mouth correction factor (A.K.A. "end correction") applicable to each specific case, and the "spring" is the air behind that. The only thing that changes between implementations, is the method for calculating the mass of the air "plug" in the neck of the device, and the method for calculating the resilience of the air spring behind.

For example, the equation for perforated panel is very similar to the one I already gave for slats:

fo = 200*sqrt( p / (t + (d*0.8))*(D) )

p = perforation percentage (percent open area)
t = panel thickness, in inches
d = hole diameter, in inches
0.8 = mouth correction
D = cavity depth, in inches

(Taken from MHoA)

That works for one hole or one thousand holes.

Not too meany people that I know of build tuned studio Helmholtz resonator traps as one single hole in one single enclosure. That's wouldn't be very effective for dealing with modal issues, for example: You need a much larger effective area than can be obtained from a single hole, and a much larger volume than can be obtained from a single enclosure. That's how the basic theory is usually explained, since it it easier to understand like that, but in real-world studios the actual implementations in treatment devices are usually either perforated panel or slot walls.

The general form of the equation for all such devices is in the image below.

- Stuart -
Attached Thumbnails
DIY helmholtz resonator made easy-helmholtz-resonator-equation-lrg.jpg  

Last edited by Soundman2020; 1 week ago at 08:02 AM..
Old 1 week ago
  #82
I have to admit even though I understand parts of that formula it doesn't make it easy to understand, could we go through the process of making a trap? as in an in depth analysis of applying the forumla, that's tuned to 53Hz, cos I'm really keen on making one now after hearing this!

in MHBOA pg 221 table 12-3 it shows me how to make one tuned to 55Hz and I can do that, but how would I tweak it down by just two Hz, it says the thickness of the airspace needs to be 3 5/8", front panel 1/8" and perforation percentage 0.25% for 110Hz is at 2.22" fractional inches which I could set with my digital caliper, so am I right in presuming if I set it to 4.44" it'll be tuned to 55Hz? this halves the perforation percentage to 0.12.5% and also I plan on making one without any kind of dampening in it, is that just crazy? I could easily put 2 inch thick rockwool inside to fill roughly half of the space, as it's not expensive.

thanks for responding so far guys
Old 1 week ago
  #83
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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 -
Old 1 week ago
  #84
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Quote:
Originally Posted by mirochandler View Post
Yes, only on the place with the highest loudness.
Thanks for all the suggestions guys.... just a little feedback:

- to treat a peak, all you have described works well (I have a peak at 20hz ... I've built two HR correctly tuned, placed in the position in which there is the the highest pressure at 20hz and yes... they work.. and I can see clearly in the Waterfall)

- the same thing done at 26hz where I've a great null..... and ... no improvement no effectiveness
Old 1 week ago
  #85
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mirochandler's Avatar
Quote:
Originally Posted by Metz71 View Post
Thanks for all the suggestions guys.... just a little feedback:

- to treat a peak, all you have described works well (I have a peak at 20hz ... I've built two HR correctly tuned, placed in the position in which there is the the highest pressure at 20hz and yes... they work.. and I can see clearly in the Waterfall)

- the same thing done at 26hz where I've a great null..... and ... no improvement no effectiveness

Let me see a measurement, the room dimensions and the 26Hz HR dimensions.
How large is the 20Hz HR ?
Old 1 week ago
  #86
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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
Old 1 week ago
  #87
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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
How can you have a mode at 20 Hz in a room that is only 7.5m long? It would have to be 8.6m long in order to have a mode at 20 Hz. With a 7.5m room, the lowest possible mode is 23 Hz, which is rather different from 20 Hz.

Quote:
- the same thing done at 26hz where I've a great null..... and ... no improvement no effectiveness
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 -
Old 1 week ago
  #88
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Yannick's Avatar
 

Quote:
Originally Posted by Soundman2020 View Post
How can you have a mode at 20 Hz in a room that is only 7.5m long? It would have to be 8.6m long in order to have a mode at 20 Hz. With a 7.5m room, the lowest possible mode is 23 Hz, which is rather different from 20 Hz.
my room is 6m9 long, the calculated longitudinal mode is 24.9 Hz. My first measurement shows a mode at 25.06 Hz (the walls flexing a bit ?).

After half of the treatment, the longitudinal mode is at 23,6 Hz !

Do not forget, treatment (with rockwool) makes the room bigger acoustically, as the speed of sound changes inside the rockwool.

This raises the question: what happens with your calculated mode, once you introduce a big helmholtz device (or four) in the room ? Experience seems to show the modes shifts, and the device does not work properly, hence tuning in room is necessary ?
Old 1 week ago
  #89
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Quote:
Originally Posted by Soundman2020 View Post
How can you have a mode at 20 Hz in a room that is only 7.5m long? It would have to be 8.6m long in order to have a mode at 20 Hz. With a 7.5m room, the lowest possible mode is 23 Hz, which is rather different from 20 Hz.

- Stuart -
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
Attached Thumbnails
DIY helmholtz resonator made easy-000-starting-point-900-mil.jpg  
Old 1 week ago
  #90
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... and here with two HR tune at 20hz ..
Attached Thumbnails
DIY helmholtz resonator made easy-002-2hr-ang-sx.jpg  
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