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Helmholtz for the people!
Old 15th September 2019
  #1
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Helmholtz for the people!

Hi everyone!
I am building a couple of Helmholtz-resonators to adress the problem around 50Hz that is shown in the attachment. Should I target the peak at 49Hz or the more early energy at 54Hz? Measurements are done with Harbeth P3esr that drop slowly after 70Hz but I am about to upgrade to bigger speakers and this problem needs to be taken care of.
Attached Thumbnails
Helmholtz for the people!-untitled.jpg  
Old 15th September 2019
  #2
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Jason Foi's Avatar
 

I would recommend a large helmholtz array targeted to multiple Fc. Varried slat width/gap/depth over a large surface area could work well if you dont have the space for a large velocity trap.
Old 16th September 2019
  #3
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Measure your room with a sub

Acoustic Treatment - Bigger Room
Old 16th September 2019
  #4
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akebrake's Avatar
 

Mains Hum?

Quote:
Originally Posted by hankg82 View Post
Hi everyone!
I am building a couple of Helmholtz-resonators to adress the problem around 50Hz that is shown in the attachment. ...
Your mains (household electricity) doesn't happen to be 50Hz?

Hum is steady amplitude (no decay) and shows like horisontal ridges on a waterfall. (50 or 60 Hz plus harmonics) depending on country. Pic 1
Decaying mode(s) close to that frequency.

Pic 2: REW's got a nice RTA where you can spot such problems.(No sweep needed)
NOTE: This Example from another measurement.

Questions:
What microphone? What kind of walls? Size and shape of your room?

Best

PS: Always measure ONE speaker at a time.
L+R optionally
Attached Thumbnails
Helmholtz for the people!-mains-hum-.jpg   Helmholtz for the people!-rta-hum-50-100.jpg  
Old 4 weeks ago
  #5
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Thanks for the reply!
I have measured the room before with a sub and with Harbeth 7es3 and the problem looks pretty much the same only with higher amplitude. The room also has some issues at 18hz but my plan is to buy Harbeth 30.1(or something similar) that does not "activate" this frequency. I did like the 7es3 but there was something in the treble that did not sound right to me. 30.1 and p3esr sounds a lot better in this area.
Never thought the AC could result in a 50hz problem like this and I do live in Sweden where the mains are 50hz but the problem seemed to be the room. I completed one of the resonators and targeted 51hz.
The results were more than satisfying(with only one resonator). I will have three of them placed between the speakers. I also think I will cover them with fabric that will lower the Q a little bit or perhaps different port lengths to cover a wider range as suggested.
Attached Thumbnails
Helmholtz for the people!-20190917_204741.jpg   Helmholtz for the people!-nohelmholtz.jpg   Helmholtz for the people!-helmholtz.jpg  
Old 4 weeks ago
  #6
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GregAnikin's Avatar
 

Quote:
Originally Posted by hankg82 View Post
Thanks for the reply!
I have measured the room before with a sub and with Harbeth 7es3 and the problem looks pretty much the same only with higher amplitude. The room also has some issues at 18hz but my plan is to buy Harbeth 30.1(or something similar) that does not "activate" this frequency. I did like the 7es3 but there was something in the treble that did not sound right to me. 30.1 and p3esr sounds a lot better in this area.
Never thought the AC could result in a 50hz problem like this and I do live in Sweden where the mains are 50hz but the problem seemed to be the room. I completed one of the resonators and targeted 51hz.
The results were more than satisfying(with only one resonator). I will have three of them placed between the speakers. I also think I will cover them with fabric that will lower the Q a little bit or perhaps different port lengths to cover a wider range as suggested.
Fabric used only for esthetic purposes it’s not going to lower the Q at all.
Old 4 weeks ago
  #7
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I tought lower airvelocity in the port ment lower Q? And slightly higher fres.
Old 4 weeks ago
  #8
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GregAnikin's Avatar
 

Quote:
Originally Posted by hankg82 View Post
I tought lower airvelocity in the port ment lower Q? And slightly higher fres.
Now imagine how low can thin fabric make your Q? Like a 1/20 of of a Hz?
Old 4 weeks ago
  #9
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Can you tell us the thickness of that panel and the dimension of the holes? And the space between them?

Last edited by JayPee; 4 weeks ago at 02:45 PM.. Reason: Grammar
Old 4 weeks ago
  #10
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Sjeff's Avatar
 

Nice!
Old 4 weeks ago
  #11
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Quote:
Fabric used only for esthetic purposes it’s not going to lower the Q at all.
Actually, it will. The most effective place for absorption in a Helmholtz resonator, is inside the neck itself, where it acts directly on the slug of air. And immediately outside the hole, in the "neck correction" region, that slug is still slamming around happily, so the effect of absorption can be very good there for absorption placed directly over the holes.

Simple experiment: blow across the top of an empty beer bottle to make it "sing". Then put a piece of cloth over the hole, or a small tuft of cotton wool inside the neck of the bottle, and blow again.... You'll be surprised at what a difference it makes. It increases the damping quite noticeably, thus lowering the Q.

Quote:
Now imagine how low can thin fabric make your Q? Like a 1/20 of of a Hz?
He's talking about lowering the Q, not changing the frequency. Fabric over the hole can indeed lower the Q, without affecting the frequency too much.

Quote:
I tought lower airvelocity in the port ment lower Q? And slightly higher fres.
It will change the Q mostly (if it is thick enough and absorbent enough), but probably won't have a large effect on the tuning.

You might need something a bit thicker than typical fabric, maybe something like upholstery fabric for sofas, but experiment with a few different types and thicknesses until you get the effect you want. Also, you might not need to cover all of the holes: maybe cover two or three of them, and leave the others open. Since you got such an excellent effect on the problematic frequency, you don't want to loose that. I would suggest experimenting with different types of fabric, and covering different numbers of holes, until you get the best effect.

Congratulations on your very successful Helmholtz device! That's actually hard to do (design and tune one accurately), but you did it very well, and it is obviously working very well.

You could also try moving them around a bit, to see if you can improve the effect. Maybe sliding them up towards the ceiling, or sliding them left/right. There probably won't be a lot of difference, but its worth trying.


- Stuart -
Old 4 weeks ago
  #12
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GregAnikin's Avatar
 

Quote:
Originally Posted by Soundman2020 View Post
Actually, it will. The most effective place for absorption in a Helmholtz resonator, is inside the neck itself, where it acts directly on the slug of air. And immediately outside the hole, in the "neck correction" region, that slug is still slamming around happily, so the effect of absorption can be very good there for absorption placed directly over the holes.

Simple experiment: blow across the top of an empty beer bottle to make it "sing". Then put a piece of cloth over the hole, or a small tuft of cotton wool inside the neck of the bottle, and blow again.... You'll be surprised at what a difference it makes. It increases the damping quite noticeably, thus lowering the Q.

He's talking about lowering the Q, not changing the frequency. Fabric over the hole can indeed lower the Q, without affecting the frequency too much.

It will change the Q mostly (if it is thick enough and absorbent enough), but probably won't have a large effect on the tuning.

You might need something a bit thicker than typical fabric, maybe something like upholstery fabric for sofas, but experiment with a few different types and thicknesses until you get the effect you want. Also, you might not need to cover all of the holes: maybe cover two or three of them, and leave the others open. Since you got such an excellent effect on the problematic frequency, you don't want to loose that. I would suggest experimenting with different types of fabric, and covering different numbers of holes, until you get the best effect.

Congratulations on your very successful Helmholtz device! That's actually hard to do (design and tune one accurately), but you did it very well, and it is obviously working very well.

You could also try moving them around a bit, to see if you can improve the effect. Maybe sliding them up towards the ceiling, or sliding them left/right. There probably won't be a lot of difference, but its worth trying.


- Stuart -
Cotton in the bottle - yes, piece of fabric in one layer - no, done that even with a t-shirt fabric, sounds good.
Old 4 weeks ago
  #13
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My intensions are to lower the Q just a little bit.
"https://www.gearslutz.com/board/showpost.php?p=7240613&postcount=51"
Feels like Helmholtz resonators are most effective with narrow banded slow decaying frequencies. I have completed all three resonators and with the P3esr it´s not really necessary with all three. With more than two I see no further improvement in the time domain. I will wait with further changes until i have larger speakers in place and see how it measures. As JayPee hinted.
I used this formula and it seemed to be very accurate.
"http://www.mh-audio.nl/Acoustics/HHReso.html"
cavities 27,7x28x16cm
port 6,5x3,6cm
Old 3 weeks ago
  #14
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Maybe they are not designed in a proper way (targeting the wrong frequencies) , hence my question about the design here and/or they are not positioned correctly in the room.
Old 3 weeks ago
  #15
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Synthpark's Avatar
Quote:
Originally Posted by hankg82 View Post
Thanks for the reply!
I have measured the room before with a sub and with Harbeth 7es3 and the problem looks pretty much the same only with higher amplitude. The room also has some issues at 18hz but my plan is to buy Harbeth 30.1(or something similar) that does not "activate" this frequency. I did like the 7es3 but there was something in the treble that did not sound right to me. 30.1 and p3esr sounds a lot better in this area.
Never thought the AC could result in a 50hz problem like this and I do live in Sweden where the mains are 50hz but the problem seemed to be the room. I completed one of the resonators and targeted 51hz.
The results were more than satisfying(with only one resonator). I will have three of them placed between the speakers. I also think I will cover them with fabric that will lower the Q a little bit or perhaps different port lengths to cover a wider range as suggested.
Your Q is too high. Several layers of fabrik will help. I prefer insulation inside.
Old 3 weeks ago
  #16
Gear Nut
 

Quote:
Originally Posted by hankg82 View Post
My intensions are to lower the Q just a little bit.
"https://www.gearslutz.com/board/showpost.php?p=7240613&postcount=51"
Feels like Helmholtz resonators are most effective with narrow banded slow decaying frequencies. I have completed all three resonators and with the P3esr it´s not really necessary with all three. With more than two I see no further improvement in the time domain. I will wait with further changes until i have larger speakers in place and see how it measures. As JayPee hinted.
I used this formula and it seemed to be very accurate.
"http://www.mh-audio.nl/Acoustics/HHReso.html"
cavities 27,7x28x16cm
port 6,5x3,6cm
As soon as you add more HH resonators, they couple to each other and also to the tuned modes in a very complicated way To better understand whats happening we have to step out from GS Sabin box and accept that axial modes is a amplitude problem and not a energy problem
Here are some great examples (from Breakthenorms studio) how tuned high Q phase shifting HH boxes distribute the axial room mode without absorbing it http://diffusor.com/PDF/Break-2.pdf
The regulars was confused  How to treat nulls 2

As you noticed, its normally enough with one high Q HH tuned on the same frigile resonance http://diffusor.com/PDF/robin_hood_effect_varitune.pdf

If you want to dig deeper we recommend Mendel Kleiners thesis
http://ancsystems.narod.ru/jasa/1315_1.pdf mentioned in this post www.gearslutz.com/board/11533618-post211.html Ignore Northwards wrong assumption that its a special case , as the thesis can be applied generally to any number of HH boxes
If we enlarge mr Kleiners model and use same vocabulary , the room volyme becomes the speaking HH resonator coupled to silent HH boxes (coupled to each other ) tuned on the mode
If you built enough of them and make them tuneable, you can use them as a passive EQ as we normally do
Old 3 weeks ago
  #17
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Quote:
.... and accept that axial modes is a amplitude problem and not a energy problem ... Ignore Northwards wrong assumption ...
Beware: You are now entering ....
Attached Thumbnails
Helmholtz for the people!-twilight-zone-logo.jpg  
Old 3 weeks ago
  #18
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Do not worry, I have already ignored Northward completely and accepted that other stuff.
If it´s enough with one resonator I´ll be very happy. Then I can use that space for absorption or diffusion instead. My girlfriend really likes the way that the living room has transformed into something..... different. I have been trying to explain that if you look at it through a waterfall graph it makes more sense.
Old 3 weeks ago
  #19
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Northward's Avatar
Quote:
Originally Posted by Soundman2020 View Post
Beware: You are now entering ....
Hahaha, yes ! The acoustics Twilight zone.

Where amplitude and energy now have nothing in common...

You are now leaving the Sabin box - where "The amount of energy carried by a wave is related to the amplitude of the wave. A high energy wave is characterized by a high amplitude; a low energy wave is characterized by a low amplitude"

Sometimes it's good to be ignored.
Old 3 weeks ago
  #20
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thomlin's Avatar
Quote:
Originally Posted by hankg82 View Post
Do not worry, I have already ignored Northward completely and accepted that other stuff.
Amen, and good luck!

I think some egos in this forum are so blown up, they don’t even need a fuse to explode.
Attached Thumbnails
Helmholtz for the people!-14308f8a-f9b9-4bf6-a8cb-25c0186d96ea.jpg  
Old 3 weeks ago
  #21
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Regardless of egos, there is real acosutic science based on the laws of the universe... then there is voodoo acoustics, where energy is not related to amplitude, WC Sabine was absolutely wrong, some resonators "speak" while others are "silent", damping the mouth correction zone of a resonator does nothing, diffusion is good way to treat modes, and snake oil is a required ingredient to make the whole thing work .... ... Yup. For sure....

Personally, I think I'll stick with the egos! At least they are logical, and make sense... ... and they don't use pixie dust or unicorn hair to get the amazing results their room designs achieve! They just use real acoustic science, in the real world... far away from the Twilight Zone.


- Stuart -
Old 3 weeks ago
  #22
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thomlin's Avatar
Quote:
Originally Posted by Soundman2020 View Post
Regardless of egos, there is real acosutic science based on the laws of the universe... then there is voodoo acoustics, where energy is not related to amplitude, WC Sabine was absolutely wrong, some resonators "speak" while others are "silent", damping the mouth correction zone of a resonator does nothing, diffusion is good way to treat modes, and snake oil is a required ingredient to make the whole thing work .... ... Yup. For sure....

Personally, I think I'll stick with the egos! At least they are logical, and make sense... ... and they don't use pixie dust or unicorn hair to get the amazing results their room designs achieve! They just use real acoustic science, in the real world... far away from the Twilight Zone.


- Stuart -
I notice the rhetoric is similar to a certain president. Repeat something enough times, and let’s hope people think it must be the truth.

I don’t see the connection between snake oil and diffusion. But I definitely recognize a snake when I see one, even in a forum like this. And I can link to sites online where a certain name is showing up with unhappy customers.

Regarding Matts I haven’t heard of unsatisfied customers, only competitors bashing him in this forum.
Ugly and unfairly.
Old 3 weeks ago
  #23
Gear Addict
 

Quote:
Originally Posted by thomlin View Post
I notice the rhetoric is similar to a certain president. Repeat something enough times, and let’s hope people think it must be the truth.

I don’t see the connection between snake oil and diffusion. But I definitely recognize a snake when I see one, even in a forum like this. And I can link to sites online where a certain name is showing up with unhappy customers.

Regarding Matts I haven’t heard of unsatisfied customers, only competitors bashing him in this forum.
Ugly and unfairly.
Go for it, Thomlin!
Old 2 weeks ago
  #24
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ehh... egos and presidents. I was being a little sarcastic. Dangerous i guess when you are not comfortable with the language. I appreciate all information and not ignoring anyone. I can decide for myself what to believe in. Usually it is the microphone with small changes to the acoustics that influences me the most.
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