PVC pipe Helmholtz resonator

[aapee]

Hi guys and gearslutz

After seven broadband absorbers and two tube traps and three (super) chunks everything seems and sounds very good in my control room - small but very well treated now. Very smooth responce thumbsup .

One question for you acoustic minded guys. I have narrow 10 db peak at 209 Hz which is still left. I red that some hiend -hobbiests have done Helmholtz resonators from standard PVC pipe. I will try to add one picture of them with this thread. Could you please give me right dimensions for this problematic 209 Hz? The length and diameter of this kind of pipe.

Would these help? The peak is quite narrow. I suppose the second harmonic of the ceiling high...

AaPee

[aapee]

Have all acoustic experts gone for vacation?

AaPee

[Ethan Winer]

AaPee,

I doubt that tubes such as those in the photo will do you any good at all. A better approach is broadband bass trapping. The more you have, the better the results.

--Ethan

[aapee]

Originally Posted by Ethan Winer

AaPee,

I doubt that tubes such as those in the photo will do you any good at all. A better approach is broadband bass trapping. The more you have, the better the results.

--Ethan

Thanks Ethan for your answer!

I really have enough broadband bass trapping in my room (thinking of the practical size of the room ). Seven pieces of 4" 4x2 panels, two large tube traps and three super chucks. Only the narrow pike at 209 Hz. Those hollow pipes do not work as a Helmholtz resonator, do they? Any other solutions for narrow pikes?

AaPee

[matt thomas]

I expect ethans right

however i assume the theory on these would be that they should be a quarter wavelength of the freq you want to dampen.

If the speed of sound is 330 m/s, then a 209 hz wave would have a wavelngth of 1.58 meters, and quarter of that will be 395 mm.

maybe..

actually on second thoughts since you said this "I suppose the second harmonic of the ceiling high..." then the length of pvc pipe should be quarter of the height of your room, which doesn't add up, maybe it is the third harmonic, so it'll be a sixth of the height of your room..

its late..

narco

[aapee]

Originally Posted by narco

I expect ethans right

however i assume the theory on these would be that they should be a quarter wavelength of the freq you want to dampen.

If the speed of sound is 330 m/s, then a 209 hz wave would have a wavelngth of 1.58 meters, and quarter of that will be 395 mm.

maybe..

narco

Thanks Narco!

Do you have any thoughts about the diameter?

AaPee

[Tim Farrant]

I would build a slat resonator, running 209Hz through my little resonator calculator, I come up with this;

Slat thickness = 19mm (3/4")
Slat width = 110mm (4.3")
Slot between slats = 5mm (0.2")
Depth behind Slats = 150mm (6")

Hope this is some help.

Tim.

[aapee]

Originally Posted by Tim Farrant

I would build a slat resonator, running 209Hz through my little resonator calculator, I come up with this;

Slat thickness = 19mm (3/4")
Slat width = 110mm (4.3")
Slot between slats = 5mm (0.2")
Depth behind Slats = 150mm (6")

Hope this is some help.

Tim.

Great Tim!

What should be the size of this resonator? Dimensions? Do you have any general picture of it?

AaPee

[rufus13]

TT's are concrete forms. They take paint/texture well and are likely cheaper than large diameter pvc. Home Despot or other big box.

[aapee]

http://www.audioholics.com/techtips/...rabsorber2.php

Here I found one page of them...

AaPee

[Tim Farrant]

Originally Posted by aapee

Great Tim!

What should be the size of this resonator? Dimensions? Do you have any general picture of it?

AaPee

I am not sure of the size of your room or the extent of the peak, but maybe two resonators mounted in corners something like 600-800mm wide and 1800mm high. There are plenty of pictures on the net regarding how to build these things if you do a Google.

Best of luck with it
Tim.

[Ethan Winer]

AaPee,

> Only the narrow pike at 209 Hz. <

Have you measured at various places around the room to confirm that the frequency is always the same? This is very important!

> Those hollow pipes do not work as a Helmholtz resonator, do they? <

Who knows. Even if they do work, they're not large enough to be useful IMO. Acoustic treatment, and bass traps in particular, requires a large amount of surface area. If you go to that manufacturer's web site you'll find some, let's say, "surprising" audiphoole type claims.

--Ethan

[aapee]

Yes Ethan, the same spike is all around, varies a few db's but seems to be some kind of resonance.

Those pipes don't have any industrial manufacturer, they were more or less DIY project. So there are no any claims or official measurements. That was the reason why I asked this question in this forum.

Are those fancy pipes only urban legend or one unsucsessful DIY project?

Is there any way to count if they would work or not?

AaPee

[jeff deff]

Helmholtz resonators have been around since the Greeks and Romans. They were used in Medieval churches. Pots were imbedded in the walls and filled with ash. The ash functions as a dissipative agent to widen the bandwidth in which the resonator is active. They work.

There was an experiment carried out at Riverbank Acoustical Laboratories where they measured the effectiveness of an empty coca cola bottle as an Helmholtz resonator. They determined that it has an absorbtion of 5.9 sabins at 185 Hz. The only trouble is the bandwidth in which the coke bottle effectively works is only 0.67Hz. That equals a "Q" of 276. Imagine dialing in that on a parametric EQ.

I learned all this by reading "The Master Handbook of Acoustics" by F. Alton Everest. I haven't finished it yet, and it has some heavy mathematical equations, but I've learned a lot by reading this book. Not everything about acoustics is intuitive.

Another really interesting fact I learned was (sorry to go off topic), in the ocean at a depth 4,200 feet, there is a strange acoustical phenomena that occurs. Warmer water towards the surface of the ocean pushes soundwaves downward by means of refraction. The higher pressure of the water towards the bottom of the ocean refracts soundwaves upward. At the depth of 4,200 feet an equilibrium between these two forces occurs creating a sound channel that allows sound to travel huge distances without losing much energy. Some experimentors detonated a 600 lb. explosive in Australia and recorded the sound in Bermuda 3.71 hours later. Pretty cool stuff.

[Ethan Winer]

AaPee,

> Yes Ethan, the same spike is all around, varies a few db's but seems to be some kind of resonance. <

Okay but I still think you'll do better with broadband absorption because it will hit that frequency, and also give you even more bass trapping at other frequencies. And extra bass trapping is always welcome.

> Those pipes don't have any industrial manufacturer, they were more or less DIY project. <

Those tubes are a DIY version of the Argent Room Lens:

www.soundstage.com/noisy14.htm

The company seems to be gone now, which is just as well because I put this, and all the claims made for it, squarely in the "snake oil" category of audiophile products. Audiophiles sometimes report hearing a change when they put a few of these things in their room, but all I can imagine these doing is adding a nasty resonant ringing as sound waves in the room excite the pipe's resonance. Sometimes people confuse "different" with "better," and that's most likely the case here too.

--Ethan

[Snatchman]

I don't have a question just yet about my room ( under construction, nothing fancy), but when I do, I feel very fortunate to have great guys as Ethan, Tim, Myfipie, etc. here to help me along the way...! Thanks guys for taking time out of your busy schedule to advise!...... ..( SOT)

[aapee]

Thanks Ethan and every other also!

I think, I will give up this PVC pipe resonator... OK to plan B...

AaPee

[Robobo1]

The Loudspeaker Design Cookbook

All you ever wanted to know...

http://www.amazon.com/gp/product/188...718521?ie=UTF8

A friend of mine built some pretty damn good speakers years ago, starting off only knowing basic fundamentals of acoustics, and I believe he used this book as his main reference.

[AdamZuf]

Originally Posted by Ethan Winer

I still think you'll do better with broadband absorption because it will hit that frequency, and also give you even more bass trapping at other frequencies. And extra bass trapping is always welcome.

--Ethan

I just discussed this today with my acoustics teacher and he said that the more broadband the Helmholtz resonator, the less it is efficient in the center frequency I'll try to target. Do you agree?

Here are a few measurements I made after absorbing first reflections on walls... The results are quite similar with sweeps or steps, these are the step measurements:



My Genelec 1029A where I sit:


Selah Audio's Carnelian where I sit:



Selah Audio's Carnelian at about 2.5m:


The main trap I'm considering is for where I sit, tuned for 75Hz, and another tuned for 62Hz, hopefully both will affect the peaks/nulls at the harmonics. I'll also cover most of the area around me with Rockwool like material (see below), which will be spaced 10cm from the wall (that's for sure for the side walls), creating a kinda linear NRC around 0.9 down to ~250Hz, according to manufacturer (the panels I've used for measurements were the old ones I made from fivex 1cm thick fiberglass boards, spaced from walls not in a very consistant manner :-) but I think it's good enough to know where the problems are...

My current plan my mixing studio (21.10.2007):




RED: bass traps, maybe with some diffusion elements
PURPLE: absorbers, spaced 10cm
BLUE: Skyline like diffusers

The big green boxes are the desk and monitors (ignore the green box on the left). The wall behind them is actually a window, and I wish I could make something that can move easily on a curtain rail so I also get some air inside from time to time...Probably it would be wize to make bass traps and first reflection absorbers on that wall, but I can't apply them on the corners of it. I do have my mains behind the desk (mounted on big PA horn subwoofers I don't use.. wonder if they make some difference...), but still got about 50cm space from behind them to the window.

There is also the wall I've hiden in order for you to see, here is a render with the treatment:


The sofa (where me/the client enjoys the music, and where I measured the Carnelian speakers, shown above) is about below the lamp (that tube you see models it very accuratly).

Advice on what to do with the bass traps and the window...

[Ethan Winer]

Adam,

I still think you'll do better with broadband absorption, though it would help if you included dimensions. A tuned absorber on the rear wall will be at the wrong frequency as soon as you move forward or back a few feet in your chair.

--Ethan

[AdamZuf]

Hi Ethan,
Here are the dimentions (and sorry some of the panles aren't showing, ignore that, just mind the dimentions and the position of the desk):


The recommendation for bass traps is first on the front croners, then in the back.. But it seem you disagree about the back wall issue, am I missing something, or do you disagree with this common opinion?

Would you absorb or diffuse next to the rear wall? I think I'll have the chance to record overdubs or small stuff in the rear part of the room, I never heard massively diffused sound, on the other hand, maybe it's wise to mix diffusion and absorbtion in the rear part as well..?

Do you think that the bass traps on the side walls will be sufficient? quite a lot of them... At the moment I have half tubes with lots of fiberglass and wooden backs behind the desk next to the front wall, rotated to diffuse as well... It's quite ugly but I don't really have the funds to do that part for now, I'll just put a curtain behind the desk.

Considering that in the work position I should target the 75Hz related problems and in the sofa position (below the lamp) I should target the 62Hz related problems, do you think I should position the traps accordingly?

Do you have any advice about what type of bass traps (type or bandwidth) should one position in different positions in a control room?



Thanks :-)
Adam

[Ethan Winer]

Originally Posted by AdamZuf

But it seem you disagree about the back wall issue, am I missing something, or do you disagree with this common opinion?

Not sure what you mean, but I don't disagree about the rear wall being useful for bass trapping. My company's QRD diffusor is meant for rear walls, and it's as much a bass trap for low frequencies as it is a diffusor for mid and high frequencies.

Would you absorb or diffuse next to the rear wall?

It depends on your budget. Good absorption costs less than good diffusion, but this room is definitely large enough to benefit from diffusion.

maybe it's wise to mix diffusion and absorbtion in the rear part as well..?

That's exactly my point above.

Do you think that the bass traps on the side walls will be sufficient? quite a lot of them

The more bass trapping you can manage, the better. Always.

Considering that in the work position I should target the 75Hz related problems and in the sofa position (below the lamp) I should target the 62Hz related problems, do you think I should position the traps accordingly?

Without being there it's difficult to say. Again, I favor the brute force approach - as much broadband bass trapping as possible.

Do you have any advice about what type of bass traps (type or bandwidth) should one position in different positions in a control room?

There's a huge amount of advice about this at my company's site, linked under my name below. That advice applies regardless of whether you plan to build or buy, or what commercial brand or DIY design.

--Ethan

[AdamZuf]

Originally Posted by Ethan Winer

Not sure what you mean, but I don't disagree about the rear wall being useful for bass trapping. My company's QRD diffusor is meant for rear walls, and it's as much a bass trap for low frequencies as it is a diffusor for mid and high frequencies.

So I gather that you meen that narrow band bass trapping is relatively less efficient there then in other spots?

It depends on your budget. Good absorption costs less than good diffusion, but this room is definitely large enough to benefit from diffusion.

And regardless of price? I just do a skyline-like diffuser from scraps of wood from the carpenter- I'm don't think they will go by the skyline formula as I can only measure the height of the scraps, but it still will give some diffusion, and in very high frequencies that's for sure! Even if I don't bother with cutting with my table saw, and just break some of the pieces by the hand to different sizes, the broken tips will further diffuse higher frequencies I think.

Without being there it's difficult to say. Again, I favor the brute force approach - as much broadband bass trapping as possible.

The Mondo Trap, how is it built? Is it more efficient then a Helmholtz resonator?
It is the most practical that I stay with Helmholtz resonators in the top corners, as I want to add diffusion elements over the panels, so I can't tune the resonant frequency according to panel width. I can make them very broad band by creating a very narrow port length.

Considering broadband approach, would you still stay with the low values I wrote for center frequencies? The low range of the resonator will be in unheard frequencies, but yet, the biggest problems lie down there (62Hz, 75Hz) and logic tells that they must be the center of a broadband bass traps, which will still treat the first and second harmonic in a reasonable degree, I assume..

Adam

[AdamZuf]

What's the thinest material you would use for constructing a helmholtz resonator?

What is the widest band type of bass trap? (the Helmholtz types, panel absorber, regular corner absorbtion etc)

[Ethan Winer]

Adam,

I've never messed with Helmholtz designs so I don't have anything to offer except that I generally avoid them.

Yes, MondoTraps are more effective in 99 percent of the rooms GS type folks have.

If you are determined to try a tuned trap, you might want to ask at John Sayers' site because he has more experience with them. Here's the link:

Recording Studio Design :: Index

--Ethan

[rlantis]

Originally Posted by Tim Farrant

I would build a slat resonator, running 209Hz through my little resonator calculator, I come up with this;

Slat thickness = 19mm (3/4")
Slat width = 110mm (4.3")
Slot between slats = 5mm (0.2")
Depth behind Slats = 150mm (6")

Hope this is some help.

Tim.

FYI, other than the correct formula per "The Master Handbook of Acoustics" by F. Alton Everest, (see discussion Acoustics Forum &bull; View topic - SLAT TYPE HELMHOLTZ RESONATOR FORMULA )
unfortunately most sources for this calculation have a "+" where there should be a "*".

I suppose this is why your calculated resonance is not correct? Your listed dimensions would actually yield a calculated 694 Hz resonance. You'd need to change the slot width to only .059" in order to hit 209 Hz. Considering the near impossibility of maintaining enough precision (.001") with such a dimension to force the desired high-Q resonance, a more practical option might be
(Slot Width Inches) r = 0.118 (3.0 mm)
(Slat Width Inches) w= 4.000 (101.6 mm)
(Effective Depth of Slot Inches) d = 0.400 (10.2 mm)
(Airspace Depth Inches) D = 6.4 (162.6 mm)
(Resonant Frequency Hz) f= 209

[rlantis]

Originally Posted by rlantis

Your listed dimensions would actually yield a calculated 694 Hz resonance.

Sorry about that... pasted the wrong spreadsheet cell...
the wrong formula doesn't calculate to 694 Hz... it actually yields a 196 Hz resonance.
Just how narrow is your 209 Hz notch?

[Manzanares&Sanmi]

Hi: What kind of program to design your studio use? Thanks a lot

Originally Posted by AdamZuf

I just discussed this today with my acoustics teacher and he said that the more broadband the Helmholtz resonator, the less it is efficient in the center frequency I'll try to target. Do you agree?

Here are a few measurements I made after absorbing first reflections on walls... The results are quite similar with sweeps or steps, these are the step measurements:



My Genelec 1029A where I sit:


Selah Audio's Carnelian where I sit:



Selah Audio's Carnelian at about 2.5m:


The main trap I'm considering is for where I sit, tuned for 75Hz, and another tuned for 62Hz, hopefully both will affect the peaks/nulls at the harmonics. I'll also cover most of the area around me with Rockwool like material (see below), which will be spaced 10cm from the wall (that's for sure for the side walls), creating a kinda linear NRC around 0.9 down to ~250Hz, according to manufacturer (the panels I've used for measurements were the old ones I made from fivex 1cm thick fiberglass boards, spaced from walls not in a very consistant manner :-) but I think it's good enough to know where the problems are...

My current plan my mixing studio (21.10.2007):




RED: bass traps, maybe with some diffusion elements
PURPLE: absorbers, spaced 10cm
BLUE: Skyline like diffusers

The big green boxes are the desk and monitors (ignore the green box on the left). The wall behind them is actually a window, and I wish I could make something that can move easily on a curtain rail so I also get some air inside from time to time...Probably it would be wize to make bass traps and first reflection absorbers on that wall, but I can't apply them on the corners of it. I do have my mains behind the desk (mounted on big PA horn subwoofers I don't use.. wonder if they make some difference...), but still got about 50cm space from behind them to the window.

There is also the wall I've hiden in order for you to see, here is a render with the treatment:


The sofa (where me/the client enjoys the music, and where I measured the Carnelian speakers, shown above) is about below the lamp (that tube you see models it very accuratly).

Advice on what to do with the bass traps and the window...

[DanDan]

I built a Helmholtz trap once.
Acoustics Forum &bull; View topic - Helmholtz formula, is this incorrect?
It was based on one featured in the MHOA.
In that book a single Trap was claimed to have a spectacular effect on a 47Hz mode. I checked with the author who claimed the experiment was quite genuine and successful. Based on this assurance I went ahead and as best I could exactly duplicated the unit, but for my own mode, 35 Hz.
My single, ( tunable) trap had no measureable effect whatsoever.
I believe such devices need to be in multiples to function at all. e.g. a whole wall of them.
DD

[whitesnake]

Originally Posted by DanDan

I built a Helmholtz trap once.
Acoustics Forum &bull; View topic - Helmholtz formula, is this incorrect?
It was based on one featured in the MHOA.
In that book a single Trap was claimed to have a spectacular effect on a 47Hz mode. I checked with the author who claimed the experiment was quite genuine and successful. Based on this assurance I went ahead and as best I could exactly duplicated the unit, but for my own mode, 35 Hz.
My single, ( tunable) trap had no measureable effect whatsoever.
I believe such devices need to be in multiples to function at all. e.g. a whole wall of them.
DD

Dan, Can you post some pictures with measures of your Helmholtz resonator. I tried to create this type of device, but I can detect his resonance frequency.