Broadband Helmholtz resonators?

Hi, I searched for this but found no info. On the Helmholtz resonator calculator in http://www.mhsoft.nl/AcousticTreatment.asp , it turns out that you can make a resonator very very wide range by using a very small port length. I talked with my acoustics teacher (on which I tend to trust, but nobody knows everything), and he told me that the efficiency of the Helmholtz resonator goes down when you broaden the bandwidth. This makes sense, an analogy I can think of is that no matter how you play with voltage or current, the power limit is the same. However, I would like to know someone else's opinion. I understand that the boundries of the ideal resonator are totally stiff. That's why I thought of something like the sketh attached, using 2 or 2.5 cm MDF for the frame and 0.35 cm masonite for the port, and add stifness to it with these angles sold for closets. Please tell me what you think Thanks Adam

Anyone ?

Adam, the drawing doesn't make sense to me - so try to do it like a cad drawing would be - give me a plan view and a section through the trap - what I am looking at doesn't look like a helmholtz trap to me at all........ Rod

your prof is right, the use of damping inside the trap increases the bandwidth which results in less efficiency at the target design frequency. but depending on your need, broadband is more likely to help you except in the case where you have a single specific problem frequency. as far as materials, my understanding is that its more a matter of the dimensions that cause the effect rather than the stiffness or frame rigidity. the ridigity and mass help avoid potentially other effects like resonances, flaking, loss of effeciency due to vibration changing the relationships of the parts, etc. in your diagram, it looks like a "bottle" type resonator, and the brackets would help hold the port (mouth) in place but you might be better served by using some 2x2 instead as you can screw and glue it. this may be safer than potentially having metal hardware come loose. in theory the calculations assume total stiffness, in practice this doesn't happen but using reasonably stiff materials the effect wouldn't be material. lastly, check your calculations on the mouth... what frequency do you think you are targeting?

rod gervais wrote:

Adam, the drawing doesn't make sense to me - so try to do it like a cad drawing would be - give me a plan view and a section through the trap - what I am looking at doesn't look like a helmholtz trap to me at all........ Rod

Therender doesn't include the main panels that enclose the box, with proportions roughly as a book- kinda thin.

gullfo wrote:

your prof is right, the use of damping inside the trap increases the bandwidth which results in less efficiency at the target design frequency. but depending on your need, broadband is more likely to help you except in the case where you have a single specific problem frequency.

The question is, how unefficient does it get.

in your diagram, it looks like a "bottle" type resonator, and the brackets would help hold the port (mouth) in place but you might be better served by using some 2x2 instead as you can screw and glue it. this may be safer than potentially having metal hardware come loose. It's plastic, and I don't screw, I glue with hot melt glue gun. Never came loose to me, but I never build a bass trap OTOH...

lastly, check your calculations on the mouth... what frequency do you think you are targeting?

I have problems with the 62 and 75Hz and some of their harmonics, hope to fix the harmonics once the foundamental is fixed. But I can also go broadband. I should probably note that behind the speaker I have my old ugly fiberglass panels (to be covered with a curtain), stacking 4 panels a side forms a kind of a bass trap... but I won't pt them in a visible place, and still there are these problems, I hope to do it elegantly with lots of flat or corner resonators. On the resonators I'll place either absorbers or diffusers. I need advice on how to do the bass trapping thing in regard of quantity, placement and of coarse, tuning. I added a sketch of the room. The green rectangle is my desk. There's also a listening sofa in the middle of the room in which the the bump around 62Hz gets really serious, and I wonder which tuned trap should I put where, in case I'm going for the narrower band versions. Anyway, I do wish to go for minimal amount of trapping. The reason I wish to go for Helmholtz is because I don't want to suck further high frequencies from the room, I'm much more into the diffusion concept into the rear part of it.

the mode you're seeing might be related to your room height. how high is your room? you might be better served with a panel trap to get the frequencies this low. use the rigid fiberglass for corner traps, first reflection points, and/or a cloud. the slat resonator will be pretty large to get down to 60-70hz. you can also scatter/stagger your wall and ceiling absorbers so they have gaps between them. this improves edge effects (assuming they don't have solid edges) and increases diffusion in the space.

The room is 2.6m high, and the calculation says 66Hz, and its harmonics: 132,198 and 264Hz, actually I can't see this pattern in my measurements, It's more likely the 30Hz of the 5.67m creating a problem in the 60Hz region- I forgot to mention that the bottom left side of the room becomes the corridor - a long one. I don't know if I can seal it right now, do you think it's critical? Is there a need to performt a measurement at its end to see if it corrilates to the problems in the room? A panel trap- you meen a sealed panel that resonates, right? What's the bandwidth of these things? Anyway, you can see that the Helholtz resonator (and NOT a Helmholtz slot resonator, if anyone here has confusions) can be very small and hit the target well, both as broadband and as specific frequencies shooter. Why not use it? About the panel trap- I'm not sure if the local wood suppliers can tell me something reliable about the exact weight of the woods- plus I can't control the frequency and Q- notice how with the Helmholtz resonator I always stay in the same box dimentions, I just play with the tube length and diameter, and the traps are standart size and thin- very elegant. I can make all of the traps and start experimenting, I'm sure I can learn a lot from this! I can even seal it and it will become a panel trap :P Adam

A big question is how much of bass trapping is adaquate? I heard more then once that the more you have, the better. I can go nuts and fill parts or my whole ceiling with resonators, and it will cost indeed, but the question is how much is adaquate. Well, probably I'll start with the corners. I do have quite a bit of fiberglass leftovers (not panels, loose stuff), The question is whether I should use these or build helmholtz resonators. Hmmm... It makes sense that in the "top" part of the room (where I mix) I'll use the fiberglass in order to better the RT60 measurement, that is the two corner broadband absorbers parallel to my desk (on wall/ceiling junction), or should I do more of the broadband type in the room? How's that plan for a broadband absorber? The red symbols the velvet covering, so I added the wooden strips so there will be no bumps and stuff from below. I can also add carpet (How's it called, "felt"?) on top of the strips to avoide bumps and get a smooth surface. Good plan?

I talked with my acoustics teacher (on which I tend to trust, but nobody knows everything), and he told me that the efficiency of the Helmholtz resonator goes down when you broaden the bandwidth. This makes sense, an analogy I can think of is that no matter how you play with voltage or current, the power limit is the same. However, I would like to know someone else's opinion.

Sorry, no opinion. Have a look at pdf page 13 in BBC RD 1992-10 Lab test results. Andre

Thanks for the article! The crucial mistake they made about evaluating the Helmholtz resonator is using a port too long. It narrows the Q. I think I'll stick to the Helmholtz resonators, thanks. Adam

just curious, how much volume does your proposed Helmholtz resonator have? generally speaking, passive absorption requires volume in order to absorb low frequencies and the Helmholtz resonator will be no different.

I'll probably broadband with few resonators, I assum you're talking about total volume? I still don't know how much is sufficient, so I'll start with 4 helmholtz on ceiling/wall junction along the side walls and back wall, and I also have 2 porous absorbers in the fronet corners and will build some more. Interesting what will be more efficient, I guess that experimenting with broadband Helmholtz will yield better results that will compliment regular absorbers, which leaves a lot of low mid, and I hate that non linear, over absorbed sound. Using porous will only help that disgusting sound (-: Maybe if the Helmholtz thing works fine, I should blast the hell out of the place with these on all corners, and balance them with some more absorbers/diffusers later, starting with absorbers on all first reflections (except back wall) Very interesting yet time consuming, this project (-:

AdamZuf wrote:

About the panel trap- I'm not sure if the local wood suppliers can tell me something reliable about the exact weight of the woods- plus I can't control the frequency and Q- notice how with the Helmholtz resonator I always stay in the same box dimentions, I just play with the tube length and diameter, and the traps are standart size and thin- very elegant. I can make all of the traps and start experimenting, I'm sure I can learn a lot from this! I can even seal it and it will become a panel trap :P

the thing is, the BBC report included the calcs for volume as well as the effects of box resonances so you may have a panel trap regardless. as far as panel traps, you cut a piece of the material 12x12 and weigh it... Q is controlled by the damping, distance of the insulation from the panel (closer is lower Q), and frequency by panel size, box depth, panel thickness, etc. overall though, you will need to put the traps where they work most effectively. preferably across the tri corners, and then across whatever other corners you need to. one good approach is poly cylinders across the corners, floor to ceiling. diffusion and bass trapping.

I'm a bit confused about panel absorbers. I can see that the leading calculation values are the panel mass and the depth of the sealed air cavity behind it. However, in Acoustic calculator at least, the panel size is "recommended". What does it meen? Is this related to a corresponding resonant frequency of the inner volume? How should I apply and use the panel size factor to my benefit? And how can I accuratly calculate a corner panel trap?