Helmholz resonator contruction

[Robert Wooden]

I will start with the fact that I am not sure which forum to post my question to? So, here we go. I am building a Helmholz resonator to absorb frequencies below 250Hz. I am a newbie at this and I am still searching for knowledge. I understand the the Helmholz principle of adjusting the duct length to absorb the frequency it is designed the atone. The duct and the chamber behind it is a complicated formula.
So, what I am designing is a sound deadening (very small) building to house a portable generator for use during power failures. (We get too many here and I have thrown away too many steaks because the freezer thawed.) I know from experience that specific mineral wool insulation are great sound absorbers above 250 Hz or so. Not the best below 250Hz. Sound analysis of my running generator levels pike at about 130 Hz. A sound level that Helmholz resonator works best at. So I read. So, a well insulated (small) building sitting over my portable generator will muffler many of the higher frequencies.
I found a YouTube video of a sound engineer who built a generator shed that included a Helmholz resonator incorporated into it. His video showed sound levels averaging 8-10 decibels below many other generator shed videos. I was intrigued. In further study I found a spreadsheet of formulas to design Helmholz duct sizes and chamber sizes. Similar formulas are used in the construction of stereo speaker boxes. So, I post my questions here to see what anyone might think.
Here they are:
Does the chamber (behind the duct) have to be a specific shape? The formulas I found where for a cylindrical shape chamber. I converted it for a square chamber volume. Speakers are square, therefore their chambers are square but, I cannot find a specific answer for cylindrical or square.
Does the chamber need to be lined with acoustic insulation? Speakers are always lined with various types of insulation so I am fairly comfortable that lining the chamber is correct.
Finally, if this is the wrong forum then where should I post these types of questions?
Thoughts anybody?

 

[Carl Klinkenborg]

Blocking sound and absorbing sound are two very different animals. I think I would be correct in stating that you don't much care about the sound level within the enclosure, but what escapes it... Whilst absorbing sound makes the room/enclosure itself inherently 'dead', sound level is unlikely to be reduced more than about 4-6dB by absorption alone. Your efforts would probably be better spent in trying to block the (broadband) noise rather than starting with absorption.

Wall construction plays a considerably bigger part in blocking noise than does absorption around the source. To a certain extent, mass is king (some of my house walls are 1m thick stone/lime, so I can attest to their effectiveness!) however 'lightweight' double-skin walls with absorption inbetween can be very effective.

A 10kw generator can range from about 70-90dBA so the scale of the noise reduction required is considerable - do you have specs. on any you are considering? You would need probably 50-odd dB isolation; not that easy in an enclosure which cannot be completely sealed.

Helmholtz resonators can be very effective at absorbing sound but have a high Q if undamped, but damping reduces their effectivity.
Membrane absorbers could be a consideration, but you would need to know the noise sound spectrum of the generator within the enclosure before you could even begin to address absorptive measures.

A further consideration is to isolate generator from the floor slab to reduce ground-borne flanking noise entering your house.

Finally, the matter of allowing air in and exhaust/heat out of the enclosure is going to pose considerably more of an acoustic problem than any of the above! Whatever the solution, the open-ended ducting is going to add considerable size to your "(very small) building...".

 

[Robert Wooden]

Thanks for the reply, Carl.

You are correctly interpreting my un-educated (sound knowledge) descriptions. I am in unknown territory when it comes to sound deadening/absorption. The idea is to reduce the noise the generator creates at ten (10) feet or so away, thereby reducing the noise pollution my neighbors are so politely tolerating. I believe (in this video I found) the creator of the video's logic thought was if he could remove the frequency inside the box it will reduce the Db sound resonating through the box.

My shed (box) construction plan match most of your points you expressed in your reply. I appreciate your suggestions. (I am still in design mode for now so it would be easy to make any needed changes.) The sound goal will be to get down to 50-60 decibels or quieter at a distance of 10 fett or so. Once the specific construction design needs are finalization I will discuss specific details in a later thread when I get the Helmholtz questions worked out.

A massive amount of air (800-1200 cfm) needs movement through the "box" with air cooling inlets and outlets dampened by baffling.

I am not sure what you meant by "have a high Q if undamped", specifically the reference to "Q", what is that, please?

What is the better shape for the chamber rectangular or cylindrical?

Does the chamber need to be lined with acoustic dampening material (i.e. insulation)?

 

[Art Welter]

A massive amount of air (800-1200 cfm) needs movement through the "box" with air cooling inlets and outlets dampened by baffling.

The "box" itself will have resonant modes "room modes" .
The inlets and outlets make the box a Helmholtz resonator.
The inlets and outlets have pipe resonances.

I am not sure what you meant by "have a high Q if undamped", specifically the reference to "Q", what is that, please?

A "high Q" is a narrow frequency band, say 1/6 octave, a Q of 8.7, a 1/3 octave is 4.3, one octave 1.141.

Q factor vs bandwidth in octaves band filter -3 dB pass calculator calculation formula quality factor Q to bandwidth BW width octave convert filter BW octave vibration mastering slope dB/oct steepness EQ filter equalizer cutoff freqiency - sengpielau

bandwidth in octaves vs Q factor -3 dB band filter pass filter quality factor calculator calculation formula BW bandwidth conversion and converter convertor conversion width bandwidth octave slope dB/oct steepness convert filter BW resonance curve octave vibration mastering EQ filter equalizer...

sengpielaudio.com

What is the better shape for the chamber rectangular or cylindrical?

Rectangular works fine for low frequencies and makes construction easy.
That said, Sonotubes (concrete forms) are cylindrical and relatively cheap.

Does the chamber need to be lined with acoustic dampening material (i.e. insulation)?

Yes, since you want to absorb the Helmholtz resonance.
The dampening material will change the virtual box volume.

Helmholtz Resonator Calculator

Find the resonant frequency of an acoustic cavity with out Helmholtz resonator calculator!

www.omnicalculator.com

You might fit bass traps with adjustable "ports" in each room corner.

 

[Robert Wooden]

Thank you Art Welter. Your post was very informative.

Thanks for the Sonotube suggestion. That's a great idea. I invision Sonotube with MDF end caps adhered onto each end then the duct in the center of one of the ends. Then, poops into my head what about PVC for smaller diameter chambers? The smallest Sonotubes are 6" in diameter. PVC I have found as large as 12" diameter. For budget sake most home centers offer PVC up to 4" dia. in short lengths and I have seen (personally) Sonotube are 4 foot lengths in home centers.

Rectangular works fine for low frequencies and makes construction easy.
That said, Sonotubes (concrete forms) are cylindrical and relatively cheap.

I am looking to absorb frequencies below 250 Hz or so. With your suggestion of Sonotube, are you thinking that a round chamber shape may be slightly better performance than a square or rectangle chamber?

I know I can absorb higher frequencies (250+ Hz) with an acoustic mineral wool insulation that is going to line the inside of my small (4 ft. x 4 ft. x 4 ft.) building that surrounds the generator.

Any thoughts on how to calculate the space displaced inside a chamber by the sound absorption material?

 

[Art Welter]

I am looking to absorb frequencies below 250 Hz or so. With your suggestion of Sonotube, are you thinking that a round chamber shape may be slightly better performance than a square or rectangle chamber?

I don't really know.
Ultimately you are trying to turn sound energy into heat, and the movement of the sound absorber walls can contribute to that, so depending on the design, the cylinder, being stiffer per material thickness may be less effective.

Any thoughts on how to calculate the space displaced inside a chamber by the sound absorption material?

No, and I'm not sure that a strict Helmholtz based approach is best for anything other than a fairly narrow band (high Q) frequency absorption, which is unlikely in your case.

As GIK Acoustics (maker of the TurboTrap Pro) puts it: "The ways in which density affects performance of absorption materials is counterintuitive. After long development and testing, we found the optimal combination of two very specific material densities, arranged in specific ways, offering more effective deep bass absorption than single-density materials used in most conventional bass traps. "

You might try a better exhaust muffler rather than trying to externally reduce the low frequency noise.

 

[Robert Wooden]

Thank you Art Welter for your time.

In general, after these conversations, my design is moving in a direction I am comfortable with. Next, I need to look into local zoning requirements to see what the city I live requires before building this "box".

As this design evolves I am anxious to proceed, weather permitting and holiday travel mixed in.

I am going to begin working on drawings and plans. More soon.