Most people refer to bass traps as friction energy absorbers. Audio energy is transformed into heat. Helmholtz do not work this way.
I’m fully aware of confirmation bias. This is why I measure and measure at least 4 times.
This is why I measured 4 times and tossed out the anomalies for every change I made.
At high end audio shows you can reliably identify rooms that sound bad by counting the Number of empty beer and wine bottles lying around. Empty Wine and liquor bottles make excellent Helmholtz resonators. You just need to find precise locations for them. The high end has many types of resonators available, including Mpingo discs, crystals, tube traps, Tibetan singing bowls, tiny little bowl acoustic resonators, to name a few.
An easy way to find peaks of high SPL (at least 6 dB above average room SPL) in the room is use an empty sealed medium size box 8x8x8 or similar that has some holes punched in one side and a test tone 315 Hz or 200 Hz for example. Or lower frequencies if you prefer. Then while the test tone is playing at moderately high volume walk around the entire room and when you feel the box vibrate that is where there‘s a peak. Room corners, behind speakers, behind listener position, etc. Find room echo, standing waves, first and second reflections, what have you.
An easy way to find peaks of high SPL (at least 6 dB above average room SPL) in the room is use an empty sealed medium size box 8x8x8 or similar that has some holes punched in one side and a test tone 315 Hz or 200 Hz for example. Or lower frequencies if you prefer. Then while the test tone is playing at moderately high volume walk around the entire room and when you feel the box vibrate that is where there‘s a peak. Room corners, behind speakers, behind listener position, etc. Find room echo, standing waves, first and second reflections, what have you.
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@geoffkait : good and simple idea, worth the test... Some kind of "Acoustic Resonance Detector". 😎
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If you are skeptical and don’t expect to hear any difference wouldn’t that be the opposite of confirmation bias? 😲
They are not traps they are resonators tuned to a particular canceling frequency and it is calculated by volume and taper.
They don't magically jump into the hole the hole cancels at a given rate per number of resonators in a given area.
Placement of a resonator is no different than putting batting in the bottom of a cabinet with a brick on top of the batting.
It has to be distributed throughout the enclosure (or room) to work properly. Concentration of openings in a given
area will tune that area with TAPERING effect from that position. You can tune 10 frequencies in a 10cf enclosure
or tune one frequency 10 times. 10 times being and extremely aggressive taper.
EX 120-140hz and 130 being the target or 125-135 and again 130 being the same peak but a different taper.
Nothing works better on peaks in a lively room. 20-300hz is the key to room semi-harmony and distortion.
Tame the bottom and the rest will be a lot easier to work with if you have to, at all. How picky are you?
I'm 68 how picky are my ears? 😎
Regards
I’m pretty sure he’s just being an ***. He described building a Helmholtz and walking around waiting for it to vibrate when a certain frequency stimulates the box. It’s obvious he doesn’t understand how they work, and that they have to be tuned specifically to a specific frequency not only that the frequency that they are tuned at has an incredibly sharp Q and not a range of frequencies.
I have never been to an audio show, but from what I gather, very few rooms are actually treated well and Helmholtz resonators are used again at a very specific problem frequency that you have. There’s no way to know what that problem frequency may be in if you don’t know the room, you will be presenting in. It would be much better to bring friction obsorbers to target wideband absorption if you’re not sure what problem frequencies you will have.
I will run some more before and after tests, but I have a feeling his mind is made up regardless of the facts that are presented to him.
Your statement regarding confirmation bias is incomplete. In this example you give so you hear a difference or do you not? Otherwise there’s no confirmation in the confirmation bias. Just bias…
Your measurements are very close to mine, with similar Helmholtz resonators (boxes, in my case) - after the treatment not much initial SPL reduction of the room resonance (about 3 dB), with better waterfall diagram. It is OK, do not push further - active area (pipe diameter) is just too small to be effective (there are membrane resonators with big area and they are very effective). Just use mineral/glass wool instead of polyfill.
Thanks, Sonce. I was thinking the effective area was too small to make an effective difference. So, I figured if I just had more of them....I will look into membrane resonators but that seems like a much more difficult build than cutting a pvc pipe in half and adding a neck to it, lol! Any good diy instructions to follow online you know of?
Helmholtz resonators are fairly broadband, so you actually don’t have to tune them very precisely. You just don’t need very many large resonators, however they need to be placed carefully. The secret Is the nozzle. You can experiment with bottles to you know, get the hang of it. As Bob Dylan says at the end of his records, good luck to everyone.
Not detailed DIY instruction, but very useful:
chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://dogbreath.de/misc/PlaneAbsorberResonance.pdf
Why are you here? What's your purpose? Are you really this bored? For those that don't know. HHR are extremely high Q, need to be large and can go anywhere where there is high SPL in the very narrow problem frequency.
Helmholtz Resonator Calculator, volume is only one variable, guys. Let’s take the tiny little bowl resonator for example. A few of those 1” wide acoustic tiny bowl resonators improve bass response in the room considerably. Does that surprise you? They also reduce RFI. Does that surprise you, too?
https://www.omnicalculator.com/physics/helmholtz-resonator
https://www.omnicalculator.com/physics/helmholtz-resonator
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Oh well... 🤔
I was just speaking about the interest of that idea only as a sensitive - but effectively aperiodic - resonance detector, in the sense that if it vibrates there, chances are there's an acoustic energy presence there. Possibly a simple inflated balloon would even be a more sensitive detector for that purpose... Of course, afterwards, you need to qualify the resonance in frequency to built your tone pipe trap, but at least, you already have an idea where to place it for better efficiency...
Again, it's an empirical unguaranteed assumption from me, OK ? 😉
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Fair enough. Or just spend the money on an a $100 mic and know what frequencies are doing what where with free software like REW.
Yes... Less empirical, for sure ! 😉
Now that I have a new computer, my audio softwares are all obsolete now, and don't want to operate on an emulated machine either... I have an ARTA license, an external soundcard, still my old analogue measurement acquisition box (to be modified for a new setup with the new machine), two measurement microphones, a level calibrator, plus there's new interesting softwares like REW and many other...
I will have to reinvest time to learn to set up all that stuff again, and its quite long, and I am lazy 😒... I shouldn't !!! 🙄🙂
Pictures of the past...
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Love it man! You've been at this a while! Some nice Cornwalls there. Always wanted to try some. Closest I got was Chorus Speakers. You'd pic REW up in a flash! Very intuitive.