# What's the frequency, Hermann?



## MarkusBonk (Jun 5, 2010)

Hi All,
So, while I am at it - how do you measure the resonant frequency of a Helmholtz resonator? Seems to be a bit of a paradox to me trying to measure the resonant frequency of something you are trying to tune to a room mode - where you will have a peak anyway - and if you try to measure it where you don't have a peak in the room probably nothing will happen.

On the last two I built I put the microphone inside started RWE rta and thumped them.
The results seem to be fairly accurate for the resonant frequency:









I am not sure about the Q though.
Anyone have a better method?

Markus


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## bpape (Sep 14, 2006)

Thumping a Helmholz will tell you little to nothing other than the resonance of the box and front panel itself which is not where it's tuned. The formulas for building either slotted or patterned hole Helmholz resonators are very well proven. If you build them as specified, they will tune to what the formulas tell you.

Bryan


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## MarkusBonk (Jun 5, 2010)

Its a box, all the walls have the same thickness and material. It has one hole. I have a measurement, which I have posted. You haven't told me how to measure just said I am wasting my time and should blindly follow some formula.

Now, as I see it, I have applied an impulse to the resonant construction, which will, after the chaos has passed away leave it swinging at its resonant frequency. Below is a listening position measurement and guess what I see...









But like I said I want to know how to prove its working correctly. A good engineer will know that after you have designed, and built something the next step is to check that it is indeed doing what is it supposed to.

Markus


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## bpape (Sep 14, 2006)

The only real way to test something like this is to do a before and after measurement of the room around that frequency to see if there's an impact being made. That also assumes that what you've built has enough absorption capability, regardless of frequency, to be measurable.

Wasn't trying to be a jerk previously, just that we know very well and people HAVE tested the results of properly constructed treatments per the existing formulas. 

Bryan


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## MarkusBonk (Jun 5, 2010)

This is very encouraging: how did those people determine the resonant frequency of their resonators?


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## bpape (Sep 14, 2006)

I have no idea other than sticking a mircophone down into the port and measuring max resonance. For smaller ports that's going to be very difficult.

Bryan


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## event horizon (Nov 12, 2009)

Perhaps i'm being slightly simple here, my apologies if that is indeed the case 

I hope i'm correct in thinking that what you are building (or have built) is effectively a ported speaker but with no drive unit, so just a box & helmholtz resonator. You can easily design this using WinISD & any driver you care to choose that will likely work with the required size of box.

You can then experiment with box Q by seeing how the peak of the resonance (rear port gain) varies according to box size & tuned frequency. If you increase the box size for a given tuned frequency the Q will be higher & affect only a narrow band, make it smaller & the Q will lower & it'll have a broader bandwidth.

Hope i was right & good luck :T


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## bpape (Sep 14, 2006)

That's absolutely correct - IF you're building a single box with a single hole (port). Some Helmholz designs us multiple holes of a specific diameter through a specific thickness and spaced a specific distance apart (hole pattern).

In those cases, the thickness of the front plate becomes the port length (plus a little fudge factor for how it actually acts. )

Bryan


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## MarkusBonk (Jun 5, 2010)

bpape said:


> I have no idea other than sticking a microphone down into the port and measuring max resonance. For smaller ports that's going to be very difficult.
> 
> Bryan


I see we have been talking at ends. Its a box with one port. So now the big question: how do you excite it?


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## MarkusBonk (Jun 5, 2010)

Hi Mark

You have put it in a nutshell. The difficult bit is measuring the resonant frequency. You see the box is in a room which insists on changing the frequency response of any signal put into the room. Then the box is built of wood, which is as far as I know a good conductor of sound - means I will get sound passing through the box to the inside as well as the sides vibrating...

Although I have never read any post anywhere on how to measure the resonant frequency I cannot imagine this is a new problem for us few who haven't got an anechoic chamber.

Markus


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## bpape (Sep 14, 2006)

MarkusBonk said:


> I see we have been talking at ends. Its a box with one port. So now the big question: how do you excite it?


Play tones 1Hz apart with a mic in the port with all tones at the same level and see where you get the excitation.

Bryan


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## MarkusBonk (Jun 5, 2010)

bpape said:


> Play tones ... with all tones at the same level


Isn't that a catch 22 - if I could play tones all at the same level in the room I wouldn't need an absorber

Markus


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## bpape (Sep 14, 2006)

What I mean is don't adjust the volume. Ideally, you'd take a speaker, the receiver, and the panel outside so the room has nothing to do with it.

In room, just do it over by a boundary so you get max buildup from pretty much every note to minimze the variation. If the mic is IN the port, difference in where the tuning is will overwhelm any room variations - especially when close to a boundary where everything is boomy.

Bryan


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## event horizon (Nov 12, 2009)

MarkusBonk said:


> Hi Mark
> 
> You have put it in a nutshell. The difficult bit is measuring the resonant frequency. You see the box is in a room which insists on changing the frequency response of any signal put into the room. Then the box is built of wood, which is as far as I know a good conductor of sound - means I will get sound passing through the box to the inside as well as the sides vibrating...
> 
> ...


Hi Markus, do you happen to know the internal dimensions of this box & the diameter of the port & it's length? The internal port diameter will help & the external diameter will let us know how much volume of the box it has taken up. We can then calculate the absorbers resonance frequency :T Wood thickness would just put the icing on the cake...If you know the wood thickness then external dimensions would do just as well.

What you need is a function generator my friend, you'll then be able to test the thing just like Bryan suggests by altering the frequency but not the volume by small increments 

Bests, Mark.


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## MarkusBonk (Jun 5, 2010)

Hi Mark,


event horizon said:


> ... do you happen to know the internal dimensions of this box & the diameter of the port & it's length?


Certainly. General shape is a prism with the hole in the top.
Outside dimensions are 0.43m * 0.43m * 1.7m; thickness of wood = 0.018m i.e the prism has an internal volume of 0.5 * (0.43-(2*0.018))^2 * (1.7-2*0.018) = 129 liters. 

The hole radius is 0.046m and its length is the thickness of the wood = 0.018m

Markus


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## event horizon (Nov 12, 2009)

129L box with a port diameter of 9.2cm & a port depth of 1.8cm gives a resonance at 42.5Hz :T


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## MarkusBonk (Jun 5, 2010)

bpape said:


> If the mic is IN the port, difference in where the tuning is will overwhelm any room variations - especially when close to a boundary where everything is boomy.
> 
> Bryan


Here are two measurements done with a 20-200Hz sweep using REW with the resonator standing in a corner:


Measured inside the body of the hh-resonator



Measured in the neck of the hh-resonator port

What I think is immediately obvious is that all those peaks do no originate from the hh, but some are simply the room. Now is the hh resonant frequency coincides with a room mode you simply won't be able to tell which one is from the resonator.

Using sine waves won't help because they will simply follow the SPL curve in the plots above - I have already tried that - REW has a signal generator built in.

Looking at those curves what would you say was the resonant frequency and Q factor and why?

Markus


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## MarkusBonk (Jun 5, 2010)

event horizon said:


> 129L box with a port diameter of 9.2cm & a port depth of 1.8cm gives a resonance at 42.5Hz :T


Only trouble is 42.5Hz doesn't coincide with any of the peaks, as you can see from the previous graphs.


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## event horizon (Nov 12, 2009)

True, it appears to correspond pretty strongly with two rather large dips, one in each graph :scratch:

Don't ask me what's going on here as i have never built something like this  However if we can find out what actually is happening i'll probably attempt something similar to operate at 40Hz where i have a particularly nasty room mode :gulp:

In fact looking at the first graph the dip appears to be at a slightly higher frequency than in graph 2. Graph 2 is of the microphone in the port so it says & this would effectively lower the tuned frequency due to making the port smaller, it appears to actually have done so on close inspection.

I'm not sure what we are looking for here but something is deffinately going on around the 42Hz region


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## MarkusBonk (Jun 5, 2010)

event horizon said:


> Graph 2 is of the microphone in the port so it says & this would effectively lower the tuned frequency due to making the port smaller


The mic has a cross-section very similar to a slightly thicker biro. Both measurements have the mic reducing the port cross-sectional area by about 0.8 cm^2, which reduces the effective radius to 0.0457m and is negligible.

Playing with the numbers I think the tube length correction you are using is incorrect - its too low - I think you should be using the double flanged values. But even with a higher correction the results of the calculator are not satisfactory - there isn't a peak there.

So, as I see it, there is nothing else to it other than measure the resonant frequency and the Q. Its just a question of how?

Markus


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## event horizon (Nov 12, 2009)

Well, other than doing something similar to the old bottle trick, blowing a constant stream of air accross the port the only way i can think of is to install a driver into the enclosure  Scrub that...

I'm not sure if you could excite it by placing a drive unit at a 90 degree angle from the port & directly next to it to excite the thing, it might be worth a try. At least if that worked you'd get a better idea of the Q of it as you'd be able to alter the frequency with something like a function generator.

Or you might get away with facing a driver into the port but keeping it a fixed distance away of say ½" to 1" & see if you can excite it that way, again using a function generator & amp to vary the frequency & help determine the resonators' Q.

Sorry if i can't be of more help, this is new to me & effectively i'm just taking slightly educated guesses


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## MarkusBonk (Jun 5, 2010)

Hi 


event horizon said:


> ... the only way i can think of is to install a driver into the enclosure  Scrub that...


Actually, that was something I was wondering about: Somehow couple a driver to the resonator and then measure the resistance at various frequencies


Markus


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## MarkusBonk (Jun 5, 2010)

event horizon said:


> Well, other than doing something similar to the old bottle trick, blowing a constant stream of air across the port ...


I tried that using a vacuum cleaner didn't work - in my case - perhaps someone else has had more luck.
I have also tried bursting balloons above the port

Markus


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## event horizon (Nov 12, 2009)

Can you provide a picture of the resonator? More specifically the port area & where it is on the box?

I'm just wondering if it'll tend to spread the tuning frequency somewhat. You see you can definately say that a port with a decent length of tube will resonate at a single frequency simply because the air in the port will truly act as a mass. Where as if you are relying on just the wood thickness you might find that instead of a single slug (mass of air) because the port is so wide compared to it's depth that it might tend to act like water going down a plughole & pull lots of air adjacent to it & kind of destroy the resonant behavior?

It was just a thought, i'm probably a country mile off the target :whistling:


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## MarkusBonk (Jun 5, 2010)

I don't see where this is going, but here's a picture









Markus


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## event horizon (Nov 12, 2009)

Ah, i see  It's ok, for some reason i got the impression that the port was at the corner of some kind of diamond shaped enclosure, obviously i was way off of the mark :coocoo:

So it went nowhere very fast indeed...


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## MarkusBonk (Jun 5, 2010)

So what is the answer to the question I posed in the original post?


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