What's the frequency, Hermann? - Home Theater Forum and Systems - HomeTheaterShack.com

Old 07-04-10, 12:47 PM Thread Starter
Shackster

Join Date: Jun 2010
Location: Munich
Posts: 70
What's the frequency, Hermann?

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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Old 07-04-10, 04:28 PM
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Bryan Pape

Join Date: Sep 2006
Location: Wildwood, MO (St. Loui
Posts: 5,211
Re: What's the frequency, Hermann?

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

I am serious... and don't call me Shirley.

Bryan Pape
GIK Acoustics
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Old 07-04-10, 05:08 PM Thread Starter
Shackster

Join Date: Jun 2010
Location: Munich
Posts: 70
Re: What's the frequency, Hermann?

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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Old 07-05-10, 08:27 AM
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Bryan Pape

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Location: Wildwood, MO (St. Loui
Posts: 5,211
Re: What's the frequency, Hermann?

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

I am serious... and don't call me Shirley.

Bryan Pape
GIK Acoustics
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Old 07-05-10, 11:38 AM Thread Starter
Shackster

Join Date: Jun 2010
Location: Munich
Posts: 70
Re: What's the frequency, Hermann?

This is very encouraging: how did those people determine the resonant frequency of their resonators?
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Old 07-05-10, 11:40 AM
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Bryan Pape

Join Date: Sep 2006
Location: Wildwood, MO (St. Loui
Posts: 5,211
Re: What's the frequency, Hermann?

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

I am serious... and don't call me Shirley.

Bryan Pape
GIK Acoustics
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Old 07-05-10, 11:53 AM
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Mark

Join Date: Nov 2009
Location: East Midlands England UK
Posts: 129
Re: What's the frequency, Hermann?

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
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Old 07-05-10, 12:12 PM
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Bryan Pape

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Location: Wildwood, MO (St. Loui
Posts: 5,211
Re: What's the frequency, Hermann?

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

I am serious... and don't call me Shirley.

Bryan Pape
GIK Acoustics
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Old 07-05-10, 02:41 PM Thread Starter
Shackster

Join Date: Jun 2010
Location: Munich
Posts: 70
Re: What's the frequency, Hermann?

Quote:
bpape wrote: View Post
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?
MarkusBonk is offline
Old 07-05-10, 03:04 PM Thread Starter
Shackster

Join Date: Jun 2010
Location: Munich
Posts: 70
Re: What's the frequency, Hermann?

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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