# helmholtz resonator and questions about frequencies



## Guest (Aug 29, 2007)

Hi, 

I have a "spike" of 38 hz in a room of 11X13X8 feet which has broadband absorption.
I have 6 plastic containers of 20 litres each, these containers were once used for antifreeze storage. 
By using 1/2" MDF and making round tops for the plastic containers, I can then drill holes to absord at 38 hz.

Here are my questions:

1) the plastic containers are only 1/8" thick, will 38 hz pass right through buckets? 
From what I've read low frequencies can pass through sheetrock 
2) Another option might be to use 60 litre metal drums, would this work?
3) would rectangle boxes made from 1/2" MDF be able to remove 38 hz, I've heard that the "Q" factor is not as efficient 
4) Do you know of a web site that shows how to make a low frequency helmholtz resonator (drum style)

My speaker for testing my room has a range of 25hz to 20khz (7" diameter speaker)
I use white noise to test the room and a rode mike, and the RAD software. 

5) I also have another problem frequency of 154 hz which may be related to the 38 hz spike, because of the harmonics.
When I use a speaker of 5 1/2" instead of the 7", the problem frequency becomes 180 hz instead of 154 hz, why does the problem frequency increase with a smaller speaker?
Also when I press the low cut 75 hz (18 db /OCT), using 7" speaker, this also causes the problem frequency to shift to a higher value (154hz spike becomes a 185hz spike).
I'm assuming that I should build my panel absorbers based on the values of the best speakers.

Thank you for your help,
I hope there's not too many questions! 

:rubeyes:


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## Ethan Winer (Jul 21, 2006)

Richard,

In a room that size I wouldn't worry about dealing with 38 Hz at all. There's very little musical content that low and - as much as I hate to suggest this - if it's really bothering you, a bit of EQ cut will help better than anything you'd build that can actually fit in a room that size.



> I also have another problem frequency of 154 hz which may be related to the 38 hz spike, because of the harmonics.


It doesn't work that way. But 154 Hz can be treated with more broadband absorption.

--Ethan


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## Guest (Aug 31, 2007)

Thanks for your advice!

I'm planning to one day build a bigger studio, if I ever build a helmholtz resonator, can sonotubes (used for pouring concrete) be used for absorbing low frequencies? 

Richard


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## Ethan Winer (Jul 21, 2006)

Richard Daigle said:


> if I ever build a helmholtz resonator, can sonotubes (used for pouring concrete) be used for absorbing low frequencies?


I don't really know. I'm not a fan of tuned bass trapping, so I never built that type or tested it.

--Ethan


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## F1 fan (Jul 6, 2006)

In the ETF manual Mr.Plumb included a design calculator for Helmholtz resonators using sonotubes.


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## Guest (Sep 4, 2007)

Thanks for your reply, I'm curious about whether or not a helmholtz of 38 hz would improve the sound in my small studio, and I might build a couple of helmholtz resonators using sonotubes.

Have a good day,
Richard


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## Ethan Winer (Jul 21, 2006)

Richard Daigle said:


> I'm curious about whether or not a helmholtz of 38 hz would improve the sound in my small studio


The most important range to trap is from about 70 or 80 Hz and up. There is very little music down at 38 Hz.

--Ethan


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## Guest (Sep 5, 2007)

Because of the room modes (or harmonics, not sure what is the correct term), I wonder if the 38 Hz spike is not causing a 38 x 2 = 74 Hz mode, and then a 38 X 3 = 114 Hz, etc ..., and because of these modes they affect other frequencies in the room, affecting the overall sound.
I tried sending a 38 Hz frequency, and "if I remember correctly", I saw the resulting harmonics of 74 hz, 114 hz, etc ... diminishing in peak as the frequency increased. Interesting as well, I tried sending a frequency of 160 hz and found that modes (or harmonics) were created at lower and higher frequencies from the 160 hz.
I'm still learning about this, and I realise the more I ask questions, the more questions I have.

Thanks for your input!
Richard


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## Ethan Winer (Jul 21, 2006)

Richard Daigle said:


> Because of the room modes (or harmonics, not sure what is the correct term), I wonder if the 38 Hz spike is not causing a 38 x 2 = 74 Hz mode, and then a 38 X 3 = 114 Hz, etc ..., and because of these modes they affect other frequencies in the room, affecting the overall sound.


Yes, room modes are related that way, but the actual response at any given cubic inch location in the room is affected by much more than the modes. So the only way to know what you really have is to measure.

--Ethan


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## Guest (Sep 6, 2007)

I measured 6 different spots in the room, and the peak ranges were between 151 to 161 Hz (peak varies between 10 to 25 db higher when referenced to other frequencies that are at acceptable levels)
I'm concerned about the peaks that were 15 to 25 db higher.
I built a 157 Hz panel absorber which helped, but some of the peaks are still too high.
I'm thinking of building a helmholtz at around 156 Hz, I'm not planning on building a 38 Hz helmholtz, because it probably won't help, as you have said.

But I hesistate on building a helmholtz because of the following questions:

My speaker for testing my room has a range of 25hz to 20khz (7" diameter speaker)
I use white noise to test the room and a rode mike, and the RAD software as I mentioned. 
When I use a speaker of 5 1/2" instead of the 7", the problem frequency centers around 
180 hz instead of 154 hz, why does the problem frequency increase with a smaller speaker?
Also when I press the low cut 75 hz (18 db /OCT), using 7" speaker, this also causes the problem frequency to shift to a higher value (154hz spike becomes a 185hz spike).
When I play my guitar the notes don't go down to 25 Hz as the 7" speaker does and I have low cut off set to 75 Hz; therefore what is the real peak frequency (or problem frequency) in the room when I play the guitar?
I could be building a 156 Hz helmholtz for nothing, because the peak frequency will shift depending on the instrument I play, this is what I don't understand.

Thanks for your help, I appreciate all the hard work you do!
Richard


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## Ethan Winer (Jul 21, 2006)

Richard Daigle said:


> I'm concerned about the peaks that were 15 to 25 db higher.


Technically speaking, peaks are never that high. Nulls can be very deep, and it can be hard to tell where one ends and the other begins.



> I'm thinking of building a helmholtz at around 156 Hz, I'm not planning on building a 38 Hz helmholtz, because it probably won't help, as you have said.


Right, a room that size needs broadband absorption, not tuned traps.



> why does the problem frequency increase with a smaller speaker?


Probably unrelated to the speaker size, and more a function of placement.



> Also when I press the low cut 75 hz (18 db /OCT), using 7" speaker, this also causes the problem frequency to shift to a higher value (154hz spike becomes a 185hz spike).


In that case I have no idea. But I suggest you measure your room with REW.

--Ethan


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