# Room bass response



## MarkusBonk (Jun 5, 2010)

Hi All,

I think, that the-powers-that-be seem to agree that the bass response of a room is determined by the room modes. Literature tells me for a room sized 18.90ft *12.72ft * 8ft the first fours modes are at 29.8Hz (1,0,0) , 44.4Hz(0,1,0), 53.5Hz(1,1,0) and 59.7Hz(2,0,0) and then goes and tells me all standing waves and about bass traps. Wonderful. So what about, say, 38Hz? Will I hear it or won't I? And how is it affect by the rooms dimensions?

Now, I am told that Helmholtz resonators are absorbers and then someone, unfortunately, mentions an empty beverage bottle and what happens when you blow across the top of it. Hmm.. doesn't seem to me that it is absorbing sound, more like the opposite: I can definitely hear something where the was no sound before.
So what are the prerequisites that it absorbs? Or could I build one that resonates at 38Hz and it would have exactly the same effect as a room mode?








The above measurement shows the base response with two Helmholtz resonators one tuned to about 32.5Hz, well actually I had it tuned a bit higher and then put some glass fiber into it, and the second tuned to about 45.5Hz. I had originally tuned the lower one to 27Hz but just didn't see any difference in the response. Anyone tell me why? Or how many of them do I need to get rid of the peak?

But, why, oh why is 60Hz giving me so much trouble?
Ok, I haven't quite told the complete story - the listening position is about 3/8 from the back wall and the room isn't rectangular - its L shape. But even assuming two rooms and combining the modes doesn't make any more modes appear near to 60Hz. (btw I am in Germany, the mains has 50Hz).

Markus


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## Babak (Mar 20, 2009)

Hi Markus



MarkusBonk said:


> But, why, oh why is 60Hz giving me so much trouble?


You answered your questions yourself, at least half of it:


MarkusBonk said:


> Literature tells me for a room sized 18.90ft *12.72ft * 8ft the first fours modes are at 29.8Hz (1,0,0) , 44.4Hz(0,1,0), 53.5Hz(1,1,0) and *59.7Hz(2,0,0)* and then goes and tells me all standing waves and about bass traps.


There are positions, where one can excite the room modes a lot (poitions where the sound pressure is the highest/the velocitiy is the lowest), and others, where the modes are not ecxited (minimal pressure, maximum velocity).

It seems that your speakers are at a positiion where the bass drivers excite the mode at 59.7 Hz quite well.

I use such a diagram to find a position, where the least modes are excited:


http://www.abload.de/img/moden518mizjo.jpg
or
http://www.abload.de/image.php?img=moden518mizjo.jpg

(sorry, I'm not allowed to upload pics yet  )

The peaks show positions with the highest pressure, the low points show those of the lowest.
I guess your speakerss are standing closer to the front wall ...
Maybe try to put them where the bass driver is about 1,40 m away from the front wall.

The same is valid vor the positions where one hears the respective frequencies the loudest / the least.
So the question is: at which position did you measure?

Cheers

Babak


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## MarkusBonk (Jun 5, 2010)

Hi

The speakers are 1.1m from the front wall and I am measuring 3.6m from the front wall

Markus


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## Babak (Mar 20, 2009)

Hi Markus,

I do not have 5 posts yet, so the system des not allow me to send any links to the graphs that I made for your room dimensions


One local minimum at 60 Hz is at 1.45m (good position for the speaker - bass membrane).
One local maximum is nea 3 m, so you are measuring close to that. The next local minimum would be at 4.4 to 4.5 m.

So try to adjust to those minima.

Another question:
How is the frequency response of your speakers at 60 Hz?
Are they linear?

If any change in position does not really help, try the tuneable bass traps called "VariBass" by Vicoustic.
You can tune them manually to the frequency of your mode.

Cheers

Babak


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## laser188139 (Sep 19, 2009)

Babak said:


> ... I do not have 5 posts yet, so the system des not allow me to send any links to the graphs that I made for your room dimensions ...


You can use the post padding thread to get yourself to 5 posts. I've read that it takes about an hour, after that, for the system to update your account.

Bill


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## MarkusBonk (Jun 5, 2010)

Hi Babak,



Babak said:


> One local minimum at 60 Hz is at 1.45m (good position for the speaker - bass membrane).
> One local maximum is nea 3 m, so you are measuring close to that. The next local minimum would be at 4.4 to 4.5 m.
> So try to adjust to those minima.


I like idea of the graphs, although I think one would need to adjust it slightly to take into account the type of mode - a tangential mode is supposedly only half as loud (whatever that means) as an axial mode.

I would have thought one sought to position the speakers so that the modes are equally excited and I assume one would want to sit at a similar position.
Using your graphs and multiplying the third mode (light blue line) by 0.5 because its an tangential mode seems to give an ideal sitting/speaker position with respect to the first modes 1.9m from the back/front walls (after adjusting for my room dimensions).

As to the frequency response of the speakers - I have until now trusted quadral but I suppose I could measure them

Markus


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## MarkusBonk (Jun 5, 2010)

MarkusBonk said:


> Using your graphs and multiplying the third mode (light blue line) by 0.5 because its an tangential mode seems to give an ideal sitting/speaker position with respect to the first modes 1.9m from the back/front walls (after adjusting for my room dimensions).


The above is of course rubbish as the graphs are only for axial modes in one dimension. However, the position of the speakers seems to be fairly good for the (2,0,0), (3,0,0) and (5,0,0) axial modes (1,0,0) and (4,0,0) are emphasized more. So that doesn't seem to explain why the (2,0,0) mode is causing such pain

Markus


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## michael123 (Jul 26, 2008)

Babak said:


> If any change in position does not really help, try the tuneable bass traps called "VariBass" by Vicoustic.
> You can tune them manually to the frequency of your mode.


Yes, you can tune, it just does not do a thing.
These are very light, around 7kg (and, cheaply made) cylinders, with foam around them and plastic tubes, aka resonators..
I see these are resonating, but the maximum I saw they reduce around 0.5db..

Maybe if I bought not a single pair, but 3 pairs...


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## Babak (Mar 20, 2009)

Hi

yes, one pair might not be enough.

and it depends on where you place them.
Their position should be on the maximum points of the mode you want to reduce.

Cheers

Babak


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## michael123 (Jul 26, 2008)

in the corner...

nice idea anyway..


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## Babak (Mar 20, 2009)

Well, it's better to double check that. :bigsmile:

So maybe you really need more of them ...
At least that's what the German distributor of Vicoustic told me.

Here are some interesting resources from Floyd Toole:

Getting the Bass Right
http://www.harman.com/EN-US/OurComp...ages/WhitePapers.aspx?CategoryID=White papers

And a calculator for the modes:
http://www.harman.com/EN-US/OurComp...Pages/Calculators.aspx?CategoryID=Calculators

I played around with this and placed the speakers and the chair on the recommended position to cancel the modes.

The result is a pretty dry bass response

Give it a try, 10 to 20 centimeters can make a difference ...

Cheers

Babak


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