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Taming room resonances between 47 and 90 Hz?

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Anonymous
March 21, 2005 3:06:41 AM

Archived from groups: rec.audio.tech (More info?)

Dear experts,

I have a couple of nasty resonances starting at 47 Hz and ending at
90 Hz in my living/listening room. I have surfed the net for solutions
and have come across numerous descriptions and vendors of bass traps.
But there is really no or very little information on how efficient
the various types are. If there are measured data at all it only
goes down to some 100 Hz.

Any leads?

--
====================================================================
Martin Schöön                    "Problems worthy of attack
                                  prove their worth by hitting back"
                                                           Piet Hein
====================================================================

More about : taming room resonances

Anonymous
March 21, 2005 2:43:23 PM

Archived from groups: rec.audio.tech (More info?)

Martin,

> I have a couple of nasty resonances starting at 47 Hz and ending at 90 Hz
in my living/listening room. <

I'm sure you have peaks and deep nulls at many other low frequencies too.
All rooms do. Some of these are modal - related to the room dimensions - but
others are positional based on your distance from the walls, floor, and
ceiling.

> But there is really no or very little information on how efficient the
various types are. If there are measured data at all it only goes down to
some 100 Hz. <

You're not looking on the right place! See my company's site:

www.realtraps.com

In particular, go to the Acoustics Info section where you'll find absorption
data down to 50 Hz as well as many published and in-house articles that
explain all about bass trapping and acoustic testing in great detail.

--Ethan
Anonymous
March 21, 2005 5:25:02 PM

Archived from groups: rec.audio.tech (More info?)

Martin Schöön wrote:

> Dear experts,
>
> I have a couple of nasty resonances starting at 47 Hz and ending at
> 90 Hz in my living/listening room. I have surfed the net for solutions
> and have come across numerous descriptions and vendors of bass traps.
> But there is really no or very little information on how efficient
> the various types are. If there are measured data at all it only
> goes down to some 100 Hz.
>
> Any leads?
>

There is only one effective method of changing these resonances and that
is to remodel your house so that the size of the room changes. Any other
claim is snake oil.

However, getting a multi -band equalizer and slightly reducing your
systems output at the resonance frequencies can help.
Anonymous
March 21, 2005 5:25:03 PM

Archived from groups: rec.audio.tech (More info?)

Robert,

> There is only one effective method of changing these resonances and that
is to remodel your house so that the size of the room changes. Any other
claim is snake oil. <

That simpy is not true. I absolutely agree that much acousic treatment is
snake oil, but it's not *all* snake oil! There are bass traps that work well
down to 40 Hz and even lower.

> getting a multi -band equalizer and slightly reducing your systems output
at the resonance frequencies can help. <

EQ can help only a little, and only at the very lowest frequencies - those
well below 100 Hz. EQ does nothing for modal ringing, nor can it deal with
the fact that low frequency response changes quite a bit around the room.
Over a span of even a few inches the response can vary by 15 dB or more. So
any correction that helps in one place can only make things worse somewhere
else.

--Ethan
Anonymous
March 21, 2005 11:57:24 PM

Archived from groups: rec.audio.tech (More info?)

On Mon, 21 Mar 2005 11:43:23 -0500, Ethan Winer wrote:
>
> I'm sure you have peaks and deep nulls at many other low frequencies too.
> All rooms do. Some of these are modal - related to the room dimensions - but
> others are positional based on your distance from the walls, floor, and
> ceiling.

I know and it is an almost trivial exercise to calculate the various
modes as long as I remember to use the speed of sound rather than the
speed of light - I am a microwave engineer. For those of you who are
not into engineering or have forgot I recommend this java application:

http://www.hunecke.de/german/rechenservice/raumeigenmod...

I think the gentleman who suggested that I have to remodel my room will
find that it only moves the resonances to new frequencies.

Here is another java app. that is kind of fun since you can choose
speakers and furnituring in the room. The modelling must be based
on some (simplistic) assumptions since you cannot specify where you
put things - not even the speakers. The low frequency resonances look
pretty plausible though (I have measured mine). Notice how little
the speaker choice does. If you think fancy cables will make any
differency you are into voodoo...

http://www.fastaudio.com/INT/rechenservice.html

> You're not looking on the right place! See my company's site:
>
> www.realtraps.com
>
> In particular, go to the Acoustics Info section where you'll find absorption
> data down to 50 Hz as well as many published and in-house articles that
> explain all about bass trapping and acoustic testing in great detail.
>
I have been to your place Ethan. That's why I added "or very little".
I think it is the only place where I have seen data below 100 Hz
but I also notice that the attenuation of even the Mondo Trap in
a corner is way down the scale at 50 Hz compared to 100+ Hz. I
freely admit that I am new to the Sabine measure and it doesn't
tell me much. I am used to dB and GHz :-)

I have read some of your articles but not all obviously. One
is called "Build a better bass trap"
http://www.ethanwiner.com/basstrap.html
and another is "Acoustic treatment..."
http://www.ethanwiner.com/acoustics.html (This one seems more detailed
than the version I read some weeks ago). Halfway down this article
I find before/after treatment curves and to me they don't look
too promising. The peak-to-valley amplitude is still 10+ dB. Is
the difference really audible?

Good articles both but they don't tell me even how good the various
traps are compared to each other - hence I still don't know what
to start testing. Putting up rigid fiberglass boards in the corners
is tempting since the investment in time and money is minimal and
I can easily remove the stuff if it doesn't help (much). (Buying
your commercial traps may be out of the quiestions since I live
in Sweden and before they have arrived here the cost of freight,
customs (?%) and tax (25%) will have made them rather expensive.)

One more acoustic java site for today:
http://www.mhsoft.nl/Helmholtzabsorber.asp

I'll stay tuned for more input.
(Back to reading Ethan's articles)

--
====================================================================
Martin Schöön                    "Problems worthy of attack
                                  prove their worth by hitting back"
                                                           Piet Hein
====================================================================
Anonymous
March 21, 2005 11:57:25 PM

Archived from groups: rec.audio.tech (More info?)

Martin Schöön wrote:

> I think the gentleman who suggested that I have to remodel my room will
> find that it only moves the resonances to new frequencies.

Not if you remove the back wall and turn the room into
an enormous tapered transmission line. :-)

--
Eiron.
Anonymous
March 21, 2005 11:57:25 PM

Archived from groups: rec.audio.tech (More info?)

Martin,

> the attenuation of even the Mondo Trap in a corner is way down the scale
at 50 Hz compared to 100+ Hz. <

I can clarify a few things here:

The absorption for MondoTraps at 50 Hz is still extremely high. When the
Sabins value is equal to the trap's square feet of front surface that means
the trap is absorbing 100 percent. So for a MondoTrap 9.5 Sabins of
absorption is 100%, and for a MiniTrap 8 Sabins is 100%. The reason the
numbers go even higher than 100% is because there's additional absorption
via the rear surface when they're mounted straddling a corner. So while the
absorption is *extremely* high between around 80-150 Hz, it's still plenty
at lower frequencies too.

Also, it doesn't take a huge amount of absorption to make a real improvement
to the peaks and nulls. If a reflection can be reduced from near 100% of the
original energy to only half that, this reduces the null depth from infinite
to only 6 dB.

> The peak-to-valley amplitude is still 10+ dB. Is the difference really
audible? <

You bet it is! Before traps the response can vary 30 dB or even more. So if
you can reduce a 30 dB span to "only" 10 dB, you have made an enormous
improvement in the response. Far better than is possible using EQ.

Understand that it's impossible to make the response in any room perfectly
flat. Even million dollar recording studios are thrilled if they can get the
response in their state of the art control room to +/- 6 dB. When you get a
chance, check out the MiniTraps demonstration video on our site. It shows
MiniTraps making a very large improvement all the way down to the 40 Hz
lower limit we tested. MondoTraps do even better, and an upcoming review in
a UK magazine shows MondoTraps making a big improvement on the 30 Hz mode in
the reviewer's room. (The reviewer emailed me a graph a few days ago.)

Another very important but often overlooked feature of bass traps is they
reduce modal ringing. I just completed a video that explains modal ringing,
and this video also shows the large improvement from adding MondoTraps to a
typical room. It's third in the list on our Videos page, which is in the
Acoustics Info section of the site.

--Ethan
!