How to kill a 100Hz open baffle resonance?

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Hi all,

Last summer I built according Kuei Yang Wang plans (65cmx105cm) an open baffle speakers (215S non bicone and a super tweeter). Since then a notice a HORRIBLE resonance at (around) 100Hz.

How to kill a 100Hz open baffle resonance?

Is it a listen room problem or a placement problem or an open baffle problem?

Any comments or suggestions are very welcome and sorry my English.

Katapum
 
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Joined 2002
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If it is a room problem you would also hear it with 'normal' speakers.....

The open baffle has two effects:

At the frequency where the path length around from front of cone to the back approaches half the sound wavelength you loose output (acoustical short), so you would loose bass pretty quickly. At the sizes you give, the path around is abouth 80cm, right? At 320m/sec sound speed, 80cm takes around 2.5 msec which would be 5 msec for a full wave or 200Hz. So your freq response starts to fall off from around 200Hz downward.

Another effect is that there is no damping whatsoever of the speaker resonant frequency. I suspect that is the problem here, that the driver has enough resonance at 100Hz to overcome the fall-off from 200Hz downwards.

Jan Didden.
 
Katapum,

the room seems to be the main culprit IMHO. I believe 3,5m is the length of your room. That leads to room modes at about 50 Hz, 100 Hz, 150 Hz, 200 Hz. 50 Hz is too low to be excited heavily by the Supravox, but wavelength for 100 Hz is very near 3,5 m!

Second culprit could be the distance of the speaker to the rear and side wall. The direct reflection of the dipole backwave from the rear wall will lead to a peak at ~170 Hz, but another reflection off the sidewall could easily drop that to 100 Hz.

Can you arrange your listening position ~85 cm from the front wall? This will give your ears minimal excitation with 100 Hz. Sitting near to the frontwall or to the middle of the room will result in maximal excitation.

Is there any chance to make the diagonal through the room your listening axis? This seems to be the best way to get rid of that nasty 100 Hz without resorting to absorbers and traps.

Rudolf
 
so is not a speakers problem… now I´m better… I spent a lot of hours to build it.

This is long-distance diagnosis, so no guarantee for that! But the dimensions of your room are a very strong argument and I never heard anyone complain about those baffles before.

And what about Jannemans question? Did you never hear this 100Hz resonance with conventional speakers in that room?
 
Hi,


I moved my listened position 50cm back and everything is better… more balanced sound with less bass and without 100Hz resonance (but I´ll check again with another conventional speakers pair). Thanks to all.


To Rudolf:
I’d to learn more about room modes and dipole reflections. Have any link?


Katapum
 
Katapum said:
I’d to learn more about room modes and dipole reflections. Have any link?

For starters you should look at http://www.linkwitzlab.com/rooms.htm
and the links given on that webpage. Don´t miss the part about "Standing waves in a rectangular, rigid room". http://www.linkwitzlab.com/faq.htm#Q31 is interesting too.

BTW: For anybody seriously into dipoles the Linkwitz site ist the BIBLE. Even if it looks very scientific in some parts, there is lots of information a lesser mind can grasp too. ;)

For some more visual clues you could resort to
http://www.rivesaudio.com/CARAquick/CARAframe.html

A google for "room modes" or "room acoustics" will provide you with enough lecture to spend all winter with.

Rudolf
 
Hi Katapum

May I make an additional suggestion? Have a look at Jon Risch's website. He has a section on room treatments (called 'Main Link to Acoustic Treatments Info and DIY Notes') that shows you how to build an absorber that can help with room problems. This might go a long way to addressing some of the basic problems in your room. His website is at:
Jon Risch's Audio DIY Notes & Tweak Info
The section you want is the second from the top.

Enjoy,
Deon
 
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