Help with decreasing resonance

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I was wondering if there was an easy way to decrease the resonance of the speakers. My speaker are about 88 lbs or 40 kg each. At high volumes I can feel the resonance in the cabinet and I am sure it is affecting the sound. Is there an easy way to decrease this resonance? I was thinking of increasing the weight of the speakers by adding heavy glass or metal plates on the sides that would also serve an aesthetic purpose. Would increasing the weight be enough or is this resonance due to an inherent design flaw of the speaker cabinet and is unfixable?

thanks in advance
well you've given very little to go on other than stating how much your speakers weigh and you perceive a problem with a resonance.
what kind of 88 pound speakers are they,fullrange,two-way,three way,sealed,bass reflex?
is your source a turntable that might not be adequately isolated or experiencing low frequency feedback?
mass loading your boxes might be the right thing but with so little to go on i'd be guessing.
They are 2 way rear ported (2 ports) bass reflex reproducing bass down to 20Hz at around -3db. two 10 inch drivers and a high freq compression driver mated to a horn. The source is an Oppo BDP-95 to active crossovers to speakers. Also, the resonance seems to come mainly from the bass at higher outputs.
well your on the right track if your trying to stop the box from resonating at higher SPL's mass loading with a dense material is the way to go.
the foam sheets aren't going to control bass energy.
is there any diagonal or lateral bracing inside especially on the 48.5 X 16.3 side panels.
your original thought on adding metal cladding or thick glass might indeed be effective providing a good bond between the two.
other options could include something like Dynamat or heavy weight felt inside the box.
have you identified the prime or band of frequencies of the offending resonance.
not knowing what your SPL measuring program is or what features it has makes it alittle hard to tell you which way to go from here.
if it has pink noise or a sweepable generator it should be fairly easy to identify as a significant bump in the response curve assuming your hitting the spl level that excites said resonance.
is this a commercially made speaker or is this something you built from a kit or designed yourself?
one brace how big and where.
can you upload any pictures of these speakers?
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I am using a cheap Behringer bass processor SX3040. It is a $100 product, but makes I the largest difference in the sound providing me with tight, articulate and well-defined bass when combined with my wyred4sound amp with 2000 damping factor. A lot of people believe in flat frequency responses, but I disagree especially when it comes to bass. The rest of the frequency is actually not lost. In order for our ears to perceive equal volume at say 50Hz and 1KHz, the actual SPL has to be 20dB higher. I am not sure if you are familiar with this frequency curve, which shows the actual SPLs needed at different volumes and frequencies for our ears to perceive an actual flat frequency response. Being a scientist I cannot ignore these facts and my actual hearing.
An externally hosted image should be here but it was not working when we last tested it.
fletcher munson curves are not lost on me.
and yes listening to systems equalized for flatness does leave alot to be desired.
even though a box is designed to have a flat response it doesn't mean it will sound flat when it's playing your favorite tunes!
(when all is right the turtles will dance)
There you go, you even know the name name of the curve, LOL. BTW, the frequency response of my speakers is not equalized, it is what it is -- just the bass processor.

Looks like the bass processor has added approximately a 20 dB equalization boost to your speaker.

Adjusting EQ to approximately match the equal loudness contours makes sense when you are listening at levels far lower than what the recording was mixed at.
The mix engineer generally uses a reasonably flat monitor system, if you are listening at the level he or she chose to mix at with a 20 dB LF boost, you are listening to a progressively more "boomy" response curve the louder your playback level is.

It does not take long to get used to a "loudness contour", but if you try equalizing the LF fairly flat for a while you may be surprised how much better it sounds at levels above 80 dB SPL or so.

In addition, the rear facing ports put out most of the LF content, it is more likely that the "resonance" you complain of in the OP is in the wall behind the cabinet unless the wall happens to be made of solid concrete.

I did not equalize the bass to the equal loudness curve. I equalized it to where the bass felt right to my ears. I had this set up without the bass processor for some time, but the speakers sounded way too lean without it. I had to compromise and add the cheap processor risking the decrease in sonic quality. However, the gains I get with it, for me, are far greater than the slight decrease in sonic quality in my LF drivers below 800Hz.

My current set up does not sound boomy at all at any volume and I do play them loud occasionally, >100db.

One of the reasons I knew about the resonance is that my Fostex tweeters, which are placed on top of the speaker boxes, would move after playing loud for a while. It is the resonance in the box that causes them to move. Also, the speakers are 2 feet away from the back wall.
Without getting into the "is the bass processor an eq system" discussion, cabinet resonances can indeed make very large audible impressions. Such are typically very high "Q", and will smear the music at the frequency of the resonances.

Large amounts of bracing will raise the frequency of the cabinet resonances, hopefully out of the woofer's range, so they're no longer a problem for you. I suggest bracing the left and right panels together with a number of bracing elements, ditto top to bottom and front to back. Expect a major improvement in sound quality.

If you want to measure what the frequency is, an old article at SB1980-3way includes hints about using an old phono cart with the needle on the cabinet and sweeping the speaker... Worth a read, in any case.
Thank you for the suggestion, that was a very interesting article, one I think I will go back a few times.

I did some more tinkering and I think the resonance is in about 200-500Hz range. When listening to Diana Krall at low to moderate volumes, I can feel the resonance with my hand on the side of the box, both with the piano notes and when she sings.

Now this doesn't sound like the mechanical transfer of the cone resonance to the cabinet as described by Mr. Linkwitz in his article, since I think the woofer resonance is way lower in the freq range. I am assuming it is cavity or air resonance in the box, which could be addressed with good damping inside. Does this sound right or am I off the mark here.

He suggests measuring the box cavity resonance by a microphone inserted into the box. I will try this through the speakers rear ports

I have to say that I have learned a lot from spending time and reading posts in this forum. I really do appreciate all of your responses and suggestions, Thank you!
bracing and or mass loading the box will help control box modes.
the other way to go is dependent on average listening level: suppress the the mid band to respect fletcher munson god rest their souls.
i would also grab a bag of poly fiber fill and heavy felt.
but that's just me.
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Now I took out my bass processor and I can sill feel the resonant modes. Finally, I remembered that I had the review for the original Klipsch RF7ii speakers from a German magazine. The impedance curve below indicates resonant modes in 250Hz and 400Hz and it seems they are pronounced.

Now knowing the offending frequencies, I would like to do something about them. I would rather avoid the bracing as I would have to take the speakers apart. Could mass loading and placing damping material inside help with these modes?

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