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How to lower the resonant frequency of a structure?

120.146.85.121
Posted on May 12, 2022 at 21:39:29
andyr
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Location: Melbourne
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 I recently built myself some 'Zero Baffle' spkrs, using:
* 4x SB Acoustics 'Satori' 6 1/2" mid/bass drivers
* and 1x Satori tweeter.

See pic:




They are 2-way active - XOs and extensive EQing is provided by my miniDSP nanoDIGI - and they hand over to subs at a choice of frequencies stored in the nanoDIGI (48dB L-R slopes): 90Hz, 110Hz, 130Hz and 150Hz.

They sound terrific but, as a result of a friend bringing over some CDs to have a listen ... I've been made aware of a problem that I would like to get fixed.

I did an experiment - using my sig-gen to feed various frequencies into the mid/bass drivers. Level was set quite high - and, starting with the sub XO set to 90Hz, I slowly wound the frequency up from 80Hz to 400Hz.

The steel frame resonated at 130Hz; changing the sub XO to 110Hz reduced the buzz a bit - upping the XO frequ to 130Hz decreased it a bit more ... and increasing the XO point to 150Hz removed the buzz (as 130Hz was now going into the sub).

My question to the brains trust is ... what do I need to do, to lower the resonant frequency of the frame which supports the drivers? (Ideally to below 90Hz!)

Now, the frame which the drivers are bolted to is as follows - made from 5mm x 13mm steel flat, bolted to a 20mm thick base, which stops them falling over. See here:




(There are 2 of these 'Us' for each spkr; sorry the pic has come out so small. :-(( )

It seems to me I have 2 options:
1. use a denser material than steel - say, bronze or brass 'flats'.
2. or increase the mass of the frame - so have solid sides instead of a frame made from flats.

Can anyone suggest the pros & cons of each?

Thanks,
Andy

 

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The same way you stop a tuning fork from vibrating. , posted on May 14, 2022 at 04:00:47
geoffkait
 Use viscoelastic "rubber bands" - wrap firmly around the structure in several places. Problem solved.

 

RE: Thank you Geoff ..., posted on May 14, 2022 at 18:56:05
andyr
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Location: Melbourne
Joined: September 2, 2000
I think you're actually telling me how to kill (damp!) the resonance - rather than lower its frequency - but that is certainly one solution that would help my problem.

But given these spkr frames:






... I'm wondering how I could attach the rubber bands?

Andy

 

RE: Thank you Geoff ..., posted on May 15, 2022 at 02:42:37
geoffkait
 Hi, it's not an isolation issue, it's a damping issue. Like a tuning fork.

 

Mass load the frame, posted on May 18, 2022 at 14:08:55
Dave_K
 If you want to lower the resonance frequency of the frame, try adding mass. The higher up you place the mass, the more effect it will have. If you can't shift the resonance such that it's an octave below the x-over, then damp the structure as well.

 

The only good vibration is a dead vibration. Nt, posted on May 18, 2022 at 15:19:08
geoffkait
 Nt

 

RE: Mass load the frame, posted on May 18, 2022 at 15:54:11
andyr
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Joined: September 2, 2000
 Thanks, Dave - yes, adding mass is something that occurred to me. :-))

Andy

 

RE: Mass load the frame, posted on May 19, 2022 at 04:43:07
geoffkait
 But where would you mass load it, even if you wanted to? There's no room at the top of the structure to mass load it and loading the base probably would not do much good. It's too bad they don't still make Techna Sonic dampers because they would be perfect, they were used to attach to speaker cabinets to reduce cabinet vibration, they even made one especially for subwoofers. I used to sell tons of them as I used them in one of my products.

 

RE: How to lower the resonant frequency of a structure?, posted on May 19, 2022 at 08:30:43
Davey
 Andy,

Why not mount the drivers on a proper baffle....like they should be??
You scheme has no benefit.....but with notable down side.

Dave.

 

RE: "Why not mount the drivers on a proper baffle ..." ..., posted on May 19, 2022 at 15:43:43
andyr
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Location: Melbourne
Joined: September 2, 2000
 Many reasons, Dave. :-))

But chiefly because they would then have much more visual impact in the room than they currently have.

(After decades of living with "room dividers", I wanted some spkrs with minimum visual imposition on the room.)

Andy

 

RE: "Well, I tried mass-loading" ..., posted on June 7, 2022 at 04:19:30
andyr
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Location: Melbourne
Joined: September 2, 2000
 and, surprisingly ... it produced worse buzzing of the spkr frame. :-((

Sure - it's against theory ... but that's how it turned out.

The U-frame each side which the drivers are bolted to was initally just made of 13x5mm steel flat - and I found it resonated when I fed a 130Hz signal into the mid/bass drivers. (Pretty peaky - the resonance was induced only between 125 and 135Hz.)

So I had a number of 13x5mm steel flat pieces cut ... and glued them to the U-frames using thin, double-sided foam tape. So this should've been damping the structure - as well as adding mass.

But the result was ... the spkr frame now buzzed from 100 to 140Hz. :-((

So I decided to get some U-frames made from 12x6 aluminium flat. The result is (despite the reduction in mass, using aluminium) ... there is no longer any vibration!

So I'm happy ... but surprised that theory didn't seem to work. :-((

Andy

 

RE: "Well, I tried mass-loading" ..., posted on June 8, 2022 at 06:33:07
geoffkait
 Just to point out that's not mass loading, what you tried. Mass loading requires mass to be located above the object that's vibrating. Increasing the total load (weight). F=ma, the load is in the direction of the force of gravity, vertical. An example of mass loading is placing a heavy slab of granite on top of a speaker cabinet. Or placing a heavy glass weight on top of a preamp. What you did wasn't really damping either, for effective damping you need viscoelastic material or some constrained layer type dampener.

 

RE: "Well, I tried mass-loading" ..., posted on June 8, 2022 at 15:00:08
andyr
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Location: Melbourne
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 Whilst I can understand that "placing a heavy slab of granite on top of a speaker cabinet" certainly is a form of 'mass-loading' ... I can't see why increasing the mass of the U-frames (from 1.7kg to 2.5kg) isn't also 'mass-loading'.

And a 'CLD' structure is "heavy-light-heavy" etc - so my use of thin foam tape with glue on each side meets this criterion. But, sure ... maybe I could've used a better (viscoelastic) material for the 'light' layer.

Andy

 

Definition of terms, posted on June 9, 2022 at 08:20:18
geoffkait
 Mass
Mass is a measure of the amount of material in an object, directly related to the number and type of atoms present in the object. Mass does not change with a body's position, movement or alteration of its shape unless material is added or removed. The unit of mass in the SI system is the kilogram (kg).

Weight
In the trading of goods, weight is taken to mean the same as mass and is measured in kilograms. Scientifically, however, it is normal to state that the weight of a body is the gravitational force acting on it and hence it should be measured in newtons (N), and that this force depends on the local force due to gravity. To add to the confusion, a weight is an artefact of calibrated mass normally made from a dense metal. So, unfortunately, weight has three meanings and care should always be taken to appreciate which one is meant in a particular context.

Force
Force is a measure of the interaction between bodies. It takes a number of forms including short-range atomic forces, electromagnetic and gravitational forces. Force is known as a vector quantity, as it has both direction and magnitude.

Load
Load is a term frequently used in engineering to mean the force exerted on a surface or body.

I would say what you did was close to constrained layer damping, but not mass loading. In the case of constrained layer damping one would use a stiff layer like metal with a viscoelastic layer. Vibration is reduced by absorbing it as shear forces in the video layer.

 

To lower the resonant frequency you make them longer, posted on June 18, 2022 at 12:02:37
Analog Scott
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 mass loading has already been suggested

 

actually what you did is not in conflict with how mass loading works, posted on June 19, 2022 at 18:27:16
Analog Scott
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 You also changed the compliance of the structure. two objects that are the same dimensions and weight may have completely different resonant frequencies depending on their relative stiffness.

 

RE: "You also changed the compliance of the structure" ..., posted on June 21, 2022 at 05:14:35
andyr
Manufacturer

Posts: 12605
Location: Melbourne
Joined: September 2, 2000
 Yes, understood. :-)

Andy

 

RE: An update! ..., posted on July 3, 2022 at 04:48:58
andyr
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Posts: 12605
Location: Melbourne
Joined: September 2, 2000
 After the above post, I tried 2 different scenarios, to reduce audible 'buzzing':
1. glueing additional pieces of steel to the U-frame structure, to a. increase mass and b. provide damping.

2. making a 2nd set of U-frames out of aluminium. Obviously, these were lighter ... but aluminium is less stiff than steel (so has a lower 'Q').

Unfortunately, neither of these efforts was successful - I could still hear buzzing from the metal frames at certain frequencies ... even though I had to turn up the volume from my sig-gen, to achieve this.

So then I decided to try solid frames - same dimensions as the metal U-frames but made from birch ply. See here:






Success at last! :-)) I can still feel the wood supports vibrating - but this does not produce any audible buzzing.

The only problem is ... wifey doesn't like the wood colour - so I have to get the wood supports sprayed red, to match the bases. :-((

Andy

 

Not surprised, posted on July 7, 2022 at 01:14:01
Analog Scott
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Joined: January 8, 2002
 wood has excellent self damping porperties. You never hear anyone say "it rings like wood." And before anyone starts pointing out that very hard dense woods ring, yes I know that. Talking about more common woods softer medium desnity woods.

 

RE: An update! ..., posted on July 7, 2022 at 06:42:49
geoffkait
 Of course any vibration of the structure is bad for the sound, right? That's why they are isolating speakers these days, or using cabinet dampeners, the cabinet vibration modulates the acoustic signal, no?

 

How to dampen slender vertical structures using mass, posted on July 16, 2022 at 05:55:11
geoffkait
 Mass per se is not dampening.

 

RE: An update! ..., posted on August 8, 2022 at 08:10:15
neolith
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Contributor
  Since: December 2, 2004
 Late to the discussion as I don't frequent this forum. In any case, your solution reminds me of discussions that occurred on the Planar Forum upteen years ago with Peter Gunn and wood frames for Maggies.



"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: Hah - yes, indeed! ..., posted on August 9, 2022 at 15:23:17
andyr
Manufacturer

Posts: 12605
Location: Melbourne
Joined: September 2, 2000
 In this latest case, I guess it simply boils down to the fact that the birch-ply structure (with its wood/glue interfaces) simply flexes in response to the vibrations created by the mid/bass driver frames and attenuates these vibrations by converting them to heat - whereas all the metal frames can do ... is continue vibrating!

Andy

 

RE: How to lower the resonant frequency of a structure?, posted on August 9, 2022 at 22:53:44
pictureguy
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Posts: 22597
Location: SoCal
Joined: October 19, 2008
 Using steel is asing for a resonant frequency buzz.

Try a non-resonant material. I suspect Alluminum is better.....if not perfcct.

Wood might be better, with a distributed mode.

If you can 'short' the vibration to ground? That may help, too.

Various damping materials may also be employed........

You have many more choices than 2 to help your problem. Some design ingeniuty will help here.
Too much is never enough

 

Steel vs aluminum, posted on August 10, 2022 at 03:28:29
geoffkait
 The stiffness of a metal depends on the modulus of elasticity, also known as Young's Modulus, which is constant for a given metal. Since Young's Modulus for steel is three times that of aluminum, an aluminum part under load will deflect three times as much as a similarly loaded steel part. Hope that helps.

 

RE: Steel vs aluminum, posted on August 10, 2022 at 08:38:55
pictureguy
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 Yet.....NO Aluminum bells...


Too much is never enough

 

RE: Steel vs aluminum, posted on August 10, 2022 at 09:41:21
geoffkait
 Ringing has nothing to do with stiffness. Go figure. Brass is a relatively soft metal and makes good bells. It's more musical. Carbon fiber makes a good viola.

 

RE: Steel vs aluminum, posted on August 10, 2022 at 12:00:18
pictureguy
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Location: SoCal
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 Didn't say it did.....
All I noted was that for a non-resonant structure.....you could do worse than Aluminum.

It's other properties of no fatigue strength have little to do with a reframe.....
Personally? I'd start with a 7000 series alloy......one of the bondable ones, so I could glue it together. Most of the 6000 series are OK for bicylce frames after broght to T6 treatment....but still and all? That's no better than mild steel tubing.....

I've alwasys felt that INGENIOUS design trumps sheer mass or the bridge style frames I see so much of.
Too much is never enough

 

RE: Steel vs aluminum, posted on August 10, 2022 at 12:19:23
geoffkait
 I'll take My steel road bike over ANY aluminum bike. Much better better physical characteristics. End of story. Good luck with your aluminum bike. In fact steel is preferred fir building large structures like uh, bridges and skyscrapers.

 

RE: Steel vs aluminum, posted on August 10, 2022 at 14:03:48
pictureguy
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Posts: 22597
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Joined: October 19, 2008
 You jump to conclusions ONCE AGAIN.
For a bike? I think Titanium may be the best metal. And than only a few select alloys....Like 6/4 which is good for dropouts OR 6/2.5 which finds use as aircraft hydraulic tubing. Among the first successful framesets, Gary Helfrich used surplus or not quite-to-spec tubing.
For STEEL? The Unavailable tubing made by EXCELL which is the ONLY steel alloy I know in bicycle use (ago) that used NICKEL in the alloy. The remainder are mainly Chrome / Moly with a few minor alloying agents addes. VERY high tensile strenght and fatigue limits. Tensile over 200,000lb and yield of 171,000. Good elongation, too which may mean a more 'compliant' ride?

This is better than your 4130 Huffy......by far.

Most people do not know that......just for example.....The very famous Reynolds 531 is exactly the same composition as the highly regarded 753r.......the difference being cold working and such.
You needed a special certification from Reyonlds to be sold this material.

Within its limits, (not to exceed fatigue limit) steel should last nearly forever. However, for Aluminum, EACH stress cycle eats into ultimate lifespan.

As an aside? I've seen the NEW Carbon Wheels. OUCH! They are VERY expensive and also massively durable. Spoke counts are now approaching Egyptian Chariot levels.....

I had this chat with my brother. He was a long haul trucker where NET WEIGHT of cargo carried is the rule. He showed me Aluminium trailers. Ravens, I think? Check LINK.

They ARE lighter, but not in proportion of the weights of the same amount of Steel V Aluminum.
Strength and longevity are the reason. Most of the trailers at the link are about 20 years old and I suspect near the end of a useful life......i'd inpsect CAREFULLY for cracks, stress risers and other career limiting defects. The extra capacity made more money for the hauler, but took years. Big Rigs even use aluminum wheels which are also much heavier than the ratio of Fe / Al weights would imply.

The first Aluminum frame I rode? VITUS. And it was awful. LIke riding a screen door. Next up was Klein / Cannondale. Both used thicker wall 'big' diameter tubing of 6061 alloy which were post treated to T6 hardness. VERY brutal ride, as you may have noticed. Big dimeter / thick wall was very rigid and necessary to avoid 'soda straw' breakage. this is when a tube 'folds' in half at a dent. Try it WITH a straw for a demo...


i'd own steel if I could afford it. Or Ti or even Beryllium......But Aluminim is abundent and cheap. As long as ridden properly, it'll last me a long time......
Look at the NEW Pinarello bikes. Like carbon graphite space ships. And priced like one!

Enjoy the ride.
Too much is never enough

 

RE: Hah - yes, indeed! ..., posted on August 10, 2022 at 15:50:52
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008



Wood can be considered a 'natural composit' and as such has multiple resonant frequencies, many of which self-cancel.


As an experiment? Send off for some samples of PLYBOO. this is a manufactured product which is made of various orientations of Bamboo in strand or cross 'grain' configurations.

Link is to STRAND while you can get EDGE or FLAT......each of which has a different presentation.

I like the Neopolititan which also happens to be the heaviest.

I'm including an idea I worked on. Rear triangulation of panel.....very rigid without being heavy or resonant. Single link to base, which if you do it right can provide variable tilt....
Other features posssible......



Too much is never enough

 

RE: Steel vs aluminum, posted on August 11, 2022 at 04:52:27
geoffkait
 Nope, wrong again. Steel is the best metal for a road bike, at least in terms of comfort, flexibility and overall enjoyment. It's you who are jumping to conclusions.

Steel provides the best ride, other metals like titanium are obviously lighter, and even stronger, which certainly deserves some consideration, but still even though the heaviest metal used in road bike construction is the best ride, you can make up for the weight penalty other ways. That's why very high grade steel bikes like my All City Mach Man are so damn expensive. The roads around here are all downhill, which helps considerably.

Titanium, measures better, rides worse. Sound familiar?

 

RE: Steel vs aluminum, posted on August 11, 2022 at 11:13:43
pictureguy
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Location: SoCal
Joined: October 19, 2008
 there is so much to 'best' ride as to render your answer as just another opinion / datapoint.

Comfort / Ride is to a large extent design driven. Aero or non-round tubes are more compliant in one direction than at 90 degrees.
Aluminim, for example, NEEDS more material than the simple weight ratio with steel would imply, because of its metalurgical characeristics. And big tubes require thicker walls.....I can't off-hand rmember the 'rule' but go too thin on the wall and you are setting yourself up for a soda straw failure.

Steel tubes are frequently 'butted' where wall thickness varies from thick at the joins.....better for brazing.....to thinner in the middle where stresses may be less.
Don't forget that OTHER characteristics / treatments matter a lot, too. The Reynolds 531 which was used in many regular bikes of the era ws identical in composition to the high-end 753r, differening only in cold working post tube fabrication.....

And you can NOT make a bike from Al or Fe using one set of plans. You should take into considertaion the material to be used in fabrication. I've even seen BAMBOO bikes. As a matter of fact? You can buy a bike KIT and make your OWN bamboo bike. A real conversation piece.

And don't forget the Fork. this is a big part of the ride equation. Different design. different materials. Even Chrome plating of steel.......ALL make a difference in ultimate ride.

so while you may like your steel bike, it is unfair to make such a blanket statement. Even those pesky carbon graphite bike have a place in all this, though $$$.

Your bike is apparently a regular...though with 'shaped' tube version of a traditional steel bike. Nothing exotic BUT is executed very well. A 4130 front fork is also quite traditional. I've lost track of the prices of this stuff, but know that 4000$ and UP are not uncommon for the very high end.

Sounds like you live in an Escher.......

Too much is never enough

 

Yikes!, posted on August 12, 2022 at 03:47:20
geoffkait
 I've opened up a hornets nest. I nominate your post for the longest logical fallacy of the week. When you ass-u-me something you make an ass out of me and Uma Thurman.

 

RE: Yikes!, posted on August 12, 2022 at 11:48:37
pictureguy
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Posts: 22597
Location: SoCal
Joined: October 19, 2008
 Yes somebody that knows materials and design. Please do a LITTLE better than a general

statement about 'logical fallacy' or whatever.....

But one thing for SURE? Enjoy your bike. No matter.

And for the Record? I think steel is great stuff for a bike. I just can't afford it. Gone from the

entry market and replaced by Mass Produced frames from who knows WHERE? Of Aluminum.

I see many Litespeed Titanium on the market. This would be a good USED move since new goodies

and wheels are easy. Handlebar tape is CHEAP and fun.
Too much is never enough

 

RE: Yikes!, posted on August 12, 2022 at 16:40:19
geoffkait
 I have much deeper and broader background in materials than you do. And my education too. This conversation can serve no purpose any more. Ta, ta

 

RE: Yikes!, posted on August 15, 2022 at 12:22:35
pictureguy
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Posts: 22597
Location: SoCal
Joined: October 19, 2008
 I'm sorry to be so.....judgmental......but you did not get your monies worth.....

You never address any questions or comments, but just pontificate....

Tell me why steel is 'the best'?
Too much is never enough

 

Never?, posted on August 15, 2022 at 14:43:36
geoffkait
 Never is a very long time. Mr. Smarty Pants.

 

RE: Never?, posted on August 15, 2022 at 15:07:43
pictureguy
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Posts: 22597
Location: SoCal
Joined: October 19, 2008
 Agree with THAT.
I figure the entire universe will be down about about 2 above absolute Zero by the time you are in the mood to'share'.......You are abrogating your duty to educate, if you can.....




Too much is never enough

 

RE: Never?, posted on August 16, 2022 at 05:58:12
geoffkait
 No one is more sharing and generous than your humble scribe. Or more knowledgable. Did you forget to take your Xanax? This is all probably a simple case of technician versus real engineer.

 

RE: Never?, posted on August 16, 2022 at 11:10:37
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 I've read your 'pontifications' on a number of subjects. And speaking of 'subjects'.....that's how you tend to treat people.
You should spend some serious 'introspection' time and try to contact your inner self. Sounds like you skipped a few doses of your Ritalin.....Or Lithium? Contact your medical professional.....
When I need to have such time? A couple days ALONE in the mountains / camping has always worked wonders. Keep it basic.

I about choked on the misuse of the word 'humble'.......than I laughed.....
Too much is never enough

 

How much?, posted on August 16, 2022 at 12:58:02
geoffkait
 How much do I owe you for the physiological analysis? How about ten cents?

 

RE: How much?, posted on August 16, 2022 at 15:17:14
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 I'm VERY glad you asked.
My advice, as is yours, is generally worth what is paid for it.
I'm under no illusions that you will listen or act on any advice, no matter how well intended.

That being said? I believe in 'pay it forward'. One of these days, you'll have an opportunity to take a deep breath and be more civil than you ever though you had IN YOU.....

Take that chance and be better for it....
Too much is never enough

 

Porpoise, posted on August 17, 2022 at 03:50:23
geoffkait
 This conversation can have no porpoise any more.

 

RE: How to lower the resonant frequency of a structure?, posted on November 6, 2022 at 06:23:38
tomservo
Manufacturer

Posts: 8615
Joined: July 4, 2002
 You can lower the resonate F by ;

Adding mass to the portion "in motion"
Reducing stiffness of the "spring component"

The portion not considered in the question posed is the "Q" or magnitude of the resonance which is lowered by adding resistance to the moving portion. Remember stiffness, mass and losses are the three things involved in a mechanical resonance.

Often the easy path here is paralleling the stiffness member with a different material with a different sound velocity and loss that (sometimes called constrained layer damping).
Usually it's the magnitude of a resonance that causes the problems as everything has stiffness, mass and internal losses.

 

RE: An update! ..., posted on July 26, 2023 at 13:34:09
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 Gee......I suggested wood in my original post...WAY above, before Geoffkait derailed.....

Glad you have discovered the benefits of wood.

You would have Hated my other suggestion if I had actually made it!
Too much is never enough

 

RE: An update! ..., posted on July 27, 2023 at 05:15:56
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 Add mass.

 

RE: An update! ..., posted on July 27, 2023 at 07:29:37
geoffkait
 Isolation is the new dampening or mass loading. Didn't anyone get the memo?

 

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