What do I need to measure speaker cab resonances?

[tone?]

Bear with me here fellas.
I’m unemployed and need some projects to keep me busy and my sanity.

I want to experiment with adding bracing to my speakers. Don’t worry it’s completely reversible. No drill holes or anything.
But I also want to do this scientifically as much as possible.

What instrumentation do I need to measure for speaker cab resonances?
I want to be able to add some bracing and then run some merriments to see if it scientifically helped or hindered.

accelerometer?
This for impedance measurements ?
Dayton Audio - DATS V3 Computer Based Audio Component Test System

Any advice would be appreciated.

addendum - bracing materials in attachments.

Btw all is fine with me. If you cared that is. Lol

Thanks!

 

[planet10]

tThe simplest, and cheapest, tool is a mechanics stethescope. The ease with which it can quickly nail down where things are ringing makes them very useful. One would have to estimate relative magnitudes.

For qualititative measures, place an accelerometer on spots where the stethescope suggest a good spot.

A laser vibrometre would be a high-end tool.

Measuring impedance may pick out box resonances, but it could also be the driver.

dave

 

[tone?]

tThe simplest, and cheapest, tool is a mechanics stethescope. The ease with which it can quickly nail down where things are ringing makes them very useful. One would have to estimate relative magnitudes.

For qualititative measures, place an accelerometer on spots where the stethescope suggest a good spot.

A laser vibrometre would be a high-end tool.

Measuring impedance may pick out box resonances, but it could also be the driver.

dave

i don’t have an accelerometer.

any links to one which can do the job easily through REW?

thanks

 

[phase_accurate]

Measuring impedance may pick out box resonances, but it could also be the driver.

The trick is to measure the driver in-box and out of the box and compare. The difficulty lies in finding the culprit that is responsible for the in box resonance(s). And at the lower end there are of course the differences that are normal due to the enclosure type that is used.

Regards

Charles

 

[tone?]

no accelerometer recommendations So I can measure resonances of the cabinet when doing the bracing?

oh and second question.
could the bracing affect the impedance in a negative manner?

 

[tone?]

I mean everyone here places bracing most likely in their speaker builds.

do you do it haphazardly? Just put it in the box and not see what it is actually doing?
if you need to place it elsewhere or more bracing or less etc


thanks

 

[phase_accurate]

Accelerometer AND impedance would be nice. Keep in mind that panel resonances are easier to be found by accelerometer measurements and internal resonances and standing waves by the impedance method.
One (famous) member of this forum once explained that he would use a phono cartridge as an accelerometer. And there are other things that could also be abused as such.

Regards

Charles

 

[tone?]

yes but my kind sir Charles.
i need some help in acquiring such an accelerometer. A link to one please?

and I’m asking again if adding bracing would affect the impedance in a negative manner.

thank you once more

 

[tmuikku]

Its a vast subject and you can find a lot just by searching around. You could also expand thinking by asking more fundamental questions like why do you have a box to begin with? 😉

 

[tone?]

i have searched high and low. Trust me.
in the day of the Internet things have become so cluttered that I have spent two days searching for answers and only got minor opinions.
zero practical solutions.

and now you are answering with a question of your own.
lol

 

[tone?]

You know it’s funny.

i have worked in IT project management for half my career.
techs LOVE to talk tech.
but at a certain point in time you have to reel them in and say. “ ok fellas , we need some practical solutions now.“
what do we actually do?

and only the really good techs will offer practical solutions. The rest will talk technical jargon for hours in circles not offering any solutions.

im used to this

 

[stv]

Bracing won't affect impedance negatively.
It will make panels stiffer and raise panel resonance to higher frequencies which might be bad for midrange.
You may also investigate about standing waves in enclosures, not related to panel resonance.

 

[tone?]

finally someone who actually read my post.

thank you for the answer.
yeah that is why I want to be able to measure it somehow to see what effect the bracing is having on the internal resonances.

as you say. I might put two bracings and the bass cleans up. But then some that sounds wrong elsewhere. I want to be able to pinpoint that.

and thanks for answering my question about impedance. Super appreciate it

 

[tmuikku]

😀 Yeah, loudspeakers are one helluva system, lots of aspects interacting. Alright, lets make a thought process

Usually the box is for a woofer to prevent sound from back and front of the cone to cancel, better efficiency for bass. Now think a box for bass for now, simplified. Take some bass woofer and model a box for it, lets not get any deeper than this. Perhaps you took 6" driver or 15" driver or anything that was familiar to you. Now, either of them will require very different size box although they could play pretty much the same wavelengths only at very different sensitivity as the other one would have more cone area.

Alright, two different drivers, two different size boxes, two different circumstances for problems to appear even though the application is pretty much the same, play some bass and have enclosure to play it efficiently not losing it in destructive interference. Common thing for both is wavelength of the bass they are supposed to play. Both are smaller than wavelength for the very low bass and at some frequency the box is half wavelength in size and internal standing waves happen which add some to the soup.

Alright, when wavelength is much longer than the size of the box what happens inside the box? Pressure changes pretty much uniformly inside. What are implications of the pressure change to the box walls? They balloon some. As there is a lot more surface area on the walls than on the radiator this might be significant (audible) if you allow them to. The ballooning depends on the pressure of course but also things like stiffness, joints on the corners, what frequencies the box (walls) resonates as the resonances would be the most audible, easily excited and would ring longer than the excitationg signal. Bracing to rescue. Bracing and other techniques are to make the box panels resonances higher up in frequency, out of bandwidth of your bass box. You could also dampen the resonances which is easier when they are higher up.

Problems arise when a bass box also plays midrange like in two way speakers, it might be impossible or at least hard to push resonances past the pass band to many kilo Hertz, especially the case with a big 15" box, much harder than with small 6" box. In this case it could be better to split the band in two, have separate bass box who is stiff not to resonate on passband and another box for the mid who then could even be a floppy one! Have resonances below the pass band. Or just use damping to kill high frequency resonances. Or have no box at all for mids because it doesn't play bass 😉 Vast subject.

Hows that turned into practice? For bass box, any bracing is good, extensive is just more work and at some point diminishing returns, use at least one between each opposing walls. You might use math or accelerometers or someone elses work to get some hunch what the panel resonances are and how much bracing is actually needed to shift them. You could also check out what makes the resonances where to put braces in order to have less braces more effective. Braces might also harm (at least do no good, only change things) if the box resonances were below pass band but now pushed up to the pass band.

Some people go crazy with it while some seem to be more pragmatic. For example mr. Geddes uses single brace for all opposing walls, so total three, tied together on the middle with shear damping included, its big two way box and cast from some material so a different case than subwoofer made from MDF or small shoebox.

You can go great extent on this but it is not necessary, just use some bracing. If it feels like there is a problem then take a closer look. Speakers have lots of problems and often solving one just creates some other problems to be solved. This is the great mystery and joy of speaker design 🙂

 

[stv]

If you don't need scientific accuracy and calibration this may be interesting:
https://www.instructables.com/Make-a-Contact-Microphone/

 

[tone?]

😀 Yeah, loudspeakers are one helluva system, lots of aspects interacting. Alright, lets make a thought process

Usually the box is for a woofer to prevent sound from back and front of the cone to cancel, better efficiency for bass. Now think a box for bass for now, simplified. Take some bass woofer and model a box for it, lets not get any deeper than this. Perhaps you took 6" driver or 15" driver or anything that was familiar to you. Now, either of them will require very different size box although they could play pretty much the same wavelengths only at very different sensitivity as the other one would have more cone area.

Alright, two different drivers, two different size boxes, two different circumstances for problems to appear even though the application is pretty much the same, play some bass and have enclosure to play it efficiently not losing it in destructive interference. Common thing for both is wavelength of the bass they are supposed to play. Both are smaller than wavelength for the very low bass and at some frequency the box is half wavelength in size and internal standing waves happen which add some to the soup.

Alright, when wavelength is much longer than the size of the box what happens inside the box? Pressure changes pretty much uniformly inside. What are implications of the pressure change to the box walls? They balloon some. As there is a lot more surface area on the walls than on the radiator this might be significant (audible) if you allow them to. The ballooning depends on the pressure of course but also things like stiffness, joints on the corners, what frequencies the box (walls) resonates as the resonances would be the most audible, easily excited and would ring longer than the excitationg signal. Bracing to rescue. Bracing and other techniques are to make the box panels resonances higher up in frequency, out of bandwidth of your bass box. You could also dampen the resonances which is easier when they are higher up.

Problems arise when a bass box also plays midrange like in two way speakers, it might be impossible or at least hard to push resonances past the pass band to many kilo Hertz, especially the case with a big 15" box, much harder than with small 6" box. In this case it could be better to split the band in two, have separate bass box who is stiff not to resonate on passband and another box for the mid who then could even be a floppy one! Have resonances below the pass band. Or just use damping to kill high frequency resonances. Or have no box at all for mids because it doesn't play bass 😉 Vast subject.

Hows that turned into practice? For bass box, any bracing is good, extensive is just more work and at some point diminishing returns, use at least one between each opposing walls. You might use math or accelerometers or someone elses work to get some hunch what the panel resonances are and how much bracing is actually needed to shift them. You could also check out what makes the resonances where to put braces in order to have less braces more effective. Braces might also harm (at least do no good, only change things) if the box resonances were below pass band but now pushed up to the pass band.

Some people go crazy with it while some seem to be more pragmatic. For example mr. Geddes uses single brace for all opposing walls, so total three, tied together on the middle with shear damping included, its big two way box and cast from some material so a different case than subwoofer made from MDF or small shoebox.

so basically what you are telling me is that.

yes there is a science to this. ( which ok is a given)
but most people just brace cabinets with a certain amount of math and not much else to really pinpoint things.

so I should brace and see what it sounds like.

lol

 

[tone?]

If you don't need scientific accuracy and calibration this may be interesting:
https://www.instructables.com/Make-a-Contact-Microphone/

honestly man.
i don’t understand why no one makes a simple accelerometer to do these measurements for speaker builders.

i think I am getting my answer.
it is kinda shot in the dark unless the manufacturer has the time and money to spend on actually seeing what the braces are doing and maybe where to put them in the optimal spot.

so I should brace the box. And brace for the best.

 

[tmuikku]

What I'm trying to say is that its more complicated than that 😀 You can start from there but as you develop the speaker further you might notice things need to change and now your box might need more or less bracing, or no box at all!

 

[tmuikku]

so basically what you are telling me is that.

yes there is a science to this. ( which ok is a given)
but most people just brace cabinets with a certain amount of math and not much else to really pinpoint things.

so I should brace and see what it sounds like.

lol

What I'm trying to say is that its more complicated than that 😀 You can start from there but as you develop the speaker further you might notice things need to change and now your box might need more or less bracing, or no box at all! so, this is as good starting point as any, use some bracing and adjust when needed. Real improvement is when you solve not just one problem but many, and sound gets very good when all problems are solved from audio related issues to your wallet and aesthetics for example. And even then you'd probably make some compromise to audio for reduced cost or better aesthetics.

Difference on DIY speaker project to IT projects is that usually there is no third party saying what the cost or looks or schedule has to be although there sometimes is, certainly marketing department and old habits and believes can be tossed at will as it is you who defines the work group its belief system, values, objectives and motivation.

Look for username augerpro, he has done quite extensive testing recently about these, damping and all. Sorry I don't have the link here.

 

[stv]

lol

tmuikku is just trying to give you some background.
don't be mad at him.

why no one makes a simple accelerometer

because the effects of panel resonance are usually rather small compared to many other factors (ok - I know, that's a dangerous statement).
and it's a rather complicated issue.

you will find plenty of accelerometers (or contact microphones), just use google.