[opacheco]

Hi again!

I have been searching for the theory behind of Inert Mass Absorber (Limp Mass absorber) design theory and l find the membrane  diaphragmatic resonator only that’s is very similar to the Inert Mass Absorber but the functionality is very diferent and the almost the only way to absorb the high pressure very low frequency waves in a room

I will be happy if anybody can help me in order to define the design procedure for get  Inert Mass Absoeber for lower frequencies about 125Hz.

Thanks anticipated !
Opacheco.

[abbey road d enfer]

Hi again!

I have been searching for the theory behind of Inert Mass Absorber (Limp Mass absorber) design theory and l find the membrane  diaphragmatic resonator only that’s is very similar to the Inert Mass Absorber but the functionality is very diferent and the almost the only way to absorb the high pressure very low frequency waves in a room

I will be happy if anybody can help me in order to define the design procedure for get  Inert Mass Absoeber for lower frequencies about 125Hz.

Thanks anticipated !
Opacheco.

Google does not return anything for Inert Mass Absorber. I guess it's something heavy that absorbs low frequencies, but unless someone uses some kind of levitation, this thing must be suspended, and thus becomes a 2nd-order resonator.

EDIT: Limp mass absorber shows results. They are resonators for which the charcateristic frequency is tuned very low and the damping is very high. As a result they act as a LF shelving absorber.

[scott2000]

I will be happy if anybody can help me in order to define the design procedure for get  Inert Mass Absoeber for lower frequencies about 125Hz.

Thanks anticipated !
Opacheco.

I guess it all depends on your space requirements too??? I looked at those VPR bass traps some time ago as well... Haven't looked into stuff lately though....

Interesting...Please update what it is you are specifically looking for and what you find out...... I'm familiar with limp mass more for sound proofing but do think I've seen something you're talking about somewhere...

[opacheco]

Google does not return anything for Inert Mass Absorber. I guess it's something heavy that absorbs low frequencies, but unless someone uses some kind of levitation, this thing must be suspended, and thus becomes a 2nd-order resonator.

EDIT: Limp mass absorber shows results. They are resonators for which the charcateristic frequency is tuned very low and the damping is very high. As a result they act as a LF shelving absorber.

Dear abbey road d enfe, thanks for your comments but I think this thing isn’t a second order system because these units aren’t resonant system in the mass-spring way!!.....or maybe these systems caan be a second order system with a Q very low or shelf type acoustic filter........I am not really sure

Opacheco

[opacheco]

I guess it all depends on your space requirements too??? I looked at those VPR bass traps some time ago as well... Haven't looked into stuff lately though....

Interesting...Please update what it is you are specifically looking for and what you find out...... I'm familiar with limp mass more for sound proofing but do think I've seen something you're talking about somewhere...

Scott2000, thanks a lot for your comments and notes.

I am really think to do a very low frequency absorber exploited the "oscillating" characteristics of this device driven by levels of sound pressure level incident at low frequency through placing it in front of some type of absorbent material such as mineral fiber or another type.......I would like to do this in a scientific way with a mathematically model.....what is the math behind of this inert mass design ??

Thanks again
Opacheco   

[opacheco]

Any idea?

Opacheco,

[abbey road d enfer]

Dear abbey road d enfe, thanks for your comments but I think this thing isn’t a second order system because these units aren’t resonant system in the mass-spring way!!.

Really? Aren't they a suspended mass?

....or maybe these systems caan be a second order system with a Q very low

That's exactly what I wrote earlier.
If you go there
https://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/743040-tims-limp-mass-bass-absorbers.html
there's a calculator that gives the frequency.

[opacheco]

Really? Aren't they a suspended mass?
 That's exactly what I wrote earlier.
If you go there
https://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/743040-tims-limp-mass-bass-absorbers.html
there's a calculator that gives the frequency.

This is a suspended mass but not all the supend mass is a resonator for audio spectrum I think?......however, the limp mass must have another treatment different instance to the membrane resonator!!......look like this Inert mass is a low shelf filter type and according to the analog electronics shelf filter have different transfer function than pass band filter!

Opacheco.

[abbey road d enfer]

This is a suspended mass but not all the supend mass is a resonator for audio spectrum I think?.

For not being a resonator, it would need to be suspended with a material that has no elasticity, only viscosity. To my knowledge, there is no such material that is convenient.
Peter Janis of Primacoustic mentions, regarding their limp mass absorbers "The rear cavity and diaphragmatic resonator does the magic on the low end."

.....however, the limp mass must have another treatment different instance to the membrane resonator!!.

Not the mass itself; the suspension needs to be so heavily damped as to smother the resonance.

.....look like this Inert mass is a low shelf filter

A mass in itself is not a filter. Only in conjunction with elasticity and/or viscosity can it act as a filter. As I wrote earlier, the only possibility is that the resonant frequency is rejected low enough and damped enough so it looks like a shelf. Similar to using a bell EQ at very low frequency to emulate a low shelf.

according to the analog electronics shelf filter have different transfer function than pass band filter!

Indeed the responses of a shelf and a bell filter are different, however they can appear to have similar effect.
Check the graph; the responses don't look terribly dissimilar, do they?

[abbey road d enfer]

However, one is a shelf, the other a bell.

[john12ax7]

Do you have a picture of what you are calling an inert mass absorber?  Or is the goal simply a usable design for sub 125 Hz absorption?

[abbey road d enfer]

Do you have a picture of what you are calling an inert mass absorber? 

It seems the vulgar name is limp mass absorber. There is a pretty good description there
https://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/743040-tims-limp-mass-bass-absorbers.html
See first post, first attachment: limpmassbass1.pdf
So far, the tuning formula is the same as for diaphragmatic absorbers.

[opacheco]

A mass in itself is not a filter. Only in conjunction with elasticity and/or viscosity can it act as a filter. As I wrote earlier, the only possibility is that the resonant frequency is rejected low enough and damped enough so it looks like a shelf.

I don't think so!....in the Impedance Analogies Force to Electric Voltage and Volume Velocity to Electric Current, every mass is like an Inductor and if you add some Rs, this become a High Pass Filter in a T section  fashion with some loss....

Opacheco.

[opacheco]

It seems the vulgar name is limp mass absorber. There is a pretty good description there
https://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/743040-tims-limp-mass-bass-absorbers.html
See first post, first attachment: limpmassbass1.pdf
So far, the tuning formula is the same as for diaphragmatic absorbers.

I don't know but J.A. Fletcher, M.A. in  BBC have mention to this equation and this is very different to the Membrane Resonator! at least it look very different  to me (I don't know if this is a equivalent form of the Membrane Resonator equation??)

I have a doubt in this equation; I suppose the SI is the Unit measurement system used that but I would like more info about the P the air pressure variable because I suppose (but I am not really sure!!) P must be the Atmospheric Pressure plus the Increment of the Pressure due to the Acoustic Pressure (dB re equivalent to the Pascals).....that will be very small quantity and think this not alter the result in any way!!....It do??

Opacheco.

[abbey road d enfer]

I don't think so!....in the Impedance Analogies Force to Electric Voltage and Volume Velocity to Electric Current, every mass is like an Inductor and if you add some Rs, this become a High Pass Filter in a T section  fashion with some loss....

Opacheco.

And how do you physically manage to suspend the mass without any elasticity?

[abbey road d enfer]

I don't know but J.A. Fletcher, M.A. in  BBC have mention to this equation and this is very different to the Membrane Resonator! at least it look very different  to me (I don't know if this is a equivalent form of the Membrane Resonator equation??)

I can't really comment on this formula since I don't understand the definition of gamma (ratio of specific heats of air?), but as I see it, it gives the frequency as a function of panel size, volume of enclosed air and mass per unit area, which is exactly what the formulae of diaphragmatic resonators are.
The big problem is there is a lot of snake oil involved in these absorbers, so you can count on their designers to make things unclear.

[opacheco]

I can't really comment on this formula since I don't understand the definition of gamma (ratio of specific heats of air?), but as I see it, it gives the frequency as a function of panel size, volume of enclosed air and mass per unit area, which is exactly what the formulae of diaphragmatic resonators are.
The big problem is there is a lot of snake oil involved in these absorbers, so you can count on their designers to make things unclear.

I am designing a couple of these units using this formulae and I will test the results........it isn't the same formule for the resonator membrane, I think!!.....may be I wrong!! but let me to test build some units and I will let you know!

Opacheco.

[PRR]

> how do you physically manage to suspend the mass without any elasticity?

Imagine a sheet of Lead. Yes, it has elasticity, due to stiffness and its pendulum action. However I think the point of this train of thought is that the diaphragm stiffness is much-less than the stiffness of the trapped air.

The ratio of specific heats appears often in air problem. One is adiabatic and there's another, isothermal? Do we give heat time to escape?

The l1*l2/V reduces to just the depth of the box.

P is just air pressure, to know air density.

The square-root means that to get low frequencies you will need a VERY deep box or VERY massy diaphragm. But something like a half-inch of Lead may not be "limp". I suppose you could use lead weights on a strong limp cloth.

[abbey road d enfer]

> how do you physically manage to suspend the mass without any elasticity?

Imagine a sheet of Lead. Yes, it has elasticity, due to stiffness and its pendulum action. However I think the point of this train of thought is that the diaphragm stiffness is much-less than the stiffness of the trapped air.

Stiffess of air is elasticity; I maintain it's a 2nd-order system. A 1st-order system would mean only friction, then I suppose we have a problem with keeping the diaphragm in place.

The ratio of specific heats appears often in air problem. One is adiabatic and there's another, isothermal? Do we give heat time to escape?

Thermodynamics is a very old story for me, so I wouldn't comment on that.

[opacheco]

Stiffess of air is elasticity; I maintain it's a 2nd-order system. A 1st-order system would mean only friction, then I suppose we have a problem with keeping the diaphragm in place.
 Thermodynamics is a very old story for me, so I wouldn't comment on that.

Again, Not all Stiffness of air or diaphragm, materials or structure is a compliance in acoustics.....it is a matter of Wavelength! The first order can be the issue in this vibrator, not resonator, it is a vibrator converting the audio energy to movement!!.....

Opacheco.

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