Negative Acoustic pressure values

[S1mul4t0r101]

Hello.
I am doing an analysis of sound propagation from a solid disc when a periodic load is applied ro it. i got the pressure values for different ferquencies with the Steady state Modal analysis but some of the pressure peaks are negative. What do t hese negative pressures mean? Are they vacuum pressure i.e. pressure below 1atm? Or are they pressure with just opposite direction? Should I consider these pressures as my sound level values?
Here are the Graphs for reference>

does this graph mean all the sound is damped?>>

 

[BridgeSmith]

Damped would be the amplitude decreases. Yes, the negative numbers are due to negative pressure on the microphone element - a momentary pressure level less than 1atm, caused by the speaker element moving away from the microphone.

 

[GregLocock]

(a) I suggest you greatly increase your frequency resolution up to 1000 Hz
(b) It is not clear what you are plotting. Is this the rms pressure at each frequency? That should always be positive. Or is the transfer function between 2 channels, in which case a negative value implies they are out of phase. If so, what are the two channels? In that case it is more common to plot the amplitude and phase separately, or even real vs imaginary to create a Nyquist or Argand diagram.

Cheers

Greg Locock


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[S1mul4t0r101]

@GredLocock, my analysis is a steel disc in an air domain and I am plotting the acoustic pressure in the air domain duing the steady state modal analysis from different observation points. I am not sure if it is the RMS pressure at each frequency point. I am just using the POR values mentioned in the field output option. I will attach the model here to further clarify my simulation.

This is the assembly>

This the load applied>

This is the air domain>

This i the disc>

 

[GregLocock]

Well if you don't know what you are plotting then I certainly don't. A modal implies there is some excitation somewhere, so I suspect you are looking at the transfer function of pressure at a point over whatever the excitation is (probably a velocity applied somewhere on the disc). In which case negative values certainly make sense. This is a standard case for which analytical solutions exist - research radiation efficiency of a disc in free air, or piston without a baffle. Is the disc flexible? If so you will also get coincidence effects, where the speed of sound matches the speed of vibration.

Cheers

Greg Locock


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[S1mul4t0r101]

"Nodal output variable POR (pressure magnitude) is available for an acoustic medium in ABAQUS (in ABAQUS/CAE this output variable is called PAC). When the scattered wave formulation is used with incident wave loading in ABAQUS/Explicit, output variable POR represents only the scattered pressure response of the model and does not include the incident wave loading itself. When the total wave formulation is used, output variable POR represents the total dynamic acoustic pressure, which includes contributions from both incident and scattered waves as well as the dynamic effects of fluid cavitation. For either formulation output variable POR does not include the acoustic static pressure, which is used only to evaluate the cavitation status in the acoustic medium.

In addition, in ABAQUS/Standard nodal output variable PPOR (the pressure phase) is available for an acoustic medium. In ABAQUS/Explicit nodal output variable PABS (the absolute pressure, equal to the sum of POR and the acoustic static pressure) is available for an acoustic medium."


This is how abaqus defines POR as. I am using Abaqus/standard. so, what does the negative value mean?

 

[rajeshs93]

Hi S1mul4t0r101

I hope you are doing well.
I found your work on acoustic analysis. The simulations you have shared are good. I was particularly interested in your approach to modeling and analyzing the acoustic behavior of plate structures. I have done the dynamic analysis.

I am simulating acoustics Analysis of steel plate is assumed to be vibrating in air.

I don’t know how to model the acoustic medium.

I need your help on how to model and simulate the acoustic medium on abaqus.
How can I extract acoustic pressure vs frequencies in Abaqus.

Please share your mail we will discuss in detail.
Can you please help me how to do it this?