I use an in-house CFD code to simulate the flow over Idealized Side Mirror. The reference paper is Caraeni2011. To compare with the reference paper, I obtain the pressure history at the monitor points. By subtracting the mean of the pressure history, the fluctuation pressure is obtained.
Sound Pressure Level is defined as follows,
https://wikimedia.org/api/rest_v1/media/math/render/svg/145443bafb64759904ba790408936f7fe1958a8a
Now, my question is: is the fluctuation pressure the same as the sound pressure? Can I directly use my fluctuation pressure to calculate the SPL?
I know SPL is a single value and it requires Root Mean Square of the pressure history. What I want to calculate is actually the spectra of SPL. Just like the following pic. The unit of frequency is dB, and I guess it's typo.
https://ws1.sinaimg.cn/large/006pwg4ugy1fj0zlypxnqj30ll0hqjsi.jpg
In order to get the spectra of SPL, I apply FFT on the fluctuation pressure history and then feed the absolute value of the FFT result as input into the SPL formula to get the spectra of SPL.
https://en.wikipedia.org/wiki/Sound_pressure#Sound_pressure_level
https://ws1.sinaimg.cn/large/006pwg4ugy1fj0zlypxnqj30ll0hqjsi.jpg
https://wikimedia.org/api/rest_v1/media/math/render/svg/145443bafb64759904ba790408936f7fe1958a8a
Conference Paper Tandem Cylinder and Idealized Side Mirror Far-Field Noise Pr...
I cannot believe that Researchgate provides no function of editing scientific formula and inserting any inline image! Only text!
to calculate sound pressure level, you need pressure fluctuation data, which later divide with reference sound pressure. so, for your question "is the fluctuation pressure the same as the sound pressure? " is not same. this is because pressure fluctuation are depends on the flow generation and while the sound pressure, it depends on the reference sound pressure in air. For your another question "Can I directly use my fluctuation pressure to calculate the SPL?", yes you can. Use the equation Sound pressure = 20 log (P'/ P0) , where P' is pressure fluctuation and P0 is reference sound pressure in air.
Dear Jingchang Shi,
Yes, indeed a calculation of the Overall Sound Pressure Level (OASPL) is based on a value of the Root Mean Square (RMS) of pressure fluctuations p'rms [Pa] (referenced to the treshhold of human hearing 2*10^-5 Pa): OASPL dB = 10 log (p'rms^2/p'ref^2) dB. The Sound Pressure Level is a frequency dependent parameter. To get it you need to make and DFFT of p'(t) [Pa] first and transfer the result to the logarithmic scale and get power spectrum of the signal (exact formulas to do that you can get from Matlab examples). As a result you get SPL(f). If you integrate SPL(f) you should get OASPL dB (valuable check). The other thing is that dB is not a unit! It is just an indication of the logarithmic scale used.
In aero-acoustics we speek of near- and far-field data. The monitored points on a surface are in the near-field, so the signal is rather a "pressure fluctuation" - not just sound, but sound+flow. What you get In the far-field is the actual sound disturbance of air.
I hope it was usefull.
Dear Oskar
After doing DFFT and power spectrum of p'(t), we get SPL (f), which is in dB/Hz. Integrating SPL(f) with df, we should get OASPL in dB. I was trying to do this as a cross check with my pressure fluctuations.
If I use Prms value for OASPL calculation, I am getting 92 dB, where as after SPL(f) integration I am getting 82 dB. What could be the mistake I am doing ? I am thinking that my simulation data (time series) length is not sufficient, that why I am missing out some frequencies and hence SPL also.
Can you please help me on this. Please find attached the timeseries data of my pressure fluctuations.
Please look in Matlab examples available in the internet. The explanation is there...
FFT, DFFT...
Take care
Oskar
Yes, I have gone through the fft part of matlab before. Based on that only my code is running. I tried with reduced no. of samples (N) also, but still getting a 10 dB difference in OASPL when using Prms (eqn: 20 log (Prms/Pref)) and when using SPL (f). (eqn: 10 log (Sum(10^(SPL/10)). Since I am getting expected OASPL from Prms value, the fluctuations seem to be correct and I should be getting same OASPL from SPL(f) integration also. I am confused why it is so.
There is a factor in DFFT that includes the length of the sampling, number of samples, etc... please look there.
Oskar
Joemon, are you shure, that numerical integration of spectrogram is correct?
Maybe for simplest integration something like this:
10log(Delta f * Sum (10^(0.1 SPL(1:length-1))
where Delta f is frequency step of spectrogram, that comes from fft.
Hi Ireneusz
Thanks for the answer. Yes, I was missing the 'Delta f' part.
I have written an answer in the following link:
https://www.researchgate.net/post/Calculation_of_OASPL_Overall_sound_pressure_level_from_PSD_of_pressure_signals
Dear all,
I am actually having a similar question (I think). I do not quite understand the relation between flow induced pressure and sound pressure (not sound pressure level). In the equation for calculating SPL, the ratio of Prms to Pref is used. What I understand is that Prms here is the pressure induced by sound wave. What I do not understand is the relationship between sound wave and the flow.
Let's take a case of combustion in an engine room. Combustion will create a sudden pressure build-up that results in flue gas flow via an exhaust pipe. Since combustion in an engine is periodic, there will be a pressure oscillation that results in oscillation of flue gas velocity.
Is Prms of this combustion pressure oscillation same as the Prms of sound wave, so that it can be used directly to calculate the SPL?
Regards,
Daniel
I am actually having a similar question I do not quite understand the relation between flow induced pressure and sound pressure
could someone answer me?
I can try answering with an example of flow around a bluff body. As the flow happens over the bluff body, due to the velocity gradients, shear stress on the surface, pressure fluctuations are developed on the surface of bluff body. These pressure fluctuations are induced due to the flow and hence termed flow induced pressure fluctuations. These pressure fluctuations can be converted to sound pressure level (SPL) using a reference pressure (Pref) and the equation 20*log(p/Pref), where p is the pressure we monitored.
SPL is a term used to quantify the obtained pressure fluctuations in terms of decibel. The input to this SPL calculation could be any pressure fluctuation, be it flow induced pressure or any other type of origin.
In short, flow induced pressure is the pressure generated due to the flow occurring and sound pressure level is the way we convert the obtained pressure fluctuations to a decibel value by comparing it with a reference value of our choice.
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