Aeroacoustic sound

[TheUbiquitous]

Hello guys,

sound power level or sound pressure level are basically defined as logarithm of a ratio of the root mean square ("effective") pressure and reference pressure. In case of sound pressure level, the reference pressure was selected as 20 microPa since it is considered as threshold of human ear. The issue is the root mean square pressure - a harmonic signal is assumed in fact. This definition, in other words, is based on the idea that human ear hears something because of oscillations of the pressure in otherwise stagnant fluid having a constant pressure (e.g. atmospheric pressure). However, let us regard the following situation:

A pressure probe is placed into a fluid having a random fluctuations with variable mean value in time. For example, imagine a pressurized container exhausting the gas into a free space through a pipe with a valve. There will be high turbulence and mean pressure value will change in time as well due to pressure decrease in the container and subsequently in the whole system. The pressure probe is placed in the pipe. Definition of root mean square does not make sense any more as mean value is changing and signal is not harmonic either. How define the sound pressure level in such a case?

[studiot]

The bel and decibel are defined in terms of the ratios of power levels not pressure levels.

Obviously power can be related to a 'pressure' (or other variable depending upon the physical nature of the signal, which may be electric etc) but it is not necessary.

Power levels do not depend upon whether the flow is turbulent or not.

[TheUbiquitous]

Yes, but the issue is the changing mean value in time, turbulent flow is a reasonable example. How to evaluate sound power level in transient random signal ?

[studiot]

28 minutes ago, TheUbiquitous said:

Yes, but the issue is the changing mean value in time, turbulent flow is a reasonable example. How to evaluate sound power level in transient random signal ?

Depends whether by 'evaluate' you mean measure or predict theoretically.

The question only has meaning in terms of measuring the instantaneous power for a defined period of time and averaging it.

The result will vary with the width of your time window.

 

[TheUbiquitous]

Thanks for the answer. I mean prediction computationally by software. Then, the value of sound power/pressure level depends on the selection of the window without any rigorous formula.. it is quite vague.