I do not see a link between reverberation time and environmental noise! Reverberation time is a concept of relevance in enclosed spaces. Environmental noise is measured outside. It is true that noise getting in an enclosed space is amplified by it, the more it is reverberant as gain is usually associated with reverberation. But it does not make reverberation time some relevant measure to environmental noise. 

I look for accurately sound level meter and reverberation time applications for measuring environmental noise coming from indoor ventilation fan 

It is better to have measurements on daytime or maybe on night time for indoor ventilation fan?

B&K sound level meter is an expensive choice, but very accurate.

B&K 2250 or 2270 

If you mean "applications" as in "smartphone apps", there are two recent articles in the Journal of the Acoustical Society of America on sound level meter apps:

http://dx.doi.org/10.1121/1.4865269

http://dx.doi.org/10.1121/1.4964639

Many thanks to all.

I found helpful comments.

 I wish to flesh out more in applications with B&K. Indeed, a very good sound level meter which I can also recommend.

Only environmental noise I take into consideration. I'm not going so far in a private side to develop an application for measurement of the reverberation time using smartphone microphone. It is a cheap way and will not bring clear results.

Also, I take into consideration very accurate choice for measurements.

Best regards to all,

Drd. Elena Zorila

Sorry @Claes, I don't use Android in research.

Maybe it is appropriate for you to concentrate on other questions.

Thank you very much.

And by the way, no fun at all.

Best regards,

Drd. Elena Zorila

I would recommend a XL2 by NTi Audio for ease of use and functionality. If the SLM has a class 1 specification, then why would we expect any difference in accuracy from one meter and another?

The most accurate way of measuring RT, known as Schroeder's method, is to play a digitally created pulse, known as TSP (Time Stretched Pulse), in a space and record the Impulse Response, then analysing that for RT. Other simple-to-use method is to use B&K Hand-held Reverberation Time Analyzer — Type 2250-F with Reverberation Time Software BZ-7227 and Qualifier Light Type 7831. Link: https://www.bksv.com/doc/bp2152.pdf

I think sound level meter and any RT measurement is not suitable for your need. Even if you are using the B&K handheld analyzer 2270. It is not enough. Alternatively, you may use the acoustic camera (Norsonic 128 ch) for measuring the far field and the P-u probe (Microflown) for the near field.  

Sound level meter type A or B.

I apologies for you @Raymond Ramousse. I thought that I wrote a question on researchGate with competent answer at this issues. Everyone can add a competent answer at some questions from topics research. Although you look to have some publications  and although I do not do research on your age and the organization you represent it is a bit shameful for you to respond by declaring yourself incompetent ... but ...

Thank you all for competent answers! I found some very good ones!

I would recommend a Class 1 Sound Level Meter. The advantage of Class 1 instruments is that they have a higher accuracy over a wider frequency range. Especially outside the human hearing threshold in regard to frequencys (under 16Hz and over 16kHz) Class 1 instruments have a higher accuracy. Depending on the application, you have to decide which accuracy class is sufficient for you. For environmental noise, there are some norms like the ISO20906 (for airport noise) that presuppose Class1 instruments. Further information about suitable instruments and about FAQs to sound measurement can be found here:

https://www.pce-instruments.com/english/measuring-instruments/test-meters/sound-level-meter-noise-level-meter-kat_40095.htm