Perhaps you should check (my guess - such systems can be greatly improved using Silicon Photomultipliers):

ACOUSTIC VELOCITY MEASUREMENTS IN THE AIR BY MEANS OF LASER DOPPLER VELOCIMETRY: DYNAMIC AND FREQUENCY RANGE LIMITATIONS AND SIGNAL PROCESSING IMPROVEMENTS

doi.org/10.1006/jsvi.1999.2516

Thank you. The flow velocites come close to the range we are looking for.

However, I think the method described in the article is too complicated for our purpose, in particular this regards

We are looking for a sensor that may be built in a wall, or a roof in order to measure flows in cavities.

However, the areticle is interesting, also because I did not know this approach beforehand. It helps me to complete the picture solidifying my current impression that sensors that we are looking for still are to be developed by research/industry.

Thanks and again and kInd regards,

Thomas

Also, you may check

Ionization flow meter https://patents.google.com/patent/US4272995A/en

Thanks again.

What we need is something that while measuring hardly disturbs the object to be measured. An objective which is not so easy to attain even when just measuring the airflow in an air duct of a ventilation system. From the patent text I cannot see how small these deveices can be made, but I fear they are quite large.

However, the following text from the patent description moreover seems to constitute majour obstacle:

The general use of this flow meter is seriously limited by the fact that the product m.d must be constant. This product is formed to be independent of the pressure and thus of the density of the gas in a wide range around one atmosphere (105 pascal), but is not independent of the temperature of the gas and is not independent of any contaminants, including water vapour.

Thanks for the efforts you took/attention you paid to this thread!

Kind regards,

Thomas