The gable end of the studio, on which the attenuators are mounted, face the lane around 50-60m away. With the 150mm vent hole the occasional car can be heard as you would expect. As soon as the covers were fixed to the attenuators the sound from outside disappeared completely. Cars now pass in silence. They work amazingly well.
I have decided to mount the fresh air fans outside the building. This is really to make access for maintenance easier and I can be sure this is the quietest option. The gable end has quite a large overhang and the vents are close to the soffit so it should be an easy job to make a weather resistant enclosure.

This is the fan I am using. It is a little noisy at full blast. 44db@1m. 150mm moves around 500m3 per H which is around 10 changes per hour full blast. My duct run is very short so there will be limited static pressure in the system, so these fans are relatively inexpensive and therefor ideal.

Temporary connection of the fresh air inlet. I experimented putting the fan outside and it is virtually silent inside at full blast with a hard connection to the exterior attenuator. Set at the lowest speed it still moves plenty of air, which doesn’t generate any air rush noise at the inlet. I will be able to rely on this for most of my cooling needs this time of year. I now have given myself another unexpected job to make an external enclosure with a flexible connection to the inlet attenuator to eliminate any risk of vibration passing. Overkill, but that is generally the name of the game....
I am also finalising the air conditioning plans and should have an update next week.
To add to an earlier point, although possibly not the place, the business aspect of such a venture relies on so many factors it is not possible to suggest whether it is viable or not. Everything from income expectation, requirement to cover costs and live, to the investment in equipment etc. A few examples are owning the real estate means no rent or debt to repay, or having equipment with no debt to service etc. There is more that can be done in a studio than recording bands or singer song writers or voice overs if you need to. My motivation is to work with as many people as possible to gain more experience, so income is not the driver. Whoever I work with, at whatever level seems to add something that improves what I do. That is enough for me, oh and I love every second of it, from the spine tingling vocal to unexpected bum note!
Cheers,