HEY, WAIT!! The TS never mentioned anything about charging a battery! This was for ssomething in the back of a truck with a 24 volt system. So why presume that it is for battery charging? And charging a battery at the back of a big truck would not make sense any way because of the resistance of all the long wires. The alternator noise (in the system) does not get loud until it is delivering current, and so there is no reason to worry about engine speed.
And with the engine not running the frequency is zero, so the filter frequency is not so very critical.

The commercial product that performs this function (switch on when alternator is running) is commonly called a “split charge relay”, which suggests that charging a battery is a very common application. Generally the said battery is 5 metres further away, down some cable which is probably not as thick is it should be, in the back of a caravan. (edit - although that might not be the case for a towing vehicle that has 24V electrics)

 

The commercial product that performs this function (switch on when alternator is running) is commonly called a “split charge relay”, which suggests that charging a battery is a very common application. Generally the said battery is 5 metres further away, down some cable which is probably not as thick is it should be, in the back of a caravan. (edit - although that might not be the case for a towing vehicle that has 24V electrics)

That reference was made by you and never acknowledged as applicable by the TS.. Hence my comment.