Technical Question: Port Ratios

[Eclipse DDS]

Hi,

I'm curious to see if anyone knows what would happen if rotor and stator of a siren had a different number of ports? What effect would this have on the sound? It certainly has some effect, as demonstrated on sirens with the rotors swapped around. https://fb.watch/vFRAiNmy2p/

[ArxCyberwolf]

B&M did this with several of their sirens, by using half-ported rotors. For example, their 15-H-2 and 15-H-4 came with 12 port stators, and 6 port rotors, while their 20-AR-7 came with 16/20 port stators, and 8/10 port rotors. This resulted in heavy undertones, as well as significantly increased sound pressure through the ports as the result of only a quarter of the siren's ports being opened at any given time. These half-ported B&Ms are significantly louder than conventional sirens according to those who have heard them like Armoturo. Note that this only will work with rotors that are half-ported.

Example of a 15-H-4: https://www.youtube.com/watch?v=wtAOVMBFDmg&t=39s
A Roblox model of a 15-H-4 by Armoturo that more clearly shows what I described: https://www.youtube.com/watch?v=KA82FoGznDM&t=9s
Ditto, but with a 20-AR-7: https://www.youtube.com/watch?v=3bncd5_fJNk&t=88s

This idea would not work well if you tried to throw mismatched ports together (say, a 10 port rotor inside a 12 port stator) because of the differences in size between the rotor and stator ports. You can tell from the video just how quiet the siren is, because the air moving through the ports is escaping without being chopped. Nirvoxide tried doing this with their 2001, by placing a 10 port XT22 rotor inside of theirs, and it had similar results to the video you sent. It mainly just blew air and didn't make a whole lot of meaningful noise.

[Eclipse DDS]

B&M did this with several of their sirens, by using half-ported rotors. For example, their 15-H-2 and 15-H-4 came with 12 port stators, and 6 port rotors, while their 20-AR-7 came with 16/20 port stators, and 8/10 port rotors. This resulted in heavy undertones, as well as significantly increased sound pressure through the ports as the result of only a quarter of the siren's ports being opened at any given time. These half-ported B&Ms are significantly louder than conventional sirens according to those who have heard them like Armoturo. Note that this only will work with rotors that are half-ported.

Example of a 15-H-4: https://www.youtube.com/watch?v=wtAOVMBFDmg&t=39s
A Roblox model of a 15-H-4 by Armoturo that more clearly shows what I described: https://www.youtube.com/watch?v=KA82FoGznDM&t=9s
Ditto, but with a 20-AR-7: https://www.youtube.com/watch?v=3bncd5_fJNk&t=88s

This idea would not work well if you tried to throw mismatched ports together (say, a 10 port rotor inside a 12 port stator) because of the differences in size between the rotor and stator ports. You can tell from the video just how quiet the siren is, because the air moving through the ports is escaping without being chopped. Nirvoxide tried doing this with their 2001, by placing a 10 port XT22 rotor inside of theirs, and it had similar results to the video you sent. It mainly just blew air and didn't make a whole lot of meaningful noise.

Ah right, that's interesting thanks. When you say that "this will only work with rotors that are half-ported", what do you mean?

If you're wondering about my application, I've been thinking about modelling a siren with a 4 port rotor and an 8 port stator.

[ArxCyberwolf]

Ah right, that's interesting thanks. When you say that "this will only work with rotors that are half-ported", what do you mean?

It means the rotor has half the port count of the stator. So like in the examples I mentioned, 6 port rotors inside of 12 port stators, or 8/10 rotors inside of 16/20 stators. Your example is also a half-port, so it'd work.

[Eclipse DDS]

Ah right, that's interesting thanks. When you say that "this will only work with rotors that are half-ported", what do you mean?

It means the rotor has half the port count of the stator. So like in the examples I mentioned, 6 port rotors inside of 12 port stators, or 8/10 rotors inside of 16/20 stators. Your example is also a half-port, so it'd work.

Ah right, that makes sense now. Thank you!