I've found that even the same kind of bell if you come to buy another one because you like the sound can sound different, maybe flatter or duller. I have a cheapo
Decathlon one (bought in Asda), the type which has the "I love my Bike" sticker on which I removed, and that's been on my Spirit for about three years now. It's a brring brring type. A passer by even said to their companion as I passed them on a local rail trail recently, "what a nice sound!"
I tried to get another one for the Rans when I bought it but having a rummage among similar bells in store, they didn't sound as good. I ended up buying a large Dutch style Dong-Ding chrome bell (why not, there's loads of room on the Rans bars!). A sort of tribute to the remote controlled one I had on the Linear. I tried several before I picked this one. If you press the lever repeatedly it gives a good warning sound, or you can just give a Dong-Ding, or if you press and hold it down it has a long resonating note which slowly fades away.
I had a Chinese Knog-a-like on my Brompton until it spontaneously disintegrated at the end of last year after three years' use. I replaced it with an old Adie made in England pinger which has been on many bikes over the decades before taking up residence in my bits box. Perhaps it's more appropriate on the made in England Brompton.
Before the demise of Wilko I'd bought a couple of their electric buzzers which are on the bars of the Spirit and Rans. They don't take up much room, run on an AA battery each and make a pleasant chirping sound which seems to get the attention of pedestrians without causing undue alarm and despondency. The bikes themselves are pretty quiet so it's good to have a back up.
There's always the "excuse me!" "coming through!" "mind your backs!" or harrumphing as an alternative.
This is on shared paths, rail trails and such like. On the road, you would need something that makes a lot of noise and probably needs to be big. Airzound and bottle? I did modify a boat horn to fit a bottle cage but it all adds size and weight for the amount of times you might need to use it.
Maybe a whistle and lanyard? Portable, but not readily identifiable as belonging to a bike.