I am recording city sounds such as high street ambiance and bus/ tube journeys. I have a zoom H6 and a choice of XY mic, MS mic and a shot gun. My overall aim is to capture and make 2 to 3 minutes long montages of the street or bus scenes I record, so all sounds matter. However when I'm just capturing a bus leaving and an ambulance suddenly arrives, although I'm happy with it the sound of the sirene can become overwhelming and I have to turn the gain suddenly very low to avoid clipping. My question is, I am doing right by constantly monitoring the gain or is there a better way?
Manually adjusting the gain during the recording is never a great idea. Your reactions are sluggish, so you'll probably have some seconds of clipped sounds that you'll have to throw away anyway, and then you have inconsistent levels that you'll have to compensate for somehow in post.
The H6 offers a couple of solutions for this: backup record and an onboard limiter. The backup record function records two files simultaneously, one with 12dB less gain. The limiter is like an auto-level that ducks sounds above a certain threshold. (There are 3 limiter presets, and they're quite vague about the differences between them but you can probably stick with the 1st, general-purpose one.)
I'd recommend preferring the backup record feature to the limiter, as then you get a clean, unprocessed take for each sound event, which you can decide to put through a limiter in the edit. It's particularly a good option if you're seeing yourself editing your takes considerably (ie. you will use the ambulance siren separately from the bus sounds). But you can even use both features together for extra safety - then your backup should record without the limiter kicking in, but in case the source is still too loud the take will still be usable.
I had just this problem when recording ing London. A siren went past. I had my limiter kicked in but the squash it applied was still audible, i.e. as the siren receeded, everything else faded back in. Personally, I would write off blended sounds like that and hang out for cleaner takes.
You might go out for a day with your gain set much lower so that you're set for a dedicated siren recording session. The standard traffic sounds might be too quiet, but the siren passbies should be usable, since sirens are much louder than most traffic.
I can't think of many instances where having your limiter on for recording wouldn't be the best idea. Although my siren story shows that a limiter doesn't always cut it, sometimes sounds can combine in such a way that the limiting is far less noticeable - a clattering, banging truck passing for instance, the short, sharp blast of a horn. Far better to have the limiter save those sounds than have to edit out clipped portions of audio.
How you want to set your gain relative to the limiter depends on how clean your preamps and mics are. I like to have my levels set so that the limiter is unlikely to engage, just using it as a safety net. Then I can boost the levels in post, hopefully without raising the noise floor to unacceptable levels. In a loud, urban environment, this shouldn't be too hard to do.
Hope this helps.