My brain is starting to overheat, but something struck me – the bangs are so close together in time that the physical distance between them becomes crucial.

As the bangs move up and down left and right banks in opposition, the sound waves generated by them may well move in and out of phase as the distance between consecutive bangs changes from more-or-less the width of the block to more-or-less the length of it. Moving in and out of phase rapidly could create the rapid amplitude variation seen (through constructive and destructive interference), which translates to a growl.

Edit Ref. Young’s double slit experiment – imagine rapidly bringing the slits closer together and apart again, and observing the brightness from a given spot as you do so. Not entirely analogous, but it helps me!

Blimey! I live and learn 🙂 Thanks.

Certainly very clear pattern here.. especially in the lower channel. Another part of the same Griffon sample..

[ATTACH=CONFIG]232231[/ATTACH]

…not more sinusoidal, surprisingly. And no pulsing?

Yes, in my second example, the pulse I was looking for doesn’t appear on the upper (left) channel. Interesting, not sure why they are different.

Give me a minute to download the Spit one..

But what is interesting is this – approx. 20 millisecond pulses, just visible in the lower track, between clear, large waves and more interference-ridden and lower amplitude waves – with the breaks clumsily marked by me :-). Could this be the ‘pulse’ effect? [ATTACH=CONFIG]232227[/ATTACH]

Hmm. OK, bit of scribbling (on a beermat!) produces a vague idea of how the firing pattern described by TempestNut looks when viewed from one side (this simplifies it for my little pickled brain). On the Merlin it jumps around, while on the Griffon it travels, first one way, then back the other. It is a ‘wave’ on the Griffon – first fore-aft, then forwards again (on the starboard ‘A’ bank). This wave is mirrored in the opposite sense on the B bank. There is no such wave on the Merlin, which instead jumps back and forth three times during the sequence.

This wave – or more of a pulse – effect might well create a ‘growl’ overtone to the Griffon Note – at – what – 1/6 the frequency of the revs?

Edit – after posting, saw Spartabus’ answer. I think that was what I was getting at, in a not so technical way? What is a pulse tuned collector – sounds very Star Trek..

Depends who ‘they’ are, or were. Most British aircraft built for military purposes during WWII had British Standard (Aircraft Series) material specifications (but not EN, as far as I know this is a much more recent set of standards) and DTD specs where no standardisation had been arrived at. I believe that the L series used to describe light alloys are/were part of the BS scheme, but I’m not sure of that.

Sorry, I misunderstood your bit about metal. Fair shout, but I don’t think original materials are necessary – in fact, modern airworthiness standards would – I understand – cause deviation from original spec in many cases anyway?

Edit – EN are European standards, incorporated into the BSi schema. Just looked it up.