How flexible should a solid top be?

Funkfingers

As per title. The soundboard of an acoustic should be expected to bow outwards under string tension. Should it be possible to produce tremolo/vibrato modulation effects by squeezing down on the board between the bridge and the end pin?

I am currently able to do this on a Naughties US-made Martin - with or without strings on. The bridge and bracing are both still solidly attached. No splits in the soundboard. No shrinkage. Nothing creaking or rattling inside.

Finally, there is a strange discolouration in the soundboard beneath the varnish. I have vague recollections that the guitar and case could have been stored in a damp or mouldy place. Would either of those explain the apparent weakness?

octatonic

Some photos would help.

Acoustic guitars work like bellows, the soundboard and back are usually not flat, I would use a 6-8mm as a guide for the arc.
But what you describe sounds more than this and certainly storing a guitar in damp conditions won't help.

Andyjr1515

The most common radius for acoustic tops is around 25 feet, but that is pretty much fixed by the braces. 

You wouldn't normally expect or want the top to rise appreciably when the string tension is applied.  It does happen sometimes on older guitars where the braces are beginning to loosen, resulting in the back of the bridge lifting and the action becoming too high to comfortably play - but that is generally seen as a problem rather than an asset

I think it is therefore pretty unusual to have a top flexible enough to perceptably change the pitch - and doing so will be putting unusually high stresses on the bracing glue surfaces.

So, if it was my guitar, I would be impressed with the flex in the top because, if it remains strong enough, that will give it its own voice...but I wouldn't tempt fate by taking it to places it was never designed to go, otherwise I might end up in a place I wasn't intending to be

ICBM

That does sound too flexible to me, although it's impossible to say for sure without hands on.

Funkfingers

Going down a string gauge has made no difference to the top bow or the flexibility issue.

Reinstating the original bridge saddle - shaved down to accommodate an undersaddle transducer strip - has lowered the action to the Martin specification height.

I am havering over whether to dehumidify the guitar or to just sell it before things come unglued.

That would be a pity as the guitar is sounding rather nice with fresh non-coated strings. 

THINKS: Do Elixir Nanoweb strings use thicker cores than D’Addario, Ernie Ball et cetera?

Andyjr1515

Funkfingers said:

Going down a string gauge has made no difference to the top bow or the flexibility issue.

Reinstating the original bridge saddle - shaved down to accommodate an undersaddle transducer strip - has lowered the action to the Martin specification height.

I am havering over whether to dehumidify the guitar or to just sell it before things come unglued.

That would be a pity as the guitar is sounding rather nice with fresh non-coated strings. 

THINKS: Do Elixir Nanoweb strings use thicker cores than D’Addario, Ernie Ball et cetera?

As I say, if it's holding good, then no reason why you can't carry on playing it.  After all, in general terms a flexible top is a good top.  
Just saying don't be tempted to use it as a poor man's whammy bar

 

Funkfingers

This afternoon, for my own amusement, I took the offending Martin to a local music shop. There, I A/B tested it against a brand new slim bodied LAG Tremontagne electro-acoustic. 

Similar proportions of solid and laminate woods. Similar synthetic bridge and nut. Vastly different electronics: LR Baggs Lyric versus Astro- LAG (in-house undersaddle piezo and pre-amp module).

Even through entirely inappropriate amplification, the Martin sounded and played better than the LAG. I shall persevere with it for the time being.

ToneControl

Funkfingers said:

As per title. The soundboard of an acoustic should be expected to bow outwards under string tension. Should it be possible to produce tremolo/vibrato modulation effects by squeezing down on the board between the bridge and the end pin?

I can do it on my workshop made pricey acoustics, perfectly normal. Some people use this is performance I think