Odd buzzing ending at 12th fret

A few days ago the A and E strings of my mandolin started buzzing and I've been unable to diagnose the issue. Here are a few findings, however

1. E string is worse than the A string.
2. Only the strings on the ground side of each pair are affected. The top string of each pair ring nicely all the time.
3. The issue first becomes noticeable when fretting on the 5th or 6th fret.
4. The issue becomes worse higher up the neck you play up to and including the 12th fret.
5. The issue stops when playing on the 13th fret or higher.

I've uploaded a quick audio clip of the issue. I play progressively up the neck for each pair of string, then string to play each string in each pair individually on the 12th fret. And then I try to compare and contrast the 12th fret to the 13th fret.

Any ideas as to what's going on? Visually I cannot see any difference between the 12th fret and the others, nor can I spot a difference between the strings in each pair.

Thanks!

It's possible (perhaps likely) that the change is lower string height. The most likely situation is that the top has flattened and thus made the bridge lower. That can be caused by low relative humidity causing the moisture content of the wood in the mandolin to be too low causing the wood to shrink.
The only-one-string-per-pair situation is probably one string per pair being cut lower in the bridge than the other. That would have been masked by higher action before whatever caused it to lower.
I suspect that a day or so at proper relative humidity will make the problem go away.

What John says. Because of where I live it is much more humid in the summer, and drier because of heating in the winter. My action will change quite a bit between summer and winter. I raise my bridge or lower it depending on the time of year. Since I like a low action I carry a gauge to set the action where I like it and chase it between the changes.

Thanks, John. Fabulous explanation. That sounds very likely. I live in New England and the RH had dropped drastically this past week.

I suspected the same (but couldn't understand why only one string would be affected, so doubted myself..), so I had installed an oasis in case humidifier along with a SensorPush hygrometer. So far, no improvement and the hygrometer has indicated the oasis has had no affect to the case's ambient RH. Hopefully a few more days and the oasis will kick in? :/

It may take as much as a couple of weeks.
You might want to go to Walgreen's and pick up a room humidifier for 30 or 40 bucks. It doesn't have to be fancy.
If you're getting real New England winter weather, the humidifier also will do you and anybody who lives with you some good also.

I'd say it could be also lifted end of fret 13. Since it is lifted at slight angle it will affect the outer string of a pair more and will stop at the bad fret. Dryness can cause looser fit in the slots and frets springing out if not radiused to fit the board. Developing fretboard hump can be also one explanation.

Yes, but let's get the instrument's water content up to acceptable levels before worrying about whether there any possible problems with the frets and fingerboard.
Go for the simple solution first.
Seasonal humidity changes in New England can be especially drastic.

I don't think a fret end would affect only one string of two different pairs of strings. Also, the buzz is apparently on all frets from 5 to 12, not just on 12.
Adrian, I think you weren't completely thorough reading this one! :)

Yes, now I see that the other strings do ring clearly.
I somehow read that the inner strings are less affected... I can still imagine a lifted fret end (or slightly bent upward) affect fretted positions from 5 up with some weird neck relief/ setup issues combined especially when the problem suddenly stops at fret 13. That in my limited experience points towards fret problem or board hump at neck joint.
Of course badly cut saddle notches are probably more common especially on cheap instruments with DIY setup.
Good pic of the fingerboard along the string length with good light direction would show if the frets are even.

All my mandolins (except the resonator and banjolin) have the standard two piece bridge (base and saddle) with the saddle resting on thumbwheels that screw up and down on posts rising out of the base. The thumbwheels can be raised to raise the saddle height. It usually doesn't take much to adjust the height for seasonal action changes.

It can be difficult to raise the saddle under string loading. Its easiest for me to do it when I reduce string tension a lot.

I don't know what it is like where you are but in South Dakota my house went from 55% humidity to 30% in a week. and it continues to drop. All instruments in cases with humidification. It makes a huge difference.

Hi all, I wanted to provide an update.

After having little success with the incase humidifier only (oasis), I invested in an humidifier for the room that I store our instruments. This has helped a bunch. I'm now able to store the instrument at +35% RH. Without it, the natural RH has tended towards 15-20% RH this winter.

Getting the humidity up has greatly helped with my instrument's intonation! However, one issue still remains. The A string still slightly buzzes when played on 5th through 12th frets. The other strings seem unaffected.

To address this last issue, should I try to be get the RH even higher? It's perhaps possible, but requires a lot of vigilance to keep the humidifier constantly full of water (it goes through about 6 gallons a day!).

Should I attempt to adjust my action? My understanding is that bass vs treble sides of the bridge sit at different heights and I'm unsure how to keep the correct ratio when I adjust the action. (This bridge height was also perfect before the winter months began, so it also makes me sad that I likely will never be able to perfectly replicate when I have to lower it again come spring.. but alas).

Thanks again for all your help!

Yes, try adjusting your bridge.
You can only "force" the humidity in your house so high. During the hardest part of the winter here in the southeast, 40% is about as high as I can get it in my home. 35% in an old New England house should be high enough.

In most cases, we set the action a little higher on the bass side than on the treble, but that is not a strict rule.
If you go to your local Ace Hardware store, they carry a 6" steel pocket rule that is graduated in 64ths of an inch. It costs about 5 or 6 bucks. You can measure your action that way. Most mandolins will play well if the measurement between the top of the 12th fret and the bottom of the strings is 4/64". If you get a lot of fret rattle at that height, the mandolin probably needs work. Sometimes we can get the treble strings down to nearly 3/64" if the mandolin has a really good, accurate fret job, the neck is really straight, and the player has a light touch.

My action is right above the 2/64 line on the E and 3/64 on the G. As rcc56 says you need good level frets. I don't play lightly tho, and the top will tell you whether you can play light or heavy. Some tops will take a heavy hand, even with low action, more than others. I have wood heat here in the midwest and am able to keep my humidity between 45 and 50 percent with the humidifier on low. My house is small and fairly tight tho, a big drafty house is a different story.