Just a pondering of mine - why is it that double stops will only stop buzzing when they're prefectly in tune? how does the violin "know"? is it to do with how the violin and strings are made? and what about the kinds of music where the intervals aren't the same - do they just not use violins - and equal temperament?

I'm sure someone on here will give a very long-winded account of how the frequencies of sound waves work and how they can either combine, or start to cancel each other out (you'll hear the two frequencies beating against each other when this happens).

Buzzing and beating has nothing to do with the instrument per se, is all down to the laws of physics and how harmonics work and how your ear hears the sound - sound being nothing other than air pressure variation, which your eardrum picks up, carried by the inner ear and processed by your brain into what we call 'sound'.

Certain notes simply don't have anything in common with each other, no common harmonics, these would also include chords where one or both notes are slightly out of tune. They sound to us, discordant. The minor second is one such discord. But if you take perfect fourths or fitfths for instance, they share harmonics and sound pleasing to our ears, these are condordant.

(Slightly off-topic, but this was one of the reasons Concorde received the name she did. The aircraft was pleasing, harmonious in design and appearance).

Double stops where one note is slightly 'off' will sound displeasing. For those just starting to learn double stops, working out which note it is that's out of tune is often a cause of difficulty. Eventually your ears will learn to tune in to these in much the same manner as you learned to tune in to individual notes. Unless you are in ensemble, where your tuning obviously needs to combine with others, or, even more so when playing with piano accompaniment, equally tempered double stops on the violin (or any other bowed string instrument) are not necessary. Keep them pure.

Each pitch generates a sound wave at a particular frequency. These are like the ripples in a pond if you throw a stone in, and like ripples have peaks and troughs. Concert A 440Hz is producing 440 ripples per second.

If you play two notes together which are exactly the same frequency, the peaks and troughs will match exactly so the sound is pure and in tune.

However if one note is 440Hz and the second is, say, 445Hz, the peaks and troughs won't quite match, and every so often the trough of one will meet the peak of the other and cancel it out. This is interference and produces an audible beat - think of the choppiness in the water that you see if different-sized ripples (from different stones) overlap.

If one note is exactly twice the frequency of the other (e.g. one is 440Hz, the other is 220Hz), the peaks and troughs will still be in step, so the sound is still pure and in tune; the notes are an octave apart.

A quick google threw up this site on beats. The text may be a bit technical but the illustrations are quite good if they don't make you cross-eyed, and there is also a sound sample.

Sorry, that was more long-winded than I intended!

It may well not be that the interval between notes is causing the buzz - if you are in the early stages of double stopping the problem is more likely a physical one to do with finger placing - are both your fingers really holding the string down firmly against the fingerboard or is one string 'rattling' a bit? Could you be catching a nail - or another part of a finger - against one of the strings? Anther possibility might be , if your chinrest is a little loose and you are gripping more with your chin when trying to get two fingers down, is the chinrest touching or vibrating against the tailpiece?