Sound isolating earbuds can mean several things to someone considering what earbud to buy.

There are 2 distinct technologies that are very different but often confused with one another.  There is technology called sound or noise canceling and there is technology called sound or noise isolating.  On the surface they look very similar but when you dig deeper and look at the technology involved you will see they are very different.

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Shure SE315 Sound Isolating Modular Ear Buds Shure SE315 Sound Isolating Modular Ear Buds Paypal US $249.99 12h 11m
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Sound canceling earbuds work by using electronics to filter out the environmental sound and allow only the sounds being transmitted from the device they are connected  with to pass through to the ear.  They create a filter in essence to produce a higher quality sound.  They don't even have to let sound pass through.  Many models will filter out the outside sounds creating a simulated peace and quiet.  This comes in handy on airplane flights or when you are trying to ignore your screaming 3 year old!  OK maybe not so much with the toddler but you get the point.

Sound isolating earbuds attempt to filter out environmental noise through fit and insulation.  Its very low tech in its approach when compared to its sibling.  The key with sound isolating earbuds is finding a good solid fit along with good insulation.  Often with these style of earbuds you will find that they come with several different sizes of foam or rubber attachments.  These allow a certain level of custom fit as the size of a persons ear canal varies.  You should try each size to see what fits the best.  You are looking for a snug fit that acts like a seal but doesn't create discomfort or pain.  When you first put this style earbud on it may feel awkward and uncomfortable.  Usually this passes and you get used to it but if it continues you should either try a different size bud or an entirely different set of earbuds.  I had the hardest time finding a pair of earbuds that fit because it seemed like every earbud was painful and annoying.  But after some research and trial and error I finally found a pair that felt good.

To wrap it up just remember when looking at earbuds that there are really two different types of sound filtering earbuds.  The kind that uses electronics and usually cost much more and the other that are sound isolating earbuds that try to filter sound through fit and insulation.