Explain to me how customs provide a seal

the seal is made because the shell of the custom is exactly the same shape as your ear canal, while the seal of a universal is completely reliant on adequately filling the gap so that no other external sound gets into your ear canal. Granted, the seal of a custom is heavily dependent on the impressions made of your ear canals being moulded exactly as they were taken and sometimes the fit needs adjustment or a do-over, but once you get it right you barely feel them in your ears at all - the same cannot always be said of universals, which often need to be jammed in with massive silicone/foam tips that irritate the skin in your ears and need to be replaced fairly regularly. 

the customs are made ever so slightly larger than the impression taken from your ears, giving you both comfort and good isolation

Contrary what most people may think, isolation is not only due to the material which is used and the perfect seal that one must reach.

Thickness of the material is also key.

With customs, you actually get at least 2cm thick of acrylic/silicone material sealing your ear with a shape following your ear canal.

Because let's be honest, acrylic materials are not exactly top notch in term of soundproofing. Silicones are already better at that for example, which partially explain why you can get excellent isolation with a thin silicon universal tip and a good seal.

Customs make it possible to have good isolation with any kind of material provided they're long enough, and acrylic resins are used a lot because of their excellent mechanical resistance. Silicone customs are also available but they are softer, more permeable to solvants and grease and prone to breakage when they age, although silicone materials have come a long way during the last 10 years. They now age much better (no yellowing with uv, less hardening/drying).

Quote:

Originally Posted by bubsdaddy 

 what I don't understand is how a hard acrylic or plastic shell, even one that is molded for my ear, can seal better than a rubber or foam tip. Rubber and foam both expand to form a gasket like seal. Plastic does not

Doesn't really matter.  The rubber or foam is needed to form a seal with your ear canal.  I custom acrylic engulfs your entire ear cavity so it stays in place without the need for expansion.

so am i right to say if a custom doesnt isolate as well as olives/complys means its a bad fit?

Quote:

Originally Posted by wh85 

so am i right to say if a custom doesnt isolate as well as olives/complys means its a bad fit?

either that or it's a vented custom.

Quote:

Originally Posted by phntmsmshr 

either that or it's a vented custom.

 

usual way of remedying that is to thicken the stem? applying lacquer?

I recently had a pair of UM56's made from silicon.  Without question, these seal and isolate better than any universal tips I have tried.  I have been at this for a while, starting with ER4's when they first came out and am very used to deep penetration with universals.  Triple flange, Gliders, P-Series have been the best universals so far as seal/isolation is concerned for me -- but the UM56's best all of them.   I cannot believe that a well fitted and properly made CIEM would perform any differently.  My guess is that soft/comfort canals will do a better job at this than 100% acrylic. 

Quote:

Originally Posted by wh85 

so am i right to say if a custom doesnt isolate as well as olives/complys means its a bad fit?

 No, not at all.  A porous, foamy material will always isolate better than a harder acrylic.  It "absorbs" the sound more thoroughly.

Etymotic even states that as great as their triple flange isolate (36-38dB), their foam tips isolate even more (41dB).  I would guess a custom acrylic is in the 28-32dB range.