Memory foam ear tips can replace stock silicone ear tips to improve comfort, breathability, and isolation.The following options are available:Memory Foam Material: Comfort or FidelityComfort uses a very soft memory foam, and is ideal for long-listening sessions, sensitive ears, and deep insertion.It is recommended for most people. The memory foam was chosen for the least firmness that could stillprotect the ear from the inner core. Fidelity uses a slightly denser memory foam, providing better retention and resistance to nozzle bending.The firmer memory foam can potentially reduce the sound attenuation sometimes associated with very softmemory foam ear tips, but it will also apply relatively more pressure.Nozzle Sizes: 3mm, 4mm, or 5mmRebound ear tips safely expand up to 40%, providing an option for earphone nozzle sizes ranging from 3mm to 6mm.To find the correct nozzle size for your earphones, please see the sizing charts in the images or at the Rebound site.If your earphones are not listed, please let us know so we can find it for you. Or the Nozzle Sizing Pack can be used.3mm is interchangeable with Comply 100; 4mm with Comply 200; and 5mm with Comply 400/500.Sizes: Small (11mm outer diameter), Medium (13mm), Large (14mm), or Medium RoundThe right ear tip size is important for comfort and sound quality. Medium fits most people best, but some need smallfor a comfortable fit, or large for a proper seal. The Ear Canal Sizing Pack can be used to find the best fit.Standard shape ear tips have better isolation and can be inserted deeper. It is recommended for most people, and is availablein all sizes. Round shape ear tips may be more comfortable or have less nozzle blockage for those who prefer shallow insertion or havesharp ear canal bends. It is currently only available in 5mm and Medium size.