Disposable hearing aids were first approved and sold in volumes in 2000 under the name, "Songbird". They come in a fixed number of different configurations and last for about a month after which you must discard the aid (you can't replace the battery) and order another one if you choose.
They typically are for mild/moderate losses and are suitable for some common types of hearing loss. They are available to fit several specific types of hearing loss, but cannot be "tuned" beyond those few typical types of losses.
Pricing may make sense for trial periods, or for backup of a regular hearing aid (for example during travel, where service on your normal aid may not be convenient.
Disposable aids do not require a major investment up-front (though long term costs are probably comparable with non-disposable aids). They can be replaced through the mail (after the initial fitting by an audiologist). They may be suitable as temporary replacements or backups on trips. They do not normally contain a telecoil, multi-mic directionality, or other useful features of modern hearing aids.
Disposable aids, have a limited fitting range both acoustically and physically, so they may not be suitable for everyone.
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