A version of behind the ear devices, open fit hearing aids are best for people who have mild to moderate hearing loss. A thin tube connects the outer and inner part of the device. If you're looking for a device that's able to pick up sounds that have a low frequency, you're likely to find it with this type of device.
These devices are considered to be a form of behind the ear device, which means that they come with many of the same advantages and disadvantages of these products. However, the batteries for these devices tend to be quite small, which means that they often need to be replaced. This can create an expense, which is not likely to be covered by a warranty. In addition, the small size means that it generally is not possible to make adjustments manually.