One or two hearing aids?

It depends on how severe your hearing loss is, whether it affects one ear or both, and how big the difference between the two ears is. With one-sided or mild to moderate hearing loss, one hearing aid is usually enough; with severe and profound hearing loss in both ears, two hearing aids are the best solution for good listening and, above all, for understanding speech. The brain is built to listen with two ears.

What two hearing aids give you

• Better speech understanding in noise. The brain adds up the sound from both ears, so speech is perceived roughly 2–3 dB louder (binaural summation) — in quiet you therefore catch even a faint or distant voice better. By comparing both ears the brain can also suppress part of the background noise, by about 1–2 dB (binaural squelch). Together this makes roughly 3–5 dB in noise. (source)
• What 1 dB means. In tests of understanding sentences in noise, every 1 dB better speech-to-noise ratio brings roughly 10–30 % more correctly recognised words (depending on the test). A gain of a few decibels is therefore very noticeable in a restaurant or in company. (source)
• You hear from the “worse” side too. The head acts as a screen — it dampens sound coming from the other side, by about 3 dB for low tones and up to 16 dB for high tones (head shadow). With two hearing aids you always have one ear on the “right” side. (source)
• You can tell where sound comes from. The brain works out direction from tiny differences in timing and loudness between the two ears — with one hearing aid this is not possible. It matters for safety (cars, bicycles) and in conversations with several people.
• The hearing aids communicate with each other. Modern hearing aids exchange information wirelessly, so they are adjusted together — when you change the volume on one, the other follows. In noisy places they cooperate: the microphones of both hearing aids focus together on the speech in front of you and suppress the surrounding noise; some do this by letting one hearing aid focus on speech while the other dampens the noise (depending on the category of hearing aid).
• You protect the unaided ear. An ear that receives no sound for a long time gradually understands speech worse, even though its hearing on the audiogram does not change (auditory deprivation). As early as a 1999 study (and newer research still confirms it today), speech understanding in the unaided ear declined in 25 % of patients with one hearing aid, but in only 6 % of those with two. (source)

As MUDr. Libor Černý, Ph.D., of the Department of Phoniatrics of the First Faculty of Medicine, Charles University and General University Hospital in Prague, stresses in an interview for the Czech consumer magazine dTest, we only have spatial hearing with two ears, and in noise we always understand better with two ears, because the brain expects complete information from both sides. (source, in Czech)

The benefit differs from person to person — that is why the doctor decides, based on the examination, whether two hearing aids are right for you.