Two tones that differ only slightly in frequency and have approximately the same amplitude will produce an effect known as beats. This effect sounds like repetitive volume surges that are equal in frequency to the difference between these two tones. The phenomenon is often used as an aid for tuning instruments, because, as the beats slow down, the two notes approach the same pitch and finally stop when the pitches match. In reality, beats are a result of the ear’s inability to separate closely pitched notes. This results in a third frequency that’s created from the phase sum and difference values between the two notes.