How do combination tones arise when loud pure tones enter the ear?

When two or more pure tones of high intensity enter the ear, they generate combination tones due to the nonlinearity of the ear's mechanism.1

At high intensities, the ear's response is not perfectly linear, which causes the incoming sound wave shapes to become distorted. This distortion introduces new frequencies into the ear that were not present in the original sound source. These resulting tones are categorized into two main types:1

• Difference tones: Produced by the difference between the frequencies of the original tones (or their multiples). These are the most commonly observed combination tones and often lie below the original pitches.21
• Summation tones: Produced by the sum of the frequencies of the original tones. These are generally more difficult to perceive because they are often masked by the higher-intensity tones that created them.21

Because these tones are produced by the ear's internal processing rather than the external sound source, they are frequently referred to as subjective or resultant tones.2