3-6 Combination Tones
What is a combination tone?
A combination tone is what the ear perceives when two pure tones are placed together at high intensity levels.
These tones are NOT produced by an outside source, but are produced in the inner ear so that we think these sounds are actually occurring. Since they are caused by the ear after hearing the original sound they are often referred to as subjective tones, resultant tones, or ghost tones.
When we hear combination tones, we are recognizing the non-linearity components of our auditory system. In other words, we are noticing that our ear has imperfections and may distort the sounds we actually here.
What will combination tones sound like?
Since these ghost tones are being produced inside our ear, they will be quite faint. Often, our auditory systems hears a tone much lower in frequency than the original two pure tones. When the tone is of a lower pitch than the original tones we call it a differential tone. A combination tone that is a higher pitch than the original tones would be called a summational tone.
It is easy to hear a combination tone on the harp:
- Listen to the sound made by 7 draw: 7 Draw Audio File
- Listen to the sound made by 8 draw: 8 Draw Audio File
- Now listen to the sound when both 7 draw and 8 draw are played at the same time: 7 Draw and 8 Draw Audio File
You should hear a combination tone which is an octave lower than 2 draw on the harp. (This is the sound you should hear: Combination Tone Audio.) If you did not hear this sound try listening to the 7 Draw and 8 Draw Audio File again.
Remember, the combination tone you hear is not actually being played on the harp but your ear is "hearing" this ghost tone any way.
Example:
What is the combination frequency we hear when a musical fifth is played? (Hint: the second tone of a fifth would be 1 and a half times greater than the first frequency).
Answer: It is given that the frequency of the second tone of a musical fifth is one and a half times greater than the frequency of the original tone we know that f2 = 3/2f1
combination tone = f2 - f1
So, 3/2f1 - f1 = 1/2 f
This means that whenever a musical fifth is played, the combination tone heard will be one half the frequency of the original tone.
Combination Tones For Some Musical Intervals:
| Interval | f1 | f2 | f2-f1 |
|---|---|---|---|
| Octave | f1 | 2f1 | f1 |
| Fifth | f1 | 3/2f1 | 1/2f1 |
| Fourth | f1 | 4/3f1 | 1/3f1 |
| Major Third | f1 | 5/4f1 | 1/4f |
| Major Sixth | f1 | 5/3f1 | 2/3f1 |
| Minor Third | f1 | 6/5f1 | 1/5f1 |
What column in the graph would give us the combination tone heard?
f2-f1 would equal the frequency of the combination tone.
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