The spectrogram and information

How does sound carry information? Think about reading one word of a story. You recognize the word, but it does not tell you much about the story. When you read the whole story you put all the words together to get the meaning. The brain does a similar thing with different frequencies of sound. A single frequency by itself does not have much meaning. The meaning comes from patterns of changing amplitude in many frequencies together. Read the text aloud
Human ear decodes sound by splitting it into different frequencies
When we hear, the nerves in the ear respond separately to each different frequency. The brain interprets the signals from the ear and creates a response to the individual frequencies contained in the sound wave. You hear the individual frequencies, which you can demonstrate by listening to multiple frequencies together. Read the text aloud
Example spectrogram for soundA spectrogram is a visual representation of sound that includes frequency, loudness, and time. The vertical axis represents frequency and the horizontal axis represents time. Color is usually used to represent loudness. The brighter the color, the higher the amplitude at that frequency. The example on the right shows a sequence of sounds from 200 to 500 Hz over a period of 10 s:
  1. a loud sound at 200 Hz that lasts
    from 1 to 3 s;
  2. a softer sound at 300 Hz that lasts
    from 3 to 5 s; and
  3. a sound at 500 Hz that starts soft, gets loudest
    at 3 s, and gets softer again.
Read the text aloud
Spectrogram of a human voiceFundamentally, information is encoded in patterns that are decoded by the ear and brain. In writing, the patterns are sequences of letters and spaces. In sound, the patterns are the changes in amplitude for about 15,000 different frequencies. The spectrogram to the right is for a male voice saying, “Hello.” The word lasts from 0.1 s to about 0.6 s. You can see lots of sound below 1,500 Hz and two bands of sound between 2,000 and 4,000 Hz. Every person’s spectrogram is different, even when they are saying the same word. You might infer that the chart represents an adult female voice if the frequencies were somewhat higher than those of a man. Read the text aloud Show Touch-tone dialing
For which of the following would a spectrogram be able to represent different parts of sound?
  1. speech
  2. music
  3. bird songs
  4. all of the above

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