8d: Timbre (The First Time)

If a trumpet and a clarinet play the same note we can still tell the difference between the two instruments. Likewise, different voices sound different even when singing the same note. Why? We now know that if they are playing or singing the same pitch the fundamental frequency is the same for both so it is not the pitch that enables us to tell the difference. These differences in the quality of the pitch are called timbre and depend on the actual shape of the wave which in turn depends on the other frequencies present and their phases. Pure tones such as from a tuning fork have a pure sine wave shape and a single frequency. However the notes from musical instruments and voices are more complex and normally contain many frequencies, as we will see in the next chapter. We will also come back to other aspects of the human perception of sounds in Chapter 10 on Perception. For now the main point is that the subjective perception of pitch, loudness and timbre are each related to more than one quantity that can be measured in the laboratory. The following diagram shows some of the connections between objective (laboratory) measurements and subjective perception.

Notice that our perception of loudness is mainly determined by the intensity of the sound (energy per second per square meter) but also is influenced by frequency and waveform of the sound. Likewise our perception of pitch is mainly determined by the fundamental frequency but also influenced by intensity and waveform. Finally, timbre is determined by waveform (which is determined by the other frequencies present and their phases) with influences from intensity and the fundamental frequency. As we will see later (in a demo in class) the duration of a sound also affects how we perceive its pitch, loudness and timbre.

Simulation exercise 8A (turn in answers on a separate sheet of paper): Sound of Various Waveforms.

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