Physics Lesson 11.5.7 - Loudness and Pitch

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Welcome to our Physics lesson on Loudness and Pitch, this is the seventh lesson of our suite of physics lessons covering the topic of Sound Waves. Intensity and Sound Level, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

Loudness and Pitch

Each voice is characteristic in itself; it has its own unique features that lets us know who is talking even if we do not see the person. We are able to distinct the voice of a man from that of a woman, the voice of a kid from an adult, etc. For this, two features that are special only for sound waves comes to our help. They are loudness and pitch.

Loudness of sound is a phenomenon that depends on the sound amplitude. It is defined as the property of sound used for differentiating between the loud and faint sound. Thus, a loud voice means a voice with a high amplitude while a quiet voice has a small amplitude. Loudness is simply the sound level measured in decibels discussed in the previous paragraph.

We cannot take amplitude of sound and loudness as the same thing because loudness is directly proportional to the square of amplitude, not just to the amplitude itself. This means if the amplitude of a sound wave doubles, the loudness quadruples.

Loudness depends on the energy of sound received by a unit area of ear in the unit of time, as energy of waves (as discussed in our Physics tutorial on Energy and the Power of Waves) depend on the amplitude. Remember the formula for the energy of waves, which is true for sound waves as well:

E = m × A2 × ω2/2

On the other hand, pitch of sound is defined as the feature of sound used for differentiating between the shrill and flat sound. Pitch depends on the frequency of sound waves. Thus, a shrill voice (a high pitch voice) means it has a high frequency and a flat voice (low pitch) means it has a low frequency. For example, the voice of a woman has a higher pitch than that of a man.

Given that energy of waves depend on the frequency (ω = 2π × f), pitch depends on the energy of sound waves as well, albeit not directly (pitch varies directly with the square of frequency).

Example 5

A sound wave has an amplitude of 2 mm and a frequency of 50 Hz. If the amplitude triples and the frequency halves, what happens with loudness and pitch of this sound wave?

Solution 5

From theory, we know that when amplitude of a sound wave triples, loudness increases by a factor of 9, as loudness is proportional to the square of sound amplitude. Therefore, the new amplitude becomes 2 mm × 9 = 18 mm.

Also, from theory we know that pitch varies directly with the square of frequency. This means when the sound frequency halves, pitch decrease by a factor of 4. Therefore, the new pitch becomes 50 Hz / 4 = 12.5 Hz (it becomes infrasound).

You have reached the end of Physics lesson 11.5.7 Loudness and Pitch. There are 7 lessons in this physics tutorial covering Sound Waves. Intensity and Sound Level, you can access all the lessons from this tutorial below.

More Sound Waves. Intensity and Sound Level Lessons and Learning Resources

Waves Learning Material
Tutorial IDPhysics Tutorial TitleTutorialVideo
Tutorial
Revision
Notes
Revision
Questions
11.5Sound Waves. Intensity and Sound Level
Lesson IDPhysics Lesson TitleLessonVideo
Lesson
11.5.1Things You Already Know About Sound Waves
11.5.2Things You Need To Know About Sound Waves
11.5.3Limits of Audibility. Audible Sound. Infra and Ultrasound<
11.5.4Echo
11.5.5Intensity of Sound Waves
11.5.6Sound Level
11.5.7Loudness and Pitch

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