Sound Waves. Intensity and Sound Level

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11.2Sound Waves. Intensity and Sound Level


In these revision notes for Sound Waves. Intensity and Sound Level, we cover the following key points:

  • What are sound waves?
  • How do sound waves propagate in matter?
  • How do sound waves vary with the density of medium?
  • What is audible sound? What are the limits of audibility?
  • What are infra and ultra sound? Where they can be used?
  • What is echo?
  • What is sound intensity and what are the factors affecting it?
  • What is sound level and why do we use a logarithmic scale to describe it?
  • What are loudness and pitch and which quantities are they related to?

Sound Waves. Intensity and Sound Level Revision Notes

Sound, as a longitudinal wave causes the medium's particles vibrate back and forth. Therefore, denser the material, easier for the sound to propagates as it is easier for medium particles to find other particles to collide with.

Sound waves can be modelled as transverse waves on a screen to ease their study, i.e. to make visible the distinction between wavelength and amplitude.

Sound waves can easily penetrate through solid objects as they propagate in a longitudinal way. Yet, the speed of sound changes when it changes medium.

Humans are able to hear sounds of frequency ranging from 20 Hz to 20 kHz (20 000 Hz). These values are known as the limits of audibility. This means sound waves, which have a frequency within this range, are audible by humans.

Sound waves of a lower frequency than 20 Hz are known as infrasounds. We cannot hear them as their frequency is smaller than our lower limit of audibility. Infrasound are not used in technology as the energy they carry is practically zero.

On the other hand, sound waves with frequency higher than 20 kHz are known as ultrasound. They are widely used in technology such as in ultrasound screening (echoes), ultrasound detectors, ultrasound distance meters (when calculating the ocean depth), etc. We cannot hear ultrasound as the eardrum of humans can vibrate up to 20 000 times per second (20 kHz).

Hearing ability of a human decrease by aging. The maximum values can be reached around the age of 10.

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.

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.

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).

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