Acoustics, the science of sound
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• Examples of sound sources
• Frequency and pitch
• Decibel scale (audibility threshold, pain threshold, and harm threshold)
• Sound generation and its transfer through the air - use the particle approach
• The speed of sound
• Noise, echo and resonance
• Human audibility frequency range
• Ultrasound and infrasound

Examples of sound sources

• A guitar vibrating
• A saxophone vibrating
• Loudspeaker vibrating because of feeding oscillating electric currents
• Vocal cords vibrating e.g. a cat meowing

Frequency and pitch

If an object oscillates, the surrounding air will also oscillate. For example, if you hold a ruler firmly against a table while half the ruler is outside the table edge, and the overhanging ruler part is bent down and released, a sound is heard.

Figure 1: Pull down the ruler and then release it. The ruler oscillates up
and down and a specific tone sounds. Estimate the frequency?
 

If two metal objects are banged against one another, they also begin vibrating. This is felt if holding the metal objects. It can feel uncomfortable holding the metal objects that one may want to drop them. The metal objects set the surrounding air into vibration. The oscillating air reaches our ears and we hear a tone. The reason we can hear the sound is to be found inside our ears. Inside each ear there is an eardrum. The eardrum is a small diaphragm oscillating in pace with the sound condensations and dilutions. Nerve impulses are sent to the hearing center in the brain.

If we instead let the ruler in the previous example extent further from the table and struck, a slightly deeper tone is heard. A deeper tone occurs when the ruler oscillates fewer times per second. The ears are thus hit by fewer cycles per second. We say the tone is lower, or we have a low tone. If instead, only a small part of the ruler extends from the table edge and struck, a higher tone is heard. This tone oscillates more times per second. This makes the eardrums oscillating more cycles per second.

Hz (Hertz) is the unit of sound. It means number of cycles per second.

• More cycles per second results in a higher tone (higher frequency (Hz))

• Fewer cycles per second results in a lower tone (lower frequency (Hz))

When talking about high or low tones the term pitch is also used, i.e. high-pitched and low-pitched tones.

Decibel scale (audibility threshold, pain threshold, and harm threshold)

The decibel scale is a way to measure of the intensity (power) of a sound. The more intense a sound, the more ache in the ears. In everyday life we say that we increase the volume e.g. of a stereo equipment. The decibel scale is structured so that 20 dB is experienced twice as loud as 10 dB. 40 dB is experienced 4 times more than 20 dB.   Figure 2: Decibel scale

Sound generation and its transfer through the air - use the particle approach

Figure 3: Sound waves consist of condensations and dilutions. The curved lines above are
condensations and the spaces in between are dilutions. The air particles are crowded and alternately diluted.
The condensations and dilutions in air travel with a speed of 340 m/s.
 

 
Figure 4: In the wave-formation above each condensation is showed as a wave-crest and each dilution as a wave trough. This wave-formation is called a sine curve. The amplitude decides the intensity (power) of the sound. High amplitude means that the sound is intense. The number of wave-crests per second (cycles per second) decides whether the tone is high (many cycles per second) or low (few cycles per second).
 

The speed of sound

The speed of sound in air is 340 m/s. Mainly, the denser a substance is, the faster the sound can travel through it. The sound speed in water is 1500 m/s and in steel 5100 m/s.

Noise, echo and resonance

Noise:
As seen above on the sounding speaker there is equal distances between the sound waves. So we have a uniform tone. With noise, the lengths between the sound waves are not the same. The sound varies in an irregular manner. Furthermore, the amplitude varies in an irregular way. If experiencing the sound as unpleasant, it is called noise.

Echo:
If shouting close to a rock wall, it seems the rock wall shouts back. This phenomenon is called echo. Echo means that the sound waves bounce towards and off the rock wall and further back to the person shouting.

Resonance:
If a tuning fork is struck it produces a uniform tone. When this frequency is transferred onto another object, it too begins oscillating at the same rate. The object resonates (is in resonance with) with the tuning fork.

 
Figure 5: The tuning fork to the left is struck. Its sound waves make the other identical tuning fork to oscillate with the same frequency.
 

Try striking a tuning fork inserted in an acoustic chamber while having an identical tuning fork with an acoustic chamber close. Close with your hand the oscillations in the first tuning fork and see what happens with the second tuning fork. The second tuning fork still sounds. This phenomenon is called resonance.

Human audibility frequency range

Humans can hear frequencies between 20 and 20.000 Hz. One Hertz (1 Hz) is one cycle per second. 20 Hz means 20 cycles per second.

Ultrasound and infrasound

All sounds below 20 Hz are called infrasound. These sounds cannot be heard. Research has shown that some infrasound can make us feel bad, sick or tired.

Ultrasounds have frequencies above 20.000 Hz. These sounds as well we cannot hear. Ultrasound is used at fetal diagnostics. The ultrasound device is hold against the mother’s stomach. The sound waves bounce back from various fetal tissues inside the womb. The device does this continuously and we can see the baby moving on a TV screen.

Bats also send out ultrasounds. The ultrasounds bounce off objects located by the bat. Some of these objects are possible preys. The echo comes back to the bat. Also dogs can hear ultrasounds. Dog Whistles emit ultrasound. When blowing into dog whistles we humans hear nothing at all. However, for the dog, it sounds a lot.