Sound

(Updated on 2/10/2013: to include latest SPM 2012 questions)

1. Sound waves cannot travel through vacuum. They need a medium - solid, liquid or air - to travel. Sound travels the fastest in solids (about 5000 m/s in concrete), moderately fast in liquids (about 1400 m/s in pure water at 0 ^oC) and the slowest in air (about 330 m/s).

2. Sound waves are longitudinal waves (pl. see: diagram below): When sound travels through a medium, the particles of the medium oscillate parallel to the direction of travel of the sound. Due to the oscillations of particles about their mean positions, a series of compressions and rarefactions (not the particles) travel through the medium and to the surrounding media including the air in our ear. 

3. When the air in our ear picks up the sound waves, its particles too oscillate to create sound waves which hit our ear drum which then vibrates the three 3 little bones (the 3 oscicles), the oval window and the fluid in the cochlea of our ear. Sensory receptors in the inner lining of the cochlea then pick up the waves in the cochlea fluid and change them into electrical impulses which are interpreted by our brain as sounds of different pitch and loudness.

(SPM 2012 P2 Q10(a) @ pg 360: How drum produces sound and how our ear hears it?) 

4. Humans can only 'hear' sounds of 20 Hz to 20,000 Hz frequency.

Pitch and Frequency of Sound

Generally, there are three characteristics of sound:

• frequency affects pitch of the sound
• amplitude affects loudness of the sound
• wave form affects quality of the sound

1. Frequency - Pitch

The frequency of a wave is measured as the number of complete vibrations of particles of a medium per unit time. A commonly used unit for frequency is hertz (Hz).

Pitch is an auditory sensation in which listener assigns musical tones to relative positions on a musical scale based on the frequency of sound. A high pitch sound corresponds to a high frequency sound wave and low pitch sound corresponds to a low frequency sound wave.

A string vibrates with a particular fundamental frequency. It is possible, however, to produce pitches with different frequencies from the same string. The four properties of the string that affect its frequency are length, diameter, tension, and density. These properties are described below:

1. When the length of a string is changed, it will vibrate with a different frequency. Shorter strings have higher frequency and therefore higher pitch. 
2. Diameter is the thickness of the string. Thick strings with large diameters vibrate slower and have lower frequencies than thin ones - therefore, lower pitch. 
3. Tightening the string gives it a higher frequency while loosening it lowers the frequency. 
4. The density of a string will also affect its frequency. Remember that dense molecules vibrate at slower speeds. The more dense the string is, the slower it will vibrate, and the lower its frequency will be. Instruments often have strings made of different materials. The strings used for low pitches will be made of a more dense material than the strings used for high pitches.

Name	Frequency Range (Hz)	Characteristics
Infrasonic	0 - 20	Very low frequencies of sound that the human ear can’t detect, but you may feel the rumbling of the waves through your body.
Sonic (AKA Audio)	20 - 20 000	Normal range for human ears, although not everyone (especially the elderly) will hear to the extremes of this range.
Ultrasonic	20 000 +	Beyond normal hearing for humans, although some animals (like dogs) hear part ways into this range. Also used in medicine (e.g. ultrasound scan for pregnant women).

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Loudness and Amplitude of Sound

2. Loudness - Amplitude

The loudness of a sound depends on the wave's amplitude.
The louder the sound, the higher the amplitude. So, amplitude is also a way of measuring the energy sound wave has.


The higher the energy, the higher the amplitude resulting a louder sound.


The system used to measure the loudness of sounds is the decibel system, given the unit dB.

Range (dB)	Description	Examples
0 - 30	Very Quiet	This is the threshold of human hearing, up to the sound of a quiet whisper.
31 - 50	Quiet	This is an average quiet house, with maybe the sound of a fridge running or someone moving around.
51 - 70	Normal	Regular daily sounds like people talking.
71 - 90	Loud	This is the point where a sound becomes annoying or distracting. Vacuuming or a noisy car on a busy street are at these levels.
91 - 110	Very Loud	Most people will try to avoid being in areas this loud. Prolonged exposure can cause permanent ear damage. Temporary effects, like "stereo hiss", may happen.
111 +	Painful!!!	Even limited exposure to levels this high will cause permanent hearing loss.

Amplifier is a device to increase the loudness of sound by use of an external energy source. It drives the loudspeakers used in PA system to make the human voice louder.