A sound wave is characterized by its amplitude and frequency. Two sounds can be distinguished from one another by the following three different characteristics:
Two figures a and b represent sound waves. Both have the same frequency and waveform but the amplitude of the sound wave in figure a is more than the amplitude of the sound wave in figure b. The loudness of sound depends on the amplitude of vibration. A larger amplitude means a louder sound, and a smaller amplitude means a softer sound.
Example: If you strike a drum gently a faint sound is heard but if you strike it hard, you hear a loud sound.
Relationship between loudness and amplitude of wave: Loudness of sound is directly proportional to the square of the amplitude of the wave.
Loudness ∝ Amplitude2
Measurement: Loudness is measured on a decibel scale. The minimum loudness of sound audible at frequency 1 kHz is considered to be the zero level of sound in decibel(0 dB). It is taken to be the reference level. When loudness increases 10 times, the level of sound is called 10 decibels and when loudness becomes 100 times, its level is 20 dB. When the loudness becomes 1000 times, its level is 30 dB. The safe limit of the level of sound for hearing is from 0 to 80 dB. The sound of level 0 to 30 dB gives a soothing effect. But a constant hearing of sound level above 120 dB (which is usually unpleasant and can be considered a noise) can cause headache and immediate harm to your ears.
It is characteristic of sound that differentiates an acute or shrill sound from a flat sound. It depends on the number of vibrations per second or frequency. Each musical note has a definite pitch. If the pitch is high, the sound is shrill and if the pitch is low, the sound is flat. Two notes with the same amplitude on the same musical instrument will differ in pitch when their vibrations are of different frequencies.
Example: On a guitar, a big heavy string will vibrate slowly and create a low sound or pitch. A thinner lighter string will vibrate faster and create a high sound or pitch. In the case of flute, a lower note is obtained by closing more holes so that the length of the vibrating air column increases, thus the pitch of the sound decreases. On the other hand, if more holes are opened, the length of the vibrating air column is reduced and thus producing a higher pitch or make the sound shrill.
Quality is the characteristics that distinguish two sounds of the same pitch and same loudness. The waveform of sound is different for different sources of sound even if their loudness and pitch are the same. The sound quality that helps in identifying the object that produces sound is called timbre. For example, we can easily identify and differentiate sounds from violin and piano, even if they are played with similar pitch, duration, and intensity.
The waveform of sound produced by a tuning fork and a piano, both have the same pitch and same amplitude but they have different waveforms.
| Characteristic | Loudness | Pitch | Timbre or Quality |
| Factor | Amplitude | Frequency | Waveform |
Experiment for you to try
Take a test tube with a little water in it as shown in the figure below.
Blow air in the tube by placing your lip in the mouth of the test tube. You will hear a flat sound. Now add more and more water in the test tube so that the length of the air column above the water level decreases. Each time blow air and hear the sound.
You will notice that the sound produced becomes more and more shrill.
Inference: Pitch increases with decreasing the length of the air column.
