Hello. I am Bhooshan Thite from India. This work is lesson for week 1 of Introduction To Music Production at Coursera.org. In this text I am going to cover how sound is generated and propagated. I will also cover characteristics of sound such as amplitude, frequency and timbre.

Sound Generation and Propagation:

Sound is created by vibration in form of pressure and displacement in a medium. If two objects hit each other, they vibrate. These vibrations are passed on to the medium in form of push and pull. When there is no sound, we say that the molecules in a medium are evenly spaced.

When two objects in the medium hit each other, they vibrate thereby affecting the distribution of the molecules in the medium.

The compressions and rarefactions continue periodically until the vibration energy in the object dies out. Also, with distance the energy of compressions and rarefactions in air also reduced and hence the loudness of a sound heard is less if we are far away from it. The figure below shows a graphical way to represent the vibrations. This representation makes it easy to see, analyze and process sound.

Amplitude:

Amplitude is the measure of the range of maximum compression and maximum rarefaction of air molecules in response with a sound. In simple terms we can see amplitude as the height of the waveform we see on oscilloscope.

Amplitude is the term measurable by the computer when fed with a sound. However, the term loudness is mistakenly seen as amplitude. Loudness is the perception of sound. Because, two different sounds having the same loudness may be perceived to be differently loud.

The following diagram shows how amplitude is related to depth of modulation:

Frequency:

Frequency is number of repetitions in a unit time. For example, the frequency of a bus is 4 times an hour. As a characteristic of sound, frequency is number of cycles of vibrations in one second. Unit of it is Hertz (Hz).

Double click to hear the sounds of 440 Hz and 1000 Hz:

440 Hz 1000 Hz

The vibrations created in the source of the sound cause compressions and refractions in air. Thus, repeated push and pull by the sound source causes the air to repeatedly vibrate. These vibrations propagate through a medium and are sensed by a listener. The term pitch is mistakenly used for frequency. However, frequency can be measured as number of repetitions in unit time while pitch is the perception. Humans are ideally able to hear from 20 Hz to 20,000 Hz. However, the range of hearing capacity reduces with age.

Timbre:

It is not possible to naturally produce a sound, which contains a single frequency, be it a ‘note’ played on a musical instrument. It contains a group of frequencies, which are present in form of harmonics.

Timbre is a tone or a specific quality of a sound. Two sounds with same pitch and same loudness can have different timbre. For example, if we hear a flute producing C3 tone and a singer singing C3 tone with same loudness, they sound different to us. That is the difference between their tonal qualities, precisely their timbres.

Difference between timbres can be analyzed on spectrum analyzer. The followings are the spectrums of two sounds (guitar and piano) played on same tone (C3) and same intensity

Click to play Piano tone (C3) Click to play Guitar tone(C3)

Piano tone includes lower frequencies more as compared to Guitar. However, frequencies around 5 kHz are more in Guitar. The inclusion and intensities of various frequency components make separate timbres for both of the sounds and thereby different sound qualities.

It was a nice experience of explaining basics of sound in a detailed manner. I used audio files, photos and text in the assignment. The audio files and photos are extracted from Logic Pro 9, Audacity. For understanding some topics in detail, I have also taken help of internet in addition to the course videos.

I would like to get a feedback about what is explained in a good manner and what needs to be improved. Your feedback is valuable to me. Thank you.