In terms of physics, sound is a form of energy produced by vibrating matter.
Sound is mechanical energy that needs a medium to propagate.
Sound can travel through a medium which is solid, liquid or gas.
In terms of physiology and psychology, sound is the reception of these sound waves and their procession by the brain.
Sound waves arrive through the ears of the receptor.
A sound wave is generated by some vibrating source, propagates through a medium as a series of compressions and rarefactions, and is finally received by our ears and brain.
Characteristics of sound waves
Velocity
The speed of sound waves is NOT alwys the same.
A speed sound depends on the elasticity, density, and temperature of the medium the sound is traveling through.
For example, at 0 degree centigrade, the speed of sound in Hz (hertz) is about 331 m/s (meter per second).
Note that the MORE dense the material, the faster sound travels through it. Sound travels SLOWER through the air at 30 degrees because the air is LESS dense at 30 degrees.
Wevelength and Amplitude
What happens to the air molecules when making sounds? The vibration produces areas in which the particles are closer together and areas in which they are further apart.
The vibration creates in the surrounding area a series of alternative high pressured regions called compressions (the region of air where the molecules have been compressed together) and low pressure regions called rarefactions (de-compressions) that travel away at a certain speed.
The air molecules vibrate back and forth, but they do not travel with the wave. Sound waves trafer energy but not matter. It means the wave energe travels in this direction (propagation), not the matter.
Sound waves can be represented as a function which ranges over particle density or pressure values across the domain of distance.
The wavelengthof a sound wave is the distance between two successive crests (or troughs) of the wave.
The amplitudeof a sound wave is the maximum change in pressure or density that the vibrating object produces in the surrounding air.
The pressure is measured in pascals (Pa). Although for practical reasons the dB SPL scale is usually used for measuring sound amplitude.
Frequency and Time Period
Frequency is the number of times per second that a sound pressure wave repeats itself. These repetitions are known as cycles. Frequency is measured in hertz (Hz) or cycles per second.
The diagram representing the upper wave contains more cycles per unit of time.
Time Period is the duration of H cycles. Time Period is the time a sound wave takes to go through a compression-rarefaction cycle.
Fomulas:
The period (T) is the inverse of the frequency (f):
As the period gets smaller, the frequency gets larger, and as the period gets larger, the frequency gets smaller.
There is also a direction relation between sound speed (ν), wavelength (γ) and frequency (f):
In order to determine the properties for a given sound, it is useful to use the waveform view of sound. The waveform view is a graph of the change in air pressure at a particular location over time due to a compression wave. The waveform view is a physical representation.
Human sound perception
Physical properties
Perceptual perperties
Frequency
Pitch
Amplitude
Loudness
Waveform
Timbre
Wavelength
Time period
Duration
What is the relation between the physical properties of sound and the psychological (or perceptual) properties of sound such as pitch, loadness, and timbre:
Pitchis the equality that makes it possible to classify sounds as higher and lower.
The physical property that is related to pitch is frequency.
Loudnessis the quality that makes it possible to order sounds on a scale from quiet to loud.
The amplitude of sound waves is related to the perception of loudness.
Timbre, also known as tone colour or tone quality, describes those characteristic of sound which allow the ear to distinguish sounds which have the same pitch and loudness.
The waveform is related to the perception of timbre.
In order to use the physical waveform view to understand something about these perceptual properties, we need to identify physical properties that are related to them. However, this is not so simple!
First, the relationship between the physical properties of a sound wave and the way we perceive it is non-linear. For example, a change in frequency does not always correspond with that constant change in pitch.
Second, the way all these properties are related to each other is not so simple. For example, the frequency is rate to pitch, but the frequency also affects the loudness, the frequency also affects to the timbre, the amplitude affects to the pitch, the wave form affects to the pitch, the duration affects to the pitch and the timbre. In fact, all these properties are related to each other.
The basic concept for understanding processes such as the digitisation of a sound wave or the compression of a sound file:
Pure tone : Several experiments about human sound perception are based on pure tones. The real-world sounds are not pure tones. Pure tones can only be produced technologically.
A pure tone is a sound that can be represented by a sinusoidal waveform that is a sine wave of constant frequency, phase-shift, and amplitude. It is composed of a single frequency.
Perception of pitch
Frequency is perceived by humans as speech.
A high frequency sound wave corresponds to a high pitch sound.
A low frequency sound wave to a low pitch sound.
As described in the figure above, the relationship between pitch and frequency is not as simple linear one for frequencies above 1,000 hertz, greater change in frequency is needed to produce a corresponding change in pitch.
There are a wide range of frequencies that occur in the world, humans cannot hear all the sound waves that arrive to our ears. The frequency range of human hearing is about 20 to 20,000 hertz.
Perception of loudness
Loudness is a sensation related to the amplitude of sound waves.
To express sound amplitude in terms of pascals, we have to deal with numbers from as small as 20 to as big as 20 millions.
Our ears perceive sound intensity on a logarithmic scale, which is why sound pressure is measured in decibels (dB), specifically dB SPL (decibels of sound pressure level). This logarithmic scale makes more sense for our hearing than a linear one. The formula expresses sound pressure in dB SPL. For example, we have a sound pressure of 20,000 micropascals.
$$ SPL=20log_{10}(\frac{2,000}{20})dB=20*3=60 $$
Dividing this by a reference pressure (usually 20 micropascals) gives us 1,000. Taking the logarithm (base 10) of 1,000 and multiplying by 20 (because we're using the dB scale) gives us approximately 60 dB SPL.
The relation between the subjective quality of loudness and the physical quantity of sound pressure level is complex. This graph is called an equal loudness contour and it shows the sound pressure level required at different frequencies to achieve a consistent perceived loudness. Each curve on the chart represents a curve of equal loudness of pure tones. Two important things are:
The ear is more sensitive to high-mid frequencies than to bass frequencies. In general, humans can hear sounds at lower decibel levels between 3000 and 5000 hertz than any other frequency.
The human ear interpret changes in loudness within a logarithmic scale.
The quietest sound we can possibly hear is given as 0 dB SPL and is referred to as the threshold of hearing. The "0" does not mean that there is no pressure in the sound wave. The loudest sound that we can hear is approximately 120 dB SPL and is referred to as the threshold of pain. Anything above this is both physically painful and damaging to our hearing.
Perception of timbre
Timbre or tone quality is what differentiates two sounds of the same frequency and amplitude.
The two sound graphs have the same frequency and amplitude, yet they differ. They have different timbre!
The perceptual property of timbre is related to the physical properties of the waveform and the spectrum of sound. Timbre is influenced by the shape of the waveform as well as the spectral characteristics. For instance, the spectrum of a pure tone contributes to its timbral qualities.
These are other waveforms. All of them are similar, but all of them are different. They have the same amplitude and the same frequency, but the sound is different because they have tone quality or different timbre.
