Physics Of Sound

*sound science series # 5

Sound is made up of changes in air pressure in the form of waves. Frequency is the property of sound that most determines pitch. The frequencies an ear can hear are limited to a specific range of frequencies.

Mechanical vibrations perceived as sound travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. Sound cannot travel through a vacuum.

The audible frequency range for humans is typically given as being between about 20 Hz and 20,000 Hz (20 kHz). High frequencies often become more difficult to hear with age. Other species have different hearing ranges. For example, some dog breeds can perceive vibrations up to 60,000 Hz.

There are four main parts to a sound wave: wavelength, period, amplitude, and frequency.

frequency: how many complete waves pass a set point a second. (measured in Hetz, Hz)

Amplitude: the height of the wave from the mid line to the peak.

Wavelength:the distance from 1 peak to the next.

Frequency

Every cycle of sound has one condensation, a region of increased pressure, and one rarefaction, a region where air pressure is slightly less than normal. The frequency of a sound wave is measured in hertz. Hertz (Hz) indicate the number of cycles per second that pass a given location. If a speaker's diaphragm is vibrating back and forth at a frequency of 900 Hz, then 900 condensations are generated every second, each followed by a rarefaction, forming a sound wave whose frequency is 900 Hz.

Wavelength and Period (T; s (second)) The wavelength is the horizontal distance between any two successive equivalent points on the wave. That means that the wavelength is the horizontal length of one cycle of the wave. The period of a wave is the time required for one complete cycle of the wave to pass by a point. So, the period is the amount of time it takes for a wave to travel a distance of one wavelength Time taken by two consecutive compression or rarefaction to cross a fixed point is called the time period of the wave. or time taken for one complete oscillation in the density of the medium is called the time period of the sound wave. Amplitude (A; unit as of density or pressure)  The amplitude of a sound is represented by the height of the wave. When there is a loud sound, the wave is high and the amplitude is large. Conversely, a smaller amplitude represents a softer sound. A decibel is a scientific unit that measures the intensity of sounds. The softest sound that a human can hear is the zero point. When the sound is twice as loud, the decibel level goes up by six. Humans speak normally at 60 decibels. Pitch How the brain interprets the frequency of an emitted sound is called the pitch. We already know that the number of sound waves passing a point per second is the frequency. The faster the vibrations the emitted sound makes (or the higher the frequency), the higher the pitch. Therefore, when the frequency is low, the sound is lower. Speed of Sound The speed of sound depends on the properties of the medium through which it travels. The speed of sound in a medium depends also on temperature and pressure of the medium. The speed of sound decreases when we go from solid to gaseous state. In any medium as we increase the temperature the speed of sound increases. Speed of Sound: at 21C (70°F), 344 meters per second, 1,129 ft per second, 1,233kph, 770mph. At freezing, the numbers are 331 m/s or 1087 ft/s. The Speed of sound in water is 1480 m/s or 4856 ft/s. More than 3,000 miles per hour. Wavelength, Frequency & Speed of Sound Wavelength x Frequency = Speed of Sound, or, Wavelength = Speed of Sound / Frequency, and Frequency = Speed of Sound / Wavelength As frequency increases (becomes higher), the wavelength becomes longer. As frequency decrease (becomes lower), the wavelength becomes shorter. A chart of a few selected frequencies and their correlative wavelengths  Frequency     in Hertz Wavelength in feet and inches or metres and centimetres 20Hz 56.5ft 17.22m 50Hz 22.6ft 6.89m 100Hz 11.3ft 3.44m 400Hz 2.83ft 0.86m 1,000Hz 1.13ft 0.34m 5,000Hz 2.71in 6.89cm 10,000Hz 1.36in 3.44cm 20,000Hz 0.68in 1.72cm Quality or Timber is that characteristic which enables us to distinguish one sound from another having the same pitch and loudness.  The sound which is more pleasant is said to be of a rich Quality. Tone and Note; A sound of single frequency is called a Tone. A sound which is produced due to a mixture of several frequencies is called a Note and is pleasant to listen to. Intensity, the amount of sound energy passing each second through unit area is called the intensity of sound.  Echo; the sensation of sound persists in ou brain for about 0.1 second. To hear a distinct echo the time interval between the original sound and te reflected one must be at least 0.1 second. speed of sound 344m/s at 22° C in the air 344 m/s  ͯ  0.1 s ꞊ 34.4 m the minimum distance of the obstacle from the source of sound must be half of this distance, that is 17.2 m.  Human echolocation Human echolocation is the ability of humans to detect objects in their environment by sensing echoes from those objects. This ability is used by some blind people to navigate within their environment. They actively create sounds, such as by tapping their canes, lightly stomping their foot or by making clicking noises with their mouths. It can however also be fed in to the human nervous system as a new sensory experience. Human echolocation is similar in principle to active sonar and to the animal echolocation employed by some animals, including bats and dolphins. By interpreting the sound waves reflected by nearby objects, a person trained to navigate by echolocation can accurately identify the location and sometimes size of nearby objects and not only use this information to steer around obstacles and travel from place to place, but also detect small movements relative to objects. However, in the case of human clicking, since humans make sounds with much lower frequencies (slower rates), such human echolocation can only picture comparatively much larger objects than other echolocating animals. Mechanics Vision and hearing are closely related in that they can both process reflected signals. Vision processes photons as they travel from their source, bounce off surfaces throughout the environment and enter the eyes. Similarly, the auditory system processes sound waves as they travel from their source, bounce off surfaces and enter the ears. Both systems can extract a great deal of information about the environment by interpreting the complex patterns of reflected energy that they receive. In the case of sound, these waves of reflected energy are called "echoes". Echoes and other sounds can convey spatial information that is comparable in many respects to that conveyed by light. With echoes, a blind traveler can perceive very complex, detailed, and specific information from distances far beyond the reach of the longest cane or arm. Echoes make information available about the nature and arrangement of objects and environmental features such as overhangs, walls, doorways and recesses, poles, ascending curbs and steps, planter boxes, pedestrians, fire hydrants, parked or moving vehicles, trees and other foliage, and much more. Echoes can give detailed information about location (where objects are), dimension (how big they are and their general shape), and density (how solid they are). Location is generally broken down into distance from the observer and direction (left/right, front/back, high/low). Dimension refers to the object's height (tall or short) and breadth (wide or narrow). Notable individuals who employ echolocation: James Holman, Daniel Kish, Ben Underwood, Tom De Witte, Dr. Lawrence Scadden, Lucas Murray, Kevin Warwick *Note: all pictures thankfully shared from various sources..