PHYSICAL PROPERTIES OF SOUNDa

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BHASKAR TIWARI MOHD. RIZWAN MD. SAMMIUDDIN

M.ARCH (BUILDING SERVICES) FACULTY OF ARCHITECTURE & EKISTICS JAMIA MILLIA ISLAMIA

CONTENT• INTRODUCTION TO SOUND • WHY INSTRUMENTS SOUND DIFFERENT• SPEED OF SOUND• DOPPLER EFFECT• WHISPERING GALLERIES AND AMPHITHEATERS

Sound is a longitudinal wave.

Longitudinal waves are made up of areas where the wave is compressed together, and other areas where it is expanded.This would agree with the way that humans themselves make sounds. We force air, sometimes harder, sometimes softer, through our vocal cords. In the process the air is either squished or allowed to move freely, making the air into a longitudinal wave.

Regardless of the source of sound wave (whether it is a vibrating string or the vibrating tines of a tuning fork)

sound waves traveling through air are longitudinal wave.

The first person to discover that sound needs a medium was a brilliant English scientist known as Robert Boyle (1627–1691).

Image source: www.physicsclassroom.com

Sound is a mechanical wave.

Longitudinal waves are made up of areas where the wave is compressed together, and other areas where it is expanded.This would agree with the way that humans themselves make sounds. We force air, sometimes harder, sometimes softer, through our vocal cords. In the process the air is either squished or allowed to move freely, making the air into a longitudinal wave.

Regardless of the source of sound wave (whether it is a vibrating string or the vibrating tines of a tuning fork)

sound waves traveling through air are longitudinal wave.

The first person to discover that sound needs a medium was a brilliant English scientist known as Robert Boyle (1627–1691).

Image source: www.physicsclassroom.com

This is why you can still hear stuff, even if it’s distorted, when you are under water at a swimming pool.The speed of sound in liquids is quicker than in gases, and the speed of sound in solids is even quicker.This is because the atoms are closer together, so they transfer the sound more efficiently.You might have even seen people in movies listening for an approaching train by putting an ear on the train tracks and listening for it. The sound of the train travels faster and more efficiently through the solid train tracks.

Don’t try this! It’s very dangerous.

Sound can also travel through solids and liquids, not just gases.

Image source: https://www.fcwa.org/story_of_water/images/animation/3forms.gif

20 Hz would be very deep, low, rumbling sounds. 20 000 Hz would be a very high pitched, squealing sort of noise. (Note:. In music “pitch” means the same as frequency.)

Sound that humans can actually hear, which are frequencies from 20 – 20 000 Hz.

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 (Audio) 20 - 20000 Normal range for human ears, although not everyone (especially the elderly) will hear to the extremes of this range

Ultrasonic 20000 + Beyond normal hearing for humans, although some animals (like dogs) hear part ways into this range. Also used in medicine (e.g. ultrasounds for pregnant women).

Ultrasonic sound waves have frequencies greater than 20 kHz . As the speed of sound is constant for given temperature and medium, they have shorter wavelength.

Shorter wavelengths allow them to image smaller objects and ultrasonic waves are, therefore, used as a diagnostic tool and in certain treatments.

Internal organs can be examined via the images produced by the reflection and absorption of ultrasonic waves. Use of ultrasonic waves is safer than x-rays but images show less details. Certain organs such as the liver and the spleen are invisible to x-rays but visible to ultrasonic waves.

Physicians commonly use ultrasonic waves to observe fetuses. This technique presents far less risk than do x-rays, which deposit more energy in cells and can produce birth defects.

UltraSound in medicine

Sound waves lose energy as they travel. That's why we can only hear things so far and why sounds travel less well on blustery days (when the wind dissipates their energy) than on calm ones.

Stand some distance from a large flat wall and clap your hands repeatedly. Almost immediately you'll hear a ghostly repeat of your clapping, slightly out of step with it.

What you hear is, of course, sound Reflection, better known as an echo: it's the sound energy in your clap traveling out to the wall, bouncing back, and eventually entering your ears. There's a delay between the sound and the echo because it takes time for the sound to race to the wall and back (the bigger the distance, the longer the delay).

Sound waves are like light and water waves in other ways too. When water waves traveling long distances across the ocean flow around a headland or into a bay, they spread out in circles like ripples. Sound waves do exactly the same thing, which is why we can hear around corners. Imagine you're sitting in a room off a corridor and, much further up the corridor, there's an identical room where someone is practicing a trumpet inside. Sound waves travel out from the trumpet, spreading out as they go. They ripple out down the corridor, race along it, ripple through the doorway into your room and eventually reach your ears.

The tendency waves have to spread out as they travel and bend around corners is called diffraction.

Properties of Sound

Whispering galleries and amphitheaters

Image source: Left: The Capitol in Washington, DC has a whispering gallery inside its dome. Photo by courtesy of The Architect of the Capitol. Right: It's easy to hear people talking in this amphitheater in Iraq. Photo by Jason L. Andrade courtesy of US Marine Corps and Defense Imagery

You might not think you could hear someone whispering if they sat a long way away, but if they can make the sound of their voice bounce off something into your ears, their voice will travel much further than usual.

You are inside a building with a giant dome, the sounds you make will reflect off the curved roof like light rays bouncing off a mirror. Buildings that work this way are sometimes called whispering galleries.The dome of the US Capitol and the famous reading room in the British Museum in London are two well known examples. You can hear the same effect at work outside when you sit in a naturally curved area called an amphitheater. You can talk in a normal voice and still be heard very clearly a considerable distance away.

An animation illustrating how the Doppler effect causes a car engine or siren to sound higher in pitch when it is approaching than when it is receding. The pink circles represent sound waves.

Doppler effect

Image source: https://en.wikipedia.org/wiki/Doppler_effect

Stationary sound source

Movingsound source

Vss < Vw

Movingsound source

Vss = Vw

Movingsound source

Vss > Vw

Change of wavelength caused by motion of the source

Why instruments sound differentAn instrument (or a human voice, for that matter) produces a whole mixture of different waves at the same time. There's a basic wave with a certain amplitude and pitch, called the fundamental, and on top of that there are lots of higher-pitched sounds called harmonics or overtones. Each harmonic has a frequency that's exactly two, three, four, or however many times higher than the fundamental. Every instrument produces a unique pattern of a fundamental frequency and harmonics, called timbre (or sound quality). All these waves add together to give a unique shape to the sound wave produced by different instruments, and that's one reason why they sound different.

The other reason is that the amplitude of the waves made by a particular instrument changes in a unique way as the seconds tick by. Flute sounds are immediate and die quickly, while piano sounds take longer to build up and die out more slowly as well.

Most sound is made up of multiple frequencies. The relative intensities of these frequencies is the sound quality or timbre. A pulsing variation in loudness caused by two slightly different frequencies is called a beat. The human ear can detect beat frequency up to 7 Hz. If two waves differ by more than 7 Hz, a complex wave is heard which may be unpleasant (dissonant) or pleasant (consonant). Consonance is called a chord.The lowest frequency making up a sound is called the fundamental. Higher frequency waves are called overtones. The intensity of the overtones is usually less than that of the fundamental.

Quality of sound

Dead spots are places where destructive interference occurs from the interaction of two or more sound waves. For example, a soloist on stage sends sound waves into the audience. Some of the waves hit the walls of the auditorium, while other waves travel directly to the listeners. In some situations, a direct wave can destructively interfere with a reflected wave so they cancel each other out at that particular location. As a result, the listeners seated in those particular seats would hear nothing from that soloist. Someone sitting a few seats over from the dead spot, however, might not experience the destructive interference and would hear the soloist just fine.

In music, timbre (/ˈtæmbər/ TAM-bər or /ˈtɪmbər/ TIM-bər) also known as tone color or tone quality from psychoacoustics, is the quality of a musical note, sound, or tone that distinguishes different types of sound production, such as voices and musical instruments, string instruments, wind instruments, and percussion instruments. The physical characteristics of sound that determine the perception of timbre include spectrum and envelope.In simple terms, timbre is what makes a particular musical sound different from another, even when they have the same pitch andloudness. For instance, it is the difference between a guitar and a piano playing the same note at the same loudness. Experiencedmusicians are able to distinguish between different instruments of the same type based on their varied timbres, even if those instruments are playing notes at the same pitch and loudness.

Sound Focus and Dead Spot: When a sound wave is reflected by a concave surface, large enough compared to the wavelength, it concentrates on a spot where sound pressure rises excessively. This is called a 'sound focus'. As a consequence, sounds become weak and inaudible at some other spots, called 'dead spots'.

Sound Transmission Class (STC): To avoid the misleading nature of an average transmission loss (TL) value and to provide a reliable single figure rating for comparing partitions, a different procedure for single figure rating, called Sound Transmission Class (STC) rating, of a partition is determined by comparing the 16 frequency TL curve with a standard reference contour, the sound transmission class contour. STC ratings of some common walls and floors are given in Appendix F

Sound Pressure Level (SPL): Sound Pressure Level or Sound Intensity Level is measured in terms of the unit bel (B), which is defined as the logarithm of the ratio of the sound pressure to the minimum sound pressure audible to the average human ear. The unit decibel (dB) is one-tenth of a bel (B). Thus, Sound Pressure Level = log10 I Io bels = 10 log10 I Io decibels where, I = Sound Pressure in watt cm2, and Io = Sound Pressure audible to the average human ear taken as 10-16 watt/cm2.

Transmission Loss: Transmission loss (TL) of a partition is a measure of its sound insulation. It is equal to the number of decibels by which sound energy is reduced in passing through the structure. Units Db

REVERBERATION: It is persistence of sound in the enclosed space, after the source

ofsound has stopped. Reverberant sound is the reflected sound as a result of

improperabsorption. Excessive reverberation is one of the most common defect, withthe

result that sound once created longs for a longer durationresulting in confusion with the sound created next.

However, some reverberation is essential for improving quality

ofsound.

Transmission Loss: Transmission loss (TL) of a partition is a measure of its sound insulation. It is equal to the number of decibels by which sound energy is reduced in passing through the structure. Units Db

REVERBERATION: It is persistence of sound in the enclosed space, after the source

ofsound has stopped. Reverberant sound is the reflected sound as a result of

improperabsorption. Excessive reverberation is one of the most common defect, withthe

result that sound once created longs for a longer durationresulting in confusion with the sound created next.

However, some reverberation is essential for improving quality

ofsound.