Unit 1 Acoustics Article 1 PDF

Daisy Grace Wakefield 13 April 2015

WHAT IS SOUND?

The Physics Of Sound

What is Sound?

Sounds are vibrations that can travel through water, gas and air [11].A sound is created through action causing movement such as speaking. Speaking will cause vocal chords to vibrate and sound waves to be released from the mouth and the air molecules around the mouth to vibrate accordingly. This vibration will then cause the surrounding molecules to also vibrate and those vibrations will spread from molecule to molecule allowing the sound to travel [8]. The sound waves are compressed, get closer together, and then they are rarefied, pulled apart. This motion allows the sound to travel [9].Sound needs matter to travel through. Matter that transports a sound is called a medium such as a solid, liquid and gasses. Sound must travel through a medium as the vibrations need to spread otherwise there is no sound [11]. As sound needs a medium to travel though, there are no sound waves in space, as

BTEC Music Technology 1

Figure 1.1


there is no air. [8] As they travel through mediums, this makes sound waves mechanical waves [6][23]. there are two types of mechanical waves; longitudinal waves and transverse waves[13]. Transverse waves are waves that are perpendicular, ninety degrees, to the direction where the sound is traveling from. Longitudinal waves however, as they move through a medium, the particles move backwards and forwards in the direction the wave is travelling making sound waves longitudinal waves. Sound is also omnidirectional meaning that it can travel in every possible direction[10][14].

Compressions and rarefactions In longitudinal waves, there are parts where the molecules in the medium, for example air molecules, are close together, where they are compressed, compressions, and other regions where the molecules are spaced out, known as rarefactions.[12]

Wavelength A wavelength is a section of a sound wave. Wavelengths have high points known as peaks and low points known as troughs. A wavelength is the distance from one peak to the next or one trough to the next trough. This is known as a cycle. If the peaks or troughs are close together then this is a high frequency but if they are far apart then this is a low frequency [24][15].

Sound Wave,Sine Wave? The easiest way to demonstrate to sound wave is to look at a sine wave. This is the simplest type of sound wave but pure sine waves rarely exist in the natural world. A sine wave can help show the three main characteristics of a sound wave which is amplitude, frequency and phase [9].

Frequency and Amplitude Frequency is the rate, or number of times per second, that a sound wave cycles from positive to negative to positive again. In other words, how many times a second does the does the sound wave move from peak (crest) to peak or trough to trough. Frequencies are measured in cycles


Figure 1.2

Figure 1.3


per second or hertz (Hz). The human ear can hear frequencies ranging from 20 hertz(20Hz), which is a low, to 20,000 hertz (20kHz) which is loud. Frequencies do exist above and below 20 and 20,000 hertz but the human ear cannot detect them. As humans get older their hearing deteriorates [9]. Through age the human ear deteriorates and becomes a less eective organ or receptor.

Amplitude, or intensity of a sound refers to its strength also known as volume or loudness. Human ears can pick up a variety of volumes for example, from a mouse squeezing to an air horn. The range of human hearing is so vast, there is a scale designed to measure the intensity of a sound. This logarithmic unit is called decibels [24][7][9].

Speed of sound The speed of sound refers to how fast the waves or vibrations pass through a medium. Depending on the medium in which the vibrations are passing through can aect the speed. For example, in air, sound travels at 343 meters per second. thats 1 mile every 5 seconds. sound travels 4 times faster in water and 13 times faster through steel [11].

Sonic boom! When something goes faster than the speed of sound, for example fighter jets, its called breaking the sound barrier. Breaking this barrier creates a sonic boom. This noise is like an explosion and it is generated from sound waves that are being forced together because the fighter jet is traveling faster than sound [11]. On the decibel scale a sonic boom is around 200dB (decibels) [11][20][21].

Reverberation VS Echo Reverberation is what happens when sound bounces o something flat and hard [16]. They are dierentiated by the length of time between the initial sound and the reflected repetition [17]. With reverberation, if the distance between where the sound is released and where that initial sound can bounce o of is short,

for example a room or your hand in front of your face, the sound is reflected back to the source less than one-tenth of a second. As the delay is so short this is reverberation. Reverberations are added to music as they can give a more natural sound [17]. With an echo, the sound has a greater distance to travel, for example calling out over a valley. As it will take longer than one-tenth of a second for the sound to return, this is classed as an echo. Unlike reverberation, an echo does not add to the original sound but is heard


Figure 1.4


as a distinct repetition of the sound. This repetition is normally a lot softer or fainter and deteriorates every time it is repeated until the sound dies out. The sound is fainter due to the amount of energy lost as the sound waves travel further away from the source which is known as decay. An echo can be measured by the strength of the repetitions, time lapse between repetitions and the decay of time sound [17].

Envelopes Sounds produced by musical instruments have a volume envelope[9]. This means that all the notes playable on the instruments have their own recognisable curve where volume rises and falls over time. Certain instruments like drums start with a high level of sound but very quickly deteriorate to a lower, quieter level and then die away to silence [1][9]. Notes on instruments such as the flute and viola can be held or sustained for longer periods of time and whilst being sustained, their dynamics, piano meaning quiet and forte meaning loud, can be changed. This unique rise and fall to an instrument is called a sound envelope. There are four main parts to an envelope;

Attack

Decay

Sustain

Release

This can be reduced to an acronym of ADSR [2][9].The attack is the time taken for the sound to rise in amplitude until it reaches full amplitude. The

decay is the time taken for the amplitude level to decrease to the sustained level when the attack is over. Sustain refers to

where the sound level roughly stabilises and the release is the time taken from releasing the note in the sustained period to fall into silence. Envelopes can be an issue when recording as depending on where the sound is being recorded there could be natural reverberation of the room added to the release tail of the instruments[3][4]. This means that that envelope of a sound could continue longer than usual as the reverberation could be added to the sound. To fix this problem, you could use dierent microphones or position the microphone closer to the instrument to avoid unnecessary reverberations. The acronym ADSR are commonly found on samplers an synthesisers and this function will allow you to alter the attack, decay, sustain and release of an instruments envelope [3][5].

Phasing Phase compares the timing between two similar sound waves[9]. If two sound waves start at the same time, and have the same frequency, then they are in phase. Phase is measured in degrees, from 0 degrees to 360 degrees. If both sound waves are in phase they are measured at being 0 degrees. If they


Figure 1.5


are measured at being 180 degrees, then the waves are playing at opposite times, when one wave is at its peak the other is at its trough, and they are out of phase. When we combine two sound waves that are in phase, this create a stronger sound wave, meaning it becomes slightly louder. When the waves are out of phase, the air pressure cancels the other one out. This means that there is very little or no sound. This is phase cancellation.

Phasing can be an issue when audio signals are being mixed because if the original signal and the reflective signal are played together in a reflective room, this can cancel out the sound [9].

Standing Waves and Harmonics A standing wave is when a wave is travelling in one direction and then bounces o something like a wall and travel back. They can occur when a medium is constrained at endpoints[39]. These waves are associated with violins, cellos, guitars and other stringed instruments [38]. One kind of constraint is a ridged boundary and this is when we pin the medium down, in this case a guitar string, so it cannot be disturbed but we can disturb the medium in-between[39]. The wave produced has to be able to fit in-between the boundaries. This means that the string will only allow certain wavelengths[39]. When the string vibrates, there will be parts of the string where the string will not be moving and these points are called nodes. Parts of the string where there is a peak or a trough, where the wave is moving, are called antinodes[43][38].

When plucked, the strings frequency will start to increase each time it vibrates remembering that Frequency is the rate, or number of times per second, that a sound wave cycles from positive to negative to positive again[9]. The lowest frequency this vibration generates is called the fundamental frequency. The fundamental frequency depends on two things; the speed and the length of the string. The longer the string then the lower the frequency and the shorter the string the higher the frequency. The speed depends on the medium, in this case a string and this relates to the thickness of the string and how heavy the string is. If we increase the tension in the string we create a higher pitch and if we increase the


Figure 1.6


thickness or mass of the string this creates a lower and deeper sound [39]. Remembering that the wave has to be able to fit within the constrained medium, as the frequency increases, the string moves more and we get more nodes and this creates a harmonic series. Its harmonic because the frequencies are all integer multiples of the same frequency. If we wait a certain amount of time, all of the waves that fit in-between these two nodes will repeat the same period over and over and over again[39][24].

Beats In physics, beats are known as an interference in time. It is when you have two sounds or sound waves that almost have the same frequency but not quite. For example I could have one sound wave at 30Hz and the second at 32Hz [40].

There is something called a beat frequency, and this is equal to the dierence in two frequencies. Back to the example, the dierence between 30Hz and 32Hz is 2, therefore I would hear 2 beats every second. The further apart in frequency, the more frequent the beats are. So if my first wave was still at 30Hx and I increase the second to 35Hz I would hear 5 beats per second. This beat occurs due to the interference

between the two waves [41].

An oscilloscope can visually represent a sound wave, and if you had your two waves at 30 and 35 Hz you would be able to see them cross over creating this beat. We can change the amplitude of the waves but the frequency remains the same. When we have two waves that are close in frequency, they are alternating between periods of constructive and destructive interference which creates the beat sound. When the two waves match peak to peak is is constructive interference but when they match peak to trough, this is destructive interference. For example, if someone was tuning a piano, and they pressed down the key for middle C, they would hit the tuning fork of

middle C to see f they could hear the beats pattern, if they can, then they are at dierent frequencies, if they cannot, then they key is in tune. There are also points where the waves are pulsating so fast that the human ear no longer registers the beats pattern and interprets the sound as two separate notes [42].


Figure 1.7

Figure 1.8


Decibels Decibels (dB) are what are used to measure sound level or intensity of sound. The ear can hear everything from a mouse squeezing to a sonic boom [18]. The quietest sound that humans can hear is close to 0dB

on the decibel scale. A sound 10 times more powerful is classed as being 10dB. A sound 100 times more powerful than 10dB is 20dB and a sound 1000 times more powerful than 20dB is 30dB. Factors such as distance can aect the intensity of the sound as for example, if there was a sonic boom created by an aeroplane and you were right underneath it on the ground, the sound would be louder to you than someone 100 miles away from you. Any sound above 85

dB can aect a persons hearing. This depends on how long they had been exposed to the sound and the intensity of the sound. If you are exposed to 90dB for 8 hours or more the this can over time cause damage to the ear but if you are subjected to 140dB then the damage occurs almost instantly [19].

How Instruments Work

What is timbre? The timbre of an instrument refers to the instruments unique sound, for example if you played middle C on a flute and piano, it would be the same note but they would sound dierent as they each have their own unique sound to them [34].

Difference between music and noise Music and noise are both made up of sound waves of dierent frequencies [25]. Music is the art of arranging and climbing sounds in order to create a harmonious melody[36]. Noise is unwanted sound that is usually very loud and meaningless[36]. Noise has an irregular wave form and length and a low frequency. The main dierence between music and noise is that music is a collection of frequencies, usually from music instruments, that create a harmonious and pleasing sound where as noise is a collection of frequencies from a variety of sources that can be seen to be annoying such as screams and


Figure 1.9


loud bangs that is usually background noise [36]. A waveform is the visual representation of a sound [26]. A musical waveform has a high frequency with no sudden changes in amplitude or wavelength and therefore it produces a regular waveform but noise is unpleasant and has sudden changes in amplitude and has a low frequency and therefore irregular waveforms [27].

This is subjective. For example, one person might interpret tapping a pencil on a table as noise but someone else might find this melodic and therefore music. Again, people who like heavy metal music will find this musical but other people might interpret this as just irritating noise [27].

Instruments Acoustic instruments are ones where the should is generated through a physical object like a cello and electronic instruments are sounds created electronically though a computer[28]. Instruments can be placed into dierent categories;

Percussion

Brass

Woodwind

Strings

Percussion instruments make sounds when they are hit like a drum, shaken like a maraca or rubbed. Brass instruments are made of brass or another kind of metal and they create noise by air being blown inside the mouthpiece. The musician blows their lips to create a raspberry noise which will create the vibrations. Woodwind instruments create noise when, like brass instruments, air is blown inside them. Air may be blown between a reed and a surface like a clarinet, two reeds like a bassoon or across the edge of a mouth piece like a flute. Finally stringed instruments make a sound when their strings are plucked, bowed or sustained like a cello.[37]

The air inside a wind or brass instrument, strings on stringed instruments and the skin or surface of a percussion instrument all vibrate when played. This vibration created a sound wave in a medium and we hear these waves as musical notes [28].


Figure 1.10

Figure 1.11


Sounds created electronically are not the same as acoustic sounds. Electric sounds are created when an electrical signal is transmitted to an amplifier which is then played through speakers [28].

Strings Stringed instruments like a cello create musical notes by plucking or using a bow against the strings causing them to vibrate. The pitch is changed due to the tension in the string and its thickness. A tighter string will create a higher pitch and a thicker string will create a lower pitch [29]. A piano is said by some to be a stringed instrument as sound is created by a tiny hammer hitting the strings to create the sound. If you bowed the string then this will allow you to sustain the note. String instruments are tuned by

tightening or loosening the strings [29].

String instruments have something called a soundbox or resonator. This is usually the body of the instrument and the loudest part, due to vibrations. They also have a soundboard made from wood and also vibrates. When the strings vibrate, the vibration is picked up by the bridge upon which the strings lay. The bridge then transports these vibrations to the soundboard, which then vibrates, and uses the soundboard to amplify the sound making it loud enough for people to hear.

The length of the strings determines how fast they

vibrate as longer strings vibrate slower and create a lower pitch. The weight of the strings, being heavy or thick, will make a lower pitch that lighter thinner stings and as said before, the tighter the string the higher the note will be [30].

Percussion An instrument that is hit, scraped or shaken to create a sound is a percussion instrument. the action of hitting, scraping or shaking the instrument will cause the instrument in question to vibrate and therefore create a sound. Drums for example are composed of a head, made from a starched synthetic or animal skin and a cylindrical frame. The drum is played by hitting the skin of it which will cause vibrations to pulsate and be amplified by the cylindrical frame. The smaller and tighter the drum head the higher pitched the


Figure 1.12

Figure 1.13


drum, as the vibrations are faster. Some drums will produce a single pitch like a snare drum.

A snare drum is usually half as deep as other drums like a floor tom or rack tom. As it is smaller, this give it a higher pitch. Tom drums are around 13 to 14 inches in length but a snare drum is only 5 to 6 inches deep. A snare drum also has wires under the bottom skin of the drum. A lever called the throw o can be used to tighten and loosen these wires. If they are tightened then the wires will touch the snare drum. Hitting the head of the drum will cause the vibrations to travel down to the wires causing them to vibrate, this created the typical cracking sound associated with the snare drum. When the wires are not touching the bottom skin, the snare drum will sound similar to other drums but at a higher pitch due to its smaller depth. Some drums also have a special coated skin which is covered in a rough material. When this skin is hit, due to the material, it will slightly mue the sound. Combining all three elements, the coated skin, smaller depth and the wires under the drum, gives the snare drum its unique timbre [31].

Wind There are two main parts to a wind instrument;

Blowing air through it

Creating dierent notes by covering holes with your fingers[32].

As the musician blows air though the the mouthpiece, this creates vibrations that move thought the instrument which when combined with covering the right holes with their fingers creates dierent notes.

Wind instruments that are made when vibrations, made by air, travels across a thin piece of wood are known as reed instruments. Some have one reed like a saxophone and others have two reeds like an oboe. A flute on the other hand creates sounds when air is blown across the edge of the mouthpiece. As the air is split by the edge this causes vibrations [35]. The air hits the side of the flute, causing the air to move around to create the sound. The pitch is changed when fingers are lifted up or pressed down to release or cover holes. This action changes the frequency and therefore pitch. A higher pitch means that there are less holes covered and a lower pitch means that there are more holes covered[33].


Figure 1.14

Documents

Unit 1 Acoustics Article 1 PDF