Module 01 - Analog and Digital Audio

Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio Module 01 Analog and Digital Audio In this module you will learn about the nature of sound, soundproofing, acoustics and acoustic treatment, analog audio electronics and digital audio Learning outcomes • To understand the way in which sound behaves in air, how sound interacts with hard and soft materials; flat and irregular surfaces • To possess the basic background knowledge of recording studio acoustic design • To understand how sound is handled and transmitted as an electronic signal • To understand how an analog electronic signal is converted to, handled and stored as a digital signal, and how it is converted back to analog Assessment Formative assessment is achieved through the short-answer check questions at the end of this module Page Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio Module Contents Learning outcomes Assessment The nature of sound Frequency The decibel The inverse square law Acoustics Standing waves Acoustic treatment Soundproofing Materials The three requirements for good soundproofing Concrete Bricks Plasterboard (drywall) Glass Metal Proprietary flexible soundproofing materials Construction techniques Walls Ceiling Floor Windows Doors Box within a box Ventilation The function of absorption in soundproofing Flanking transmission Cable ducts Audio electronics Passive components Real world components Resistors in series and parallel Digital audio Digital versus analog Analog to digital conversion Problems in digital systems Latency Clocking Check questions Page 1 10 11 14 16 18 18 19 19 20 20 20 21 21 22 22 23 23 24 25 25 26 27 27 28 29 30 32 33 34 34 36 38 40 40 42 Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio The Nature of Sound We all know the experience of sound, and we all learned in school that it is a vibration of air molecules that stimulates our eardrums People who work with sound every day tend not to think about the science of sound and take it for granted But unless you have assimilated a good understanding of the nature of the medium in which you work, how are you ever going to make it really work for you? Sound starts with a vibrating source, commonly the vocal folds (formerly known as the ‘vocal cords’ or sometimes ‘vocal chords’), musical instruments and loudspeaker diaphragms, as far as we are concerned Let us think of a loudspeaker diaphragm It vibrates forwards and backwards and pushes against air molecules On a forward push, it squeezes air molecules together causing a ‘compression’, or region of high pressure On pulling back it separates air molecules causing a ‘rarefaction’, or region of low pressure The compressions and rarefactions travel away from the diaphragm in the form of a wave motion Vocal folds - illustration courtesy University of California, Berkley Wave motions are all around us, from the water waves we see in the sea (best viewed from a ship the breaking effect near the shore disguises their true nature), to all forms of electromagnetic radiation such as x-rays, light, microwaves and radio waves The child’s toy commonly known as the ‘slinky spring’ can display a wave very much like a sound wave The slinky is a spring – the metal versions work best – of around 15 cm in diameter and perhaps m long when lightly stretched If two people pull it out and one gives a sharp forward and backward impulse, the compression produced will travel to the end of the spring and – if the other person holds his or her end firmly – reflect back This demonstrates a longitudinal wave where the direction of wave motion is in the same direction of the motion of the actual material (we can call the motion of the material the ‘particle motion’) A sound wave is a longitudinal wave Page ‘Slinky’ spring - photo by Roger McLassus (GFDL) Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio Contrast this with a water wave where the wave moves parallel to the surface of the sea, but water molecules move up and down This is a transverse wave Electromagnetic waves are transverse waves too One feature that the water wave demonstrates perfectly is that if you look out from the side of a ship at a piece of flotsam riding the wave, the wave appears to travel from place to place, carrying energy as it does so, but the flotsam simply bobs up and down Other than wind or tide acting directly on the flotsam itself, it will bob up and down all day without going anywhere This is true of sound too A sound wave leaves a loudspeaker cabinet, but this doesn’t mean that air travels away from the cabinet The air molecules simply vibrate forwards and backwards, never going anywhere (When air molecules travel from one place to another that is called, in purely technical terms, a wind!) If this were not so then either a vacuum would develop inside or around the cabinet and there would be a danger of asphyxiation Obviously this doesn’t happen Oddly enough, if you put your hand in front of a bass loudspeaker you will feel a breeze, if not a full-on wind, on your hand This is an illusion since you feel the air molecules when they press on your hand, but not when they pull back In a transverse wave, such as a water wave, the direction of particle motion is at right angles (‘perpendicular’) to the direction of wave motion In a longitudinal wave, such as sound, the direction of particle motion is parallel to the direction of wave motion Although the longitudinal wave in the slinky spring is similar to a sound wave, it doesn’t quite tell the whole story The slinky wave is confined within the spring whereas a sound wave spreads out readily It is possible to think of each air molecule (actually oxygen, nitrogen and an increasing amount of carbon dioxide) that vibrates under the influence of a sound wave as a sound source in its own right Page Transverse wave created on a string Photo courtesy Union College Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio Molecules are of course very small, and it is a feature of small sound sources – or point sources – that they emit sound equally in all directions, or ‘omnidirectionally’ So where light travels over great distances in straight lines, sound merely has a tendency to follow a straightline path, and readily spreads out from that path in an ever-widening arc, particularly at low frequencies [Regarding point sources - it is also worth considering the example of a small loudspeaker emitting a low frequency tone If the speaker is small in comparison with the wavelength being emitted, then it will have the characteristics of a point source and will obey the inverse square law - sound pressure halves for every doubling of distance from the source.] Frequency To compare the range of frequencies in human experience, a satellite TV signal - for example - has a frequency of around 10 to 14 GHz The Olympic Games have a frequency of nanohertz (they happen once every four years!) • hertz (Hz) means one cycle of vibration per second • 1000 Hz = kHz • 1,000,000 Hz = Megahertz (1 MHz) • 1,000,000,000 Hz = Gigahertz (1 GHz) Sound comes in virtually all frequencies but our hearing system only responds to a narrow range The upper limit of young human ears is usually taken to be 20 kilohertz (kHz) (twenty thousand vibrations per second) This varies from person to person, and decreases with age, but as a guideline it’s a good compromise If a sound system can handle frequencies up to 20 kHz then few people will miss anything significant At the lower end of the range it is difficult to know where the ear stops working and you start to feel vibration in your body In sound engineering however we put a figure of 20 Hz on the lower end We can hear, or feel, frequencies lower than this but they are generally taken to be unimportant Page Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio Frequency is related to wavelength by the formula: velocity = frequency x wavelength This applies to any wave motion, not just sound The velocity, or speed, of sound in air is a little under 340 meters per second (m/s) This varies with temperature, humidity and altitude, but 340 m/s is a nice round number and we’ll stick with it If you work out the math, this means that a 20 Hz sound wave travelling in air has a wavelength of 17 metres! The extreme physical size of low frequency sound waves leads to tremendous problems in soundproofing and acoustic treatment At the other end of the scale, a 20 kHz sound wave travelling in air has a wavelength of a mere 17 mm Curiously, the higher the frequency the more difficult it is to handle as an electronic, magnetic or other form of signal, but it is really easy to control as a real-life sound wave travelling in air Low frequencies are easily dealt with electronically, but are very hard to control acoustically Page Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio The decibel The concept of the decibel a convenience that allows us to compare and quantify levels in the same manner through different media In sound terms, decibels can be used for every medium that can store or transport sound or a sound signal, for instance • real sound travelling in air • electric signal • magnetic signal • digital signal • optical signal on a film sound track • mechanical signal on a vinyl record A change in level of dB means exactly the same thing in any of these media Without decibels we would have to convert from newtons per square metre (sound pressure), volts, nanowebers per square metre, etc Decibels have another advantage for sound The ear assesses sound levels logarithmically rather than linearly So a change in sound pressure of 100 µN/m2 (micro-newtons per square meter) would be audibly different if the starting point were quiet (where it would be a significant change in level) then if it were loud (where it would be hardly any change at all) A change of dB is subjectively the same degree of change at any level within the ear’s range [Sound pressure is measured in newtons per square meter You may think of the newton as a measure of weight One newton is about the weight of a small apple.] An important point to bear in mind is that the decibel is a ratio, not a unit It is always used to compare two sound levels To convert to decibels apply the following formula in your scientific calculator: 20 x log10 (P1 /P2 ) …where P1 and P2 are the two sound pressures you want to compare So if one sound is twice the pressure of another then P1 /P2 = The logarithm of (base Page Optical film soundtracks, variable density and variable area Illustration by Iain F Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio 10) is 0.3, and multiplying this by 20 gives dB Actually it’s 6.02 dB but we don’t worry about the odd 0.02 This is useful because we commonly need to, say, increase a level by dB, but it doesn’t actually tell us how loud any particular sound is because the decibel is not a unit The answer to this is to use a reference level as a zero point The level chosen is 20 µN/m2 (twenty micro-newtons per square meter), which is, according to experimental data, the quietest sound the average person can hear We call this level the ‘threshold of hearing’ and it can be compared to the rustle of a falling autumn leaf at ten paces We quantify this as dB SPL (sound pressure level) and now any sound can be compared with this zero level Loud music comes in at around 100 dB SPL; the ear starts to feel a tickling sensation at around 120 dB SPL, and hurts when levels approach 130 dB SPL If you are not comfortable with math, it is useful to remember the following, which apply to both sound pressure and voltage (but decibels work differently when referring to power): • -80 dB = one ten thousandth • -60 dB = one thousandth • -40 dB = one hundredth • -20 dB = one tenth • -12 dB = one quarter • -6 dB = one half • dB = no change • dB = twice • 12 dB = four times • 20 dB = ten times • 40 dB = one hundred times • 60 dB = one thousand times • 80 dB = ten thousand times • Threshold of hearing = dB SPL • Threshold of feeling = 120 dB SPL • Threshold of pain = 130 dB SPL Page This fruit weighs approximately newton Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio Do you need to understand decibels to be a sound engineer? The answer is, “Yes - to a point” You need to be able to relate a change in decibels to a fader movement, and from there to an image in your aural imagination of what that change should sound like In addition to that, you’ll get producers telling you to raise the level of the vocal “a bit” How many decibels equal “a bit”? Only the experience you will gain in the early years of your career will tell you Page Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio The inverse square law There is more to find out about the inverse square law Here is an interesting point The maximum rate of decay of a sound as you move away from it is decibels per doubling of distance (the sound pressure halves) This is simply due to the spreading-out of sound - the same energy has to cover an ever greater area If the sound is focused in any way, by a large source or by reflection, then it will fade away at a rate less than dB per doubling of distance This fact is of great importance to PA system designers The ultimate focused sound source is the old-fashioned ship’s speaking tube Sound is confined within the tube and can travel over 100 meters and hardly fade away at all Page 10 ... newton Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio Do you need to understand decibels to be a sound engineer? The answer is, “Yes - to a... even heavy-duty springs The mass of the slab is important as the mass-spring system Page 22 Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio will... 34 36 38 40 40 42 Audio Masterclass Music Production and Sound Engineering Course Module 01: Analog and Digital Audio The Nature of Sound We all know the experience of sound, and we all learned