! Acoustics For Musicians! Maximilian Crosby! Music Technology!
! ! ! ! ! ! Acoustics For Musicians! Maximilian Crosby! Music Technology! ! © ! What is sound?! ! Acoustics For Musicians! Sound is essentially vibrations. These vibrations can effectively move through anything. When you hear sound using your ears, Your ears have a tiny diaphragm inside of them and the vibrations or Sound Waves that make contact with it make it vibrate in such a way that it will send an electrical signal up into your brain thus the sound. There is an unfathomable amount of different sounds that you could here mainly because everything makes a different sound. ! ! But how does the diaphragm not just in year ears but in microphones tell different sounds? Lower sounds or lower frequencies make long but short Sound Waves. Where as a high pitched sound would make really short but tall Sound Waves. The picture to the right will demonstrate. Lower frequency sounds will travel further simply because the Sound Waves... are longer! For example say you have a normal brick wall. Higher frequencies will not travel through the wall because the waves are simply not long enough to travel through it but lower frequencies will travel through it because the waves are long enough to pass through. That’s why when your neighbor is playing music and 2 O‘clock in the morning all you here is the thud of the base. ! ! Speaking of thud of a base. Why do we feel Sound Waves? Why do we feel the thud of the Kick Drum if you stand next to it? Why does the ground shake at a loud gig? This is called a Sound Pressure Level or SPL. SPL is a force generated by the Sound Waves. A lower Sound Wave will have a higher SPL than a higher Sound Wave simply because it will need a bigger surface area to generate. The actual strength of the SPL is generated by a combination of surface area and amplitude. ! ! Amplitude is commonly known as volume and is measured in decibels. The Human ear can only withstand a certain amount of amplitude before you damage it’s diaphragm. Anything above 75 decibels is actually damaging your diaphragm. Anything above 130 decibels is louder than a Jet Engine and will defiantly damage your diaphragm. There is nowhere in the universe where there is no sound or no amplitude. Even in space there is more than 0 decibels because of the big bang. Everything is moving so even though you may not be able to hear it there is sound. ! ! When a sound is made there will always be Harmonics in the Sound Wave. What are Harmonics? Harmonics are different Sound Waves being generated from a Sound Wave. Pure sound waves are divisions and multiples of a Sound Wave. For example... I’f a Sound Wave has a Frequency of 200. Even Harmonics from that Sound Wave will be the © Frequency of 400, 800, 1600, Ect Ect Ect. Odd Sound Waves will be all the other Frequencies. 201, 202, 203 Ect Ect Ect. Without Harmonics things would just sound dull.! So to sum things up Sound has 4 main characteristics. ! ! 1. Frequency. The human ear can here from 20 to 20,000hz.! 2. Amplitude. The volume of the Sound Waves.! 3. SPL or Sound Pressure Level is a force generated by the Sound Waves! 4. Harmonics is the other Frequencies generated when you make a sound.! ! ! Sound Waves Through Air:! ! ! ! Above is Air in static state.! Above is Air with Sound Waves traveling through.! ! ! ! Above is a Sound Wave or Sign Wave.! ! Sound Waves travel through the molecules in the air. If you were playing Sound under water sound would travel roughly 4x faster than it would in the air simple because there are much more molecules in water. Solids like some metal sound will travel up to 10x faster! © Frequency:! Frequency is one of the main characteristics that define sound. It’s one of the biggest and easiest ways we can tell apart a sound from another. As regards to the meaning in purely musical terms frequency is the same thing as pitch. But in the scientific world Frequency is much more. ! ! Frequency is the number of waves per unit time. For example a very low frequency means that the wave length is very long creating a lower pitch and a high frequency would have a very thin wave length, this creates a high pitched sound. If you look at a sign wave diagram Frequency applies to the X exis. It’s much like hot and cold temperatures. When something is Hot the molecules are vibrating and banging into each other at an extremely fast rate. Where as if something is cold it’s the opposite. The Molecules are very slow. ! ! Frequency has a massive spectrum From 0Hz to tens of thousands of Hz but the human ear can only hear from 20hz to around 20,000Hz. Some lucky people can hear up to around 24,000Hz but that’s the limit our ears can possibly go to. Animals ears can hear differently though. Some can hear higher pitches and some lower. For example a whale’s voice to us sounds very low in the frequency band but to them there voices sound much higher! This is because there ears are designed that way. So in relation we sound more like birds chirping to whales. The same thing with birds but reversed. There ears are adapted to hear there chirps at a lower frequency. Whales probably can’t even hear birds and vice virsa because they’re both probably out of each others frequency range.! ! Amplitude:! Amplitude applies to the Y axis instead of the X of a diagram of a sign wave. Amplitude is the magnitude of the wave - how high it goes on the Y axis. This effectively means that the sign wave is louder. It has more volume, more decibels. (see apparent noise levels on the next page)! ! To further answer the question, what is amplitude? You could ask, why is an amplifier called an amplifier? It’s in the name! A signal taken from an electric guitar is very small. To small to be heard without an Amplifier. What an amplifier does is it boosts the signal electrically to a sufficient Voltage/Wattage so that it’s healthy to be played through speakers. ! ! Sound Pressure level:! Sound Pressure level or SPL is the pressure of the sound waves through air. It’s the physical intensity of sound. Bass waves tend to have a high SPL. Ever stood next to a kick drum and felt the thud every time to drum is hit? That’s SPL. ! ! SPL is evident in every sound wave it’s just most of the time especially in higher frequencies you can’t feel it. If you concentrate enough you can normally feel it if your playing music through any set of loud speakers. If you can’t feel it just by standing there put your hand on the speaker. Then you’ll feel it! (Sound Pressure and sound Pressure level on the next measured in Pa) ! ! SPL can be measured in dB but can more accurately be measured in Pa or Pascal. Pascal is the SI derived unit of pressure. It is a measure of force per unit area. Defined as one newton square meter. © Phase! ! If Sound Waves are out of phase, one Sound Wave is out of time with the other. If 2 Sound Waves ware in Phase you’d hear one sound. more or less. If they weren’t in Phase you’d hear more than one sound as this diagram shows. ! If you were to put an SM57 on top and under a Snare drum you’d need to reverse the phase on the bottom mic. Why? because if you didn’t the microphones would create catastrophic feedback because they’re facing into each other. Reversing the phase completely rules this out as they’re at opposite ends of the frequency band at all times so they don’t interrupt each other. ! If you’re clever you can do some really funky things with phase. Ever wanted an annoying sound to just be gone? Well by using phase you can! If you set up a microphone, Find the sound you don’t like with EQ, Flip the polarity and play it back through some speakers you will find that particular sound will no longer be present. This is because the two sounds are now ruling each other out. Your ear simply can’t hear them. This doesn’t happen when you flip the polarity on a snare though because the sound is differant as one Mic is on top and one is under the snare. In order to rule out a sound the sounds need to be exactly the same. ! ! ! ! ! ! ! ! ! ! ! ! ! ! Apparent Noise Levels ! ! These are the dBs of all the every day sounds you hear. Remember there is no such thing as 0 dB.! Auditory threshold at 1KHz = 2x10-5 Pa / 0 dB! Breathing = 6.32x10-5 Pa / 10 dB! Whisper = 2x10-4 Pa / 20 dB! Computer Fan = 25 dB! Fridge = 2x10-3 Pa / 40 dB! Small Office = 50 dB! Large Office = 55 dB! Laughter = 2x10-2 Pa / 60 dB! Hair Dryer = 70 dB! Side Of Motorway = 75 dB! Motorcycle = 6.32x10-1 Pa / 90 dB! MP3 Player Max Volume = 2 Pa / 100 dB! Orchestra = 6.3 Pa / 110 dB! Hearing damage = approximately 20 Pa / 120dB ! Starts To Hurt = 63.2 Pa / 130 dB! Pain. Gunshot Close Range = 140 dB! Jet engine at 30 meters away = 632 Pa / 150 dB! Instant Perforation Of EarDrum = 160 dB! Stun Granade = 6,000 Pa / 170 dB! Theoretical Limit for undistorted sound = 101,325 Pa / 194 dB! © ! Speed Of Sound ! The Speed Of Sound is 344 meters per second at sea level at 20 Degrees. It’s different if your at a higher altitude or at a different temperature. It’s also different if sound is traveling through something other than air. Sound travels 4x faster through water and 10x faster through some metals. This is because the molecules in those materials are more dense. The less distance molecules have to travel to bang into the next means the sound is traveling faster. Air is obviously a very thin substance but through some metals sound is so much faster.! ! Infra and Ultra Sound! Infra sound is sound that is below 20dB. The human ear cannot hear below 20dB. Our hearing becomes less sensitive as the frequency becomes lower. Ultra sound is higher than 20Khz. Some females can here over 20Khz because of genetics. Dogs can also hear Ultrasound. That’s why you get Ultrasound whistles for dogs. ! ! Black and White Noise! The human ear can hear white sound. That’s roughly anything between 20hz to 20Khz. Anything below or above that I called black sound because we cannot hear it. ! ! Timbre! Timbre is the character of a sound which enables us to disguise between different musical instruments including the voice. When we hear to different instruments, for example acous<c guitars, playing the exact tone or sound playing at the same frequency and loudness the notes will sound different which is because of <mbre. Fletcher Munson Curve ! The Fletcher Munson curve or the equal loudness contorts, show the response of the human ear throughout the frequency range (20Hx-‐20KHz) ! ! ! ! ! ! ! ! Acous<cs: © ! 1. Harmonics! No harmonics = Pure tone. Harmonics = color to sound.! 500 - 1,000 - 2,000 - 4,000 etc etc = True Harmonics. ! ! 2. Sound Pressure Level. ! The amount of pressure of air that is generated. Eg bass drum. ! ! 3. Frequency ! (cycles per second) 500hz signal = 500. 10,000hz = 10k. ! ! 4. Amplitude. ! dB. in a quite room on average 50dB. Jet engine on average = 130dB. 130dB = Ear pain barrier! ! ! Human ear = 20 - 20k hearing range. speed of sound 344MPS sea level 20c.! sound waves 4x slower though plastic. 10x faster though metal. ! 2x faster through sea. ! ! Calculation for wave length = Wave Length = Speed of Sound (344) over Frequency.! 344 over 50 = 6.88m. ! The Human Ear:! ! Here is a diagram of the Human ear. I have labeled all of the main parts of the human ear one to twelve.! ! 1. The top of the outer ear is folded over. This is called the Helix. The shape of the outer ear is the shape it is because it helps to capture sound waves. ! ! 2. This part of the ear is called the Antihelix. This is the bit in the middle that sticks out. Like I said the shape of the outer ear is the shape it is because it’s designed to capture sound.! ! 3. This part of the ear is called the Lobule. It’s commonly known as the ear lobe. ! ! 4. This is the Crest of the Helix! © ! 5. this is called the External Auditory Meatus or the External Auditory Canal ! ! 6. This is where the Eardrum or the Tympanic membrane is right at the back of the ear canal. This is a thin layer of skin that vibrates when the sound waves collide with it! ! 7. This is the Auditory Ossicles. This part of the ear is a very important part of the ear. This part contains three bones that pick up the vibrations from the ear drum and effectively amplifies it. These three bones are called Hammer (malleus) this bone is closest to ear drum, Anvil (incus) This is the middle bone and the Stirrup (stapes) this is the bone furthest away from the ear drum but is connected to the Anvil and the oval window. These three bones are positioned in such a way that the vibrations coming from the Ear drum are amplified and become more substantial. ! ! 8. This is the Oval Window. The Oval Window is an opening in the Cochlea. This allows the vibrations from the Stirrup to travel through the window and into the Cochlea. ! ! 9. This is the Cochlea. The Cochlea is one of the main part of the inner ear that contains all the organs. The vibrations that are coming in through the window are sent through liquid in the Cochlea. The Cochlea is wound up in a spiral. If you were to unravel it, It would be quite long. The thinest part of the Cochlea picks up the high frequency sound waves and the thickest and deepest part at the other end picks up all the low frequency sound waves. This is because low frequency sound waves travel further then high frequency sound waves. There for you need more area in order to effectively pick the frequencies up.! ! ! 10.This is the Semicircular Canals. In this part of the ear the sound waves are picked up by millions of little hairs. All hairs are in bunches of 3 and they very from short to long. Short for the high frequencies and long for the low frequencies. These hairs send messages to the Eighth Nerve.! ! 11.This is the Eighth Nerve. These stems send electrical signals picked up by the vibrations in the hairs. These signals are sent up into the brain and that is how the brain recognizes the sound. ! ! ! About The Human Ear:! ! The Diaphragm in the human ear moves less than a tenth of an atom. ! The bones are in this order: Hammer, Anvil and then the stirrup. The stirrup is connected to the window and the window transmits the sound waves through the liquid in the cochlea.! ! The tube of the ear is called the Pinna. ! The sound comes in and the shape of the ear give us a 3D sound.! There are 3 bones in the human ear that help us to hear. They are called the Hammer, Stirrup and the Anvil.! ! © The cochlea is very long because that allows us to hear lower frequencies. Imagine a sound that is 100Hz. That is very long (around 6.5m) so the cochlea needs to be as long as possible to hear those sound waves. ! ! The follicles in the ear are different lengths. When you hear a really high pitched sound when you go to bed at night that is the sound of one of the hairs in your ears dying. That hair is no more and you can no longer hear that frequency. The hairs are of deferent lengths to pick up deferent frequencies. There are 23-28,000 hairs in the human ear each picking up a different sound. ! ! The base is thiner and the apex in the human ear. ! ! The collection of the bones are called the oclus. The coclia had “the oval window” to transfer the vibrations into the liquid in the coclia. ! ! Sound Colors:! ! ! ! © Black Noise:! Black noise is everything that you cannot hear. ! ! White Noise:! White Noise is basically everything you can hear. Ever hear static? That’s what white noise sounds like. White noise is a constant sound.! ! Pink Noise:! Pink noise is very high pitched. Ambulances, Fire engines and the Police all commonly use pink noise in they’re sirens. ! ! Green Noise:! Green noise is supposedly the “background noise of the world.” Green noise is much like Pink noise but with a hump around 500Hz.! ! Violet Noise:! Violet noise is also called purple noise. ! ! Orange Noise:! Orange noise is quasi-stationary noise. bands of zero energy are centered about the frequencies of notes in whatever scale is of interest.! ! Grey Noise:! Grey noise is close to white noise but it has a loudness curve instead of a sustained noise level.! ! Blue Noise:! Blue noise is also called azure noise. Blue noises density increases 3dB every octave. ! ! Brown(ian) Noise:! In fields that adopt precise definitions, the terminology "red noise", also called Brown noise or Brownian noise , will usually refer to a power density which decreases 6 dB per octave with increasing frequency.! ©
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