Acoustics

The Physics of Sound Sound can travel through any compressible medium such as gasses, liquids & solids. Sound cannot travel in a vacuum as there are no molecules/no medium for the vibrations created by sound sources to travel through. There is no sound in space. Sounds (Mechanical Waves) travel as longitudinal waves and travel through solids as transverse waves. Sound waves are generated from a sound source such as a coin hitting the ground which creates vibrations within the sound sources media. As long as the sound source vibrates, the vibrations travel away from the sound source Omni-directionally at the speed of sound which creates the sound wave. Sound waves consist of areas of high and low pressure called compressions and rarefactions. Sounds will travel faster through solids over water or gasses most of the time. Speed of sound through air 343 Meters / Second Speed of sound through water 1482 Meters / Second Speed of sound through aluminium 6420 Meters / Second

When objects exceed the speed of sound they create what we know as a Sonic Boom. There is a great example of Sonic Booms in Jet Planes that exceed 343 Meters / Second because the sound waves travelling away from the sound source are condensed together then expand so rapidly they create the sound of an explosion. Sound volume is measured in Decibels after Alexander Graham Bell the inventor of the Telephone. Generally as a rule, a human whispering to another human from about a meter away is around 10dB. If a gun was to be fired close to your ear it would be around 140dB. When the decibels of a sound are too high it will cause physical pain to the human ear. When a sound causes us pain it is usually because this sound is above 130dB which we call the threshold of pain. Low volume sounds which we can barely hear like a quiet whisper are usually around 5dB which we call the threshold of hearing.

Musical Instruments Musical instruments create sound when part of them vibrate rapidly. For example, the column of air inside of a wind instrument, the string on a string instrument or the skin of a drum when beaten cause vibration which produce sound waves in the air, which we hear as musical notes. There is also a very important characteristic of sound which we know as pitch. Pitch is altered by changes of frequency. The way frequencies rise or drop is by the amount of cycles of compression and rarefactions which are put into one word as oscillations. The faster or slower the sound waves are oscillating, the higher or lower the frequency will be. Pitch is measured in hertz. The perfect human hearing range is from 20Hz to 20,000Hz.

The easiest way to explain this is to think about an Acoustic Guitar and how they work. If you get a plectrum and strum the thick low E string it will create a low pitch frequency but if you strum the high e string it will make a much higher pitch sound. This is because the high e string is a lot thinner and lighter than the low E string so it can vibrate a lot faster creating a lot more compression and rarefaction cycles. The way that acoustic guitars create sound is mainly through the sound hole which is on the body of the instrument. When you play a string on the guitar, the sound waves travel through the body of the guitar and enter into the sound hole where they bounce around inside before leaving the sound hole where they were amplified.

If you look at that diagram of the acoustic guitar and look toward the tuning pegs at the head of the body, you can see that these are where the strings are tightened or loosened which which when combined with the thickness of each string makes the pitch of each string higher or lower. This is pretty much how all string instruments work. Wood Wind instruments such as the saxophone, clarinet, bassoon etc have a completely different way of making sound. These instruments contain a reed which vibrates as the musician playing the instrument blows into it. These instruments have tone holes along their bodies to make sure that each note makes the correct tone. When one of these holes is covered by the musician, it prevents air from escaping that hole. For example if the closest hole to the musicians mouth is covered it will lower the tone of the instrument as the distance that the air travels through the instrument is increased as air exits through each open hole a little bit at a time.

Drums work differently than both string & woodwind instruments in the fact that acoustic drums use a skin which when struck causes a vibration to reverberate inside the drum until it dissipates. The size and shape of the drum are the main factors in which tone comes from which drum.

Human Hearing Human audio perception, one of the five major senses, is the ability to detect sound by vibrations, changes in the pressure of the surrounding medium with the ear. Sound can be heard through gasses, solids and liquids. Hearing is performed by the auditory system. The vibrations are detected by the ear and transduced into nerve impulses that the brain picks up. For hearing to be possible it requires molecules in the world around the human detecting the vibrations.

Hearing Mechanism In the human ear there are three main components: The outer ear, the middle ear and the inner ear. The Outer Ear The visible part of the ear; the Pinna, which is there to focus the sound waves being detected through the ear canal and into the ear drum (tympanic membrane). In humans, the tympanic membrane separates the outer ear to the middle ear as well as receiving sound waves from the air and transmitting them to the ossicles in the middle ear. Basically the ear drums job is to amplify vibrations travelling through the air into vibrations in fluid in the inner ear.

Middle Ear In the middle ear you have a small air filled drum which is next to the ear drum. Contained in this drum there are three tiny bones altogether called the ossicles which have the malleus, incus and stapes (easy way to remember is the hammer, anvil and stirrup as they are shaped a little like these objects). These bones help aid the transmission of vibration from the ear drum to the inner ear. The Stapes sends sound waves into the inner ear through the oval window. The oval window is a flexible membrane which seperates the air filled middle ear away from the fluid filled inner ear. There is another flexible membrane called the round window which allows for the smooth displacement of the inner ear fluid when sound waves enter.

Inner Ear Inside the inner sit the sensory organs for hearing and balance. The cochlea which is a spiral shaped tube full of two clear fluids; one called Perilymph and the other Endolymph. Inside the cochlea lies the The Organ of Corti, the sensory receptor which holds the hair cells, the nerve receptors for hearing. The vibrations caused by the middle ear push into the oval window in the cochlea. The force of this movement causes the fluids contained in the cochlea to stimulate the tiny hair cells. The individual hair cells respond to specific frequencies which depending on the pitch which travel through into your inner ear, only pitch specific hair cells are stimulated. The hair cells which receive these signals are changed into nerve impulses and are then sent out into the brain by the cochlea part of the auditory nerve. This nerve then carries the impulses received by the cochlea into a relay station of sorts inside the brain which are then carried into other brain pathways which end in the auditory cortex.

Space Design In a space where music is performed or recorded you have to take into account the design of the space because sound travels for longer causing more reverb in bigger emptier spaces. Sound will seem a lot clearer in a space where there is less reverb or certain materials on the walls and floor to absorb sound waves and deaden the vibrations. If you were building a hall for opera to be performed, youd need a a big space with a lot of reverb available. If you were building a smaller space for a vocal booth you wouldnt want that reverb so you would try to deaden the sound. Reverb is caused by sound waves bouncing off of shiny, reflective surface. The longer the sound travels, the more reverb there is. Because sound travels in an omni-directional manner it will fill a room and reflect off of anything it reaches. If you have a room with a reflective surface on the walls and floor then you will get a lot more sound bouncing around. If the wall is treated with some sort of sound proofing then you would get a lot less reverb as the sound waves would be absorbed.

The perfect recording space requires a lot of work. You have to manually construct nearly everything. Starting with soundproofing the floor and make it a floating floor which helps a lot with noise isolation as well as drastically reduce vibration and the spread of resonance; especially low frequencies which travel further and tend to be much louder and boom if not controlled properly.

When you are sound proofing walls there are different factors to take into account other than just making floating walls with the many different layers of materials to help with noise isolation and sound absorption.

One of these other factors to consider when creating a space, soundproofing and isolating a space to record in are Standing Waves (Room Modes). Standing waves are created when a sound is reflected back into its own path which causes phase differences which in turn interfere with the amplitude response of a room.

When there are major problems with these standing waves they will actually create increases & decreases in volume within different frequencies. There is a correlation within the wavelengths which are effected by standing waves to the distances between the opposing surfaces. If you had an empty 10 x 7 bedroom that you wanted to turn into a recording studio it would be very productive to sound proof the room in some way. Although it is very helpful to soundproof a room, if there is too much absorption in a room it can actually be bad for your recording. You want to soundproof a room where you are recording so that any signal the microphone picks up is only coming from the sound source and not being reflected off the walls or flooring and changing the true sound from the sound source which makes it harder for the producer to master the mix. Sound proofing gives the producer a flat mix with truer sound. There are multiple different methods used to deaden sound and make a clearer sound source recording. One of these methods is to use acoustic foam which is easy to apply to the walls in your recording room.

These panels also help to reduce flutter-echoes (slap-echoes) by absorbing reflections that without the foam there to stop them, would have a long delay time. The sound that isnt absorbed bounces off of these panels in an erratic fashion which stops a lot of the sound waves from reaching the recording microphone. What flutter-echo essentially is; is sound waves bouncing around in a small space giving the illusion of an echo which makes the sound seem as if it was played/recorded in a much larger space. There is another way to prevent flutter-echo and that is with diffusion. How diffusion works is by preventing the standing waves from reflecting in a straight line and reaching your ear too soon. This prevents that smearing effect you hear when you have the reverb and the sound from the speakers coming towards you at the same time.

Another acoustic room treatment technique is to use Bass traps in the corners of the room youre transforming into a recording room/mixing room as corners are where the low end frequencies have their highest and lowest points of pressure.

These bass traps help to control the low frequencies that can become overbearing in a mix while also helping to prevent low frequencies from leaking out of the room and annoying any neighbors or housemates.