Acoustical Physics PHY 1100
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CHM 25500 - 001
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This 5 page Class Notes was uploaded by Werner Hagenes on Saturday October 3, 2015. The Class Notes belongs to PHY 1100 at Belmont University taught by Robert Magruder in Fall. Since its upload, it has received 15 views. For similar materials see /class/217963/phy-1100-belmont-university in Physics 2 at Belmont University.
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Date Created: 10/03/15
Introduction Acoustical Physics Physics 1 1 00 As you go through this course please keep in mind the following 1 This is a physics course and not a music course 2 You are not expected to have had physics before or any background except algebra Topics will be developed and discussed as needed 3 In this introduction there may be terms and concepts you have not heard before That is okay We will study these concepts and terms as they are needed You are not expected to understand them at this point 4 Sound and perception ofsound is a very broad and complex eld In large part this is due to our interpretation of sound based on individual likes dislikes ability to hear as well as cultural social and psychological aspects 5 You will need to study outside of class or lab The content modules go well beyond what is given in the book 6 Different authors and practicing sound industry people can use different terms to mean different things Physicists may use variations in definitions from audio engineers 7 Stay current with the material Sound The rst thing that needs to be J is an r quot 39 J J39 of sound To do this we must rst consider that there are three descriptions of sound used that are related but at times are used in different conteth and have different meanings 1 Sound 2 Sound waves 3 Auditory response of the ear Sound itself is the presence of small rapid changes in air pressure These rapid changes are in general have a frequency or pitch and consequentially are of a sinusoidal nature While we have not defined sinusoidal at this point we can note that it implies that these changes are formed by vibrations of some object or system For these pressure changes to be considered as sound they are in a speci c frequency range Note sound has frequency and is in general created by vibrating systems These are two crucial facts that will dictate some of the topics we will discuss Change in air pressure itself is not necessarily sound For example changes in air pressure with the weather changes are not sound We do not hear a high pressure roll in but we do hear thunder The key is vibrations and frequency which will take up shortly To sum up what he is trying to say Sound can be de ned as Vibrations through an elastic medium with frequencies ranging from around 20 Hz to 20000 Hz The audible range of frequencies in the human ear Sound waves are the method by which sound propagates through a medium and transfers energy without the transfer of matter Again by the connection to wave a sinusoidal nature is implied In general we expect the sound waves to have frequency and the waves to be created by a vibrating system with the same frequency The auditory response of the ear is the biophysical process by which sound is heard This process includes not only the sensor the ear but also the brain and the interconnecting nervous system This response involves a great deal of perception and interpretation both active and passive by the individual The area that deals with this is referred to as psychoacoustics As we move through this course we will deal with all three of these descriptions What causes sound We have said that sound is changes in air pressure and it is a wave It is a particular type of wave a longitudinal wave The meaning of longitudinal will be addressed in a future module We want to focus here on what causes these pressure variations in air or other medium A lot of things can cause pressure changes for example a ute or recracker can produce pressure changes that can be sensed as sound In principle anything that causes air to vibrate in the correct frequency range can cause sound There is a key word here and it is vibrate In many cases if not most the vibrations are very subtle and invisible to the unaided eye While we can see the vibrations of guitar string as it plays we cannot see the vibrations of a blackboard as someone draws their ngernails across it to produce a rather irritating screech nor can we see the vibrations of pressure changes in the air We and see a cone on a speaker moving though clearly we cannot count the vibrations like on a swing pendulum or a mass on a spring vibrating However all vibrating systems have a few things in common and the frequency of their vibrations are determined by physical parameters of the systems vibrating Okay Vibrations and things that allow Vibrations to be passed to the air can be sound sources W TF What is he trying to say Some are abrupt noises a recracker some are annoying a rattling noise in your car some are wonderfully pleasant your favorite music But vibrations and systems that cause vibrations are one of the keys to understanding sound and we will focus a lot of attention on simple vibrating systems Almost all if not all musical instruments use some sort of simple vibrating system to create notes how could it be almost all And not all The human voice is no exception We also have to consider that everyday descriptions of sound are not always correct and often we can receive incorrect impressions because of misconceptions about sound Lets specifically focus on two such descriptions pitch and loudness Very often we use pitch to mean frequency But pitch can depend on loudness or volume of the sound as it is perceived by the individual auditory system Pitch then is a perceived term while frequency is a measurable quantity that can be determined In the same way loudness is what is open referred to as volume or intensity However loudness depends on the individual and their auditory response whereas the intensity is a measurable quantity Perhaps some of you have worked with sound intensity levelSL or SIL which is measured in decibels or dB While this is a relative measure it is based on intensity whereas loudness is perceived quantity Relative or perceived loudness pitch quantity Measurable quantity intensity frequency Pitch and loudness can be affected by the intensity and frequency of sound as well as the presence of other sounds Common examples of these effects are the formation of beats masking effects echoes and the Doppler Effect Many more exist The harmonics present in a musical note can greatly affect our perception of the sound that is present We will study these effects during the semester While we may all disagree as to what constitutes music notes are produced by specific kinds of physical systems called simple harmonic oscillatorsSHOs Understanding the physical principles behind these kinds of systems is crucial to understanding the formation of musical notes and sound in general Moreover we can use this information to learn why different artist techniques produce different sounds on the exact same instrument Now while we may have established some operational definitions or unfinished descriptions a great amount of detail has been left out This course proposes to do three things or 3 objectives 1 Fill in the details of our operational descriptions 2 Explore the physics related to these operational definitions and 3 Answer several questions a How does sound effect my life Fquot What is it about sound that greatly enriches my life sometimes and why sometimes but not always c Why does the perception enjoyment of sound depend on the surroundings d How do instruments work in a broad sense The rst thing we must start with is the basic fundamental physics necessary to pursue the three objectives listed above
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