Pitch Whenever you’re playing a musical instrument, you make particles in its immediate vicinity vibrate. How is it that an instrument can produce both high and low tones? Pitch What affects pitch? Observation 1 • You will need a small plastic box without a lid, • 2 rubber bands of different thickness. Instruction • Put rubber bands around the box and make sure that the distance between them is sufficient to grab each of them separately with your fingers. • Pull on the thinner rubber band with your finger. Assess whether the sound produced is high or low. Then pull on the thicker rubber band and compare the sound produced to the previous one. • Grab any rubber band approx. 1 cm from the edge of the box and pull on the longer end. Assess whether the sound produced is higher or lower than the first sound. Summary The shorter the section of a rubber band that is made to vibrate, the faster it vibrates and the higher is the sound it produces. In addition, thicker the band, lower the sound it produces. A study on how high and low tones are produced Source: Tomorrow Sp. z o.o., licencja: CC BY 3.0. Pitch The faster the body vibrates, the higher sounds it produces. If you take a look at a guitarist playing their instrument, you’ll be able to see that they press strings in such a way so that only parts of them vibrate. This way, the speed (frequency) of vibrations changes. The more vibrations per second, the higher the sound. Source: Chris Friese, licencja: CC BY 2.0. https://en.wikipedia.org/wiki/Guitar#/media/File:Acoustic_guitar_parts.png The pitch of a vibrating string depends on four things. • The mass of the string: more massive strings vibrate more slowly. On steel string guitars, the strings get thicker from high to low. On classical guitars, the size change is complicated by a change in density: the low density nylon strings get thicker from the E to B to G; then the higher density wire-wound nylon strings get thicker from D to A to E. • The frequency can also be changed by changing the tension in the string using the tuning pegs: tighter gives higher pitch. This is what what you do when you tune up. • The frequency also depends on the length of the string that is free to vibrate. In playing, you change this by holding the string firmly against the fingerboard with a finger of the left hand. Shortening the string (stopping it on a higher fret) gives higher pitch. The body The body serves to transmit the vibration of the bridge into vibration of the air around it. For this it needs a relatively large surface area so that it can push a reasonable amount of air backwards and forwards. The top plate is made so that it can vibrate up and down relatively easily. It is usually made of spruce or another light, springy wood, about 2.5 mm thick. On the inside of the plate is a series of braces. These strengthen the plate. An important function is to keep the plate flat, despite the action of the strings which tends to make the saddle rotate. The braces also affect the way in which the top plate vibrates. https://www.westfarthingwoodworks.com/how-to-make-an-acoustic-guitar-series-part-thirteen-attaching-the-top-plate/ The air inside The Helmholtz resonance of a guitar is due to the air at the soundhole oscillating, driven by the springiness of the air inside the body. Sound volume Pitch depends on the frequency with which the source vibrates. On the other hand, whether sound is loud or soft depends on the strength of vibrations. For instance, the harder you pull on a guitar string, the stronger the vibration (the greater the amplitude of vibrations) and the louder the sound produced. This will not cause a change in pitch. Displaying sounds https://pages.uoregon.edu/dlivelyb/phys101/Lab8_S07.pdf Sound waves can be displayed graphically using a microphone and an oscilloscope. • When sound waves enter the microphone, they make a crystal or a metal plate inside it vibrate. • The vibrations are changed into electrical oscillations, and the oscilloscope uses these to make a spot oscillate up and down on the screen. Question 3b/133 https://pages.uoregon.edu/dlivelyb/phys101/Lab8_S07.pdf What can we use oscilloscopes for? • We can use oscilloscopes for looking at all kinds of signals in all kinds of ways. • Plug in a transducer and you can use an oscilloscope to measure almost anything. • Transducer converts one kind of energy into another. • a microphone (a type of transducer that converts sound energy into an electrical signal) to study sound signals with an oscilloscope; • a thermocouple (a transducer that converts heat into electricity) to study temperature changes • a piezoelectric transducer (which generates electricity when you squeeze it) to study vibrations—such as a person's heartbeat. Oscilloscope https://academo.org/demos/virtual-oscilloscope/ Spectrum Analyzer https://academo.org/demos/spectrum-analyzer/ Beats Beats are the periodic and repeating fluctuations heard in the intensity of a sound when two sound waves of very similar frequencies interfere with one another. https://www.physicsclassroom.com/Physics-Interactives/Waves-andSound/Beats/Beats-Interactive Volume of sounds The larger the distance from the source of sound, the more it will lose its intensity. Vibrations are reduced until they finally disappear. Sound volume is measured in decibels (dB). Sounds louder than 130 dB can cause ear pair and even permanently damage your hearing. Volume of sounds produced by certain objects Source: Arcturus, CHG, Dariusz Adryan, Ellywa, Rafael.lcw0120, Skwanem, The cat, Σπάρτακος, http://commons.wikimedia.org licencja: CC BY 3.0. Echo If the guitarist that you’re observing gives a concert at a large hall, you may hear an echo. Where does it come from? Sounds can reflect from large surfaces, e.g. walls, mountain slopes or a dense forest wall. Sound reflected from an obstacle is called an echo. Certain animals can use this phenomenon to orient themselves in their surroundings.
