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Describing sound - Physics narrative
Physics Narrative for 11-14
A Physics Narrative presents a storyline, showing a coherent path through a topic. The storyline developed here provides a series of coherent and rigorous explanations, while also providing insights into the teaching and learning challenges. It is aimed at teachers but at a level that could be used with students.
It is constructed from various kinds of nuggets: an introduction to the topic; sequenced expositions (comprehensive descriptions and explanations of an idea within this topic); and, sometimes optional extensions (those providing more information, and those taking you more deeply into the subject).
Core ideas of the Sound topic:
- The source-medium-detector model and hearing
- Measuring and describing sound is all about amplitude and loudness
- Frequency, pitch ranging and speed is all about speed, frequency and wavelength
The ideas outlined within this subtopic include:
- Hearing and sound
- Pitch and loudness as perceptions
- Sound as travelling vibrations
Imagine you're working at your desk upstairs. You become distracted by a news programme from a downstairs radio: starting to pay attention you realise that you can clearly hear every single word.
Not surprising in some ways… but then again there's something rather fundamental to notice here. The sound starts from the radio's speaker downstairs and reaches you at the top of the house. The sound travels up the stairs, back along the landing and then up along another set of stairs to your lair. Not only could you hear the news up-stairs and down-stairs, but you could probably hear it outside as well.
In day-to-day living we are surrounded by all kinds of different sounds. Just stop and listen. The lived-in world is awash with sounds: noise; music; speech; birdsong. Some of these, such as the noise from traffic or the sound of people talking or the radio playing are just there and we often don't even notice them. Of course, sounds don't just exist as a kind of backdrop to our lives. They also provide us with information. For example, you quickly learn to recognise, by the sounds, when bacon is cooking too quickly, when the kettle is coming to the boil or when the car is misfiring.
How are we able to hear?
Vibrations from source to detector – that's how we hear
So, how is it that you are able to hear the radio playing in all parts of the house or the bacon sizzling or the notes from the guitarist at a concert? The guitarist is up there on the stage and you are in the audience. What happens to allow you to hear the sounds from the guitar, which is at least 20 metre away?
The first step in answering this question is to treat the act of hearing as involving a chain from source (which is vibrating) to medium (which enables the vibrations to pass) to detector (which in this case is you!) For vibrations to travel from source to detector there must be particles of matter in the gap, and these form the medium. If there are no particles then there is nothing to carry the sound from source to detector.
The medium transmits information from the source to the detector.
Source and detector
All sources of sound have the same kind of to and fro motion. All sources of sound vibrate. It is an interesting exercise to
think of a sound (yes, any sound!) and trace it back to the vibration that is producing it.
Here are some examples: voice, vibration of vocal cords; notes from a sitar, vibration of a string; toot of a trumpet, vibration of lips at mouthpiece.
The vibrating source acts on the medium around it, setting the medium moving to and fro, in a way that matches the motion of the source.
The detector also vibrates as it is set in motion by the medium adjacent to it. The motion of the detector matches the to and fro motion of the medium, so the sound is carried from source to detector – transmitted by one block of particles acting on the next.
Density varies as the vibration travels
What happens in the medium to allow the sound to pass? If the medium is air, the air particles are initially in a state of random motion.
Here we show what happens as the cone of a loudspeaker acts on the air in front of it. As you can see, the to and fro motion of the loudspeaker cone produces changes of density in the air.
As the cone moves forwards, the air particles are squashed together to make a region of high density. As the cone moves backwards, the air particles in front of it spread out to form a region of low density.
The animation helps to visualise the patterns of high and low air density. These are first created directly in front of the loudspeaker cone and then farther away, as the disturbance in one
block of air affects the next
block of air. In this way the sound travels out from the loudspeaker cone through the surrounding air.
What sound is
As the disturbance of high and low-density regions travels out through the air, each block of air moves backwards and forwards following the motion of the loudspeaker cone. If you're not sure about this, step back to the previous screen and review the animation there. Each block of air moves backwards and forwards. It is not the case that the block of air that starts directly in front of the loudspeaker cone ends up at your ear. The sound travels through the air but the air itself simply moves backwards and forwards.
You might ask the question:
Teacher: But what exactly is the sound?
Well, as we've emphasised, the cone moving to-and-fro is not the sound. Rather it is the source of the sound.
Equally, the detector is not the sound.
The sound is actually the disturbance travelling through the air, the to-and-fro motion of the air that creates the pattern of high and low-density regions. That is what we detect with our ears.
Making connections to energy descriptions
Loudspeakers enable you to hear by turning the to-and-fro movements of electrical currents, which encode the sound, into the to-and-fro movements of air. Later, and remotely, your ear is affected by these to-and-fro movements. The loudspeaker is working remotely on your ear. While it is working, you can hear a sound. Once it stops working, the sound stops. The sound must happen over time: you cannot freeze it. So sound, or, more precisely, hearing a sound, is a process. So it cannot be sensibly thought of as an energy in a store, which is as a result something that has happened (there is much more on this in the SPT: Energy topic).
In everyday life this makes perfect sense: You buy a sound system rated by watts, not joules.
That is a clue that making connections between hearing and energy descriptions is best done through power. The
sounding is a pathway that empties or fills stores of energy.
Our ears are very sensitive, so the power is tiny (150 W of illumination in a room is bright, but 150 W of sound system running at full power is just painful). So as a means of filling or emptying stores sound is very ineffective. But for those who like a neat systematic view sound is a pretty good match for the mechanical working pathway.
Hearing sounds is linked to the energy description via power in pathways—in fact it's best thought of as the mechanical working pathway, because of the mechanisms at either end.
The human ear
The way in which humans hear is rather complicated, and explaining it fully involves aspects of physiology, psychology and acoustics. A simpler description concentrates on the action of the ear as it converts sounds to a sequence of nerve impulses which are transmitted to the brain. The ear's ability to do this allows you to detect, all at once, a range of sounds, from loud to soft and from a high-pitched squeak to a low-pitched growl.
The ear consists of three parts: the outer ear, the middle ear and the inner ear. Each part serves a specific purpose in the task of detecting and interpreting sound.
The outer ear collects and channels sound to the middle ear. The middle ear transfers the to and fro motion of the air (which is the sound) into matching vibrations of the bone structure of the middle ear. These vibrations are transmitted into a to and fro vibration in the fluid of the inner ear. This vibration is changed into nerve impulses, which are then transmitted to the brain as electrical signals.
The board game
Mouse Trap provides a good basis for imagining how the ear works.
Looking every so often
How a stroboscope works to allow study of high-frequency vibrations
A big problem in trying to figure out what's going on as we hear sounds is that the vibrations tend to be too fast to see directly. One way to make the effects visible is to use a stroboscopic device.
A stroboscope allows you to see the vibration through a series of snapshots, spaced so that you catch the vibration a little later on in its cycle each time.
This gives a slow motion view of what is happening – slow enough for the eye to see. You can strobe either by flashing a light, thereby illuminating the scene for very short bursts, or by putting a rapidly opening and closing shutter in front of the detector – often the eye. You can even strobe just by waving the outstretched fingers of your own hand in front of your eye. All you need to
strobe is to take regular snapshots of a scene, looking only at regular intervals.
Infrasound and ultrasound
Explaining the terms infrasound and ultrasound, used for vibrations that we cannot hear
Our ears can detect only a relatively small range of frequencies of vibration. Beyond what we can hear there is:
- Infrasound, where the rate of vibration is too low for us to hear
- Ultrasound, where the rate of vibration is too high for us to hear
This full range of frequencies of vibration, from a few vibrations per second to many vibrations per second, is called a spectrum. Just as there is the electromagnetic spectrum of light (complete with infrared, the visible region, and ultraviolet) so too there is the spectrum of sound.
There is more about frequency in episode 02.
Sounds heard by humans and other organisms
Not all organisms are sensitive to the same range of vibrations. That's why some dog whistles are inaudible – to us.
These are approximate values. You cannot just ask a ferret if he can hear a particular sound. Nor are the organisms equally sensitive to sound over the whole range of their hearing.
Here we provide more information for your interest.
Sound is often encoded in another form, before being decoded and converted back to vibrations in the air that we can hear. Producing these codes involves several steps to capture the vibrations of the air as a continuous record over time.
Long ago, the original wax recordings relied on ingenious mechanical devices to carve grooves into the surface of the record, with each vibration in the air corresponding to a curve cut in the wax. To play back the record you had to follow these curves at the same speed that they were cut, again with a clever mechanical device. This time the movement of the tracking object was amplified to make the air vibrate enough to enable hearing. Vinyl records are a mass produced version of the same coding and decoding process.
Things have moved on since then. Now we have microphones to transform the original vibrations of the sound into electrical vibrations, which are converted into an analogue signal that can be recorded onto magnetic tapes or disks. Or we might add another layer of coding and digitise these electrical vibrations, storing the result as a series of numbers, which are ultimately coded as a set of on and off states.
This whole process is reversed when we want to re-hear the sound: from the code to the electrical vibrations, then finally setting a loudspeaker vibrating to and fro, resulting in a sound from source (loudspeaker) to detector (us).
Sound consists of audible vibrations, which travel from a source to a detector.
These vibrations travel out from the source at a speed that depends on the medium, which must contain particles. The medium must therefore be a solid, liquid or gas. Sounds do not travel through a vacuum.