When a membrane that is stretched over a circular frame is struck, energy is supplied, which again causes the membrane to vibrate in a number of modes simultaneously.

The first six modes in which the circular membrane can vibrate are shown in Figure 20. The diagrams comprise circles that are conce
Author(s): The Open University

If a solid rectangular bar is excited by striking it, energy is supplied that starts the bar vibrating transversely. The bar will vibrate in a number of modes simultaneously since the striking action supplies energy over a range of frequencies. The motion of the bar will be the superposition of the standing-wave patterns of the excited modes.

Assume for the moment that the rectangular bar is supported in such a way that both ends are free to vibrate and the effects of the supports can b
Author(s): The Open University

You saw in the previous two sections that stringed instruments and wind instruments possess primary vibrators that have harmonically related natural frequencies. As a result, these two classes of instruments produce notes that have a well-defined sense of pitch.

In this section, I want to briefly introduce you to some primary vibrators that don't have harmonically related natural frequencies. Specifically we shall take a look at a rectangular bar, a circular membrane and a circular plat
Author(s): The Open University

The third configuration of air column that we shall consider is that enclosed by a conical tube. Figure 17 shows the normal modes of vibration for a conical tube plotted in terms of pressure. As you would expect, there is a pressure antinode at the closed tip of the cone and a pressure node at the open en
Author(s): The Open University

In the previous two sections on standing waves in cylindrical tubes, we assumed that at an open end there must be a pressure node. In fact, the pressure node (and the corresponding displacement antinode) actually lies a small distance outside the tube. The effect is that the air column behaves as though it were a little longer than it really is by an amount called the end correction. Because of this end correction, the resonance frequencies will be a little lower than originally expect
Author(s): The Open University

The frequencies at which standing waves can be set up in an air column enclosed by a cylindrical tube that is open at both ends can be determined quite easily. Because both ends are open to the atmosphere, the pressure at these positions always remains at atmospheric pressure. In other words, there is no fluctuation in the pressure at the open ends so they must be pressure nodes (think â€˜no-deviationâ€™ in pressure). So, as we saw with the string fixed at both ends, the length L of th
Author(s): The Open University

You learned in the previous section that for standing waves to be set up on a string there must be reflection. A travelling wave reaches the end of the string and is reflected. This results in a second travelling wave, which moves back up the string in the opposite direction to the first wave. The two travelling waves interact to produce a standing wave.

Standing waves are set up in an air column enclosed within a tube in a very similar way. Again there must be reflection. In this case,
Author(s): The Open University

When a string is bowed, plucked or struck, energy is supplied that starts the string vibrating. The string doesn't just vibrate in one single mode; instead, it vibrates in a combination of several modes simultaneously. The displacement along the string is the superposition of the standing-wave patterns corresponding to those modes. For example, if the string vibrated only in the first and second modes, the displacement at a given instant of time might appear as shown in Author(s): The Open University

We still haven't answered the question of how standing waves are set up on a string. To do so we need to return to our string, fixed at one end and held in someone's hand at the other end. Imagine now that instead of sending a single pulse along the string, the person flicks their hand up and down periodically and sends a sinusoidal wave along the string. This wave gets reflected and inverted at the fixed end and travels back towards the person holding the string. There are now two waves of t
Author(s): The Open University

If standing waves are set up when two travelling waves moving in opposite directions interact, then how are standing waves set up on a string and why are they set up only at certain frequencies?

To help answer these questions, I want you first to imagine a length of string that is fixed at one end and held in someone's hand at the other. Suppose the person holding the string flicks their end of the string in such a way that an upward pulse is sent along the string.

As the pulse pa
Author(s): The Open University

You learned earlier that when a musician plays a note on an instrument, they supply it with energy that causes the primary vibrator to oscillate at certain specific frequencies. In Section 5 we are going to look at what determines these specific frequencies for some of the primary vibrators found in different instruments.

In Unit TA212_1 Sound for music technology: an introduction, we talk about travelling waves: that is, waves that propagate outwards away from their sourc
Author(s): The Open University

For a player to be able to sound a musical instrument, there must be a means of inputting energy to set up the vibration. This energy may be introduced in a short, sharp burst or continuously over a period of time.

In the case of brass instruments such as the trumpet and trombone, and woodwind instruments such as the flute and oboe, the player feeds in energy by blowing air into the instrument. The energy can be supplied in a short burst â€“ in which case short-lived â€˜staccatoâ€™ note
Author(s): The Open University

Musical instruments come in all shapes and sizes and produce an enormous variety of different sounds. Yet, with the exception of certain electronic instruments, the basic physical principles by which sound is produced are the same for all instruments â€“ including the human voice. In this section, I shall introduce some of these principles. These will then be expanded upon over the rest of the unit.

Remember I told you that when a musician plays an instrument they cause it to vibrate. T
Author(s): The Open University

After studying this course, you should be able to:

• Explain correctly the meaning of the emboldened terms in the main text and use them correctly in context

• Identify whether a given sound source can be classed as a musical instrument and explain why (Activity 2)

• Identify the primary vibrator and any secondary vibrators in the most common types of instrument (Activity 3)

• Appreciate that, when a note is played, a musical in
Author(s): The Open University

In filtration, the partially treated water is passed through a medium such as sand or anthracite, which acts as a 'strainer', retaining the fine organic and inorganic material and allowing clean water through. The action of filters is complex and in some types of filter biological action also takes place. Sand filters are used in water treatment to remove the fine particles which cannot be economically removed by sedimentation. They have been effective in removing Cryptosporidium, a pr
Author(s): The Open University

Water for public supply can be obtained from underground sources by wells sunk into aquifers, or from surface sources such as purpose-built reservoirs or lakes (collecting rainwater run-off or water from streams) and rivers. The safety of the water is of utmost concern â€“ several million people die each year after consuming contaminated water. The primary aim in water treatment is the elimination of any pathogenic micro-organisms present. All the above-mentioned sources can be subject to pol
Author(s): The Open University

Water in its 'natural' state supports a complex, yet fragile, ecosystem. The ability of natural watercourses to sustain aquatic life depends on a variety of physical, chemical and biological conditions. Biodegradable compounds, nutrients and dissolved oxygen must be available for the metabolic activities of the algae, fungi, bacteria and protozoa which are at the lowest level of the food chain. In addition, plant and animal growth cannot occur outside narrow ranges of temperature and pH. Susp
Author(s): The Open University

Most of the major cities and harbours in the world are located on estuaries. The estuarine ecosystem is a unique intermediate between the sea, the land and fresh water.

A rather precise definition of an estuary is 'a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which sea water is measurably diluted with fresh water derived from land drainage'. This excludes large bays with little or no freshwater flow, and large brackish seas and inland
Author(s): The Open University

A great many biological species and individuals occur in normal streams. These often differ markedly in their sensitivity to environmental factors, and likewise the tolerances of various species to different types of pollution vary considerably. The major groups of organisms that have been used as indicators of environmental pollution include bacteria, fungi, protozoa, algae, higher plants, macroinvertebrates and fish. The benthic 'bottom living' macroinvertebrates are particularly suitable a
Author(s): The Open University

Algae are photosynthetic organisms that are generally aquatic; they are primary producers. Many freshwater algae are of microscopic size, but when amassed can be seen as a green, brown or blue-green scum. Blue-green algae are capable of producing toxins and these have caused the death of wild animals, farm livestock and domestic pets which have consumed the contaminated water. The toxins can produce a painful rash on human skin. The extract below shows what happened off the west coast
Author(s): The Open University