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2 Construction with lipids

The cell membrane is constructed from lipids. Chemically, lipids are a rather varied group of compounds that include all the substances you might already think of as fats or oils. What they have in common is that they are all insoluble in polar liquids like water, but soluble in organic (carbon-based) solvents: by this I mean the sort of smelly solvents you tend to find in paints, glues and degreasing agents; chloroform is one example. Lipids make up the fatty components of living organisms a
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Learning outcomes

After you have completed this unit you should be able to:

  • describe and give examples of how self-assembly enables construction ‘from the bottom up’ in natural materials;

  • explain what is meant by primary and higher-order structure in proteins and give examples;

  • give examples of the range of functions carried out by proteins within cells;

  • describe how a combination of strong and weak bonding within biopolymers and lipids is used to
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Acknowledgements

Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence

Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:

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6 Radiation

All the primary vibrators we discussed in the previous section can to some extent communicate vibrations to the surrounding air and hence radiate sound. However, some radiate sound better than others. Air columns, for example, radiate sound quite well. Even though only around 1% of the energy possessed by a vibrating air column is radiated away, this is enough to produce a clearly audible note.

Similarly, circular membranes and circular plates are also good sound radiators. They have a
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5.13.4 Pitches of notes produced by percussion instruments

We have seen that none of the rectangular bar, the circular membrane and the circular plate have harmonically related natural frequencies. It may not surprise you to learn, therefore, that instruments containing these primary vibrators tend to produce notes that don't have a very well-defined sense of pitch.

This is certainly true in the case of the cymbal, which has a circular plate as its primary vibrator. Whether a single cymbal is struck with a drumstick or two cymbals are crashed t
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5.13.3 Circular plate

By now you shouldn't be at all surprised to learn that when a circular plate that has an outer rim that is free to vibrate is struck, the plate will vibrate in a number of modes at the same time.

The first four modes of vibration of a circular plate with a free edge are shown in Figure 21. As with
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5.13.2 Circular membrane

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 concen
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5.13.1 Rectangular bar

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
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5.1 Standing waves

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
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4 Excitation

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 – in which case short-lived ‘staccato’ note
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10.2 Adding decibels

A feature of decibels is that adding two decibel values is equivalent to multiplying the ratios they represent. To see how this comes about, consider another context in which a decibel measurement is often used, that of signal amplification.

In Figure 28, the triangular symbol represents
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10.1 Introduction

For a variety of reasons, not least the very wide dynamic range of human hearing, the decibel (symbol dB) is often used as a unit for the amplitude of sound waves. The decibel is also used in other contexts, such as specifying the amplification of amplifiers or the degree to which a signal is affected by noise. In the context of sound, the use of the decibel as a unit captures something of the subjective impression of the way loudness changes with amplitude.

The decibel unit has
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9.3 Summary

The nominal frequency range of human hearing is 20 Hz to 20 kHz, though most people cannot hear to 20 kHz. However, the pitches used in music correspond roughly to frequencies in the range from 20 Hz to 2.5 kHz. Generally, musical tones are not pure sine waves but are mixtures of sine waves with frequencies that can extend well beyond 2.5 kHz. However, although they are mixtures of sine waves, they are usually heard as having a single pitch.

The dynamic range of human hearing refers to
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6.4 Summary

Amplitude refers to the size of a sine wave. It can be defined in various ways, but a standard definition is that it is the maximum value of a wave's departure from its average value. (The average value of a sine wave lies midway between its peaks and troughs.) The size of a sine wave is sometimes also expressed as a peak-to-peak amplitude, which is the vertical distance from peak to trough.

Root-mean-square (r.m.s.) amplitude is a way of specifying the size of a sine wave so that compa
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6.3 Root-mean-square amplitude

One drawback of the amplitude as I have defined it is that although it allows the relative sizes of sine waves to be compared, it does not give a good idea of what a sine wave can deliver in absolute terms. For instance, a sine wave with an amplitude of 10 volts has twice the amplitude of one with an amplitude of 5 volts. But is a power source that delivers a sine wave with an amplitude of 10 volts as powerful as, say, a 10 volt battery? Could you use it to drive a bulb and get the same illum
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4.3 Summary

The speed of sound in air, symbol v, is approximately constant at 340 metres per second. (You do not need to memorise this value.) As temperature increases, the speed increases slightly.

Speed, frequency and wavelength are related by the formula v = f × λ. Other forms of this relationship are f = v/λ and λ = v/f. Because the speed is approximately constant, it follows that frequency and wavelength are inversely pr
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2.7 Summary

Pressure in the air is related to how closely packed the molecules are. Other things being equal, more closely packed molecules are at a higher pressure than more dispersed molecules. Sound is associated with fluctuations of the air pressure caused by local disturbance. Fluctuations of pressure travel outwards away from the disturbance, carrying energy imparted by the disturbance.

A simple form of local disturbance to air pressure is a vibrating tuning fork. It generates a pressure wave
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2.4 Period

You saw in Section 2.3 that the prongs of the tuning fork vibrate cyclically. You also learned that a cycle of the prongs' vibration is a complete sequence of motion up to the point at which the motion starts to repeat itself. Another term for this repetitive kind of motion is periodic motion. The t
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2.3 Pressure waves and cycles

In this section we shall be looking at the behaviour and properties of pressure waves in the atmosphere.

Sound originates from the motion or vibration of an object. Let's look at an example of a sound wave generated by a vibrating tuning fork. The prongs of the tuning fork move backwards and forwards cyclically. A cycle is a complete series of movements up to the point where the movement starts to repeat itself. As the prongs of the fork vibrate back and forth they push on neighbouring
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2.2 Pressure in the atmosphere

The sounds we hear generally consist of rapid fluctuations of air pressure in the atmosphere that surrounds us. Sound can also be transmitted through other media, for instance water, so not all sound consists of fluctuations in air pressure. However, for the purposes of this discussion I shall confine myself to sound in air.

These fluctuations in air pressure are caused by a local disturbance to the air pressure, which might be sudden and transient – for example, when a paper bag is b
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