the jordan loudspeaker manual chapter 2

7/29/2019 The Jordan Loudspeaker Manual Chapter 2

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Chapter 2: THE PROPERTIES OF SOUND

The sensation we know as sound is produced when air is vibrated at an intensityand frequency within the range of the human ear.

These vibrations cause rapidly alternating changes in the air pressure whichtravel away from the source rather like ripples from a stone dropped in water,but at a speed of 344 metres per second or 720 miles per hour. When thesepressure changes reach our ears they vibrate our eardrums in a mannercorresponding to the vibration of the source, which via the remarkable andcomplex mechanisms of the middle and inner ear, stimulates the auditory nerveto the cortex, The cortex area of the brain may generally be regarded as amental data base which translates incoming stimulatory patterns from ourvarious senses into recognisable information.

This data base is partly inherited from our early ancestors together with ourother senses as part of our survival mechanism, and partly from learned

intellectual, emotional and sensory experiences all of which determine ourperception and response. Another name for this learned response isconditioning and the listeners comment mentioned earlier about realistic soundnot sounding like loudspeakers, is a good example commercial conditioning.

Sound intensity is a measure of the vibrating air pressure at the ear. Loudness ishow we perceive this and varies with the individual. Typically the ears of anormally healthy young person are incredibly sensitive and can detect somesounds where the amplitude of air vibration at the ear is less than one tenthousand millionth of a centimetre. This is equivalent in terms of energy to theintensity of light produced at the eyes of a 50-watt light bulb at a distance of

3,000 miles in free space. The lowest level of sound intensity we can hear isknown as the threshold of hearing.

At the other end of the scale, if the sound intensity is high enough it can causepain, permanent damage to the hearing (often incurred by disco sound levels),

The intensity ratio between the thresholds of hearing and pain is about1,000,000 to one. However our auditory system does not sense this enormousratio but tends to compress it logarithmically in terms of decibels, which, in thiscase is 120 dB. A table of decibels is given in the appendix. Ref. 1. In music, theratio of intensities between the quietest (pp) and the loudest (ff) is about 4500to one. (73 dB). Intensity ratio is also referred to as dynamic range. It should

be noted that rock or disco music may be very loud but have, by definition, avery restricted dynamic range.

Frequency or Pitch

This is the rate at which the alternating air pressure changes are repeated. Thelowest frequency we can hear as sound is about 20 complete rise and fall

pressure change cycles per second. This is written as 20 Hertz or 20Hz Thehighest frequency we can hear varies with age and sex but is normally taken as20,000 Hz or 20 kHz (kilohertz). The frequency response of the ear is far from

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even and its maximum sensitivity occurs between 3 - 4 kHz. The musical termfor frequency is pitch. Another aspect of frequency is wavelength. Referringback to our ripples in water, the wavelength would be the distance between twosuccessive peaks. In the case of sound it will be the distance between twosuccessive pressure peaks. The wavelength at any frequency equals the speed ofsound, (344 metres per second) divided by the frequency.

The wavelength at 20 Hz for example is 17.2 metres (156 ft.) at 1000 Hz it is0.344 metres (1.1,28 ft.) and at 20 kHz it is 0.0172 metres (0.67 inches).

Harmonics

These determine 'character' of the sound - e.g. the different sound quality of aviolin and a trombone even when playing the same note or the differencesbetween two voices singing the same note. The differences are due to the factthat in nature there is no such thing as a pure single frequency note. Any singlemusical note consists of a fundamental frequency plus overtones of harmonicallyrelated frequencies, which are simple multiples of the fundamental frequency,

and it is the relative magnitudes and phase of these overtones that define thetonal character or timbre of the instrument.

PhaseThis is a measure of time alignment between the fundamental and harmonicfrequencies. If this is altered within the reproducing chain both the quality ofsound and sense of spatiality will be impaired. Below is shown the waveform ofa pure tone and its second harmonic. (Fig:1a). The resulting summation isshown b. The effect of adding the third harmonic is shown, c. In d, the effectof time, (phase), shifting the second harmonic is shown and the resultingsummation, e which can be seen to be very different from b.

Fig: 1 a Fig 1b



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The waveforms of several musical instruments and the male voice are shownbelow, Fig:2. a: violin, b: flute, c: oboe, d: clarinet, e, The vowel, a as infather. f, electronic pulse. g, bell.

Fig: 2 a b c

d e f g

It has been shown that by Fourier transform that any repetitive waveform can bemathematically analysed as a fundamental and a series of harmonically related

sine waves.

Transient Sounds

These basically are sounds, which start very suddenly. Examples are tappingsounds, pistol shots, and castanets. Some instruments such as the piano pluckedstrings, and the triangle produce sounds, which start quickly and die awayslowly; Fig; 2l. An electronic pulse; Fig; 2k, can produce sounds, which start andstop quickly. Some continuous sounds, which are rich in harmonics, may be

considered in many respects as a continuous series of transient sounds producedin rapid succession as can be heard in the lower registers of trombone.

Resonance

Most things, including mechanical, electrical and acoustical elements tend tohave one or more natural resonant frequencies at which they will most readilyvibrate. If we tap a wine glass it will ring at a very well defined pitch and thesound will take some seconds to die away. This is called the resonant frequency.Tapping a cup will produce a far less well-defined note and tapping the table willelicit no discernible note at all.

The basic requirements for a mechanical resonance are mass and springiness orcompliance, which is the inverse of stiffness. These determine the resonantfrequency. The duration and quality of the tone, referred to as the Q, which isdefined as the ratio of the mass to the external or internal frictional losses.

The wine glass is an example of a high Q resonance because of its low internalfriction. Touching the rim of the glass introduces a relatively high frictional lossand the ringing becomes subdued and of short duration.

This is referred to as damping and the damping factor is defined as the inverse

of the Q. (Q originally referred to the Quality of the tuned circuits used in theearly days of radio and defined by the reactance of the inductor divided by theresistance.



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In the case of the cup and more so the table, these have multiple resonantfrequencies, which are not harmonically related but are highly damped.

Dropping a tin can or scraping the inside of a saucepan produces examples ofsound produced by multiple resonances, which are neither harmonically related

nor highly damped.

Again there are some sounds, which have no discernible resonances for examplethe roar of traffic, the sound of the wind and the sea. The wind only howls whenit strikes solid objects and sets up oscillations due to eddy currents.

Another way of regarding these sounds is to consider them as continuum ofinfinite resonances over a very broad frequency band.


the jordan loudspeaker manual chapter 2

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