the jordan loudspeaker manual chapter 7

7/29/2019 The Jordan Loudspeaker Manual Chapter 7

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SOUND DISTRIBUTION

Sound distribution is the practical aspect of directivity as described for the idealpiston and is a direct function of both dimensions and frequency. The limiting

conditions are shown in Fig 14, a, b and c.

Fig. 14.

.

a b c

A point source (a) may be regarded as one having dimensions very much lessthan wavelength of the highest frequency being considered. Radiation will bespherical and omni-directional and the sound intensity will vary with the inversesquare of the distance. The point source is the most easily located

A flat diaphragm, (b), which is infinitely large or, in practice very much largerthan the wavelength of the lowest frequency considered, will radiate plane wavesparallel to its surface and the distribution will be extremely directional. There willbe no definable location of the sound source, only its direction. The decrease inintensity with distance from the source will be due only to the frictional losses inthe air, which, within the confines of domestic locations will be small.

It should be noted that directivity and wave front curvature are inverse indicationsof the same cause and since the listener has two ears attached each side of afrequently moving head there is evidence to suggest that wave front curvaturecan be perceived and provides part of the information allowing us to judge thedirection and distance of a sound source. This further suggests that the soundfrom loudspeakers might be interpolated back to point sources behind the actualradiating surface.

Loudspeaker drivers fall between the above two extremes and the illustration,(c), shows the difference in wave-front curvature from both a cone driver and adome tweeter respectively together with the predicted interference effects whenthe tweeter is placed at the acoustic centre of the cone and radiating the samefrequency at the crossover point. This effect becomes worse for spatiallyseparated tweeters.


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SOUND LOCATION.

Our ability to locate sound is due to the fact that we have binaural hearingwherein each ear receives a slightly different image of the sound source. Thesedifferences are principally in terms of time and intensity. . For a sound sourceimmediately ahead of the listener, both ears receive the sound at the same time

and at the same intensity. If the sound source is to either side then there will be adifference of both time and intensity between each ear. These are referred to asthe intramural time difference, ITD, and the intramural intensity difference, IID.Binaural hearing, evolved as a natural survival mechanism and since pure toneswere not part of the natural order, these are the most difficult to locate but this isstill possible since our response to ITD embraces a response to extremely smalldifferences in phase. The most easily locatable sounds are those that are transientwith sharp leading edges such as snapping sounds or clicks.

Binaural hearing also allows us to select and concentrate upon a one specificsound among a multiplicity of sound sources. A convincing demonstration of this

is to listen to someone speaking in a noisy environment first using both earsnormally and then with one ear covered. The immediate impression with one earcovered is that the wanted sound is being masked by the noise whereas the useof both ears results in a much greater separation and intelligibility of the speakersvoice.

STEREOPHONIC REPRODUCTION.

The advantages of the single, wide band cone for stereophonic sound havealready been outlined and, from the foregoing, can now be looked at in furtherdetail. It has been seen that for a sound source immediately ahead of the listener,both ears receive the sound at the same time and at the same intensity.

Therefore, if the listener is subjected to identical sounds from loudspeakersequidistant either side of his head he will perceive a virtual centre image. Withconventional near spherical distribution, movement of the listener to a positionhalf way between the centre and the left loudspeaker the sound intensity fromthat speaker will increase four times, due to the inverse square law of near-fieldspherical wave fronts whilst the intensity from the right will decrease to less thanone half. In addition to this to this is the time delay difference with the result thateven a small movement away from the centre will cause the image to smear tothe nearest loudspeaker. Similar smearing will also accompany shifts in thereproduced image.

Stereophonic imaging can be significantly improved by utilising the specificdirectional properties of the wide band driver where a preferred distribution is forthe axial response to rise gradually with frequency, countered by a correspondingfall at about 30 degrees. The increased wave-front curvature reduces the squarelaw effect and, if the speakers are then positioned so that their axes cross in frontof the listening area, a movement of the listener either way will be toward theaxis of the further speaker and away from the axis of the nearest speaker. This, ineffect, tends to stabilize the imagery over a larger listening area. This is explainedby reference to Figs 15, a, b and c.


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Fig 15.

a b c

The vertical axes show magnitude and the horizontal axes show time. Forillustrative purposes a simple step function is used, Fig 17a. It has been shown byFourier that any repetitive waveform can be analysed into a series of harmonicallyrelated sine waves. Fig 17 b shows the result of adding the first, second and third

harmonics. Fig 17 c shows the addition of adding up to the twelfth harmonic. Thisshows that the leading edge of the step is comprised of the upper harmonics. Ittherefore follows that any reduction in magnitude of these harmonics results in aneffective reduction in both intensity and delay in time.

THE VERTICAL LINEAR ARRAY.

Multiple arrays of similar units offer number offer significant advantages over asingle driver:

1) The power handling is increased in proportion to the number of drivers

2) A large radiating surface is provided whilst retaining the performanceadvantages of individual low mass cones thereby combining the qualities of bothcone and planar systems.

3) Small wide band drivers are available designed specifically for arrayapplication. These are assembled into close fitting rectangular chassis to minimisethe combing effect.

4) A vertical linear array of such drivers, correctly positioned, can offer anoutstanding level of stereophonic reproduction.

The feature of the vertical linear array is that it whilst that in directivity thehorizontal plane is similar to that of a single driver, it is very restricted in thevertical plane. This increases the forward efficiency and minimises floor to ceilingroom reflections. In addition, the near field distribution pattern for the upperharmonics tends to be cylindrical. At 2kHz the near field of a 30 centimetrevertical linear array is approximately 2 metres will maintain a near field cylindricalwave front up to a distance of at 2kHz. Above this the near field will extend indirect proportion to frequency. From previous argument it can be seen that thiscan result in further stabilisation of stereophonic imagery Again it is important forthe axes of the arrays to cross well in front of the listening area.

Whilst it is beyond the scope of this work to delve further into the relevantpsycho-acoustic theory, the is no doubt that a vertical linear array of four small


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wide band drivers is capable of an exceptional sound stage with total fill-inbetween the loudspeakers with no loss of positional integrity.

This performance is effective over a large listening area. and together with theintrinsic sonic qualities of array configurations, this arrangement fully meets theloudspeaker reference standards specified on the first page of this work.

the jordan loudspeaker manual chapter 7

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