A transducer can be affected dramatically by the presence of other objects, but the effect is highly frequency dependent. In Figure 3.35(a) a high frequency is radiated, and this simply reflects from the nearby object because the wave- length is short and the object is acoustically distant or in the far field. However, if the wavelength is made longer than the distance between the source and the object as in (b), the object is acoustically close or in the near field and becomes part of the source. The effect is that the object reduces the solid angle into which radiation can take place as well as raising the acoustic impedance
Figure 3.35 (a) At high frequencies an object is in the far field. (b) At low frequencies the same
object is in the near field and increases velocity by constricting the radiation path.
the transducer sees. The volume velocity of the source is confined into a smaller cross-sectional area and consequently the velocity must rise in inverse proportion to the solid angle.
In Figure 3.36 the effect of positioning a loudspeaker is shown. In free space (a) the speaker might show a reduction in low frequencies which disappears when it is placed on the floor (b). In this case placing the speaker too close to a wall, or even worse, in a corner (c), will emphasize the low-frequency output. High-quality loudspeakers will have an adjustment to compensate for positioning. The technique can be useful in the case of small, cheap loudspeakers whose low-frequency response is generally inadequate. Some improvement can be had by corner mounting.
It will be evident that at low frequencies the long wavelengths make it impossible for two close-spaced radiators to get out of phase. Consequently when two radiators are working within one another’s near field, they appear acoustically to be a single radiator. Each radiator will experience a doubled acoustic impedance because of the presence of the others. Thus the pressure for a given volume velocity will be doubled. As the intensity is proportional to the square of the pressure, it will be quadrupled.
Figure 3.36 Loudspeaker positioning affects low-frequency response. (a) Speaker in free air
appears bass deficient. (b) This effect disappears when floor mounted. (c) Bass is increased when mounted near a wall or corner.
The effect has to be taken into account when stereo loudspeakers are installed. At low frequencies the two speakers will be acoustically close and so will mutually raise their acoustic impedance causing a potential bass tip- up problem. When a pair of stereo speakers has been properly equalized, disconnecting one will result in the remaining speaker sounding bass light.
In Figure 3.37 the effect of positioning a microphone very close to a source is shown. The microphone body reduces the area through which sound can escape in the near field and raises the acoustic impedance, emphasizing the low frequencies. This effect will be observed even with pressure microphones as it is different in nature to and adds to the proximity effect described earlier. This is most noticeable in public address systems where the gain is limited to avoid howlround. The microphone must then be held close to obtain sufficient level and the plosive parts of speech are emphasized. The high signal levels generated often cause amplifier clipping, cutting intelligibility.
When inexperienced microphone users experience howlround they often misguidedly cover the microphone with a hand in order to prevent the sound from the speakers reaching it. This is quite the reverse of the correct action as the presence of the hand raises the local impedance and actually makes the howlround worse. The correct action is to move the microphone away from the body and (assuming a directional microphone) to point it away from the loudspeakers. In general this will mean pointing the microphone at the audience.
In Figure 3.38 a supra-aural headphone (one which sits above the ear rather than surrounding it) in free space has a very poor low-frequency response
Figure 3.37 Bass tip-up due to close microphone positioning. A suitable filter will help intelli-
gibility.
Figure 3.38 Supra-aural headphones rely on the bass tip-up in the near field to give a reasonable
bass response.
because it is a dipole source and at low frequency air simply moves from front to back in a short circuit. However, the presence of the listener’s head obstructs the short circuit and the bass tip-up effect gives a beneficial exten- sion of frequency response to the intended listener, whilst those not wearing the headphones only hear high frequencies. Many personal stereo players
incorporate a low-frequency boost to further equalize the losses. All prac- tical headphones must be designed to take account of the presence of the user’s head since headphones work primarily in the near field.
A dramatic example of bass tip-up is obtained by bringing the ear close to the edge of a cymbal shortly after it has been struck. The fundamental note which may only be a few tens of Hertz can clearly be heard. As the cymbal is such a poor radiator at this frequency there is very little damping of the fundamental which will continue for some time. At normal distances it is quite inaudible.
3.21 Refraction
If sound enters a medium in which the speed is different, the wavelength will change causing the wavefront to leave the interface at a different angle. This is known as refraction. The ratio of velocity in air to velocity in the medium is known as the refractive index of that medium; it determines the relationship between the angles of the incident and refracted wavefronts. This doesn’t happen much in real life, it requires a thin membrane with different gases each side to demonstrate the effect. However, as was shown above in connection with the Doppler effect, wind has the ability to change the wavelength of sound. Figure 3.39 shows that when there is a wind blowing, friction with the earth’s surface causes a velocity gradient. Sound radiated upwind will have its wavelength shortened more away from the ground than near it, whereas the reverse occurs downwind. Thus upwind it is difficult to hear a sound source because the radiation has been refracted upwards whereas downwind the radiation will be refracted towards the ground making the sound ‘carry’ better. Temperature gradients can have the same effect. As Figure 3.40(a) shows, the reduction in the speed of sound due to the normal fall in temperature with altitude acts to refract sound away from the earth. In the case of a temperature inversion (b) the opposite effect happens. Sometimes a layer of air forms in the atmosphere which is cooler than the air above and below it. Figure 3.40(c) shows that this acts as a waveguide because sound attempting to leave the layer is gently curved back in giving the acoustic equivalent of a mirage. In this way sound can travel hundreds of kilometres. Sometimes what appears to be thunder is heard on a clear sunny day. In fact it is the sound from a supersonic aircraft which may be a very long way away indeed.
Figure 3.39 When there is a wind, the velocity gradient refracts sound downwards downwind of
Figure 3.40 (a) Temperature fall with altitude refracts sound away from the earth. (b) Tempera-
ture inversion refracts sound back to earth. (c) Cool layer in the atmosphere can act as a waveguide.