Level Difference Cl

Right

Dashed curves - speech, head Free to move. Solid line - impulses, head immobilized.

direction of sound can be moved about by using two loudspeakers and adjusting the time delay and level between them.

Clearly a stereo image, where a sound is perceived as originating between two loudspeakers, is difficult to maintain. The center image shifts to one side when one sound arrives only a few milliseconds earlier. Thus true stereo imaging is limited to a relatively small listener region close to the centerline between two carefully balanced loudspeakers. In a large room, such as a church or theater, a true stereo image can seldom be achieved.

Directional cues are best introduced by placing loudspeakers near the location of origin of the sound. For example, in motion picture sound systems, three loudspeaker clusters are arranged behind the screen in a left-center-right configuration, and the sound is panned to the proper level during the mix. In theme park attractions localization loudspeakers are placed in or near animatronic figures to provide a directional cue, even when most of the sound energy may be coming from a separate loudspeaker cluster.

Binaural Sound

It is possible to reproduce many of the three-dimensional spatial attributes we hear in real life by recording sound using a dummy head with microphones in the ears and listening to the sound through stereo headphones, one for each microphone. This recording technique is referred to as dummy head stereophony or binaural reproduction, and is used in the study of concert hall design as well as in highly specialized entertainment venues. The results are startlingly realistic, particularly when the sound sources are located behind and close to the head.

When sounds are recorded binaurally, events that occur on the side or to the rear of our head are clearly localized. Sound sources located in front sound like they originate inside our head, overhead, or even behind. Several explanations for this phenomenon have been offered: 1) the effects of the pinnae are not duplicated when the playback system is a pair of headphones, 2) headphones affect the impedance of the aural canal by closing off the tube, and 3) the cues available from head motion are not present.

Figure 3.34 Equal Loudness Curve for Delayed Signals (Kuttruff, 1973)

Critical level difference between a delayed and undelayed signal which results in a perceived equal loudness of both signals (speech,). (Meyer and Schodder, 1152,)

Figure 3.34 Equal Loudness Curve for Delayed Signals (Kuttruff, 1973)

Critical level difference between a delayed and undelayed signal which results in a perceived equal loudness of both signals (speech,). (Meyer and Schodder, 1152,)

TIME DELAY, msec

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