Fluid Dynamics and Acoustics Group


  Virtual Source Imaging

Any sound reproduction system that attempts to give a listener a sense of "space" must somehow make the listener believe that sound is coming from a position where no real sound source exists. For example, when you sit in the "sweet spot" in front of a good two-channel stereo system, you know that it is possible to "fill out the gap" between the two loudspeakers. If you pass two identical signals to both loudspeakers then you will ideally perceive the sound as coming from a position directly in front of you. If you then increase the input to one of the speakers, the sound will be pulled sideways towards that speaker. This principle is called amplitude stereo, and it has been the most common technique used for mixing two-channel material ever since the two-channel stereo format was first introduced. However, it is intuitively obvious that amplitude stereo cannot create virtual images outside the angle spanned by the two loudspeakers. In fact, even in between the two loudspeakers, amplitude stereo works well only when the angle spanned by the loudspeakers is 60 degrees or less. 

Geometry / 60 deg.


Virtual source imaging systems work on the principle that they "get the sound right" at the ears of the listener. A real sound source generates certain interaural time- and level differences that are used by the auditory system to localise the sound source. For example, a sound source to left of the listener will be louder, and arrive earlier, at the left ear than at the right. A virtual source imaging system is designed to reproduce these cues accurately. In practice, loudspeakers are used to reproduce a set of "desired" signals in the region around the listener's ears. The inputs to the loudspeakers must be determined from the characteristics of the desired signals, and the desired signals must be determined from the characteristics of the sound "emitted" by the virtual source. 

An example illustrates this concept. Suppose we want to use two closely spaced loudspeakers to give a listener the impression that sound is coming from a virtual source at a position of 45 degrees to the left relative to straight front. This is is well outside the angle spanned by the loudspeakers. Let us say that we want the listener to hear a short "click" coming from that direction. If we imagine that the click is played back through the virtual source, then we can derive, through measurements or synthesis, the sound pressures that are produced at the ears of the listener. We refer to these signals as the desired signals. Since the listener's torso, head, and pinnae (outer ears) modify incoming sound waves, the desired signals do not have the same shape as the original click. Once the desired signals are known, it is then possible to calculate the loudspeaker inputs. This turns out to be a surprisingly difficult problem, but it can be solved by using sophisticated digital filter design techniques. 

Geometry / 10 deg.


Input for virtual source

Inputs to loudspeakers

The signals at the ears