FIELD

The present disclosure generally relates to the field of binaural sound synthesis; and more specifically, to binaural sound synthesis for sound that is closer to the listener than the near-field boundary.

BACKGROUND

The human auditory system modifies incoming sounds by filtering them depending on the location of the sound relative to the listener. The modified sound involves a set of spatial cues used by the brain to detect the position of a sound. Human hearing is binaural, using two ears to perceive two sound-pressure signals created by a sound.

Sound is transmitted in air by fluctuations in air pressure created by the sound source. The fluctuations in air pressure propagate from the sound source to the ears of a listener as pressure waves. The sound pressure waves interact with the environment of the path between the sound source and the ears of the listener. In particular, the sound pressure waves interact with the head and the ear structure of the listener. These interactions modify the amplitude and the phase spectrum of a sound dependent on the frequency of the sound and the direction and the distance of the sound source.

These modifications can be described as a Head Related Transfer Function (HRTF) and a Head-Related Impulse Response (HRIR) for each ear. The HRTF is a frequency response function of the ear. It describes how an acoustic signal is filtered by the reflection properties of the head, shoulders and most notably the pinna before the sound reaches the ear. The HRIR is a time response function of the ear. It describes how an acoustic signal is delayed and attenuated in reaching the ear, by the distance to the sound source and the shadowing of the sound source by the listener's head.

A virtual acoustic system is an audio system (e.g., a digital audio signal processor that renders a sound program into speaker driver signals that are to drive a number of speakers) that gives a listener the illusion that a sound is emanating from somewhere in space when in fact the sound is emanating from loudspeakers placed elsewhere. One common form of a virtual acoustic system is one that uses a combination of headphones (e.g., earbuds) and binaural digital filters to recreate the sound as it would have arrived at the ears if there were a real source placed somewhere in space. In another example of a virtual acoustic system, crosstalk cancelled loudspeakers (or cross talk cancelled loudspeaker driver signals) are used to deliver a distinct sound-pressure signal to each ear of the listener.

Binaural synthesis transforms a sound source that does not include audible information about position of the sound source to a binaural virtual sound source that includes audible information about a position of the sound source relative to the listener. Binaural synthesis may use binaural filters to transform the sound source to the binaural virtual sound sources for each ear. The binaural filters are responsive to the distance and direction from the listener to the sound source.

Sound pressure levels for sound sources that are relatively far from the listener will decrease at about the same rate in both ears as the distances from the listener increases. The sound pressure level at these distances decreases according to the spherical wave attenuation for the distance from the listener. Sound sources at distances where sound pressure levels can be determined based on spherical wave attenuation can be described as far-field sound sources. The far-field distance is the distance at which sound sources begin to behave as far-field sound sources. The far-field distance is greatest for sounds that lie on an axis that passes through the listener's ears and smallest on a perpendicular axis that passes through the midpoint between the listener's ears. The far-field distance on the axis that passes through the listener's ears may be about 1.5 meters. The far-field distance on the perpendicular axis that passes through the midpoint between the listener's ears may be about 0.4 meters. Sound sources at the far-field distance or greater from the listener can be modeled as far-field sound sources.

As a sound source approaches the listener, the effects of the interaction between the listener's head and body and the sound pressure waves become increasingly prominent. The difference in sound intensity between the listener's ears is called the Interaural Level Difference (ILD). A sound that is moving toward the listener along the axis that passes through the listener's ears will increase in intensity at the ipsilateral ear and simultaneously decrease in intensity at the contralateral ear because of head shadowing effects. The ILD starts to increase at a distance of about 0.5 meters and becomes pronounced at a distance of about 0.25 meters.

The difference in sound arrival time between the listener's ears is called the Interaural Time Difference (ITD). The ITD also increases rapidly as a sound source moves toward the listener, and the difference in distances from the sound source to the listener's two ears becomes more pronounced. Sound sources at distances where the effects of the listener's head and body become prominent can be described as near-field sound sources. Sound sources that are less than about 1.0 to 1.5 meters from the listener need to be modeled (to simulate how a listener would hear them) with binaural filters that include these near-field effects.

Modeling of sound sources with binaural filters that include near-field effects can be effective for distances of about 0.25 meters or more. As the desired location for the sound source gets very close to the listener, e.g. less than about 0.25 meters, binaural filters that include near-field effects begin to produce binaural audio signals that have been found to be subjectively undesirable. Head shadowing effects may become so prominent that the sound becomes inaudible at the contralateral ear, producing an uncomfortable feeling of occlusion in the contralateral ear.

SUMMARY

If it is desired to reduce the distance to a perceived sound source so as to place the sound at a location between the listener's ears (also referred to as an in-head location), binaural filters that are based on HRTFs are no longer applicable. That is because HRTFs are derived from microphone measurements that detect the sound arriving from sound sources that are placed at a distance from a listener's head, where the detected sound has of course been altered by the listener's head and shoulders. The measurements for deriving HRTFs may be made using microphones located at a listener's ears or in the ears of a dummy head or acoustic manikin.

It would be desirable to provide a way to synthesize binaural audio signals (that would drive respective earphone transducers at the left and right ears of a listener) for a virtual acoustic system, to create the illusion of a sound source moving toward or away from the listener between i) the end of the effective range of near-field modeling and the center of the listener's head or another in-head location.

In a device or method for rendering a sound program for headphones, a location is received for placing the sound program with respect to first and second ear pieces. If the location is between the first ear piece and the second ear piece (an in-head location), then the sound program is filtered to produce low-frequency and high-frequency portions. The high-frequency portion is panned according to the location to produce first and second high-frequency signals. The low-frequency portion and the first high-frequency signal are combined to produce a first headphone driver signal to drive the first earpiece. A second headphone driver signal is similarly produced, by combining the low-frequency portion and the second high-frequency signal to produce a second in-head signal. The sound program may be a stereo sound program. The device or method may provide for rendering of the sound program at a location between the first ear piece and a near-field boundary. The location may be variable over time, so that the method can for example move the sound program gradually from an in-head position to an outside-the-head position, or vice-versa (e.g., from outside-the-head to an in-head position.)

The above summary does not include an exhaustive list of all aspects of the present disclosure. It is contemplated that the disclosure includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, as well as those disclosed in the Detailed Description below and particularly pointed out in the Claims section. Such combinations may have particular advantages not specifically recited in the above summary.

