Басқа қатысты трансферлік функция: Ыстық пен кеңістіктегі дыбысты қабылдау ерекшеліктері

Response that characterizes how an ear receives a sound from a point in space

A head related transfer function (HRTF) is a response that characterizes how an ear receives a sound from a point in space. As sound strikes the listener, the size and shape of the head, ears, ear canal, density of the head, size and shape of nasal and oral cavities, all transform the sound and affect how it is perceived, boosting some frequencies and attenuating others. Generally speaking, the HRTF boosts frequencies from 2–5 kHz with a primary resonance of +17 dB at 2,700 Hz. But the response curve is more complex than a single bump, affects a broad frequency spectrum, and varies significantly from person to person. A pair of HRTFs for two ears can be used to synthesize a binaural sound that seems to come from a particular point in space. It is a transfer function, describing how a sound from a specific point will arrive at the ear (generally at the outer end of the auditory canal). Some consumer home entertainment products designed to reproduce surround sound from stereo (two speaker) headphones use HRTFs. Some forms of HRTF processing have also been included in computer software to simulate surround sound playback from loudspeakers.

Humans have just two ears, but can locate sounds in three dimensions – in range (distance), in direction above and below (elevation), in front and to the rear, as well as to either side (azimuth). This is possible because the brain, inner ear, and the external ears (pinna) work together to make inferences about location. This ability to localize sound sources may have developed in humans and ancestors as an evolutionary necessity since the eyes can only see a fraction of the world around a viewer, and vision is hampered in darkness, while the ability to localize a sound source works in all directions, to varying accuracy,
regardless of the surrounding light. Humans estimate the location of a source by taking cues derived from one ear (monaural cues), and by comparing cues received at both ears (difference cues or binaural cues). Among the difference cues are time differences of arrival and intensity differences. The monaural cues come from the interaction between the sound source and the human anatomy, in which the original source sound is modified before it enters the ear canal for processing by the auditory system. These modifications encode the source location and may be captured via an impulse response which relates the source location and the ear location. This impulse response is termed the head related impulse response (HRIR). Convolution of an arbitrary source sound with the HRIR converts the sound to that which would have been heard by the listener if it had been played at the source location, with the listener's ear at the receiver location. HRIRs have been used to produce virtual surround sound. The HRTF is the Fourier transform of HRIR. HRTFs for left and right ear (expressed above as HRIRs) describe the filtering of a sound source (x(t)) before it is perceived at the left and right ears as xL(t) and xR(t), respectively. The HRTF can also be described as the modifications to a sound from a direction in free air to the sound as it arrives at the eardrum. These modifications include the shape of the listener's outer ear, the shape of the listener's head and body, the acoustic characteristics of the space in which the sound is played, and so on. All these characteristics will influence how (or whether) a listener can accurately tell what direction a sound is coming from. In the AES69 2015 standard, the Audio Engineering Society (AES) has defined the SOFA file format for storing spatially oriented acoustic data like head related transfer functions (HRTFs). SOFA software libraries and files are collected at the Sofa Conventions website.

The associated mechanism varies between individuals, as their head and ear shapes differ. HRTF describes how a given sound wave input (parameterized as frequency and source location) is filtered by the diffraction and reflection properties of the head, pinna, and torso, before the sound reaches the transduction machinery of the eardrum and inner ear (see auditory system). Biologically, the source location specific prefiltering effects of these external structures aid in the neural determination of source location, particularly the determination of the source's elevation.

A basic assumption in the creation of a virtual auditory space is that if the acoustical waveforms present at a listener's eardrums are the same under headphones as in free field, then the listener's experience should also be the same. Typically, sounds generated from headphones are perceived as originating from within the head. In the virtual auditory space, the headphones should be able to "externalize" the sound. Using the HRTF, sounds can be spatially positioned using the technique described below. which runs physical simulation on a full 3D mesh of the head, and EAC, which uses a neural network trained from existing HRTFs and works from photo and other rough measurements.

Recordings processed via an HRTF, such as in a computer gaming environment (see A3D, EAX, and OpenAL), which approximates the HRTF of the listener, can be heard through stereo headphones or speakers and interpreted as if they comprise sounds coming from all directions, rather than just two points on either side of the head. The perceived accuracy of the result depends on how closely the HRTF data set matches the characteristics of one's own ears, though a generic HRTF may be preferred to an accurate one measured from one's one ear. Some vendors like Apple and Sony offer a variety of HRTFs to be selected by the user's ear shape. Windows 10 and above come with Microsoft Spatial Sound included, the same spatial audio framework used on Xbox One and Hololens 2. On a Windows PC or an Xbox One, the framework can use several different downstream audio processors, including Windows Sonic for Headphones, Dolby Atmos, and DTS Headphone:X, to apply an HRTF. The framework can render both fixed position surround sound sources and dynamic "object" sources that can move in space. Apple similarly has Spatial Sound for its devices used with headphones produced by Apple or Beats. For music playback to headphones, Dolby Atmos can be enabled and the HRTF applied. The HRTF (or rather, the object positions) can vary with head tracking to maintain the illusion of direction. Qualcomm Snapdragon has a similar head tracked spatial audio system, used by some brands of Android phones. YouTube uses head tracked HRTF with 360 degree and VR videos. Linux is currently unable to directly process any of the proprietary spatial audio (surround plus dynamic objects) formats. SoundScape Renderer offers directional synthesis. PulseAudio and PipeWire each can provide virtual surround (fixed location channels) using an HRTF. Recent PipeWire versions are also able to provide dynamic spatial rendering using HRTFs, however integration with applications is still in progress. Users can configure their own positional and dynamic sound sources, as well as simulate a surround speaker setup using existing configurations. The cross platform OpenAL Soft, an implementation of OpenAL, uses HRTFs for improved localization. Windows and Linux spatial audio systems support any model of stereo headphones, while Apple only allows spatial audio to be used with Apple or Beats branded Bluetooth headsets.