Are we able to detect the direction or the distance from which a sound comes with just one ear?
I don't know if the inner ear works as a single point sensor or if it has multiple sensitive regions, like an array of sensors, like the retina.
Or we can only do it if we use both ears? In that case, the source would only be partially located on a 3D space.
Would it be useful to make headphones with multiple speakers for each ear? (Not for different frequencies but at different locations).
The eardrum is a single sensor: The sound pressure which vibrates the eardrum propagates as a single signal in the hammer bone which is attached to the inner surface of the eardrum, i.e. there is no several bones which would detect the pressure on different locations of the tympani (which would probably be inefficient anyway).
Direction-dependent mono-aural cues: Horizontal Spatial hearing cues are mainly driven by differences in time and amplitude of the sound wave in the two ears, while vertical spatial hearing is mostly driven by direction-dependant spectral signature of the head and pinae (HRTF).
--> Studies with single-sided deafness individuals show that it is possible to learn to localize sound sources from HRTF cues in the horizontal planes too.
Agterberg MJ, Hol MK, Van Wanrooij MM, Van Opstal AJ, Snik AF. Single-sided deafness and directional hearing: contribution of spectral cues and high-frequency hearing loss in the hearing ear. Front Neurosci. 2014 Jul 4;8:188. doi: 10.3389/fnins.2014.00188. PMID: 25071433; PMCID: PMC4082092:
To investigate whether single-sided deafness listeners rely on monaural pinna-induced spectral-shape cues of their hearing ear for directional hearing, we investigated localization performance for low-pass filtered (LP, <1.5 kHz), high-pass filtered (HP, >3kHz), and broadband (BB, 0.5–20 kHz) noises in the two-dimensional frontal hemifield. We tested whether localization performance of single-sided deafness listeners further deteriorated when the pinna cavities of their hearing ear were filled with a mold that disrupted their spectral-shape cues. [...] Several listeners with single-sided deafness could localize HP and BB sound sources in the horizontal plane [...]. Localization performance of these listeners strongly reduced after diminishing of their spectral pinna-cues.
This literature review about spatial hearing with impaired ear points some laboratory limitations to the above findings:
it will be necessary to show that it extends to more realistic and challenging listening situations than those typically used in the laboratory, and that the benefits include not only a recovery in sound localization accuracy, but also improved speech-in-noise perception.
The Agterberg et al. 2014 paper also reviews the studies with non-human single-sided deafness and how inborn versus later deafness are involved.
The Cochlea that translates sound/vibrations into chemo-electrical (nerve) signals is only a single sensor and therefore directionality cannot be obtained by time-delay estimations.
However, the ear is on one side of the head, and therefore the sound intensity varies when the sound arrives from the side of the ear or from the other side of the head. Therefore, the brain could learn to use this intensity difference to assess the sound direction. Changing the direction of the head while hearing the sound could provide sufficient variations in sound intensity to learn this. The presence of the outer ear channelling the sound would support/amplify such intensity variations.
Using the sound intensity to obtain sound direction allows us to assess if sound is coming from behind.
How good such direction estimate will be depends on the training data set of the human neural network (also known as brain).
Yes, to some extent. The shape of human ears filters sound differently depending on direction, see https://en.wikipedia.org/wiki/Head-related_transfer_function
The shape of the ear helps with broadband sounds. If you have a cat you can see a similar effect by testing its ability to find the height of a sound source. It can accurately find the source when using a broadband source (hiss), but single freqeuncy (whistle) will confuse it.
There are different sound cues we naturally learn to be able to localize sound sources. These cues may be related to the sound energy (amplitude) and to the time or frequency domains.
One that quickly comes to mind is the interaural time difference (ITD), which "is the difference in arrival time of a sound between two ears". Also, there is its amplitude counterpart, interaural level difference (ILD), but they won't serve us well in the proposed scenario of single-sided deafness (SSD). However, they are usually accompanied with another cue, which is filtering, due to sound diffraction around the head.
In classical physics, diffraction is explained by the Huygens-Fresnel principle1. Put in a practical way, since wavelengths that are relative large compared to the size of head will be able to "go around it", the spectral content of low frequencies will be better preserved between ears, while high frequency content will be attenuated. Also, important filtering happens around the pinna.
On this way, without ITD and ILD because of the single-sided deafness, the high frequency sounds are easier to localize than the low frequency ones, as pointed in the paper by Agterberg et al. brought by @Noil.
ITD, ILD and the filtering resulted from diffraction around the head are useful for horizontal plane localization; if the sound source is horizontally centered in relation to the receiver, i.e. it is on the median plane, the sound arrives at both ears, ideally, at the same time, with same levels and filtering. The diffraction from the pinna is then important to the sound localization at the median plane and single-sided deafness (SSD) is less of a hindrance.
However, it is important to keep in mind the Blauert's "determining frequency bands" effect, which is commented, for example, in Psychoacoustics2 (a main reference in the field). The effect states that some frequency bands, regarding the median plane, will always be perceived as coming from a specific orientation. There are more recent studies referring to Blauert's original paper that confirmed the effect for some of the frequencies.
The Wikipedia page on sound localization seems relatively complete on the whole matter. Regarding the list of sound distance cues, only the last two (ILD and moving sound sources, depending on the scenario) appear to be affected by SSD (this is more of a personal thought/analysis, though).
As from the Wikipedia page on the sound distance cues, let's take the direct/reflection cues. If in a room, there may be a close speaker and a resulting clear speech, as also a distant speaker and less intelligible speech due to reflections. Also, voice timber changes if the speaker is speaking either quietly or loudly. These, accompanied with the arriving sound level and ambient noise, help determine if the speaker is close or far away (apparently independently of SSD). It is relatively easy to imagine some of these scenarios, but speech is something we're very familiar with. So knowing the sound already, having an expectation or experience, may influence our perception. Also, we move and turn our heads when trying to localize the sound source. In summary, depending on the matter, I think it's important to think on real life scenarios and that there may be a lot of variables involved.
If in a virtual reality environment, there seems to be no need for extra speakers as is usual to convolute the original/dry signal with an appropriate HRIR3 (head related impulse response), which applies the sound cues we talked about depending on source-receiver orientation. Also, technology that understands head movements and changes the HRIR accordingly would be of great benefit. It is important to keep in mind, though, that different people have different personal HRIRs and things like how the headphones couple on someone's head may interfere in the final perception.
@Noil already talked about how the sound that gets into the inner ear is a single signal (independently if there are multiple sound sources and from different directions). However, I'd like to point out that yes, there are multiple receptors in a latter stage. The basilar membrane, inside the cochlea, vibrates differently depending on frequency, causing differences on the excitation of the multiple hair cells that send electrical signals to the brain.
