An auditory hallucination is the experience of hearing spontaneous imaginary noises    . The most common examples of this include hearing clips of sound such as imagined music   , voices        , tones  , popping   , and scraping  , but can also be an infinite variety of other potential noises that are stored within one's memory. In terms of their behaviour, these sounds will often be based on noises which were expected to occur or have been genuinely heard on a frequent basis within the external environment. For example, a person may repeatedly hear a knock at the door when they are expecting a visitor or may hear music which they were listening to earlier on in the day. However, at other times, auditory hallucinations may also present themselves as consisting of completely novel and alien sounds that are unlike anything which could currently occur within the external environment. Auditory hallucinations are often accompanied by other coinciding effects such as auditory distortion    and auditory enhancement   . They are most commonly induced under the influence of moderate dosages of hallucinogenic compounds  , such as psychedelics, deliriants, and dissociatives. However, they can also occur less commonly under the influence of stimulant psychosis, cannabinoids, and during sleep deprivation   . This effect can be broken into two specific sub-types which are described and documented below:
An internal auditory hallucination can be described as the perception of hallucinated audio that sounds as if the specific location of its source does not have a particular sense of distance or direction attributed to it and that the sound is instead occurring within a person's own head. This is in contrast to external auditory hallucinations, which sound as if they are occurring seamlessly within the external environment as if they were actually happening. The experience of this effect can be broken down into three distinct levels of intensity. These are described and documented below:
Internal auditory hallucination
- Fleeting hallucinations - At the lowest level, internal auditory hallucinations generally consist of ill-defined, distant, and subtle sounds that stop as soon the person notices them. For example, a person may hear vague voices, music, and tones within their own head but find that they are not remotely discernable in terms of their given details.
- Partially defined hallucinations -At this level, the hallucinations can be directly noticed without them immediately stopping, but are not fully defined in terms their detail. This means that, although fully audible, they will still sound partially muffled and distant. For example, a person may hear voices or music within their own head but find that they do not sound quite as crisp and detailed as similar sounds which can be heard in real life.
- Fully defined hallucinations - At this level, the hallucinations have become completely realistic and lifelike in terms of the detail of their sound. For example, a person may hear fully defined voices, music, or other sounds within their own head that are complex and fully defined in terms of their details.
An external auditory hallucination can be described as the perception of a hallucinated noise that occurs seamlessly within the external environment as if the specific location of its source has a particular sense of direction and distance attributed to it. This is in stark contrast to internal auditory hallucinations that sound as if they occur exclusively within one's own head. This effect is capable of manifesting itself across the 4 different levels of intensity described below:
External auditory hallucination
- Partially defined embedded hallucinations - At the lowest level, the hallucinated sounds lack clarity and may be indistinct, muffled, and difficult to make out. They are also only heard embedded within real sounds occurring within the external environment. For example, a person may hear subtle music or voices embedded within the sound of the wind, cars, and rain. These hallucinations sound as if they are coming from an unspecifiable direction somewhere within the external environment.
- Partially defined separate hallucinations - At this level, the sounds remain only partially defined, but are heard on a separate layer of their own instead of only manifesting themselves as embedded within other noises. They will also begin to sound as if they are occurring from a vaguely specifiable direction or source within the external environment.
- Fully defined separate hallucinations - At this level, the sounds become fully defined in their clarity, meaning that the content of the hallucinations can be recognized and heard perfectly as if they were actually occurring. They will also sound as if they originate from a specific location within the external environment in a manner which is so lifelike that it will often confuse the person into believing it was a real sound.
Sinister hallucinatory voices
- Auditory hallucinations: a review of psychological treatments | https://www.schres-journal.com/article/S0920-9964(98)00052-8/abstract
- Paracusia (Medical dictionary) | http://medical-dictionary.thefreedictionary.com/paracusia
- The clinical spectrum of musical hallucinations (ncbi) | https://www.ncbi.nlm.nih.gov/pubmed/15546592
- The Omnipotence of Voices: A Cognitive Approach to Auditory Hallucinations (The British Journal of Psychiatry) | https://www.cambridge.org/core/journals/the-british-journal-of-psychiatry/article/omnipotence-of-voices/BF121CBA135062075365F28B5AA0ED94
-  N Stanciu, C., & M Penders, T. (2016). Hallucinogen Persistent Perception Disorder Induced by New Psychoactive Substituted Phenethylamines; A Review with Illustrative Case. Current Psychiatry Reviews, 12(2), 221-223. | http://www.ingentaconnect.com/contentone/ben/cpsr/2016/00000012/00000002/art00013#expand/collapse
- Al-Assmar, S. E. (1999). The seeds of the Hawaiian baby woodrose are a powerful hallucinogen. Archives of internal medicine, 159(17), 2090-2090. | https://jamanetwork.com/journals/jamainternalmedicine/article-abstract/521620
- Nichols, D. E. (2016). Psychedelics. Pharmacological reviews, 68(2), 264-355. | https://dx.doi.org/10.1124%2Fpr.115.011478
- Lu, B. Y., Woofter, C., & Escalona, R. (2004). A case of prolonged peyote-induced psychosis resolved by sleep. The Journal of clinical psychiatry, 65(10), 1433. | https://www.ncbi.nlm.nih.gov/pubmed/15491253