Different personalised sound technologies use different scientific principles to customise sound and can be categorised according to two main parameters:
subjective measurement vs. objective measurement
accounting for a subset of the listener's hearing or their environment vs. accounting for the listener's full hearing system.
Subjective tests, which are a part and parcel of every clinical Audiometry procedure specifically used to determine a person's hearing, require the listener to give feedback based on what they perceive. A common type of subjective hearing test requires the user to push a button if they can hear a tone that is played at a particular frequency and amplitude. On the other hand, objective measurements automatically measure one or more feedback signals and do not require any input from the user. Some personalised sound technologies measure a subset of the user's hearing, such as the acoustics of the user's outer ear, whereas other technologies offer a full hearing measurement. Several reviewers have noted that these different technologies vary in accuracy and usability.
Subjective, full hearing measurement
Historically, hearing aids were used to enable people with hearing loss to understand speech better by measuring their hearing and adjusting devices based on an Audiogram. However, recent developments in the field are enabling listeners with mild to moderate hearing loss, who do not necessarily need hearing aids, to substantially improve their enjoyment of music and sound, in addition to better speech recognition. The establishment of a new class of hearing aids in the U.S. marketed directly to consumers and regulated by the Over-the-Counter Hearing Aid Act of 2017 reflects these developments. New devices for personalising music listening have been introduced that employ subjective, full hearing tests. The user responds on the mobile application interface if they perceive tones. Oftentimes, the assessment tests each ear separately. Examples of products that offer subjective personalised sound are headphones by EVEN and Audeara, which provides a subjective hearing tests that take between 90 seconds and 15 minutes to complete and the Aumeo Headphone Personaliser dongle that can be connected to most media players and provides a two-minute subjective hearing test. The Mimi mobile app allows the user to undergo a six-minute subjective hearing test and the app then adjusts the mobile device's audio output based on the test results. Unlike the Mimi technology, Skullcandy's Crusher ANC headphones use a subjective, full hearing measurement and then adapts audio output through the user's headphones, not their mobile device. Skullcandy's hearing measurement takes under 3 minutes to complete on a mobile application and was developed by Swedish software company Audiodo.
Objective, full hearing measurement
Personalised sound technologies with objective, full hearing measurement claim to analyse the user's full hearing without the user manually responding to testing tones. The Nuraphone by Nura and are the only available consumer product that performs an objective measurement of the listener's full hearing system in order to enhance the music listening experience, primarily targeting listeners with normal hearing. The Nuraphone's proprietary technology automatically monitors otoacoustic emissions to measure the user's hearing in 1–2 minutes. The Nuraphone adapts its frequency response based on the user's hearing, allowing the user to hear more detail when listening to music.
Objective, subset hearing measurement
There are several products that provide personalised sound by measuring some acoustic properties of the user's environment without actually measuring their hearing. As an example, the AKG N90Q headphones measure the acoustics of the user's outer ear. The headphones then adjust their EQ based on the measurements. The smart speaker personalises sound based on the acoustics of the room where the speaker is placed. The Sonos Play:5 measures the room's acoustics by playing testing tones, while the user's phone detects how these tones sound, and the speaker's EQ is adjusted.
