These categories of speech sounds involve a transitional aspect during their articulation. In trills for example a flexible articulator vibrates repeatedly against an immobile articulator so that a somewhat rattling sound quality is obtained. In an alveolar trill the tip of the tongue vibrates against the roof of the mouth just behind the upper teeth. The actual number of vibrations is variable. Languages can have trills at various places of articulation, but the alveolar trill is the most common type. Other places of articulation are labial and uvular. In a labial trill, the lips first create a closure which is followed by 2-3 trill periods. This trill type has been attested linguistically in only few places in the world such as in Central Africa, Papua New Guinea and Vanuatu (Rangelov, 2019). In a uvular trill it is the uvula which trills against the back of the tongue. All these trills can be both voiced and voiceless.
Taps or flaps consist of a single ballistic movement of a flexible articulator against an immobile articulator. Both terms are often used interchangeably, but some phoneticians make a distinction. Taps and flaps distinguish themselves from trills in that they have only one moment of contact between the articulators.
Rangelov, T. (2019). The bilabial trills of Ahamb (Vanuatu): acoustic a articulatory properties. Proceedings of the ICPhS
Visual illustration of the alveolar trill