it is actually the sound of different harmonic components of same sound coming into dominance at different times. John W. Coltman, in a detailed analysis of flute acoustics, describes two types of warbles in Native American flutes: One "of the order of 20 Hz" caused by a "nonlinearity in the jet current", and a second type "in which amplitude modulation occurs in all partials but with different phases". The first type is analyzed by Coltman in a controlled setting, but he concluded that analysis of the second type of warble "is yet to be explained". The warble can be approximated by use of vibrato techniques. The phase shift that occurs between different harmonics can be observed on a spectrograph of the sound of a warbling flute. Written music Written music for the Native American flutes is often in the key of F-sharp minor, although some music is scored in other keys. However, the convention for this music written in F-sharp minor is to use a non-conforming key signature of four sharps, creating what is known as "Nakai tablature". Note that the use of finger diagrams below the notes that is part a high percentage of written music for the Native American flutes is not necessarily part of Nakai tablature. The use of a standard key signature for written music that can be used across Native American flutes in a variety of keys classifies the instrument as a transposing instrument. Music Extensive ethnographic recordings were made by early anthropologists such as Alice Cunningham Fletcher, Franz Boas, Frank Speck, Frances Densmore, and Francis La Flesche. A small portion of these recordings included Native American flute playing. One catalog lists 110 ethnographic recordings made prior to 1930. These recordings capture traditional styles of playing the instrument in a sampling of indigenous cultures and settings in which the instrument was used. However, the legal and ethical issues surrounding access to these early recordings are complex. Because of incidents of misappropriation of ethnographic materials recorded within their territories, Indigenous communities today claim a say over whether, how and on what terms elements of their intangible cultural heritage are studied, recorded, re-used and represented by researchers, museums, commercial interests and others. During the period 1930–1960, few people were playing the Native American flute. However, a few recordings of flute playing during this period are commercially available. One such recording is by Belo Cozad, a Kiowa flute