Welcome to the homepage of Frans A. Bilsen

Professor emeritus in biophysics and acoustics at Delft University of Technology.
I am still engaged in some interesting stuff, along with a special item for the grand children. Please, mouse click the topic you are interested in:

Repetition Pitch Dichotic Pitch Hearing Aids Acoustics and Biophysics
Classical Mechanics Model Trains

For your convenience, PDF files of papers not easily available:
Bilsen 1966, Bilsen 1967, Bilsen 1967/68, Bilsen & Ritsma 1967/68, Bilsen & Ritsma 1969/70,
Ritsma & Bilsen 1970, Bilsen 1973, Bilsen & Raatgever 1973, Bilsen 1972, Bilsen & Wesdorp 1974,
Bilsen & Raatgever 1983, Bilsen 1994, Bilsen 1995, Bilsen & Dols 1995,
Bilsen 2001, Bilsen & Raatgever 2001, Bilsen & Raatgever 2002

frans at Chantilly castle in 1993 
 at Chantilly castle in 1993

Note on Bilsen 2001, "The case of the missing central spectra". Using a better (unbiased) psychophysical procedure, the latter pilot results reported on DRP could not be confirmed. On the contrary, the results by Bilsen and Goldstein (JASA 1974) could be confirmed.

Repetition Pitch

A pitch sensation often occurs in free nature when the sound of a sound source reaches the ear of an observer directly and, at the same time, after being reflected against a sound-reflecting surface. This phenomenon has been named Repetition Pitch (RP), because the addition of a true repetition of the original sound to itself is the basic requirement (Bilsen, 1966). RP corresponds to the reciprocal value of the time delay between the original and the repeated (reflected, delayed) sound. RP is most salient when the original sound is wide band and pitch-less itself (for example: white noise). Probably the first written report of the phenomenon dates from Christian Huygens (1693), who observed such a pitch in the sound from a fountain reflected against the steps of a large stone staircase in the garden of the castle of Chantilly in France (see Oeuvres Complètes, Vol.10, Correspondance no.2840, pages 570-571, see Gallica; see DBNL, see also RPnature). A similar example stems from the Mayan step pyramid in Chichen Itza (see ocasa; for a detailed explanation see  RPglide). In free field, one might also be able to observe a gliding pitch when a plane flies over, by moving one's head towards and from the ground. Or more nostalgic, when one approaches a steam locomotive blowing off steam. In music, the phenomenon is sometimes deliberately created by electronic means (delay and add) to superimpose a pitch or coloration effect on the original music (see Flanging). In room acoustics and sound recording, the phenomenon often causes an unwanted coloration of the original sound. Blind peoples might use RP to locate obstacles by clicking the street surface with their cane, thus producing a wide-band impulsive sound that is reflected by an obstacle. Numerous experiments in the past, in the field of psychological and physiological acoustics, aimed at understanding the perceptual and neurological processes underlying pitch perception in general, RP in particular. Both temporal and spectral models of pitch extraction were proposed (RP-prediction; see also References).

     Pictures drawn by F.M.M, Bilsen 

Sound examples:

Three key publications:

Dichotic pitch

A dichotic or binaural pitch may be observed when continuous white noise is presented by headphones to the left and right ear of a listener. Given a particular interaural phase relationship between the left and right ear signals, a sensation of pitch occurs. In other words, stimulation of either ear alone gives rise to the sensation of white noise only, but stimulation of both ears together produces the pitch. Generally, a dichotic pitch is perceived somewhere in the head amidst or separate from the background noise. To be more specific, the dichotic pitch is characterized by three perceptual properties: pitch value, timbre, and in-head position (lateralization) of the pitch image. The first binaural pitch phenomenon reported in the literature is the so-called Huggins Pitch (HP) (Cramer and Huggins, 1958). In this case, the interaural phase relationship consists of a rather sharp phase shift over 2 pi radians in a narrow-band frequency region of the white-noise spectrum. The sensation is of a fluctuating pure tone. Probably the strongest dichotic pitch ever is created by having several harmonically-related phase-shift regions. It is therefore called multiple-phase-shift pitch (MPSP, Bilsen, 1976; see also sirl for a similar stimulus). Experiments on dichotic pitch were motivated by the study of pitch in general and of the binaural system, being crucial for sound localization and separation of sound sources (see cocktail party effect). Various models were developed in the past. Especially, the so-called CAP-CS model  seems successful in predicting both the pitch value and pitch-image position (see References).
Sound examples*:
* sound tracks taken from the audio compact disc with booklet "Demonstrations of Dichotic Pitch" by F.A. Bilsen and J. Raatgever (Delft, 2002),
* It may be evident that dichotic-pitch stimuli should be listened to properly with good-quality headphones only.

Three key publications:

Hearing aids

One of the problems with conventional hearing aids is malfunctioning in situations with disturbing background noise like, for example, in a cocktail-party or meeting-like situation. Therefore, around 1983, I proposed a project at the Delft University of Technology aiming at a new type of hearing aid that would enable hard-of-hearing peoples to focus on a particular voice amidst of other disturbing voices and/or noises. In general, three strategies seem feasible: 1) the use of two identical conventional hearing aids, provided that both ears have degraded about equally, 2) the use of a conventional hearing aid supplied with binaural-like signal processing, and 3) the use of one or two conventional hearing aids with a strong directional microphone. Given the knowledge and experience available in the TUD Sound Control Group, the latter option was chosen and this has resulted in the "hoorbril", a pair of spectacles with an array of microphones mounted either on the legs (endfire array) or on the body itself above the glasses (broadside array). Financial support was supplied by the Dutch Organization for Scientific Research (NWO) and the Philips Company. Today, the company Varibel produces and sells an hoorbril based on the endfire principle. See also Babble for similar products.

Specific publications:

Acoustics and biophysics

Here you will find a list of publications in english by coworkers of the former research group named "Biologische Natuurkunde" (Biophysics, from 1967 headed by Prof.dr. G. van den Brink) and re-named "Akoestische Perceptie" (Perceptual Acoustics, untill 2002 headed by Prof.dr.ir. F.A. Bilsen) on acoustical topics,.

Classical mechanics

Basics summarized
In classical mechanics, the movement of rigid bodies is generally described by two analogous vector equations: F = dp/dt for translation of the center of mass, and M = dL/dt for rotation around the center of mass, with F the total external force, p the momentum (dutch: impuls), M the moment of forces (koppel of krachtmoment), and L the angular momentum (impulsmoment).

TippeTop explained
As an example, we consider the intriguing movement of the so-called tippe top or Kelvin top after its inventor Lord Kelvin (Sir William Thomson). Such a top essentially consists of a spherical body and a cylindrical stem, with the center of mass displaced from the center of the sphere. After having been put into rotation around its axis of symmetry vertically, the top gradually changes its movement and eventually it flips over into a stable rotation on and around the stem. We have to conclude that the rotation has changed sign, such that the angular momentum vector L still has its original vertical position. Further, the center of gravity has moved upwards, apparently at the cost of a decrease in magnitude of L. This unexpected behavior is explained by the action of a small friction force at the contact point of the top with the ground surface. Many publications in the past have been devoted to the explanation of the complex movement of the tippe top. Here we propose a qualitative explanation using the above equations only. For further details and demonstrations see TippeTop1 and TippeTop2

Spinner showing precession
This recent hype flywheel thing can also learn us a lot about classical mechanics movie A Spinner is mounted on a pencil suspended by a rope. It is started rotating firmly. Its angular momentum L is directed horizontally along the pencil. A moment M due to the gravital force works perpendicular on L. It can only change the direction of L, not its magnitude. The resulting rotation of L in a horizontal plane is called precession.

Model trains

Enjoy a movie (Bilsstadt1) or slide show (Bilsstadt2) of our model train setup.