THE ORGANS OF THE SPEECH

The organs of the speech produce almost all the sounds needed for language. These organs are divided in 2:

Passive articulators: those which remain static during the articulation of sound. (Upper lips, upper teeth, alveolar ridge, hard palate, etc.)

Active articulators: those which move towards these passive articulators to produce various speech sounds in a different way. (Uvula, lower jaw, lower teeth, lower lips, etc., the most important active articulator is the tongue)

1-nasal cavity

2-lips3-teeth

4-aveolar ridge

5-hard palate

6-velum (soft palate)

7-uvula

8-apex (tip) of tongue

9-blade (front) of tongue

10-dorsum (back) of tongue

11-oral cavity

12-pharynx

13-epiglottis

14-larynx

15-vocal cords

16-trachea

17-esophagus

The nature of voicing
The mucosal wave: For voiced sounds, the vocal cords are held together by the action of the arytenoid cartilages, but they are held together less tightly than for a glottal stop.

When air is forced up the trachea from the lungs, at a certain pressure it is able to force its way through the vocal cords, pushing them open. As air passes through the glottis, the air pressure in the glottis falls, because when a gas or liquid runs through a constricted passage, its velocity increases (the Venturi tube effect). This increase in velocity results in a drop in pressure of that gas or liquid (the Bernouilli principle). Because of the drop in pressure, the vocal cords snap together, at the lower edge first, closing again. The cycle then begins again. A single cycle of opening and closing takes in the region of 1/100th second: therefore, the cycle repeats at rates in the region of 100 times per second. This rate is too rapid for the human ear to be able to discriminate each individual opening/closing of the vocal cords. However, we perceive variations in the overall rate of vibration as changes in the pitch of the voice, "pitch" being the perceptual correlate of acoustic frequency.

Voicelessness: If the vocal cords are held apart, air can flow between them without being obstructed, so that no noise is produced by the larynx. In voiceless fricatives such as [f], [s], [ʃ], [ʂ], [θ ], [ç], [x], and [ χ], the vocal cords are held apart. If there is a sufficiently high rate of airflow through the open glottis, a quiet disruption of the air results (whisper). The glottal fricative has whisper phonation, as do whispered vowels, and the aspiration portion of voiceless aspirated stops such as English /p/, /t/, or /k/ in pre-vocalic position. The IPA diacritic [° ], written below a symbol, indicates such voicelessness. Voiceless vowels, nasals and liquids can be transcribed using that diacritic. For stops and fricatives there are separate letters for voiced and voiceless sounds, e.g. [b] (voiced) vs. [p]. In these cases, the voicelessness diacritic can be used to denote a (possibly partially) devoiced realisation of a phoneme that might otherwise be expected to be voiced, such as the pronunciation of the / ɹ/ in /tɹeIn/, "train", in which the /ɹ/ may be devoiced due to its following a voiceless, aspirated /t/.

Breathy voice or murmur: This phonation is a combination of breath and voice, which occurs if the vocal cords do not close completely along their entire length while they are vibrating; the air which flows through the remaining aperture adds whisper to the vocal cord vibrations.
Creaky voice or creak: In this kind of voicing, the vocal cords are stiffened, so that they are very rigid as they vibrate.

Click on this link to watch another useful explanation.
http://www.rsc-ne-scotland.ac.uk/eolympics/organs_of_speech/home/index.htm

http://en.wikipedia.org/wiki/Speech_organ
http://tutorpal.com/ipa/organs.html
http://www.phon.ox.ac.uk/jcoleman/phonation.htm

ASSIMILATION

Assimilation has a very precise meaning when it’s related to studies of languages. Is a common phonological process bye which the phonetics of a speech segment becomes more like another segment in a word. In other words it’s when a letter (sound) is influenced by the letter (sound) before or after it so that it changes its sound and/or spelling. The word assimilation it self it’s said to be assimilated; it is derived from the latin prefix ad- meaning to and simil- meaning like but, instead of being adsimilated, it has the easier pronunciation of assimilated.

A common example of assimilation is “don’t be silly” where the /n/ and /t/ are assimilated to /m/ by the following /b/, in many accents the natural sound is “dombe silly”.

Assimilation can be synchronic being an active process in a language at a given point in time or diachronic being a historical sound change. There are 4 configurations found: the increase in phonetic similarity may be between adjacent segments or between segments separated by one or more intervening segments; the changes could be in reference to a preceding segment or a following one. Even when all four occur, it changes in regard to a following adjacent segment account for virtually all assimilatory changes. Assimilation to an adjacent segment are vastly more frequent than assimilation to a non-adjacent one.

If a sound changes with reference to a following segment, it is called “regressive assimilation”, the changes with reference to a preceding segment are called “progressive assimilation”. A lot of people find these terms very confusing because they seem to mean the opposite of the intended meaning. To avoid the problem exist a variety of alternative terms. “Regressive assimilation” is also known as right to left, leading or reciprocal assimilation. “Progressive assimilation” is known as left to right or preservative, lagging or lag assimilation.

Occasionally two sounds may influence one another in reciprocal assimilation. When such a change results in a single segment with some of the features of both components, it is known as coalescence or fusion.

1. / t / changes to / p / before / m / / b / or / p /
2. / d / changes to / b / before / m / / b / or / p /
3. / n / changes to / m / before / m / / b / or / p /
4. / t / changes to / k / before / k / or /g/
5. / d / changes to / g / before / k / or / g /
6. / n / changes to /ŋ/ before / k / or / g /
7. / s / changes to /ʃ/ before /ʃ/ or / j /
8. / z / changes to /ʒ/ before /ʃ/ or / j /
9. /θ/ changes to / s / before / s /

http://slb-ltsu.hull.ac.uk/awe/index.php?title=Assimilation_(phonetic)
http://www.btinternet.com/~ted.power/assimilation.html
http://en.wikipedia.org/wiki/Assimilation_(linguistics)