question: Why does tapping wood and tapping metal produce different sounds?
comment: you know when you put a ruler on the edge of a table and push it down and let it go and it makes that woioioioinggggg sound? And if you vary the length of it off the table, the sound changes and the speed at which the end of the ruler goes up and back down changes? This is the natural frequency of the ruler when it is that length of the table and thus produces a certain sound. Now certain materials also will have a natural frequency such as wooden or metal tables and that's why they make different sounds. They make different frequencies because they also will only take in set amounts of energies.
passages: Though he was inspired by Drobner, Penderecki’s timbre system goes another step further, in which “it is of no importance whether metal, wood, and leather are represented by a vibrator or by an inciter, both colliding bodies being of equal weight as primary materials.” This means that “if a given body can be a sound source—that is, if it is made of one of the three materials capable of performing this function (m, w, l) --then it becomes a sound source regardless of whether it is hit, rubbed or plucked or itself hits, rubs or plucks.”
   Three factors relating to the walls of a resonator will affect how it functions: the material it is made of, the thickness of its walls, and the type of surface it has. The resonance characteristics of a musical instrument obviously will vary with different materials and the amount of material used will have some effect.
   BULLET::::- Tubular bells or chimes
  
  111.24 Percussion vessels.
  
  BULLET::::- Boungu
  
  BULLET::::- Chuk
  
  BULLET::::- Cymbals
  
  BULLET::::- Crash cymbal
  
  BULLET::::- Hank drum
  
  BULLET::::- Hi-hat cymbal
  
  BULLET::::- Hang
  
  BULLET::::- Kagul or tagutok
  
  BULLET::::- Ride cymbal
  
  BULLET::::- Slit drums:
  
  BULLET::::- Slit gong
  
  BULLET::::- Splash cymbal
  
  BULLET::::- Steelpan or steel drum
  
  BULLET::::- Udu (also an aerophone)
  
  BULLET::::- Wood block
  
  111.241 Gongs - The vibration is strongest near the vertex.
  
  111.241.1 Individual gongs.
  
  BULLET::::- Babendil
  
  111.241.2 Sets of gongs.
  
  BULLET::::- Agung or agong
  
  BULLET::::- Bock-a-da-bock
  
  BULLET::::- Gandingan
  
  BULLET::::- Kulintang or kolintang
  
  111.242 Bells - The vibration is weakest near the vertex.
  
  BULLET::::- Bell tree
   Different mallets are used primarily to alter the timbre of the mallet instrument being played. Generally, mallets composed of softer materials will stick to the instrument for longer as they bounce off of it, which gives a deeper sound made up of lower frequencies. Harder materials tend to bounce off quicker, and as they stick to the instrument for a shorter amount of time, they tend to be able to excite more of the higher frequencies, giving the sound a higher pitch with more overtones. Mallet choice is typically left up to the performer, though some compositions specify if a certain sound is desired by the composer. 
   Timbre composition with acoustic instruments is usually very subtle. Minute altercations in performance change the sound produced by the instrument. Percussionists use specific sticks and mallets to yield desired timbres. Players of wind instruments alter the tone simply by changing the pressure of their lips. One could argue that the act of choosing which instrument to play or write for is in itself timbre composition.
  
  Timbre composition is used in vocal techniques such as throat-singing where the main focus of the music is timbre as opposed to pitch.
   Different species of wood have different strength levels, and the exact strength may vary from sample to sample.
  
  Section::::Properties of wood.:Dimensional stability.
   Similar to two-hand tapping, "selpe" technique is used in Turkish folk music on the instrument called the "bağlama". 
   Section::::Scrapers.
  
  This can be a wood, metal or plastic instrument which has ridges on its body. Often known as Guiro, rhythms are created by running a thin stick up and down the ridges at different speeds. Gourds or bamboo have traditionally been used as they have a resonant hollow body and can easily be cut with ridges. A common type from Asia is a carved wooden frog which has ridges cut on its back and its mouth and belly hollowed out.
  
  Section::::Cowbells.
   These factors can be influenced by the shaping of the wood. A good luthier understands how to shape the wood to directly influence these acoustic properties.
  
  Guitar players can actively vary the timbre by the way they pluck the string—by changing the nail or finger angle, the distance from the bridge, etc. A good instrument helps the musician produce widely varying timbres and have yet have good resonance. Nevertheless, a "default" colour and response is identifiable for notes in the different registers and define the instrument's sound.
  
  Section::::Skills.:Styles of "sound aesthetic".
   In simple terms, timbre is what makes a particular musical sound have a different sound from another. For instance, it is the difference in sound between a guitar and a piano playing the same note at the same volume. Both instruments can sound equally tuned in relation to each other as they play the same note, and while playing at the same amplitude level each instrument will still sound distinctively with its own unique tone color. Experienced musicians are able to distinguish between different instruments of the same type based on their varied timbres, even if those instruments are playing notes at the same fundamental pitch and loudness.
   However, these type of use is not considered communication in the strictest sense.
  
  Section::::Production of vibrational cues.
  
  Vibrational cues can be produced in three ways, through percussion (drumming) on the substrate, vibrations of the body or appendages transmitted to the substrate, or, acoustical waves that couple with the substrate. The strength of these cues depends mostly on the size and muscular power of the animal producing the signal.
  
  BULLET::::- Percussion:
   BULLET::::- 112.2 Scraped Idiophones – The player causes a scraping movement directly or indirectly; a non-sonorous object moves along the notched surface of a sonorous object, to be alternately lifted off the teeth and flicked against them; or an elastic sonorous object moves along the surface of a notched non-sonorous object to cause a series of impacts (this group must not be confused with that of friction idiophone
  
  BULLET::::- 12 Plucked idiophones such as the jew's harp
  
  BULLET::::- 13 Friction idiophones such as Rub rods
  
  BULLET::::- 2 Membranophones
   An auditory–motor interaction may be loosely defined as any engagement of or communication between the two systems. Two classes of auditory-motor interaction are "feedforward" and "feedback". In feedforward interactions, it is the auditory system that predominately influences the motor output, often in a predictive way. An example is the phenomenon of tapping to the beat, where the listener anticipates the rhythmic accents in a piece of music. Another example is the effect of music on movement disorders: rhythmic auditory stimuli have been shown to improve walking ability in Parkinson's disease and stroke patients.
   There are two types of tones: pure tones and complex tones. The musical note produced by a tuning fork is called a pure tone because it consists of one tone sounding at just one frequency. Instruments get their specific sounds — their timbre — because their sound comes from many different tones all sounding together at different frequencies. A single note played on a piano, for example, actually consists of several tones all sounding together at slightly different frequencies.
  
  Section::::See also.
  
  BULLET::::- Larynx
  
  BULLET::::- Phonation
  
  BULLET::::- Respiration (physiology)
  
  BULLET::::- Speech
  
  BULLET::::- Speech and language pathology
  
  BULLET::::- Speech perception
   Two or more complementary sonorous parts are struck against each other.
  
  111.11 Concussion sticks or stick clappers (nearly equal thickness and width).
  
  BULLET::::- Clapsticks
  
  BULLET::::- Claves
  
  111.12 Concussion plaques or plaque clappers (flat).
  
  BULLET::::- Clapper
  
  BULLET::::- Guban
  
  BULLET::::- Paiban
  
  BULLET::::- Pak
  
  BULLET::::- Slapstick
  
  111.13 Concussion troughs or trough clappers (shallow).
  
  BULLET::::- Devil chase
  
  111.14 Concussion vessels or vessel clappers (deep).
  
  BULLET::::- Spoons
  
  111.141 Castanets - Natural and hollowed-out vessel clappers.
  
  BULLET::::- Castanets
  
  BULLET::::- Krap
  
  111.142 Cymbals - Vessel clappers with manufactured rim.
  
  BULLET::::- Chap
  
  BULLET::::- Ching
  
  BULLET::::- Clash Cymbals
  
  Section::::Directly struck (111).:Percussion idiophones (111.2).
   Different pieces of wood cut from a large tree may differ decidedly, particularly if the tree is big and mature. In some trees, the wood laid on late in the life of a tree is softer, lighter, weaker, and more even-textured than that produced earlier, but in other trees, the reverse applies. This may or may not correspond to heartwood and sapwood. In a large log the sapwood, because of the time in the life of the tree when it was grown, may be inferior in hardness, strength, and toughness to equally sound heartwood from the same log. In a smaller tree, the reverse may be true.
   The physical characteristics of sound that determine the perception of timbre include spectrum and envelope. Singers and instrumental musicians can change the timbre of the music they are singing/playing by using different singing or playing techniques. For example, a violinist can use different bowing styles or play on different parts of the string to obtain different timbres (e.g., playing sul tasto produces a light, airy timbre, whereas playing sul ponticello produces a harsh, even and aggressive tone). On electric guitar and electric piano, performers can change the timbre using effects units and graphic equalizers.
   Many factors affect sustain for a given instrument. For example, sustain in guitars is determined by factors including body construction (hollow versus solid), body woods, neck woods, the placing of strings (through the body or atop the body), string gauges and pickup design. Also, there are dedicated effects pedals for electric and bass guitars capable of prolonging the sustain-phase of a tone. 
   The degree to which the choice of woods and other materials in the solid-guitar body (3) affects the sonic character of the amplified signal is disputed. Many believe it is highly significant, while others think the difference between woods is subtle. In acoustic and archtop guitars, wood choices more clearly affect tone.
   Indirectly stuck idiophones produce sound resulting from an indirect action of the performer as opposed to the directly struck idiophones. The player himself does not go through the movement of striking; percussion results indirectly through some other movement by the player. This category is divided in two main sub-categories: shaken idiophones and scraped idiophones.
  
  Section::::Indirectly struck (112).:Shaken idiophones or rattles (112.1).
  
  The player makes a shaking motion
  
  112.11 Suspension rattles - Perforated idiophones are mounted together, and shaken to strike against each other.
  
  112.111 Strung rattles - Rattling objects are strung in rows on a cord.
   Because Penderecki understood timbre primarily as a function of the materials, the timbral categories in his sonorism are based upon the most common materials used in the assembly of instruments and accessories of traditional symphonic orchestras: metal, wood, leather, felt, and hair. Though any of these materials can act as both vibrators and inciters, felt and hair are the least effective vibrators due to the fact that they would be inaudible when vibrated. This then means that at least one of the two sound-generating bodies must be made of metal, wood, or leather, referred to as primary materials. In Penderecki’s timbre system, the three primary materials may (and do) interact with one another, whereas felt and hair can only interact with one of the primary materials and not themselves.
   The mallet shaft is commonly made of wood, usually birch, but may also be rattan or fiberglass. The most common diameter of the shaft is around . Shafts made of rattan have a certain elasticity to them, while birch has almost no give. Professionals use both depending on their preferences, whether they are playing with two mallets or more, and which grip they use if they are using a four-mallet grip.
   First a segment of a hardwood tree trunk or thick branch is taken and stripped of its bark. Holes are then bored into the log in a straight line, from one end to the other, optionally leaving some space at each end. What remains in between the holes is then chiseled out, forming the characteristic slit. After this, the log continues to be hollowed out through the slit. Both the shape of the slit and the extent that the log is gutted will affect the tone and pitch of the pate.
   Mallets used as drumsticks are often used to strike a marimba, xylophone, glockenspiel, metallophone, or vibraphone, collectively referred to as mallet percussion. The sticks usually have shafts made of rattan, birch, or fiberglass. Rattan shafts are more flexible than the other materials. Heads vary in size, shape, and material; they may be made of metal, plastic, rubber, or wood, and some are wrapped with felt, cord, or yarn. Heavier heads produce louder sounds, while harder heads produce sharper and louder sounds, with more overtones.
  
  Section::::Toys.
   Psychoacoustic experiments from the 1960s onwards tried to elucidate the nature of timbre. One method involves playing pairs of sounds to listeners, then using a multidimensional scaling algorithm to aggregate their dissimilarity judgments into a timbre space. The most consistent outcomes from such experiments are that brightness or spectral energy distribution , and the "bite", or rate and synchronicity and rise time , of the attack are important factors.
  
  Section::::Tristimulus timbre model.
  