The Abicana site- music instrument principles
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How does musical instruments make sound and modulate the tune of the sound
Sound is periodic increase and decrease of the presure in a medium that propagate through the medium with a certain speed, the speed of sound in that medium. The medium can be a solid substance, a fluid or a gas, for example air. If you is positioned at a certain place in that medium and measure the pressure, you will observe that it increases or decreases with a certain frequency. If you measure where the pressure is highest or lowest along a path in that medium, you will find that those points occur at a specific distance from each other. You will also find that the higest or lowest pressure intervals are travelling at the speed of the sound, or that they are passing by with the speed of sound.
A phenomenon that is increasing and decreasing periodically and travelling that way, is called a wave, and since the pressure variates in a sound, a sound is called a pressure wave. When sound is travelling through the air and reaching the ear, it will be heard as a tune and height. The tune depends upon the sound frequency, while the height depens upon how strongly the pressure increases and decreases.
To make a sound, something must vibrate with a certain frequency, and the vibrating object must be in contact with a medium so that the virbration can cause periodic increase and decrease of pressure in that medium. When the vibration causes sound in another medium, energy is transfered.
The longer away from the source the sound gets, it will be diluted and sounds weaker. Also the ordered vibration that sound is, will also gradually be transfered to disordered molecular movements, which is heat.
But the ear recognizes the intensity of sound according to a logaritmic scale. That means that a sound that have been diluted or decreased manyfold, will will still be heared only slightly weaker.
If the sound is not sustain with som kind of work subjected upon the vibrating object, all of it will eventually be heat and the sound seaces. When the sound decreases, the energy is not lost, but will be transfered to heat.
In a music instrument you will have such a vibrating object and you have some way or some mechanism that works mechanically upon that vibrating object. The vibrations can then be transfered to the air around, thus making ssound.
The primary vibrating object will usually also be connected to a greater object, usually filled with air.
The primary vibrating object also sets the greater object in vibration and you then have a greater vibrating surface that makes sound in the air around. Such a greater object is called a resonance box. The resonance box makes it possible to deliver more energy in form of sound out into the air.
The frequency of vibration in an object is dependent both on the stiffness of that object and of its size. The stiffer and the smaller the object is, the more frequent will the vibration and the sound be, or lighter, or higher up in the scale, will the tone be heard as. To be able to make tunes of various hight, a music instrument must have one or more of these ways of variating the vibration:
- Several vibrating objects with different size and stiffness.
- Objects where smaller or greater parts of them are used to make sound.
- Objects where the stiffness is modulated with smaller or greater mechanical pull on them.
- A resonance box that can vary or modulate its inner space
In a string instrument, strings of various sizes are the promary vibrating object.
In many of them the player set a string in vibration by pullings sidewise with his fingers or by rubbing sidewise at the string with a bow. In these instruments the tune hight can also be modulated by varying the size of the vibrating part of each string. This is usually done by pressing a string down at some surface with a finger at various places on the string.
These ibnstruments also have an ounter resonance box that the strings are mounted upon. Guitars and violins are typical examples of this type of instrument. Harps work according to the same principle, and here the strings are suspended in a thick hollow frame that also serve as the resonance box.
In pianos and the like the strings are suspended in a frame inside the resonance box, and the vibrations are made by hammers blowing at the strings, or hooks pulling sidewise at the strings. The hammers are set in motion by the player by means of external tangents that are connected to the hammers with some mechanism.
In many blow instruments the primary vibrating object is the lips of the player together with a mouthpiece and a tube extending from the mouthpiece. The player makes the vibration by blowing through his lips that are held teight togeather. The tune is modulated by shuting in or out tube sections by means of a mechanism operated by the fingers. The vibration makes sound in the air inside the tube. The tube ends in a wide siphone that is serving the role as resonance box. Typical examples of this kind of instruments are trumpets, tubas and trombones.
In other blow instruments the primary vibrating object is a mouthpiece with a narrow slot or with two blades. The player makes vibrations by blowing through the narrow space in the slot or between the blades.
After that there is a tube that serve as resonance object and as tune modulator. The tune is modulated by opening or closing holes in the tube. The part of the tube from the mouthpiece to the first open hole serve as a primary resonance box that defines the tune. The rest of the tube togeather with some widening or siphone serve as a secondary resonance box. Examples of these instruments are flutes, saxophones, fagottes and clarinettes.
The reason why blowing through the narrow slot or lips makes sound, is that the blowing makes turbulence in the air inside the slot. To make turbulence, the player must blow with some intensity. The turbulense is caused because the air that is near to the walls inside the slot will pass slower due to friction, than the air passing mid in the slot. Then whirls are constantly created and stopped inside the slot, which makes vibrations.
In some instruments, like xylophones and metallophones, the primary vibrating objects are bars of various length made of wood or metal. The vibrations are made by hitting the bars with a hammer. The bars are typically mounted at the top of the resonance box, or they are mounted on a frame with resonance tubes extended downards from each bar.