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loudspeaker.md

An introduction to the physics of the loudspeaker

1. a few concepts about the sound Sound is a sequence of waves of pressure that propagates through compressible media such as air or water. People hear the sound when it reaches their ears. Without the media to propagate the waves, there would be no sound.

2. Three physical quantities of the waves.

   1. Amplitude(A): The amplitude of the wave
   2. Frequency(F): The number of oscillations per unit time (the frequency range that human can hear is 20-200000Hz)
   3. Period(T):The time need to finish a full oscillation. It is related to the frequency by T=1/F

3. Three elements of sound

   1. Loudness: Sound loudness is a subjective term describing the strength of the ear's perception of a sound. It is intimately related to sound intensity but can by no means be considered identical to intensity. The sound intensity must be factored by the ear's sensitivity to the particular frequencies contained in the sound.
   2. Pitch: the frequency of the sound. A sound could have multiple components of different frequencies.
   3. Timbre: it is then a general term for the distinguishable characteristics of a tone. Timbre is mainly determined by the harmonic content of a sound and the dynamic characteristics of the sound such as vibrato and the attack-decay envelope of the sound.

4. Different types of the loudspeaker
   The loudspeaker is a electronic devise that convert energy forms between the electric energy and mechanical energy. There are several common types of loudspeakers according how they work:

   • Electrodynamic loudspeaker
   • Electrostatic loudspeaker
   • piezoelectric loudspeaker
   • Plasma loudspeaker

5. The mechanics of the electrodynamic loudspeaker
   The electrodynamic loudspeaker (field coil loudspeaker) is a dynamic loudspeaker in which the magnetic field is produced by an electromagnet rather than by a permanent magnet. The magnetic field is used to convert the electric energy to the sound energy. It is the most common type of loudspeakers that are currently used and has a history of more than 80 years since its first sale in 1926. A structural characteristic of the electrodynamic loudspeaker is that it has a moving coil. Because of its great performance, it has many variants and multiple applications, including high, middle and low range loudspeakers.

   1. Alnico: Alnico has two classes: metal Alnico and ferrite Alnico The major technical requirement of ALnico is the magnet performance (BHmax), which is typically measured in units of megagauss-oersteds (MGOe). This quantity tells how much magnetic field energy the material could store per unit volume.
      • ferrite ALnico is normally used in external magnetic circuit. The major benefit is its low cost and easy magnetization. The disadvantage is that such magnetic circuit has big size and heavy weight.
      • metal Alnico (particularly rare earth metal Alnico) has advantages in high magnet performance. It is mostly used in internal magnetic circuit due to its small size and light weight. But it has a high production cost and is hard to magnetization.
   2. 导磁件：T yoke、FRONT PLATE, etc: 导磁件 are mostly made of steel. The overall requirement for them is a high permeability. One technical feature is the carbon content percentage. The less carbon, the better. But less Carbon also means higher cost.
   3. vibration system (produce vibrations and make sounds): The voice coil is the heart of the loudspeaker. Upon switching on the circuit, it moves in the magnetic field and vibrates the cone. The voice coil is made of 骨架 and wires. The types of voice coils:
      1. Copper wires -- low electrical resistance, good ultimate tensile strength, easy Welding and applications in various loudspeakers.
      2. Aluminum wires -- high electrical resistance, weak ultimate tensile strength, hard Welding and main applications in high range loudspeakers.
      3. Copper-clad aluminum wires -- have advantages from both copper wires and aluminum wires but are expensive. They are mainly used in loudspeakers with extreme requirements.
         The surface of voice coil is wrapped with the enamel insulation layer and the adhesive. The materials of the the enamel insulation layer and the adhesive are selected according to the power of the loudspeakers. There are two types of wires, LOCK wires and SV wires, currently used:
      • LOCK wire -- for low power and low temperature resistant loudspeakers
      • SV wires -- for high power or high temperature resistant loudspeakers

   The voice coil 骨架: basic materials | surface coasting | temperature resistance

   4. The spider is one of the most important components of a loudspeaker. Its major functions are

      1. Ensure the voice coil in the right position
      2. When the voice coil is forced to move, keep the vibration system moves along the axial direction only and a good ability to restore the balanced status.
      3. The spider should have a good air permeability. But the holes density cannot be too small. It has to ensure that the air under the spider gets out properly to prevent the lose of accuracy due to pressure, while keep dust from going through.
      4. the compliance of the spider together with the compliance of the cone determines the total compliance of the loudspeaker, which has major effect on the resonant frequency of the loudspeaker.

   5. The materials of the spider:

   materials	benefit	drawback
   C (100% cotton)	soft, cheap	poor moisture resistance, fatigue
   PC (POLYESTER/cotton)	good strength, good fatigue resistance, good deformed resistance	more expensive than cotton
   NC (NOMAX/cotton)	better than PC in strength and fatigue resistance	poor glued, twice as expensive as cotton
   CONEX	high temperature resistant, good flame retardant, good fatigue resistant, good stability	poor glued, the most expensive

   6. Diaphragm is a major component of the electrodynamic loudspeaker: A diaphragm is generally, but not exclusively, cone shaped and made of paper (in low range, middle range, high range or full range loudspeakers). We normally called it 'cone'.

      1. The functions and requirements of the diaphragm: The diaphragm is a important component of the vibration system. An electrodynamic loudspeaker uses the vibration of the voice coil to vibrate the connected diaphragm and produces sounds through the vibrations of the air in th front and back of the diaphragm, then radiates the sounds. Among all the components of the loudspeaker, the diaphragm is the most critic to the sound quality.

   7. Connectors: basket, 编织线、接线板、压边等 supplementary parts

6. 1. characteristic impedance

      1. Impedance curve: Given a electric signal at any frequency, there is a corresponding impedance. The curve represents the impedances at different frequencies is called the 'impedance curve'.
      2. Characteristic impedance: We define the characteristic impedance as the first minimum impedance after the maximum at the low frequency region. The impedance of a loudspeaker should not be lower than 80% of its characteristic impedance in its working frequencies. Otherwise the compatible devises would not work properly.

   2. The characteristic sensitivity of the loudspeaker The characteristic sensitivity of the loudspeaker at a given frequency is defined as the loudness of the output sound measured at distance of 1 meter in unit of dB under the working power of 1 watt. The unit is dB.
      The average characteristic sensitivity is the average of the characteristic sensitivity over the working frequency region. if not otherwise specified, the sound pressure level (SPL) we usually talk about is the average characteristic sensitivity.

   3. The working frequency region. The line below the average characteristic sensitivity by 10 dB meets the characteristic sensitivity curve at two points. Assume the low frequency point is F1 and the high frequency point is F2. The frequency range between F1 and F2 is called the 'effective frequency region'.

   4. Audio distortion