Loudspeaker

General information

A loudspeaker is a electromechanical Transducteur intended to produce its S starting from an electric signal . One employs sometimes also this term to indicate the apparatus intended for the sound level of which it is one of the principal components (in this case, to refer to the article " Speaker ").

Principle and history

To produce sounds, it is necessary to make vibrate Air. A loudspeaker is kind of engine, generally it consists of a winds electric mobile bathing in the Magnetic flux of a Aimant. The reel is interdependent of a mobile membrane in paperboard or matter Synthétique, tended on a framework which solidarizes the whole. When one runs an electric current in a wire which is placed in a Magnetic field, this wire is subjected to forces known as of Laplace, which can put the wire moving. The loudspeaker transforms initially the current that one assigns to him moving of the reel. This one puts moving the membrane of the loudspeaker, which itself puts the air moving. The sound perceived by a man or an animal being by nature a Vibration of the air, the loudspeaker “transforms the electric current into sound. ”

There exist several types of loudspeakers:

electrodynamic,
Isodynamic,
electrostatic,
Piezoelectric
with Plasma.

The moving-coil loudspeaker, covering approximately 99% of the market, has today still a relatively simple operation for a technology of mass.

The loudspeaker is an invention already centenary. December 10th 1877, the first patent concerning a loudspeaker with mobile reel was granted to C.H Siemens. Nowadays, the loudspeaker corresponds to a very varied range of technologies: one as well finds loudspeakers in greetings cards as on Amplificateur S of power for concert.

Electrodynamic loudspeaker of the type

Principle of operation

It functions according to the following principle:

an engine transforms electrical energy into mechanical energy;
this engine transmits this mechanical energy to the membrane;
the membrane transmits the mechanical energy to the ambient air - from where the sound.

The engine is made up as follows:

a unit generating a magnetic field B permanent (invariable in time) in a given space called air-gap. This space is determined by the mechanical geometry of the engine (see further). The magnetic energy source is generally permanent magnet of toric type, low relative thickness compared to its surface, polarized in the axial direction. The field is concentrated in the cylinder head (plate stuck on the back face of the magnet), then reorientated by the cylindrical central core towards the face opposite of the magnet. The field opposite before is directed in the plate of field. Space between the plate of field and the core is named air-gap.
a reel of wire (of copper or aluminum, sometimes money) on a cylindrical component (out of paper, aluminum, Kapton, glass fiber or a composite of these materials) is plunged in this air-gap in the axial direction.

When a current traverses this reel, because of tension which one fixes has his terminals, a force (F) tends has to make leave the reel the field B of the air-gap in the axial direction. this force is defined by the formula F=B*L*i*Sin alpha (Loi of Laplace), B being the field in the air-gap, L the length of coil lead introduced into the air-gap, I the current traversing the reel, and the angle alpha which is equal to the angle between the vector B and vector I: thus in the case of the high speaker, Sin alpha will be always equal has 1. The force F is proportional to I, if B is constant (what is the case if the magnetic unit is well conceived) and if L is constant whatever the displacement of the reel: it thus should be taken care that the reel is assembled symmetrically compared to the field, so that the length of wire which leaves the air-gap is compensated by that which returns in the air-gap.

Cooling of the loudspeaker

Double energy conversion, electric-mechanics then mechanics-acoustics is not perfect. Electromechanical conversion is done with a loss of output primarily by Joule effect.

To fight against this heat allows:

to maintain the characteristics electric of the reel mobile.
not to reach the point of curie of its magnetic engine in order to keep the constant magnetic flux.
not to involve a physical damage on component wire the reel.

These three parameters are important to maintain the distortion lowest possible during the displacement of the reel. The distortion appears when the movements of the reel are not any more one function linear of the current. The solutions of the manufacturers all are based on one of the two following approaches:

the reduction of consumption.
thermal dissipation.

The reduction of consumption

The reduction of electricity consumption is done by increasing the magnetic fields or by increasing the number of whorl which is in this fields (see the formula of the Inductance), which makes it possible to reduce the amperage while having a noise level are equivalent.

By reducing the mass of the membrane or in softener the suspension, one reduces the load and thus consumption too.

Thermal dissipation

Heat is manufactured by the mobile reel according to formula R*I ². with:

R: resistance while continuous current of the reel
I: intensity in amp of the current crossing the reel.

This heat is blocked by the membrane and the spider for the front part. With the back its evacuation is slowed down by the plates of fields and the core or a cylinder head. To evacuate it, there exist several solutions which are often used jointly.

a membrane radiator . The mobile reel fixed at the membrane can transmit heat to the latter if thermal resistance between these last is low. This system is used on the AE1 of the company Acoustic Energy. The metal membrane acts like a radiator. A AE1 admits 200 Watts effective for one 90 Misters. Additional radiators . By adding radiators on the circumference of the cylinder head of the loudspeaker, one evacuates the heat collected by the plates of fields and the core. At Acoustic Boston, this solution is especially used on the tweeters.

By introducing a metal stem into the core connected to a radiator, heat is led by this thermal footbridge towards the outside of the enclosure. Moreover it makes it possible to rigidify the box while behaving like a cleat.

Of the ventilation shafts . By boring the core power station or the cylinder head one creates a passage of air. The heat transmitted to the core by the reel is evacuated by this puit. It is Vented Gap Cooling of JBL, cross-section. This system offers a cooling limited to the part of the reel close to the front part. JBL invented a solution which ventilates the reel by three small wells in the central periphery of the core. The air reaches the heart of the reel directly, it is Super Vented Gap cross-section of JBL. Dynamic compression is maintained with a few dB. A loudspeaker having a reel of low diameter as the EV will undergo a compression of about 7 dB. evolution of dynamic compression according to time for a given power JBL TAD EV.

A simpler solution is to right through bore the back plate and the core or the cylinder head. The hole leads to the back plate of fields. Several solutions of evacuations are possible. Cf Forum audiorountable

the watercooling . While inserting a Ferrofluide in the air-gap, one decreases thermal resistance compared to the air and one transmits best heat to the plates of fields before and to the core which evacuate it then. It is a solution installation at Dynaudio and on many tweeters as at Audax. The particles make approximately 10 Nm and are containing Fe3O4. This matter is also a brake with the displacement of the reel also decreasing the total output of the engine. The acceptable power on the other hand made a jump and the curve of impedance for the tweeter are very deadened offering consequently a filtering to possible weaker slope. This solution spread in many manufacturers. example of ferro fluids

side ventilation . While having the nonjointed circular magnets around the reel, one facilitates the passage of the air towards outside, thus decreasing the temperature. Focal adopts the multi-magnets on these systems high-end. By installing a space between the salad bowl and the spider, to ensure a circulation of air, the reel is cooled on its front part as on the range stratos of Triangle

Electrostatic loudspeakers

The membrane planes which will reproduce all the frequencies is covered with a conducting film charged with a potential difference with several thousands of volts continuous compared to the mass (approximately 5000 volts). The membrane is put moving by nonmobile drivers, fixed all around the membrane. These wire are traversed by the audio signal, whose tension is increased by a transformer, they attract or push back thus statically the membrane. As the membrane reproduces all the frequency band, there is no distortion known as of Crossover.

See too

Pregnant (audio)

Simple: Loudspeaker

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