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Home Theater Handbook

Loudspeakers

Part 3


See also: all questions, part 1, part 2

What is the difference between active and passive subwoofers?

Subwoofers are distinguished as passive or active, with the later having an integrated amplifier, which further reduces the load on our main amplifier and improves bass performance.

Subwoofers use the same enclosures as regular loudspeakers, with all the advantages and disadvantages that each enclosure offers. Active subwoofers have dedicated power amplifiers, tailor made to reproduce low frequencies. These amplifiers must have a lot of power to be able to drive the woofers to deep bass, especially if the input signal is boost­ed, to make up for the decreased sensitivity of our hearing of bass frequencies. Most subwoofers have a line level elec­tronic crossover prior to the amplifier, so once the signal is amplified, it is sent directly to the woofer without any addi­tional crossover intervening in the signal path.

What are the principles of operation of electrodynamic speakers?

Electrodynamic speakers are the most popular speakers and they come in varied shapes, size and cost. Our familiar cones and domes characterize electrodynamic speakers. They are the diaphragms that generate the sound and usually the only visible parts of a loudspeaker. The electrodynamic speakers' operation is based on the principles of electromagnetic induc­tion. According to this, when a conductor moves in a magnet­ic field it experiences forces that result in the generation of an internal electric field and potential differences at its ends.

What component is at the heart of electrody­namic speakers?

At the heart of the electrodynamic speak­ers there is a strong permanent magnet, cylindrical in shape, with a cylindrical shaft (the pole) at its center. Between the pole and the magnet there is a space of a few millimeters, in which a very strong and homogeneous magnetic field exists.

Where is the voice coil placed?

In the space between the magnet and the pole we place the voice coil, which is a conductor tightly wrapped in coils. The voice coil is free to move in the magnetic field and is sup­ported by an elastic suspension, which makes sure that the coil does not touch the pole and behaves as if it is floating.

What is the function of the voice coil?

When the audio signal in the form of alternating current is conducted through the coil, forces are generated that cause the coil to move back and forth. On the outside of the coil a diaphragm is attached, the size of which determines the low­est frequency that can be reproduced. The diaphragm moves and sound is generated.

What is the use of the special liquid (ferrofluid) contained in the tweeters?

For low and medium frequencies different size cones are used, while for high frequencies hemispherical dome shaped diaphragms, with a diameter equal to the coil, are common. The high frequency speaker is called a tweeter. Many tweet­ers contain a small quantity of a special liquid (ferrofluid) in the space between the voice coil and the magnet, to damp the movement of the coil and to cool it. It also acts as a lubri­cant and aids in keeping the coil at the center of its path. The ferrofluid contains microscopic particles of magnetic materi­al mixed in special oil and is kept between the coil and the magnet by the magnetic field.

What is the correlation between the diaphragm diameter and the driver’s mass?

Larger diameter cones require larger size voice coils and magnets. This creates speaker drives with large mass and inertia, which require more power to drive. In an effort to minimize the mass of the diaphragm while keeping the required rigidity, synthetic and exotic material are often used, such as polypropylene, Kevlar, titanium etc.

What is the operation principle of magnetostatic speakers?

Based on the same principles as electrodynamic speakers, but following a different application technique, the magne­tostatic speakers use a large and flat plastic diaphragm, which is traversed by conductors and is placed in the middle of two magnetic and parallel surfaces. This low-mass plastic diaphragm acts simultaneously as the voice coil, resulting in lower inertia and improving dramatically the efficiency of the speaker.

What are ribbon speakers?

A special category of magnetostatic speakers is the ribbon speaker, in which a metallic strip is suspended between the opposite poles of two magnets. Ribbon speakers are very sensitive and are primarily used for the reproduction of high frequencies.

What is the difference between electrostatic and magnetostatic speakers?

Electrostatic speakers also use a large plastic membrane, with the difference that it is covered by a metallic layer through which runs a very high potential and it is placed between two fixed stators that are fed with reverse polarity copies of the audio signal.

What is the characteristic of electrostatic speakers?

The audio signal creates a variable electrostatic field between the membrane and the stators, which generates a pull-push force on the membrane. This force moves the membrane and generates sound. Electrostatic speakers have excellent audio performance. However, the need for large dimension membranes and for huge amounts of power makes electrostatic speakers an expensive choice for the few.

What do we mean by "active" speakers?

Speakers are usually passive units. However it is becoming very popular to use active speakers, which are speakers with an integrated amplifier, in many cases discrete for each drive. This results in the optimal matching of the amplifier with the speaker, better efficiency and increased control of the motion of the cone.

Active speakers exhibit exemplary control, range and fidelity without the need to use exotic material.

How are the drivers of a dipole speaker placed?

Most speakers are direct radiating, with all of their drives placed on the same side of the cabin and radiating towards the same direction. Dipole speakers have drives placed on opposite sides of the cabin, radiating in opposite directions but with the same phase and for this they diffuse sound better, creating a richer surround field, "easier" listening environment, albeit not always with the best accuracy. Dipole speakers are the preferred surround speakers for THX certified home theater set ups because they simulate the surround speaker arrays of movie theaters.

Why do common speakers interfere with the TV?

As a result of home theater setups, loudspeakers found their way next to television sets. Common speakers however generate a strong magnetic field that interferes with the TV, especially the cathode ray tube and erase magnetic storage media.

How do we magnetically shield a speaker?

As a result of the potential problems caused by magnetic fields, loudspeaker companies manufacture magnetically shielded speakers to protect the television sets and susceptible magnetic storage media.

The magnetic shielding of speakers is achieved by enclosing the magnet of the speaker inside a steel enclosure, or by placing a second magnet with reverse polarity behind the speaker to cancel the stray magnetic field.

What are the advantages of a subwoofer/satellite speaker system?

  • The subwoofer/satellite system is very easy to install, it requires less space than common speakers do and it matches easier with most decorations. In addition it can achieve excellent audio performance.
  • The subwoofer can easily hide in the room leaving only the small satellites visible.

What is the difference between two and three way speakers?

The crossover circuit of a two-way speaker splits the audio frequencies to two parts. The lower frequencies are driven to the woofer while the high frequencies are sent to the tweeter.

Three way speakers have a crossover circuit that splits the frequencies to three parts. The lows go to the woofer, the middle frequencies go to the middle frequency speaker and the highs go to the tweeter.

What is speaker sensitivity why is it important?

  • The sensitivity of a speaker indicates how efficiently the speaker converts the electrical signal to sound.
  • A more sensitive speaker plays music louder when it is driven by a specific signal. A sensitive speaker utilizes the power of an amplifier much more efficiently.
  • An increase of the sensitivity by 3 dB (for example from 91 dB @ 1W m to 94 dB @ 1 W m) entails doubling of the reproduced audio level from the same input signal! So weak amplifiers must be connected to sensi­tive loudspeakers. Be careful though not to be misled by size, as many small speakers have lower sensitivities than larger speakers

What is the use of speaker stands?

  • The low and middle frequencies are very directional. In addition speaker off axis response is inferior to on axis behavior. Loudspeakers therefore sound better when the tweeters are at the same height as the listener's ears.
  • Small speakers must be placed on appropriate stand to reach the appropriate height that brings their tweeters at ear level when seated.




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