Vented enclosures often have complicated arrangements of shelves inside resulting in the sound from the driver having to travel through a tunnel before it emerges from the port. One type is called the "labyrinth" which means the shelves are parallel and horizontal, and another is called "transmission line", where the shelves are at angles other than horizontal. These enclosures are very popular because they allow smaller drivers to reproduce lower frequencies than they would with a simple vented enclosure. This is called "bass extension". The speaker cabinets usually look tall and thin, and quite elegant. The "horn loaded" speaker uses shelves which result in the sound coming out like a megaphone. They are very sensitive and are useful with low power amplifiers (this would be a good choice to consider if you have fallen in love with one of the single ended triode amplifiers described in the preceding section). Sensitivity is measured in terms of dB/watt/meter or dB/2.83volts/meter which means that a certain loudness (1 dB or decibel) will be achieved with a standard power (1 watt or 2.83 volts) at a standard distance from the speaker (1 meter). If you have a high powered amplifier (200 watts per channel), this is not a major concern, but if your amplifier is low powered (20 - 40 watts per channel), sensitivity becomes important. Speakers with ratings above 90dB/watt/meter are considered sensitive, while ratings below 90 are not as sensitive. Horn loaded speakers, for example, can reach levels above 100dB/watt/meter, and in the early days of motion pictures, theaters used this type of speaker because the amplifiers only had about a 10 watt output. Acoustic suspension speakers, on the other hand, are usually not very sensitive because the air inside the sealed box acts like a spring, retarding the speaker cone from moving outwards and resisting the cone moving inwards. Although they give up sensitivity, acoustic suspension speakers gain in bass "tightness" because of this spring-like control. However, sensitivity ratings can be somewhat misleading because the measurement will depend on the impedance variations of the speaker across its audio spectrum. Therefore, speaker sensitivity ratings should be viewed in a general sort of way, and not in absolute terms. It is easy to get caught up in comparing and matching specifications, so, as always, try out any combination in the store before purchasing it, regardless of what it says on paper.
Ribbon and Electrostatic Speakers
There are a handful of special design speakers which don't use cones. Instead, they have thin foils suspended between magnets or metal sheets. With a ribbon speaker, the musical signal is applied to a foil ribbon, and the varying electrical charge placed upon it by the music causes it to be attracted or repelled by the magnets, moving air in doing so, and thus reproducing the sound. A variation on this consists of a foil attached to a large flat membrane, and it is the membrane which is suspended. Such designs are called planar-magnetic (example shown on right). Electrostatic speakers (example shown on left), by comparison, have a plastic membrane, coated with something like powdered graphite, suspended between two perforated metal sheets. A positive voltage (several thousand volts) is connected to the membrane, and the musical signal, the voltage of which is increased by a transformer in the base of the speaker, is applied to the perforated sheets. The varying signal in the metal sheets attracts or repels the membrane, and the music is reproduced. These ribbon and electrostatic speakers reproduce midrange and upper frequencies with superb clarity, but are not very good at the low frequencies (below 100Hz). Therefore, they usually have standard cone type speakers in a separate cabinet at the base (see photos) to serve as woofers. Ribbon speakers, planar magnetic speakers, and electrostatic speakers are dipolar in nature.
Inside the speaker cabinet is an additional component, called the "crossover network". This is made of one or more resistors, capacitors, and inductors (see schematic diagram of a typical circuit on the right). The property of a capacitor is to pass high frequencies, but to impede low frequencies, while inductors pass low frequencies and impede high frequencies. Resistors are used to balance the loudness between the various drivers. The components are wired so that only high frequencies are sent to the tweeter, midrange frequencies sent to the midrange driver, and low frequencies to the woofer (in the case of a three way design). The crossover network, named for the fact that it crosses over frequencies to the proper driver, has connections to the binding posts on the rear of the speaker, so that the signal passes through the crossover network, and then to the speaker drivers. The property of sending low frequencies to the woofer is called "low pass", and the property of sending high frequencies to the tweeter is called "high pass". You may see these terms used, for example, in setting the low pass frequency that a subwoofer crosses over at, sending all signals below this frequency to the subwoofer amplifier, and the high pass frequency, above which, signals are sent back to the main speakers.