AN3340

Overview

Doc ID 18453 Rev 1

3/24

2

Overview

2.1

What is a loud speaker

A loudspeaker, or often called a speaker drive, is an electro-acoustic transducer. It is able to

convert an electrical signal coming from an audio power amplifier into the movement of a

diaphragm. Hence the diaphragm of the speaker moves in accordance with the variations of

the electrical signal to cause pressure waves in the air which the listener detects as sound.

A lot of speaker models and designs are available on the market. The simplest and most

diffused solution consists of a single diaphragm which is able to reproduce a large portion of

the audio frequency range. This sort of device is named a “full-range speaker”. The

frequency response of a full-range speaker and its cost are driving a lot of TV manufacturers

to select this kind solution in a considerable part of their products, mainly in low and mid

level markets.

The audio performance of a full-range speaker can be improved using a multi-way speaker

system where two or more speakers are connected via a filter so that each speaker

reproduces only a specific portion of the audio frequency range. A very diffused example of

this solution is represented by a two-way speaker system where a large diameter speaker

(woofer) reproduces the low frequencies while another speaker manages the mid and high

frequencies. A further step is made using three speakers: a woofer for the low frequencies, a

mid-range speaker for the middle frequencies and a tweeter speaker to reproduce only the

high frequencies.

Of course for, a multi-way solution each speaker must be appropriately driven with the

correct portion of the audio band. For a three-way loud-speaker system, for instance, a low-

pass, a high-pass and a band-pass filter must be designed and implemented to properly

drive each loud speaker. These filters, called crossover filters, can be implemented with

passive or active components.

A passive crossover filter is made using only inductors and capacitors and it is connected

between the output of the power amplifier and the loud-speaker system. It is generally

placed in the speaker cabinet.

In the active solution the crossover filter is implemented using active components, such as

operational amplifiers or DSPs (in a digital system) and it is implemented just following the

pre-amplifier. In this solution each speaker is driven using a dedicated power amplifier. It

can be a very expensive solution.

2.2

Why compensate a loud speaker

In the flat-screen TV market the incessant tendency to reduce the thickness of the TV set

forces the speaker manufactures to design new components with very small dimensions.

The result of this process impacts negatively on the quality of the sound reproduction

because the frequency response of this sort of full-range speaker is very narrow and the

sensitivity is quite low.

In high quality flat-screen TV sets the loud-speaker system is often implemented with a

2.1-channel active solution. Here, two channels are dedicated to reproduce the mid and

high frequencies (for each left and right channel) while the lowest portion of the audio band,

usually below 200 Hz, is reproduced in mono using a single large speaker (a subwoofer)

enclosed in a suitable box. Each speaker is driven with a dedicated power amplifier (five