Audio Filters: The functioning and classification of speakers – Part 4

In the previous tutorial, we discussed the classification and working principle of microphones. We learned that the microphone is an input transducer that converts sound waves into electrical signals. These audio signals are then amplified by a preamplifier and transferred to a main audio unit.

The unit may include an amplifier and/or filter circuit, or have circuitry that connects to a computer to save and store the audio. The amplified or stored audio is played back using another type of transducer – a speaker.

A speaker is a type of output transducer that converts an electrical signal into an audio signal. Speakers are typically placed in a rectangular or square cabinet or box, although there are many types of boxes.

The cabinet and speaker

You may question the need for a cabinet or box to house a speaker, but it is an important element of this type of transducer.

Speakers produce sound through a diaphragm, a thin paper-like membrane that moves back and forth in the presence of a sound wave (you'll remember the diaphragm from the microphone tutorial). Now, the front surface of a speaker, which is typically shaped like a cone, will produce a high air pressure sound wave. However, the back surface of this cone reduces air pressure.

The sound generated by the front and rear speakers is the same, only the phase differs – lower frequency sound (coming from the back) produces a longer wavelength than higher frequency sound (coming from the front). The lower frequency sound wave will diffract more, essentially canceling out the front and back waves, which mutes the bass frequency.

So this is why there is always a need to properly encapsulate the speakers. But the shape of this box affects the sound quality (discussed below).

There are also different types of speakers/transducers, which produce different types of audio frequencies. Each transducer is called a driver and the cabinet is known as a speaker.

Since a single speaker is unable to provide a full range of audio frequencies, a speaker offers driver units to support the frequencies. Essentially, a speaker driver is an electro-acoustic component that makes a speaker work.

The three different driver units include:

1. Tweeter – for high frequency ranges
2. Mid-range driver – for mid-frequency ranges
3. Woofers – for low frequency ranges

Types of speakers
Speakers can be classified in several different ways, including based on amplification, construction, and enclosure.

Amplification
In terms of amplification, there are passive and active speakers.

• Passive speakers – do not have built-in power amplifiers and therefore must be connected to an external power amplifier to obtain high power output. Most speakers available on the market are passive, which allows for their small size.
• Active speakers – include a built-in power amplifier for high and low frequency sounds. Often called powered speakers, these assets live up to their name and are typically heavy and bulky. As a result, they are used less frequently than passive speakers.

Construction

Classification of speakers based on their construction includes conventional, electrostatic, and ribbon speakers.

Conventional speakers – the most common type of speaker. It consists of a voice coil connected to a diaphragm and a permanent magnet. When an oscillatory electrical current passes through the coil, it creates a magnetic force, thanks to the permanent magnet. This generates a mechanical movement in the diaphragm, which produces sound waves.

Electrostatic speaker – designed with its diaphragm between two rigid, perforated metal plates. The diaphragm, which is a thin metal sheet, has the opposite charge to the perforated metal plates. As a result, each plate attracts the diaphragm with a force equal and opposite to the total force on the diaphragm, which is zero.

However, this force changes when an audio signal is applied to the metal plates. Thus, when plate 'A' is more positive than plate 'B', the diaphragm moves towards plate 'A' and vice versa. Each time an audio signal changes sign, it causes a push-pull movement between the diaphragm and the plates. This “flexion” produces sound.

These types of speakers are less responsive to low-frequency signals and tend to be heavy and expensive. Not surprisingly, they are used less frequently than conventional speakers.

Ribbon speakers – composed of a tall, thin metallic ribbon (usually aluminum), which is suspended between two magnetic plates. When an audio signal is applied to the tape, it is repelled and attracted by the magnetic plates (similar to the way an electrostatic speaker's plates react with its diaphragm).

This generates a vibration that produces sound. These types of speakers are also less responsive to low-frequency signals, but because the ribbon is so light, it responds quickly to any signal change.

Casing

Speakers can also be classified based on the type of box or cabinet they have and there are five common types: enclosed/sealed, ported, open air, bandpass, and transmission line boxes.

Closed or sealed (acoustic suspension) – an airtight enclosure, so that when the driver moves back and forth, the air pressure will continually change. This increases pressure on the diaphragm, causing it to move back and forth rapidly – ​​producing clear, accurate audio.

Ported (bass reflex) – this type of cabinet has a hole in the front, which equalizes the pressure inside and outside the driver. While it does not require extra power to operate, it typically results in less clean or clear audio output. But this type of speaker has less distortion compared to other speakers.

Free air (infinite baffle) – the woofer is mounted on a baffle, which isolates the front wave from the rear. The deflector or board extends so far that even the longest wave can never reach the front wave. As this system cannot be implemented in practice, the free-air enclosure is typically a large, sealed box that requires low power and has little distortion.

Band pass – the speaker is placed in an enclosure and sound emanates from the integrated ports. There are different types of bandpass cabinets depending on the order (such as 4th, 6th, and so on).

In a 4th order bandpass, for example, one side of the speaker is placed in a closed enclosure while the other side is in a ported enclosure. In this case, the woofer sounds louder than the bass reflex, but has a narrower frequency response.

Transmission line (acoustic maze) – the back wave of this speaker passes through a complex maze (an irregular network path), which is of fixed length and dampens the sound. This allows for greater control and use of speaker power, providing a strong, low-frequency response that can reach the subsonic range (>45Hz).

Horn – This enclosure involves a horn-shaped casing that extends from a narrow to a wide diameter, facilitating the transit of sound waves through the speakers. Ideally, the size of the horn in this case should be suitable for the frequency range of the audio output. For example, a higher frequency should use a larger horn.

The horn housing is best suited for tweeters and mid-range drivers. For woofer drivers, folded horn enclosures can be used and take up less space.

These are the most common types of enclosures used for speakers, but there are several other options available. Examples include dipole, wall, push-pull, quarter and taper wave cabinets and others.

In the next tutorial, we will learn about the basics of audio filters.