In the previous tutorial, working principle and classification of microphones was discussed. The microphone is an input transducer which converts sound waves into electrical signals. The audio signals from a MIC are amplified by a preamplifier and transferred to the main audio unit. The audio unit might comprise of amplifier and/or filter circuit or may have other circuitry to store audio to a computer. The amplified or stored audio is reproduced using another type of transducer which is called speaker.
The Speaker is a type of output transducer which converts the electrical signal into audio signal. Speakers are enclosed in a rectangular or square shape cabinet. The shape of the box affects the quality of the sound. The cabinet consists of different types of transducers/speakers which produce different types of audio frequency. Each transducer is called as ‘driver’ and the whole cabinet is known as ‘loudspeaker’.
Fig. 1: Typical Image of Speaker
As a single speaker is incapable to provide the whole range of audio frequency so the loudspeaker consists of different driver units which are as follow –
1) Tweeter (for High-frequency range)
2) Mid-range driver (for Middle-frequency range)
3) Woofers (for Low-frequency range)
Types of speaker
The speakers can be classified in many different ways. On the basis of amplification, they can be classified as follow –
1) Passive Speaker
2) Active Speaker
Passive Speaker – The passive speaker does not have built-in power amplifier. So they need to connect to an external power amplifier for high power output. Most speakers that are commercially available are passive type due to their small size.
Active Speaker – This type of speaker has inbuilt power amplifier for high and low frequency. So they are also called as Powered Speakers. But they are bulky as compared to the passive one so they are less used commonly.
On the basis of the construction, speakers are classified as follow –
1) Conventional Speaker
2) Electrostatic Speaker
3) Ribbon Speaker
Conventional speaker – The dynamic speaker is the most common type of speaker. This consists of a voice coil which is attached to the diaphragm (a very thin membrane like a paper or plastic) and with a permanent magnet. When an oscillatory electric current is passed through the coil, due to the strong magnetic field of the permanent magnet an oscillatory magnetic force is developed on the coil. This generates a mechanical motion to the diaphragms which produces the output sound wave.
Fig. 2: Image showing Construction and Working of Conventional Speakers
Electrostatic Speaker – In this type of speaker, the diaphragm is wide and placed in between two rigid perforated metal plates and the diaphragm is covered with a very thin metal sheet. The diaphragm and the two perforated metal plates are oppositely charged, each metal plate attracts the diaphragm with an equal and opposite force to the total force on the diaphragm is zero. But this force changes when an audio signal is applied to the metal plates. When plate ‘A’ is more positive than plate ‘B’ then the diaphragm moves towards plate ‘A’ and vice versa. Each time the audio signal changes its sign, this creates a push-pull movement to the diaphragm and this flexing produces the sound. These kind of speakers respond less to low-frequency signals; additionally, they are bulky and costly so their use is less as compared to conventional speakers.
Fig. 3: Image showing Construction and Working of Electrostatic Speakers
Ribbon speakers – These speakers have a tall, thin and narrow ribbon of very thin metal like aluminum which is suspended in between two magnetic plates. When an audio signal is applied to the ribbon it is repeled and attracted by the magnetic plates. This generates a vibration and thus produces sound. These type of speakers respond less to low-frequency signals, but due to the lightweight and low mass of the ribbon, they respond fast to the signal changes.
Fig. 4: Image showing Construction and Working of Ribbon Speakers
The speakers are also classified on the basis of their casing. On the basis of the enclosure, they are classified as follow –
1) Closed Enclosure
2) Ported Enclosure
3) Free Air Enclosure
4) Band Pass Enclosure
5) Transmission Line Enclosure
Need of Enclosure –
The speakers or drivers produce sound by the diaphragm which is moving back and forth. The front surface of the cone of the speaker when move forward, produces a sound wave of high air pressure but the back surface cone lowers the air pressure. The sound radiated from back and front of the speaker is same except their phase. The lower frequency sound wave has large wavelength than high-frequency sound wave, that’s why the lower frequency sound wave diffracts more and cancel out the back and front waves. This silences the bass frequency. That is why there is always need to properly enclose the speakers.
Closed / Sealed (Acoustic Suspension) Enclosure – This is an airtight enclosure in which the air from the enclosure is not escaped out. When the driver moves back and forth, it will constantly change the air pressure in the driver. This will put more pressure on the diaphragm, which takes extra power. But this extra pressure makes the diaphragm move back and forth faster, which gives crisper and more accurate sound.
Fig. 5: Image of Closed or Sealed (Acoustic Suspension) Enclosure for Speakers
Ported (Bass Reflex) Enclosures – In this enclosure there is a hole in front of it which equalizes the pressure inside and outside of the driver. So it will not take extra power. But it will result into less accurate output, thus the low-frequency signals are reproduced less precisely. Usually, this type of enclosure has less distortion compared to other enclosures.
Fig. 6: Image of Ported or Bass Reflex Enclosure for Speakers
Infinite Baffle / Free-Air Enclosures – In this type of enclosure, the woofer is mounted on a baffle which isolates the front wave to the back wave. The baffle or the board extends so much that even the longest wave can never meet from the front wave. However, practically this system cannot be implemented. In real life, the infinite baffle is merely a large sealed box. It needs low power and has less distortion.
Fig. 7: Image of Infinite Baffle or Free Air Enclosure for Speakers
Band Pass Enclosures – The band pass enclosure has different types depending upon the order like 4th, 6th and so on. The speaker is placed in the cabinet and the sound comes out from the ports.
Like in a 4th order band pass, one side of the speaker is placed in a closed enclosure while another part is in a ported enclosure. In this type of enclosure, the woofer plays louder than bass-reflex but has a narrow frequency response.
Fig. 8: Image of Band Pass Enclosure for Speakers
Transmission Line Enclosures – This enclosure will make a labyrinth (an irregular network of the path)at the back of the speaker. The back wave of the speaker passes through this labyrinth which has fixed length, which is related to the resonant frequency of speaker in the free air. So coming waves will be in the phase to the front waves of the speaker. This way it has a great low-frequency response which even can reach to the subsonic range of frequency (>45Hz).
Fig. 9: Image of Transmission Line Enclosure for Speakers
Horn Enclosure – In this type of enclosure, there is a horn shaped casing extending from narrow diameter to broader diameter. In such enclosure, it is easier for sound waves to transit through the speakers. The size of the horn in such enclosure must suit to the frequency range of the audio output. Greater is the frequency, larger should be the size of the horn. So, this enclosure type best suit for tweeter and mid range drivers. For woofer drivers, folded horn enclosures can be used. By folding, the enclosure takes less space despite working as a horn enclosure.
Fig. 10: Image of Straight and Folded Horn Enclosure for Speakers
There are various other types of enclosure out there like dipole enclosure, quater wave enclosure, tapered quater wave enclosure, in wall enclosure, push pull enclosure etc. The most common type of enclosures are discussed here.
In the next tutorial, the basics of audio filters will be discussed.
