UNIT-1
Audio Fundamentals and Devices: Basic Characteristics of sound signal, Microphone –
working principle, sensitivity, nature of response, Types of microphone, loud speaker- working
principle, Woofers and Tweeters, Characteristics. Types of loud speaker. Sound recording.
1.1 LOUDSPEAKERS
A loudspeaker is a transducer which converts electrical signals of audio frequency into sound
waves of the same frequency. It is also called as output transducer or reverse transducer.
A loud speaker's performance is determined by the following characteristics:
1.2 CHARACTERISTICS
Efficiency:
✔ It is defined as the ratio of output sound power to the input audio (electrical power).
✔ Its value depends on proper matching of the mechanical impedance with acoustical
impedance of the air volume being disturbed. (Some manufacturers quote the efficiency in
terms of sensitivity which is defined to be the input signal required to give a sound pressure
level of 0.1 Pa or 1 microbar at a distance of 1 metre from the loudspeaker.)
Noise:
✔ The unwanted sound, not contained in the input signal but present in the output of a
loudspeaker is called noise produced by the loudspeaker (the mechanical parts may vibrate
at some resonant frequency, causing noise).
✔ Signal-to-noise ratio or SNR of the system which is de- fined as ratio of signal output to
the ‘output of noise in the absence of signal’. Frequency Response: It indicates the
loudspeaker's response for the audible frequency range of sound. Ideally, the response of a
loudspeaker should be flat within ± 1 dB for the frequency range of 16 Hz to 20 kHz.
✔ The mass of the diaphragm assembly have high frequencies which are attenuated; and due
to series compliance, low frequencies are attenuated. Moreover, the movable system may
have some natural resonant frequency within the audible range and the output at that
frequency will be emphasized.
Distortion:
✔ Any change in frequency, phase and amplitude complexion of the output sound as
compared to the input audio signal is called distortion.
, ✔ Frequency and phase distortions may result due to mass and compliance effect. Amplitude
or non-linear distortion will result due to non-uniformity in the magnetic field in which the
coil moves.
Directivity:
✔ It is the ratio of actual sound intensity at a point (in the direction of maximum intensity) to
the sound intensity that would have been available there, had the loudspeaker been
unidirectional.
Power :
✔ It is the maximum audio power (indicated in watts) for which it is designed. Power more
than the maximum will damage the speaker.
Impedance :
✔ The input impedance of the loudspeaker is represented in ohms and is an important
parameter, as its matching with the impedance of source amplifier is necessary for the
optimum efficiency.
1.3 CRYSTAL LOUDSPEAKER
Fig 1.1 Crystal type speakers
✔ Rochelle-salt crystals have the property of becoming physically distorted when a voltage
is applied across two of their surfaces.
✔ This property is the basis of the crystal type of speaker driver.
✔ The crystal is clamped between two electrodes across which the audio frequency output
voltage is applied.
✔ The crystal is also mechanically connected to a diaphragm. The deformations of the crystal
caused by the audio frequency signal across the electrodes cause the diaphragm to vibrate
and thus to produce sound output.
, ✔ Crystal speakers have been impractical for reproduction of the full audio-frequency range
because the input impedance is almost completely capacitive. Thus it is difficult to couple
Power into them.
✔ At high audio frequencies, the reactance becomes lower and the relative amount of power
smaller.
✔ In the base range, stresses on the crystals are very great, and the crystals have been known
to crack under stresses.
1.4 DYNAMIC LOUDSPEAKER
✔ To provide very strong magnetic field for high wattage speakers, an electromagnet is used
instead of a permanent magnet. . Its construction is shown in Fig. 1.2.
✔ Loudspeakers of more than 25 watts and up to a few hundred watts are of the
electrodynamics type.
✔ The strong and steady magnetic field is produced by a large field coil wrapped around a
core.
✔ The shape of the magnet is pot type with the south pole in the centre and the north pole
in the periphery.
✔ The special shape of the core allows magnetic flux to remain concentrated in the annular
gap between pole pieces.
✔ The voice coil is wound on fiber or aluminum (to keep it light in weight). It is placed in the
annular gap.
✔ The audio signal from the amplifier's output transformer is applied to the voice coil. This
signal causes a varying magnetic field.
✔ The resultant interaction between the magnetic fields (one due to electromagnet and the
other due to audio current in the voice coil) produces mechanical vibrations (motor action)
in the coil assembly, which correspond to the audio signals.
✔ The vibrations of the coil are transmitted to the attached cone which create sound waves in
the air in the listeners' area, and hence radiate sound energy directly.
, Fig. 1.2 Dynamic Loudspeaker
Advantages
✔ Higher power can be obtained
✔ Frequency response is better (40 Hz to 5000 Hz)
Disadvantages
✔ Power supply needed for field coil
✔ Heavier weight for the same amount of magnetic field
1.5 ELECTROSTATIC {CONDENSER/CAPACITOR) LOUDSPEAKERS
✔ This type of speaker operates on the principle that a dc voltage between two parallel metal
plates causes these plates to attract or repel each other.
✔ The amount of attraction or repulsion depends on the applied voltage l f one of the plates
is a flexible metal, it will bend.
✔ But the amount of attraction and repulsion is not directly proportional to the voltage
applied.
✔ For example, considering the movable and fixed plates of Fig1.3 with no voltage applied.
Now suppose we apply a slowly varying ac voltage to both plates.
✔ As the voltage increases from zero the potential difference between the two plates also
increases.
✔ This in turn produces an increasing force of attraction between the plates, so that the
movable plate tends towards the fixed plate.
✔ As the ac voltage decreases once more to zero, the attractive force decreases, and the
movable plate moves back to its original position.
Audio Fundamentals and Devices: Basic Characteristics of sound signal, Microphone –
working principle, sensitivity, nature of response, Types of microphone, loud speaker- working
principle, Woofers and Tweeters, Characteristics. Types of loud speaker. Sound recording.
1.1 LOUDSPEAKERS
A loudspeaker is a transducer which converts electrical signals of audio frequency into sound
waves of the same frequency. It is also called as output transducer or reverse transducer.
A loud speaker's performance is determined by the following characteristics:
1.2 CHARACTERISTICS
Efficiency:
✔ It is defined as the ratio of output sound power to the input audio (electrical power).
✔ Its value depends on proper matching of the mechanical impedance with acoustical
impedance of the air volume being disturbed. (Some manufacturers quote the efficiency in
terms of sensitivity which is defined to be the input signal required to give a sound pressure
level of 0.1 Pa or 1 microbar at a distance of 1 metre from the loudspeaker.)
Noise:
✔ The unwanted sound, not contained in the input signal but present in the output of a
loudspeaker is called noise produced by the loudspeaker (the mechanical parts may vibrate
at some resonant frequency, causing noise).
✔ Signal-to-noise ratio or SNR of the system which is de- fined as ratio of signal output to
the ‘output of noise in the absence of signal’. Frequency Response: It indicates the
loudspeaker's response for the audible frequency range of sound. Ideally, the response of a
loudspeaker should be flat within ± 1 dB for the frequency range of 16 Hz to 20 kHz.
✔ The mass of the diaphragm assembly have high frequencies which are attenuated; and due
to series compliance, low frequencies are attenuated. Moreover, the movable system may
have some natural resonant frequency within the audible range and the output at that
frequency will be emphasized.
Distortion:
✔ Any change in frequency, phase and amplitude complexion of the output sound as
compared to the input audio signal is called distortion.
, ✔ Frequency and phase distortions may result due to mass and compliance effect. Amplitude
or non-linear distortion will result due to non-uniformity in the magnetic field in which the
coil moves.
Directivity:
✔ It is the ratio of actual sound intensity at a point (in the direction of maximum intensity) to
the sound intensity that would have been available there, had the loudspeaker been
unidirectional.
Power :
✔ It is the maximum audio power (indicated in watts) for which it is designed. Power more
than the maximum will damage the speaker.
Impedance :
✔ The input impedance of the loudspeaker is represented in ohms and is an important
parameter, as its matching with the impedance of source amplifier is necessary for the
optimum efficiency.
1.3 CRYSTAL LOUDSPEAKER
Fig 1.1 Crystal type speakers
✔ Rochelle-salt crystals have the property of becoming physically distorted when a voltage
is applied across two of their surfaces.
✔ This property is the basis of the crystal type of speaker driver.
✔ The crystal is clamped between two electrodes across which the audio frequency output
voltage is applied.
✔ The crystal is also mechanically connected to a diaphragm. The deformations of the crystal
caused by the audio frequency signal across the electrodes cause the diaphragm to vibrate
and thus to produce sound output.
, ✔ Crystal speakers have been impractical for reproduction of the full audio-frequency range
because the input impedance is almost completely capacitive. Thus it is difficult to couple
Power into them.
✔ At high audio frequencies, the reactance becomes lower and the relative amount of power
smaller.
✔ In the base range, stresses on the crystals are very great, and the crystals have been known
to crack under stresses.
1.4 DYNAMIC LOUDSPEAKER
✔ To provide very strong magnetic field for high wattage speakers, an electromagnet is used
instead of a permanent magnet. . Its construction is shown in Fig. 1.2.
✔ Loudspeakers of more than 25 watts and up to a few hundred watts are of the
electrodynamics type.
✔ The strong and steady magnetic field is produced by a large field coil wrapped around a
core.
✔ The shape of the magnet is pot type with the south pole in the centre and the north pole
in the periphery.
✔ The special shape of the core allows magnetic flux to remain concentrated in the annular
gap between pole pieces.
✔ The voice coil is wound on fiber or aluminum (to keep it light in weight). It is placed in the
annular gap.
✔ The audio signal from the amplifier's output transformer is applied to the voice coil. This
signal causes a varying magnetic field.
✔ The resultant interaction between the magnetic fields (one due to electromagnet and the
other due to audio current in the voice coil) produces mechanical vibrations (motor action)
in the coil assembly, which correspond to the audio signals.
✔ The vibrations of the coil are transmitted to the attached cone which create sound waves in
the air in the listeners' area, and hence radiate sound energy directly.
, Fig. 1.2 Dynamic Loudspeaker
Advantages
✔ Higher power can be obtained
✔ Frequency response is better (40 Hz to 5000 Hz)
Disadvantages
✔ Power supply needed for field coil
✔ Heavier weight for the same amount of magnetic field
1.5 ELECTROSTATIC {CONDENSER/CAPACITOR) LOUDSPEAKERS
✔ This type of speaker operates on the principle that a dc voltage between two parallel metal
plates causes these plates to attract or repel each other.
✔ The amount of attraction or repulsion depends on the applied voltage l f one of the plates
is a flexible metal, it will bend.
✔ But the amount of attraction and repulsion is not directly proportional to the voltage
applied.
✔ For example, considering the movable and fixed plates of Fig1.3 with no voltage applied.
Now suppose we apply a slowly varying ac voltage to both plates.
✔ As the voltage increases from zero the potential difference between the two plates also
increases.
✔ This in turn produces an increasing force of attraction between the plates, so that the
movable plate tends towards the fixed plate.
✔ As the ac voltage decreases once more to zero, the attractive force decreases, and the
movable plate moves back to its original position.
