Sound Transducers are devices that manipulate air that generates sound, whether audible or unheard, using electrical energy to create mechanical vibrations.
Sound is the generalized name given to "acoustic waves".These acoustic waves have frequencies ranging from just 1 Hz to tens of thousands of Hertz, and the upper limit of human hearing is in the range of 20 kHz (20,000 Hz).
The sound we hear consists mainly of mechanical vibrations produced by an Audio Converter used to create acoustic waves, and an environment is needed to be transmitted through air, liquid or solid in order for the sound to be "audible".
Also, real sound does not need to be a continuous frequency sound wave, such as a single tone or note, it can be an acoustic wave consisting of a mechanical vibration, noise or even a single sound pulse such as "explosion".
Audio Convertersinclude both input sensors that convert sound into electrical signals, such as microphones, and output actuators that recycle electrical signals into sound, such as speakers.
We tend to think that sound exists in the frequency range from 20 Hz to 20 kHz (a typical speaker frequency response), where only the human ear can detect it, but sound can go far beyond these ranges.
Audio converters can also detect and transmit sound waves and vibrations from very low frequencies called infra-sound to very high frequencies called ultrasound.But in order for an audio converter to detect or produce "sound", we first need to understand what sound is.
What is Sound?
Sound is basically an energy waveform produced by a kind of mechanical vibration, such as a tuning fork, and has a "frequency" determined by the source of the sound; for example, a bass drum has a low frequency sound. the bell has a higher frequency sound.
A sound waveform has the same characteristics as an electric waveform with Wavelength ( ε ), Frequency ( ε ) and Speed ( m/s ).Both the sound frequency and waveform are determined by the vibration or producing the sound originally, but the speed depends on the transmission medium (air, water, etc.) that carries the sound wave.The relationship between wavelength, speed and frequency is as follows:
Sound Wave Relationship
- Wavelength – The time period of a full cycle in seconds, ( ε )
- Frequency – The number of wavelengths per second in Hertz, ( ε )
- Speed – m/s is the speed of sound that passes through a transmission environment in -1.
Microphone Input Converter
The microphone is also an audio converter that can be considered an "audio sensor".This is because it produces an electrical analog output signal proportional to the "acoustic" sound wave that acts on its flexible aperture.This signal is an "electrical image" that represents the properties of the acoustic waveform.In general, the output signal from a microphone is an analog signal in the form of a voltage or current, which is proportional to the actual sound wave.
The most common types of microphones that can be used as audio converters are Dynamic , Electric Condenser, Ribbon and newer Piezo-electric Crystal types.Typical applications for microphones as an audio converter include audio recording, replication, broadcasting, as well as body scanners where phone, television, digital computer recording and ultrasound are used in medical applications.An example of a simple "Dynamic" microphone is shown below.
Dynamic Moving Coil Microphone Audio Converter
The structure of a dynamic microphone is similar to that of a speaker, but vice versa.It is a moving coil type microphone that uses electromagnetic induction to convert sound waves into electrical signals.A permanent magnet has a very small thin wire coil that hangs inside its magnetic field.When the sound wave hits the flexible diaphragm, the diaphragm moves back and forth in response to the sound pressure that acts on it, causing the connected wire coil to move within the magnetic field of the magnet.
The movement of the coil within the magnetic field causes a voltage to be induced in the coil, as defined by Faraday's Electromagnetic Induction Law.The output voltage signal from the coil is proportional to the pressure of the sound wave that acts on the diaphragm, so the louder or stronger the sound wave, the larger the output signal, making this type of microphone design pressure sensitive.
Since the wire coil is usually very small, the range of movement of the coil and the connected diaphragm is also very small, producing a very linear output signal that is 90 o out of phase according to the sound signal.Also, since the coil is an inductor with low impedance, the output voltage signal is also very low, so a kind of "pre-amplification" of the signal is required.
Since the structure of such a microphone resembles the structure of a speaker, it is also possible to use a real speaker as a microphone.
Obviously, the average quality of a speaker will not be as good as that of a studio-type recording microphone, but the frequency response of a reasonable speaker is actually better than a cheap microphone.In addition, the coil impedance of a typical speaker differs from 8 to 16Ω.Common applications where speakers are often used as microphones are in intercoms and radios.
Speaker Output Converter
Sound can also be used as an output device to produce a warning sound or act as an alarm, and speakers, bells, horns and sirens are all types of audio converters that can be used for this purpose with the most commonly used audibility type of output sound.
Speakers are audio converters that are classified as "audio actuators" and are the opposite of microphones.Their task is to convert complex electrical analog signals into sound waves as close as possible to the original input signal.
Speakers are available in all shape, size and frequency ranges with more common types: moving coil, electrostatic, isodynamic and piezo-electric.Moving coil type speakers are by far the most widely used speaker in electronic circuits, kits and toys, and therefore this type of audio converter that we will examine below.
The principle of operation of the Moving Coil Speaker is the opposite of the "Dynamic Microphone" that we examined above.A thin wire coil called a "speech or sound coil" hangs within a very strong magnetic field and is attached to a paper or Mylar cone called "diaphragm", and itself hangs on the edges of a metal frame.Then, unlike the microphone, which is a pressure-sensitive input device, this type of audio converter can be classified as an output device that produces pressure.
When an analog signal passes through the speaker's audio coil, an electromagnetic field is produced and its power is determined by the current flowing from the "sound" coil, which is determined by the sound control setting of the driving amplifier or by the moving coil driver.The electromagnetic force produced by this field resists the main permanent magnetic field around it and tries to push the coil in one direction or another, depending on the interaction between the north and south poles.
Since the sound coil is permanently connected to the cone/diaphragm, this also acts as a tandem and produces a sound or note, causing a distortion in the air around its movement.If the input signal is a continuous sinus wave, it will move in and out like a piston that pushes and pulls the air while the cone is moving, and a continuous single tone will be heard representing the frequency of the signal.The power and therefore the speed at which the cone moves and pushes the air around it produces the height of the sound.
Since the speech or sound coil is essentially a wire coil, it has an impedance value, such as an inductor.For most speakers, this value is between 4 and 16Ω and is called the "nominal impedance" value of the speaker measured at 0Hz or DC.
Keep in mind that it is important to always pair the amplifier's output impedance with the speaker's nominal impedance to achieve maximum power transfer between the amplifier and speaker.Most amplifier-speaker combinations have a low efficiency rating of 1% or 2%.
Although discussed by some, the choice of the good speaker cable is also an important factor in the efficiency of the speaker, since the internal capacitance and magnetic flux characteristics of the cable vary with the signal frequency, thereby causing both frequency and phase distortion.This has the effect of weakening the signal.In addition, large currents flow from these cables with high power amplifiers, so small thin bell wire type cables can overheat over long periods of use and again reduce efficiency.
The human ear can usually hear sounds from 20Hz to 20kHz, and the frequency response of modern speakers, called general purpose speakers, is adapted to work in this frequency range.
However, for high-performance high-efficiency (Hi-Fi) sound systems, the Frequency response of the sound is divided into different small sub frequencies, thereby improving both the efficiency of the speakers and the overall sound quality as follows:
Generalized Frequency Ranges
|Descriptive Unit||Frequency range|
|Alt-Woofer||10Hz to 100Hz|
|Bass||20Hz to 3kHz|
|Middle class||1kHz to 10kHz|
|Tweeter||3kHz to 30kHz|
In multi-speaker enclosures with a separate Woofer, Tweeter, and Mid-range speakers coexist in a single enclosure, a passive or active network of "switching" is used to ensure that the audio signal is accurately divided and reproduced by all different subunits.
This transition network consists of resistors, inductors, capacitors, RLC passive filters or op-amp active filters whose transition or cutting frequency point is finely tuned according to separate speaker characteristics, and the multi-speaker "Hi-fi" type design is given below.
Multi-Speaker (Hi-Fi) Design
In this tutorial, we looked at different Audio Converters that can be used to both detect and create sound waves.Microphones and speakers are the most commonly found audio converters, but there are many other types of audio converters that use piezoelectric devices to detect very high frequencies, hydrophones designed to be used underwater to detect underwater sounds, and sonar converters that both transmit and receive sound.