The VU Meter ( Volume Unit Meter) is the simple analogue moving coil meter used in audio equipments to measure the intensity of sound in Volume Units(VU).The level of measurement is in dB and time in seconds. TheVolume unit is ZeroVU when One Milli Watt Sinewave power flows through a 600 ohms resistor at 1000 cycles per second.The design of the VU meter is based on the way by which human ear perceives sound in volume.VU meters are well suited to indicate continuous audio signals than the fast transient audio signals.
The typical VU scale range from -20 to +3 and the rise and fall time is 300 milli seconds.This means that when a sinewave of zero amplitide is applied to the VU meter, it will take 300 milli seconds to reach the zero scale.
Sound to RMS
RMS ( Root Mean Square) is the measure of average voltage levels of electric signals.The VU meter can display the approximate RMS voltage level in the audio signals. As a result, sustained audio signals give higher meter reading than the brief higher voltage, even though both of them have maximum voltage levels.The reading in the VU meter depends on both the amplitude and duration of audio signal peaks.The Fall back time ( time taken to return the needle to zero) is around 300 milli seconds.The RMS value closely resembles the average sound level perceived by the human ear.VU meter is sensitive to both positive and negative half cycles of audio signals, making it an ideal device to meaure the sound levels.
How it Works?
Working of VU meter is too simple. As a rule in Physics, when current passes through a coil, a magenetic field develops around it which is proportional to the current flowing through the coil. The VU meter has a coil made up of low guage copper wire wrapped around an iron core. The iron core with the coil is fixed so as to move freely in the meter. A needle is attached to the core which can indicate the audio level when the core move back and forth.The copper wire ends are connected to the terminal posts in the meter to provide audio signals.
The circular shaped core with the coil is placed around a permanent magnet. So when current flows through the coil,a magnetic field develops around the coil which repels the magnetic field of the permanant magnet. Since the core with coil is set to move freely, it move back and forth depending on the strength of magnetic field developed. That is, higher current creates high magnetic field leading to fast movement of the core. Since the needle is connected to the core, movement of the core causes reflection of the needle to indicate the levels of audio signals. With appropriate calibration, it is easy to measure the audio levels in terms of dB.
What is Decibel?
Decibel (dB) is the unit to measure the intensity of sound.A decibel is the 1/10 of a Bel. That is, 1 Bel is 10 deciBel. The Bel can be defined as the lagarithm of the ratio of two power quantities of 10:1 and for two field quantities in the ratio.The Field quanty can be voltage, current, electric field, velocity, charge density or sound pressure.The sqaure of field quantity in linear systems is proportional to the power quantity. Power quantity is the power or quantity directly proportional to the power such as energy density, sound intensity etc.