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Live Sound R einforcement

Live Sound R einforcement. Audio measurements. Live Sound R einforcement. One of the most common terms you will come across when handling any type of live sound equipment is frequency response.

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Live Sound R einforcement

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  1. Live Sound Reinforcement Audio measurements

  2. Live Sound Reinforcement • One of the most common terms you will come across when handling any type of live sound equipment is frequency response. • Frequency response is a components ability to produce audio output within a particular frequency range.

  3. Live Sound Reinforcement • Sound equipment will have a graphed measurement of it’s output according to frequency in comparison to the input signal. • The input signal is measured in dB (decibels) and the output is graphed as a curve through the audio spectrum

  4. Live Sound Reinforcement • Here is a frequency response graph for a Shure condenser mic, it shows that the mic can reproduce the majority of the audio spectrum at 0dB.

  5. Live Sound Reinforcement • If you look at the left side of the graph, you’ll notice that the dB scale is a range from a negative number up to 0 and then upwards to the positive dB reading.

  6. Live Sound Reinforcement • Most of the frequency response measurements start at a negative dB input range and level out around 0dB. • In fact higher end stereo receivers use this same scale, if you have a receiver with this scale you'll notice that at 0dB it is very loud. • The positive side of this scale will not reach the same inverse negative value on the receiver.

  7. Live Sound Reinforcement • So why is 0dB the loud end of a volume scale? • 0dB is a reference point that indicates a components ability to still sound good without any distortion with full power delivered. • The volume control is an attenuator, it limits the power delivered to the speakers which is why it is a negative value.

  8. Live Sound Reinforcement • It is negative because with the volume knob you are controlling how much the signal is attenuated. • You are not turning your stereo up when you are turning it up -- you are throttling it less, at the zero point, you are not throttling it at all

  9. Live Sound Reinforcement • Anything above 0dB is essentially pushing the speakers ability to reproduce sounds with clarity, instead you start to hear distortion, we call this clipping since the top of the wave form is clipped off.

  10. Live Sound Reinforcement • Here is another chart but with a different input value, it is in dB SPL, what is SPL?

  11. Live Sound Reinforcement • Notice the curve runs along an SPL level similar to a home Hi Fi stereo, in this case 80dBSPL is the reference point.

  12. Live Sound Reinforcement • Here is a graph of a speaker, we can see the amplitude response or how much the frequency varies, at the beginning and end of the chart the frequencies drop off this is called roll off.

  13. Live Sound Reinforcement • Roll off is the point at which a component cannot reproduce certain frequencies, in this case it starts at about 30Hz and rolls off again at about 10KHz. • This means we will not hear anything beyond 10,000Hz coming out of the speaker, or more accurately it won’t sound good at all until it gets to where you’ll hear nothing.

  14. Live Sound Reinforcement • If we look at the frequency response of a sub woofer we would want to see it provide a flat line from 20Hz to a minimum of 200Hz the lower half of the frequency spectrum with the same input power. THIS CHART INDICATES THAT THIS SUB WOOFER WILL SOUND GOOD THRU THE 15 TO 200Hz RANGE AT 112SPL

  15. Live Sound Reinforcement • Now that we know how frequency response is determined what happens when we get further form the source of the sound? • The inverse square law says that each doubling of distance from the sound source results in a four fold reduction of sound power, equal to 6dB loss. • So if you move from 4’ to 8’ from a sound source the sound level is reduced by 6dB.

  16. Live Sound Reinforcement • What would the total dB loss be if you moved from 4’ to 32’ away from a sound source?

  17. Live Sound Reinforcement • Another factor to consider when talking about dB loss is attenuation in air. • The higher the frequency the greater the loss is over a long distance, conversely the lower the frequency the lower the loss. • Lower frequencies travel further because of the greater sound pressure pushing the air molecules.

  18. Live Sound Reinforcement • The way we measure amplitude of a signal is called RMS (root mean square). • RMS represents an average value to measure amplitude and it represents the equivalent intensity of DC electricity over a full cycle to describe the average strength of a wave.

  19. Live Sound Reinforcement • RMS value equals .707 or 70.7% times the peak amplitude of a wave form. • What is the RMS value of a peak amplitude of 100 volts?

  20. Live Sound Reinforcement • VU: volume units, based on the hearing process • PPM: peak program meter, supplements a VU meter to measure frequency spikes that the human ear ignores but could cause distortion in sound equipment. • Loudness levels are measured in PHONS.

  21. Live Sound Reinforcement • Another term that you may already be familiar with is impedance. • Impedance is total opposition to AC, that includes resistance, inductance and capacitance. • Speakers are rated in impedance and the unit of measure is ohms (Ω).

  22. Live Sound Reinforcement • That brings us to the various dB measurements used in sound reinforcement, (incidentally the dB is used through out all of low voltage to represent signal strengths). • In its simplest terms a dB is a comparison of two intensity levels on a logarithmic scale

  23. Live Sound Reinforcement • If we dig a little deeper the decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level. • A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities. • A decibel is one tenth of a bel, a seldom-used unit SEE THE LINK WHAT IS A DECIBEL FOR A MORE IN DEPTH UNDERSTANDING

  24. Live Sound Reinforcement • dBu is a measurement based on voltage used to quote normal operating levels and maximum capabilities of components. 0dBu = .775 volts RMS. • dBm is based upon a computation of power capabilities in low impedance circuits. 0dBm = 1milli-watt (.001watt RMS)

  25. Live Sound Reinforcement • dBV is a measurement based on voltage, dBV = 1 volt RMS. • dBv is also a measurement based on voltage but a dBv is .775 volts RMS. This is exactly the same as a dBu, however dBu has replaced dBv. • 0dBV = 2.2dBu/dBv.

  26. Live Sound Reinforcement • There are three basic gain structures and audio signal strengths can be grouped into basic intensity ranges; mic level, line level and power amp or speaker level. • Mic level have outputs measured in millivolts, mics range between -70dBu to -30dBu gain. • Line levels (mixers, EQs, etc.) range around +2dBu to +8dBu gain, remember that 0dBu is our reference point.

  27. Live Sound Reinforcement • Power amp output levels can be upwards to a 1000 watts (about 90 volts into 8Ω), • At each stage of the signal processing procedure we are boosting the signal strength from the microphone through the mixer and EQ and finally through the power amplifiers out to the speakers. • Louder is not better, clarity of the different frequencies and balancing of all the instruments is the key to great sound, other wise you just have varying levels of distortion.

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