Sound management by lt ns wickramasinghe b sc e & e

SOUND MANAGEMENT

26 February 2014 [email protected]/[email protected]

what your ears capture Sound is a vibrationin an elasticmedium such as air,water, buildingmaterials and earth.

Sound is thephysical phenomenonthat encourages thesense of hearing. It isgenerated by vibratedbodies in the form ofwaves rarefaction ofcompression andrarefaction in the air.

- Wave Length

- Amplitude

Rarefaction

Compression


Interferencethere can be two kinds of interference patterns;

Constructive- Two waveforms are added together and creates a louder sound

Destructive - Occurs when two waves are out of phase and creates a diminished waveform


Standing WavesA standing wave is the result of the wave reflecting off the end of the tube

(whether closed or open) and interfering with itself and sound is produced in an instrument by blowing it, only the waves that will fit in the tube resonate, while other frequencies are lost.

The longest wave that can fit in the tube is the fundamental, other waves that fit are overtones


OvertonesOvertones are the other frequencies besides the fundamental that exist in

musical instruments. Instruments of different shapes and actions produce different overtones


Sound has two main characteristics;

Frequency – measured in Hz (Hertz)

Pressure level – measured in dB (decibels)


Quality of sound depend on following;

Pitch- Pitch is roughly equivalent to the frequency of a sound wave.Frequencies are measured in vibrations per second, or Hertz(Hz).Higher pitches vibrate more quickly, whereas low pitchesvibrate more slowly


Amplitude- determines the intensity, or loudness the size of the vibration


102 = 100

log10(100) = 2

In engineering the argument of the logarithm isalways a ratio of two quantities having the samedimensions. In mathematics the argument of alogarithm function is simply a number. Nodimensions involved.

Example


Gain Level10 log (x)

1 0 dB

2 3 dB

3 4 dB

4 6 dB

5 7 dB

6 7 . 7 dB

7 8.4 dB

8 9 dB

9 9 dB

10 10 dB

200 23 dB

100 20 dB

80 19 dB

40 16 dB

20 13 dB

16 12 dB26 February 2014 [email protected]/[email protected]

logb(mn) = logb(m) + logb(n)

logb(m/n) = logb(m) – logb(n)

logb(mn) = n · logb(m)


dB can be used to express the absolute level of the physical quantity. Thedecibel is also commonly used as a measure of gain or attenuation, theratio of input and output powers of a system, or of individual factors thatcontribute to such ratios.

Examples;

Voltage Acoustics Audio electronics

Antenna measurements

dBV(Rms) dB SPL dB(mW) dBi

dBmV(Rms) dB SIL dBTP dBd

dBμV dB SWL dBq

dB HL


The power P(dBm) in dBm is equal to 10 times the base 10 logarithm of the power P(mW) in mill watts (mW) divided by 1 mill watt (mW):

P(dBm) = 10 · log10( P(W) / 1mW)

dBmV (decibels relative to one mill volt) is a measure of the signal strength in wires and cables at RF and AF frequencies. This unit is defined in terms of root-

mean-square ( rms ) alternating current ( AC ) signal voltages in circuits in which the impedance is a pure resistance of some specified value.

P(dBmV) = 20 · log10( P(V) / 1mV)


dBm – dBm = dB

dBm + dB = dBm dBm – dB = dBm


1 dBm

0 dBm

3 dBm

+


What will happened when this knobs are rotate

0

3

6

9-9

-6

-3

0 dBm

3 dBm

6 dBm

9 dBm-9 dBm

-6 dBm

-3 dBm


Take two short audio samples and play them, Humanlisteners can detect the difference between two soundsources that are placed as little as three degrees apart, aboutthe width of a person at 10 meters. Or play them30microseconds apart


Convex Surface

Corners

Concave Surface


θ1 θ2

θ3 θ4P1P3

P1 = P2 = P3 θ1 = θ2 = θ3 = θ4

P2

Smooth Surface Absorption Surface

θ1 θ2

θ3 θ4

P1P2 P3

P1 > P2 > P3 θ1 = θ2 = θ3 = θ4

Diffusion Surface

P1

P2P3

P4P5

P1 = P2 = P3 = P4 = P5


The purpose of acoustic treatment is to improve the quality of sound in a room/studio.

Diffusers TrapsAbsorbers


Two long sides and two

short sides from planks of

planed (using 15mm thick,

125mm wide, 2.4m)

then glued and screwed the

corners.

Rockwool and foam fit

inside the frame.

To secure the 30 mm

Rockwool slab, then fitted

20mm wooden batten

around the inside of the

frame, set back from the

front edge by 35mm

It would sit 5mm behind

the front of the frame. This

meant that when the

Auralex foam was glued.26 February 2014 [email protected]/[email protected]

Main Isolation Window

Dry Wall

Isolation Wall

Shell Wall

2” Regiform Gap

2”x2” wooden poles

Cable Duct


Main Window Arrangement

Wooden poles


Dynamic Microphone - A paper cylinder, onto which fine copper wire iswound, is connected to a membrane which moves under the force of soundpressure created by the sound source. This coil is in a narrow gap with a highmagnetic field created by a permanent magnet. When the coil moves in thismagnetic field, it produces a voltage identical to the sound causing themovement.


Crystal Microphone - The crystal microphone uses a thin strip ofpiezoelectric material attached to a diaphragm. The two sides of thecrystal acquire opposite charges when the crystal is deflected by thediaphragm. The charges are proportional to the amount of deformationand disappear when the stress on the crystal disappears.


Ribbon Microphones - The airmovement associated with the soundmoves the metallic ribbon in themagnetic field, generating an imagingvoltage between the ends of the ribbonwhich is proportional to the velocity ofthe ribbon - characterized as a "velocity"microphone.

Condenser Microphones- Sound pressurechanges the spacing between a thinmetallic membrane and the stationaryback plate. The plates are charged to atotal charge


Directionality

Frequency Response

Impedance

Handling Noise

Transient Time

Sensitivity

Work through each of these characteristics and determine your needs


Omni directional

Captures sound equally from all directions.

UsesCapturing ambient noise; Situations

where sound is coming from many directions; Situations where the micposition must remain fixed while the sound source is moving.

Cardioid

Cardioid means "heart-shaped", which is the type of pick-up pattern these mics use. Sound is picked up mostly from the front,

but to a lesser extent the sides as well.

UsesEmphasising sound from the direction the mic is pointed whilst leaving some latitude

for mic movement and ambient noise.


HypercardioidThis is exaggerated version of the cardioid

pattern. It is very directional and eliminates most sound from the sides and rear. Due to the

long thin design of hypercardioids, they are often referred to as shotgun microphones.

UsesIsolating the sound from a subject or direction

when there is a lot of ambient noise; Picking up sound from a subject at a distance.

BidirectionalUses a figure-of-eight pattern and picks

up sound equally from two opposite directions.

UsesAs you can imagine, there aren't a lot of

situations which require this polar pattern. One possibility would be an

interview with two people facing each other (with the microphone between

them).26 February 2014 [email protected]/[email protected]

Frequency response refers to the way a microphone responds to differentfrequencies. It is a characteristic of all microphones that some frequencies areexaggerated and others are attenuated (reduced)

More importantly, it should be noted that a flat frequency response is not always the mostdesirable option. In many cases a tailored frequency response is more useful. For example, aresponse pattern designed to emphasise the frequencies in a human voice would be wellsuited to picking up speech in an environment with lots of low-frequency background noise.


All microphones have a specification referring to their impedance. This spec may be written on the mic itself (perhaps alongside the directional pattern), or you may need to consult the manual or manufacturer's website.

You will often find that mics with a hard-wired cable and 1/4" plug are high impedance, and mics with separate balanced audio cable and XLR connector are low impedance.

There are three general classifications for microphone impedance. Different manufacturers use slightly different guidelines but the classifications are roughly:

Low Impedance (less than 600Ω) Medium Impedance (600Ω - 10,000Ω) High Impedance (greater than 10,000Ω)

Note that some microphones have the ability to select from different impedance ratings.


Remember that the diaphragm works by converting vibrations fromsound waves into an electrical signal. Unless the microphone hassome sort of protection system, the diapragm can't tell thedifference between a desirable sound wave vibration and anyother sort of vibration (such as a person tapping the microphonecasing). Any sort of vibration at all will become part of thegenerated audio signal.

If your mic is likely to be subjected to any sort of handling noise orvibration, you will need a mic which will help prevent this noisefrom being picked up. High quality hand-held mics usually attemptto isolate the diaphragm from vibrations using foam padding,suspension, or some other method. Low quality mics tend totransfer vibrations from the casing right into the diaphragm,resulting in a terrible noise.


One of the most important factors in amicrophone is it's ability to respondto rapidly changing sound waves. Thisis known as "transient response".

The acoustic sound wave physicallymoves the diaphragm. The amount oftime it takes for this to happendepends on the weight of thediaphragm material. Of course, theresponse time is longer for theheavier diaphragm material of thedynamic to react over thelightweight, thinner condenserelement.

Transient time of Condenser Mic, Ribbon Mic and Dynamic Mic


Sensitivity expresses the microphone's ability to convertacoustic pressure to electric voltage. The sensitivity states whatvoltage a microphone will produce at a certain sound pressurelevel. A microphone with high sensitivity will give a high voltageoutput and will therefore not need as much amplification (gain)as a model with lower sensitivity. In applications with low soundpressure levels, a microphone with a high sensitivity is requiredin order to keep the amplification noise low.

According to the IEC 268-4 norm, the sensitivity is measured inmV per Pascal at 1 kHz (measuring microphones at 250 Hz).


TRS Balance Connector

XLR Balance Connector


TS Unbalance Connector

RCA (Radio Corporation of America)connector, sometimes called a phonoconnector or cinch connector, is a type ofelectrical connector commonly used to carryaudio and video signals.


XLR Balance Cable This means that much of the electromagneticinterference will induce an equal noise voltage ineach wire. Since the amplifier at the far endmeasures the difference in voltage between thetwo signal lines, noise that is identical on both wiresis rejected. The wires are also twisted together, toreduce interference from electromagneticinduction.


1/4″ on the other hand comes in two styles: balanced TRS (tip, ring, sleeve) and unbalanced TS (tip, sleeve).

To be continued with Public Address Systems


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Electrical and Electronic Engineer


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Sound management by lt ns wickramasinghe b sc e & e

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