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SOUND INSTALLER'S GUIDE

SHURE BROTHERS INCORPORATED 222 Hartrey Avenue

Evanston, Illinois 60204

Copyright 1975, Shure Brothers Incorporated AL507 Printed in U.S.A.

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RELATIVE LOUDNESS LEVELS OF COMMON SOUNDS

I~)\ •• SPL / POWER AT EAR (watts/cm~)

DECIBELS

140 Strong rock vocal (1" from 50-hp siren (100') microphone)

Threshold of pain 130 10-3

Pneumatic chipper (5') Jet takeoff (200')

200 120 10 '~

Loud rock music (on stage)

Hammering on steel plate (2')

110 10·:; Thunder Boiler factory

Can-manufacturing plant 20 100 Heavy truck (90')

Subway passing

10.0 Riveting machine (35')

Very loud music (classical)

90 10-7 10-hp outboard motor ,(50')

Heavy street traffic (5')

Inside bus Loud music (classical)

2 80 10.8

Stenographic room Heavy traffic (25-50') Average factory Speech (1')

::)

Average automobile 70 10-9

Conversation (3') Department store Noisy office

Background music Average office 0.2 60 10-10

Quiet residential street

50 10.11

Average residence Minimum street noise

0.02 40 10-12 Very quiet radio at home

Very soft music

30 10-13 Country house

Quiet auditorium

Quiet whisper (5') 0.002 20 10-14

Quiet sound studio background noise level

Leaves rustling 10 10.111

Anechoic room

) Threshold of hearing

0.0002 0 10·J (I

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TABLE OF CONTENTS

1a. dB to voltage ratio, dBM to dBV conversion factor .... . ... . . ........ ........ .. ..... . . . . . 4

1b. dB to power ratio... .. .... .. .. . . ......... .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5

2. Microphone power rating, voltage and impedance . ... .. ....... . .... .. .. . ..... .... . ... .. 6

3. Microphone output voltage, dB SPL, and voltage rating .. .. .. .... . ... ... . ..... . .... .. . ... 7

4. Amplifier output power, voltage, and load impedance, power formula .. ... . ... . ....... ... . 8

5. Constant voltage 70.7 and 25 volt speaker load, power to impedance . .... ... . ..... . ...... . . 9

6. Measuring input and output impedances, formulas and test circuits ..... .. . ......... . . 10 & 11

7a. Speaker line resistance, length and gauge . . ... .. .. . . . . . .. ........... . .... . .. ... .. . . 12

7b. Power available at speaker after line loss ....... ... . . . . . .... .. . .... . .......... ... .. 13

8. Balanced low impedance attenuator ........ .. ... .. ... . . .. .. ... . .... ... . ..... .... .. . . ... 14

9. Audio connector wiring (3 pin and phone plug) . . . . .... . ... . ..... . . . ............... . . ... 15

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dB TO VOLTAGE RATIO

VOLTAGE RATIO

3 4

30 32 34 J6

DECIBELS Je "0 ~ ~

(FIGURE 1a)

1a. The above chart converts decibels (dB) to a voltage ratio.

Example: An amplifier that has a voltage gain of 10 times (the output volt­age is 10 times larger than the input voltage) has a gain of 20 dB (from the chart). The formula for finding a dB voltage ratio is:

E1 20 log10 E2

Note that the dB ratio can be negative. This means that the ratio is less than unity. A -6 dB change represents a voltage ratio of 112. The chart shows a 6 dB change as a 2 times change. If the voltage ratio numbers are inverted (2 becomes 112) the dB's are read as being negative. Since dB's are only a ratio, using them requires a reference. The two com­monly used dB references are: one volt (OdBV) and one milliwatt (OdBm). dBV: The dB reference for dBV is one volt with any load impedance. A + 10 dBV figure is (10 dB above 1.0 volts) 3.2 volts. dBm: The dB reference for dBm is one milliwatt. To convert a dBm figure to volts requires a known load impedance. This impedance is generally, but not always, 600 ohms. Some ac voltmeters have scales calibrated in dBm. These scales are only valid when measuring the voltage across a 600 ohm load. The voltage for 0 dBm (1 mW) across a 600 ohm load is 0.77SV. VU me-

,,- .

ters are generally calibrated for a 0 reading at +4 dBm on a 600 ohm load. C) This is the same as 1.228V. J Converting dBm to dBv: To change a dBm reading on a 600 ohm load to dBV, subtract 2.218 from the dBm reading. Example: +4 dBm (across a 600 ohm load only) = 4 - 2.218 = 1.782 dBV. .

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dB TO POWER RATIO

POWER RATIO

3 4

DECIBELS

(FIGURE 1b)

1b. The above chart converts decibels (dB) to a power ratio. Example: Find the dB difference between a 50 watt and a 100 watt amplifier The power ratio is

100 2 t' - == Imes 50 This is a 3 dB (from the chart) change. The formula for finding a dB power ratio is:

Pl 10 10glo P;

Note that a 6 dB voltage change is the same as a 6 dB power change. The ratios are different because power is proportional to the voltage squared. This 6 dB change was produced by a doubling (2 times ratio) of voltage, re­sulting in a quadrupling (22 = 4 times ratio) of power.

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MICROPHONE POWER, VOLTAGE AND IMPEDANCE

OPEN CIRCUIT

VOLTAGE RATING

dBV -110

-60

-40

-30

- 20

dBM POWER RATING

10 I'b

(FIGURE 2)

IMPEDANCE IN OHMS EIA

RATING NOMINAL

100 ,000 80,000

60.000

40,000

20,000

10,000 8000

6000

4000

2000

1000 800

600

60

40

2

2. The above nomograph converts between microphone output level power rating, output impedance, and open circuit output voltage. The two power ratings are: dB referenced to 1 milliwatt (0 dBm) with 10 microbars of sound pressure using the nominal microphone impedance, and the EIA rating in dB relative to 1 milliwatt (0 dBm) with .0002 dynes per square centimeter of sound pressure using the EIA rating microphone impedance. Using either of the microphone output level power ratings and the imped­ance, the open circuit output voltage rating in dB relative to one volt (dBV) can be found. Example: A 150 ohm microphone rated at -58 dB (relative to 0 dBm for 10 Ilb). A line from the 150 ,ohm impedance through the -58 dBm/10 Ilb intersects the open circuit voltage at -80 dBV. Example: A 15,000 ohm, -150 dB EIA rated microphone. The impedance used is the nearest EIA rating impedance (as indicated by the triangles): 9,600 ohms. A line from 9,600 ohms through the -150 dB EIA gives a -60 dBV open circuit voltage rating.

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MICROPHONE OUTPUT VOLTAGE, dB SPL, AND VOLTAGE RATING

dav ACTUAL OPEN CIRCUIT OPEN

VOLTAGE CIRCUIT SOUND RATING OUTPUT PRESSURE

VOLTAGE dBV Ilb dB SPL

0 10,000 lS4

-20 134

-40 114

-60 94

-60

-80 O.1mV -80 1.0 74

10ftv -100

-100 1.0ftv -120 0.1 S4

O.1ftV -140

-120 .01 34

-140 .001 14 (FIGURE 3)

3. The above nomograph is used to find the actual open circuit output voltage from a microphone for a given sound pressure and microphone open circuit voltage rating. The nomograph can also be used to find the input signal to noise ratio (SIN) if the microphone preamplifier equivalent input noise (EIN) is also known. Example: A microphone with an open circuit voltage rating of -80 dBV (as found by using the previous nomograph) is used at a sound pressure of 134 dB SPL (1,000 J.'b). A line through these two points crosses the actual open circuit output voltage at 0.1V (-20 dBV). This output voltage may approach the input clipping level of the microphone preamplifier. If the input clipping level of the preamp is only 0.03V (-30 dBV) then an input atten­uator of at least 10 dB is required to avoid overloading (clipping) the preamp. On the other hand if the preamp input clipping level is 1.0V (0 dBV), then a margin (headroom) of 20 dB exists. Signal to noise (SIN) considerations: a typical low noise preamp has an equivalent input noise (EIN) of -129 dBV. In this case the input signal (actual open circuit output voltage) was -20 dBV. The input SIN is 129 - 20 = 109 dB, a very good figure. Example: The same microphone (-80 dBV) used at a much lower sound pressure level, 74 dB SPL (1 J.'b). The nomograph gives an actual open circuit output voltage of 0.1 mV (-80 dBV). This is well below the input clip­ping level of preamps, but the signal to noise ratio is poor. The input SIN for the same preamp is 129 - 80 = 49 dB. This low signal to noise ratio is marginal and may be unacceptable for some applications. The only solution to improve the SIN is to move the microphone closer to the sound source, or to have the sound source increase its output (sing or play louder).

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AMPLIFIER OUTPUT POWER, VOLTAGE, AND LOAD IMPEDANCE

WATTS

1000 900

800

700

600

500

400 350

ffi 300 ~ 250

2 200

150

100 75 SO

25

10

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/ /' / / . /..,/ /~ ..... ----

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V /

/ / / /

/ 7 / /

v / /"

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V -'" 17

L ~ / .... /

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~ --~ ~ 5 1 o

10 20 30

VOLTS

(FIGURE 4)

2n 4n

/ /' / ./

/' V

/' ./ Sn

1/ L v

./ V

./ ~ 16n

./ ----~

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----- 64n

-------so 60 70

4. Amplifier output power (P) in watts vs. amplifier output voltage (E) for various speaker load impedances (2).

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Example: To deliver 100 watts of power to a 4 ohm speaker load requires 20 volts rms.

P E2 =z

E = ...;pz

E = -v100 x 4

E = 20 volts rms

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CONSTANT VOLTAGE 70.7V AND 25V VOLT SPEAKER LOAD POWER TO IMPEDANCE

POWER LOAD AT FULL OUTPUT

1000~ ~ffit~9J~~~~~~=S~~~~~~~SEff~~~m5ER~~~ 7

6 ~

3

2

1000 9 8 7 6

i 'III -' 10~ w 8 Z 7 ::::; 6

4

3

2

10 . 9 8 7 6

5

4

3

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POWER IN WATTS

(FIGURE 5)

5. The above chart is used to convert 70.7V and 25V constant voltage speaker loads from watts to impedance.

Example: A 10 watt load on a 70.7V line. The 10 watt vertical line inter­sects the 70.7V curve at the 500 ohm horizontal line. The load impedance is 500 ohms.

Note that if the impedance of a constant voltage line is measured, either with an impedance meter or by the technique described under input imped­ance, the amplifier ~ower requirements can be verified by the chart.

Example: An unknown constant voltage speaker line measures 10 ohms. From the chart, note that the 10 ohm horizontal line intersects the 25V curve at 62.5 watts and the 70.7V curve at 500 watts. This 10 ohm line is probably a 25 volt line.

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AF OSC

6a.

10n

MEASURING INPUT IMPEDANCE (Z)

z = RE2 El - E2

R

IN

(FIGURE 6a)

TEST DEVICE OUT

NORMAL LOAD

The figure above shows the connections and voltage test points for meas­uring the input impedance of a device. If the device under test is an ampli­fier or powered unit, it should be turned on. If the device is passive, such as an equalizer or filter, it should be terminated with its normal load. The test tone from a 600 ohm audio frequency signal generator (oscillator) is connected to the 1 kilohm and 10 ohm resistors to provide a low impedance source (10 ohms). Note that the 600 ohm line output and test tone from a Shure SR101 Audio Console or M67 Microphone Mixer may be used as the tone oscillator. The output of the oscillator is adjusted to approximately 1V (or +4 dBm, 0 VU). Choose resistor R to be about the same size as the input impedance to be measured.

Example: Measure a low impedance microphone preamp input.

Choose R = 150 ohms

Measure El = 10 mV, E2 = 8 mV

RE2 z = El - E2

z = (150) (8) = 600 ohms (10 - 8)

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NORMAL SOURCE

MEASURING OUTPUT IMPEDANCE (Z)

El - E2 Z = RE2

Note: If El is much greater than E2 (at least 10 times greater) then the above equation simplifies to

IN

Z =

TEST DEVICE OUT

•

R

1 AF OSC

NO INPUT SIGNALS

c_ (FIGURE 6b)

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6b. The figure above shows the connections and voltage test points for meas­uring the output impedance of a device. If the device under test is an am­plifier or powered unit, it should be turned on. If the device is passive, such as an equalizer or filter, its input should be terminated in its normal source. The test tone is connected to the resistor R and is adjusted to approxi­mately 1V (or +4 dBm, 0 VU). Choose R small enough to be able to measure E2, yet large enough to avoid distorting the oscillator output.

Example: Measure the output impedance of an amplifier.

Choose R = 1,000 ohms

Measure El = 1V, E2 = .01V

Note since El is much greater than E2, the simplified equation may be used.

Z = -El

Z = (1000) (.01) = 10 ohms 1

11

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Cii w a: A. 0 0 ... (,) c w z ::::; a: w ~ < W A. t/)

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SPEAKER LINE RESISTANCE

OHMS

10

9

8

7

6

5

4

3

2

1

V1 GAU ~E

/ [7

~PEAK R WE 16 G "UGE

~OPPE fl WIR

/ V ~AUGE (AW( )

L

/ V V V

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V V V [7 1 ~ GAU t3E

/ /

V V V V

/

/ / / V V

V ~ ~ 12 G" UGE

/ /V V ~ V

V ... ~ L.--V / / /' ~ l.---...-

I V ~ ~ ~ 10 G ~UGE

/ ....... ---)~ V V /' ..... ---~ ~ L.--............ ---~ ~ /-::: ~ ~

I--

~ 0 500 1000 1500 FEET

SPEAKER LINE TOTAL LENGTH (distance from amplifier to speaker)

(FIGURE 7a)

7a. The above chart is used to find the resistance of the speaker line for copper wire of various gauges (AWG) given the length of the wire run from the amplifier to the speaker. This chart gives the resistance for a pair of wires (hot and common); for the resistance of a single conductor, divide the chart resistance by two.

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Example: For a 500 foot run of 16 gauge pair, enter the chart at the 500 foot vertical line and follow this vertical line to the intersection of the 16 gauge line. Read the resistance of the horizontal line: 4 ohms. For very long dis­tances (or very short), the chart can be scaled by multiplying (or dividing) both the distance and resistance by ten or some other convenient scale factor.

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..... N --W (,) Z 0( C W D.

! 0 0( 0 .... II: W ~ 0( W D. en

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OHMS

POWER AVAILABLE AT SPEAKER, AFTER LINE LOSS

-0.5 dB -1 dB 89% 79%

-2 dB 63%

-3 dB 50%

20~--~--~~r-~--~--~~--r---~---.>r--.---.----.---.

15

10 -6 dB 25%

5

o 10 OHMS

SPEAKER LINE DC LOOP RESISTANCE (R)

(FIGURE 7b)

7b. The speaker line between an amplifier and speaker causes a loss of avail­able power. When the amplifier and speaker are located next to each other and connected by a very short piece of heavy gauge wire, maximum (100%) power is transferred from the amplifier to the speaker and no loss (0 dB) occurs. When the amplifier is remotely located from the speaker, consider­able available power can be lost. To use the chart, the speaker load im­pedance and speaker line resistance are required. To find the speaker line resistance, use the length and gauge on the previous chart.

Example: A 16 ohm speaker is to be used with a 4 ohm (500 foot, 16 gauge) speaker line. Find the intersection of the 16 ohm speaker horizontal line and the 4 ohm line vertical line. These lines intersect at the -2 dB, 63% curve. The available power lost is 2 dB and only 63% of the original power is delivered to the speaker. This means that a 100 watt amplifier will deliver only 63 watts to the speaker. Consider the economy of using a 63 watt am­plifier located next to the speaker vs. a remote 100 watt amplifier. It is good engineering practice to limit the power loss to 1 dB. In ' this case, chang­ing the wire gauge to 12 gauge reduces the power loss to under 1 dB.

For high resistances (or very low), the chart can be scaled by multiplying (or dividing) both the speaker load and line resistance by ten or some oth­er convenient scale factor.

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BALANCED LOW IMPEDANCE ATTENUATOR

BALANCED 11 ATTENUATOR

Attenuation 1500 to 1500 6000 to 600n

Ri Rz RI Rz

5 dB 535 45.6 2142 182 10 289 107 1155 427 15 215 204 860 817 20 183 371 733 1485 25 168 665 672 2659 30 160 1185 639 4739 35 155 2108 622 8432 40 153 3750 612 14999

Choose closest 5% resistors for approximately .5 dB accuracy.

(FIGURE 8)

8. For low impedance microphones (150 ohms) use the 150n to 1500 columns.

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For 600 ohm lines use the 6000 to 600n columns.

Example: A 15 dB low impedance microphone attenuator (150 ohms) would use two 220 ohm (R1) and two 200 ohm (R2) resistors. Note that the resistors are rounded off to the nearest standard 5% value.

If the impedance of the attenuator is other than 150 or 600 ohms, the resistor values shown in the chart should be scaled according to the ratio of the chart impedance to the desired impedance.

Example: . For a 50 ohm, 15 dB attenuator, use the ratio of:

150 = 3 = R1 (chart) 50 1 R1 (desired)

R1 (desired) = R1 chart 3

R1 = 215 = 72 ohms 3

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White

Shield

AUDIO CONNECTOR WIRING

~,

I ' I \

I I \ /

~L...---V __ : :

I I IBlaCk 9 ....... _A

__

V

___ ::

(FIGURE 9)

9. For 3 pin professional connectors, pin 1 is always the shield (ground). This pin is longer in the female connectors, and connects first. The above figure shows the balanced wiring configurations. For unbalanced connections, the hot wire is connected to either pin 2 or 3. Some manufacturers' high imped­ance input connections use pin 3 (Shure Brothers Incorporated), while oth­ers (Bogen) use pin 2. In some cases it is desirable, or necessary, to connect the unused pin to pin number 1 (g round). For 2 circuit phone plugs, the shield is connected to the sleeve (S) or barrel. The hot wire is connected to the tip (T). This wiring is used for unbalanced connections and monophonic headphones. For 3 circuit phone plugs, an additional connection, the ring (R), is added. This provides a third connection for balanced connections or for stereo headphones. The present EIA standard (RS-331) for stereo headphones recommends connecting the tip to the right channel and the ring to the left channel. However, the standard industry practice is just the opposite and almost all equipment conforms. For stereo connections, the standard practice is:

Right - Ring Left - Tip

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