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Shure Apps Tech Tip: Microphone Distance Factor

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What is the Distance Factor for a microphone? In brief, it means that a directional microphone may be placed farther away from a talker than an omnidirectional microphone and still produce similar audio results. This assumes two microphones of equal quality and sensitivity.

As an example of the Distance Factor, let's consider a simple application: recording a talker's voice in a meeting room. Through experimentation, an omnidirectional mic is found to produce an acceptable recording when placed 2 feet away from the talker.

Acceptable recording = minimal level of background noise in relation to the talker's voice level. Rule of thumb: the talker audio should be at least 20 dB louder than the background noise.

Now try a cardioid microphone in place of the omnidirectional. The Distance Factor for a cardioid is 1.7. This means the cardioid may be placed 1.7 times the distance of the omnidirectional and produce the same audio quality. In this example, the cardioid may be located 3.4 feet away (2 feet x 1.7) from the talker and produce an acceptable recording. The Shure KSM141 is the perfect microphone for this experiment as it can be switched from omnidirectional to cardioid.

Next, try a supercardioid mic in place of the omnidirectional mic. The Distance Factor for a supercardioid is 1.9. So it may be placed 3.8 feet away (2 feet x 1.9) from the talker and produce an acceptable recording.

Then, put a hypercardioid microphone in place of the omnidirectional. The Distance Factor for a hypercardioid is 2.0. It may be placed 4 feet away (2 feet x 2.0) and produce an acceptable recording.

Finally, try a shotgun microphone in place of the omnidirectional microphone. The Distance Factor for a typical shotgun is 3.0, which allows the microphone to be placed 6 feet away (2 feet x 3.0) from the talker and produce an acceptable recording.

Remember that the Distance Factor is a multiplication function that directly relates to the audio quality obtained with an omnidirectional mic in a given acoustic environment. If an omnidirectional mic must be used at 1 inch from the talker for acceptable results in a noisy setting, then a hypercardioid mic must be used at 2 inches for the same results... not, not, not the 4 feet mentioned in the previous example above.

IMPORTANT: The increase in Distance Factor for a directional mic is due to its greater rejection of ambient (background) noise, not due to any increase in sensitivity to the desired sound source. In other words, the directional mic does NOT reach out and grab the sound emanating from the talker's mouth. Really, it does not…

When a mic is placed farther from the talker, more amplification is necessary to maintain the same output level. In a public address application, it is loudspeaker positioning that often dictates microphone location and overrides the Distance Factor in determining the maximum distance from microphone to talker.


SUMMARY OF DISTANCE FACTOR
Omnidirectional = 1 Cardioid = 1.7 Supercardioid =1.9
Hypercardioid = 2.0 Shotgun = 3.0

 

 

Shure Tech Tip: A Small Slice of Ohm's Law

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The sound system designer required the power consumed (in watts) from each Shure product in the design. This information was required to satisfy the local electrical inspector that the sound system would not overload the electrical circuits in the vintage building. In most cases, the watts rating can be found on the Shure User Guide for the product. If it is not listed, it can be calculated; all that is required is simple multiplication. First, here is a lesson in basic electronics using water as the analogy:

Current, measured in amperes, is the amount of water flow.

Voltage, measure in volts, is the water pressure.

Power, measured in watts, is the amount of work the water can do.

Water power is "water pressure" multiplied by "water flow."

Electrical power is "voltage" multiplied by "current."


Example 1: A kitchen toaster needs 120 Vac (volts alternating current) to operate and consumes 10 amperes. 120 multiplied by 10 = 1,200 watts.

Example 2: The Shure UA844SWB needs 18 Vdc (volts direct current) to operate and consumes 3 amperes. 18 multiplied by 3 = 54 watts.

Example 3: The Shure ULX4 needs 15 Vdc to operate and consumes 0.550 ampere (550 milliamps.) 15 multiplied by 0.550 = 8.25 watts.

The math also works in this way:

Example 4: The SCM800 is rated at 24 watts when connected to a 120 Vac source. 24 divided by 120 = 0.2 ampere or 200 milliamps.

If the voltage and amperes are known, the wattage can be calculated. If the wattage and voltage is known, the amperes can be calculated.
 

   

Wired Guitar + Wired Microphone = Electric Shock

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A Tech Tip from Shure Applications Engineering:  

The musician was bewildered…more than usual. "When my guitar is connected to my amp with a cable, and I sing into a wired mic, I get an electrical shock through my lips. If my guitar is wireless, it does not happen. If my mic is wireless, it does not happen. Why do I get shocked when the guitar and the mic both use a cable?"

Of course, the mic gets blamed because it touches the lips. But the culprit is not the mic, nor the cables. The culprit is the guitar amp.

Because of the electric design of many vintage guitar amps, it is not uncommon for a small amount of current (120 VAC) to "leak" onto the amp chassis.

Read more: Wired Guitar + Wired Microphone = Electric Shock

   

Wireless Mic Antenna Placement - Closer Is Better

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A history lesson: when Shure introduced the Vagabond professional wireless mic system in 1953, its primary purpose was to replace the 20 foot cable attached to the microphone. The Vagabond was not expected to reliably transmit a signal for hundreds of feet. In the 1970s, wireless mics began to grow in popularity, particularly for in-studio TV production (think "person presenting the weather") and for Las Vegas stage shows. But even then, the transmission distance was relatively short.

As wireless mic technology improved, the transmission distances increased. Eventually, the pro audio world began to think of a wireless mic as replacing a 200 foot cable run, not just a 20 foot mic cable. So antennas were moved farther away from the stage - often ending up by the mixing console for the sake of convenience. This antenna relocation method worked well for three decades, primarily because there just were not many RF (Radio Frequency) signals in the air.

But now in 2013, the trend is beginning to reverse. The reason is the...

Read more: Wireless Mic Antenna Placement - Closer Is Better

   

Proper use of the Shure UA874US directional antenna

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Shure Apps Tech Tip: Correct use of the UA874US Directional Antenna

Recently, a local theatre group was staging a Broadway musical. During technical rehearsals, there were ongoing problems with the wireless microphone system and I was asked to consult with the production crew. As the theatre was near my home, I stopped by one afternoon.

There were ten channels of Shure UHF-R in L3 band, and twelve channels of UHF-R in the H4 band. All receivers were UR4D+ units, and were grouped in their respective frequency bands, i.e., all L3 units in one group and all H4 units in the other group. I was pleased to find that the operating frequencies had been properly coordinated. But then I saw the antenna set-up and immediately knew the source of the nagging problems.

Here is what I found:

Four UA874US antennas located 60 feet from the stage with direct line-of-sight to the stage;

Two antennas for the L3 receivers; two antennas for the H4 receivers;

All antennas mounted vertically;

All mounted side by side with six inches from antenna to antenna;

All four antenna BNC cables were less than 10 feet in length;

All antennas had gain settings of +12 dB;

One antenna had no LED illuminated.
 

Before I continue, test yourself. There are three major errors in the antenna setup. (Insert "Jeopardy Final Answer" theme music here.)

Time is up. Here are the errors:

Read more: Proper use of the Shure UA874US directional antenna

   

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