Debunking Common Audio Myths



When we decided to devote this issue to dispelling common audio myths and legends, we had no trouble impaneling a group of experts. Culled mostly from Shure's Applications Engineering Group, these individuals devote their days, and sometimes their nights, to setting the record straight. Chief among this talented group is Tim Vear, who not only wrote the book on audio for houses of worship (you can download it here) but also served as our primary mythbuster and spokesperson.







False.
It should always be possible, if the system is designed properly to get adequate Gain Before Feedback. But proper design may not be obvious to the casual sound system user.

The primary determinant of gain before feedback in a sound system is a question of two main distances that can be identified. One of them is the distance between the mouth of the person using the microphone and the microphone itself. The other is the distance between the microphone and the nearest loudspeaker.

In order for a sound system to have good Gain Before Feedback, the distance between the microphone and the loudspeaker has to be much, much, much greater than the distance between the microphone and the mouth of the person using it – or the sound source. We're talking about ratios of 10 or 20 or 30.

That means if you are two feet from the microphone, you are not going to get any reasonable gain before feedback from that sound system unless the loudspeakers are fifteen times that distance – that means 20 or 30 feet away or farther. It's really why you get the best Gain Before Feedback in any sound system if you're very close to the microphone. If the microphone is within a couple of inches of your mouth or closer, you can get adequate gain. If it's too far away, you cannot. This is called Potential Acoustic Gain, which is governed by principles of physics.

The loudspeakers have to be much farther away from the mic than the sound source. This is especially important with choir mics. By definition, the choir mics are at some distance from the mouths of the people singing — perhaps 3 or 4 feet, maybe even farther. And unless the loudspeakers are 30 or 40 or 50 feet away, you are not going to be able to get GBF in that sound system and that's why, ultimately, you can't get a lot of gain out of choir mics. You can achieve moderate amounts of gain, but you'll never get rock and roll sound levels out of choir mics because they're just too far away from the sound source.

False.
This isn't true and it hasn't been true since the late 1950s. In a sound system of modern design, the load impedance or the impedance of the device that you're plugging into has to be significantly higher than the source impedance, which is the device that you're plugging in.

For example, a microphone has a source impedance of about 150 ohms. The device that you're plugging the mic into needs to have an input impedance five to ten times greater. If it were a mixer, for instance, it would have to have an impedance of at least 1000 to 1500 ohms. If you look at the actual specifications of mixing consoles, you'll see that the actual input impedance of a so-called low impedance mic input is typically about 1500 ohms. If you match the impedance, you get less level and less headroom – the systems are not designed to work that way any longer. Matching, generally, is not an issue.

More false than true.
If the connections between devices are short – that is, the cables are a few feet long, things are in the same rack, close to each other, if there isn't a huge amount of radio or other electromagnetic interference in the vicinity and there aren't big ground potential differences between the devices that you're hooking up, then an unbalanced hook up is perfectly fine.

The audio quality between a balanced and an unbalanced hookup is not intrinsically different. The place where a balanced hookup is preferred is if the connecting distances are large, the devices are far away from each other, and if there's a likelihood of large amounts of radio frequency interference or other electromagnetic interference on the connections or if there's significant possibility of ground potential differences between the devices.


False for several reasons.
Phantom power by definition puts a DC voltage on both conductors of a balanced microphone cable; Pin 2 and Pin 3, if you actually looked at it. In a dynamic microphone, Pin 2 and Pin 3 are hooked up to the opposite ends of the voice coil that's part of the microphone cartridge. Since the DC voltage in phantom power is by definition equal, there's no voltage difference across the voice coil. No current will flow from through the voice coil from the phantom source. The voltage is the same at each end.

If the dynamic microphone is wired properly and the phantom power is operating normally, the phantom power has absolutely no effect on the behavior of the mic.

Note, though, that if there is a fault with the internal wiring of the dynamic microphone or the cable that's being used, there's still no possibility of damaging the microphone because even if a wiring fault causes current to flow through the voice coil of a dynamic microphone, the phantom power sources are current-limited. That means the maximum amount of current that you can draw from a phantom power supply even into a complete short circuit is not enough to damage the voice coil. It will sound funny – typically, the level will go way down and the low end will disappear and it will sound thin and nasty. The user will know that the microphone sounds terrible. Fix the wiring fault and the microphone will be fine.

False.
Think of it this way: A radio station is broadcasting a signal from a big antenna somewhere. Ten thousand people are driving around the city in their cars and all their radio receivers are tuned to the same radio station. They can all pick up the signal without any interaction. It's perfectly fine for a single transmitter to transmit to multiple receivers, an unlimited number of receivers, on that same frequency.

This is a typical application for in-ear personal monitor systems. There is a single transmitter and there are multiple musicians onstage who are tuning in to it to listen to the mix. In a church setting, it's very often the case that there are assisted listening systems where there's a single transmitter transmitting the signal from the PA to the congregants who are using the receiver devices. No problem.

The reverse situation would be multiple transmitters all trying to feed into the same receiver. Under those conditions, the multiple signals on a radio frequency cause massive interference with each other. None of them will be picked up clearly and the receiver will tend to latch on to one or the other, depending on which one is a little bit closer to the receiver - or shift back and forth between them. You'll get noise and odd sounds.

There's often a little bit of confusion between wireless microphone and in-ear personal monitor systems. With personal monitor systems, the bodypack is a receiver. In a wireless microphone system, the handheld microphone or the bodypack are the transmitters.

False.
In absolute terms, condenser mics have some characteristics that dynamic microphones don't have. For instance, very wide frequency response, very flat frequency response and very high sensitivity. For an application that requires these things, a condenser microphone would be a better choice. But in that case, it might sound better because it captures a wider range of the original sound source in a more high fidelity fashion.

There are many sources that don't benefit from the flatness or the frequency range of a condenser microphone. A good example is miking close up vocals for pop music. Its hardly necessary to use a microphone that has a frequency response from 20 hertz to 20 kilohertz to pick up the a sound that only has a frequency range of maybe 100 Hz to 14 or 15 kHz. The sound system that is reproducing it might not even have that wide a range.

Particularly with pop music, a flat frequency response is not going to give you the presence or the ability to cut through a mix of other amplified instruments like guitar, drums and so forth. For example, an electric guitar plugged straight into a sound system has a very dull, bassy sound. But when it's plugged into a guitar amp that has a shaped response designed for that instrument, you get all of the brights and textures and exaggerated midrange response you want to hear in an electric guitar. A microphone with a shaped response works the same way and is often going to give you a better sound quality for that application.

Same for a kick drum — it doesn't benefit from a flat response mic in contemporary or pop music applications. You want something with a little bit of shape to give it the oomph or the snap you need to define that drum. You're not trying to get a high fidelity sound. You're trying to get a particular sound.

There are numerous examples where a dynamic microphone has a more appealing or preferred sound. Snare drums. Electric guitar amplifiers. Kick drums. Close up rock & roll vocals. Certain percussion instruments. They all benefit from a dynamic microphone's shaped response.


False.
Reach is not a specification of a microphone. Mic users have a concept of reach as the ability of a microphone to reach out and grab the desired sound in the midst of some ambient undesired noise conditions. They believe that some microphones can pick up from farther away than other microphones.

The reality is that microphones do not reach out and grab the sound from a distance. They merely measure pressure variations right at the diaphragm itself. The microphone doesn't "know" anything about what is happening at any distance from itself. For this reason, if you try to characterize a microphone's "reach", it's almost completely dependent on the ambient acoustic conditions around the microphone.

Here's an example: Take a microphone to the Superbowl on a Tuesday morning at 2AM in the middle of July. There's nobody there. They've turned off the air conditioning and it's a huge quiet box. You put your microphone at one side of the stadium and drop a nail on the concrete on the other side of the stadium. Yes, the microphone will pick up the sound of that nail a couple hundred yards away because there's no ambient noise. Go back on Superbowl Sunday in the middle of the fourth quarter when the opposing quarterback is lining up to call the play. Put your microphone down on one side of the stadium, clear out the beer vendors and drop the same ten-penny nail on the concrete. Can you hear the nail? What changed? Same mic, same nail, same concrete, same building. But the ambient noise level is now 100 decibels higher.

The reach of the microphone, if you can even call it that, is mostly dependent on the ability of the microphone to pick up sound in the middle of all that noise. No microphone has a "reach" that is defined independent of ambient noise.

The one specification of a microphone that loosely corresponds to the concept of reach is directionality or the microphone's polar pattern. The directional characteristic of a microphone describes how much sound it picks up from ambient sources compared to how much it picks up on-axis.

The numbers are there, but they're not huge. The difference between how much ambient noise an omni-directional and a hypercardioid microphone will pick up in the same conditions is only about 6 decibels. (The hypercardioid mic picks up 6 decibels less ambient noise than an omni.) Because of the Inverse Square Law of Sound, if I double the distance between the sound source and the microphone, the level of the sound source drops by six decibels at this greater distance. The ambient noise stays the same. If an omnidirectional microphone picks up a certain ratio of ambient noise to on-axis sound at one foot away from a sound source, then a hypercardioid microphone can be used at two feet from the sound source and still pick up that same ratio. This is NOT because the hypercardioid is more sensitive to the on-axis sound but because it is 6dB less sensitive to the ambient noise.

In that sense, the hypercardioid has more "reach". But neither one will work at great distances in the presence of any significant background noise. They just measure little pressure variations right at the diaphragm.

False.
The truth is, a wireless system's range of performance is greatly affected by physical obstacles – any intervening structure, body, wall that are in the path of the radio waves.

There are two classes of obstacles. The first class is a metal obstacle. Radio waves do not penetrate metal obstacles if the obstacle is large compared to the wavelength of the signal. For UHF systems, that's about one and one-half feet. So, if the metal obstacle is larger than a foot and a half square, it will completely block the radio wave from passing beyond it. That includes things like file cabinets, refrigerators, and metal doors.

Churches and interior spaces have decorative metal grillwork, metal ducts on the air handling equipment, metal diffusers hanging down, metal deflectors on the lighting grids. Even when these object have holes in them, they will block the radio waves just like a solid object if the holes are smaller than the wavelength.

A classic example is a microwave oven. The front glass door contains a metal grid – the holes in the mesh are very small compared to the microwaves, which are several inches long. It keeps the microwaves in the oven but you can still see inside because light waves have a very short wavelength - in the angstrom range - and can pass through.

Another metal-blocking problem occurs when customers mount their receivers and the receiver antennas inside a metal equipment rack. Unfortunately this happens a lot, particularly when the receivers and the receiver antennas are in an enclosed metal cabinet. Amazingly, they may still get ten or twenty feet of range out of that — sometimes not even that. Shure's Application Engineering Department gets a call every couple of days from a user trying to keep equipment safe and secure this way. Radio waves can't penetrate metal obstacles — that's important to remember.

The other class of objects that attenuate radio waves are non-metal. Drywall. Masonry. Human bodies. Furniture. Wall paneling. They all reduce the strength of radio waves. The human body is a tremendous absorber of radio energy. It's very important to make sure that the receiving antennas are well above the heads of the audience –so that the receivers can se the transmitters as directly as possible.

Metal and non-metal obstacles can significantly affect wireless performance. Line-of-sight is a rule of thumb.


False.
The truth is that high power can have detrimental effects, particularly when using multiple wireless systems. In that case, the lowest power setting should be used.

Multiple transmitters interact with each other to create interfering signals called intermodulation products. The higher the power of the transmitters, the higher is the level of interference from these products. The net effect is that the total number of wireless systems you can use is greatly reduced. An example: with a typical transmitter power level of 10 milliwatts (ten-thousandths of one watt) you may be able to use up to 30 or 40 systems at one time. If you bump them up to 100 milliwatts (10 times the power), the increased intermodulation products may reduce the usable number of systems to as few as 10.

This is why in theatrical applications where they are 30 or 40 channels at a time, low-power transmitters are used. High power transmitters are only recommended when there are a few transmitters, or maybe even just one. They're not concerned about interfering with other equipment – they are trying to cover a very long range or there may be a high level of background radio noise to get above.

Shure's UHF-R system, for instance, is switchable from 10 to 100 milliwatts. We run into problems sometimes with customers who set all their transmitters to 100 milliwatts and run into all kinds of problems. The other obvious effect of high power is that it drains the batteries much faster. High power users are generally broadcasters.


False.
For most performance applications, lower radio frequencies are better. Comparing VHF to UHF:

VHF
Around 200 Megahertz
A typical VHF station operates at about 100,00 watts and has a coverage radius range of about 60 miles.

UHF
Between 500-800 Megahertz
A 60-mile coverage radius requires transmitting at 3,000,000 watts.

Because wavelengths get shorter as frequencies get higher, wavelengths of UHF systems are about a foot and a half long and wavelength in the VHF range are about five feet long. Not intuitively obvious, it turns out that radio waves travel better through obstacles when they have longer wavelengths. It's one of the reasons that submarines use wavelengths that can be measured in miles.

Because the wavelengths are shorter, UHF systems have shorter antennas which people like. There are also more frequencies to use in that band — about ten times as many.

All things being equal, the longer wavelengths will travel farther and better through things. Within the UHF range – between 600 and 800 megahertz, there really isn't any preference on where to operate – there's no significant difference.

False.
DTV: February 17, 2009 is the completion date for transition from analog television broadcast to digital television broadcast (DTV). All broadcast television stations will be required to operate in what are now channels 2-51. Only DTV stations will remain on the air. The analog stations will be gone. The former TV channels 52-69 are going to be reallocated for other purposes; one of them is Public Safety, which will be using Channels 63, 64, 68 and 69. The rest of that spectrum will be primarily used by communications services in devices that resemble cell phones.

Wireless microphone or personal monitor system products that operate in these former television bands may begin to suffer more interference from these services. That doesn't mean that the systems won't work any longer. But users will probably have to change frequencies to avoid these new services.

There's still likely to be a fair amount of open spectrum in different places around the country on some of the frequency ranges. In some places though, it may be difficult to operate as many systems as previously because there may not be enough spectrum for all of it. If users have frequency-agile systems, they'll be likely to continue using that system with very few problems in the foreseeable future.

Shure will not sell equipment that operates on channels 52-69 after February 2009 and we haven't sold equipment in that range for some time now.

DTV is completely separate from the White Spaces issue. The only common denominator is the date – February 17, 2009.

White Spaces: Even when all the remaining TV stations are in the range of channels 2-51, there will be plenty of open channels in most places. There won't be 51 channels on the air in every city. But there may be 15 to 33 open TV channels scattered around the country in different cities. These are the licensed channels that wireless microphones and personal monitors currently occupy.

There is a White Spaces Coalition that includes Microsoft, Intel, Google, Hewlett-Packard and several other companies proposing that a class of consumer electronic devices operate in the unused portion or the open television channels. These devices would be unlicensed, that is, anyone could operate such a device without obtaining an FCC license. Cell phones, cordless phones, and wireless laptop computers are examples of unlicensed devices. Historically, the nature of consumer electronic devices has been to heavily populate the unlicensed bands in which they have been allowed to operate. This includes the 49 MHz band, the 900mHz band, the 2.4 GHz band and several others. The concern is that new devices could disrupt professional audio users because their operation would be unpredictable.

The FCC's issue is deployment of rural broadband Internet access, using unused TV channels to provide service. Cable is expensive to run to these areas and satellite is a downlink only, so rural high-speed broadband access via wireless transmission offers a good solution.

Shure, as part of a group of concerned audio users and manufacturers, has organized a very strong campaign within Congress and the FCC to make sure that the interests of wireless microphone users are protected. There are various technical schemes that are being proposed for these unlicensed devices to be able to detect the operation of current users of the spectrum in order to avoid interference. Pending the outcome of FCC testing, the initial target date proposed for introducing these devices is February 18, 2009 and that's the only nexus with the DTV issue.

The task of the FCC is to weigh the needs of the current users of the spectrum (broadcasters and audio professionals) versus potential new users (consumers)–and make sure that the incumbents are not disrupted. We believe that wireless microphones will get appropriate consideration from the FCC.

The best advice for wireless audio users is to purchase frequency-agile systems.

Note: Shure posts updates on White Spaces on the company's website, a good way to stay up-to-date on the most recent developments.

See note in Letter From the Editor regarding recommended books, sites and articles.