Evaluating a wireless microphone system
While operating a wireless microphone system can be complicated at times, it is practical to perform a few simple tests (without the need for specialized test
testing equipment) to get a feel for the key features of a wireless microphone.
A wireless microphone is still a "microphone" by definition. Its sole design goal is to produce accurate audio
frequency signals for a variety of applications. The fact that a microphone is "wireless" means that it can be used without the need for an attached connecting wire.
Before you decide to purchase or rent a system, the following tests are recommended to help you evaluate the quality of your particular wireless microphone system
. Each test will examine the system for particular types of performance and problems. It is best to perform as many of these tests as possible (but not all of them) in order to get an overall
assessment of the quality of the system, as you may find that certain designs perform well in
some areas and poorly in others. Performing only one or two tests is not enough to get a better overall
estimate.
"Car Key Test"
This is a popular test among high-end wireless manufacturers. This simple test reveals how well a wireless microphone can handle high-band audio transients, while at the same time reflecting the quality of the audio processing link in the overall system.
Connect a headset or an audio system to a wireless system that can completely block out whistling at very high SPLs. Ideally
the headphones or audio system will allow you to listen intently to the audio
output of the receiver while isolating the acoustic sound produced by the keychain. Set the input gain on the transmitter to a normal value according to the average speaking volume.
Close to the microphone and gently shake the keychain to make it jingle. Shake the keychain a foot or so away from the microphone, and then slowly move away from the microphone while
shaking it until you are 8 to 10 feet away from the microphone. Listen to the audio coming out of the receiver. Does it sound
like a keychain shaking or a crushed bag of potato chips?
Next, have someone speak over the wireless system while jiggling the keys. Pay attention to the distortion in the speaker's voice. Hold the key
a foot or so away from the microphone, then 8 to 10 feet away, and pay attention to the effect of the change in the speaker's voice.
This is a very difficult test for any wireless microphone except one connected by a wired cable
. The results you hear will tell you whether the startup and delay times of the input limiters and compression expanders in this design are
excellent, and will give you an idea of the kind of audio quality you can expect from a wireless system in real life.
Metal keys swinging loosely on a key ring produce a lot of high-frequency transient sound. Wireless systems that fail this test
often also distort the gravelly sound of the human voice in normal applications. Often listeners don't notice these high-frequency transients,
because the fricatives don't have a specific frequency point and are more like random noise. Distorted random noise still sounds like noise, so it's not
easy to detect. However, in this keychain test, in many of the audio signals coming out of the failed wireless mics, what would have been
a crisp keychain clattering sound did not have a clear sound quality on the receiver's end, and instead you heard a muffled sound
as if someone's hand was placed between their mouth and the microphone. The keychain test will remind you to listen carefully for any distortion in the sound. The key
string test will also show the audio circuitry of a wireless microphone that has been disrupted by ultrasound. The peak energy of a crunching key is actually centered at
30khz, which is above the range of human hearing. If the circuitry in the transmitter doesn't filter out the ultrasound, the compression expander
will respond incorrectly. Since the tooth fricatives in the human voice also contain ultrasonic waves, this is an experiment of practical significance.
Because sounds you can't hear cause the intensity to fluctuate up and down, overloading the ultrasound will make the fricatives sound harsh.
The low-frequency "knock test"
This test reveals whether a wireless system's intrinsic signal-to-noise ratio and compression extenders are capable of handling low-frequency audio
frequency signals well. The "intrinsic inherent signal-to-noise ratio" demonstrates the signal-to-noise ratio of the wireless microphone itself before it is optimized by the compression expander
.
This test requires listening in an extremely quiet environment with minimal background noise. Put the transmitter and microphone
in a different room from the receiver, or use high-isolation headphones to listen to the receiver's audio output. In either case
there will always be minimal background noise heard near the microphone. Background noise of sufficiently high intensity will invalidate this test.
Set up the system at normal sound intensity, then place the transmitter and microphone on a table or counter. Clench your fists and gently
tap the table (not your knuckles). We want to create a low-intensity, low-frequency
"knock" around the microphone in this way to initiate compression expander processing on the wireless system.
Try varying the amount of tapping you do with your fist on the desktop, trying to find as low an intensity as possible that will just kick off the compression-extender processing, while listening carefully to the audio signal coming out of the receiver. When you "tap" the desktop, you'll hear a background sound like a hiss or a whoosh mixed in with the tapping sound
Our idea is to listen to the sound when you "tap" the desktop, and then listen to the sound when you "tap" the desktop. The idea is to listen to how much background noise is released into the wireless system when a "knock" occurs, and also
to see if the "knock" heard in the wireless system is the same as in real life.
This test shows a very clear difference in performance between a single-band compression spreader and a dual-band compression spreader with dnr filtering
, and also reveals the signal-to-noise performance of the wireless system.
Set the transmitter gain to normal volume levels during the test, and you'll hear results that are very similar to what you'll see in real-world use
.
Although not a standard test method, the results are just as interesting. First set the transmitter input gain adjusted to minimum,
then turn the receiver output to maximum, and finally perform the knock test. The only reason for doing this is to help understand how much noise the system is actually suppressing in normal
use, and to emphasize how important it is to get the transmitter input gain adjusted to the right place
.
A wireless mic system design that uses a lot of pre-emphasis/de-emphasis as noise attenuation is likely to do very well in the "knock test," but the same system is likely to fail miserably in the "keychain test" that precedes it. and then the same system may well have been a major disappointment in the preceding "keychain test".
Checking the Input Limiter Range
For this test, you'll need to make a bit of noise into the microphone, but be able to monitor the receiver's output in a very quiet environment
. It's best done by two people. The purpose of this test is to hear if the transmitter input limiter does a good job
of handling audio peaks that are just above average strength.
Set up the wireless system at an average sound intensity so that the system can reach instantaneous peaks of sound
when fully modulated, with the microphone two feet from the speaker's mouth. Gradually move the microphone closer to the speaker's
mouth as he speaks in an even tone. Keep the microphone on the side of the mouth when it is very close to the mouth to ensure that no sharp gasps enter the microphone. If the
transmitter has a poor limiter, or no limiter at all, the signal will get louder and louder, and then as the loudness increases, the signal
starts to distort. In a system with a good limiter, the sound will go all the way up to the maximum, and then even if you hold the microphone
closer, the sound will stay at a proper volume level.
When the mic is moved closer to the speaker's mouth, the change in distance may cause the sound color to change, but the system should be able to handle a lot of overload without
distortion. You can also test a limiter by yelling into the microphone, but
keep in mind that the speaker's characteristics will change as they go from talking to yelling. Some wireless system designs try to prevent overload by providing a lower microphone gain for the user
. This compromise will produce a poor signal-to-noise ratio when the RF signal becomes weak
. Sharp audio peaks produced by hand clapping or other means are also a good test
of how good a limiter is.
"Walking test"
As the name suggests, the test involves one person walking and talking at a transmitter while another listens to the output of a receiver
.
There are two different "walk tests" for a wireless system
Checking maximum operating range
Checking proximity silence and diversity performance
Before performing any of these tests, a wireless microphone system should be set up as it would be in a real-world application. The microphone and transmitter
must be in the same position as they would be on the speaker's body in actual use, the receiver must be connected to any
other equipment that needs to be connected, and the power supply and antenna must be connected and placed as they would be in actual use. If the system is not connected in this
style, the results of the walk test will not be of any practical significance. Don't remove the transmitter or the antenna from the receiver to taste
test the simulated extreme operating range, as this will change the way some receivers work, such as some models of electrosonics
Checking the maximum operating range
Classical walk tests are all about finding out how well you can walk with a transmitter on before the distortion in the output of the system is so severe that it becomes unworkable.
How far. You can keep walking until 8 to 10 instances of distortion have occurred and define the distance at that time as the range limit. Or,
Based on self-estimates, keep going until the accumulation of distortion or hiss reaches a point where you can't tolerate it. When comparing two or
multiple different wireless systems, it's very important to repeat the exact same line for each walk test, placing the receiver and transmitter on the body in the same interconnect at the same
position, and applying the same criteria for defining the range limits, otherwise
it's not a valid comparison.
Even if the system's maximum range is just above what you would normally need, the test will reveal how selective the receiver is
and how well the system is able to handle weak signal conditions.
Short-range test of silent and diversity reception performance
The "short-range" walk test is designed to check how well a receiver handles multipath transmission nulls that occur at closer operating ranges with stronger RF signal reception conditions.
The short-range walk test is designed to check how well a receiver handles multipath transmission nulls that occur at closer operating ranges with weaker signal reception conditions. Do not make the situation worse
by removing the antenna from the transmitter or receiver, as this will render the validity of the test meaningless.
Set up the same wireless system as above, but don't look for areas with too many multipath reflections, such as areas with many metal filing cabinets
or lockers, small to medium-sized metal buildings, metal trailers, and so on. Place the receiver antenna within a few feet of a metal surface
to increase the multipath offset on the antenna. The antennas on a diversity receiver each require at least 1/2 wavelength to obtain the maximum benefit of diversity
receiving technology. If the receiver cannot be configured in this manner in a practical application, then place the antennas in
the same position as they will be used.
Walk around the area while wearing the transmitter and speaking to try to find the
locations where distortion or silence (audio muting) is occurring. Moving the transmitter within a few feet of a metal surface might help produce the conditions needed for multipath frequency runs.
The purpose of this test is to see if the system is susceptible to frequency runs and also to look for impact noise that occurs during a run that does not output a high
sound pressure level. An effective diversity system will make finding frequency runs difficult, which will tell you things like how effectively the diversity
receiver circuitry works. In the unlikely event that a runout does occur at the receiver
with a large average RF signal strength, the receiver should simply mute the audio during the runout and not allow any noise or noise pulses to be emitted at the same time.
In close quarters, active mute processing in the receiver is the best solution, as it will eliminate the noise pulses generated
by the runout. However, it will also reduce the maximum operating range as in previous tests. Less active silence allows for a maximum operating range,
but usually allows for a noise pulse to be output during close frequency runs.
These two tests illustrate the dilemma of a traditional silent system not being able to balance both near and far operating ranges, while
that way, automatically optimizing itself between near and far operating ranges.
After both types of walk-throughs are completed, you'll have a clear idea of what to expect in real-world applications.
Some systems may offer excellent maximum range characteristics, but prove noisy in close-range, multi-path situations. Other systems
may perform well in proximity tests, but are poor at maximum operating range tests. Of course, the ideal wireless system
microphone mics will perform well on both counts.
A-b test via cable connection
Get two identical microphones, one connected to an audio cable and the other to a wireless system, in order to perform a listening test.
The trick to doing this is to make both mics listen at exactly the same intensity. Even if there is a slight difference in intensity, the human ear will not be able to
recognize this audible difference in frequency.
Place the microphones at equal distances from the sound source or the human mouth so that the same signal can enter both microphones. Switching back and forth between a cable connection
and a wireless connection allows the listener to compare the sound from the different setups. It's best to do this test blindfolded, of course, as there's no way for the listener to identify which setup is being monitored, and then make a note of the results.
As a "physical check," swap the two mics around, listen again, and see if there are any discernible differences in the mics themselves that are as subtle as in the first
comparison
.