Recently, I arrived early for a party.  The host was working in the kitchen 20 feet from where I stood.  “Have a seat and I will tell you what’s been going on,” she said.  I settled into a nearby comfy chair and we chatted.  The house was quiet and it was no problem to converse even though the talker/transmitter (the host) was located 20 feet from the listener/receiver (me).

Later that evening, the house was filled with guests when the host decided to continue the earlier conversation.  But to hear each other we had to be one foot apart…the talker/transmitter had to be much closer to the listener/receiver.   What changed?

The answer:  the ambient (background) noise level had increased due to the many conversations in the room.  For me, the host’s voice was the signal that I wanted to hear.  All other sounds were noise, because these were signals that did not interest me.  Yet when I arrived at the party and the noise level was quite low, I could be 20 feet away and still have a conversation.

This is a practical demonstration of Signal-to-Noise Ratio.  The signal is the voice of the host; the noise is everything else.  The noise is uncorrelated (random) acoustical debris.  If the level of the noise is greater than the level of the signal, I cannot understand what is being said.  Everyone has experienced a poor Signal-to-Noise Ratio at a party, or a rock concert, or sporting event.  Noise creates an acoustical fog that makes it difficult, or impossible, to understand what is being said.

Here is a related question: Why might a Shure wireless mic system work properly over a distance of 400 feet in the Utah desert, while the same exact Shure wireless system will work properly for only 50 feet in New York City?  As the wireless mic system did not change, what did?

The answer: the ambient (background) RF (Radio Frequency) noise level increased due to the many sources of RF transmission in New York City.  Examples are numerous TV station transmitters, two-way radio signals, broadcast radio signals, Wi-Fi, data networks, plus the ubiquitous smartphone, owned by every New Yorker and consistently in use.  These signals are uncorrelated waves of electro-magnetic debris.  Even though most of these RF noise sources do not operate on the same frequency as the Shure wireless system, these sources raise the level of the RF garbage.  This makes it more difficult for the Shure transmitter (talker) to be heard by the Shure receiver (listener).  Just like what happened at the party, the receiver has a tough time sorting out the desired transmitter signal from the undesired noise.

In an environment with a high level of RF noise, the receiver (or the receiver antennas) must be located closer to the transmitter.  This improves the RF Signal-to-Noise Ratio.  The receiver then can pick up the desired transmitter because the signal is now stronger than the noise.

A wireless receiver can be made more selective (improved ability to sort out the signal from the noise) by adding “front-end RF filters.”  Located after the antennas, these filters reduce the level of RF noise sent into the receiver.  Effective and efficient filters are expensive thus they are primarily found on higher priced wireless systems.

Related link on this subject:

http://www.shure.com/uploads/publication/upload/396/us_pro_antenna_setup_ea.pdf

Ever think about what good sounds really means? You’re probably thinking good material, excellent musicianship, and the right equipment, but it’s really a little more scientific than that.

Most problems in live performance are directly related to fidelity, intelligibility, and loudness. If one or more of these basic measures of sound quality isn’t right, then your audience can’t really hear the music you’ve worked so hard to perfect. Let’s look at them, one by one.

Fidelity

Is it true? This is mostly determined by the overall frequency response of the sound arriving at the listener’s ear. It must have sufficient frequency range and uniformity to product realistic and accurate speech and music. All parts of the audio chain contribute to it: a limitation in any individual component will limit the fidelity of the entire system.

Intelligibility

Is it understandable? This is a function of the overall signal-to-noise ratio and the direct-to-reverberant sound ratio at the listener’s ear. All this really means is that the “signal” — which is the desired sound from the sound system — must be at least 20 decibels louder than the noise and reverberation level at the listener’s ear to be intelligible.

What makes a room “live” or “dead”? Here’s where direct-to-reverberant ratios comes in. It’s determined by the acoustics of the room and the direction of the loudspeakers. Reverberation time is the length of time that a sound persists even after the sound source has stopped. A high level of reverberant sound interferes with the intelligibility of the sound since your audience won’t be able to determine where one sound stopped and another started. On the other hand, a very low level of reverberant sound can create a lifeless acoustic environment: a dead room.

Loudness

Most musicians find this concept the easiest to understand and apply: optimum volume levels must be achieved without unwanted distortion or feedback. A sound system used by a rock-and-roll band demands that attention be paid to Potential Acoustic Gain; in other words, the amount of amplification that can be delivered before the screeching howl of feedback occurs. The position of microphones and loudspeakers — as well as room acoustics — all play a role.