• Signal Levels in Sound Systems

  • Perhaps the most common complaint with a sound system is signal level issues. There are times when the signal is too weak and perhaps even more frequently the sound is so strong it is distorted. When a signal is too weak, it is generally self-correcting as the solution is to simply turn it up (to a point). If a microphone is being used, that tipping point will generally manifest itself with the shrill sound of ringing which is commonly known as feedback. The term feedback, shortened from “acoustic feedback”, is describing the phenomena of certain frequencies coming from the loudspeaker and re-entering the microphone to be re-amplified in an endless loop.  The only way to stop this once it begins is to interrupt that loop, preferably by turning down the microphone. Otherwise, the system will continue to increase the signal level until something breaks (most likely the loudspeaker).

    One of the worst things you can do is to cup your hand over the business-end of the microphone; this causes a resonant cavity which can also accelerate the howling.  The solution can be to place the talker closer to the mic.  How often have we seen a presenter hold a microphone at their waist as though it were a thing to fear? By cutting the distance between the microphone and the mouth in half, we get just under a doubling in signal level. Cut that distance in half again and it will be just over twice as loud.  This is free amplification and the simplest thing to do. For most amateur presenters, this is counter intuitive as they tend to first hold the microphone further away and speak softer when they hear the ring begin.  Doing this requires more electronic amplification which only promotes runaway feedback. The best option is to teach the speaker that if feedback is heard, to move the mic closer and to simply speak louder. This allows the system gain to be turned down so it becomes stable again.

    On the other hand, strong signals require a bit of discussion. There are essentially three signal levels in professional audio: microphone, line level and loudspeaker. None of these levels are absolute, simply generalizations. Take a look at the table below for reference; remember the AA battery is 1.5 volts DC (which is about the middle of pro line level).

    System Type Low Level (V) Typical Level (V) High Level (V)
    Microphone 0.00002 0.0002 0.006
    Professional Line Level 0.008 1.2 10
    Consumer Line Level 0.003 0.3 3
    200 Watt Power Amplifier 0.4 4 40
    Sound System Signal Levels from Audio System Design and Installation by Philip Giddings

    Outputs have to be placed into an input that is expecting that signal level. All too often we see the output of a loudspeaker being placed into a microphone input. Perhaps you scoff at the thought of an amplifier output being plugged into a microphone input. What is the output of a headphone jack but loudspeaker? When this happens, the sound heard is heavily distorted because the microphone input is being overpowered by the signal presented.  Otherwise known as “clipping”, in which the top and bottom of the wave form is broken off as the input is simply overwhelmed with signal.  A re-inspection of the signal level chart will show that if we are careful and it is the only option this interface can be done.  Turn down the headphone output and go into a line level input capable of handling that strength of signal. An even better solution is to use a direct box, which is an interface between all of these signal levels.  Doing this provides isolation, impedance matching and protection between the two devices.

    By matching signal levels appropriately, unwanted distortion and possible damage to your sound system is eliminated. Do you need help selecting the correct direct box? Contact ESCO today and we will be happy to help!

    Source: https://en.wikipedia.org/wiki/Clipping_(audio). By No machine-readable author provided. CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=453964

    Phil Hodson is currently the director of Project Engineering with ESCO Communications having been with the company for over 28 years. He has 45 years in the communications field with experience from touring to designing. He also holds a BASEET and is one of the few NICET Level III certified people in Low Voltage Communications in Indiana.