Aircraft Audio

Clear, unambiguous communications, whether between flight crew members or ground controllers, is of paramount importance. There are four (4) distinct types of audio in aircraft and at times they can become confusing. They are:

RECEIVED AUDIO This is audio coming from communications radios and/or navigation equipment for identification. This audio usually has an output impedance of 500 ohms (phones) or 3.2 ohms (speaker).

INTERPHONE AUDIO This is audio heard on the headphones and is typically conversation between crew members. This Intercom System (ICS) can either be voice activated (VOX) or push to talk (PTT) activated. Like RECEIVED AUDIO phones, the output impedance is 500 ohms and is designed to drive a headset directly if necessary. No communications are beyond the aircraft and no transmitters are keyed.

SIDETONE AUDIO This is audio heard in the headphones of a crewmember while transmitting. This audio provides a feedback to the pilot of the clarity of the transmission. Note that this audio level is not adjustable by the pilot. What you hear is what you get.

MICROPHONE AUDIO This is the low level audio (0.15 to 0.25 V AC) created by speaking into a microphone and is used for all communications. In the civilian world, the microphone is typically powered by 9 V DC or more with a series resistance between the voltage source and the microphone. When speaking into a microphone, the audio waves strike its diaphram and cause it to oscillate, changing its resistance. This changing resistance in series with the the fixed resistance creates a small AC voltage to appear at the junction of the two resistors. This is capacitively coupled to an amplifier and routed as necessary in the audio system.

So how do you test the integrity of these audios in an aircraft? Simple. Load the mic and phone jacks with a resistor until the measured voltage is half its unloaded value. This is the impedance of the circuit. For phones it will vary between 150 and 600 ohms (measuring AC voltage), for a speaker between 3.2 and 8 ohms (measuring AC voltage) and for a microphone audio circuit, between 150-600 ohms (measuring DC voltage).

How much audio power is there? Measure the phones or speaker AC voltage when loaded to one-half its unloaded voltage. Call this “V”. Power = V2 / Select Resistor Value

Example: Pilot reports that his headphone audio is too low to hear in flight, OK on ground (without flight noise, vibration, etc.). Technician powers up avionics, tunes in stable audio source with volume full CW. The measured AC voltage without a headset installed is 2.2 V AC. The voltage drops to 1.1 V AC with a 140 ohm resistor in place of the headset phones. Therefore, the audio power is: (1.1 V AC) 2 / 140 ohms = 0.0086 Watts or 8.6 mW. You typically need at least 25 mW headphone audio for a noisy environment, therefore, you have confirmed the audio squawk.

Example: Pilot reports his sidetone is garbled and weak on all radios and others have difficulty understanding him on intercom. Technician powers up avionics, selects appropriate comm on the audio panel and measures the mic audio DC bias voltage without a microphone installed. The measured DC voltage is 12.0 V DC. The voltage drops to 6.0 V DC with a 420 ohm resistor in place of the microphone. Therefore, the basic mic bias circuitry in the aircraft is operating properly. You may have a defective microphone.

You can use this principle of loading an avionics signal source to equal one-half its unloaded value to determine the serviceability of many circuits. All that you need is a resistor decade box with extended wires and a voltmeter.