General Information:
 
FAA TSO Approval - HMEC 25/45 (PDF)
JTSO - C58a (PDF)
JTSO - HME 100 Series (PDF)
JTSO - HMEC 25 Series (PDF)
JTSO - HMEC 300 Series (PDF)
JTSO - HMEC 45 Series (PDF)
Noise Reduction Rating - HME 100 Series (PDF)
Noise Reduction Rating - HMEC 25 Series (PDF)
Noise Reduction Rating - HMEC 300 Series (PDF)
How Noise Affects Human Hearing, Communication and Job Performance (PDF)
History of Sennheiser Aviation Products (PDF)

Frequently Asked Questions...and the Answers:

Attenuation Active?
The reduction of noise, particular in the low frequency range by using active noise compensation electronic circuitry. (Measured in db)

Caliper Pressure (or contact pressure)?
This is the amount of force applied to the sides of the head while wearing headphones. A tight coupling between the headphone and the ear is necessary in order to maximize passive attenuation. However, too much pressure can make the headphone uncomfortable to wear for long periods of time. Caliper pressure is measured in Newton’s (N) and could range from 1.6 N (loose fit) to 10 N (snug fit).

Circumaural?
The Circumaural headphone encircles the ear creating a seal thereby effectively reducing ambient noise from getting to the ear (this is known as passive noise reduction). This type of headphone is most effective in a high noise environment where it's necessary to eliminate as much of the environmental noise as possible for effective communication.

Condenser (Electret) Microphone?
A microphone that is based on an electrically charged diaphram/backplate assembly, which forms a sound-sensitive capacitor. Here, sound waves vibrate a very thin metal or metal coated-plastic diaphragm. The diaphragm is mounted just in front of a ridged metal or metal-coated-ceramic back plate. This assembly is known as a capacitor (condenser), which has the ability to store a charge or voltage. When the element is charged, an electrical field is created between the diaphragm and the back plate, proportional to the spacing between them. It is the vibration of this spacing, due to the motion of the diaphragm relative to the back plate that produces the electrical signal corresponding to the sound picked up by a condenser microphone. The construction of a condenser microphone must include some provision for maintaining the electrical charge or polarizing voltage. An Electret condenser microphone has a permanent charge, maintained by a special material (Electret) deposited on the back plate or on the diaphragm.

Dynamic Microphone?
They employ a diaphragm/voice coil/magnet assembly, which forms a miniature sound-driven electrical generator. Sound waves strike a thin plastic membrane (diaphragm), which vibrates in response. A small coil of wire (voice coil) is attached to the rear of the diaphragm and vibrates with it. The voice coil itself is surrounded by a magnetic field created by a small permanent magnet. It is the motion of the voice coil in this magnetic field that generates the electrical signal corresponding to the sound picked up by a dynamic microphone. Dynamic microphones have a simple construction and are economical and rugged. They can provide excellent sound quality and good specifications in all areas of microphone performance. They handle extremely high sound levels: it is almost impossible to overload a dynamic microphone. They are relatively unaffected by extremes of temperature humidity.

Diaphragm?
The thin membrane in a microphone, which moves in response to sound waves.

Electret?
A material (such as Teflon) that can retain a permanent electric charge.

Gain?
The amount of increase in audio signal strength, often expressed in dB

Mic Gain Control?
A level control on a microphone preamplifier that sets gain and is used to prevent overload of a preamplifier.

Pre-Amp?
A low-noise amplifier designed to take a low-level signal (such as that from a microphone) and bring it up to normal line level.

Why is it a good idea to have a mic gain adjustment on your microphone?
This gives the technician the ability to make adjustments for minor anomalies that may occur during the installation of the audio system. While the microphone may work fine, often a small adjustment may make a big difference in the total performance of the microphone.

Maximum SPL (Sound Pressure Level)?
SPL is the loudest sound that can be produced by the headphones. Maximum SPL is measured in decibels (dB). A typical rating would be 118 dB. The higher the number the louder the headphones can get.

Minimum terminating impedance?
This is the lowest input impedance rating the mic input of the radio can have. The microphone may not function properly if the input impedance of the radio is too low.

NRR (Noise Reduction Ratings)?
NRR is a rating of the passive reduction and this is by a set standard (ANSI). However, there are no present standards for measuring ANR. Each Manufacturer uses their own standards. Sennheiser uses the standards and results of an independent laboratory that is accredited by the National Institute of Standards and Technology. One should be very careful in assuming that all manufacturers use these results in a fair and unbiased way.

NoiseGard on/off switch?
This is the switch that allows you to turn the active noise compensation circuitry on or off. It comes with a two colors LED for-power on and low battery-on headsets using a battery pack. NOTE: If you should turn off the NoiseGard system there is no loss of, or reduction of, the received audio and you still retain the passive capabilities of the headset. You may have to increase the receiver volume to compensate for the increase of ambient noise due to the loss of the NoiseGard feature. The LED is located on the battery pack of the headsets. The battery pack in included in-line on the HMEC25/45 series and is an optional accessory on the HMEC300 series.

Nominal impedance?
This is the amount of opposition to alternating current. The ideal nominal impedance of a headphone would exactly match the output impedance of the source. Differences between the output impedance of the source and the impedance of the headphone could result in louder or softer volume than expected.

Output voltage?
This is similar to sensitivity except it takes into account additional electronics used to amplify the signal. This is typically measured over a range of frequencies and will include the term + or - 3 dB. Rather than using 1 Pascal, this specification may also include the input level used such as 114 dB at a distance of 6 mm.

Overload?
To put too much signal level into thereby causing distortion

PJ055 Headphone Plug?
Standard ¼ in. plug, found on most aircraft headsets.

PJ068 Microphone Plug?
Standard 5.25mm plug found on most aircraft headsets. On some Sennheiser headsets (HMEC25/45 series KAP version only) the tip of this plug can be used to power (28volt aircraft only) the NoiseGard power supply.

Sensitivity?
This is a measurement of how much voltage is produced by a microphone for a specific measured input (1 Pascal). Sensitivity is measured in micro volts (uV) or millivolts (mV). A typical rating for a dynamic microphone would be 1 mV, for an Electret would be 3.5 to 10 mV, and an Electret with amplifier would be 170 mV. The higher the number the stronger the signal produced by the microphone.

Supra-Aural?
The Supra-Aural headphone does not encircle the ear but rests on the ear. Because the ear cup is smaller it is generally lighter and more comfortable. This type of headphone is recommended for an environment where ambient noise is not excessive and where noise reduction can be effectively reduced for communication clarity using active (electronic) noise reduction or where noise reduction is not a factor in choosing a headphone.

TSO?
FAA technical standard order. JAA The European Joint Aviation Authority. HMEC 25 and 45 Series headsets. The success of Sennheiser to receive these approvals, C57a and C58a go a long way in providing evidence of the superior quality and performance of Sennheiser products. That is why they are used in over a hundred Air Carriers all over the world both on the flight deck and in the cabin.

Transducer?
A device that converts energy from one form to another. Example: A microphone converts sound waves into electrical energy. An earphone converts that electrical energy into sound waves.

XLR-5 Connector?
This is a single connector that can be used in place of the PJ055 headphone and the PJ068 microphone plugs on some Sennheiser headsets. Pin 5 of this plug can also be used to provide aircraft power for the NoiseGard power supply on some Sennheiser headsets.

XLR-3 Connector?
This is the connector that is used to power the NoiseGard power supply. It provides 12-35VDC from the aircraft. It is used in conjunction with the PJ055 and the PJ068 plugs.

Sennheiser Nomenclature Key

H = Headphone
D = Dynamic Transducer
M = Microphone
E = Electret
C = Active Noise Cancellation
K = Kit

300 and 100 series = closed cup (Circumaural) Headphone
25 series = Supra-Aural (closed back) headphone
45 series = Open-Aire® Supra-Aural headphone

KA = PJ068 connectors
CA = 5 PIN XLR connector
KAP = PJ type connector, MIC jack powered (28V Aircraft)
CAP = 5 PIN XLR, Pin 5 Powered (28V Aircraft)
KAX = PJ Type connector, XLR-3 Pin Powered (12-35V)