Appendix 3 A calibration signal is particularly necessary when the microphone is used with amplifiers other than a standard sound level meter or when a recorder is used.
This "end-to-end" calibration should be made both at the beginning and end of a test run, and at any other time where there is a possibility that the system gain may have been changed.
It is important in all test qerations to maintain an accurate log of all conditions: microphone placement, weather conditions if outdoors, system channel connections (if more than one channel), all attenuator and calibrated amplifier gain settings, time of day and date, source and distance from source to microphone, etc. When a tape recorder is used, the log information should be recorded vocally on the tape.
While the PISTON-PHONE calibrator is an essential part of any acoustic measurement program, it does not give an adequate check of microphone, amplifier and recorder frequency characteristics. The instrumentation and procedures required for full calibration are beyond the scope of this discussion, but some provision should be made for periodic recalibration of system components by the manufacturer or by a reliable and well-equipped standardization laboratory.
CALIBRATION A preflight sensitivity check should be used to adjust the gain of the sound level meter to match the output of the acoustic calibrator as adjusted for atmospheric pressure. A "warm-up" time of at least 1 minute should be allowed before checking the sensitivity of the sound level meter. If a tape recorder is used, the sensitivity checks shall also be recorded.
If an in-flight acoustic sensitivity check is used, it should be taken when the aircraft has reached the desired cruise altitude and the aircraft's internal pressure is at the desired value. The indicated sound pressure level of the output of the acoustic calibrator should be noted; the gain of the sound level meter should not be adjusted in flight if the indicated level is not the same as the acoustic calibration level obtained before takeoff. If necessary, cabin pressure should be noted so that adjustments for differences in air pressure may be made.
Appendix 4 APPENDIX 4
EXAMPLE CALCULATION OF PSIL PSII = I500 + 11000 + 12000 * T.O. Power Normal Cruise Power Approach Power Octave Band Avg. Meas. Avg. Meas. Avg. Meas.
Cntr. Freq. Data Data Data 63 106.4 103.0 102.8 125 114.5 111.6 110.0 250 110.0 109.2 100.5 500 99.1 95.8 86.5 1000 84.6 80.1 73.9 2000 81.2 78.4 73.2 4000 76.9 73.8 74.8 8000 76.1 74.1 73.7 db (C) 116.2 113.8 110.9 db (A) 104.3 102.7 96.6 PSIL 88.3 84.7 77.9 From the above it can be seen that the takeoff and normal cruise power noise levels exceed a PSIL of 78 and speech interference can be expected in the cockpit in those flight regimes. The db (A) in all three flight regimes also exceed the recommended level of 88.
* L is the noise level (flat) at the specified octave band center frequency.
Appendix 5 APPENDIX 5
MODIFICATIONS OF SIGNAL-TO-NOISE RATIOS An easy speech intelligibility concept to grasp is that the louder the speech is in comparison to the background noise, the easier it is to understand.
obviously, there are practical limits to the concept, but through most of the range of audible sound pressures, this statement about the speech-to-noise or the signal-to-noise ratio (S/N) is true. (Where both speech and noise are extremely quiet or extremely intense, nonlinearities arise. For the cockpit-noise situation, one may confront a degree of high-intensity An improvement in S/N, then, will serve to improve the intelligibility of speech.
The most direct approaches call for an increase in absolute signal level or a decrease in absolute noise level. One may also try to create relative differences between the signal and the noise levels.
The difficulty with trying to decrease cockpit noise levels at the source has already been discussed. However, it should be noted that noise attentuation materials are available for light aircraft. The use of inflatable door seals nd acoustic blankets can reduce interior noise levels. Nevertheless, the most ttective option may be to increase signal levels or modity the relationship between signal and noise.
Signal levels can be increased by increasing the gain of an intervening amplifier (for electronically transmitted communications), or by moving the talker and listener closer together. Research has shown a deterioration of intelligibility with an extremely weak or strong vocal force.
Hearing protectors for aviators can provide protection against hearing loss that results from noise exposure and improves speech intelligibility.
perform the intelligibility improvement task in two ways. The lesser of these is that they lower the overall intensity of the sound that enters the human auditory system into a middle range of sound pressures where the system operates optimally. (Note that hearing protectors do not remove sound; they only decrease its intensity). The other way is selective filtering which can be effective in some noise environments.
some precautions are necessary, though, before one elects to use hearinc rotectors for the purpose of improving voice communication. First, a well-sealed, well-fitted protector is necessary. Second, some auditory functions are changed by the introduction of hearing protectors into the transmission system. For example, some people report a decrease in the ability to make fine pitch discriminations, many people report a decrease in the ability to judge the azimuth of a sound source. However, the human auditory system rapidly accommodates itself to environmental change of all sorts, so one can assume that with a bit of practice these functions can be brought back into the normal range. Third, because one adjusts one's vocal effort to overcome the 1
Appendix 5 noise one hears, hearing-protector wearers (since they hear less noise) usually don't speak loudly enough. Persons who wear hearing-protectors must train themselves to speak more loudly.
In most cockpits where noise is a problem, the noise spectrum tends to have the same shape as the average speech spectrum. As a result, one cannot count on selective filtering to improve speech intelligibility. Whatever changes are made in one spectrum will be made similarly in the other. The S/N stays about the same. Thus, in cockpits with similar noise and speech spectrums, the improvement in speech intelligibility for pilots and copilots who wear hearing protectors is probably limited to the small amount that arises from bringing signal intensities into the linear, middle frequency range where the auditory system works better.
A microphone may help some, because if it is held close to the mouth, it is somewhat like reducing the distance to the ear. Considerably more improvement in S/N can be obtained by using noise cancelling microphones in communication systems. The noise-cancelling microphone is built to accept sound from the front, the back, or the top. In a fairly homogeneous sound field, approximately the same ambient-noise wave form enters from both sides, serving to cancel much of the effect of the noise on the microphone diaphragm. A talker, though, directs his or her speech to one side only, so the cancellation effect for speech is far less than for noise--if the user understands the proper way to use the microphone. Covering the rear vents with the hand diminishes the cancellation effect. Holding the front of the microphone more than a few inches from the lips of the talker permits the speech to enter the back with nearly as much intensity as enters the front, thus cancelling speech as well as noise. Another potential loss of S/N improvement results from the reflex that leads a talker to speak with enough effort to be heard above the noise: if the talker expects to be heard (by the microphone) at a distance of 3 inches rather than 3 feet, he or she is likely to reduce vocal effort accordingly.
Miniature headsets have come into use among pilots in recent years.
The headsets, which are worn over the ear, conduct sound to the microphone diaphragm via a hard, plastic tube that is hinged so that it can be moved about Although these headsets are not noise-cancelling devices in the usual sense, the tip of the plastic tube can be moved so close to the talker's lips as to make a significant improvement in S/N over face-to-face communications in the same noise environment. Again, the likelihood of improvement is a direct function of how much vocal effort is exerted and of how close the tube is to the mouth; it the tube has been moved out of the way las it needs to be for eating or drinking), any S/N improvement will be markedly diminished.
Some headsets are equipped with circumaural muffs which attenuate the cockpit noise and enhance the S/N for electronic communications. This type of ear muff furnishes some hearing protection and acts somewhat like an ear plug in normal cockpit voice communications. Headsets equipped with the better designed circumaural muffs may attenuate cockpit noise more that 20 dB. These headsets used with noise cancelling or boom microphones and an intercom system can substantially enhance the S/N and markedly improve crew communications.
Appendix 5 Proper use requires holding the noise-cancelling microphone so that the vents are not blocked, holding it close to the mouth, and speaking as loudly as if the listener were a few feet away. When the microphone is used properly, it can make a significant difference in S/N.
It should be noted that increasing the gain of an amplifier or trying to do selective electronic filtering will make no useful change in the S/N; it will stay the same as it was at the face of the microphone whose sounds are being amplified or filtered. If the S/N is poor to begin with, amplifying both the speech and the noise cannot make the situation any better. Also, electronic filtering is no different in its effect than the acoustic filtering that a hearing protector does: if the spectrum of the noise and the spectrum of the speech are similar, selective filtering will not help.
Additional information on aircraft audio system characteristics and standards No. Do-170, Audio Systems Characteristics and Minimum Performance Standards, Aircraft Microphones (Except Carbon), Aircraft Headsets and Speakers, Aircrait Audio Selector Panels and Amplifiers, January, 1980.