![\"\"](\"http:\/\/cafe.foundation\/blog\/wp-content\/uploads\/2010\/07\/CAFE-EAS-IV-081-528x438.jpg\" "\\"CAFE")
<\/a>Dr. Charlotte Whitfield, NASA aeroacoustic expert<\/p><\/div>\n
When people write embittered emails to their local FAA office, or complain to the 911 operator, they are likely to be experiencing an unacceptable EPNL (Effective Perceived Noise Level) or SEL (Sound Experience Level).\u00a0 <\/a>As implied in\u00a0both terms, this can be a highly subjective response to the perceived sound level<\/a>, and varies from person to person.\u00a0 A pilot hearing a light twin passing overhead, for instance, may be excited by the sound, while a non-flying neighbor may be annoyed at the noise disturbing her attempt to meditate or quiet her child.<\/p>\n Both terms relate to not only the noise intensity, but\u00a0the second includes\u00a0its duration.\u00a0 A short burst of sound may be a temporary annoyance, but long-term exposure to that same sound can be damaging to the auditor.\u00a0 Sound level is normally measured in what auditory scientists call the A-weighted decibel scale (dBa), a logarithmic scale.\u00a0 Roughly speaking, sound intensity doubles for every three dB increase.\u00a0 Prolonged exposure to levels below 70 dBa are not harmful to the average human, but even short exposure to greater sound levels can cause hearing loss.<\/p>\n \u00a0Examples of potentially harmful sound levels <\/a>include:<\/p>\n For perspective, a person standing less than a foot away has to shout to be heard over a 95 dB noise level, which can cause hearing loss with only 45 minutes daily exposure.\u00a0 If that person has to shout in your ear to be heard, the sound level is over 105 dB, and can cause permanent hearing loss after only five minutes exposure.\u00a0 This should alert us that wearing those headphones and earplugs is an important part of maintaining our health.\u00a0 Airplanes, until we\u00a0achieve all-electric flight, are noisy.<\/p>\n Steady loading noise and thickness noise are related to the rate at which a blade passes through the air.\u00a0 For propeller or helicopter blade, steady loading noise results from cyclic pressure changes as this passage occurs.\u00a0 Thickness noise results from the regular parting of the air.\u00a0 Both types of noise are related to blade passing frequency (BPF) and its harmonics.\u00a0 This can be shown by the equation:<\/p>\n BPF = B x RPM \/ 60<\/p>\n in which B<\/em> is the number of blades that pass a fixed point (such as the side of a fuselage on a twin-engine craft \u2013 or a cowling air opening on a single-engine airplane) multiplied by the number of times the propeller turns per minute, divided by 60, to give the frequency in Hertz (cycles) per second.\u00a0 In a steady loading state where the airplane is stationary or moving at a steady speed, the propeller generates \u201csteady loading\u201d noise.\u00a0 Changes in rpm or variations in atmospheric conditions (random gusts, for instance) can cause \u201cunsteady loading\u201d noise.\u00a0<\/p>\n \u00a0L. Gutin\u2019s equations used in NACA papers by Harvey H. Hubbard and Leslie W. Lassiter <\/a>show a method for predicting the blade passing frequency and its harmonics in steady loading.\u00a0\u00a0<\/p>\n Ms. Whitfield recommended several routes of study for those interested in pursuing the matter.\u00a0 Hawkings and Lawson <\/a>used Gutin\u2019s equations as a basis for performing time domain methods in their analysis.\u00a0 Farassat<\/a> applied computational fluid dynamics to the problem.\u00a0\u00a0Again, many of these papers are in the CAFE Foundation technical library.\u00a0<\/p>\n At a practical level, well-known precepts apply.\u00a0 Ms. Whitfield\u2019s recommendations include turning a large propeller slowly to reduce disk loading and noise, and adding thin, stiff propeller blades.\u00a0 These require either a high-torque, slow motor, or a geared, high-speed motor in the electric aircraft of the future, and add mechanical and electrical problems that require solutions.\u00a0 As her talk suggested, these problems require careful analysis and rigorous mathematical foundations for their resolution. <\/p>\n\n