During studies on the physiological effects of noise on human beings, it came to light that sales of tranquilizers, sleeping pills, antacids, cardio-vascular and anti-hypertensive drugs increased after an exposure to aircraft noise in the vicinity of airports. It is not known how many people are affected due to noise-induced-hearing damage.
Noise has become a silent hazard.With the aircraft industry poised to expand unprecedented, the number of aircraft flying is likely to double in next 20 years. The question one would ask would be: “Where are the noise regulatory bodies and what are they doing? What will happen in such a scenario?”

No amount of forced regulation will help. What can improve the situation is the re-design of aircraft for reducing noise levels.

Engineers observe that the overall shape of passenger aircraft has not changed much over the last five decades. Noise has not been a critical aircraft design target in the past.

Incidentally NASA has identified aero-acoustics as one of the 10 critical areas of science and engineering and started laying thrust on revolutionary concepts in aerodynamic design.

It has rightly set a goal of achieving a reduction of aerodynamic noise by as much as 30EPN (Effective Perceived Noise) dB relative to 1977 levels by the year 2022.

Experts opine that the configuration of such an aircraft has to be radically different from current designs. Recently, researchers from Cambridge University and Massachusetts Institute of Technology (MIT) unveiled a major silent aircraft design initiative based on revolutionary concepts.

This team aims to design a quiet aircraft which will produce a noise level of 63 dB (A) outside airport perimeter which is nearly 25 dB lesser than current aircraft, equal to that of a dishwasher! They also predict that such aircraft, when realized, will give 149 passenger-miles per UK Gallon of fuel compared to 120 for the best current aircraft in this range and size. Not just quiet, but fuel-efficient too!

Sound power is measured on a logarithmic scale – the deciBel. If one can reduce sound by 10 dB it means sound energy is reduced to 1/10th. In the same vein, a reduction of 25dB corresponds reducing sound energy to 1/500th of its original value.

Human ear responds to sound non-linearly. Suppose we have two grinders producing a certain noise level. If we switch one grinder off, half the sound energy is produced, but we barely perceive a reduction in noise.  That’s why aircraft noise has to be reduced by as much as 25dB to have appreciable differencel.

What makes a silent aircraft? There are three major noise sources in an aircraft: the engines, the under carriage, and the airframe – the physical structure of the plane. The engines are the obvious sources of noise with their huge fans at the front, high-speed machinery and jet exhaust.

Unlike conventional aircraft, the engines for the silent aircraft will be located above the wings, shielding the listeners on the ground. Presently, all aircraft have engines underneath the wings and hence sound tends to be reflected downwards. Shift in the position of the engines on the airframe poses many engineering challenges in aircraft design. Also, to absorb engine noise, acoustic liners will find place in engine inlet/exhaust.

Apart from redesign of the aircraft, effect of noise on humans can be minimized by certain operational techniques. For example, flying the aircraft high for as long as possible and/or keeping the aircraft at low engine power for longer time periods will reduce the overall noise reaching the ground.

Another effective technique is adopting the Continuous Descent Approach (CDA) in which aircraft stays at higher altitude at lower thrust and descends continuously. Current aircraft descend in steps allowing cruise at different heights before landing. Added advantage of CDA is that the aircraft consumes less fuel and gives out lesser emissions. We need greater integration of engine and mainframe.

The logical procedure involves optimizing the aircraft frame without considering noise as part of design. Noise analysis of this reference frame is then carried out using latest software and subsequently, noise is introduced as a constraint. The change in performance is quantified and compared to that of current equivalent aircraft. The reference configuration is re-optimized for the target noise level of 63 dB(A).

With the technology challenges numerous, this silent aircraft is expected to be a reality only by 2030.
The author is Director, Naval Science and Technological Laboratory, DRDO, Visakhapatnam 530027, and President, Acoustical Society of India.

Why ginger is good for nausea
Whether on a ship, in a car or on an airplane, most people have experienced the miseries of motion sickness. While the cause is always the same -- conflicting sensory signals going to the brain -- the list of potential remedies is vast. One of the oldest, ginger, is among those backed by the most evidence.

A study in the journal Lancet involved 36 people highly susceptible to motion sickness. The researchers had the subjects take either two capsules of powdered ginger, an antinausea medication or a placebo, and then, 20 minutes later, spin on a motorized chair for up to six minutes. Taking ginger delayed the onset of sickness about twice as long as taking the medication. The study also found that half the subjects who took ginger lasted the full six minutes, compared with none of those given the placebo or the medication.

A study by Danish scientists looked at 80 naval cadets prone to seasickness and found that those given one gram of ginger powder suffered less in a four-hour period then those given a placebo.

Precisely how ginger works is unclear, but at least one study suggested that one of its active compounds, 6-gingerol, enhances "gastrointestinal transport."
NYT News Service