Re: Engine noise

Date:         05 Jan 97 03:22:35 
From:         D.P.Rhodes@lboro.ac.uk (Darren Rhodes)
Organization: Loughborough University
References:   1
Followups:    1
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On 27 Dec 96 13:32:25 , "Robert M. Sherry" <sherryr@a.cs.okstate.edu>
wrote:

>So just out of curiousity, are there different standards for
>sound *frequency* in the stage 2/3 specs, or is it purely a
>measure of decibels? Given what I know from tinkering with
>speakers, a low frequency sound may not be percieved as as
>loud as one of higher frequency, even if both are of the
>same power. So are engine manufacturers more likely to try
>and block the high frequency fan whine than the overall
>exhaust roar?

The difference in tone between the DC-9 and F100 is mainly due to the
difference in engine bypass ratio. The DC9 engine has a bypass ratio
around 1.0. The F100 uses R-R Tay engine with bypass ratios around
3.0. Lower bypass ratios mean more high frequency noise which the
human ear can pick more easily.

When certificating aircraft they use the Effective Perceived Noise
Level (EPNL) which is measured in EPNdB. First the sound pressure
levels are corrected for tones (e.g. high frequency noise) producing
the a Perceived Noise Level (PNL) in PNdB. The duration of the noise
is then taken into account by integrating the noise energy
(integration limits are -10dB from max level) to the EPNL.

As commented in another post the small difference is noise level you
heard between a stage 2 and a stage 3 is probably because the stage 2
aircraft was lighter and hence climbed faster away from you.

Many airlines also derate the take-off power to increase engine life.
If there is sufficient runway length available or the aircraft is
light they may only use 70% of max T/O power for take-off.