Mechanical Engineering homework help. MAE$101B, Spring$2020$Homework/Units$and$dB
(show$all$work)
1) Convert$as$indicated$giving$answer$with$proper$sign$
figures.
a. 7.550×103 km$to$feet
b. 285$K$to$degrees$Fahrenheit$
c. 6.85×104 ft$lbf$to$joules
d. 14.7$lbf/in2 to$pascals
e. 77.7$slug/ft3$to$grams$per$cubic$centimeter
2) Using$the$rules$for$expressing$SI$units,$express$each$of$the$
following$in$correct$form$if$given$incorrectly
a. 11.5$cm’s
b. 475$N
c. 9.5$m/s/s
d. 5000$K
e. 8,000$pa
f. 25$amp
g. 100.1$m$m
h. 62.5$j
i. 300$degrees$Kelvin
3) Compute$the$power$output$of$a$225$hp$engine$in
a. British$thermal$units$per$minute$and$
b. KiloVwatts
4) Convert$a$mass$flow$rate$of$195$kg/s$(typical$airflow$
through$a$turbofan$engine)$to$slugs$per$minute
5) An$acoustic$wave$has$a$sound$pressure$level$(SPL)$of$80$
dB$re$1µbar (Pref=1µbar).$Find$
a. The$sound$pressure$level$re$1µPa (Pref=1µPa)
b. The$sound$pressure$level$re$20µPa ( Pref to$20µPa)
i. Hint:$SPL=20*log10(P/Pref)
ii. 1µbar=0.1$N/m2=105µPa
c. If$the$new$SPLnew reading$is$18$dB$down$(62dB) re$
1µbar,$what$is$the$ratio$Pnew/P$equal$to$(P$pertains$
to$the$Pressure$at$80$dB)
Extra&Credit:$A$graduate$student$working$for$SpaceX$is$
interested$in$purchasing$microphones$to$measure$sound$
pressure$levels$(SPL)$for$the$development$of$the$human$
capsule.$The$output$of$any$sensor(microphones,$
accelerometers) is$an$electrical$signal.$These$electrical$
quantities$are$often$specified$in$terms$of$“levels”.$The$voltage$
level$is$defined$as$VL(re$Vref)=20log10(V/Vref).$In$order$to$
convert$between$electrical$and$acoustic$(pressure)$quantities$
the$sensor$manufacturer$furnishes$the$sensitivity$data.$The$
sensitivity,$Mo,$of$a$microphone$is$defined as$Mo=(V/P)$
(V=voltage,$P=pressure).$$This$sensitivity$is$usually$given$in$dB,$
and$this$term$is$usually$referred$as$“sensitivity$level”$ML(re&
Mref)=20log10(Mo/Mref).$Please$show$the$graduate$student$that$
in$order$to$obtain$SPL$from$the$sensor’s$voltage$reading,$the$
following$equation$can$be$applied
SPL(re$Pref)$=$VL(re$Vref)VML(re&Mref)+20log10(Vref/(Pref *$Mref))
