# Buzzed this morning

#1

This morning, just after 7AM in Austin, I was buzzed by a plane. It sounded like it was very low. The pitch change from the doppler effect was very fast, and the sound volume change was also very fast. I listened to the radio for a while, half way expecting to hear a report of a crashed plane.

I didn’t see it, but this got me to wondering about how I could tell how close it was given only a sound recording. I figured:

Let:
S = speed of sound

I = speed of plane toward observer (-I is then speed of plane away after it passes)
P0 = the pitch of the propeller’s sound in Hz in the plane’s reference frame
PI = the pitch of the propeller in Hz from the listener’s reference frame.

I think PI = S * PO / (S - I)

If R = the ratio of the incoming pitch to the departing pitch (both of which would be asymptotes assuming no collision), then

R = (S + I) / (S - I)

Solving for I we get:

I = (R - 1) * S / (R + 1)

The volume of the sound is inversely proportional to the square of the distance (sound pressure is inversely proportional to distance). Graphing the volume over time, the sound of a passing plane looks like a bell, with the peak being where (but not when, because the sound takes a while to travel to the observer) the plane is at closest proximity.

I think what needs to be done is to compare how much time elapsed from where this graph changes from 1/2 the peak height to the peak height. That establishes a time frame and a distance ratio. We then compare this with the known speed of the plane in the first step to get the distance, but I haven’t worked that part out, yet. I’m not sure what the proper units would be on the sound recordings volume. Would that correspond to sound pressure or sound intensity?

#2

Buzzed, yes. Definitely buzzed. No doubt.

#3

At the least!

#4

A neighbor posted this:

It was a Cessna 172, heading due east…very fast, at 7:12 a.m. I believe
it was the traffic plane for KASE/KVET ? This is not the first time either
! Last time, the pilot did the same thing…following the same course and
just as low, between Teakwood and Ohlen. I would guess his altitude was
probably 250-300 ft. AGL !

How can I find out what the height requirements are for here? Possibly relevant, there was a very low ceiling at the time, and light fog.

#5

Flying low and fast to get somewhere does NOT equate with “buzzing”!

#6

OK. The jokes were funny, but now I’m serious. How can I find out what the minimum height is a Cessna 172 may fly over a residential neighborhood?

And, BTW, the title to the thread was an attempt to put some levity on this situation, which, in my opinion, is actually a serious one. The plane was low and loud enough to wake several neighbors up from their sleep. I was already awake but still groggy, but not after the plane flew by.

Weather conditions were very light fog and a low ceiling.

#7

It doesn’t equate to good judgment either.

#8

If the radio station operates under part 91, then FAR 91.119 applies (1000ftAGL).

#9

Are there any airports, no matter the size, near your observation point?

Minimum altitude would be either 1,000 or 500 feet, depending on whether it’s over a congested or non-congested area

"Sec. 91.119 — Minimum safe altitudes: General.
Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:

(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.

(b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.

© Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.

(d) Helicopters, powered parachutes, and weight-shift-control aircraft. If the operation is conducted without hazard to persons or property on the surface—

(1) A helicopter may be operated at less than the minimums prescribed in paragraph (b) or © of this section, provided each person operating the helicopter complies with any routes or altitudes specifically prescribed for helicopters by the FAA; and

(2) A powered parachute or weight-shift-control aircraft may be operated at less than the minimums prescribed in paragraph © of this section."

#10

Shouldn’t that be “an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft” instead of AGL?

#11

The nearest airport (about 8 miles) is Executive airport, to the east of the observation point. It’s runstrip is oriented roughly NW/SE, so doesn’t align with the path of the airplane. The next closest airport is Austin Bergstrom, about 10 miles away. Commercial traffic landing there generally are further east and flying N/S, unless they need to land northbound, in which case, they’ll fly around the airport and do a u-turn to head upwind. When that happens, they fly directly overhead southbound.

The plane in question was eastbound.

#12

Only over a congested area.

OK, how would you describe the area in which you “observed” the flight, congested or non-congested? Be honest.

Could you share the observation location with us? Approximate will suffice. The subject aircraft may have been following ATC instructions to fly below surrounding class B airspace.

#13

Ah, OK. Smack dab in the middle of the Wooten neighborhood. Zip 78757. Teakwood and Mullen intersection is probably very close.

#14

This is not particularly relevant, except for those of us who live in the area and remember the event, but the flight path is about 2 miles south but in the opposite direction of the plane that flew into the IRS building in February 2010.

#15

I’m not saying it was or wasn’t 250-300 ft AGL because I wasn’t there. How did you determine the altitude? Are you experienced at estimating altitudes? Was there a reference point you could use to estimate the height? Do any trigonometric functions to determine the altitude?

#16

I got the information from a neighbor. I have no idea how he got his information. It seemed to match what I heard, though, but without a recording, it’s only speculation. I can say this, though, the time it took for the bulk of the Doppler effect to transpire was on the order of a second or two. Since the Doppler effect has asymptotes, it’s hard to peg this down, too, I realize.

Out of curiosity, I search youtube for doppler effect to see if there was something approximating what it sounded like. Surprisingly, the first link I tried was actually pretty good.

The frequency is way off, of course. Ignore that. Pay attention, instead, to how fast the pitch changes. The very first example is the closest, but it wasn’t quite that fast. Maybe if the transition time doubled it would be about right. It was definitely much faster than the second example. The perception of the event was that the pitch took 1-2 seconds to change. That transition time is what I’m going by. I’ll ask my neighbor how he arrived at the height.

#17

Dunno what ExCalbr’s description would be but my description would be flight within the yellow boundaries that identify towns and cities on a VFR sectional

#18

Yep. It’s about at the intersection of the outer purple circle and the solid yellow of Austin right above the word AUSTIN on this map:
flightaware.com/resources/airport/KAUS/sectional

In chatting with someone about this, we discussed how there was a low ceiling that morning, and if VFR were in effect, they would have had to fly pretty low to be under the clouds. Maybe they got caught by surprise by the weather (but the weather was very consistent with the forecast).

#19

I found a youtube video that sound approximately right. Different plane, but the Doppler transition speed sounds about right.