# We hear from 20-20,000 Hz. Why can't I hear some sort of steady pitch from an idling jet?

**URL:** <https://boards.straightdope.com/t/we-hear-from-20-20-000-hz-why-cant-i-hear-some-sort-of-steady-pitch-from-an-idling-jet/614587>\
**Category:** Factual Questions\
**Created:** [March 2, 2012, 9:31pm UTC](https://boards.straightdope.com/t/we-hear-from-20-20-000-hz-why-cant-i-hear-some-sort-of-steady-pitch-from-an-idling-jet/614587 "2012-03-02T21:31:28Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\
**Post date:** [March 2, 2012, 9:31pm UTC](https://boards.straightdope.com/t/we-hear-from-20-20-000-hz-why-cant-i-hear-some-sort-of-steady-pitch-from-an-idling-jet/614587/1 "2012-03-02T21:31:28Z")

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See subject. Maybe it’s just me.

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**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\
**Post date:** [March 2, 2012, 9:36pm UTC](https://boards.straightdope.com/t/we-hear-from-20-20-000-hz-why-cant-i-hear-some-sort-of-steady-pitch-from-an-idling-jet/614587/2 "2012-03-02T21:36:11Z")

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I think you’ll have to clarify your question a bit (or I’m misunderstanding it). I live and work across the street from an airport. I hear idling jet engines all the time. They’re really not all that loud though.

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**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\
**Post date:** [March 2, 2012, 9:42pm UTC](https://boards.straightdope.com/t/we-hear-from-20-20-000-hz-why-cant-i-hear-some-sort-of-steady-pitch-from-an-idling-jet/614587/3 "2012-03-02T21:42:41Z")

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There are many sources of noise from jets: fans, combustors, mixing of the exhaust plume, scrubbing of the jet on nozzle surfaces, etc. Only those associated with rotating machinery (mostly fan noise) would be dependent on a physical phenomenon occurring at a particular frequency. And some fans _are_ particularly “tonal”, but not all. The rest of the noise sources are atonal, because they are products of turbulent processes that created broad-spectrum, nearly white noise.

A C-5 Galaxy, with its TF39 engine, has one of the most distinctively tonal sounds of any engine I’ve heard, especially on approach. But most engines have their fan tones drowned out by the other noise sources.

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**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\
**Post date:** [March 2, 2012, 9:55pm UTC](https://boards.straightdope.com/t/we-hear-from-20-20-000-hz-why-cant-i-hear-some-sort-of-steady-pitch-from-an-idling-jet/614587/4 "2012-03-02T21:55:01Z")

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Cool. Thanks. Ignorance fought.

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**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\
**Post date:** [March 2, 2012, 9:58pm UTC](https://boards.straightdope.com/t/we-hear-from-20-20-000-hz-why-cant-i-hear-some-sort-of-steady-pitch-from-an-idling-jet/614587/5 "2012-03-02T21:58:46Z")

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Now that the question has been answered, I was wondering if we can hear the same phenomenon in pencils due to their 6 sides. When I spin them I get a humming sound, and when I speed up the rotation the humming seems higher in pitch. I’m wondering if it comes from the 6 sides of the pencil, or from perhaps the 30+ sides of the ferrule, or perhaps from something else.