Resonance frequency question

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Mike - 19 Jul 2007 19:15 GMT

Hello, very armchair physicist here...had a question about an
observation I had. Seems a bit odd...but it relates to the group...when
I am in a metal bathroom stall I find that when I hum certain
frequencies, the walls resonate and amplify the sound. What is the
physics behind this? Not trying to be annoying, just curious?

Mike

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Salmon Egg - 19 Jul 2007 19:56 GMT

On 7/19/07 11:15 AM, in article MPG.2109664e425be211989683@216.75.63.64,

> Hello, very armchair physicist here...had a question about an
> observation I had. Seems a bit odd...but it relates to the group...when
[quoted text clipped - 3 lines]
>
> Mike

Although the same material is available for acoustics, there is a much large
literature available for microwaves and electromagnetic radiations. Look up
cavity resonators, Q, impedance, and mode excitation. The same wave
fundamentals apply whether you are talking electromagnetics, sound, quantum
mechanical probability amplitude, or water waves.

Bill
-- Fermez le Bush--less than 18 months to go.

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angelo Campanella - 20 Jul 2007 14:53 GMT

> On 7/19/07 11:15 AM, in article MPG.2109664e425be211989683@216.75.63.64,
>>Hello, very armchair physicist here...had a question about an
>>observation I had. Seems a bit odd...but it relates to the group...when
>>I am in a metal bathroom stall I find that when I hum certain
>>frequencies, the walls resonate and amplify the sound. What is the
>>physics behind this? Not trying to be annoying, just curious?

A perfectly good acoustics question.

The following is a bit off-topic, I think.

> Although the same material is available for acoustics, there is a much large
> literature available for microwaves and electromagnetic radiations. Look up
> cavity resonators, Q, impedance, and mode excitation. The same wave
> fundamentals apply whether you are talking electromagnetics, sound, quantum
> mechanical probability amplitude, or water waves.

    Returning to acoustics, Mike is experiencing the effects of standing
waves in a relatively absorption-free environment. The sound bounces
from one hard surface to another. A resonance or strngthening of the
sound occurs at the frequency where a round trip of sound of exactly one
full wavelength occurs over the longest dimension of the room.

    My shower stall does the same thing. For a height of 8 feet, the lowest
frequency resonance is at 1100/8/2=69 Hz, with higher order mode
frequencies of twice and three times, etc, that.

    In width, the frequency of the first lateral mode for a 36" width is at
183 Hz, etc.

    Then there are the skew modes. Most books on acoustics ("room
acoustics") will provide a formula to calcualte the frequency of all
possible modes.

        Angelo campanella

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Salmon Egg - 20 Jul 2007 21:29 GMT

On 7/20/07 6:53 AM, in article
L73oi.178073$Sa4.103977@bgtnsc05-news.ops.worldnet.att.net, "angelo
Campanella" <a.campanella@att.net> wrote:

> Returning to acoustics, Mike is experiencing the effects of standing
> waves in a relatively absorption-free environment. The sound bounces
[quoted text clipped - 12 lines]
> acoustics") will provide a formula to calcualte the frequency of all
> possible modes.

Just because there is a mode, doe not mean that it gets excited.
Where in the standing wave you are affects the radiation impedance. Q of the
mode is also important.

Another related consideration is that of speckle. That commonly gets treated
as an optical phenomenon but is also present in acoustics.

Bill

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If intelligent design trumps evolution, please explain hemorrhoids.

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Natural Science Forum / Physics / Acoustics / July 2007

Resonance frequency question