what is the "coincidence frequency"?

[dzhoucam]

Hi,

Recently, I am begining to work for a project about sound transmission loss through a sandwich panel. Many literatures says coincidency freq. is that the trace wavelength matches the free bending wave in the panel. I feel confused. What is the trace wave? What's the relation with incident wave?

Thanks.

 

[tomirvine]

The critical frequency is the frequency at which the airborne acoustic wavelength matches the plate bending wavelength. This is also called the coincident frequency.

This is the frequency at which a plate with free boundary conditions has its highest loss factor. It is also the frequency at which the plate is most suspectible to acoustic excitation.

The acoustic wave is a longitudinal that has the following relationship:

wave speed = frequency x wavelength

The bending wave, however, has a more complex relationship between frequency and wavelength, as shown in Cremer, Heckl, Ungar, Structure-borne Sound, 2nd Ed, Chapter 6, equation (56).

Tom Irvine

http://www.vibrationdata.com

 

[MikeyP]

It's tricky to explain without a diagram.

Trace wavelength is the the wavelength of the acoustic wave "projcted" onto the panel. So if an acoustic wave of wavelength lanbda is impinging on the panel at an angle theta to the panel normal, then the wavelength "seen" by the panel is lambda/sin(theta). This is the trace wavelength.

The definition you give in your question

the frequency where "the trace wavelength matches the free bending wave in the panel"

is not quite correct. That is a definition of "trace matching" as given by Cremer and Heckl (see Tom's post for the full reference). The trace wavelength of the acoustic wave can match the wavelength of the bending wave at ANY frequency above the critical frequency (by varying the angle of incidence of the wave).

The critical frequency corresponds to the lowest frequency where trace matching can occur. At this frequency, theta MUST be 90 degrees (so called "grazing incidence&quot. At grazing incidence the acoustic wavelength and the trace acoustic wavelength are the same.

Bear in mind that a sandwich panel may well be orthotropic and hence have different critical frequencies in each direction!

M