What exactly is this resonance curve showing?

[Kashim]

Hi I had a quick question. From what I understand resonance is when a natural frequency of an object is matched by the driver frequency however in this graph it seems as though resonance is occurring at all the frequencies around the natural frequency just not to a great extent.

http://perlgeek.de/talks/2010/yapceu-p6-realworld/resonance.png

So according to this graph damping reduces the maximum effect that can take place, i.e. the maximum amplitude possible at THE natural frequency, however increases the number of frequencies at which 'partial' resonance can occur. This doesn't make sense to me as I thought that resonance can only occur at ONE frequency, the natural frequency.

This ties into why I don't understand when people refer to 'resonant frequencies' I thought there can only be one.

Sorry if I'm unclear and if you don't understand anything I wrote please let me know and I'll try to clarify.

 

[mfb]

You get large oscillations close to the resonance frequency, too.

This doesn't make sense to me as I thought that resonance can only occur at ONE frequency, the natural frequency.

No.
For systems with low damping, the peak in the graph can be very sharp, so it can look like a single frequency, but in real systems it is always a frequency range.

This ties into why I don't understand when people refer to 'resonant frequencies' I thought there can only be one.

There can be many resonance frequencies, related to different oscillation modes of the system.

 

[Dale]

Resonance is a phenomenon where at a particular frequency some energy transfer is particularly effecient. "The" resonant frequency is the frequency at which the energy transfer is most efficient, so it is a single frequency, the peak of the plot you showed. Energy transfer generally occurs at other frequencies also, it is just less efficient than at "the" resonant frequency. For frequencies that are very close to the peak resonance frequency the energy transfer is very close to as efficient as at the resonant frequency.

Note, it is possible for there to be more than one peak.

 

[Kashim]

Ok thanks a lot guys :)

Edit: Sorry 1 more. Are harmonic frequencies (multiples of the fundamental) the same thing as resonant frequencies?

 

[Philip Wood]

A single 'lumped' mass on a spring will have a single natural frequency. The natural frequency is also the resonance frequency - the frequency of driving force for maximum amplitude.

A distributed mass system, for example a guitar string or a drum skin will have several (strictly infinitely many) natural frequencies. In the case of the guitar string (but not for the drumskin) the natural frequencies are multiples of the lowest frequency or 'fundamental'. All natural frequencies are resonance frequencies: the system will respond strongly to applied oscillating forces of those frequencies.