# Frequency Response Function

The output to input relationship of a structure. Mathematically, it is the Fourier transform of the output divided by the Fourier transform of the input. It is also the transfer function measured along the j axis in the s-plane.

### Co-Incident

Another name for the real part of the frequency response function.

### Dynamic Stiffness

The frequency response function of force/displacement.

### Effective Mass

The frequency response function of force/acceleration.

### Imaginary Part

A plot of the imaginary part of the frequency response function versus frequency. For a single-degree-of-freedom system, the magnitude is a maximum or minimum at the damped natural frequency.

### Real Part

A plot of the real part of the frequency response function versus frequency. For a single degree of freedom, the magnitude is zero at the damped natural frequency.

**See also: **Effective Mass, Frequency Response, Frequency Response Matrix, Impulse Response, Modal Analysis.

**Subjects: ** Noise & Vibration