# Dictionary:*Q*

**1**. **Q**uality factor, the ratio of 2π times the peak energy to the energy dissipated in a cycle; the ratio of 2π times the power stored to the power dissipated. The seismic *Q* of rocks is of the order of 50 to 300. *Q* is related to other measures of absorption (see below):

*Q*=α

*V*/π

*f*=αλ/π=

*hT*/π=δ/π=2Δ

*f*/

*f*

_{r},

where *V*, *f*, λ, and *T* are, respectively, velocity, frequency, wavelength, and period (see Sheriff and Geldart, 1995: 60, 177). The **absorption coefficient** α is the term for the exponential decrease of amplitude with distance because of absorption; the amplitude of plane harmonic waves is often written as

*Ae*

^{–αx}sin2π

*f*(

*t*–

*x*/

*V*),

where *x* is the distance traveled. The **logarithmic decrement** δ is the natural log of the ratio of the amplitudes of two successive cycles. The last equation above relates *Q* to the sharpness of a resonance condition; *f*_{r} is the resonance frequency and Δ*f* is the change in frequency that reduces the amplitude by 1/√2. The **damping factor** *h* relates to the decrease in amplitude with time,

*A*(

*t*)=

*A*

_{o}

*e*

^{–ht}cosω

*t*.

See also Figure A-2. **2**. The ratio of the reactance of a circuit to the resistance. **3**. A term to describe the sharpness of a filter; the ratio of the midpoint frequency to the bandpass width (often at 3 dB). **4**. A designation for *Love waves* (q.v.). **5**. Symbol for the *Koenigsberger ratio* (q.v.). **6**. See *Q-type section*.