+ There are different metering standards, most of which are available in Ardour. In short:
+</p>
+
+<dl>
+ <dt>Digital peak-meter</dt>
+ <dd>A <dfn>Digital Peak Meter</dfn> displays the absolute maximum signal
+ of the raw audio PCM signal (for a given time). It is commonly used when
+ tracking to make sure the recorded audio never clips. To that end, DPMs
+ are always calibrated to 0 <abbr title="DeciBel Full
+ Scale">dBFS</abbr>, or the maximum level that can be represented digitally
+ in a given system. This value has no musical reason whatsoever and depends
+ only on the properties of the signal chain or target medium. There are
+ conventions for <dfn>fall-off-time</dfn> and <dfn>peak-hold</dfn>, but no
+ exact specifications.
+ <p>
+ Various conventions for DPM fall-off times and dBFS line-up level can be
+ chosen in <kbd class="menu">Edit > Preferences > GUI</kbd>.
+ </p>
+ </dd>
+
+ <dt>RMS meters</dt>
+ <dd>An <dfn><abbr title="Root Mean Square">RMS</abbr>-type meter</dfn>
+ is an averaging meter that looks at the energy in the signal. It
+ provides a general indication of loudness as perceived by humans. Ardour
+ features three RMS meters, all of which offer additonal peak indication.
+ <ul>
+ <li><dfn>K20</dfn>: A meter according to the K-system introduced by Bob
+ Katz, scale aligned to -20 dBFS, rise/fall times and color schema
+ according to spec.</li>
+ <li><dfn>K14</dfn>: Same as K20 with scale aligned to -14 dBFS.</li>
+ <li><dfn>K12</dfn>: Same as K20 with scale aligned to -12 dBFS (since 3.5.143).</li>
+ <li><dfn>Peak + RMS</dfn>: standard RMS, customizable via
+ <kbd class="menu">Edit > Preferences > GUI > Metering</kbd></li>
+ </ul>
+ </dd>
+
+ <dt>IEC PPMs</dt>
+ <dd><dfn><abbr title="International Electrontechnical Commission">IEC</abbr>-type
+ <abbr title="Peak Programme Meters">PPM</abbr>s</dfn> are a mix between DPMs and
+ RMS meters, created mainly for the purpose of
+ interoperability. Many national and institutional varieties exist (<abbr
+ title="European Broadcasting Union">EBU</abbr>, <abbr title="British Broadcasting
+ Corporation">BBC</abbr>, <abbr title="Deutsche Industrie-Norm">DIN</abbr>).
+ <p>
+ These loudness and metering standards provide a common point of
+ reference which is used by broadcasters in particular so that the
+ interchange of material is uniform across their sphere of influence,
+ regardless of the equipment used to play it back.
+ </p>
+ <p>
+ For home recording, there is no real need for this level of
+ interoperability, and these meters are only strictly required when
+ working in or with the broadcast industry. However, IEC-type meters have
+ certain characteristics (rise-time, ballistics) that make them useful
+ outside the context of broadcast.
+ </p>
+ <p>
+ Their specification is very exact, and consquently, there are no
+ customizable parameters.
+ </p>
+ </dd>
+
+ <dt>VU meters</dt>
+ <dd><dfn><abbr title="Volume Unit">VU</abbr> meters</dfn> are the dinosaurs (1939)
+ amongst the meters. They react very slowly, averaging out peaks.
+ Their specification is very strict (300ms rise-time, 1 - 1.5% overshoot,
+ flat frequency response). Ardour's VU meter adheres to that spec, but for
+ visual consistency it is displayed as a bar-graph rather than needle-style
+ (more below).
+ </dd>
+</dl>