Editing Reverberation (section)

=== Measurement ===

[[File:RT60 measurement.jpg|thumb|240px|right|Automatically determining T20 value - 5dB trigger - 20dB measurement - 10dB headroom to noise floor]]
Historically, reverberation time could only be measured using a level recorder (a plotting device which graphs the noise level against time on a ribbon of moving paper). A loud noise is produced, and as the sound dies away the trace on the level recorder will show a distinct slope. Analysis of this slope reveals the measured reverberation time. Some modern digital [[sound level meter]]s can carry out this analysis automatically.<ref>{{Cite web|url=https://www.nti-audio.com/en/applications/room-building-acoustics/reverberation-time|title=Reverberation Time|website=www.nti-audio.com}}</ref>

Several methods exist for measuring reverberation time.  An impulse can be measured by creating a sufficiently loud noise (which must have a defined cut-off point). [[Impulse noise (audio)|Impulse noise]] sources such as a [[Blank (cartridge)|blank]] pistol shot or balloon burst may be used to measure the impulse response of a room.

Alternatively, a [[Colors of noise|random noise signal]] such as [[pink noise]] or [[white noise]] may be generated through a loudspeaker, and then turned off. This is known as the interrupted method, and the measured result is known as the interrupted response.

A two-port measurement system can also be used to measure noise introduced into a space and compare it to what is subsequently measured in the space. Consider sound reproduced by a loudspeaker into a room. A recording of the sound in the room can be made and compared to what was sent to the loudspeaker. The two signals can be compared mathematically. This two port measurement system utilizes a [[Fourier transform]] to mathematically derive the impulse response of the room. From the impulse response, the reverberation time can be calculated. Using a two-port system allows reverberation time to be measured with signals other than loud impulses. Music or recordings of other sounds can be used. This allows measurements to be taken in a room after the audience is present.

Under some restrictions, even simple sound sources like handclaps can be used for measurement of reverberation <ref>{{cite journal |last1=Papadakis |first1=Nikolaos M. |last2=Stavroulakis |first2= Georgios E. |title=Handclap for Acoustic Measurements: Optimal Application and Limitations.  |journal=Acoustics |year=2020 |volume=2 |issue=2 |pages=224–245 |doi=10.3390/acoustics2020015 |doi-access=free }}</ref>

Reverberation time is usually stated as a decay time and is measured in seconds. There may or may not be any statement of the frequency band used in the measurement. Decay time is the time it takes the signal to diminish 60&nbsp;dB below the original sound. It is often difficult to inject enough sound into the room to measure a decay of 60&nbsp;dB, particularly at lower frequencies. If the decay is linear, it is sufficient to measure a drop of 20&nbsp;dB and multiply the time by 3, or a drop of 30&nbsp;dB and multiply the time by 2. These are the so-called T20 and T30 measurement methods.

The RT<sub>60</sub> reverberation time measurement is defined in the [[ISO]] 3382-1 standard for performance spaces, the [[ISO]] 3382-2 standard for ordinary rooms, and the [[ISO]] 3382-3 for open-plan offices, as well as the [[ASTM]] E2235 standard.

The concept of reverberation time implicitly supposes that the decay rate of the sound is exponential, so that the sound level diminishes regularly, at a rate of so many dB per second. It is not often the case in real rooms, depending on the disposition of reflective, dispersive and absorbing surfaces. Moreover, successive measurement of the sound level often yields very different results, as differences in phase in the exciting sound build up in notably different sound waves. In 1965, [[Manfred R. Schroeder]] published "A new method of Measuring Reverberation Time" in the ''Journal of the Acoustical Society of America''. He proposed to measure, not the power of the sound, but the energy, by integrating it. This made it possible to show the variation in the rate of decay and to free acousticians from the necessity of averaging many measurements.