APPLICATION NOTE

Reverberation Timewith XL2 Sound Level Meter

The XL2 Acoustic Analyzer measures the reverberation time T in octave and one-third octave band
resolution. The step-by-step procedure to measure the reverberation time is described in the XL2user manual. This application note describes additional technical basics and details to be observed
for standard compliant measurements.

Index
What is Reverberation Time?

2

Distance Microphone to Speaker13

Why measure Reverberation?

3

Microphone Position

13

Measurement Methods T20 & T30

3

Distance between Microphones

13

Measurement Accuracy

5

Speaker & Microphone Positions

14

Sound Sources

6

Reporting

14

Measurement

11

Typical Results

16

People in the Room

12

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What is Reverberation Time?
Reverberation time is the time required for sound to “fade away” in a room.
Sound waves will repeatedly bounce off reflective surfaces such as the floor,
walls, ceiling, windows or tables and return to the listener position. This is particularly noticeable in a church, for example, where the sound may be heard forseveral seconds while it fades away.

In principle, reverberation time is the time required for the sound pressure level
to decrease by 60 dB after the sound stimulus signal abruptly stops. The sketch
below visualizes this basic principle of a reverberation time measurement.

Steady Sound Source

Sound turned OFF

Sound Level (dB)0Natural Decay of Sound

- 60

T

Time
Level Decay by 60 dB

The reverberation time measurement is defined in the following standards• ISO 3382-1 for performance spaces• ISO 3382-2 for ordinary rooms• ASTM E2235

The actual measurements span across 63 Hz to 8 kHz in octave band resolution
and 50 Hz to 10 kHz in 1/3 octave band resolution. Rooms have individual absorption capabilities for each frequency, so the reverberation time values withineach frequency band will vary.

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Why measure Reverberation?
Reverberation is a key parameter when qualifying the acoustic status of a room.
Particularly, too much reverberation may have a negative impact on the intelligibility of speech. On the other hand, too little reverberation will reduce therich, warm acoustic sound from an orchestra in a concert hall. Also, the soundpressure level from noise sources can be enhanced while the privacy of a room
can be decreased by reverberation in a room. Furthermore, reverberation time ismeasured to determine the room absorption correction value required in many
acoustic measurements, such as sound insulation and sound power.

Measurement Methods T20 & T30
By way of example, say the ambient noise floor in a room is 45 dB. To measure alinear decay of 60 dB, we need to ensure that the decay measurement ends 10
dB above this noise floor (so the linearity of the sound decay is not significantly
influenced by the ambient noise). An additional 5 dB headroom is required to detect the start of the decay. Adding these levels together, we require a test signal
level of 45 + 60 + 10 + 5 = 120 dB across the whole frequency spectrum. Thisis technically often not feasible, particularly at low frequencies.

Level [dB]
0 dB
-5 dB
distance tonoise floor

20 dB level range to obtain
the reverberation time
-25 dB-35 dBTime

Reverberation Time Measurement with T20 Method

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In practice, therefore, we measure the time taken for the reflections to decay
by 20 dB or 30 dB in accordance with the ISO 3382-1 standard. If the decay is
acceptably linear, these readings can then be linearly extrapolated to a decay of
60 dB. The reverberation time measurement method used is identified as T20
or T30 respectively.

T20• The measurement requires a relatively small dynamic measurement range of~35 dB above the noise floor for each frequency band.
• RT60 (T20) = 3 x time measured for a 20 dB decay

T30• The measurement requires a dynamic measurement range of ~45 dB above
the noise floor for each frequency band.
• RT60 (T30) = 2 x time measured for a 30 dB decay

For a lower measurement uncertainty (i.e., better measurement accuracy)• T30 is better than T20• 1/1 octave band resolution is better than one-third octave resolution• 5 cycles are better than 3 for each measurement position• 5 measurement positions are better than 3 in the room
However, if the background noise is too high and/or the sound source is notloud enough to create 45 dB above the noise floor, then T20 may be the better
option.

Mathematically, reverberation time is calculated from a linear least-squares regression of the measured decay curve.
• If the overall reverberation time is short (e.g. < 0.3 seconds) the room acoustic is referred to as being “dead”. For example, a room with thick carpets,
curtains and upholstered furniture may have such an acoustic character.
• If the overall reverberation time is long (say more than 2 seconds) the room
acoustic is referred to as being “live” or “echoic”. For example, a large empty
room with painted plaster walls and a tiled floor may have such an acoustic
character.

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Measurement Accuracy
The ISO 3382-2 standard specifies three levels of measurement accuracy. The
main difference concerns the choice of signal source and the number of measurement positions.

Survey method• There are no directivity requirements on the signal source.
• Measurements typically carried out in octave band resolution.
• The frequency range should cover at least 250 Hz to 2 000 Hz.
• The nominal measurement accuracy is assumed to be better than 10 % foroctave bands.
• A short excitation or an impulse signal may be used as an alternative to theinterrupted noise signal. Pink noise provides a better measurement accuracy
than an impulse signal.

Engineering method• For measurements in performance spaces: An omnidirectional speaker withidentical radiation characteristics in all directions is required.
• For measurements in ordinary rooms: There are no directivity requirementson the signal source.
• Measurements may be carried out in octave band resolution or the optional
one-third octave band resolution (part of the XL2 Extended Acoustic Pack).
• The frequency range should cover at least 125 Hz to 4 000 Hz in octave
bands, or 100 Hz to 5 000 Hz in one-third octave bands.
• The recommended duration of excitation of the room equals the measured
reverberation time in each frequency band.
• The nominal measurement accuracy is assumed to be better than
— 5 % in octave bands— 10 % in one-third octave bands

Precision method• For measurements in performance spaces: An omnidirectional speaker withidentical radiation characteristics in all directions is required.
• For measurements in ordinary rooms: Requires an omnidirectional speaker.
• Measurements may be carried out in octave band resolution or the optional
one-third octave band resolution (part of the XL2 Extended Acoustic Pack).

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• The frequency range should cover at least 125 Hz to 4 000 Hz in octave
bands, or 100 Hz to 5 000 Hz in one-third octave bands.
• The duration of excitation of the room needs to be sufficient for the soundfield to have achieved a steady state before the source is switched off.
• The reverberation time measurement accuracy is assumed to be better than
— 2.5 % in octave bands— 5 % in one-third octave bands

Sound Sources
Reverberation time measurements require a diffuse sound field in the room. Thismeans that the sound energy of the test signal has to be distributed uniformly.
A speaker with an omnidirectional radiation characteristic should be used for
accurate measurements using a gated pink noise test signal. The recommended signal source kit consists of the DS3 Dodecahedron Speaker and the PA3Power Amplifier. Survey method measurements may be performed with an existing installed speaker system using the Minirator MR-PRO as the signal source.
Alternatively, an impulsive sound source may be used in accordance with ISO
3382-1. The ASTM E2235 standard does not allow impulsive sound sources.

DS3 Dodecahedron Speaker Kit
Sound insulation and precise reverberation time measurements require the useof an omnidirectional sound source. Omnidirectional sources radiate soundequally in all directions. Loudspeakers mounted on the surfaces of a polyhedron
will give such a uniform, omnidirectional radiation.
The PA3 Power Amplifier includes a pink noise signal generator. This test
signal is played through the DS3 Dodecahedron Speaker into the room under test. The test signal may be switched on and off by remote control. The
test signal should be played for a long enough time period to ensure that abalance between injected and absorbed acoustic energy has been reached,i.e., the sound should be given enough time to reach all reflective surfaces in
the room. The duration of the pink noise test signal should thus be at least
equal to or longer than half of the measured reverberation time for each frequency band. As the room can never be “over-saturated” with sound fora reverberation time measurement, to be on the safe side, play the noisefor least the time period of the estimated reverberation time test result.
Each time the test signal stops, the sound level meter recognizes this interruption, measures the decay time, and calculates the reverberation time
automatically.

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APPLICATION NOTE

DS3 Dodecahedron Speaker

The international standards ISO 3382-1:2009 and ISO 16283-1:2014 specify
the directivity response of omnidirectional speakers. To give an adequate approximation of uniform omnidirectional radiation, it is stated that the dodecahedron (12 faces) is the preferred polyhedron.
The DS3 Dodecahedron Speaker is such a powerful sound source with omnidirectional radiation characteristics. It consists of 12 loudspeakers of the same
type, mounted to a dodecahedron-shaped chassis. The optimized frequency
spectrum delivers a high level of equalized sound energy for precise reverberation time measurements. The emitted sound power is constant throughout themeasurement and for several consecutive hours.
In the design of the NTi Audio Dodecahedron Loudspeaker enclosure, consideration was given to the practical advantage of transporting a smaller, lightweight
enclosure, and providing sufficient sound power output, while ensuring a flat
frequency response, and optimizing the spectral uniformity - and all this at anaffordable price.
The omnidirectional DS3 Dodecahedron Speaker should be positioned at the
typical sound source locations for the room e.g. for classrooms, at the front from
where the teacher normally speaks. In performance spaces a minimum of two

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APPLICATION NOTE

speaker positions shall be used at a height of 1.5 m above the floor.
For small rooms or in case there is no typical sound source location, one source
position should be in a room corner.
In very large rooms (e.g. concert halls) you may operate several Dodecahedron Speakers in parallel. For example, using two speakers increases the soundpressure level in the room by 3 dB.

Impulse Sound Sources
Typical impulse sound sources are, for example, a starter pistol, starter clapper
board or bursting balloon. Such sound sources may be used for survey methodmeasurements in performance spaces. The sound level meter measures the
decay time and calculates the reverberation time automatically.
Various starter pistols may be used to generate an impulse in the room and trigger the reverberation time measurement. For example, customers may use thefollowing models, both shooting blank or gas using a 9 mm caliber:• HW37, manufacturer Weihrauch• Chiefs Special, manufacturer Smith & Wesson

Warning: Using such a pistol in a closed room with approx. 800 m3 may generate an impulse sound pressure level LZImax of up to 125 dB.

Advantage• lightweight and portable

Disadvantages• may be inappropriate in the presence of children or in a church• may not be permitted on airplanes• may leave a smell of gunpowder residue in the room
• may unsettle dust in the room; e.g. requires cleaning after the measurements

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Sample Gun Shot Spectrum

Alternative impulse sound sources are, for example, balloons, a clapper board
or hand clapping. The challenge is that these sound sources typically generate
insufficient sound energy in the lower frequency bands. Thus, the reverberation
time cannot be measured in these bands.
The larger the balloon, the deeper frequencies it will cover, and the more soundenergy it can produce. Make sure you use higher-quality balloons that are fit for
purpose. Cheap children party balloons can be difficult to blow up, and may
burst prematurely (at worst, in front of your client!). Also allow sufficient time,
as a balloon of 1 meter diameter could take up to 5 minutes to inflate with anelectric balloon inflator.
Hand clapping is often used for a quick initial indication of the reverberation timein the room. Take a look at the following videos of handclapping in different rooms:
https://www.nti-audio.com/en/support/know-how/how-to-get-a-feeling-for-rt60-value

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APPLICATION NOTE

Minirator MR-PRO
The Minirator MR-PRO is a test signal generator that can be used for survey
method measurements. The generated audio signal may be played through anexisting speaker system, e.g. in a large venue, or a portable speaker in one of
the room corners. The MR-PRO generates a gated pink noise with a low crest
factor. The on/off cycle time is adjustable from 1 to 10 seconds. The MR-PRO
connects to the installed power amplifier and the test signal is played throughthe installed speakers. In addition, the MR-PRO offers a Glide Sweep signal witheditable cycle time (< 1.5 seconds for reverberation time measurements using
the XL2 Acoustic Analyzer).

Minirator MR-PRO

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Measurement
The XL2 measures the energy decay spanned across 63 Hz to 8 kHz using the
Schroeder method in octave resolution. The optional Extended Acoustic Pack
enables one-third octave band resolution from 50 Hz to 10 kHz.

XL2 Acoustic Analyzer with
M4261 Measurement Microphone

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Pink Noise Source
Using a pink noise signal source, we recommend an excitation of the room
equaling the measured reverberation time in each frequency band. This is a simplified guide, e.g. if the reverberation time is 3 seconds, then power on the pinknoise test signal for 3 seconds and off for 3 seconds. In detail the ISO 3382-1
standard states that the sound has to radiate at least half of the measured reverberation time for the engineering and precise methods. Due to the randomnessof the pink noise, it is recommended to average over at least three decays ateach position in order to achieve an acceptably low measurement uncertainty.
The sound level meter averages the readings of all cycles automatically.

Impulsive Sound Source
Using an impulsive sound source, the XL2 has a feature that discards reverberation time results when a level overload condition occurs. This is in accordancewith the ISO 3382 standard. Sometimes you might not be able to reduce thelevel of an impulse sound source; e.g. in a small room using a starter gun. In order to still get a reverberation time result in such a situation, this overload feature
can be disabled by loading an empty file with the name “RT60allowOVLD.txt” inthe root directory of the instrument. Note that such an overload condition may
affect the reverberation time result.

People in the RoomIn many rooms, the number of people present can have a strong influence on
the reverberation time. Reverberation time measurements should be made in aroom containing no people. However, a room with up to two persons present
may be allowed to represent its unoccupied state, unless otherwise specified.
If the measurement result is used for correction of a measured sound pressurelevel, i.e., for sound insulation or sound power, the number of persons present
in the room should be the same for that measurement.

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Distance Microphone to Speaker
The measurement shall not be strongly influenced by the direct sound from the
speaker. Therefore, the formula below defines the minimum distance between
the omnidirectional sound source and the measurement microphone.

Minimum Distance D = 2*√ (V/(c*T))
with

V = Volume of the room [m3]c = Speed of sound [m/s] (at temperature of the room)
T = Reverberation time of the room [s]For example, in a small hall, 10 meters by 10 meters with a height of 5 meters,
and an expected reverberation time of 3 seconds, the microphone must be atleast 1.4 meters away from the sound source.
D = 2*√ ((10*10*5)/(342*3)) = 1.4 m

Microphone Position
In performance spaces the measurement microphones shall be positioned at
the typical listener locations in the room, e.g. 1.2 m above the floor is the typicalear height of seated persons. The selected positions shall represent the entire
space in the room. Symmetric microphone positions should be avoided.
The minimum distance to the nearest reflecting surface, such as a floor, wall or
ceiling, shall be at least a quarter of a wavelength of the lowest frequency measured. e.g. 1 m to measure down to 100 Hz. For measurements down to 50 Hz,
use a minimum distance of 2 m to the nearest reflecting surface.

Distance between Microphones
The microphone positions shall be at least 2 m apart for measurements down
to 100 Hz. Use at least a distance of 4 m between microphone positions formeasurements down to 50 Hz. This recommendation is based on at least a half
wavelength of the lowest frequency measured.

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Speaker & Microphone Positions
The minimum number of speaker positions, microphone positions and measurements in a box-shaped room, using the interrupted noise measurement method,
depends on the required accuracy.

SurveyEngineering

Precision

Speaker Positions

1

2

2

Microphone Positions

2

2

3

Measurements

2

6

12

Decays per Measurement

1

2

3

More positions should be used in case of a more complicated room geometry.
The measurement starts by pressing the start/stop button on the Acoustic Analyzer and ends by pressing the start/stop button. It is recommended to measure
at least three decays at each position; this reduces the statistical measurement
uncertainty in the report.

Reporting
The Acoustic Analyzer stores all data on an SD card for direct transfer to a computer. Data reports and log files are stored in plain text format, which can be
opened with any text editor (Notepad, Wordpad, etc). The data is tab-delimited,so dropping the .txt file into a spreadsheet application will conveniently show theresults in columns.
Alternatively, an excellent reporting tool is provided by NTi Audio. The Room
Acoustics Reporter is a PC software for the generation of professional reverberation time measurement reports. The software supports acousticians and experts in the visualization and detailed evaluation of measurement data recordedwith the XL2 Acoustic Analyzer.

The following sample report presents the reverberation time in third-octave resolution. For simplification, a single figure mid reverberation time may be calculated by averaging the following bands:• Octave band: 500 Hz, 1000 Hz• One-third octave band: 400 Hz - 1250 Hz

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Typical Results
These are typical recommended reverberation time measurement results:

Location

VolumeRecommended

Recording Studio< 50 m3

0.3 s

Classroom< 200 m3

0.4 - 0.6 s

Office< 1`000 m3

0.5 - 1.1 s

Lecture Hall< 5`000 m3

1.0 - 1.5 s

Concert / Opera Hall< 20`000 m3

1.4 - 2.0 s

Church2 - 10 s

Commonly, the reverberation time can be reduced with the introduction of absorbing materials such as thick carpets, curtains, upholstered furniture, or dedicated sound-absorbing panels. Furthermore, the presence of people in a room
reduces the reverberation, and therefore produces a lower reverberation timevalue compared to the unoccupied room.

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