Reporting and source qualification (ISO 3382-1)

Most of ISO 3382-1 is about how to measure. This page is about the four tables it prints and the rules that go with them: what a source has to be before it may be used, how many seats a survey has to reach, how the bands are averaged into the one number a hall gets quoted by, and what the report shall say.

A source that is not omnidirectional measures a different hall depending on which way it faces. Table 1 says how far from omnidirectional it may be, band by band, and 4.2.1 says how to find out.

A source qualifies at 125 Hz and fails at 4 kHz, and Table A.1 does not average every quantity over the same bands.

The survey is made on a turntable, or from measurements every 5 degrees if there is none. Either way the readings are combined into gliding 30 degree arcs, and each arc is compared with a reference that is “a 360 degree energetic average in the measurement plane”. Source and microphone are at least 1,5 m apart throughout.

import numpy as np
from phonometry import room

bearings = np.arange(72) * room.DIRECTIVITY_STEP_DEG
pattern = 7.5 * np.cos(8.0 * np.deg2rad(bearings))
pattern += 2.4 * np.cos(16.0 * np.deg2rad(bearings))

deviation = room.gliding_directivity_deviation(94.0 + pattern)
print(round(float(np.max(np.abs(deviation))), 2))   # 6.89  dB
print(room.source_directivity_limit(4000.0))        # 6.0   dB

Table 1 prints six bands, 125 Hz to 4 kHz, and no more, so a survey in the 63 Hz or 8 kHz octave has no printed limit to be held to rather than the nearest one:

from phonometry import room

print(room.MAX_SOURCE_DIRECTIVITY_DEVIATION_DB)
# {125.0: 1.0, 250.0: 1.0, 500.0: 1.0, 1000.0: 3.0, 2000.0: 5.0, 4000.0: 6.0}

Two things about that survey are not settled by what is printed. The first is a contradiction: 4.2.1 and the caption of Table 1 both call its values the maximum acceptable deviation, and A.4 calls them “the minimum limits given in Table 1” when it asks for a marginal source to be measured again at three orientations. Read literally, A.4 asks for the extra work on the sources that need it least. The second is an omission: “gliding averages, each covering six neighbouring points” does not say whether the six lead, trail or straddle the arc they are reported against. Over a full turn that is a relabelling, so it moves the bearing each deviation is reported against by up to 15 degrees of the pattern and not the deviation itself, but the bearing is what A.4 asks a marginal source to be turned about. Both are in the errata register, and gliding_directivity_deviation says in its docstring which readings it took.

Table A.2 prints three rows: 500 seats want 6 microphone positions, 1 000 want 8 and 2 000 want 10. They lie exactly on a straight line in the logarithm of the seat count, two positions per doubling, which is what lets a hall between the rows be answered at all.

from phonometry import room

print([round(float(room.minimum_receiver_positions(n)), 2)
       for n in (500, 1000, 1400, 2000)])
# [6.0, 8.0, 8.97, 10.0]
print(round(float(room.minimum_receiver_positions(5000)), 2))   # 10.0

The line stops where the table does. A.4 asks for “a minimum of between 6 and 10 representative microphone positions”, so a 5 000-seat arena does not get thirteen positions on the strength of three rows, and a 200-seat studio does not get four. A.4 adds the geometry the table does not: at least three on-stage source positions in the normal case, a source height of 1,5 m, a microphone height of 1,2 m at audience seat locations, positions evenly distributed over all the seating, and more of them wherever a hall breaks into balconies and under-balcony areas.

Qualify the source first. Drive it with octave bands of pink noise in a free field and keep the microphone at least 1,5 m away. On a turntable, average its output over gliding 30 degree arcs; without one, take a reading every 5 degrees and average six neighbouring readings into each arc. Every arc is then held against the 360 degree energetic average in the measurement plane: from 125 Hz to 4 kHz, none of them may leave Table 1.

Measure with a type 1 instrument to IEC 61672-1, IEC 61260 filters and an omnidirectional microphone, preferably with a diaphragm no more than 13 mm across, its sensitivity calibrated if $G$ is wanted. Normally use at least three source positions on the stage, more with a large stage or an orchestra pit, for instance one on the centre line and two at equal distances either side, with the acoustic centre 1,5 m above the floor. Take the minimum number of microphone positions from Table A.2, 6 for 500 seats up to 10 for 2 000, at 1,2 m above the floor at seats, about 2 m apart at least, spread evenly, and add more where balconies split the seating. Without synchronous averaging, hold the source at least 45 dB over the background in every band, or at least 35 dB when only $T_{20}$ is wanted, and average repeats of any response that does not repeat exactly. Measure a source close to its Table 1 limits again, turned in at least three steps, and average arithmetically. Report the source and microphone positions with their heights, preferably on a plan, the stage furnishing and the occupancy, and the rest of the fifteen items of 9.2.

Every quantity in Annex A gets quoted as one number, and A.5 asks for the index “m” on the symbol when it is. What the “m” averages is not the same for every quantity: Table A.1 gives each row its own band set, and A.5 prints two examples for exactly that reason, $G_m$ over the 500 Hz and 1 kHz octaves and $J_{\mathrm{LF}m}$ over four bands from 125 Hz.

import numpy as np
from phonometry import room

octaves = np.array([125.0, 250.0, 500.0, 1000.0, 2000.0, 4000.0])
strength = np.array([6.2, 5.4, 4.8, 4.4, 4.1, 3.6])
lateral = np.array([0.20, 0.26, 0.31, 0.28, 0.24, 0.19])

print(round(room.single_number_average("G", strength, octaves), 4))    # 4.6
print(round(room.single_number_average("J_LF", lateral, octaves), 4))  # 0.2625
print(room.TABLE_A1["G"].averaging_bands_hz)      # (500.0, 1000.0)
print(room.TABLE_A1["J_LF"].averaging_bands_hz)   # (125.0, 250.0, 500.0, 1000.0)

One of the seven rows carries two symbols. The table prints the early lateral energy fraction as ”$J_{\mathrm{LF}}$ or $J_{\mathrm{LFC}}$”, one row for the two weightings of Equations (A.14) and (A.15), so "J_LFC" is taken wherever "J_LF" is and reads the same four bands, the same just-noticeable difference and the same range.

import numpy as np
from phonometry import room

octaves = np.array([125.0, 250.0, 500.0, 1000.0, 2000.0, 4000.0])
lateral = np.array([0.20, 0.26, 0.31, 0.28, 0.24, 0.19])

print(round(room.single_number_average("J_LFC", lateral, octaves), 4))  # 0.2625
print(room.perceptibly_different("J_LFC", 0.20, 0.26))                  # True

Footnote a of the table makes one row an exception to the arithmetic: the late lateral sound level is energy averaged, through Equation (A.17). And two of its just-noticeable differences are not plain numbers either. The early decay time’s is printed as “Rel. 5 %”, a fraction of the value rather than a difference in seconds, so the same 60 ms means nothing in a two-second hall and is plain in a half-second one. The late lateral level’s is printed as “Not known”, and the library refuses to invent one.

from phonometry import room

print(room.perceptibly_different("EDT", 2.0, 2.06))    # False
print(room.perceptibly_different("EDT", 0.5, 0.56))    # True
print(room.TABLE_A1["L_J"].just_noticeable_difference) # None

A.5 also offers a coarser presentation, the octave pairs: 125 Hz with 250 Hz for a low-frequency result, 500 Hz with 1 kHz for a mid one, and 2 kHz with 4 kHz for a high one, all arithmetic. That is a different product from the single number and the two must not share a code path: they are the same two bands for the five monaural quantities and different band sets for the lateral ones.

import numpy as np
from phonometry import room

octaves = np.array([125.0, 250.0, 500.0, 1000.0, 2000.0, 4000.0])
lateral = np.array([0.20, 0.26, 0.31, 0.28, 0.24, 0.19])

pairs = room.octave_pair_averages(lateral, octaves)
print({k: round(v, 4) for k, v in pairs.items()})
# {'low': 0.23, 'mid': 0.295, 'high': 0.215}
print(round(room.single_number_average("J_LF", lateral, octaves), 4))  # 0.2625

A.5 notes that lateral energy fractions in the 4 kHz octave are not usually thought to be subjectively important, so the high pair means little for them.

Clause 9.1 prints two ways to quote one number for the reverberation time and no third: the average of $T_{30}$ over the 500 Hz and 1 kHz octave bands, or “averages over the six one-third-octave bands from 400 Hz to 1 250 Hz”.

from phonometry import room

print(room.MID_FREQUENCY_OCTAVES_HZ)
# (500.0, 1000.0)
print(room.MID_FREQUENCY_THIRD_OCTAVES_HZ)
# (400.0, 500.0, 630.0, 800.0, 1000.0, 1250.0)

What a one-third-octave analysis must not do is average the two one-third-octave bands that happen to be called 500 Hz and 1 kHz. That is two thirds of an octave against the two octaves either printed route covers, a third of the band, and it is neither of them. The accredited fiche takes the six-band route when the analysis is in thirds and names it on the sheet.

Clause 9 is normative, unlike every annex on this page, and 9.2 is its only requirement about what is written down rather than what is measured. It lists fifteen items, a) to o), and TEST_REPORT_ITEMS carries them in order: the conformity statement, the room’s name, place, plan and volume, its seating and upholstery, the shape and material of walls and ceiling, the occupancy, the state of any variable equipment, the curtains in a theatre, the stage furnishing, the temperature and humidity, the apparatus, the signal, the coverage with its positions and heights, and the date and the organization.

from phonometry import room

print(len(room.TEST_REPORT_ITEMS))     # 15
print(room.TEST_REPORT_ITEMS[3])
# volume of the room, with an explanation of how it is defined if the room
# is not completely enclosed

Most of that is metadata rather than measurement, and it reaches the accredited fiche through the ReportMetadata container the room acoustic parameters page fills in.

• Covered

  The printed tables of ISO 3382-1:2009 and the rules around them: Table 1 with the gliding arc survey of 4.2.1, Table A.1 with its per-quantity averaging bands, its one energy-averaged row and its one relative just-noticeable difference, Table A.2 with the line its three rows lie on, the octave pairs of A.5, the two single-number routes of Clause 9.1, and the fifteen items Clause 9.2 says a test report shall include.

• Not covered yet

  The procedures around them: the three-orientation repeat A.4 asks of a source close to its Table 1 limits, the sectional averaging A.5 suggests for a large hall, and the plotting conventions of Clause 9.1, which are about paper rather than about numbers.