The concept of reverberation. Standard and optimal reverberation time. The influence of the reverberation time on the acoustic properties of the hall.
Reverberation-
gradual fading of the sound after
turn off the sound source.
Standard
and optimal reverberation time.
Standard
reverberation time -
reverberation time during which
sound pressure level standard
500Hz tone is reduced by 60dB
after turning off the sound source. Time
reverb -T.
Depends
from: the volume of the room, FTE. Calculated
at frequencies of 125, 250, 500, 1000, 2000, 4000 Hz.
Formula
Sabina.
T=
(c) ν-volume
A=
FTE. (should be related to
materials
hall decoration)
α-
average sound absorption coefficient
(if
α
Formula
Airing:
Sgen-
area of all internal
surfaces.
φ(α)
= -ln
(l-α)
is the mean function
coefficient
sound absorption.
(from
tables).
Optimal
reverberation time -
the time at which in the room of this
destination the best conditions are created
audibility.
Permissible
discrepancy between the calculated and optimal
reverberation time
10%.
Influence
reverberation time on
acoustic properties of the hall.
characterizes
the general loudness of the room. Too bad,
when long or short reverb.
Small reverberation - the sound does not go to the hall.
(Small
reverberation - “Dry” hall). long
reverberation time - boom.
3.
Structure of early reflections and its influence
on the acoustics of the hall (assignment of points,
calculation of delay of serial
reflections, acoustics requirements for
direction of arrival and delay time
reflections).
Early
reflections-
reflections arriving at the listener from
delay time compared to
direct sound no more than 50ms for speech and
80ms
for
music. Structure of early reflections
checked at three points located
along the axis of the hall and the corresponding front,
middle and rear seating area
Structure
early reflections.
Purpose
points.
S-source
sound
1
(2,3) - the middle of each zone
Payment
delays of successive reflections.
Produced
using geometric (ray)
constructions at 3 points located
along the axis of the hall and the corresponding front,
middle and rear seating areas.
(SB+B1)-
S1
S1-straight
Ray
B1-reflected
path
Requirements
acoustics to direction of arrival and time
reflection delays.
Direction
the arrival of reflections depends on the shapes and
hall sizes.
Permissible
useful reflections are received
to the listener with T delay, compared
with direct sound no more than 50ms. These reflections
complement the direct sound of the source, improving
audibility and speech intelligibility
clarity and transparency of the sound of music.
1.
V
speech rooms for
good speech intelligibility: delay
first reflection versus direct
sound did not exceed 20ms. With the same
everyone should come late
subsequent beams.
2.
Optimal sound for music and
maximum spatial effect
her perceptions: following the direct sound
the first reflection comes (from the side
walls) after 25-35ms, the next
15-20ms, after which the time structure
starts to thicken.
3.
Halls
multipurpose:
delay of the first reflection, according to
compared to direct sound (as well as
intervals between visits
following reflections) should not exceed
20-30ms.
Reverb Time Calculation
The formula for drama theater is used to calculate the reverberation time.
Twholesale = 0.36 logVSt - 0.1= 0.36lg 1053.70 - 0.1 = 0.99 s
Figure 4.3.1 shows the resulting reverberation time in an empty hall after overlapping surfaces.
Fig.4.3.1.
The graph shows the recommended reverberation time of 1 s (red straight line in the center). The black curved lines are the limits that the reverb time should be within.The blue line is the resulting reverb time after the materials were applied. At 500 Hz there is a rise, from 500 Hz there is a sharp fall, so the reverberation time is out of range.
2. Calculation of the average sound absorption coefficient
Sound waves carry mechanical
energy received or from a source
sound (sound energy). Falling on
any surface, sound waves
reflected from it, losing part of their
energy. This process is called
sound absorption, and the ratio of the absorbed
in this case, the energy to the incident - by the coefficient
sound absorption a, which is dimensionless
size. With complete absorption of the incident
energy α= 1, and with its total reflection
α = 0. Sound absorption coefficient
some surface depends on its
material and located behind it
designs, on sound frequency and angle
falling sound waves. With acoustic
room calculations are usually used
averaged for different angles of incidence
surface sound absorption coefficients,
corresponding to diffuse sound
field.
To calculate the hall reverberation time
must be pre-calculated
air volume V, m3, total area
internal surfaces Scommon,
m2common, m2. and total FTE
(equivalent sound absorption area)
A
If any surface has
area S and sound absorption coefficient
α , then the quantity A = α×S is called
equivalent sound absorption area
(EPS) of this surface.
From the definition of sound absorption it follows,
that the FTE is the area of completely absorbing
the sound of the surface that absorbs
the same amount of sound energy
as well as the given surface S. If S
measured in square meters,
A has the same dimension.
To some objects of complex shape and
relatively small size
(e.g. armchairs and listener) concept
sound absorption coefficient difficult
applicable, and sound-absorbing properties
such an object is characterized
its equivalent sound absorption area.
The total FTE at the frequency for which the
calculation is found by the formula
(9)
where
—
the sum of the product of the areas of the individual
surfaces S, m2, on their coefficient
sound absorption α for a given frequency,
is determined by formula (8);
—
sum of FTE, listeners and seats, m2;
αDOB- coefficient
additional sound absorption, taking into account
additional sound absorption caused by
penetration of sound waves into various
cracks and holes, fluctuations of various
flexible elements, etc., as well as absorption
sound lighting fixtures and other
hall equipment.
Sound absorption coefficients of different
materials and structures, as well as FTE
listeners and chairs are given in app. II (table.
one). Values given in the table
obtained by measuring the reverb
method giving the sound absorption coefficient,
averaged for various directions
falling sound waves. These values
taken on average according to different data with
rounding.
Additional sound absorption coefficient
αextfor multi-purpose halls
the category under consideration on average
can be taken equal to 0.09 at a frequency
125 Hz and 0.05 at 500 ¸ 2000 Hz. For
halls in which the conditions are strongly expressed,
causing additional sound absorption
(numerous slots and holes on
internal surfaces of the hall,
numerous flexible elements - flexible
lampshades and lamp panels, etc.),
these values should be increased by approx.
by 30%, and in the halls where these conditions
weakly expressed, about 30% decrease.
After finding AOVRcountedα- average sound absorption coefficient
the inner surface of the hall on this
frequency:
(10)
Energy density calculation
The model of the sound field in the stationary mode from the point of view of geometric theory will be taken in the form:
where e is the total sound energy density; eD is the direct sound energy density:
eN is the energy density of the first sound reflections:
eR is the diffuse sound energy density:
RA = 0.63 W is the power of the sound source;
With = 1.22 kg/m3 is the air density;
With = 340 m/s is the speed of sound;
? = 4.8 is the coefficient of axial concentration;
is the mean square of the sound pressure.
Substituting the obtained values eD, eR ieN in formula (3.7) we find the numerical value of the total density of sound energy, which is equal to:
Knowing the value of the density of sound energy e find the intensity I and intensity level LI.
where I = 10-12 corresponds to zero intensity level.
According to the graph of curves of equal loudness (Fig. 2.8), it can be seen that the intensity level LI equal to 105 dB corresponds to a volume level of 100 phon, which is in the field of auditory perception of the human ear. Not above the threshold of touch and not below the threshold of hearing. For good perception, the required sound level is at least 85 phon.
