[Reader-list] the space interlude/corridor spaces/6th posting

[sayandeb mukherjee]

i regret for the delay for it took time to prepare
scientific and theoretical explanations on
acoustics/acoustic responses of different kinds of
corridor spaces


6TH POSTING 

 
BEFORE dealing with the psychoacoustics of these
spaces it is important to give an account on the
acoustic behavior or acoustic responses of these
spaces.
Corridors are generally interspersed with doors which
are once again the interfaces of individual
flats/rooms/enclosures. The sounds that are emanated
from the indoors (i.e. the rooms or flats) are
transmitted to some extent through the walls and
closed doors to reach this common passage. If the
doors are open, like it remains in colleges, hospitals
or mostly public spaces/unprivatized spaces, the sound
from the source directly reaches the passage. But a
person who’s passing through the passage and is away
from the door also hears a portion of the incidental
sound. That is due to reasons – a. a portion is
transmitted through the walls and b. a major portion
is diffracted at the ends of the opening of the door.
Unlike optical shadow, sound shadow is not so defined
because of the diffractive properties of sound. Refer
to the figures drawn (will be posted later). It shows
how a listener, in spite being away from the room, is
able to hear sound sourced from inside the
rooms/enclosures.
The diffractive properties differ throughout the
entire audio-frequency spectrum. A low-frequency / low
mid-frequency content in the sound is diffracted more
than the high / high mid-frequencies. Also, the high
frequency sounds are more directional than the low
frequency sounds. Hence the high frequency sounds
generally don’t reach the ears of the listener who is
off / oblique from the direction of the source. The
low frequency-content of the source gets more
scattered and diffracted while reaching the ears of
the listener distant from the direct opening of the
room. As the high-frequency content is depleted a
person perceives a booming or wooly sound of a speech
when he traverses in the corridor-like-spaces.
We construct four cases:

CASE A:  Where the corridor is having one side open

This scattered diffracted and transmitted sound passes
away to the opposite opening unreflected or
unobstructed. In these kinds of spaces, not much dense
sound-field is generated.  The sound generated from
the space doesn’t confront much
reflections/refractions (for there is no substantial
reflective surface) and the listener hears it almost
dry, or without being reverberated. This substantially
detriments the depth of the sound-field and hence the
space appears acoustically colorless. In this single
loaded corridor the property that is distinguishable
from the double-one is the mammoth interference of the
extraneous sounds – the environmental sound elements
that give a characteristic spatial definition to the
corridor space. In the single loaded corridors, that
can be called a verandah, the visual attributes along
with ambient audible qualities provides a positional
reference to the space. So the listener is not thrown
into an abstract field with no clues of time and
space, whereas he is well posited for the re-assurance
of the environmental associativity. Even if there are
reflections, reverberations of the sounds generated
from verandah that would immensely get masked by the
ambience coming from the open side and the audio
qualities doesn’t become audible until and unless gets
diminished to a considerable extent. So when this
ambience gets diminished, the sonic-scape   of this
veranda takes a different look/shape. The depth of the
audio field increases and there happens a resurgence
of the sounds and its reflections generated from the
verandah. This happens in the late hours of the night,
for example in the case of a college which is located
beside a road with heavy traffic plying during the
daytime, the acoustic details of the traffic gets
prominence with the redemption of the traffic during
night and attains more clarity with the fall of the
night and as silence prevails in the surrounding. The
incidental sounds like footsteps with its length
stretched throughout the verandah, door banging, some
people talking standing in the verandah acoustically
establishes that it is a single loaded corridor. 

CASE B:   Where the corridor space is having walls on
both the sides

As discussed earlier, that the sounds generated from
inside the rooms/enclosures/flats connected to this
common interface – the corridor undergo 1.transmission
through the walls of the corridor, 2.diffraction from
the openings of the rooms (like doors, windows)
thereby penetrating this double loaded corridor space.
A major portion of the generated sound remains inside
the room as they are reflected/bounced back to the
other walls of the room getting absorbed therein. The
sounds leaking out from the room, after losing most of
its high frequency content (as stated earlier) and a
portion of it being transmitted and diffracted reaches
the opposite wall of the corridor which acts as a
reflective surface. The intensity of reflection
depends on three factors primarily – (i) the intensity
of the source, (ii) the dimension of the corridor and
(iii) the reflective properties of the walls. 
The intensity of reflection is directly proportional
to the intensity of the source and the reflective
qualities of the wall and is inversely proportional to
the distance between the walls. The louder the source,
the more it will undergo reflections from the walls.
Generally, the sounds are not so impulsive or loud
(like an explosion) to sufficiently generate higher
order images. 
Since the sound is being generated at a considerable
distance from the passage, inside the room and also
because sound is inversely proportional to the
distance, its energy content gets appreciably
diminished by the time it reaches the common space.
Hence, the reflected image of this leaked out sounds
is too weak to reach the opposite wall of the
corridor. 
The possibility of second order images, (i.e.
reflected images from reflected sounds) further
reduces as the distance between the corridor walls is
increased. With the increase in the dimension of these
spaces, for the sounds arriving from these openings,
its early reflections from the surfaces will take some
time to get depleted and a considerable time elapses
before the sound content decays to an imperceptible
level. 
The corridor spaces are mostly comprised of masonry
walls with cemented surfaces sometimes coated with
plaster of paris. When it is uncoated with plaster of
paris then the texture is not so regular to produce
specular reflections like that of light and the
surface is porous enough to render diffusion for
certain range of the audio spectrum. 
[About Specular reflection – specular reflection is a
mirror-type reflection, similar to the reflection of
light from a mirror.  In specular reflection, the
incidental sound beam is reflected off the reflecting
surface as per Snell’s law. For specular reflection to
occur, the surface irregularities in the texture
should be smaller than wavelength of sound. And higher
the frequency of the sound wave, the smaller will be
its wavelength. So, with surface irregularities of
smaller dimension specular reflection of high
frequencies with small wavelength will be affected
more than low frequencies.
About Diffuse reflection – in diffuse reflection, the
incidental sound is reflected equally in all
directions causing a uniform scattering of sound. For
diffuse reflections, the reflecting surface must be
irregular and heavily textured. The dimensions of the
irregularities should be not less than or
approximately equal to the wavelength of sound. Thus,
for a wall to provide diffuse reflection at 1 kHz
(?=wavelength approximately equal to 0.3m or 1ft), its
surface irregularities should be of the order of 0.3m
(1ft). Surface irregularities of a few cm will provide
specular reflection at 10 kHz frequency. Now a sound
with a frequency of 100 Hz will have wavelength
approximately 3m or 10ft. With this wavelength, the
sound will be specularly reflected from a wall with
surface irregularity of dimension 0.3m or 1ft. In
other words, a 100 Hz sound will not see these
irregularities and the wall will behave as a smooth
wall. On the other hand, a 1 kHz sound will be
diffusedly reflected from this surface. At a frequency
of 10 kHz, with a wavelength of approximately 30mm
(nearly 1 inch), each individual irregularity will be
large enough to function as an independent reflector.
Therefore, sound will be specularly reflected from
each surface irregularity thereby providing some
scattering of sound (since the surface irregularities
are oriented in different directions). ]
For small little surface irregularities like it
remains in corridor walls in most cases, the high and
somewhat high-mid frequencies of the sound spectrum
are more diffused than the lower end. This attenuates
the harsh reflections of the higher end (or mutilates
the acoustic glare caused by the reflectivity of the
walls). Because of this diffusive property of sound, a
portion of it is evenly scattered thereby reducing its
energy so that it doesn’t reach the opposite wall of
the corridor after being reflected. And as mentioned,
the high frequencies contained in the sound will be
more diffused than the lower thereby making the sound
more muffled giving space to the boomy lower
frequencies to get partially or fully specularly
reflected and travel in the walls of the corridor. The
residual portion of this leakage from individual
openings gets absorbed in the wall. Now the quantity
of absorption depends on the absorption coefficient of
the walls
contd.
[Absorption coefficient determines the strength of
absorption of the walls; this provides a value needed
for the qualitative analysis of the acoustic materials
used for acoustic treatment of studios, theatres,
auditoriums.]  
Mostly for corridor walls, unless it is a special
case, the walls are made of concrete with a required
number of coatings and sometimes for affluent/well off
places it would probably be finally coated with
plaster of paris. The concrete surface with coarse
texture and no plaster will have an average absorption
coefficient 0.34 (0.36 at 125Hz, 0.31 at 500Hz, 0.29
at 1 kHz, 0.25 at 4 kHz) partially absorbing the
incidental sound thereby evading unusual reflections
to happen. The absorption by these kind of walls are
better than marble or glazed tiles or metallic
surfaces which will have acoustic glare thereby
rendering unusual fluttering of the sound and
sometimes discrete early reflections to happen which
would be very annoying for the listener.  
When the sound source or the sound-activity is
happening ‘in’ the corridor then, 1.the intensity of
the sound would be more in comparison to the sounds
generated from inside the room and meeting this space.
When the observer himself is the acoustic centre and
if he makes an appreciably loud sound, then the sound
wave instantaneously reaches the opposite walls of the
corridor. Here the two parallel walls act as two
parallel reflectors. As we know, when there are two
parallel reflectors, we will obtain an infinite number
of images of the source since each image works as a
source for the other reflector. This may be confirmed
by standing between two parallel mirrors; an infinite
number of the self will be seen. 
This is simply another way of stating that the sound
will be reflected back and forth between two parallel
reflecting walls infinite number of times before
exhausting to inaudibility. 
Now imagine a sound source(s) located between two
reflective parallel walls 15m apart, as shown in
FIGURE (to be posted later). Obviously, this situation
produces an infinite number of images of the source.
The first-order images, image I1 and I2, are behind
wall 1 and wall 2 respectively. The second order
image, I12, is the image of I1 and is formed behind
wall 2. I21 is the image of image I2 behind wall 1.
Similarly, I121 and I212 represent third order images
and so on. 
If we determine the distance between images, we find
that the distance between successive order images
increases by 30m – twice the distance between walls.
Thus, the first order images are 30m apart, second
order images are 60m apart, and third order images are
90m apart and so on. 
Since the speed of sound is 344m/s, the time gap
between each successive reflected sound will be
87milliseconds. This, according to the Haas effect,
will produce echoes. Since these echoes recur after a
regular interval of 87 milliseconds, they produce a
flutter effect; hence this phenomenon is called
flutter echoes. 
If the distance between walls were 5m, successive
order images would be 10m apart. Therefore the time
gap successive reflections would be 10/344, i.e. 29
milliseconds, which (according to the Haas effect)
should not be perceived as echoes. However, the
flutter is heard all the same. The reason lies on our
ears being extremely sensitive to periodic repetition
of sounds
..contd.


thanking you

sayandeb mukherjee







SAYANDEB MUKHERJEE
  FT#308, SUBBARAJU TOWERS,
  ROAD NO.4, VIJAYAPURI COLONY,
  KOTHAPET, HYDERABAD
  PIN: 500 035
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