"...better components sound better under the right conditions, but the sonic advantages they provide can be swallowed up by bad acoustics"
The Forgotten Component - Room Acoustics
In order to understand the phenomena of the room acoustics correctly, it is necessary
to imagine the propagation of the sound with deep and high frequencies differently.
The border between both frequency ranges depends on the space volume.
For a car the critical frequency lies with approximately 400Hz, for a normal living
room with approximately 180Hz and for a concert hall with approximately 30Hz. The
transition between both ranges is flowing.
The far above critical frequency the space dimensions are much larger than the wavelength
of the sound. Therefore the model of the geometrical acoustics applies:
- Outgoing from an acoustic source the sound spreads radially and by the space walls
is then reflected.
- With each reflection loses a sound jet, depending upon condition of the walls, a
more or less large portion of its energy.
With the help of this conception one can deduce a set of spaceacoustic criteria.
The most well-known criterion is the reverberation time. It is the measure for the
halligkeit of an area. In close relationship with it stands the volume, which an
acoustic source in the area produces. The longer the reverberation time is, the
more largely is the volume.
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Differently than when hearing over headphones the sound is substantially affected
during the rendition over loudspeakers by the hearing area.
The acoustic waves reach the ears not only on direct way but additionally by various
reflections at walls, cover and soil.
The size and geometry of the area, the kind of the space surfaces and the spatial
arrangement exert substantial influence on the loudspeaker sound.
Buying overbuilt machined connectors and oversized speaker wire when the fundamental
room acoustics are poor is not a real good return on your investment.
Ironically the sonic benefits of high end cables and tweaks are largely masked by
poor room acoustics.
I heard that if you paint your CD's black, put an heavy stone on the top of your
CD player and stick a carrot in your ass while drinking ginseng tea your stereo
will never sound better!
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Reverberation time in my livingroom, not that bad...
A Helmholtz Absorber for 63Hz, absorber panels for 500Hz and 2kHz should do the
job, i hope.
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Picture taken by Timbre @ www.euphonia-audioforum.se
On my last vacation in Sweden i visit the Listening Room of Audio Concept in Stockholm.
I promise you, that is the best Listening Romm i ever heard!
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What is reverberation time
The reverberation time gives a reference point for the musical and linguistic suitability
of an area and helps with the determination and the removal of space resonances.
The reverberation time RT60 indicates the length of time after disconnection of
a noise level, until this level faded away around 60dB. The reverberation time is
naturally also strongly periodical.
The more deeply the frequency, the longer is with most areas the reverberation time.
Here you can calculate the reverberation time
for your room
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| Standing Waves |
Standing waves exist in all kind of rooms. The shape of the room, the dimensions
of the room, and the relationship between the dimensions of the room, are important
parameters that will determine the frequencies around which the phenomenon exists
as well as the distribution of these standing waves. But how do they occur?
Imagine a sound source. When the sound is emitted the sound is emitted the sound
wave will propagate in all directions if no obstacles in sight. This will of course
happen with the speed of sound. Now, if the sound source is placed inside a room
the sound wave will hit the boundaries of the room.
If the boundaries consist of acoustically hard (reflective) surfaces, the sound
is reflected. If the angle of incidence is 90º the sound will be reflected right
back where it came from. Under certain circumstances the sound wave will meet itself
again. For instance if the sound is reflected between to parallel walls. |
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This Becomes a problem, when the sound wave not only meets itself, but when it meets
itself in phase. And this will happen when the distance between the walls is half
a wavelength of the radiated sound wave. Or one whole wavelength - or 1 ½, 2 ½ and
so on.
This phenomenon is called standing waves. Actually the
sound wave is not standing. But it is experienced like that because the sound pressure
maxima and minima are positioned in fixed places in the room.
The sound field is initially radiated having a radical wave front but within a few
reflections the sound field has obtained a plane wane front.
More info:
How Does Listening Room Acoustics Affect Sound Quality?
Listening Room Acoustics: Room Modes & Standing Waves Part I
Bass Trapping Ideas for Non-Ideal Spaces
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| Room Resonance Control: Bass Traps |
Sound is conveyed through waves in the air. Waves that exist between a pair of surfaces
can create standing wave resonances whenever the distance between the surfaces is
any even multiple of one-half of the wavelength.
At resonant frequencies (tones), the sound is louder and decays much more slowly
than at non-resonant frequencies, causing uneven tonal quality and interference
with clarity.
Resonant frequencies occur mainly in the bass range, due to the relationship between
the wavelengths of low-frequency sounds and the typical sizes of listening rooms.
Every room has its associated resonant frequencies. Rooms built using preferred
dimensions ratios have potentially more even distributions of these resonant frequencies.
Room built with angles walls or ceilings have more complicated resonant modes than
typical rectangular rooms and the resonances can be potentially less severe.
But, no matter what the size or shape of the room, resonant frequencies can be controlled
through the use of Bass Traps. |
| Reflection Control |
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Controlling room reflections is fundamental to creating accurate sound reproduction
in any room.
In addition to utilizing precisely selected panels addressing comb filter and flutter
problems, it is also generally desired to include the proper combination of absorption
and diffusion to control sounds reflected throughout the room.
The desired balance of absorption and diffusion is obtained through selection of
appropriate absorptive material and proper placement to create diffractive diffusion
and/or multiple time-delayed specular diffusion.
The proper placement and selection of panels to attain the desired reflection control
is determined on a case-by-case basis due to the large number of variables involved.
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A special technique developed by Manfred Schroeder is very capable in making a smooth and controlled diffusion. |
| Rear Wall Cancelletion |
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When the loudspeaker is set up at some distance in front of a wall, reflections
from the wall may occur and influence the perceived frequency response. This could
result in comb filtering if all frequencies produced by the monitor were radiated
in all directions. But the monitors are typically only omni- directional at low
frequencies. The result of the reflection is a single or few dips in the frequency
response perceived in front of the monitor. The frequency response may look like
this: one cancellation at the frequency that has a wave length of four times the
distance to the rear wall.
The dip – or cancellation frequency – is dependent on the distance to
the wall. If the distance is 1 m, the first dip frequency with a wavelength of 4
m.
l = c/f
Where :
l = wavelength (m) (or ft)
c = speed of sound [m/sec] (or ft./sec.)
f = frequency (Hz) |
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Hence:
4 = 344 / f
F = 86 Hz |
A closer position will result in a cancellation at a higher frequency. This is then
limited by the frequency where the monitor becomes directional and does rot radiate
sound to the rear.
A farther position will result in cancellation at a lower frequency. This is limited
by the distance being so long, that the reflected sound is attenuated due to the
long extra path travel.
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Master Handbook of Acoustics
by: F. Alton Everest, Ken Pohlmann
Amazon Price: $26.37 (as of 08/28/2009)
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If you are really interested room acoustics as it relates to music and audio, this is the book for you. It's over six hundred pages but it is not written like a physics grad student book full of theorems and derivations and it's also not dumbed down to a point where you can't use the information other than for clever conversation. It's full of practical examples and information related music recording and listening.
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Build a high quality measurement and recording microphone by Siegfried Linkwitz (ext. link)
About Room acoustics from Siegfried Linkwitz (ext. link)
All About Room
Design (ext. link)
Setting Up Speakers In A Rectangular Room
Calculate Room Modes
Amount of Standig Waves In A Rectangle Room
Standing Waves Calculation Program (ext. link)
Cancellation Frequencies
Calculate Q and Bandwidth
Reverberation Time Calculation
Calculate optimal Room Dimension
Calculate your Bass Trap, Panel Absorber, Slot Absorber,
Skyline Diffusor or Quadratic Residue Diffusor
Basics about Diffusers
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Acoustics
