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donderdag 22 juni 2017
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    Acoustics

"...better components sound better under the right conditions, but the sonic advantages they provide can be swallowed up by bad acoustics"



What is most important for Music Enjoyment?

A. Recording
B. Listening Room
C. Source
D. Amplification
E. Speakers
F. Wires, Rack & Acc.
G. Girls

  


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.

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!




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.





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!





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

Please read also Prediction of Reverberation Times [ PDF - 530kB]



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 90 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.
  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 11/2, 21/2 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



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
 
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.


A special technique developed by Manfred Schroeder is very capable in making a smooth and controlled diffusion.



Rear Wall Cancelletion
 

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)
   

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.






Master Handbook of Acoustics

by: F. Alton Everest, Ken Pohlmann

Amazon Price: $26.37 (as of 08/28/2009)
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.




The moral:
The moral, if any, could be that the obvious is that something you do not really know you will not miss. If you are the impact of such a thing as "good dynamics" have never really suffer, you will not miss this dynamic if it is not there.

But what is also obvious is the reverse: if you do not really realize how good you sound, then it seems to be that you do not fully appreciate. This is certainly psychologically explicable. So you can be fixated on a bass line from a song that happens to be a bit odd is included. A relative strength of 3 or 4dB difference between two successively excited bass is clearly audible and you think you have while you are actually basprobleem the peculiarity of a specific recording is heard.





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