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Tommy_tucker

bookcase design for dampening

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On 23/11/2019 at 12:54 PM, Tommy_tucker said:

Is there more to it than that?

It's a complex question .... but not really.

 

It's easy enough to figure out the major issues a room will have without taking (m)any super complicated measurements of it.

 

The main issues are always bass.  Bass requires large "treatments" ... and so it really depends on what lengths the person is prepared to go to, and $.

 

If you have an entire wall worth of space to dedicate, and 30cm or so ....  and you're prepared to make a bit of a mess, then fantastic.

 

The next step is to answer "what are you trying to fix".... as the answer to this really does affect what you might do.

 

 

Most people need (lots of) absorption, but don't want it to work in the frequencies above about 400Hz  (as this will make room sound dead).   IMVHO putting the absorption behind the speakers works great.

 

It difficult to tell you exactly what to design/build ..... but if you are going to build shelves over your entire front wall, then you could.

  • Fill over the front wall as deep as you can with fluffy absorption (30cm deep+ if you can)
  • You could leave the first 10, or even 20cm from the wall as air gap .... and not lose significant performance
  • Cover over the absorption with nice looking acoustically transparent fabric (for the parts where you will see it)
  • Build shelves, etc. in front
  • Have some (although not necessarily all) of the backs of the shelves open - so sound can pass through to the absorber

 

Another option is to build absorption into just the corners ..... you can find people calling this a "superchunk" style.

 

Another option is to build a device which works by bending (like the thing another poster here showed photos of in his corner)....   but the performance of these devices is much harder to predict, and work over a narrower range.

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Clearly much more to this than I had realised, and much more space required as well.  Thanks again for the time spent in responding.  Ultimately I dont think I have an additional 30cm to spare behind the bookcase, the bookcase itself is already eating into valuable living room space and my wife definitely wont understand the project. 

Starting to wonder whether I should stick with the ARC room correction and be happy.  

My daily browsing of the classifieds for a pair of larger floor standing speakers may have to come to an end, wonder what I will do with my free time now. 😉

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6 hours ago, Tommy_tucker said:

Clearly much more to this than I had realised, and much more space required as well.

A quick primer on acoustic treatment:

Most rooms benefit from treatment to absorb bass frequencies, and the more rigid the room, the more the bass frequencies bounce around before decaying (boomy bass that hangs around - called ringing).

 

Below around 250Hz in normal rooms, the sound bounces around like the ripples after dropping a pebble into a fish tank.

The sound waves have a pressure component and a velocity component, and the pressure and velocity are inversely related.

Consider a golf ball bouncing off a brick wall - at the point of impact, the velocity is zero, but the pressure is highest, and then the golf ball accelerates off the wall but eventually slows down.

Sound waves behave similarly - they have maximum pressure and minimum velocity at room boundaries, and their wavelengths determine where their pressure and velocity is highest away from the boundaries

  • wavelength x 0 away from the boundary (on the boundary) is lowest velocity/highest pressure
  • wavelength/4 away from the boundary is highest velocity/lowest pressure
  • wavelength/2 away from the boundary is again lowest velocity/highest pressure
  • wavelength x 3/4 away from the boundary is again highest velocity/lowest pressure

There are 2 main types of treatment - velocity traps and pressure traps. You may also come across Membrane traps - these are a type of pressure trap.

 

Velocity traps are your typical insulation batt (absorption) trap. They operate by slowing down the high velocity air particles and converting the energy to heat within the fibres of the batt. They are "broad band" devices - absorbing over a wide range of frequencies.. They are best placed where velocity is highest (ie 1/4 wavelength away from boundaries) but still work anywhere where velocity > 0.

 

Some quick calcs show why you need large/deep absorption traps to operate at low frequencies - at 150Hz, wavelength=2.3m, and maximum velocity is at wavelength/4 = 0.6m away from the boundary!!

 

You can understand from this that absorption needs to be large and deep to be effective down low, and realistically gets too big to manage bass frequencies much below 150Hz or so - but it works wonders 150Hz - 500Hz to clean up mid-high bass.

 

Not surprisingly pressure traps should be placed in the room where there is maximum pressure - ie on boundaries.

Pressure traps tend to be "narrow band" devices - ie they work (absorb) over a narrow frequency range, and need to be designed for the specific problem frequency to be absorbed - if your room has multiple problem frequencies, you would likely require multiple pressure traps each tuned to a different problem frequency.


In one of my posts above I mentioned constructing a new gyprock wall over the existing brick - this would operate as a large "membrane" pressure trap.

The usual lightweight construction of homes these days with Gyprock on studs is great for absorbing/transmitting low bass (not so great for neighbours/other house occupants). The additional focus on thermal performance usually helps acoustic performance.

IME lightly constructed rooms don't have low bass issues, and some absorption targeted at lower frequencies 150 - 500Hz with a few bands of EQ cut below 150Hz works wonders.

 

Achieving the same result in a rigid/brick room is vastly harder - especially if you like to listen at higher volumes.

 

8 hours ago, Tommy_tucker said:

Ultimately I dont think I have an additional 30cm to spare behind the bookcase, the bookcase itself is already eating into valuable living room space

but perhaps you could go the full depth of the book case at the top bottom and sides with absorption and/or a limp mass membrane, ie the bookcase is framed with acoustic treatment.

You'd need to "design" the limp mass so it was working for you, but no "design" work would be needed for just absorption - just make it as wide as you can get away with (top bottom and sides of the book case) and as deep as the book case, covered in nice fabric.

 

Mike

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