Dust Sniper (quiet extractor system)

Step 3 Noise reduction - background information (and some theory you can skip)

So when I was building this, I spent a while researching about noise, sound and jazz, if you're into that kind of thing too read on... If you are a sound wizz already, look away. If you are bored easily and/or just want to hammer things together you can skip it too.

So what Is sound anyway?

Every school kid will tell you sound is basically the result of things getting excited and vibrating. Everything vibrates anyway, but if you do something like hit the table in front of you (assuming there is one), you change the way it vibrates and it passes on that vibration to the air. In the air we can imagine a series of high and low pressures, which in turn vibrate the internal gubbins of our ear - causing us to register what we call sound.

As with most subjective things in life we humans like to try and measure and quantify these vibrations. The quantity most often used to measure the “strength” of a sound wave is the 'sound pressure level' (SPL or sometimes Lp, not to be confused with 'sound power level') measured with respect to a standard reference pressure of 2 ×10−5Pa.  

SPL is expressed in dB (or Decibels) which are a logarithmic unit, so that for every 6 dB decrease in volume, the sound is perceived as being half as loud. 
 

Blocking out Nasty Sound (noise)

When an airborne sound wave encounters a solid blocking its path, it effectively bashes into it, the disturbance causes the solid to vibrate. This vibration is transmitted through the solid. Now on the other side, the surface acts as a new emitter by disturbing the air and producing a new sound wave. By this process the sound effectively passes through the barrier. The efficiency of the transmission depends on the physical properties of the solid in particular, its mass.

How much of the sound is blocked out? Well, if L1 dB is the sound pressure level on the noise source side of the partition and L2 dB that on the other side, then the Sound Reduction Index (SRI) or Transmission Loss (TL) is defined as:

TL = SRI = L1 - L2  dB  
 

The transmission loss, or SRI, varies with mass and frequency. In general the higher the frequency the better the sound is blocked, hence the higher the SRI will be. There are exceptions to this when partitions start vibrating at their resonance frequencies. More on that later, for now all we need to know is that:  

1. For precision work (or for special noises with a particular frequency content), the SRI index is quoted for particular frequencies, normally in octave bands.

2. For many purposes and for convenience, the SRI is quoted as a single number, which is the average SRI between the frequencies 100 – 3,150 Hz. The resultant sound level is then quoted in dB(A). (A) presumably standing for average. 

The Mass Law

The so called 'mass law' simply states that by increasing the mass of a partition, we increase the transmission losses or SRI of the partition proportionally. So mass is generally a good thing when we are trying to reduce sound (think about the useful properties of lead). The mass law however only applies to a given material, over a specific range of frequencies. It could be, for example, that a deep bassy noise (low frequency) travels through a panel with very little reduction in volume even when you increase the mass of a panel. Indeed it is often the case that low frequency noise transmission is more effected by the stiffness of a material.

Again, this all depends upon the material in question. A lead curtain's behaviour, for example, is essentially mass-law controlled over the entire audible frequency range. For a more geeky explanation along these lines, check out "Engineering Noise Control: Theory and practice, Fourth edition, David A. Bies and Colin H. Hansen (2009)" 

For us, the mass law is a good demonstration of the compromise we are going to make between light weight and sound reduction. "[We] should rule out the use of low density fibreglass (such as insulation batts used in house ceilings), as well as typical polyester blankets. In fact polyester blankets are likely to be completely ineffective." (Bies & Hansen, 2009 p 386). Although if we can compress them a lot and have them to hand anyway, it is a different story...

Building less symmetrical and more random please

As with double or triple glazing, it is important not to have all the panes the same thickness, as this accentuates the dip in the TL (transmission loss) curve at critical frequencies. The same goes for our purposes when we construct a double wall box. It is better to use different materials as well as thicknesses for the different layers. That way we will block out a broader range of frequencies.

While preventing resonance by mixing materials and shapes is good, it is also well worth incorporating an air (or foam) gap, which prevents the direct transmission of vibration. Vibration is easily transmitted to other materials by mechanical coupling - avoid if possible. 
 

"Acoustic isolation is generally accomplished by providing as wide a gap between the panels as possible and by filling the gap with a sound-absorbing material, while ensuring that the material does not form a mechanical bridge between the panels." (Bies & Hansen, 2009, page376)

Absorbing Sound

The nature of the surfaces on which the sound wave falls determines how much will be absorbed. Hard rigid non-porous surfaces like glass, marble or concrete, provide the least absorption and are thus the best reflectors. Soft porous surfaces and those which can vibrate absorb more of the sound. When sound energy is absorbed it is converted into heat energy, but this energy is very small so no need to worry about overheating caused by sound.

The amount of sound absorbed is proportional to the area of the material concerned. So if S is the sound absorbed and A is the area of the exposed material, we can say that S is proportional to A. In general this means that rough surfaces are better at absorbing than finely finished ones. Further,

 S = aA
 

where:  a is the Absorption Coefficient.

The Absorption Coefficient is a number always less than 1 (because it has no units, it is a ratio) and is small for a material that reflects most sound and large for a material that absorbs most of the sound incident upon it. It is determined by the amount of sound absorbed by a material divided by the sound energy arriving at the surface (so a = absorbed sound energy / incident sound energy). Just for interest the table below (from the Sengpielaudio website) shows a bunch of absorption coefficient values for various materials. As you can see, different materials are better or worse at absorbing different frequencies.
 

Floor Materials	125 Hz	250 Hz	500 Hz	1000 Hz	2000 Hz	4000 Hz
concrete or tile	0.01	0.01	0.15	0.02	0.02	0.02
linoleum/vinyl tile on concrete	0.02	0.03	0.03	0.03	0.03	0.02
wood on joists	0.15	0.11	0.10	0.07	0.06	0.07
parquet on concrete	0.04	0.04	0.07	0.06	0.06	0.07
carpet on concrete	0.02	0.06	0.14	0.37	0.60	0.65
carpet on foam	0.08	0.24	0.57	0.69	0.71	0.73

 

Seating Materials	125 Hz	250 Hz	500 Hz	1000 Hz	2000 Hz	4000 Hz
fully occupied - fabric upholstered	0.60	0.74	0.88	0.96	0.93	0.85
occupied wooden pews	0.57	0.61	0.75	0.86	0.91	0.86
empty - fabric upholstered	0.49	0.66	0.80	0.88	0.82	0.70
empty metal/wood seats	0.15	0.19	0.22	0.39	0.38	0.30

 

Wall Materials	125 Hz	250 Hz	500 Hz	1000 Hz	2000 Hz	4000 Hz
Brick: unglazed	0.03	0.03	0.03	0.04	0.05	0.07
Brick: unglazed & painted	0.01	0.01	0.02	0.02	0.02	0.03
Concrete block - coarse	0.36	0.44	0.31	0.29	0.39	0.25
Concrete block - painted	0.10	0.05	0.06	0.07	0.09	0.08
Curtain: 10 oz/sq yd fabric molleton	0.03	0.04	0.11	0.17	0.24	0.35
Curtain: 14 oz/sq yd fabric molleton	0.07	0.31	0.49	0.75	0.70	0.60
Curtain: 18 oz/sq yd fabric molleton	0.14	0.35	0.55	0.72	0.70	0.65
Fiberglass: 2'' 703 no airspace	0.22	0.82	0.99	0.99	0.99	0.99
Fiberglass: spray 5''	0.05	0.15	0.45	0.70	0.80	0.80
Fiberglass: spray 1''	0.16	0.45	0.70	0.90	0.90	0.85
Fiberglass: 2'' rolls	0.17	0.55	0.80	0.90	0.85	0.80
Foam: Sonex 2''	0.06	0.25	0.56	0.81	0.90	0.91
Foam: SDG 3''	0.24	0.58	0.67	0.91	0.96	0.99
Foam: SDG 4''	0.33	0.90	0.84	0.99	0.98	0.99
Foam: polyur. 1''	0.13	0.22	0.68	1.00	0.92	0.97
Foam: polyur. 1/2''	0.09	0.11	0.22	0.60	0.88	0.94
Glass: 1/4'' plate large	0.18	0.06	0.04	0.03	0.02	0.02
Glass: window	0.35	0.25	0.18	0.12	0.07	0.04
Plaster: smooth on tile/brick	0.013	0.015	0.02	0.03	0.04	0.05
Plaster: rough on lath	0.02	0.03	0.04	0.05	0.04	0.03
Marble/Tile	0.01	0.01	0.01	0.01	0.02	0.02
Sheetrock 1/2" 16" on center	0.29	0.10	0.05	0.04	0.07	0.09
Wood: 3/8'' plywood panel	0.28	0.22	0.17	0.09	0.10	0.11

 

Ceiling Materials	125 Hz	250 Hz	500 Hz	1000 Hz	2000 Hz	4000 Hz
Acoustic Tiles	0.05	0.22	0.52	0.56	0.45	0.32
Acoustic Ceiling Tiles	0.70	0.66	0.72	0.92	0.88	0.75
Fiberglass: 2'' 703 no airspace	0.22	0.82	0.99	0.99	0.99	0.99
Fiberglass: spray 5"	0.05	0.15	0.45	0.70	0.80	0.80
Fiberglass: spray 1"	0.16	0.45	0.70	0.90	0.90	0.85
Fiberglass: 2'' rolls	0.17	0.55	0.80	0.90	0.85	0.80
wood	0.15	0.11	0.10	0.07	0.06	0.07
Foam: Sonex 2''	0.06	0.25	0.56	0.81	0.90	0.91
Foam: SDG 3''	0.24	0.58	0.67	0.91	0.96	0.99
Foam: SDG 4''	0.33	0.90	0.84	0.99	0.98	0.99
Foam: polyur. 1''	0.13	0.22	0.68	1.00	0.92	0.97
Foam: polyur. 1/2''	0.09	0.11	0.22	0.60	0.88	0.94
Plaster: smooth on tile/brick	0.013	0.015	0.02	0.03	0.04	0.05
Plaster: rough on lath	0.02	0.03	0.04	0.05	0.04	0.03
Sheetrock 1/2'' 16" on center	0.29	0.10	0.05	0.04	0.07	0.09
Wood: 3/8" plywood panel	0.28	0.22	0.17	0.09	0.10	0.11

 

Miscellaneous Material	125 Hz	250 Hz	500 Hz	1000 Hz	2000 Hz	4000 Hz
Water	0.008	0.008	0.013	0.015	0.020	0.025
People (adults)	0.25	0.35	0.42	0.46	0.5	0.5

So you get the idea.  Armed with all that knowledge you are ready to scrounge up some free materials and get building, right?