Acoustic treatment.

[EDN]

I got some beautiful new headphones the other day which have made me accutely aware of how crappy sounding my monitoring setup is, predominantly down to my complete lack of acoustic treatment (and budget monitors).
So to remedy this I am thinking of building some bass traps from this stuff to clean up the low end problems in my room.
I was wondering (if anyone knows about these things) if it would be worth layering some of this stuff (so a few layers folded) over the top of it to help with mid/high end reflections.
Any thoughts or ideas?

[gnome]

There are great videos of how to make sound panels using burlap and insulation. I'm not sure about bass traps though.

[gravity]

i think you can create some quite effective bass traps in the form of helmholtz absorbers out of rockwool, roofing felt and some bits of wood. i remember researching into this when i was at uni. lemme find the articles...

edit - here we go:

http://www.soundonsound.com/sos/jul98/a ... tics1.html
http://www.soundonsound.com/sos/aug98/a ... ustic.html
http://www.soundonsound.com/sos/sep98/a ... tic_3.html
http://www.soundonsound.com/sos/oct98/a ... stics.html

PANEL ABSORBER

The panel absorber is the easiest and most predictable bass trap to design and build. It consists of a simple wooden frame over which is fixed a thin, flexible panel such as plywood, hardboard, barrier mat or even roofing felt. Fibreglass or mineral wool is often fixed inside the frame to help damp the system by absorbing energy, since the more the trap is damped, the wider the frequency range over which it will work. The resonant frequency is a function of cavity depth and mass per square foot of the panel material, so it's easy to calculate the necessary dimensions using a simple formula (shown in the box elsewhere in this article).

The actual area doesn't make any significant difference to the operating frequency, but obviously the more you want to reduce the low-frequency reverberation time, the larger the area of panel you'll need in the room. To understand better the effect of a given area of absorber, consider that a perfectly efficient full-range trap would affect the sound in the same way as an open window of the same size (but obviously without the associated problems of sound leakage).

Filling the cavity with fibreglass or mineral wool tends to lower the resonant frequency by up to 50 per cent as well as doubling the effectiveness of the trap. It also lowers the Q of the trap so that it is effective over a wider frequency range. A typical panel-type trap is effective for frequencies around one octave either side of the centre frequency, which at least has the advantage that you don't have to be absolutely accurate to get results.

As the surface of the panel may also reflect higher frequencies, curved panel traps have been constructed to simultaneously absorb bass frequencies and diffuse higher ones. However, it may be easier just to cover the front face of the trap with a layer of acoustic foam to extend its usefulness to the mid and high end of the audio spectrum. Figure 1 shows the construction details of a conventional panel absorber. Because the amount of damping affects the tuning of the trap, you may want to test the finished trap to see what effect it's actually having. You can discover the resonant frequency by sticking a cheap contact mic on to the panel's surface, then plugging the mic into a preamp or mixer with a VU meter. Play a test tone from an oscillator or test tone CD using loudspeakers, and vary this around the frequency the trap is designed for until you get a maximum meter reading. This will be at the trap's resonant frequency.

HELMHOLTZ TRAPS

Another type of tuned trap, popular in broadcast studios and older recording studios, is the Helmholtz resonator. This is essentially an enclosure with an aperture, not dissimilar in terms of its physics to a bottle -- and just as a bottle has a very specific resonant frequency (which you can hear if you blow over the hole), so too does a Heimholtz resonator. A bottle has a very narrow bandwidth, but by introducing an absorbent material such as fibreglass or mineral wool into the neck to reduce the Q, the operating range can be widened. While you don't see many studios full of bottles (at least not ones being used as bass traps), it is possible to simulate the effect of hundreds of tuned bottles with a perforated panel over an air space.

By fixing a perforated wooden panel over a frame and putting an absorbent material inside the space created, a resonant bass trap is formed with each perforation acting as a single 'bottle' in our virtual bottle array. Again, there is a fairly simple formula to determine the operating frequency (see box).

Figure 3 shows the construction of the Helmholtz trap. By varying the percentage of perforation, the design can be applied to both the bass and mid range. However, predicting the performance of these traps is difficult because the Q or bandwidth varies depending on the amount of internal damping.

The other problem is getting the right perforation percentage. Common pegboard is usually used in mid traps rather than bass traps. For example, pegboard with 3/16-inch holes on a one-inch matrix has a perforation percentage of 2.75 per cent. Fixed over a four-inch air gap, this gives a resonant frequency of a little over 400Hz. Boards of different perforation percentages may be available from specialist acoustic suppliers, but are not readily available from conventional builders merchants.

Like a panel absorber, adding an absorbent material lowers the resonant frequency slightly and also broadens the resonant peak. Instead of employing perforated board, it's theoretically possible to use a series of slats to create a slotted panel with the correct perforation percentage, but the calculations for such an absorber don't always predict a precise result, so I'd recommend some form of test to verify the final resonant frequency.

Helmholtz resonators were once very widely used both in broadcast and recording studios, but panel traps with limp membranes seem more widespread in modern designs.

Tuned traps should be placed on the walls corresponding to the room modes that you wish to attenuate. Your can tackle floor-to-ceiling modes by placing a trap on the ceiling. Traditionally, bass traps are placed close to corners where there is MOVABLE SCREENS


Portable acoustic screens are useful because they can be used to modify the sound of a small part of a room for the recording of, say, a vocal, drums or acoustic guitar. These screens are generally built with a polished wood or synthetic laminate surface on one side and a mineral wool or foam absorber about four inches thick on the other. They are supported by simple wooden legs and, by turning either the hard or the absorbent side towards the performer, a live or dead environment can be created. Drum booths can be made of a set of tall screens with another screen balanced on the top to form a roof. For drums, acoustic guitars and so on, the live side is normally used, and for vocals, the dead side. The diagram is this box shows the construction of a simple acoustic screen.

These screens are only effective down to around 250Hz on their absorbent side, but that's usually adequate for the purpose.


an area of high pressure.

[macc]

That Wickes stuff is alright, but you may as well go for the real shizzle - Rockwool RW3, and RWA45.

http://www.dobuilddirect.com/www.dobuil ... o.php?p=64

I get mine from those boys - call them and see what you can haggle out of them. Great service.

If the slabs are too big then one of these beauties will sort that out before you wrap em;

http://www.amazon.co.uk/Kenwood-Electri ... 190&sr=1-1

Hot knife through butter!

[macc]

BTW - a helmholtz absorber is a good suggestion but for a typical domestic space (where I assume the OP is), you need enough room to be able to, you know, get into the room to work and so on

[EDN]

Do you know anywhere where I can get that stuff from which doesn't require me ordering 5 packs of them?
Thats a bit out of my price range atm cause I only need about 3/4 traps for now.
*edit* scratch that, found somewhere *edit*

[paradigm_x]

That Wickes stuff is alright, but you may as well go for the real shizzle - Rockwool RW3, and RWA45.

http://www.dobuilddirect.com/www.dobuil ... o.php?p=64

I get mine from those boys - call them and see what you can haggle out of them. Great service.

If the slabs are too big then one of these beauties will sort that out before you wrap em;

http://www.amazon.co.uk/Kenwood-Electri ... 190&sr=1-1

Hot knife through butter!

beat me to it. Went to wickes to have a look as ones local but they dont even put the density on the packaging!

(OT)

[macc]

Roffles!