As some of you may or may not know this month marks the 1st birthday for your boy music weapons. So to celebrate we are having a bunch of sales and announcing some other cool news. So without further delay here are the current sales we have enabled in celebration of this momentus occasion:
100$ OFF DRUM WEAPONS2, DRUM WEAPONS 3 AND SOUND ARSENAL WHEN PURCHASED TOGETHER WITH THE COUPON CODE
75$ OFF DRUM WEAPONS 3 AND SOUND ARSENAL WHEN PURCHASED TOGETHER WITH THE COUPON CODE
50$ OFF WHEN YOU BUY DRUM WEAPONS 2 AND DRUM WEAPONS 3 TOGETHER WITH THE COUPON CODE
100$ OFF WHEN YOU BUY DRUM WEAPONS 3, BANGIN BASS AND SOUND ARSENAL TOGETHER WITH THE COUPON CODE
In other news we have begun accepting BITCOINS as a payment option for all products on our store. Also the price of our hit plugin and what put us on the map “drum weapons 2″ has been permanently reduced to $99. The maschine expansion and reason refill versions of drum weapons 3 are shipping. The music weapons:loaded product containing all of our products has been updated to include all dw3 products and is even better of a deal as it was before.
Drum weapons 3 has a about to be released update to 3.02 which fixes a silly bug in the audio engine where white noise would appear randomly on a few patches, as well as fixing the 1 in a thousand dropped midi notes bug that was completely random as well. We also made it so the pads on the dw3 gui light up when the plugin is triggered via midi input from sequencer or external midi messages in addition to the keyboard module in the gui window guiding you for mapping (as requested). All this makes drum weapons 3 surely the most groundbreaking hiphop plugin ever created. There will also be a small amount of new patches added. Glitchy gui elements like the faders glitching out fixed as well.
Bangin Bass will be updated to 1.05 to benefit from the newly redesigned audio engine, as well as random patch mapping and pitch fixes for the bass library and a few patches will be added as well to round out the product.
Sound Arsenal will be updated to 1.05 to benefit from the new audio engine and completely eliminate all tiny bugs.
VSDJ will be updated to 1.03 to benefit from the new audio engine and completely eliminate all tiny bugs.
3 “Pure Vinyl” expansions will be added to the store for drum weapons 3, and possibly others.
The drum weapons 2 library will be ported into drum weapons 3 kits, and will be available as an expansion for dw3 users. It will be free of charge for users of both dw2 and 3.
Bangin Bass 2 has begun its development stage, building on the knowledge and skills we have attained building our last few plugins we are able to bring that expertise into developing a true one stop hiphop and urban music bass patch plugin. The patches will be better, there will be multi velocity layers, there will be more patches, it will have the new audio engine, it will have alot more options for editing on the gui including mono/poly and filters and more editing right on the gui. Of course it will look alot cooler as well
Hip-Hop ONE, our upcoming (fairly distant future) multichannel multitimbral multi out full fledged hiphop sound module is beginning its first stage. At its core it will be a single plugin to host all your music weapons plugins in one place with up to 16 instruments in each plugin instance patches from dw2/3, baning bass, sound arsenal, vsdj etc will all be available for use in hip-hop ONE. Along with a full featured 16 channel mixer with eq dynamics and time based effects. It will take on a look similar to the final drum weapons 3 gui, and we are even looking into adding mp60 and sp1200, tape, and tube emulation builtin as well as a basic sampler. We also hope one day (with the proper funding) to be able to part a hip-hop ONE -esque product into hardware format or at the very least offer a customized controller for the plugin. So yeah Hip-Hop ONE is BIG, but we no time table for that and it is pretty far into the future but it is our long term goal at music weapons to materialze that so you heard it here first!
Within a week or two i am launching my new site to buy and sell instrumentals in light of my new production work i see its only fitting to start a new era of that as well. So even though its our birthday here it music weapons, it is indeed you that will benefit from our birtday so its kind of an oppostie birthday wehre we give you stuff haha
The microphone preamp is possibly one of the most important component in the whole recording chain apart from the microphone itself. Between them, these two units bring the minuscule voltage coming from the diaphragm up to the 1+volts that we work with. From that point on everything is operating at normal levels and the signal can be recorded, EQ’d etc. without problems with noise and distortion. The typical mike preamp can have the following controls.
I said “can have” because not all do. First there is the mike input level knob. This is the gain control for the preamp.
A special note here – Before you start to set the mike input level you must set the console up for unity gain. This involves first setting the console output faders to Zero, then the channel fader to zero. If you are going out a group put that fader to zero. This first step is vitally important because a console is capable of increased noise and distortion if not setup with correct gain structures. If you have a little Mackie or something which doesn’t have a separate control over monitoring turn your amp and speakers down. Basically if you run the output faders low you have to get the gain from somewhere so you turn up the mike preamp which is capable of adding noise and distortion.
If you find that you are fully counterclockwise and still have too much signal you must insert the Pad. The pad drops the level by 10 – 20db (depending in the console) and stops the preamp from overloading. Some consoles will include a phase reversal button which is a very handy option to have. There is also a button for Phantom Power which will supply power to your mikes if required. (See microphones) It often comes as a single on/off switch on the rear of the console. There is often an optional line input knob with an associated mike/line switch. This allows you to trim the level of the line inputs. Finally you will probably find a mike/line button that allows you to adjust the level of the line input individually. The flip button is only on certain consoles. It swaps the two main faders over (line and monitor), More about this option later when we look at monitoring..
Before we look at compressors and limiters we must understand the term Dynamic Range. The Dynamic Range of a sound is the range between the quietest section and it’s loudest section or in the case of a recorder the range between the noise floor and the point of distortion. You know how loud a Symphony Orchestra can get yet you also know how quiet it can get. An Orchestra has a wide Dynamic Range.
The meters above show a dynamic range of 72db. On a home cassette recorder the quiet section in this track would be below the noise of the tape recorder and all you would hear through the quiet passage would be tape hiss. The distance from the loudest section to the point of distortion is called the Headroom. If distortion is reached at +6db then we currently have a 4db headroom. To reduce the dynamic range you could ride the whole track with a fader and turn it up when it’s too low and pull it back when too high or your can use a compressor.
In the diagram above unity gain means that what you put in you get out. In the 2:1 ratio example When the signal is above the threshold the signal output is reduced in a ratio of 2db in will give 1db out when the compression ratio is set at 2:1, so you have saved 1 db off the top of your dynamic range and you can turn it all up by 1 db without effecting the headroom. In a more severe case like the 20:1, which is more commonly called limiting, for a 20 db rise in signal only 1 db comes out. The compressor and limiter can be used together in one unit where the compressor works in the 2 – 15:1 range whilst the limiter stops the extreme transient peaks in the signal in the 15 – 20:1 ratios which is why it is often called a Peak Limiter.
In the above graph the threshold of the limiter has been raised so that the main program material will be compressed above the threshold of compression at 2:1 and above the limiting threshold it will be 20:1. A compressor is a gain reduction device, therefore all compressors have a make up gain control so that if you are using 3db of gain reduction you can turn the output by that amount and still retain the same headroom.
In the diagram above the transition from unity gain to compression at the threshold of compression/limiting is gradual instead of a straight line. This is called a Soft Knee threshold and is much smoother.
The Meter on a compressor can usually be switched to read either the input level, output level or the amount of gain reduction. It is advisable to check that the input level is correct before you start adjusting the threshold and setting the compression ratio etc.
The attack time determines how quickly the the compressor reacts to signals above the threshold. Signals have short sharp peaks called Transients that can easily trigger a compressor to act. The attack time determines how long the peak should be above the threshold before compression takes place. These short transients are important in the clarity of a sound but don’t effect the loudness of the sound. The aim of compression is to make the instrument sound louder, to squeeze the dynamic range, therefore you may wish to lengthen the attack time and let the transients through (to be dealt with by a limiter if necessary) and the compressor will then be working on sustained levels above the threshold.
The release time determines how quickly the compressor lets go, or restores normal gain. If the release is too fast for the amount of gain reduction applied then the return to normal gain over and over as the signal moves above and below the threshold can cause what is known as pumping because the gain structure is changing rapidly. It is advisable to ask the player to play sustained notes and set the release so the change of gain is smooth. Instruments that have long sustaining notes like bass guitars should tend to use a slower release times than sharp percussive instruments like percussion. Most of the new generation compressors now have an Auto button that leaves it to the compressor to work it out, and they usually do it fine.
Take a look at a typical compressor and its controls:
The left section is the Noise Gate section. It has controls for the threshold at which the gate opens, the release time variable and a switchable fast/slow attack control. The centre section is the compression section with the standard controls over threshold, ratio, attack and release. The Peak/RMS switch determines how the compressor will track the signal i.e. its peak content or its RMS content. The Auto button is often an option where the compressor works out the attack and release times itself by analysing the program material. The hard/soft switch determines the Knee setting. The meter can read input or output levels plus it can read the amount of gain reduction. Finally there is the makeup gain control (Often just labelled output level) The link button is there if there are two compressors in the unit . Stereo Compressors have a link facility that makes one of the two compressors a master. (Usually the left compressor). All the controls on the master effect the slave compressor, so they both operate together. If the compressors weren’t linked any strong signal on the right would be gain reduced and the stereo image would move because centre panned instruments would vary in their left to right balance so when compressing a stereo signal make sure the compressors are linked.
The Electro and Warm options are computer additions not found in a stand alone analogue version. As you can see the threshold is below the peak signal so gain reduction is taking place as indicated in the attenuation meter. The ratio is set at 2.90:1 and there has been no make up gain applied.
The attack time is set to 3.66ms and the release is at 214ms and the control on them is manual (not auto).
The expander is a compressor in reverse. There are two types of expander. In some, signals above the threshold remain at unity gain whereas signals below the threshold are reduced in gain, whereas in others the signal above the threshold also has the gain increased. Therefore you can use an expander as a noise reduction unit. Set the threshold to be just below the level of the player when playing. When the player stops the signal will fall below this threshold and the signal is reduced in gain thus reducing the noise or spill.
The drawing above shows the different actions of compressors and expanders. The expander in the drawing is increasing gain above the threshold and reducing gain below the threshold.
Most recording in popular music today has had heavy compression. Recording are loud and in your face! As well as most of the components of a track being individually compressed the whole mix overall has been compressed and limited before going to CD. I don’t think that’s a bad thing.
A limiter is just a severe compressor where the compression ratios are high. On some units like the DBX 160 and the Aleisis compressors an additional Peak limiter control with a LED that flashes is supplied, but units like the Aphex Dominator are pure limiters and are very sophisticated in how they attack and control peaks and you can get some pretty hot “brick wall” mixes through them.
A De-esser is a frequency selective compressor/limiter that compresses only at a predetermined frequency. If set to the frequencies around the sibilance area of a vocal (4kHz – 8Khz, it varies between men and women,) the vocal will be compressed only at those frequencies which will reduce the sibilance. Sibilance is the peaks of high frequencies created by ‘S’, ‘T’, ‘C’s etc.
The new generation
The new generation compressors, expanders gates etc. in the new computer programs are worth a mention here. These compressors have one outstanding advantage over the stand alone compressor. They can read the signal ahead of time by extracting the signal from the hard disk ahead of time, analysing it and then outputting it in real-time. They know what is going to happen next which gives them a distinct advantage in maintaining smooth control over the signal.
The diagram above shows how a gate works on level. When the signal falls below the threshold the gate reduces the level to the specified reduction level. The attack time here determines how quickly the gate will open and the release time determines how fast it will close. Some gates have a Hold function that allows you to tell a gate to hold open for a set time once it is open and then the release time can take over and close the gate. This facility can stop the gate opening and closing quickly due to peaks. It can also be used as an effect, especially if it is put over the return from a reverb unit. If you have some reverb on say a snare, and you put a gate over the reverb return signal, you can get the hold function to hold the reverb open for a period set by the hold function and then to quickly close it by using a fast release. This effect is called Gated Reverb and is now a standard program in most reverb units.
A gate can also be set to be triggered by something else via a side chain. For example, if you put a gate over a room ambience mike you could use the snare mike to trigger it to open when the snare was hit and to close when the snare stopped. This is called Gated Ambience. Another effect is to put a hihat feel into the side chain and modulate the gate to open and close on a synth sound. The effect is a modulating synth with the attack and release times controlling the modulation.
Gates can also so linked so that one controls the other and when one opens the other opens as well. (Like the compressor) This is used in stereo gate situations like over stereo toms.
I got this chart off the web and it gives you an idea of how the different materials absorb sound at different frequencies.
Remember that full absorption is 1 whilst full reflection is 0
Absorption coefficients of common building materials and finishes Floor materials 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz carpet 0.01 0.02 0.06 0.15 0.25 0.45 Concrete (unpainted, rough finish) 0.01 0.02 0.04 0.06 0.08 0.1 Concrete (sealed or painted) 0.01 0.01 0.02 0.02 0.02 0.02 Marble or glazed tile 0.01 0.01 0.01 0.01 0.02 0.02 Vinyl tile or linoleum on concrete 0.02 0.03 0.03 0.03 0.03 0.02 Wood parquet on concrete 0.04 0.04 0.07 0.06 0.06 0.07 Wood flooring on joists 0.15 0.11 0.1 0.07 0.06 0.07 Seating materials 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz Benches (wooden, empty) 0.1 0.09 0.08 0.08 0.08 0.08 Benches (wooden, 2/3 occupied) 0.37 0.4 0.47 0.53 0.56 0.53 Benches (wooden, fully occupied) 0.5 0.56 0.66 0.76 0.8 0.76 Benches (cushioned seats and backs, 0.32 0.4 0.42 0.44 0.43 0.48 Benches (cushioned seats and b 0.44 0.56 0.65 0.72 0.72 0.67 Benches (cushioned seats and backs, fu 0.5 0.64 0.76 0.86 0.86 0.76 Theater seats (wood, empty) 0.03 0.04 0.05 0.07 0.08 0.08 Theater seats (wood, 2/3 occupied) 0.34 0.21 0.28 0.53 0.56 0.53 Theater seats (wood, fully occupied) 0.5 0.3 0.4 0.76 0.8 0.76 Seats (fabric-upholsterd, empty) 0.49 0.66 0.8 0.88 0.82 0.7 Seats (fabric-upholsterd, fully occupie 0.6 0.74 0.88 0.96 0.93 0.85 Reflective wall materials 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz Brick (natural) 0.03 0.03 0.03 0.04 0.05 0.07 Brick (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) 0.1 0.05 0.06 0.07 0.09 0.08 Concrete (poured, rough finish, unpai 0.01 0.02 0.04 0.06 0.08 0.1 Doors (solid wood panels) 0.1 0.07 0.05 0.04 0.04 0.04 Glass (1/4" plate, large pane) 0.18 0.06 0.04 0.03 0.02 0.02 Glass (small pane) 0.04 0.04 0.03 0.03 0.02 0.02 Plasterboard (12mm (1/2") paneling on 0.29 0.1 0.06 0.05 0.04 0.04 Plaster (gypsum or lime, on masonry) 0.01 0.02 0.02 0.03 0.04 0.05 Plaster (gypsum or lime, on wood lath) 0.14 0.1 0.06 0.05 0.04 0.04 Plywood (3mm(1/8") paneling over 31.7mm 0.15 0.25 0.12 0.08 0.08 0.08 Plywood (3mm(1/8") paneling over 57.1mm 0.28 0.2 0.1 0.1 0.08 0.08 Plywood (5mm(3/16") paneling over 50mm( 0.38 0.24 0.17 0.1 0.08 0.05 Plywood (5mm(3/16") panel, 25mm(1") fi 0.42 0.36 0.19 0.1 0.08 0.05 Plywood (6mm(1/4") paneling, airspace, 0.3 0.25 0.15 0.1 0.1 0.1 Plywood (10mm(3/8") paneling, airspace 0.28 0.22 0.17 0.09 0.1 0.11 Plywood (19mm(3/4") paneling, airspace 0.2 0.18 0.15 0.12 0.1 0.1 Absorptive wall materials 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz Drapery (10 oz/yd2, 340 g/m2, flat aga 0.04 0.05 0.11 0.18 0.3 0.35 Drapery (14 oz/yd2, 476 g/m2, flat agai 0.05 0.07 0.13 0.22 0.32 0.35 Drapery (18 oz/yd2, 612 g/m2, flat agai 0.05 0.12 0.35 0.48 0.38 0.36 Drapery (14 oz/yd2, 476 g/m2, pleated 0.07 0.31 0.49 0.75 0.7 0.6 Drapery (18 oz/yd2, 612 g/m2, pleated 0.14 0.35 0.53 0.75 0.7 0.6
Fiberglass board (25mm(1") thick) 0.06 0.2 0.65 0.9 0.95 0.98 Fiberglass board (50mm(2") thick) 0.18 0.76 0.99 0.99 0.99 0.99 Fiberglass board (75mm(3") thick) 0.53 0.99 0.99 0.99 0.99 0.99 Fiberglass board (100mm(4") thick) 0.99 0.99 0.99 0.99 0.99 0.97 Open brick pattern over 75mm(3") fiber 0.4 0.65 0.85 0.75 0.65 0.6 Pageboard over 25mm(1") fiberglass bo 0.08 0.32 0.99 0.76 0.34 0.12 Pageboard over 50mm(2") fiberglass bo 0.26 0.97 0.99 0.66 0.34 0.14 Pageboard over 75mm(3") fiberglass b 0.49 0.99 0.99 0.69 0.37 0.15 Performated metal (13% open, over 50mm 0.25 0.64 0.99 0.97 0.88 0.92 Ceiling material 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz Plasterboard (12mm(1/2") in suspended 0.15 0.11 0.04 0.04 0.07 0.08 Underlay in perforated metal panels (25 0.51 0.78 0.57 0.77 0.9 0.79 Metal deck (perforated channels,25mm(1" 0.19 0.69 0.99 0.88 0.52 0.27 Metal deck (perforated channels, 75mm(3 0.73 0.99 0.99 0.89 0.52 0.31 Plaster (gypsum or lime, on masonary) 0.01 0.02 0.02 0.03 0.04 0.05 Plaster (gypsum or lime, rough finish 0.14 0.1 0.06 0.05 0.04 0.04 Sprayed cellulose fiber (16mm(5/8") on 0.05 0.16 0.44 0.79 0.9 0.91 Sprayed cellulose fiber (25mm(1") on s 0.08 0.29 0.75 0.98 0.93 0.76 Sprayed cellulose fiber (25mm(1") on 0.47 0.9 1.1 1.03 1.05 1.03 Sprayed cellulose fiber (32mm(1-1/4") 0.1 0.3 0.73 0.92 0.98 0.98 Sprayed cellulose fiber (75mm(3") on s 0.7 0.95 1 0.85 0.85 0.9 Wood tongue-and-groove roof decking 0.24 0.19 0.14 0.08 0.13 0.1 Miscellaneous surface material 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz People-adults (per 1/10 person) 0.25 0.35 0.42 0.46 0.5 0.5 People-high school students (per 1/10 0.22 0.3 0.38 0.42 0.45 0.45 People-elementary students (per 1/10 p 0.18 0.23 0.28 0.32 0.35 0.35 Ventilating grilles 0.3 0.4 0.5 0.5 0.5 0.4 Water or ice surface 0.008 0.008 0.013 0.015 0.02 0.025
RT60 relates to intelligibility. Diffractors reduce pronounced reflection by breaking up the sound wave before reflecting it back. This does not reduce reverberant energy, but does reduce echo spikes that may otherwise exceed -60db of direct, thus lowering RT60 and improving intelligibilty, but not necessarily improving the listening environment for music.
Low frequencies are big waves, consider that a 50Hz wave is 6.6m (21′ 8”) and a 30Hz wave is 11m (36ft) long! That’s 11m peak to peak -There’s a lot of guys around here who would love to surf a wave like that! So to stop it requires special techniques.
There are basically two ways to control low frequencies.
- Acoustic Hangers. This is a system of fibre board panels that are wrapped with insulation and are hung freely using wire or rope. The large hangers 1.8m x 500mm work in the low frequency range whilst the panels 1.2m x 300mm effect the low mid frequencies. It is common to have up to a 1.2m space at the rear of the control room with the large hangers whilst the smaller hangers are effective if suspended in the ceiling cavity created by a false ceiling.
- Panel Absorbers. A panel of plywood or particle board is placed over an air cavity with insulation glued to the back of the panel. The panel has a resonate frequency and when it occurs in the room it resonates and the insulation absorbs the energy.
The above drawing shows the rear of a typical control room design. The fibreboard panels are suspended from the ceiling with the sizes varying to give a broadband absorption field. They can also be hung behind a false wall in the studio as in the following drawing.
False Wall with Acoustic Hangers
A panel absorber is created when you place a sheet of plywood or fibreboard, with insulation glued to the back of it, over an air cavity. The panel will have a resonate frequency of its own, tap it and you will hear it. When it is placed over a sealed cavity, and insulation is attached to the back, everytime it hears its own note it resonates and the air in the cavity resonates and the insulation absorbs the resonance, hence absorbing the frequency! It is important to note that here we have an absorber that reflects the high frequencies and attenuates the low. With the hangers all that exposed insulation absorbs the high frequencies as well so the panel absorber has a place in the studio. The two factors determining the frequency of absorption are:
- The mass or density of the panel.
- The depth of the air cavity, i.e. depth of the sealed timber frame.
A panel absorber is made like this:
You can apply different shaped front panels
The other great advantage of panel absorbers is that they can have angled or curved fronts so when mounted on a wall or the ceiling they stop parallel wall interference and prevent standing waves creating diffusion.
You can even tune this absorber by placing a contact microphone on the plywood panel which is plugged into a real-time analyser and blasting the panel with white noise or a swept tone with a speaker. When the frequency = the panel’s resonate frequency the panel will vibrate and the frequency will show up on the real-time analyser. The thicker the plywood the lower the frequency and the greater the depth (depth of the timber box) from the wall the lower the frequency. Using fibreboard as an alternative tends to create a low-mid absorber.
You can create a broadband low frequency absorption wall by building a series of sealed boxes with different depths with each box being only 1m x 1m (3′ x 3′). With a variety of different thickness of plywood you can cover the whole low frequency range. It looks good too. You can also alternate the fronts between panels and slats. (See helmholtz resonators)
For absorption coefficients and panel thickness check out the absorption coefficient chart.
You can create a variable panel absorber by splitting the box into two boxes and placing hinges on one side so that it opens fully as per the following diagram:
VARIABLE PANEL ABSORBER
The variable panel absorber allows you to change the acoustics in a room. A wall of these absorbers can quickly change a room’s acoustics from live to dead. A variation is to have a slat resonator in the bottom box so that when the box is opened it reveals a slat resonator so you end up with a wall of alternating low-mid absorbers and high frequency absorbers. If you can only afford the space for one studio this is an excellent addition as you can change the room acoustically to cover all situations.
The scale along the bottom of the chart shows the frequencies from 16Hz to above 16kHz. When engineers talk about the high mids they are referring to the frequency range from 1kHz to 8Khz, roughly.
The centre frequency is around 750Hz and the Gain Increase (boost or cut) is around 18db. The Q Factor is the width of the frequencies effected by the boost and is measured in octaves. A high Q is narrow and a low Q is wide.
There are two kinds of equalisers, Parametric and Graphic and each can control a number of bands.
The Graphic Equaliser
You will note that there are slider controls for each frequency and the scale along the base shows which frequency. The scale along the top states how many db change has been made at each frequency and it can be positive or negative (boost or cut). A typical graphic equaliser does not have any controls over the Q factor of each boost, it is normally pre-set.
The Parametric Equaliser
The left unit is a typical high end console analogue equaliser whereas the right one is a new generation computer program digital ones. The left one has a switchable peak/shelf High frequency control. It has two sweepable mid bands with variable Q and a peak/shelf low frequency control. The computer version has 4 Bands each with it’s own centre frequency, Q width and gain. The resultant EQ curve is displayed as well. (It’s a digital EQ) The mid bands of the analogue version are usually divided into two sweepable bands the the low – mid covering 100Hz – 4Khz with the other covering 600Hz – 15Khz (typically – it varies from console to console) You will note that the digital unit is sweepable from 20Hz to 20KHz in all bands.
If you look at the coefficient of absorption figures for the various products you will note that whilst some attenuate the highs some also attenuate the low mids as well. 100mm (4″) fibreglass for example not only absorbs high frequencies but it also works down into the low mids depending on how thick it is.
The other main factor is what are the highs in your room doing? Consider the fact that your high frequencies are coming from your speakers which have a directivity factor. In a standard multi – speaker system the highs are coming from the tweeters or horns. Both these units have a fan shaped dispersion of around 30 degrees. And create what is referred to as the on axis off axis effect. Stand in front of a speaker and you hear all the highs but go 30 degrees off axis and the highs start to reduce to the point that if you are 90 degrees off axis the highs are eliminated completely (apart fro highs that reach you my reflection from some other surface.)
Take a look at this plan of a control room:
Control Room Plan
The dotted lines indicate the axis of the high frequency projection. Note that the engineer is sitting on axis to the speakers yet someone sitting to the right of the console is off axis to the right speaker but still on axis to the left speaker. The high frequencies are reflected by the opposing walls (in this case glass doors). The idea of this control room design is make sure (by angling the walls) that the high frequencies from the right speaker are not reflected back into your left ear.
Once the sound passes the engineer the rear of the control room absorbs the sound and it doesn’t come back to the engineer.
If you look at the absorption coefficients of various materials you will notice that some of the fibreglass products absorb low-mid frequencies very efficiently as does a panel absorber with a fibreboard panel instead of a plywood panel. But the best low mid absorber (and the best looking) is the helmholtz resonator – often called a slat resonator.
The helmholtz resonator (named after a Mr Helmholtz who discovered it) can best be demonstrated by taking a normal soft drink bottle and blowing over the mouth of the bottle – a note is produced. Now place some cotton wool in the bottle and try again. You will notice the note has reduced- well not really, the note is produced but the wool absorbs the resonance and turn the sound energy into heat! Imagine, if you lined a whole wall with bottles of various sizes, all filled with insulation material. You would now have a low-mid (200 – 500Hz depending on the bottle size) absorbing wall that as well as absorbing the low mids would also reflect or diffuse the high frequencies. I haven’t tried it yet but it would be worth trying if you are short of cash because bottles are cheap. The Romans used to do it using clay jars which they placed around their theatres.
The helmholtz resonator is often called a slat or slot resonator because you can create a helmholtz resonator by building a wall with slats of timber separated by slots as in the following diagram
|Further more, our scientists have created a formula with which we can tune the resonator to a specific frequency. If we vary the depth from the wall, slat width, slot width (and the slat depth) we can create a wall that is a broadband low-mid frequency absorber. The beautiful thing about these absorbers is that they still reflect high frequencies, in fact they will diffuse them which is even better.|
|As you can see a slat wall like this can break up parallel walls thus stopping standing waves. Because the distance from the front to the back is varying from 300mm to 100mm or around 12 degrees, the wall becomes a broadband absorber. So simple yet so effective! I’ve seen some beautiful looking ones where you cut the slots out of a sheet of quality particle board with a timber veneer.
Another form of helmholtz resonator is created using perforated plywood – i.e. plywood with hundreds of holes in it. We call it pegboard in Oz, you see it in hardware stores holding up tools etc. If you place a panel of this over an air cavity like in a panel absorber not only do the little holes act like bottle necks the whole panel acts as a low frequency panel absorber!
The formula for calculating the helmholtz resonant frequency is:
f = resonant frequency in Hertz (Hz)