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Real Traps Mini Trap & Micro Trap

Acoustic Panels
Published September 2004
By Martin Walker

Real Traps Mini Trap & Micro Trap

These acoustic panels not only offer sonic improvements over simple acoustic-foam treatments, but are also very easy to install.

It always amazes me that so many musicians are prepared to spend thousands setting up their studios without investigating any sort of treatment to sort out their acoustic problems. If there are peaks and troughs in the frequency response of your monitoring environment, then you'll be misled while you're mixing and your finished tracks won't sound good on other systems.

In the past, acoustic treatment often had to be done before installing your studio equipment. However, several companies including Auralex, Primacoustic, and Real Traps have now designed easy-to-use modular acoustic treatment systems which can be added after your gear has already been set up. While such systems don't provide a ruler-flat frequency response, they are a vast improvement over nothing at all!

Modular Acoustic Treatment

Real Traps have been offering acoustic treatment panels in the US for a little while now, under the direction of Ethan Winer. Their first design was a panel absorber with a solid wood surround and back panel, and a flexible plywood front panel. Within the case, a fairly rigid fibreglass block (similar to loft insulation, but highly compressed) absorbed the incoming sound energy, as well as lowering the resonant frequency and 'Q' of the trap to make it useful over a wider frequency range. Real Traps made the absorber available as a set of stand-alone modular products with a standard two-foot width, tuned to centre frequencies of 60Hz, 100Hz, or 180Hz. Each was available in three heights to accommodate different ceilings. This design proved very popular in the US because the units could be easily hung on the walls of an existing studio from mounting bars, providing acoustic treatment with the minimum of down time. However, the panels were, of necessity, rather heavy, making them virtually impossible to ship around the world.

The Mini Traps under review here take the modular concept further, but this time the panels have been made only 3.25 inches thick, bringing the weight down to just fifteen pounds, which is why Sonic Distribution here in the UK have been able to import them for the European market. This also makes them light enough to be hung from a picture hook, so installation becomes even quicker and easier. This time Ethan abandoned the low-Q tuned resonator approach in favour of using a hybrid of rigid fibreglass and membrane absorber that looks like a huge acoustic tile. The fibreglass block is mounted in a slotted metal frame with an felt-like acoustically transparent fabric covering, designed to expose the maximum amount of block to incoming sound. Since the fibreglass is three times as dense (six pounds per cubic foot) as most competing foam products, it's also claimed to be far more effective at absorbing lower frequencies. Mini Traps can absorb a wide range of frequencies right down to about 90Hz, below which their effectiveness falls off, although they still provide some useful absorption right down to 50Hz.

Although understandably cagey about revealing his design secrets, Ethan did mention to me that his custom-manufactured fibreglass panels have a dual-layer membrane bonded to them that increases their absorption up to three times at the lowest frequencies. At the same time, the front surface is intentionally semi-reflective at higher frequencies, so you can use enough traps to cure your low-end problems without killing the high end (as many people do by covering their studio walls with egg boxes or thin acoustic tiles). In fact, the Mini Trap design has proved to be so effective that Ethan has now discontinued his original panel traps!

The Long & The Short Of It

This review took me longer than expected, because after some initial acoustic tests I ended up turning my entire studio contents round by ninety degrees! I hasten to add that this wasn't because of the Mini Traps, but it certainly helped them to perform even better. Since you may face the same scenario, let me explain why I did this. To get the best stereo imaging, you should always set up your gear as far as possible such that each side of the room is a mirror image of the other about your listening position — this ensures that the reflections from the side walls are similar. However, this often leaves a choice of whether to point your speakers down the short or long dimension of the room.

Before you get serious about acoustic treatment, it's worth trying your gear in different orientations. As you can see quite clearly from this frequency response plot in my studio using minimal acoustic treatment, pointing the speakers down the shorter dimension (green trace) resulted in terrible dips at 70Hz and 120Hz, and these remained no matter how much I adjusted both speaker and listening positions. Turning the studio round 90 degrees so the speakers fired into the longer dimension (white trace) reduced these frequency dips significantly, even before acoustic treatment.Before you get serious about acoustic treatment, it's worth trying your gear in different orientations. As you can see quite clearly from this frequency response plot in my studio using minimal acoustic treatment, pointing the speakers down the shorter dimension (green trace) resulted in terrible dips at 70Hz and 120Hz, and these remained no matter how much I adjusted both speaker and listening positions. Turning the studio round 90 degrees so the speakers fired into the longer dimension (white trace) reduced these frequency dips significantly, even before acoustic treatment.

Firing into the short dimension means the side walls are further away from the speakers, thus reducing the impact of side reflections, and with my original setup it also meant I could face a huge double-glazed window, giving me a nice view of the garden! However, it also meant that the rear wall was closer to the speakers and, sadly, a few measurements with Rightmark's Audio Analyser showed that the reflections from this wall were causing problems in my small 12.5 x 10 x 7.25-foot studio. Therefore the garden view had to go, and the local plumber had to be called to move a radiator so that I could set up my gear firing down the long dimension. The difference was remarkable, giving the Mini Traps a much flatter initial response to work on — sometimes some initial reorganisation can reap huge acoustic dividends!

Installation

Sonic Distribution sent me four Mini Traps in two huge boxes, plus a further box of five even lighter and thinner (1.35-inch) Micro Traps. Both traps are available in either black or white to suit your particular studio decor, and I chose the white. The traps looked very smart once I'd got them unpacked, and were light enough for one to be carried in each hand. I also found them less obtrusive compared to some of the more colourful moulded-foam designs, which is an important consideration if you want to install them in a living room, for instance.

The ideal place for the Mini Traps in most small studios is in the corners, since this is where you normally find the most bass energy for them to work on. However, unlike panel traps (the main working surface of which is the front panel and which are therefore best placed flat against a wall), Mini Traps work more effectively if you place them across a corner, since the air can then get at the back as well as the front, thus doubling the active surface area and extending absorption to lower frequencies. Because of their comparatively light weight, the traps can be hung horizontally or vertically using normal picture wire threaded through one of the six slots spaced across the rear mounting bars. They will need a fixing more substantial than a picture hook to support them, but a single screw and wall plug did the job for me.

In many rooms, placing the traps vertically across the four corners at about head height is the best option, but they can also be hung across the upper wall/ceiling boundary if you've got a door in the way, or in a ceiling corner. If you need more than four Mini Traps (and the more you install, the flatter your bass response is likely to get) you can fit two in each corner by mounting one vertically at head height and a second horizontally at ceiling height.

You can also mount them on side walls or the ceiling, to treat the main wall reflections. Micro Traps are suitable for this role if your bass end is already OK, even though their effectiveness falls off significantly below about 300Hz. Once again, wall-mounted traps will be significantly more effective if you space them away from the wall so that their rear surfaces become active. With ceiling traps you can just suspend them using longer wires, but for the walls the easiest thing is to place the three-inch-thick foam blocks found at each end of the packaging behind the traps. There's a detailed instruction sheet with each box of Mini Traps to explain the various options.

Standing Waves

Nowadays there are some excellent nearfield monitors available, but you won't really hear their quality in an untreated room. Each of the room dimensions (width, depth, and height) will result in standing waves at a specific frequency, along with all its harmonics. A simple formula to calculate the lowest mode in Hertz is 6720Hz/L, where 'L' is the length (in inches) being considered. So, for instance, if your room is eight feet wide, it will have its lowest mode at 6720Hz/96, or 70Hz, plus there'll be modes at 140Hz, 210Hz, 280Hz, and so on. You get another set of modes determined by the room depth, and a third set according to its height. Apart from these axial modes, you also get tangential modes that involve two dimensions, and oblique modes that involve three, but the axial ones are the strongest).

Since the wavelengths at these frequencies fit exactly into the room, you'll find much higher sound levels at the room boundaries (and particularly in the corners, where two or more sets of modes coincide), plus a series of low and high levels (minima and maxima) spaced between the boundaries. If you're lucky, or have built the room with acoustics in mind, the three sets of Axial modes will be smoothly spaced from the lowest room-mode frequency to several hundred Hertz, beyond which their numerous peaks and troughs can be effectively treated using acoustic tiles. If not, some frequencies from the different mode series will coincide, causing even larger peaks and troughs in your frequency response. The very worst case is a cubic room, since all three mode series will be identical, but thankfully there are published lists of preferred ratios for room dimensions that ensure relatively smoothly spaced modes.

Ethan Winer's Modecalc utility will plot your room modes and highlight any problem areas. In my studio the dimensions are such that in the most difficult area to treat (below about 200Hz) the modes are reasonably evenly spaced, and bunching only occurs at around 220Hz, 400Hz, and 450Hz. The worst shape is a cube, since all three modal sets will be identical, so you'll get huge problems at these frequencies.Ethan Winer's Modecalc utility will plot your room modes and highlight any problem areas. In my studio the dimensions are such that in the most difficult area to treat (below about 200Hz) the modes are reasonably evenly spaced, and bunching only occurs at around 220Hz, 400Hz, and 450Hz. The worst shape is a cube, since all three modal sets will be identical, so you'll get huge problems at these frequencies.

Ethan Winer himself has written a tiny Modecalc utility for Windows and DOS into which you can enter the dimensions of your room and see a simple graphic display of the axial mode spacing (and any pile-ups), as well as a selection of preferred ratios. You can download Modecalc for free from www.ethanwiner.com. This will help you identify the main problem areas, although your bass response will also depend on where you sit within the room, and therefore the distance of your head from the various room boundaries. Whatever frequency you choose, there will be a partial null point at one quarter wavelength from each wall, which explains why moving your listening position can also shift the frequencies of peaks and dips in the measured response — not all problems are solely due to room modes.

Testing The Space

Because I was reassembling my studio from scratch, I had the opportunity to measure the room acoustics in a fairly empty state, using some sine-wave sweeps courtesy of Rightmark's Audio Analyser. I could also perform waterfall tests using ETF Software's ETF 5. My ATC SCM10 speakers have a very good reputation for flat response, so I wasn't concerned about them clouding the results. As I had expected, my small studio sounded pretty 'honky' without any acoustic treatment, ringing like a bell at some frequencies. The worst resonant peaks proved to be at 43Hz, 130Hz, and 180Hz (all related to the front/back room dimension), with a further peak at 80Hz (because of the ceiling height) and dips at both 56Hz and 113Hz (because of the side-to-side room dimension).

One of the beauties of the Mini Traps is the instant gratification — you can prop them roughly in position within a few seconds and listen to the improvements. According to Ethan's lab tests, they work particularly well between about 90Hz and 150Hz if placed across the corners, so I wasn't expecting them to cure the 43Hz problem, but even after placing just two Mini Traps across the front corners, the modes at 80Hz, 130Hz, and 180Hz were far more controlled, showing a much shorter overhang on ETF 5 's low-frequency waterfall plot. Adding the other two traps in the rear corners improved these results still further. However, the traps also made a tiny, but nevertheless measurable, improvement to the 43Hz mode, which is pretty impressive for such a compact solution! A flatter response would need significantly more treatment, but subjectively my room had already been transformed — the bass end was now significantly tighter and smoother.

These waterfall plots from 20Hz to 200Hz were taken before installing four Mini Traps. You can see the initial ringing at 43Hz, 86Hz, 130Hz, and 180Hz — there are long 'plumes' at these frequencies carrying on forwards into the plot.These waterfall plots from 20Hz to 200Hz were taken before installing four Mini Traps. You can see the initial ringing at 43Hz, 86Hz, 130Hz, and 180Hz — there are long 'plumes' at these frequencies carrying on forwards into the plot.After the Mini Traps had been fitted, the top three plumes at 86Hz, 130Hz and 180Hz all but disappeared, and even the one at 43Hz became noticeably shorter. The dips between the various peaks were also less pronounced, resulting in a tighter and cleaner bass end.After the Mini Traps had been fitted, the top three plumes at 86Hz, 130Hz and 180Hz all but disappeared, and even the one at 43Hz became noticeably shorter. The dips between the various peaks were also less pronounced, resulting in a tighter and cleaner bass end.

Most users will get good performance from the Mini Traps with this corner placement, but since I was already impressed enough to seriously consider buying the review models I spent a considerable amount of time fine-tuning their positions, and found moving them by even six inches resulted in slightly different results. After further careful adjustments in my studio, I ended up with the first two traps in the front corners suspended about eighteen inches from the ceiling; the third slightly higher in one of the rear corners; and the fourth midway between the speakers on the front wall. A door in the fourth corner meant that I couldn't place a Mini Trap there, and putting it across the ceiling/wall boundary proved less effective.

The Micro Traps were very useful in taming mid-range and high-end reflections above about 500Hz, and are rather less obtrusive than similarly performing acoustic tiles of several times the thickness, although they are significantly more expensive here in the UK. However, for me, the Mini Traps are the stars of the range.

Acoustic Absorption

Unfortunately, however well spaced they are, your room modes will still cause peaks and dips in your frequency response unless you treat them. This needs absorption to soak up some of the extra energy at problem frequencies (mostly below about 250Hz), stopping it being reflected back into the room. Many musicians place a comfy sofa at the back of the room, which certainly helps with the rear reflections, but the best place for absorption is where the energy levels are highest, which tends to be in the corners of the room where modes from the different dimensions 'pile up'. Anything placed here will improve ringing at these frequencies and even out the frequency response.

Measuring the absorption of a particular material at different frequencies isn't easy without proper lab facilities, and even then differing testing methods can give rather different results, particularly at frequencies below 250Hz, where the effectiveness of thinner and more lightweight acoustic materials falls off. As I had some Melatech acoustic foam tiles in my studio of the same dimensions (four foot by two foot by four inches thick) as the Mini Traps, I tried them in exactly the same positions, and can confirm that they had little effect below 250Hz, although I can't personally comment on other products using higher-density foam.

However, when having his Mini Traps and Micro Traps lab-tested, Ethan went to the extra expense of having some rival foam-based products measured under exactly the same conditions to provide the more comparative results you can see in the graph below.

As you can see from this graph of Absorption versus Frequency, the response of the Micro Traps is similar to normal three-inch thick foam tiles, so these are therefore more suitable for wall-mounting to tame side-wall and ceiling early reflections. The Mini Traps, on the other hand, are effective down to about 200Hz when mounted flat against a wall, and down to below 100Hz when mounted across a corner.As you can see from this graph of Absorption versus Frequency, the response of the Micro Traps is similar to normal three-inch thick foam tiles, so these are therefore more suitable for wall-mounting to tame side-wall and ceiling early reflections. The Mini Traps, on the other hand, are effective down to about 200Hz when mounted flat against a wall, and down to below 100Hz when mounted across a corner.

Trap Star Or Pony & Trap?

Over the last 15 years I've tried a variety of modular solutions to bass trapping, including resonant tubes and panel traps, and none of them have had performance anywhere near that of the Mini Traps. The Mini Traps looked good, were incredibly easy to fit, and made a significant improvement to my studio acoustics at the most troublesome frequencies — they even had some effect down to about 50Hz! The Micro Traps are also very useful for treating wall reflections, and don't have to be glued in place, making both products ideal for studios where appearance is important and where the treatment may need to be taken down or moved at some point.

In the US you can buy Mini Traps direct from Real Traps for $179.99 each (currently about £100 in the UK), and Micro Traps for $119.99 each (around £67), plus shipping charges at cost (generally around $20 per trap). This seems very reasonable. Unfortunately, European readers face much higher shipping costs, because the Mini Traps are just so bulky to import, resulting in a UK price of £252 each when buying four Mini Traps at a time (the realistic minimum quantity), or £238 each for eight or more.

Even if you were able to import them into the UK yourself, the price for each of eight Mini Traps would end up about £222 once you added shipping, duty, and VAT. However, Sonic Distribution provide free shipping anywhere in the UK or Europe, and they also offer a free planning service to work out the best places for traps in your own studio. Furthermore, they have a loan scheme so that you can try out any of their products that don't need permanent fixing, so you can see how effective the Mini Traps are in your own studio at no risk. Ultimately only you can decide how much the Mini Traps are worth to your studio. Despite costing about the same in the UK as an esoteric mic channel, four Mini Traps would probably prove far more beneficial overall for your studio.

You can of course try a DIY approach by buying four-inch-thick high-density rockwool slabs and covering them in muslin or hessian. However, the most appropriate materials often seem to be hard to find locally, can cause skin irritation until covered, and furthermore (judging by various comments I've found on web forums from users who made DIY versions and compared them to the real Mini Traps), the latter are significantly more effective, considerably smarter, and far more convenient! I've put my money where my mouth is, and not only are the four review Mini Traps staying put in my studio, I've also added another two!

Published September 2004