Studio SOS: RFI & Hum Problems

The nearby transmitter mast was spoiling Gordon's acoustic guitar recordings. However, experiments with kitchen foil showed that the problem could be solved by creating an earthed shield around the body of the guitar's pickup system.

The SOS team visit the West Midlands, where Gordon Giltrap's home studio needs help, nestling as it does in the shadow of one of the UK's most powerful TV and radio transmitters.

Gordon Giltrap's unique acoustic guitar style has earned him a place amongst the world's top players and his career has taken many turns, from gigging with his own highly successful band in the '70s to touring either alone or with other genre-leading musicians, examples being Rick Sanders, violin player with Fairport Convention, or jazz guitarist Martin Taylor. Gordon also featured as the minstrel in Cliff Richard's Heathcliff musical and has a string of albums to his credit, but it's only in recent years that Gordon has taken the idea of recording his own music seriously.

Because most of Gordon's compositions don't need much in the way of overdubs, he bought himself a Vestax HDR6 six-track hard disk recorder, which also has in-built mixing capabilities. Teaming this with a small Mackie analogue mixer, a budget Lexicon MPX100 reverb and an AKG C414 microphone (now augmented by a pair of SE Electronics SE1s), he recorded and mixed a number of critically acclaimed albums, but on moving house he called us in to see if we could solve a few problems he was having.

Eliminating Radio-frequency Interference

When Gordon told me he was suffering radio interference on his acoustic guitar pickup systems, and that he lived less than half a mile away from one of the most powerful radio and TV transmission masts in the UK, I thought we were going to have our hands full. On top of this, the room/monitor setup wasn't working as well as it should, and the whole system was afflicted by a low level of hum. Furthermore, because the room (a former bedroom) was acoustically untreated, there were reflection problems colouring both the monitoring environment and the miked sound of the guitar. We felt this might require two visits, one to assess the problems and another to bring together all the solutions.

Here you can see Paul building a couple of MDF-backed acoustic foam panels to tame room problems in Gordon's studio. First the MDF was sawn to size and sanded down, before having holes drilled in the corners, through which string was tied to allow wall mounting. After a brief glance at the instructions on the can, spray adhesive was applied to both MDF and foam, and the surfaces finally pressed firmly together.Upon our arrival, Gordon demonstrated the radio interference problem using one of his acoustic guitars fitted with a Fishman Rare Earth pickup system comprising a magnetic pickup and a small mic with integral preamp. Radio signals are of course far too high in frequency to be audible, but as soon as they encounter any circuitry with semiconductors, the radio signal gets rectified, and if the signal is from an AM transmitter then the programme material becomes audible. Although FM transmissions are harder to accidentally convert into meaningful audio, and digital signals all but impossible, they can still produce noise-like signals in the audio band. The only real cure is either to saw down the transmitter (hardly practical in this case!) or to remove the offending radio-frequency signals before they can reach any active circuitry.

Small parallel capacitors or ferrite sleeves fitted over cables can be very effective in attenuating radio-frequency signals without significantly affecting the wanted audio, though there are only around four or five octaves separating the audio and the carrier frequency of some of the long-wave radio stations, so something sharper than a 6dB/octave passive RC (Resistor/Capacitor) filter might be needed in some situations.

After trying clip-on ferrite chokes on the guitar lead and on the cable inside the guitar joining the pickup to the jack socket, it became clear that very little improvement was being made. This really didn't surprise me, as I felt the RF was most likely being picked up in the windings of the magnetic pickup and then being rectified by the pickup preamp circuitry, which is housed in the same physical unit. The other way to remove RF is to prevent it getting to vulnerable parts in the first place, something that can be achieved by placing a conductive screen around the part and then grounding it. To verify this experimentally, (see opening photo) I wound aluminium baking foil around the pickup assembly leaving the top open (the part facing the strings) and then used an ordinary jack lead to temporarily connect the foil to the ground of the guitar's output jack, simply by holding the cable in place. The result was better than we could have hoped for, and the phantom radio programme in the background was silenced.

Of course baking foil is not really a permanent solution, because even if you could fix it in place using double-sided adhesive tape, it's very difficult to make a reliable electrical connection to it and it can only be soldered using special flux. The more professional solution is to use either conductive paint or adhesive-backed copper foil, the latter being my preference, as you can solder directly to it. This could be linked via a thin wire to the ground side of the battery compartment or to the ground of the output cable. Since several of Gordon's guitars would need to be modified in this way, we suggested that he get his usual guitar technician to do the job for him based on our solution.

Gordon subsequently ordered some adhesive foil and modified one of the pickups himself and found that the screening worked OK, but that if he grounded it to the battery compartment the battery became permanently live and the battery drained very quickly. To do the job properly would involve running a wire to the 'earthy' side of the output jack, but as Gordon was now getting good results using only microphones, he hadn't got around to fixing this by the time of our second visit.

Rearranging The Studio

The next step was to look at the arrangement of equipment within the room. Because of a centrally placed radiator beneath the window, Gordon had decided to set up his equipment along a side wall, but this meant the setup was acoustically asymmetrical, with the window to his left and a solid wall with a door to his right. The monitors were perched on fairly unstable tall CD racks right in the corners, which usually leads to an inaccurate and unpredictable bass response, and, as there were no curtains fitted at the time, there were significant flutter echoes between the window and the opposite wall.

I felt that the gear could be moved round so that the mixer would be below the window, in front of the radiator — this was unlikely to be turned up very high when the studio was in use, because of the heat generated by the equipment. As a precaution, foil backed foam of the type normally put behind radiators to reduce heat loss through the wall could be hung over the front of the radiator to reduce the amount of heat radiated towards the equipment when the radiator was on. We recommended that the speakers be placed on solid stands rather than the CD racks and be moved a little way inwards from the corners of the room to minimise bass problems.

With the studio equipment arranged symmetrically around the window, flutter echo problems could be addressed using acoustic foam, and once again Paul Eastwood of the Audio Agency managed to find us a quantity of blue Auralex panels, plus a couple of sets of foam corner bass traps in purple, which would go nicely with Gordon's room.

I was a little worried about sticking panels directly to the wall, just in case Gordon decided to move again in the near future, so instead suggested that we hang the panels on the rear wall and to each side of the listening position after first fixing them to MDF or hardboard using spray adhesive. This would allow the panels to be hung from nails, just like pictures. We had to fix the bass traps (which had to go in the front corners, as the door at the rear of the room was right in one corner) more permanently using spray adhesive, but Gordon was happy about doing this.

Gordon had been mixing his tracks using the internal mixer in the Vestax, and though this can be driven from an external MIDI hardware controller such as a Kenton Spin Doctor, Gordon had been using the menu buttons and LCD window — not very hands-on! However, there were just enough inputs on the Mackie mixer to accommodate all six direct outs from the Vestax, which we felt would be easier and just as good sonically (if not better), so rather than set up a hardware controller, which had been my first thought, we went this route instead.

Gordon was also interested in getting hold of a simple, relatively inexpensive mic preamp/front end and eventually settled on an SPL Gold Mic, which he had installed by the time of our second visit — he says he is extremely pleased with it.

The Return Visit

By the time of our second visit, Gordon had already moved the room contents round by 90 degrees and bought proper Atacama speaker stands. The speaker stands employ two thin vertical rear tubes and a larger-diameter front tube between the base plate and top plate, and the front tube was quite resonant. To cure this potential ring we suggested filling the tube with dry 'playbox' sand, which would mean unbolting the top or bottom plate and filling the tube, using the cut-off top of a plastic drinks bottle as a funnel.

Hugh installs the newly made wall panels, using a spirit level for a neat result, while Paul carefully sticks acoustic foam bass traps directly into the front corners of the room using more spray adhesive.The base plate was fitted with four adjustable spikes, but these were very difficult to adjust — the result being that the tall speaker stands tended to rock slightly on the carpet. We suggested buying a couple of small concrete garden slabs and using those to provide a firm high-mass base on the carpet, placing and levelling the stands on top of the flat surface. The speakers could be secured in place on top of the stands with Blu-Tac or similar.

Gordon also had some sheets of quarter-inch MDF prepared, to which we could stick the foam panels, so we were pretty much set to go. However, because of the furniture in the room and the fact that some of the wall space was being used to hang guitars and pictures, we reasoned that a single four-foot by two-foot panel on each side, and one on the rear wall, would be sufficient. This meant cutting one of his MDF panels in half (we originally anticipated using four half-metre tiles per side), after which we drilled holes in them, through which we could pass string or wire for hanging them up, rather in the style of paintings.

Without the excess reflections which were colouring the room sound, the guitar sound benefited from placing a reflective surface beneath the guitar, adding some lively early reflections.The string was fitted to the panels first, then the foam was glued into place using the spray adhesive provided (Note, I did read the instructions on the can first!). Around three inches of foam protruded from each edge of the MDF sheet, meaning the MDF was quite invisible once the panels were hung in place.

The foam bass traps were glued directly to the walls using the same adhesive, and this process takes a little care as the adhesive grabs on contact — you definitely need to line up the traps carefully before pushing them into place!

Once the Auralex foam was fitted, we played some of Gordon's CDs and immediately discovered a tighter, more focused sound that was more even at the bass end.

The room was carpeted, so we suggested using the spare sheet of MDF to place on the floor to add a few reflections while recording the guitar with microphones, and Gordon tested this by playing a few songs with the MDF beneath his chair while listening carefully to the tone of the instrument. As expected, the guitar sounded a little more lively with the MDF in place and, because the Auralex foam had tamed the worst of the room's coloration, the acoustic sound of his instrument was much more pleasing than before the room was treated.

The original setup, along the side wall.The final arrangement, shifted to the front wall for better acoustic symmetry.

Summary

As with many of our previous visits, what was initially described as one problem actually turned out to be several smaller problems working together. The hum and interference problem was cured in the main system by using balanced connections between balanced pieces of equipment, while the monitoring was improved by using proper stands, moving the speakers away from the corners and fitting some strategically positioned acoustic foam.

A bedroom 12 feet square is never going to be transformed into an acoustically perfect recording and mixing room, but the application of a modest amount of acoustic foam to the sides and rear of the room plus the fitting of four corner bass traps improved the situation very significantly. The window, of course, remains reflective at mid-range and high frequencies, but Gordon is planning to fit vertical blinds, which can help break up reflections to a worthwhile extent when they are set to their half-open position. The sum of all these small improvements transformed a badly behaving room with noisy equipment into one in which Gordon should be able to continue to make high-quality guitar recordings, free from electrical hum, radio interference or excessive room coloration.