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Research Projects > Acoustic Ground Hum Elimination
Acoustic Ground Hum Elimination
A simple technique to eliminate ground loop hum in a terrestrially based hydrophone monitoring system.
Jack Kassewitz* - SpeakDolphin.com - Miami, Florida
Robert Lingenfelser -Marine Mammal Conservancy - Key Largo, Florida www.marinemammalconservancy.org
Mark Weber-Sound Specialist, Inc.-Miami, Florida *Author for correspondence (e-mail J. Kassewitz: info @ speakdolphin.com)
Hydrophones have been used for the recording and documentation of ocean events for many years. However there is an inherent problem whether you are recording on a boat or monitoring from a land based research site - ground hum. This paper will outline a unique technique that eliminates ground hum on land and propose a similar technique for recording while on a boat.
Hydrophones are used worldwide to monitor underwater sounds. Hydrophones, underwater microphones, are a sound-to-electricity transducer for use in water or other liquids, analogous to an ear for listening to underwater sound. Such a transducer can sometimes also serve as a projector (emitter), but not all have this capability, and may be destroyed if used in such a manner.
There is an inherent problem whether you are recording on a boat or monitoring from a land-based research site - ground hum. We were faced with a project that needed to record, and periodically playback, pristine extended frequency sounds for research analysis, 24 hours a day for 7 months.
Our subject was Castaway, a pregnant female Atlantic bottlenose dolphin (Tursiops truncatus), stranded on the morning of November 11th, 2006 at Castaway Cove in Florida. After that stranding Castaway was eventually quarantined to the Marine Mammal Conservancy's (MMC) rehabilitation facility in Key Largo, Florida, where it was determined that the dolphin was hearing impaired.
In isolation, Castaway and her calf would not have the normal "chatter" of a dolphin pod to assist the calf in its language development - both in utero and also after birth. The mother's hearing impairment raised questions as to her ability to teach the calf on her own. Therefore, an environmental enrichment plan was implemented to play pre-recorded dolphin sounds to the unborn calf through underwater speakers. We were simultaneously monitoring and recording any responses from the dolphin with high quality hydrophones.
During her quarantine at MMC, Castaway demonstrated predatory stunning and killing abilities with sound bursts. We were fortunate to not only have eye witnesses during an event in which the dolphin stunned and killed a mullet, but also to able to record this crucial acoustical data without hum corruption because of the grounding technique described in this paper.
Our biggest audio challenge was to record clean samples for measurement and analysis. These recordings of Castaway's vocalizations, echolocations and predator stunning are of significant value to the study of dolphin communication. It was a rare opportunity to be able to record while Castaway was giving birth, the calf's first breath, vocalizations, and development of a signature whistle.
A self-powered (two 9v batteries) hydrophone, in less than three feet of water, was placed adjacent to the birthing pen constructed for Castaway's impending delivery. The cables were strung on an upright pole and then passed through PVC conduit openings into a research shed on land adjacent to the birthing pen. Hydrophone XLR connectors terminated at the Grace Lunatec V3 premium microphone preamp and the digital 192k/24 bit signal delivered interconnected via 50 ohm AES/EBU digital cabling to the Sound Device 722 HD digital recorder running at 192k/24 bit.
Immediately upon connecting this XLR hydrophone cable hum was quite prevalent and loud which would make any recording useless. In fact, the hum was so loud that could not monitor any of the sounds Castaway made or potentially any of her expected calf's sound production. (See picture 1a)
To remedy this problem we dropped a 1 meter copper grounding rod into the salt water at the pen and connected it directly to the recording equipment chassis ground via a GB 10 grounding block. (See figure 1).
A ground rod is a copper metal shaft used for establishing earth-ground. These rods are typically 8 feet in length, 5/8 inches in diameter, driven into the earth and grounding wires are clamped onto the sunken shaft to establish electrical ground potential with the earth. Ground rods should be free of paint or any other non -conductive material and should be not less than 1/2 inches thick. Most applications call for a ground system of 25 ohms or less. While, as a practical matter, you may not get to 0 ohms, you certainly can get to 25 ohms or less if the ground rods are properly installed (See Picture 2).
We changed this usual configuration to a water-ground. Although not necessary for non-ferrous copper, zinc sacrificial anodes can be attached at both ends to prevent electrolysis from attacking the recording equipment. The rod was dropped into the water beside the birthing pen. A10 awg solid copper wire clamped onto the middle of the rod was run to the grounding block in the research shed. At the recording equipment a chassis-ground was established by soldering an empty BNC connector shell (one with no center conductor) onto a length of the 10 awg ground wire leading from the GB10 ground block. This BNC connector was plugged onto an unused wordclock output on the Sound Devices 722 recorder to establish chassis-ground with the electronics. For a recorder without an available BNC we would have either terminated the ground wire through pin 1 of an XLR connector and used any available I/O XLR port or at the very least just twisted the ground cable onto a connector release tab or alligator clipped to any exposed chassis potential piece of metal.
The results were instantaneous and remarkable! (See picture 1c) There was absolutely no electronic hum even though numerous other electronic research devices were running in close proximity as well as the air conditioners and security video system. (Please note that the air-conditioner was grounded separately) Studio quality sound was achieved only after a water- ground was established.
Note: See how the central lines in the upper portion of these waveforms differ. Picture 1a is without grounding protection and 1b is with earth-ground and 1c is with water-grounding. There is still residual hum in 1b; 1c is completely free of hum.
In dolphin research, where sound quality and analysis are critical, a special signal ground known as a "technical ground" (or "technical earth") usually must be installed. This is not the same thing as an AC power ground and no appliance ground wires are allowed any connection to it as they may carry electrical interference (See Figure 2). Great care has to be taken to prevent any AC-grounded appliances (air conditioners, etc), from making contact to the technical ground as a single AC ground connection to the technical ground will destroy its effectiveness. With the recent proliferation of digital multi-track capable, high sampling/bit rate enabled hard disc recorders the installation of high-quality audio grounding has become mandatory. Any ground-introduced interference tends to be cumulative and obscures many of the frequencies being recorded. This creates noise levels that may have been satisfactory for older analog systems but become intolerable and skew the results beyond scientific recognition for today's exacting standards. We revised this idea by creating a water-ground which differed structurally only in the fact that rather than being driven into the ground it was placed in the water adjacent to the birthing pen.
The majority of available hydrophones are wired in an unbalanced electrical configuration making them very susceptible to interference due to lack of shielding and the hum rejection offered with balanced twisted pairs. Water often has a different electrical potential than the electrical ground potential of the recording equipment being used. This relationship causes ground loop hum and unwanted noise in hydrophones. In order to avoid this interference from taking place on the recording, the water must be properly connected to the chassis ground of the hard disk recorder. In the case of recording from a boat a copper wire immersed in the water and connected to the chassis-ground will solve the problem. You may use the same grounding wire as described below and connected to a small metal plate for weight so that submersion over the side of the boat is guaranteed.
A Sound Device 722 is a high-definition two-channel digital audio recorder. The device records audio to an internal hard drive, CompactFlash cards, or external FireWire Drives. It writes and reads uncompressed PCM audio at 16 or 24 bits with sample rates between 32 kHz to 192 kHz. Additionally, compressed (MP3) audio playback from 64 kb/s to 320 kb/s is possible.
The 722 implements a non-compromised audio path that includes Sound Devices' (next-generation) microphone preamplifiers. Designed specifically for high bandwidth, high bit rate digital recording; these preamps set a new standard for frequency response linearity, low distortion performance, and low noise.
The hydrophones used were Model 8178-7 Hydrophones: We currently are using two matched and calibrated Model 8178-7 hydrophones. They are wideband, omnidirectional underwater sensors with a frequency response from 20 Hz to 200 kHz. The spatial response is omnidirectional in orthogonal planes with minor nulling along the axis looking up the cable.
Hydrophone Sensors: The sensor is a small lead zirconate titanate (PZT) spherical sensor. It is fairly thick walled and will withstand several thousand psi of pressure. It is reverse poled on one hemispherical half and series wired to increase sensitivity. The element center is approximately ½" from the end inside the rounded dark polyurethane end.
Hydrophone Electronics: The electronics include a low noise, wideband FET input preamplifier, a pass filter at 15 Hz to reduce geophysical noise, a power supply filter, and power input protective circuits. The amplifier is included inside a metal EMI filter and is encapsulated inside the PVC plastic body of the hydrophone assembly.
Hydrophone Cable: The cable is 45 foot underwater shielded cable. There are four conductors inside a metalized shield. On the top end, the power lines are terminated in a small dual 9 volt battery clip set.
Hydrophone Acoustic Parameters: These hydrophones are provided with calibration that indicates that the hydrophone is flat down to about 20 Hz at about -168.5 dB re 1 v/uPa. The response from 2 kHz to about 10 kHz is -168.5 dB, and then the response rolls off gradually at about -172 dB. From 80 kHz to about 140 kHz, the response is about -174 dB, then the response climbs again to about -170 dB by 180 kHz. After about 200 kHz, the response rolls off gradually to about 250 kHz, then drops rather sharply at about -12 dB per octave.
Grace Lunatec V3 Signal Processing: The signal from the hydrophone is routed through a Grace Lunatec V3 mic-preamp. The Grace Lunatec V3 has an ultra-low distortion 24-bit A/D converter. It has flexible outputs including SPDIF (RCA) and two sets of AES3 (XLR). Sample rates are 44.1, 48, 88.2, 96, 176.4 and 192 kHz. The Grace Lunatec V3 is also equipped with a unique analog noise shaping dither circuit, or ANSRTM, which can be applied for word-length reduction when sending signals to a 16-bit digital recorder. Grounding Rod: A grounding rod is a metal shaft used for grounding. These solid copper rods are meant to be driven into the ground. Most codes call for a ground system of 25 ohms or less. (In our case the rod was just laid in the water) While, as a practical matter, you may not get to 0 ohms, you certainly can get to 25 ohms or less if the ground rods are properly installed. The resistance of this ground or water is intended to be 25 Ohms or less, but is rarely tested. To verify the resistance of ground or water, it should be tested with instruments using the fall-off-potential method. (See picture 2)
Grounding Block (Electrical Isolator): Experiments using hydrophones require that your electrical equipment not add to the data that you are trying to collect. This simple and cost effective grounding block will localize and organize the ground circuit in your system. The GB-10 comes with all the hardware needed to mount the block to your isolation table and attach up to 10 single leads. (See figure 1)
It was our goal to create a hydrophone recording system that was beyond music studio quality for precise research measurement as well as applications demanding very broad extended frequency response and extreme dynamic ranges. The first part of this applied research had to overcome electronic hum and its damaging effect to our research data. By using a simple grounding rod and ground block inter-connected by copper cable and terminating at water's ground potential we were able to eliminate ground loops and potential interference.
Our current spectrographic analysis shows no observable electronic hum in the sounds we are recording. (See picture 3.)
This relatively simple answer has allowed us to record continuously for weeks at a time with no appreciable sound distortion. The results from these pristine digital recordings are currently being analyzed and prepared for publication. We have seen the remarkable first echolocation from the calf as well as robust vocalizations only moments after birth. The results of this research are forthcoming over the next few months.
For questions contact: info @ speakdolphin.com