Acousonde™ Technical Questions

Updated January 2016

The nine most frequently asked questions

How long can the Acousonde record?
As of 2011 the standard Acousonde storage capacity increased from 16 GB to 64 GB. New Acousonde units may be ordered with up to 128 GB (3B model) or 256 GB (3A model). At these capacities the Acousonde is typically power limited, not storage limited.

For sampling rates under 26 kHz, typical of sampling with the low-power channel, the standard A-size lithium cell should be sufficient to record for anywhere from 6 to 14 days depending on the ambient temperature and the extent to which auxiliary sensors are sampled.

For the fastest sampling rates, longevity may still be constrained by available storage. At 232 kHz sample rate, for example, 64 GB of storage capacity should be sufficient to record a maximum of 38 hours. However, the increased power demands at fast sample rates increase susceptibility to cold, battery contact resistance, and battery condition; thus even at fast sample rates power remains a concern.

This being said, a 128-GB Acousonde 3A tested at 464 kHz sampling rate in January 2012 filled 89 and 100 percent of storage (34 and 38 hours, respectively) in two tests running on a standard A-size lithium cell. A new battery was used for each test. The first test occurred at room temperature, the second at an average of 7.5°C (the better performance at colder temperature may reflect variability among new batteries). These tests indicate that the longevity of a 128-GB Acousonde sampling at 464 kHz is nearly balanced between storage-limited and power-limited. Thus the 64-GB configuration operating at fast sample rates is probably storage limited except in adverse ambient conditions.

Do I really need a Palm? What kind of Palm should I get?
You must have a Palm to program the Acousonde. There is no alternative at the present time.

Any Palm III or later PalmOS-compatible PDA is fine. We have tested the Palm III, IIIx, IIIxe, V, VII, m100, m125, m500, T|X, Tungsten T3, and Z22, as well as the Zire 21, 31, and 71; the Handspring Visor Deluxe and Visor Edge; and the Kyocera 7135 smart phone. In addition, customers have used a Palm Tungsten E2, a Palm Treo smart phone, and a Sony Clié. All have worked with the Acousonde and/or with its predecessor the Bioacoustic Probe. We are confident that any PalmOS-compatible PDA made by Palm, Handspring, or Sony will suffice.

Further, you do not need the cable or software for synchronizing the Palm with your personal computer, as the Acousonde can transmit its companion PalmOS application directly onto the Palm via infrared. If you have a rechargeable model you will of course need the charging cable (usually molded together with a wall-socket power adapter).

We recommend, if possible, using a Palm that operates from store-bought batteries rather than on an internal rechargeable battery. Store-bought batteries are more convenient in the field and there is no dedicated charger and cable to keep track of. Not only that, but the rechargeable battery is often the first thing to go wrong with a Palm as it ages. Models that operate on store-bought batteries include all models of the Palm III series, the Handspring Visor, Visor Deluxe, and Visor Neo, and all models of the Palm m100 series except the m130 (the m100, m105, and m125).

Cetacean Research Technology stocks Palm units specifically for sale to Acousonde customers. Customers have also found new Palms through Amazon.com and other retailers, and used Palms at swap meets and through online classified ads. A used Palm is perfectly adequate to operate the Acousonde; however you must test it. We have seen several used Palms with faulty batteries and touchscreens.

We are aware that, as a discontinued technology, Palms will become gradually harder to find. As of early 2016, however, Amazon.com still offers new Palms for sale, and we are confident that Palms will be easily obtainable for several years to come.

We regret that we are unable to offer advice more specific than this, or to confirm unconditionally that any specific PDA model not listed above will work.

What kind of battery do I need?
The Acousonde is designed for use with the Saft LS17500 size A 3.6V lithium primary cell. This battery is rated at 3.6 Ah with a maximum recommended continuous current of 130 mA. It is technically classified as "non-restricted for transportation" but that expression is misleading; regulations do prohibit transporting this battery under certain circumstances. Please see the transportation FAQ.
Can I re-use a battery? Will a partially-used battery be good enough?
Given the critical importance of reducing weight, the Acousonde design uses the smallest battery that can still power high-frequency sampling in temperate waters. This design criterion makes the Acousonde power limited. We therefore recommend that you don't re-use a battery for mission-critical recordings after it has powered significant standby or recording time.
Can the Acousonde's firmware be updated after delivery?
Yes. Firmware updating takes place via infrared and requires an Apple OS X personal computer with a specialized serial-and-infrared dongle combination. Please see the documentation available under downloads for detailed instructions on updating firmware.

In some circumstances we can preload a PalmOS handheld with the Acousonde firmware and mail it to the user, where it can be used to update one or more Acousonde units. Alternately, Acousonde units may be returned to us for firmware update.

Does the high-frequency option replace or augment the low-power hydrophone?
Option B003-HF, the high-frequency hydrophone option, adds a high-frequency hydrophone system to a B003A or B003B Acousonde. The low-power hydrophone comes with the Acousonde regardless of whether you have Option B003-HF or not. So, if your Acousonde is equipped with Option B003-HF, it has two hydrophones in it. You choose which hydrophone you want to use at deployment time.
What has changed from the Bioacoustic Probe?
  • Two acoustic channels instead of one
  • Low-power acoustic channel similar to Bioacoustic Probe, 9.28 kHz max bandwidth
  • New high-frequency acoustic channel samples up to 464 kHz
  • Acoustic gain choices now just 0 or 20 dB (dropped 10-dB choice)
  • Anti-alias filters can be bypassed
  • Future firmware will support "ping-pong" alternating sampling of both channels concurrently
  • Tiltmeter (accelerometer) now 3D 10-bit (compared to 2D 16-bit)
  • 3D compass provides new orientation sensing
  • Real-time clock accurate to within 1 minute a year in normal temperatures
  • Firmware supports easier and more precise time synchronization
  • Up to 256 gigabytes per-deployment recording life (compared to 1 gigabyte)
  • Full-speed USB data offloading (compared with standard infrared)
  • New technology throughout offers room for future firmware/hardware improvements
When analyzing data, how do I account for having used 20-dB gain?
As long as you import the data for analysis using a technique that applies the metadata, for example the MTRead.m MATLAB m-file, you don't need independently to compensate for gain selections made at deployment time.

Every time the Acousonde writes a new data file when sampling, it begins by writing a small amount of metadata. These metadata provide per-file calibration constants that account for all gain that is currently being applied. To put it another way, if you could compare the metadata for a file written with 0-dB gain and a file written with 20-dB gain, you would find differences. The 0-dB file would have one set of calibration constants, the 20-dB file another.

So there should be no need manually to account for the gain specified at recording time. It is already incorporated in the calibration and MATLAB file-import process.

I need ceramic sensitivity, gain, and A/D reference to calibrate the acoustics.
No, actually, you don't.

Per-unit gain specifications and sensor sensitivities come pre-installed in each Acousonde. At build time, we verify each unit's gain specifications by recording an electronically-injected test signal of known amplitude with all possible gain paths. Then, at sample time as the data are saved, the Acousonde combines the user-selected gain and fixed A/D scaling with the known hydrophone sensitivity and places comprehensive overall calibration parameters in each data file's metadata header.

So as long as you import the data for analysis in a way that accesses and applies the per-file embedded calibration values (such as with the MATLAB script available in the downloads area) you will not need to calibrate the files independently.

We do not recommend calibrating the raw data samples yourself from a priori hardware parameters. Not only would you need to track the different sensitivities of each unit you use, you would also need to keep careful note of any user-selected gain associated with each data file and whether the anti-alias filter was bypassed or not (which can also affect gain). It is easier and less error-prone to take advantage of the Acousonde's built-in calibration system.

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Palm questions

Must I install Palm synchronization drivers and applications on my personal computer?
Thank goodness, no! There are only four things you need to do with the Palm:
  • Ensure that the Palm's batteries are fresh (replaceable batteries) or charged (rechargeable batteries);
  • Ensure that the Palm's time is set correctly, which can be done directly on the Palm;
  • Load the "Acousonde" commanding app onto the Palm via direct infrared transfer from the Acousonde itself; and
  • Operate the "Acousonde" commanding app once it is on the Palm.
None of these things require installation of Palm operating/synchronizing software on your personal computer. Nor is any Palm synchronization hardware necessary, such as cradles or cables, except of course what may be required for charging rechargeable Palm units.
I found a Palm but its synchronization cradle/cable is lost.
No problem. You do not need to connect or synchronize the Palm with your PC, so there is no need for any of the synchronization equipment that originally came with the Palm. Rechargeable Palms do need their recharging gear, however, which for some Palms involves a cradle. For simplicity and reliability we recommend Palms that rely on store-bought instead of rechargeable batteries.
A Palm? Are you kidding me?
Yes, the Acousonde depends on discontinued 1998-2009 technology for commanding and interrogation. Believe it or not, no current technology matches the Palm when optimizing for the following objectives:
  • Robust, inexpensive, handheld commanding unit for use in difficult field situations
  • No cables
  • No commanding sockets on Acousonde that could compromise reliability
  • No prior pairing or other per-Acousonde setup necessary
  • Select communication with desired Acousonde easily, even with several other Acousondes nearby
  • Commanding app only needs to be maintained for one platform
  • Commanding app can be loaded onto a Palm directly from an Acousonde in the field without need for cabling or any other equipment
  • User does not need to sign up for a vendor's app "ecosystem"; in fact user does not need network connectivity at all
We have thus hesitated to change how the Acousonde is commanded not only because of the development hurdles, but also because we have been unable to identify a realistic concept that works as easily and as well.

Nevertheless, we understand that Palms will gradually become harder to find and that eventually an alternative must be pursued. A stopgap measure may be simply to write a command-line app for a laptop computer mated with infrared hardware, and indeed we already have this set up for development and testing. If all Palms disappeared tomorrow we could quickly modify this software to replace the Palm functionality.

Battery questions

Will other batteries work?
The Acousonde requires an A-size lithium primary cell to operate. The only such cell we have tested is the Saft LS17500, so we cannot recommend any other brand or model. AA-size lithium cells such as the LS14500 do not provide sufficient peak current and will not work, even though they will fit (very loosely) in the battery compartment. Alkaline batteries do not have sufficient voltage and will not work.
I can't find the Saft LS17500 at the store. Where can I get it?
That's not surprising; we have yet to find any retail store that stocks the LS17500. However some retail stores, such as Batteries Plus, may be willing to special-order the battery for you. Also, a quick search of the web will turn up dozens of distributors worldwide through whom the LS17500 may be purchased. We often order from the TNR Technical Battery Store. Please be aware that national and international regulations on shipping batteries with lithium content may restrict delivery to ground or sea transportation, which in some locations may mean lead times on the order of several weeks.
Spring contacts are unreliable, you should use clips or soldered batteries.
When designed properly, cleaned regularly, and assembled securely, spring contacts work well for all but the most shock-intensive applications. For example, six Bioacoustic Probes deployed for 1-2 weeks on northern fur seals in August 2005 all ran flawlessly, despite cold Bering Sea waters and significant movement (in this example the contacts were treated with Stabilant 22, a chemical contact enhancer.) While other approaches such as battery clips or soldered tabs do work, their reliance on user-handled wires would decrease the long-term reliability of the Acousonde. The Acousonde is a sealed instrument and if wires emerging from the sealed portion were to break, repairs would be difficult or impossible. As long as a spring system is strong, tight, and clean, it will be adequate. Note: never attempt to solder directly to batteries, particularly lithium batteries, that do not have solder tabs pre-attached by the battery manufacturer. There is an explosion risk.
Why can't you use movement- or solar-based power, instead of a battery?
Self-powering systems, such as self-winding watches and solar-powered calculators, run on power levels measured in microwatts. At present the Acousonde typically requires between 20 and 120 milliwatts when sampling, far more than a movement- or solar-based power system could provide.
Why don't you use a rechargeable battery?
The Acousonde's solid urethane encapsulation is not compatible with a permanently-installed rechargeable battery. Besides the difficulty and expense of "surgically" replacing a worn battery, rechargeables sometimes outgas during operation. Lacking a vent to the outside, this gas could deform or burst the urethane.
Do the lithium primary cells used by the Acousonde outgas? Is it hazardous?
As energy densities increase, batteries contain more energy in a smaller space, which increases their hazard. The Acousonde addresses this issue in three ways. First, the Acousonde uses batteries whose design has undergone rigorous safety testing. Second, the Acousonde's battery cap is designed so that during removal it will vent before being mechanically free. This prevents any buildup of gas in the battery chamber from affecting removal of the battery cap. Finally, the battery chamber includes redundant o-ring seals to ensure no seawater enters and shorts the battery. Nevertheless, lithium batteries must always be treated with respect. Do not allow them to be shorted by seawater or other conductive materials. Ensure that the battery compartment is clean and dry when inserting a battery. Respect hazardous-material disposal requirements for your area.
There are horror stories about batteries dying too soon.
Battery performance can vary dramatically from manufacturer to manufacturer and even from batch to batch. Manufacturer's specifications are no assurance that a given cell will perform adequately. For this reason it is impossible to guarantee the Acousonde's recording life. Always test batteries from a new batch before heading to the field. Also, dirty or tarnished battery connections substantially reduce battery lifetimes! Be sure to clean the Acousonde's battery contacts regularly, and to clean the contacts on the battery itself before use. A contact enhancer such as Stabilant 22 may help; in August 2005 Stabilant 22 was applied to the battery contacts of six Bioacoustic Probes deployed on northern fur seals in the Bering Sea, and all six filled their storage units despite cold conditions and significant movement. For more information, download the manual and check the troubleshooting section.
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Sampling rates, bandwidth, and anti-aliasing

What sampling rates are supported?
The Acousonde hardware does not allow for completely arbitrary sample rates; it has to round up to the nearest supported sample rate. Supported sample rates are determined with the formula 464 kHz / {1, 2, 3...}; thus, 464 kHz, 232 kHz, 154 kHz, 116 kHz, 92 kHz, etc. As the formula indicates, supported sampling frequencies are closer together at lower frequency values, so for desired sample rates on the order of 1 to 5 kHz the nearest supported rate will be close.

The high-frequency channel option, if installed at build time, can record at the hardware's maximum 464 kHz rate. The standard low-power channel cannot record faster than 154 kHz, but as this channel's anti-alias filter does not cut off higher than 9.2 kHz and the low-power hydrophone itself falls off above 30 kHz, the 154-kHz maximum sample rate does not impact usable bandwidth.

What's the relationship between sampling rate and data bandwidth?
Data bandwidth is determined by the cutoff frequency of the anti-alias filter.

The low-power channel features an adjustable anti-alias filter that is automatically set to no higher than 0.38 times the sampling rate (it may be set lower; 0.38 times the sampling rate is the upper limit for the filter cutoff). This adjustable filter, like the sampling rate, is only capable of certain discrete cutoff frequencies. These are determined with the formula 9.28 kHz / {1, 2, 3...}; thus, 9.28 kHz, 4.64 kHz, 3.09 kHz, 2.32 kHz, 1.86 kHz, etc. For example, for a sample rate of 25 kHz on the low-power channel, signals above 9.28 kHz will be filtered out, but for a sample rate of 20 kHz, the anti-alias filter will be set to the next lower cutoff frequency, which is 4.64 kHz.

For the high-frequency channel, if installed, the bandwidth is determined by a fixed-frequency 6-pole linear-phase filter with a cutoff at 42 kHz (down by 22 dB at 100 kHz). If you do not sample the high-frequency channel at 232 kHz or above there may be aliasing of higher frequencies within the filter band into lower frequencies, which may or may not be acceptable depending on your recording environment.

What's the highest-frequency signal the Acousonde can record?
For the low-power channel, the maximum frequency recordable is 9285 Hz (corresponding to a sample rate of 25793 Hz). For the high-frequency channel, a specific number for "maximum frequency" is difficult to state since the linear-phase anti-alias filter rolls off so slowly. The 3-dB filter cutoff is at 42 kHz, but the filter is only 22 dB down at 100 kHz.
A 9.28 kHz limit (on the low-power channel) is not high enough! Can you raise it?
There is a possibility that future software may raise the low-power channel's upper frequency limit to just over 10 kHz. Hardware limitations on that channel prevent raising the upper frequency limit any higher than that. These hardware limitations are due to design for low power consumption.

The easiest way to incorporate higher frequencies on the low-power channel is to defeat the anti-alias filter at deployment time. This is only suitable if you are confident that all significant signal content will be at frequencies below one-half of the sample rate you choose (also at deployment time), otherwise your data will include aliased artifacts.

If you need to record frequencies above 9.28 kHz without aliasing, you will need to specify the high-frequency option (Option B003-HF) when you order and use that channel when recording. Remember that, by default, Option B003-HF incorporates a high-pass "pre-whitening" filter that attenuates frequencies below 10 kHz. If your application requires flat response from a few tens of Hz or less to frequencies over 10 kHz, then customized high-frequency characteristics will be necessary (Option B003-CF).

What's the lowest frequency the Acousonde can record?
Powerful low-frequency flow noise dominates the Acousonde's expected acoustic environment -- the side of a moving animal. To keep this noise from overdriving acoustic recordings, each Acousonde acoustic channel incorporates four high-pass filters. The specific characteristics of these filters have evolved with time, but as of June 2011 the default characteristics are as follows:
  • the preamplifier input, set by the input shunt resistor and the hydrophone capacitance;
  • the preamplifier input follower stage (10 Hz);
  • the preamplifier gain stage; and
  • the mainboard input (10 Hz).
As noted in the list, high-pass filters 2 and 4 are set to 10 Hz. The other filters are set differently for the low-power and high-frequency channels. For the low-power channel, Filter 1 is 8 Hz while Filter 3 is 9.5 Hz; for the high-frequency channel, Filter 1 is 10 kHz while Filter 3 is 86 Hz.

The combination of these four high-pass filters results in a total filter response that high-passes at 22 Hz for the low-power channel and 10 kHz for the high-frequency channel for late 2011 units (earlier units had higher low-power-channel cutoff frequencies up to 38 Hz).

What are the specifications of the Acousonde preamplifier boards?
There are three high-pass filters embedded in the preamplifiers; default high-pass corner frequencies are given in the answer to What's the lowest frequency the Acousonde can record? above. Default gain for the low-power hydrophone is 14 dB, for the high-frequency hydrophone 29 dB. The output is DC coupled at a quiescent voltage halfway between the power and ground rails. Different gain-stage amplifiers are used for the low-power and high-frequency preamplifiers, as appropriate for the different bandwidth and noise-floor requirements. A single-pole low-pass filter at 284 kHz attenuates any high-frequency radio interference from the recorder's digital electronics. The input is a high-impedance JFET type.
Can gain and filtering be customized?
Gain and high-pass filtering on both channels and the high-frequency channel's anti-alias filter may be customized, within the limits of the design; please see the ordering information. Specifically, the cumulative high-pass cutoff may be shifted lower for the high-frequency channel, while the high-frequency anti-alias filter may redesigned as long as the new design does not exceed 6 poles. The low-power channel's antialiasing is provided by a single integrated circuit and cannot be modified.
Can I specify my own gains and high-pass frequencies for the Acousonde's preamplifiers?
Yes, within the limits of the design. This is Option B003-CF (for an embedded high-frequency preamp), Option B003-CH (for an embedded low-power preamp) or B003-PC (a loose preamp, e.g. for integration in a customer's external hydrophone).
You should prewhiten to improve broadband SNR.
Prewhitening is a high-pass filtering approach that gradually increases attenuation with decreasing frequency. The attenuation parameters are chosen to counteract the increase in ambient sound levels at lower frequencies typically observed in the world's oceans. Attenuating those expected low frequencies balances the recorded spectrum. Prewhitening enables significantly more preamplifier gain, and thus better signal-to-self-noise ratio at high frequencies, because overloading by low-frequency ocean noise is no longer a concern. Prewhitening is common practice when detecting weak higher-frequency signals is a goal.

The Acousonde's high-frequency channel applies a form of prewhitening by using a default ceramic-and-shunt resistor corner frequency of 10 kHz. This filter attenuates incoming signals by 3 dB at 10 kHz, by 20 dB at 1 kHz, and by 40 dB at 100 Hz. Above 10 kHz the prewhitening filter has little effect. The low-power channel, on the other hand, is intended for calibrated general-purpose monitoring applications at lower frequencies and is not suitable for prewhitening.

A linear-phase anti-alias filter? Crazy! It rolls off too slowly.
Four considerations drove the choice of a linear-phase anti-alias filter for the high-frequency channel. First, the primary signals of interest at high frequencies are odontocete echolocation clicks. An elliptic or other "brick wall" anti-alias filter would have heavily distorted these clicks and confounded time-domain analysis. Second, odontocete clicks roll off at high frequencies such that any aliased content will most likely be weaker in amplitude than the in-band portion of the click. Third, the gradual rolloff of the linear-phase filter will allow clean decimation in downstream digital processing, possibly onboard. Fourth, it is possible to bypass the anti-alias filter if desired and, potentially, to sample at a rate high enough (464 kHz) that a clean signal can be acquired with neither filtering nor aliasing. This being said, the filter choice is experimental and subject to change in the future.
What is aliasing?
Aliasing means high-frequency signals are masquerading as low-frequency signals. The phenomenon is familiar to anyone who has seen automobile wheels appear to turn backwards in a movie (unintentional aliasing), or an engine flywheel appear to be stationary when viewed with a strobe light (intentional aliasing). Just as a machine operator may be injured if he thinks the strobed flywheel is stationary and reaches to touch it, a researcher may be misled by an acoustic record that contains aliased signals, since it will show tones or noise at a low frequency that were in reality at some higher frequency.

Any incoming analog signal with a frequency above one-half the digital sampling rate (the "Nyquist frequency") will be aliased when digitized, and once the aliasing takes place, there is no way it can be undone in post-processing. Therefore digital recorders must filter out frequencies above the Nyquist frequency before digitization. However, if one is confident that one's recording environment contains no signals above the Nyquist frequency, or that any such signals are weak compared with the corresponding in-band sound levels, or if one wishes intentionally to alias incoming signals (as in the strobed-flywheel example above) then anti-alias filters can be omitted. The Acousonde provides optional anti-alias bypassing.

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Channel timing and inter-channel simultaneity

Are the auxiliary channels all sampled simultaneously?
The accelerometer channels are. The accelerometer hardware acquires all three of its channels simultaneously at the top of each sample interval.

The other auxiliary channels, however — compass, temperature, pressure, and light — are not sampled simultaneously. These channels feed to a single high-resolution auxiliary analog-to-digital (A/D) converter through a multiplexer, and only one channel can be sampled at a time. Even at its fastest the A/D requires 6 milliseconds to acquire one sample; thus, compass Y is acquired 6 milliseconds after compass X, and compass Z another 6 milliseconds after that. At a 40-Hz sample rate, this means compass Z will be sampled halfway between samples of compass X! These data may require extra postprocessing to be useful in assessing very rapid changes in 3D orientation.

Can the low-power and high-frequency channels be recorded simultaneously?
Not perfectly. The Acousonde's acoustic analog-to-digital (A/D) converter (separate from its auxiliary A/D converter) is not capable of true simultaneous sampling. However, if both channels are connected to hydrophones (i.e. Option B003-HF has been installed) the A/D converter can "ping-pong" (alternate sampling) between them. This means that the per-channel sampling rate for each of the two channels would be one-half of the A/D master sampling rate, which can be sustained as high as 464 kHz. Firmware to enable this capability is under consideration for future release; we welcome user comments as to how useful it would be.
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The sampling program

Is sampling continuous or duty-cycled?
Either. You choose between continuous or duty-cycled sampling when you program the Acousonde before deployment.
How does duty-cycled recording operate?
Duty cycling of acoustic recording may be programmed by specifying how many "duty period" minutes to record out of every "cycle period" minutes, with an optional "delay period" in minutes.

The cycle period must divide integrally into each 24-hour period; for example, 5 minutes, 15 minutes, and 60 minutes are all valid cycle periods, while 13 minutes and 57 minutes are not. The beginning of each cycle is aligned with the time of day. For example, 60-minute cycles start at the top of each hour. A cycle period cannot be longer than one day (1440 minutes).

The duty period specifies how many minutes of each cycle that acoustic recording will be active. For example, with a cycle period of 60 minutes and a duty period of 15 minutes, acoustic sampling will begin at the top of each hour and continue until a quarter after the hour (assuming zero-minute delay period; see below) at which time acoustic recording will cease until the top of the next hour.

The delay period specifies how long after the beginning of a new cycle to wait before beginning the duty period. For example, with a cycle period of 60 minutes, a duty period of 15 minutes, and a delay period of 30 minutes, recording would not begin until half past the hour and would continue until a quarter before the next hour. The delay period plus the duty period cannot be longer than the cycle period.

Fixing duty cycles to the time of day ensures that multiple units operating with the same duty-cycle parameters will record the same snapshot of time provided their clocks are synchronized, a technique known as synoptic recording.

Duty cycling applies only to acoustic recording. Once the sampling program begins, auxiliary sampling, if enabled, will be continuous.

Can the start of the sampling program be delayed so it begins later, not now?
Can sampling be triggered on an event of interest?
Not yet. However much of the programming has already been done to trigger sampling when certain auxiliary indicators -- depth, temperature, tilt, etc. -- satisfy a user-programmed condition, so we expect to provide this capability in future firmware updates.
Can the Acousonde do acoustic event detection?
The hardware can -- in fact it includes a vector floating-point coprocessor for efficient digital signal processing -- but we have no plans at present to support this feature in firmware. Please see the Customization FAQ for more information about significant customizations.
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Calibration and signal quality

How are each Acousonde's calibration data maintained and applied?
Each Acousonde unit stores its own individual calibration information. This information is customized for each unit and stored in the unit at the factory. When acquiring sensor data, the Acousonde includes its calibration information in the metadata header that begins each data file. The sensor data themselves are "raw" samples directly from the analog-to-digital converters,* but the information to calibrate them is immediately handy within the file header. In this way the per-unit calibration remains with the acquired sensor data through transfer, analysis, and archival. *Note that the depth sensor data are not strictly "raw" as they are adjusted automatically for temperature effects before being stored on board the Acousonde.
How are the auxiliary sensors calibrated? Do I need to recalibrate them?
Temperature and acceleration sensors use calibration supplied by their respective manufacturers. There should be no need to recalibrate them.

We calibrate the pressure transducer (depth sensor) ourselves when we pressure-test each unit, and also estimate the effect of temperature on the depth sensor. Only the depth offset may need occasional recalibration to assure that the depth sensor reads zero at sea level and at sea-surface temperature. The depth offset may be "zeroed" via the Palm control interface.

The compass is calibrated at the factory to determine offset and scale values adequate to adjust the three magnetic axes for matching response; this is sometimes called "sphere-ing the ellipsoid". This can be redone by the user if there is concern that the compass sensor may have suffered a change in sensitivity, for example if exposed to a strong magnetic field that the built-in automatic degaussing cannot correct. We also recommend a further calibration exercise at your field location to maximize confidence in reconstructed animal tracks. A sample exercise for use with the "TrackPlot" track-reconstruction software is described here. Note that the compass calibration process does not support accurate measurements of absolute magnetic field strength.

Is the 3B's light-level sensor adequate for bioluminescence research?
Probably not. Unlike other auxiliary channels, the light sensor (only present on the Acousonde 3B) receives power from a poorly-regulated digital output from the microprocessor. As a result, changes in processor load can induce small artifacts in the light record. The light-level sensor is intended for monitoring larger-scale light phenomena and for general concept evaluation.
How are the Acousonde hydrophone(s) calibrated?
The hydrophone(s) embedded inside the Acousonde are calibrated by their manufacturers at spot frequencies. Then, before integrating the hydrophone with the Acousonde printed-circuit board, we verify the accuracy of the board's signal conditioning and data acquisition electronics. Meanwhile, the acoustic impedance of the overmolded urethane closely matches that of seawater and is thus nearly invisible from an acoustic standpoint. We thus have confidence in the acoustic calibration data for each instrument even though we do not independently recalibrate the acoustics after the instrument is complete. This confidence is supported by the positive results of customers who have performed calibration tests.
Please provide the hydrophone frequency response.
The manufacturer of the low-power hydrophone does not supply us with a frequency response, only a sensitivity spot-check. As we do not calibrate the acoustics independently, no frequency-response chart is available. However, several customers have conducted their own calibration and verified reasonably flat response within the pass band. If you have stringent flatness requirements please contact us.
Please provide the hydrophone beam pattern.
The manufacturer of the low-power hydrophone does not supply us with a beam pattern, and we do not calibrate the acoustics independently, so no beam pattern is available. Moreover, the frequencies monitored by the low-power hydrophone are relatively low (9.28 kHz and below) and therefore the associated wavelengths are quite large with respect to the size of the Acousonde. As the hydrophone measures a scalar quantity (pressure) we do not expect significant directionality at these low frequencies. With regard to the beam pattern of the high-frequency hydrophone option, please see the next question.
How accurately does the Acousonde record ultrasonic frequencies?
Unavoidably, the Acousonde's frequency response is anything but flat at high frequencies. First, the transfer function of the high-frequency hydrophone itself may not be perfect; for example, the transfer function of the high-frequency hydrophone used in the earliest Acousondes exhibited excursions to both -20 and +20 dB as frequency reached and passed 100 kHz (we don't use that vendor anymore but it indicates the difficulty in creating a uniformly responsive high-frequency hydrophone). Second, the location of the high-frequency hydrophone amidst a circuit board and other neighboring components profoundly affects its directionality at ultrasonic frequencies. Finally, the placement of the Acousonde relative to other objects, including of course an animal to which it may be attached, impacts the overall transfer function. These factors can to some extent be characterized and compensated for, but there will be many unknowns in each deployment.
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Offloading data

What computer and operating system are required to download data?
When plugged into a personal computer via USB, the Acousonde behaves like a multivolume USB disk drive, and has tested OK with multiple Mac OS X and Windows computers. Linux, regrettably, appears to be incompatible as a USB host for the Acousonde. The Linux disk scheduler interacts with the Acousonde's multiple volumes in a way that is terribly inefficient for the Acousonde's low-power storage design and times out, aborting the mount process before it can complete.
Wait, "full speed" is much slower than "high speed" USB! How long does it take to offload data?
As of late 2012, new Acousonde units have demonstrated offload speeds of 3.3 GB/hour. At this rate the Acousonde's current standard formatted storage capacity of 63 GB (59 GiB) will offload in 19 hours (for data sampled at 25 kHz sample rate, 63 GB would correspond to 14 days of recordings). You can choose to offload selected portions of stored data, which of course will require less time.
Do I need infrared capability on my personal computer, like I did for the Bioacoustic Probe?
No. The Acousonde offloads data via USB and there is no reason to use infrared for data transfer. The Palm is the only infrared tool you need to interact with the Acousonde for normal operations. The only exception is for users who would like to update the firmware on their Acousonde themselves. Please see the documentation available under downloads for detailed instructions on updating firmware.
Why not offload the data to the Palm via infrared?
It is possible, and encouraged, to offload small text files to the Palm, such as the log file. Larger files, however, would exceed the Palm's data capacity and would take an unacceptable amount of time to transfer via infrared. In any event there is no Palm software available to view or analyze the data files from the Acousonde.
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Importing and analyzing data

How do I import the data into software that can analyze them?
The tag stores and provides data in a custom format known as "MT" format. A MATLAB program (that is, an M-file) is available to read MT files directly into MATLAB; see the downloads area. A free unsupported application for Apple's Mac OS X, "MT Viewer," is also available on request to read and inspect MT files on Macintosh personal computers; MT Viewer can also export selected data in MATLAB, ASCII, or WAV format. If neither MATLAB nor MT Viewer is an option for you, you may be able to convert MT files directly to WAV if you manually keep track of the sampling rate, and strip the leading 512-byte MT header using multipurpose audio-conversion software such as SoX . Otherwise, some custom translation software may need to be programmed; detailed information on MT format is available to facilitate such an effort. With time we hope to gather the translation tools that users have created and make them available to the community to minimize "wheel reinvention."
What exactly is MT format?
MT format is a simple single-channel format for containing time-series data. It consists of a fixed-length 512-byte header followed by an arbitrary length of 2-byte samples. The header provides information on sampling rate, start time, site, title, comments, and calibration, among other things. All header fields are in ASCII, making the headers immune to byte-swapping issues and easy to read in low-level file editors.
Why not use WAV format?
WAV format is the de-facto standard for storing uncompressed audio on computers running Microsoft Windows. WAV was never intended to store scientific data, and does not provide a standardized means of storing calibration values, time of day, site codes, comments, or other "metadata" parameters essential to scientific purposes. Although WAV is not the best native format for scientific data, some analysis packages can accept WAV input, so a few Acousonde users have developed MATLAB software to convert the Acousonde's native MT format to WAV. MT Viewer (see "How do I import the data" above) provides an export function that allows selected portions of data to be exported to WAV for analysis by other software.
Why not use Broadcast WAV format?
Broadcast WAV is an extension to WAV format that incorporates generous and flexible metadata capability, primarily for the news and entertainment industries. It offers a serious alternative for keeping scientific metadata with acoustic records; however, we are not aware that a universal scientific-metadata format has yet evolved for Broadcast WAV. So even if the Acousonde supported Broadcast WAV the compatibility with analysis software might be limited.
I've never analyzed acoustic data. How do I get started?
First, determine if you need to perform calibrated acoustic analysis. If not, there are many acoustic analysis packages available for listening to records and inspecting spectrograms. If calibrated analysis is necessary, however, we must recommend partnering with an expert. Calibrated acoustic analysis is nearly impossible for a non-specialist to do in a repeatable, standards-based manner without formal education. Even with formal education it is surprisingly easy to use techniques that are inappropriate to the signal or situation in question, leading to meaningless and irreproducible results. The use of canned software only increases this risk. A good way to know when you may be ready to analyze acoustic data on your own is when you fully understand the meaning of the unit designation "dB re 1 µPa/Hz½".
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