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Terrasonde Audio Toolbox

Test & Measurement Set By Hugh Robjohns
Published February 2000

The Audio Toolbox is available in the original Mk I case, shown here, or a revised Mk II version.The Audio Toolbox is available in the original Mk I case, shown here, or a revised Mk II version.

The technical standards of any professional, semi‑pro, or hobbyist recording studio rely on accurate calibration. Hugh Robjohns tries out Terrasonde's funky DSP‑based handheld audio test and measurement set.

When it comes to checking levels, frequency response, speaker phase and the like, most of us make do with a test CD and our mixer's meters, together with our ears, to assess the technical alignment. Although often sufficient for a quick check, this approach is hardly accurate or reliable — but most of us have better things to spend our money on than an Audio Precision test set and better things to do with our time than look for 0.1dB alignment errors!

Nevertheless, an affordable, accurate, and easy to use measurement set would have wide appeal, particularly if it provided a broad range of functionality. That is exactly what American company Terrasonde have come up with in their DSP‑based 'Audio Toolbox' — a comprehensive analogue audio test and measurement set. Now with Mark II hardware in three alternative forms and Version 2 software, the Audio Toolbox is a very powerful and cost‑effective system.

The Audio Toolbox

Terrasonde Audio Toolbox

The standard Audio Toolbox is a large handheld unit measuring about 140mm wide by 235mm deep and weighing 700 grams in its plastic case (without the external battery unit); a ruggedised all‑metal alternative is also available. A rackmounting version will be launched shortly but all versions employ exactly the same electronics and software. The device is wedge‑shaped, with a backlit 64x128 pixel supertwist LCD panel angled towards the user.

The large knob is the only operational control on the device, and operates as a shaft encoder as well as a push switch. Rotating the knob selects menu items and parameter fields, after which pressing and releasing it opens the appropriate submenu or enables the parameter to be adjusted by further rotations. To exit the parameter‑adjust mode, the knob is pressed once more, whereupon it reverts to its previous field‑selection mode. The system works well with the version 2 software and is delightfully fast and simple to use.

The rear of the box is graced with a stubby omnidirectional microphone, used for a range of acoustic measurements, and both side panels are loaded with connectors for both signal and data inputs and outputs. The output side is on the right and contains a miniature loudspeaker together with a stereo quarter‑inch headphone socket. Balanced outputs on XLR and TRS connectors are accompanied by an unbalanced phono connector, all three carrying the same signal at the same level. Three 5‑pin DIN sockets provide a single MIDI input and two outputs and are also used for data and software transfer between PC and Mac computers.

The left side of the box carries a pair of combo XLRs, for balanced stereo inputs on XLR or TRS jacks, together with a pair of phono sockets for unbalanced inputs. A 2.1mm co‑axial socket accepts DC power from a supplied wall‑wart mains module, while a pair of threaded inserts enables a supplied battery box (holding six AA cells) to fasten to the side of the unit, cleverly utilising the same DC power inlet.

An optional accessory kit provides a hard‑shell carrying case, a PC interface with a serial adaptor cable and a floppy disk containing a special ATB Upgrader program (enabling operating software upgrades), a spare battery holder, a windscreen for the built‑in microphone, and various adapter cables.


The Audio Toolbox provides comprehensive interconnection and cable‑testing facilities, and is capable of being upgraded via a Mac or PC.The Audio Toolbox provides comprehensive interconnection and cable‑testing facilities, and is capable of being upgraded via a Mac or PC.

After the 10‑second boot‑up process, the main menu page presents four submenu selections: Acoustic Analysis, Test Functions, Session Helpers, and Utility. The Acoustic Analysis submenu provides options for five measurements, the first of which is a Sound Pressure Meter. This covers the range 35‑120dBA with the internal mic, or 25‑130dBA when coupled with an external calibration mic. When used with a suitable external microphone the machine conforms to ANSI Type I specifications with options for A, B and C weighting, and offers slow, fast, impulse and peak averaging as well as long‑term SPL averaging (LEQ) and Sound Exposure Level (SEL) to permit civic or industrial 'Noise in the Workplace' measurements. Internally generated sine and square waves can be used for the measurements, as well as white or pink noise signals. The meter display is clear and easy to understand with continuous bargraph and numeric readouts, maximum peak level display and various status information.

The second program is a real‑time frequency analyser covering the full 20Hz to 20kHz spectrum in octave, 1/3, 1/6 or 1/12 octave bands. Averaging is adjustable from 1 to 60 seconds with equal‑weighting or exponential decay options. A tuneable marker identifies the frequency and level of selected signal peaks or notches and collected data can be stored in non‑volatile user memories. The system can also be focused towards the low‑frequency end of the spectrum (between 10 and 332Hz) to aid in setting up subwoofers, and has an NC (Noise Criteria) function.

An Energy Time Graph shows amplitude decay against time based on a brief pulse of sound generated by the Toolbox. This can be used to assist in the analysis of room reflections or speaker array timing and it can be calibrated in either time or distance (both metric and imperial units) with the viewing window scaled between 15 and 960mS. A zoom function enables an area of interest to be examined in closer detail if required. The time delay between the generation of the test impulse and its reception is also displayed and a moveable cursor allows amplitudes to be found at specific times in the delay sequence.

An extension of the energy‑time measurement provides a Reverb Decay Time analyser. This extrapolates an RT60 time from a self‑generated, gated burst of pink noise, even in situations where the full 60dB of dynamic range is not achievable. The noise output from the Toolbox would, typically, be routed to a control‑room monitor or a PA speaker, its level being set as loud as is practicable. The RT60 display will then show the peak acoustic level being received, the background ambient level and the difference between them. The time taken for the noise to decay from the peak level to the background ambient level is then extrapolated to a full 60dB dynamic range and the corresponding time displayed.

Finally, for this first section, a polarity tester generates a custom 'pop' test signal which can be output from the internal loudspeaker as well as the line outputs. Thus the polarity of microphones can be tested using the internal speaker as the reference source, and the internal microphone can be used to determine the polarity of loudspeaker drive units. Electronic equipment is tested by connecting between the Toolbox line outputs and inputs and polarity is indicated on screen immediately as an unambiguous + or — sign.

Test Functions

The Mk II hardware features several revisions including a slide power switch.The Mk II hardware features several revisions including a slide power switch.

The 'Test Functions' submenu contains more conventional test fodder. A Signal Generator produces sine or square wave signals from 20Hz to 20kHz. The frequency can be incremented in 1/3 or full‑octave steps as well as in fine resolution, or using an automatic sweep adjustable for both range and speed. White and pink noise sources are also available and the signal can be output through the internal loudspeaker as well as the electrical outputs, either at calibrated or adjustable levels.

A polarity test signal can be generated, as already mentioned, which resembles a square wave in the positive half‑cycle and a sine wave in the negative half‑cycle. An impedance‑testing facility is also available which, with the addition of a couple of simple test leads, can measure loads from 1Ω to 2kΩ at any specific frequency between 20Hz and 20kHz. This enables the characteristic impedance of a loudspeaker to be tested, for example.

A Level Meter provides signal‑level measurements either as a straight numeric display or as a peak‑reading or VU bargraph meter. Measurements can be scaled in dBu, dBV, or as peak, average or RMS voltages, and the dB scales can be referenced to +4dBu or ‑10dBV. A Frequency Counter can accommodate signals between 16Hz and 50kHz, although it requires a fairly clean sine wave signal for accurate frequency detection.

A Signal to Noise Ratio menu calculates the S/N figures for connected equipment, a Sweep menu providing automated amplitude/frequency response and impedance/frequency response graphs in either 1/3 or 1/12 octave resolution. The final menu in this section provides a Sample Scope — a display much like an oscilloscope — showing the waveform from the internal microphone or for one or both line inputs. Since the system is limited to audio sampling at 48kHz, the resolution of the sample scope does not compare to a good digital oscilloscope and signals above 10kHz are not always presented accurately, but the function still provides a lot of useful information when interpreted carefully. The signal sensitivity and time base can both be adjusted, and an X‑Y mode produces Lissajous figures, allowing Vectorscope‑style analysis of the stereo width and mono compatibility of a stereo signal, tape‑machine azimuth alignment, or information about the phase delays through a signal processor.

A THD+N distortion meter uses FFT analysis to determine the relationship between the fundamental of an internally generated signal (at 63, 125, 250, 500, 1k, or 2kHz) and its harmonics, thereby measuring the distortion products. The system is claimed to be accurate from 50 percent down to around 0.2 percent distortion, so is not terribly useful for checking the specs of any modern electronic equipment, but ideal for assessing loudspeaker drive units, for example.

Session Helpers

The Audio Toolbox Plus features a tough metal case.The Audio Toolbox Plus features a tough metal case.

The 'Session Helpers' sub‑menu includes an Instrument Tuner, Tempo Computer, Hum Canceller, MIDI Helper, MIDI Transmitter, and some Time Code Tools. The tuner has preset configurations for guitar (open, chorded and capo positions), 4 and 5‑string bass, violin, cello and viola, and the reference tuning point can be offset up to 10 percent from A440 or A110.

The Tempo section provides time signatures of 4/4, 2/4, 3/4, and 6/8 ranging between 45 and 240bpm in 0.2bpm steps. Low‑quality audio samples (kick drum, snare, hi‑hat, tom and clave) can be output on any beat of the count to generate a rough rhythm track. Further subsections of the Tempo Computer also provide a 'Leadin‑Laggin' display which compares the left and right inputs (or one input and the internal click) to calculate their time offset in milliseconds with a corresponding graphical display.

A Drum Replacer mode generates a user‑determined MIDI note signal from a transient input signal, complete with a user‑variable time offset, which can be used to fire a sampler, for example. MIDI channel, note and velocity are all adjustable.

The Hum Canceller works by sampling the background hum and noise from an input audio signal and then combining the sample in opposite polarity with the live input (in real time) to remove or reduce the offending hum. It is intended to operate on both 50 and 60Hz mains components and is claimed to produce up to 45dB of improvement.

The MIDI Helper displays incoming MIDI data as either text messages, raw hexadecimal codes, or as a bar chart to allow checking and fault finding — the input data is passed through to both outputs in this mode. The system is capable of splitting and routing MIDI events between the two MIDI outputs and MTC can be decoded. The unit also functions as a MIDI cable tester and can generate a wide range of MIDI data to assess, test or control other MIDI equipment.

The Time Code Tools section allows time offsets to be calculated between timecode signals input on the left and right channels, MIDI timecode to be sourced from LTC, input timecode to be regenerated (with an offset if required), true frame‑rate analysis, and even provides a timecode calculator to add or subtract timecode numbers. All six standard timecode rates are accommodated.


The last of the four main menus is the 'Utilities' section, which includes facilities to set parameters for the internal monitor amp (speaker and headphones) and includes a number of options for guitar practice applications, including distortion, echo, flanging and compression effects.

A cable‑testing facility is available for any combination of XLR, quarter‑inch jack and phono plug, connected between the relevant output socket and an appropriate flavour of left‑channel input. Balanced or unbalanced cables can be checked and the system produces a display showing how the cable is wired. The cable checking process does not rely just on simple DC testing of the cable, but also checks the passage of audio with frequency‑response tests at 1kHz and 20kHz too! To speed up cable checks, beeps (good) and buzzes (bad) are output from the speaker for audible confirmation, and by modifying a couple of cables into suitable test probes, this function can also be used as a fast continuity tester.

A submenu checks the presence and voltage of phantom power (for example, at the end of a long multicore run). Another display page relays the voltage of the Toolbox's own battery supply complete with an estimate of its useful life expectancy (for a selection of battery chemistries — alkaline, NiCd, lithium cells, or lead‑acid batteries).

A data transfer mode allows information stored in the Toolbox's non‑volatile memories to be dumped to a PC or Mac for printout or archiving. This data transfer employs the MIDI ports on the Toolbox and a serial port on the host computer. Finally, comprehensive facilities are provided to completely recalibrate every aspect of the machine, although this obviously requires high quality external test equipment and a solid understanding of what you are doing — so tweakers beware!

In Use

I have to say I found the Audio Toolbox, initially, rather fiddly to use. It is a little too big to sit comfortably in the hand and having to use one wheel to navigate the menus, select the parameters and then set their values is not the easiest or most ergonomic thing in the world. However, after just a few days of playing with the machine I soon became pretty adept with it and I have been using it regularly ever since.

Overall, I found the Toolbox to be an accurate, flexible and versatile piece of equipment, and I was surprised just how much I started to rely on it when reviewing all kinds of other equipment for the pages of Sound On Sound. Of course, a test set is only as good as its calibration, but comparing the Toolbox against an industry standard Lindos test and measurement set, I found it to be more than adequate for the vast majority of purposes, and the convenience of having such a wide range of functions available in the one compact and lightweight box was superb.

Once satisfied that I could trust it, I used the Toolbox in all manner of roles including assisting in the fine tuning of surround and stereo monitoring systems, checking and realigning the response of an analogue open‑reel machine, fault‑finding in a MIDI setup, checking the calibration of various metering systems, and testing countless dodgy cables! I even used it to assist in the tuning of a couple of guitars and to check the accuracy of a stand‑alone rhythm programmer.

The difference between the original v1.2 software and the current v2.10 is stunning — and it is a credit to the company that such improvements have been made so quickly. The Mark II hardware improves upon the earlier version in some respects, but I have reservations about the toggle power switch that has been added. A rocker or slide switch would have been far less likely to get damaged or knocked accidentally.

It would seem that the UK distributor, Rocky Road Distribution Limited, agrees and is continuing to sell Mark I units running the latest software — certainly the most cost‑effective arrangement. Mark II platforms are also available though, if required, and owners of Mark I units who really want the power switch and other tweaks can have their machine upgraded for a modest cost.

For those requiring something more rugged, the all‑metal Toolbox Plus, or the Toolbox Rackmount version would be ideal. The Toolbox Plus is supplied with a detachable microphone element on a two‑metre lead, a rechargeable lead‑acid battery system, PC/Mac serial interface leads, and a carrying case. The 2U‑rackmount version retains the same display and control‑knob design as well as the same detachable microphone unit, but brings all the connectors out onto the front panel and has an internal AC power supply (no more wall‑warts).

The only element missing from the machine is the ability to test digital equipment, which seems rather bizarre given the DSP nature of the machine. Presumably further hardware and software updates will address this issue in the future.

There can be few recording engineers who have not wished for some convenient and affordable tool to help align their equipment at some time or another. The Terrasonde Audio Toolbox is that tool and I can thoroughly recommend it.


  • Standard Audio Toolbox:
    Original Mark I case £764.
    Mark II case £840.
  • Audio Toolbox Plus (all metal case) £1405.
  • Audio Toolbox Rackmount £1522.

All versions supplied with the latest software, currently v2.10.

All prices include VAT.


  • An affordable test set.
  • Sufficient accuracy for most tasks.
  • User‑friendly.
  • Flexible and imaginative functionality.
  • Multiple hardware options.


  • No digital test functions yet.


A very fast, convenient and all‑embracing test set with an enormous range of uses around the studio or live sound arena. Emphasis is on practicality rather than perfection, but every function you could wish for is present and the Audio Toolbox provides excellent value for money in all of its guises.