Geoff Hill's Tetrahedral Test System & Loudspeaker Design

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Tetrahedral Test System

How To Accurately Measure Loudspeaker Drivers using your existing Microphones and Analysis Equipment.

Let's face it Loudspeaker Driver measurements are in a mess right now!Tetrahedral Main Assembly

We have many standards, IEC 60268-5, JIS and now AES2-1-r in draft form... All of which in theory and in practice should give the industry throughout the world reliable and consistent measurement data - but do they? Actually they don't do this, as they parts either undefined or ill defined and open to individual interpretation.

All of them in effect demand a perfect measurement environment (typically an Anechoic Chamber) free from any undesirable effects over the full frequency range of 20Hz to 20kHz+: Which it could be argued can never exist. Even worse an Anechoic Chamber is primarily designed to allow reliable detection of very quiet sounds by excluding external influences and minimising internal reflection of any sound produced within - the quietest are very quiet -15dB below the threshold of hearing (0dB SPL re 20uPa) - great any one notice we are measuring "LOUDspeakers" at any where from 60dB to 100dB+!

We are mostly using Analysis Tools based upon, MLS, TDS, Log-Sweeps and other FFT techniques more than capable of "seeing" down through noise levels 80dB or more below the fundamental and certified to +/-0.05 dB (conservatively).

All this and still as an Industry we cannot reliably and consistently measure to +/-0.5dB, between any supplier to any customer and trust that measurements will be to this level or accuracy - more typically one can expect +/-2 to 3dB!

Sorry, but this is simply crazy! We have spent and are probably still spending Million of £/$/€ on large Anechoic Chambers) - typically a small one is the size of a house and this is to measure Loudspeaker Drivers inaccurately.

There has got to be a better way - there is now: -

Introducing the Tetrahedral Test System for Loudspeaker Drivers, this sweeps away the uncertainties of:

(i) Unknown Size and Dimensions
(ii) Unknown Microphone Positioning
(iii) Unknown or variable Acoustic Absorption
(iv) Bouncy Mesh Floors and Beams reflecting sound
(v) Clutter affecting the measurements in undesired ways
(vi) Different Baffle sizes IEC and AES or the vagaries of the JIS Test Box
(vii) Unknown Driver to Baffle interface
(viii) Many Operator errors

Instead we replace these uncertainties with a Standardised easily duplicated design in a compact robust structure both mechanically and acoustically. This design is based upon three right angle triangles and can easily be placed into the corner of a room, these three triangles are faced with a fourth triangle which becomes the measurement baffle. Such a design inherently tends to minimise Standing Waves as (a) there are few parallel surfaces these are relatively small and highly damped, (b) the lowest frequencies are far to long to form them in such a compact enclosure.

Inside the design the Side Wall's and the Floor are covered with Acoustic absorption, which needs to work from 200Hz up to the limit of the System. This acoustic absorption mops up the few remaining artefacts and this combined with the thickness and strength of the structure attenuates outside interference, with a relatively close microphone position give us an unprecedented signal to noise ratio ensuring clean measurements without windowing.

The Measurement microphone is held in a fixed geometrical relationship with the measurement baffle, which in turn provides a rigid well controlled mounting face for a series of interchangeable sub baffles these being individual to each loudspeaker driver or tweeter that we wish to measure.

Having defined the physical structure, the method of mounting a loudspeaker to be tested and the microphone geometry and everything else we can then define an accurate equalisation or calibration curve to compensate for the inevitable low frequency gain of the system.

The Tetrahedral Test system is built from 24mm thick plywood, as three identical precision CNC assemblies that are bolted together to provide an immensely strong and rigid structure the internal baffle face and the faces of the individual sub baffles are faced with a 1mm aluminium skin providing an almost perfectly reflecting baffle whilst the triangular shape further breaks up edge diffraction effects ensuring the highest standards are met.

This is a Registered Design, which can be purchased directly.

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