Geoff Hill's Tetrahedral Test System & Loudspeaker Design

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Tetrahedral Internal Test System

Why do we need a new way of measurement?

Here I will hand over to a former QC Director…

“From my point of view as a QC representative the thing I found most frustrating with speaker development was that individuals and companies had their own pet way of doing things. This made life more difficult than it should be in establishing whether products or components were actually within the specification. More than an element of a black art surrounds speaker development in my opinion as somebody who is not a trained Acoustics Engineer.
I think that is the problem with speakers; you need to be very well acquainted with the theory and have a good working knowledge of the interaction of components within the speaker motor unit and then the next layer of issues caused by the mounting of said speaker. This is so you can manage and assimilate all the data and feel confident of knowing who or what is right and what is wrong and make better use of resources to resolve the real problems, be they design or manufacturing in nature..
A unified measurement approach will benefit the industry and reduce stress in the organization avoiding the inevitable blame culture if product is not as specified and will speed up problem resolution and root cause of errors or mistakes."

Why do I need the Tetrahedral Test system?

Let us ask a question? Do you have a perfect Anechoic Chamber together with a perfectly matched IEC Baffle or a JIS standard test box with a perfectly set-up microphone and environment that is calibrated to +/-0.5 dB from 20 Hz to 50 kHz, that you can repeatedly set-up up to +/-1 mm?

You're doing well! The rest of us need this system for the following reasons:

  1. Its acoustically stable, structurally strong and robust design ensures that measurements are made in exactly the same way every time.
  2. The simple freely interchangeable measurement baffles ensure consistently repeatable and reliable measurements are made quickly and easily, without high skill levels.
  3. Accuracy careful set-up of the measurement microphone together with tight dimensional control and individual chamber calibration ensure highest accuracy and repeatable results.

"An affordable reliable system and procedure that allows design and manufacturing and then QC to measure a speakers characteristics in the same way in any global location is long overdue. To be able to carry out test/measurements that are repeatable anywhere in the world and provide the same data/result if carried out to the same procedure will reduce costs, improve products.”
- Eric Lytra

The Tetrahedral Internal Test System

It struck me that some of the most consistent measurements I had made over my career had been in a corner of a Chamber. So I thought why not basically build it from four triangles.

One of the Platonic Solids is the Tetrahedron, but modify it slightly with three right angled and one equilateral triangle.

A Tetrahedral based design has many advantages:

  1. As all of the major dimensions are different it will not support standing waves at high frequencies.
  2. It is inherently very stiff and strong resisting vibration.
  3. It is assembled from three identical parts and a series of fully interchangeable measurement baffles individual to the driver, tweeter or subwoofer under test.

Tetrahedral Test System - Parts

The system is built from three identical interlocking assemblies with a main baffle bolted securely onto the front edges of these three assemblies, all of these assemblies are bolted together using M6 Bolts and internal T-Nuts.

The main construction material is to be 24 mm Plywood with Main and Measurement Baffles having a 1 mm thick Aluminium skin inside.

The inside faces or walls and floor are covered with a 100 mm thick layer of Wool and then a 50 mm layer of Acoustic Foam.

The Microphone is held rigidly in a tube in the far internal corner, at a fixed distance from the internal face of the baffles.

Sub baffles are individual for every driver. All fit into 350 main baffle hole and have a 1 mm Aluminium Skin supporting the driver and a 45 degree chamfer at the rear.

The sub baffle(s) are clamped to the main baffle by 250 kg Horizontal clamps, whilst the drive units are clamped to the individual measurement baffles by 75 kg Horizontal clamps (dependent upon the loudspeaker) that need to be measured.

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