Acoustic Measurement Test Chambers
Introducing the Tetrahedral Test Chamber, this is designed to ensure consistently stable, affordable loudspeaker and microphone measurements.
For far too long acoustic measurements have been the poor man of audio, largely out of control and terribly variable. We should ask why?
Simply we have not been able to measure loudspeakers, microphones and similar devices consistently!
So how do we do measure them?
With a Large and Expensive, Anechoic Chamber.
There's got to be a better way!
Making stable, consistent and accurate loudspeaker measurements
Geoff in front of an IEC Baffle and alongside a TTC350
What's the Advantage?
The Tetrahedral Test Chamber (TTC) ensures a precisely defined, stable and modally minimised environment. Completely free from set-up variations.
This gives precision along with highly accurate and repeatable measurements in a small volume at a greatly reduced cost compared to an anechoic chamber. An IEC Baffle, shown above with a TTC350.
Another advantage is that a TTC can measure too much lower frequencies than a conventional anechoic chamber. But a TTC is not an anechoic chamber.
An anechoic chamber attempts to absorb all reflected energy at all frequencies. In contrast a Tetrahedral Test Chamber is designed with absorption sufficient to clean up high frequencies only.
The theory has been published and is readily available see the technical page.
An equalisation curve is supplied with each tetrahedral test chamber.
The physical design of a Tetrahedral Test Chamber, is based upon the triangle amongst the strongest known structures; it differs from test boxes, by minimising parallel surfaces, which in turn reduces modal effects that seriously interfere with most measurement chambers.
The measurement microphone is fitted inside the chamber at a known and easily set distance. The loudspeaker to be tested is placed so that it's sound is directed into the chamber - towards the microphone, not out of it like a normal loudspeaker enclosure.
Individual sub baffles are then used to adapt individual loudspeaker drive units to the test chambers allowing rapid and very accurate physical location which equates to more stable measurements.
We have experience integrating TTC's into production environments in UK, Europe, USA, India and China.
How Accurate and How Consistent is a TTC?
The top graph shows the response of a single SEAS loudspeaker drive unit #7 measured fifty times with the chamber equalisation applied in a TTC900. The one-third octave curve is the error level at 95% confidence, is less than 0.2dB from 20Hz to 10kHz, even at the loudspeaker drivers resonance, up until 20kHz where the SPL drops off.
The lower graph shows the results of a series of these loudspeaker drive units #2 to #7 but measured sixty-eight times. This shows greater variation as would be expected from different loudspeakers. Note: The minimum variance shown in the mid-band is 0.25dB, at 95% confidence, is in agreement with the first graph, so these are genuine loudspeaker response variations.
These graphs demonstrate the control and stability of the measurements that a tetrahedral test chamber can provide, wherever they are used. R&D, QC, production, goods in or repair and refurbishment all can benefit equally.
Hill Acoustics was formed, to promote loudspeaker test chambers based upon the Tetrahedral Test Chamber principle expounded by our Chief Technical Officer - Geoff Hill.
Loudspeaker measurements have long been a source of excessive variability with typical tolerances of +/- 3dB being the norm, whilst a tolerance of +/-0.1dB is typical for virtually all the rest of the equipment chain.
Long frustrated by the state of loudspeaker measurements Geoff set out to provide a better alternative. Launched in 2013 we built various sizes of chambers whilst refining the theory & design. And understanding that international standardisation was a necessity we pursued this through the publication of the principles through: The Audio Engineering Society, ALMA, ISEAT and working with the International Electrotechnical Committee (IEC-TC100), on updates to IEC 60268-5, 60268-21 & 22, he worked with the AES SC-04-03 on the Loudspeaker Measurement Chambers, Information Document, AES73id-2019.
This was achieved in 2018, with the inclusion of the technique as an alternative in IEC60268-21 section 13.1(d) "A test cabinet not in accordance with annexe A.1.2 (type-A or type-B), generating a defined air load and radiation condition at the front of the transducer may be used for end-of-line testing and relative measurements (see )."
Chamber sizes (currently)
Hill Acoustics TTC's are manufactured in the UK by Spectral Measurement under license from Hill Acoustics.
Please contact us now for pricing and availability or for advice on which size to choose!
For any inquiries, questions or comments , please call: +44-(0)1353 648 888 or click this link TTC enquiry form
+1 817 5144900
Spectral Measurement Limited,
Unit 1A, Grovemere House
Lancaster Way Business Park,Ely, Cambridgeshire,CB6 3NW, UK
Geoff Hill is a highly experienced Loudspeaker Design Engineer, specialising in Design, Modelling Test and Measurement, A Member of the AES, he is author of
If you are interested in loudspeaker design, this book is now being used in companies and universities to help teach loudspeaker design. It is available online or through Routledge and all good booksellers.
Are you Interested in getting Geoff to consult on Loudspeaker Design and or Measurements?
Please send an e-mail to firstname.lastname@example.org