Consistently Stable Loudspeaker Measurements using a Tetrahedral Enclosure

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Consistently Stable Loudspeaker Measurements
using a Tetrahedral Enclosure

• A major problem for the loudspeaker and
  transducer industries throughout the world is an
  inability to rely upon measurements routinely
  exchanged between suppliers and customers.
• The Tetrahedral test Chamber is available in a
  range of sizes, offers a unique and stable test
  environment giving an opportunity to standardise
  and compare results between measurement sites.
• It rigidly defines the measurement geometry which
  together with interchangeable sub baffles,
  ensures rapid and accurate change over and
  repeatable measurements.
• With many in use from design, production, quality
  control to the customer, final results will be
  comparable throughout the world to an
  unprecedented degree.

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Our current Standards for Loudspeakers are based
around Laboratory conditions
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But we are usually not in a Laboratory when we
make Loudspeaker measurements

• And our customer makes the same measurements
  again when they receive the parts as they cannot
  trust the suppliers measurements.
• This is a major problem here in the USA as many
  of the products designed here are produced in
  China, many of these products do not work
  correctly when they reach your customers this
  can lead to delays or rejection of the assembled
  loudspeakers or drive units.
• During this time the customers are not willing to
  pay for production that does not meet their
  requirements this leads to further delays,
  mounting costs and expenses.
• This situation has occurred as most of the
  standards which we use to define loudspeaker
  measurements are fairly old and have not kept up
  with modern techniques and production.
• The only people who win in this situation are
  the Lawyers!

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Our standards are based upon individual
measurements.
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In a modern high volume production environment.
A different approach is required.
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Anechoic Chamber - Measurement

• When sufficient care is taken and you use an
  appropriate anechoic chamber with a correctly
  set-up IEC baffle or JIS test box you can get
  reliable measurements.
• These can be reproduced by another equally well
  trained, disciplined and equipped individual
  elsewhere in the world.
• So we need to ask ourselves a different question
  
• What is it about these standards that mean that
  results made using them, are so variable that in
  many cases they are useless?

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Well... Anechoic chambers vary wildly in their
size and performance and set-up...
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SEAS H1207 on IEC Baffle in Anechoic Chamber
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H1207 Measurements
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Loudspeaker Drive Unit Measurement

• I have been designing and measuring loudspeakers
  for nearly 40 years, one problem has come up time
  and time again inconsistency due to set-up
  variations.

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What do the Standards Specify?

• Nearly all of the calibration routines and
  procedures assume the equipment varies. for
  example there are still calls to recalibrate
  microphones on a daily basis
• I don't know about your experience but in my
  opinion recalibrating a microphone each day is
  very good for the Microphone Suppliers as in
  practice it leads to damaging the Microphone and
  changing at the least the measurement set up.
• In my experience the real problems are the human
  factors.
• Our current standards do not specify this!
• Instead everyone does it their way and everyone
  is convinced they and only they are right!
• It is crazy there has got to be a better way!

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We really are in Trouble!

• There is evidence that typically we can only
  measure reliably across the whole industry to
  /-3dB.
• /-3dB /- 41 or
• Halving or Doubling the Power!
• Whilst modern instrumentation is typically
  capable of
• /- 0.01 tolerance or better!

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What should we do then?

• We should standardise on a common approach
• (1) Using a range of freely available core sizes
  of Sub Baffles
• (2) Using a new method based upon a Tetrahedral
  shape
• (3) Defining the Key Parameters -
• (i) Overall shape
• (ii) Key Mechanical Dimensions
• (iii) Method of Operation and Calibration

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The Tetrahedron

• We start with a shape produced from 4 identical
  equilateral triangles

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The Tetrahedral shape

• This is sloped to fit a corner so it is now
  produced from three right angle and one
  equilateral triangle.

We add a measurement Sub Baffle and an Internal
Microphone
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A pre-production example

• This external shape is covered by EU Registered
  Design 002292532

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Boundary Element Analysis ABEC3 VACS
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Low Frequency Field Distribution ABEC 3
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Calibration Methodology

• We know from actual measurements and simulations
  that though the pressure is nearly equal
  everywhere inside the enclosure it is not flat
  with respect to frequency.
• Richard Small used this technique in his paper
  Simplified Loudspeaker Measurements at Low
  Frequencies over 40 years ago to measure low
  frequencies without an anechoic environment.
• Don Keeles paper of 1973 shows us how we can
  measure low frequencies in the external near
  field of a Loudspeaker.
• So to make accurate and reliable measurements we
  can derive a basic correction curve by measuring
  the external near-field response of a drive unit
  and subtracting this from the measured internal
  (pressure response) inside a tetrahedral test
  system and apply this as a correction curve.
• An example is shown next of a SEAS H1207 Bass/Mid
  Driver we saw earlier, but measured in the TTC
  350 small Tetrahedral Test Chamber.

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Internal Tetrahedral Measurement
Internal Microphone of H1207 in Small Tetrahedral
Test System using Klippel MI18-HL Microphone at
100mm from inside of Baffle.
We can see a low frequency rise due to the
internal Pressure Response.
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The Tetrahedron

• We start with a shape produced from 4 identical
  equilateral triangles

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Internal Tetrahedral Measurement
Internal Microphone of H1207 in Small Tetrahedral
Test System using Klippel MI18-HL Microphone at
100mm from inside of Baffle.
We can see a low frequency rise due to the
internal Pressure Response.
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External Near Field Measurement
External Microphone of H1207 In small tetrahedral
system measured in the near field, at the rear of
the Cone, at distance of 3 - 5 mm from the cone
(It is essential to keep clear of ribs and other
obstructions).
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Correction Curves
These two measurements are divided by each other.
Simply copy the inside measurement response as an
input EQU curve and measure with the external
microphone again. The result is the difference
curve in dB, which is the ratio of both
measurements we do not correct the higher
frequencies so this is flat...
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Final SEAS H1207 Result
The Result from the Klippel QC system is compared
to the result in an IEC Baffle
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TTC 350 Correction Curve
Now we show measurements using HOLMImpulse
Software and a low cost Behringher Microphone
first we show the Internal correction curve used.
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Internal Microphone Measurement
Then the Internal Measurement result note the
pressure rise at low frequencies
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All Curves
Then we show all three curves and divide the
Internal Measurement by the Correction curve to
give the Final Curve in Blue
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Final H1207 Measurement
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International Standards

• The IEC has received and authorized three new
  project proposals relating to Loudspeakers Driver
  measurements
• Germany Task Force 1 60268-5 revise to deal
  with drive units, a new standard.
• China Task Force 2 Consider a standard for
  micro-drivers, lt2" diameter.
• CEA Task Force 3 Revise IEC 61305-5 consumer
  oriented extracts appropriate parts of 60268-5
• The AES has a New Projects- X223 Loudspeaker
  Driver Correlation Chamber I have submitted
  three documents to the SC 04-03 Server
• ISEAT 2013 At this conference in Shenzhen, I met
  with many people on the Chinese Standards
  Committee they agree that an updated Standard
  is required.
• ALMA. Does ALMA want to be involved?
• So that this becomes a fully accepted
  internationally accepted measurement technique?

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Where Next?

• I see a real need for a standard measurement
  technique that can be used consistently
  throughout the world
• Starting at Design
• Then used in Production
• Checking Performance Targets are met at Quality
  Control
• Before final confirmation at the Customer(s)

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What next?Tetrahedral Test Chambers

• I invite other people to get involved we need to
  agree a common specification throughout the world
  for the measurement of loudspeaker drive units.
• I am interested in talking to people about how we
  can best move forward towards general acceptance
  and use of this technique.
• I am interested in talking to manufacturers of
  loudspeaker drivers, tweeters and micro-speakers
  to ensure we can meet your needs.
• To manufacturers interested in producing these in
  high volumes
• To the standards people to ensure we build a
  world wide consensus.

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TTC 350
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TTC 900
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Acknowledgements

• Thanks to Phil Knight for many theoretical
  discussions.
• Alan Slaughter and the production team for
  valuable suggestions, to all at the AES 51st
  Conference at Helsinki, Finland and those at the
  AES 135th Convention in New York for further
  feedback.
• For further information please see me directly or
  contact me at
• geoff_at_hillacoustics.com or our team via
  www.hillacoustics.com

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References

• 1 Alan S. Phillips, Measuring the True
  Acoustical Response of Loudspeakers, SAE
  Technical Paper, 2004-01-1694 (2004).
• 2 Richard Small, Simplified Loudspeaker
  Measurements at Low Frequencies, JAES, vol 20
  (issue 1), pp 28pp 33, (1972).
• 3 D.B.Keele,'Low-Frequency Loudspeaker
  Assessment by Nearfield Sound Pressure
  Measurement', JAES (1974)
• 4 Consistently Stable Measurements using a
  Tetrahedral Enclosure
• Engineering brief 123, presented at the 135th
  Convention of the Audio Engineering Society in
  New York (2013).
• 5 Klippel, www.klippel.de
• 6 ABEC 3 Vacs www.randteam.de
• 7 HolmImpulse, www.holmimpulse.com/holmimpulse.p
  hp