Consistently Stable Loudspeaker Measurements using a Tetrahedral Enclosure

(c) Hill Acoustics Ltd 2014 - www.hillacoustics.com

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.


Our current Standards for Loudspeakers are based around Laboratory conditions


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!


Our standards are based upon individual measurements.


In a modern high volume production environment. A different approach is required.


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?’


Well... Anechoic chambers vary wildly in their size and performance and set-up...


SEAS H1207 on IEC Baffle in Anechoic Chamber


H1207 Measurements


Loudspeaker Drive Unit MeasurementI 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.


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!


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!


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


The Tetrahedron

We start with a shape produced from 4 identical equilateral triangles


The Tetrahedral shapeThis 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


A pre-production example

This external shape is covered by EU Registered Design # 002292532


Boundary Element Analysis ABEC3 & VACS


Low Frequency Field Distribution ABEC 3


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 Keele’s 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.


Internal Tetrahedral MeasurementInternal 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.


The Tetrahedron

We start with a shape produced from 4 identical equilateral triangles


Internal Tetrahedral MeasurementInternal 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.


External Near Field MeasurementExternal 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).


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...


Final SEAS H1207 Result

The Result from the Klippel QC system is compared to the result in an IEC Baffle


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.


Internal Microphone Measurement

Then the Internal Measurement result – note the pressure rise at low frequencies


All Curves

Then we show all three curves and divide the Internal Measurement by the Correction curve to give the Final Curve in Blue


Final H1207 Measurement


International StandardsThe 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, <2" 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?


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)


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.


TTC 350


TTC 900


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

[email protected] or our team via www.hillacoustics.com


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 28–pp 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.php

Consistently Stable Loudspeaker Measurements using a Tetrahedral Enclosure

Documents

Transcript of Consistently Stable Loudspeaker Measurements using a Tetrahedral Enclosure