Effect of Thermal Compression in real world subwoofers

Whenever we at FAC design a subwoofer,we test after finishing the equalizing what the loudspeaker does at higher levels. The best method is to do a compression measurement. During this measurement,the input level of the subwoofer is increased and we check if the output level changes follows

A subwoofer after filtering

If you normalize the results to the starting value,you can get a nice picture of what the sub does in real life.

Compression test with the subwoofer shown above

There are two obvious sources of compression. First of all –in case of a ported design- the port starts to fail at higher levels due to too high air velocity,with the result that the region around the tuning frequency does not go loud enough.
The second effect is the thermal compression. The coil get’s hot,therefore the resistance rises and the result is a lower level.

But if you carefully compare the response curve of the compression measurements,you see that there is a difference. Even so the “static”measured curve looks flat,ALL compression curves –even the low level ones –show some slightly falling character.

So what happened? Well,in case of this particular measurement,the driver is measured at each individual frequency with all the defined level steps. During that period from small voltage to larger voltage,the temperature rises in the voice coil. When jumping to the next frequency,even at the lowest level,the coil is still hot and so the level is slightly reduced. So far,so good,but due to the impedance curve of the subwoofer,this change in level is not constant over frequency. At the impedance peak around 60Hz,the level difference is much smaller. As a result,this level stays more or less even at higher levels,but above and below,the levels goes down,depending on the temperature of the voice coil.

Impedance of Subwoofer without amplifgier

One the measurement below the following test was done:The green curve shows the standard measurement with a relatively low input level of 0.1V. The red one was done  again with 0.1V just after finishing a high level faster sweep  with 1.5V. The blue curve shows the same,but this time the high level sweep was done slower to give the coil time to heat up.

One could say this is a fault in the measurement and maybe it needs to be changed to avoid that effect,but if demonstrates nicely what happens in the real world with music or a movie. The coil get’s hot during peaks or heavy bottom end and the resulting response curve is changing. For the design process it means to make sure that from the frequency of the high impedance peak,the “static”response rises slightly around 1dB. That means in real live you get a better balanced behavior.

Compression curves after filter modification

This modification might not make a good subwoofer starting with a bad one,but a carefully look at thermal compression cannot be wrong and it is not a lot more work to take care about the effects.

Thanks to Christian Gather,who made all the measurements and had the initial idea.

Stitching together nearfield and farfield loudspeaker measurements –Part 1

Nearly everybody in our industry knows the AES article about nearfield measurements of Don B. Keele introduced in 1974. In his article,he described a way to make a nearfield measurement of a speaker and he compared it with the farfield response of the same speaker. He came to the following conclusion:

Based on that conclusion,a lot of speaker engineers started using the nearfield measurement of a woofer to stitch it together to a farfield measurement made in a anechoic chamber. This was to overcome the limitations of a real world chamber at low frequencies (below 100-200Hz,depending on room size).
Unfortunately,many did not take into account that a nearfield measurement of a todays size loudspeaker does not represent the farfield result even at low frequencies. The limited front baffle has a major effect on the farfield response.
Funny enough,Mr. Keele included that already in his article:

He mentioned “half space”and that means the speaker should be on a big baffle. But normally a speaker box is measured free standing in 1m distance,and that’s full space. The difference to a half space measurement is huge!

Mr. Keele showed clearly what he measured –you just have to carefully look at it.
The speaker tested in this example was a 4.5″in a cubic enclosure of roughly 22cm. The farfield result can be seen in measurement a. The nearfield result in in b. In c,the small enclosure was mounted into a bigger baffle of roughly 1.2×2.4m to “support”the the small enclosure. Only now,nearfield and farfield looked similar.

It can be said that the work of Mr. Keele is still valid and true,as long as everybody understands that nearfield and farfield measurements are only comparable if you have made the farfield under half field field conditions or found a method to recalculate a nearfield measurement into a farfield version.

A good exercise to understand the influence of the baffle to the farfield response curve is the nice little program called “The Edge”. This free goodie can be used to calculate a baffle step compensation,but also shows nicely the effect a limited baffle size has already at low frequencies. There are other nice free programs at the same site –it’s worth going there.

All quotes of Mr. Keele have been taken from his original article,the two pictures of EDGE can be found on the web site of Mr. Tolvan. Part 2 will show a way to re-calculate nearfield measurements into farfield versions.


Carsten Hicking of DigitalHighend published a page with links of websites for High-Resolution Audio downloads. Please support him if you find more and let him know,so the page can be updated. HighResDownload

On the left the company dog of DigitalHighend 🙂

See you in Munich


if you would like to meet us,you have a chance during the High-End show in Munich. The show starts on the 21st and ends on the 24th.

Here are some details:HighendSociety

You will find us at the Thorens booth,showing the new Thorens turntable.

So,see you in Munich!

Swinging Tonearm

During the development of the new tonearm for a Swiss client,we had to think about a new method to measure the resonances in a tonearm tube. exciter The literature and the Internet did not help us a lot. Black Art,Voodo,all sorts of different philosophies,but no real engineering approach. At the end,we dicided to do our own measurement setup,using our Scanning Laser Vibrometer and a self made exciter ( photo on the left).

This exciter was a NXT exciter in it’s previous life and with the weight of 6g including fixing bar close enough to a half way normal cartridge.

It was very exiting for us to see the results. First of all,it was surprising that we had strong resonances in the region around 6-8kHz and not only at a few hundred Hz as everybody was expecting.

Here are two examples:

771 Hz

7.1 kHz

Below,you can see the spectrum of the resonances in a 10 mm Aluminum tube with 1mm wall thickness.


The next picture shows the result after we added compliant mass absorbers and surface damping:

More information will be posted,once the product is out in the market. Introduction is in Munich on the High-End Show 2009

Wolfgang’s Vault

Hey,no worry,I’m not talking about the latest software release of Wolfgang Klippel’s measurement system.! This is the music section 😉

Wolfgang’s Fault
is a website with a lot of really nice live music. You can listen to if for free after registering or buy mp3 and Flac files for permanent download. It’s really fun! I spend two hours listening to some Sorry Terry &Brownie McGhee and Allman Brothers tracks after I saw it first. Sound quality is not bad at all 🙂

I found this information on Audioplus –thanks Steve!

Speakers in the Flash

One of the most powerful tools to test drive units is a Scanning Laser Vibrometer. It’s not new in loudspeaker design –CELESTION in the UK started already in the early 80th with it,but in that time it took ages to finish one driver.

Now you need half an hour for it with the powerful POLYTEC system and around a few hours with the new KLIPPEL Scanning system.

Below you can see a 20cm drive unit at 480Hz. It shows a nice combination of the surround being out of phase and a radial mode travelling through the corrugated cloth surround.

With the information about the root cause of a drivers problem,it’s much easier to think about a possible fix 🙂

Klippel Scanner
Polytec Scanner

Drive Unit testing

Let us test your drive units. We tell you how good they are and we know how to make them better. Promised.

Analyzing is a big part of our business. We therefore have a lot of special equipment available for testing purpose.

  • Klippel Analyzer for Transfer function measurement in anechoic chamber
  • Klippel Analyzer for testing dynamic behaviour and static parameters
  • Polytec Scanning Vibrometer to monitor cone and surround movement

Once we have analyzed the driver,we can help to optimize it by:

  • Designing a new magnet system,using latest simulation methods made by powerful commercial and homemade software
  • Developing new soft parts like Cones,Surrounds and Spiders with modern FE/BE simulation. Material property can be measured at our place.
  • Performing listening tests,done by very experienced engineers and music lovers.

Interested? Give it a try. The first driver test is on us 🙂

Hallo HiFi World

Welcome to our new webpage that we created to give you some information of what we do in our company. Enjoy –but be careful,it’s always somewhat BETA

Best regards

Karl-Heinz Fink