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Fletcher Munson Compensation

How can the perfect transmitter be developed without knowing all imperfections of the receiver? In other words, how can one develop the perfect sound system without a study of the human ears characteristics? We invite you to do a small experiment; play a wellknown and high quality music track at a sound level of about 80dB. Play it a few times, listen critically, and adjust your tone controls for best sound quality and tone balance. Now boost the volume to about 100dB (make sure that you don't exceed the limits of your system) You will notice a change of tone balance, and you will be tempted to make small changes in your tone settings...

During the development of our D-sub/D-Top system this phenomenon again became very clear. In the first testing phase, the technically correct system would sound super at normal (moderate) sound levels, but at more extreme levels it would sound too agressive. Tweaking the processing could make it sound super at highest sound levels, but then it would miss clarity and intelligibility at moderate levels. We encountered this problem before since this is not the first PA we developed... But the extreme dynamic range of the new system clearly called for dynamic processing, matching the frequency response to the current sound level.

Now what is causing this phenomenon?

The answer lies in human ears imperfections. Of course, the human hearing curve can be measured and plotted. This is normally done at reasonable low levels, for example to check for the need of wearing hearing devices. But when we repeat these measurements at higher sound levels, and plot the lines of equal loudness, we will see that higher sound levels produce a different curve. In 1933(!) the first scientific research in this field was done by Mr. Fletcher and Mr. Munson. In 1937 they mapped the hearing curves over a wide area of sound levels. Although more recent research produced slightly different curves, and some assumptions had to be re-adjusted, these scientists really produced good data and a solid base for future research. Therefore we would like to credit these guys by using their names for this phenomenon. In 1956 two other researchers, Mr. Robinson and Mr. Dadson mapped slightly different curves thanks to their more accurate equipment. The Robinson Dadson curves are therefore accepted and adapted by the International Standards Organisation as the basis for ISO 226:187; Normal Equal-Loudness Level Contours. (ELCC)

Conclusion so far:

Our human ear is far from linear, dynamically speaking, especially in the low frequency and high-mid frequency regions.
Fletcher Munson

Fletcher Munson diagram. For us, the area of interest lies between the 80 and 120 dB curves.

But what does this mean to sound reproduction? Most sound systems use a fixed frequency response; whether flat or curved, at all playback levels the frequency response remains unchanged. This appears to be only logical, and in most cases it is not a problem at all; most sound systems are used in a rather small dynamic range. Take a small PA-system: Generally it will be used in the upper few dB's of its dynamic range. In other words; normally it will run between 1/40th and full power. That's a dynamic range of only 16 dB. When you like the sound of this PA at 80 dB, you will probably like it at 90dB as well. But with the power available in modern high power sound systems, this dynamic range is enlarged dramatically. Our D-Top / D-Sub combination is capable of producing peak levels of 130dB (@3m) on the dance floor. The other day the same system may be used for a theatre application at 80dB max. This is a dynamic gap of 50dB, this means a power ratio of 1 to 100.000! And this is when a good old fixed frequency response is not good enough anymore. A fixed frequency response may be correct for use at 80dB OR at 130dB, but never at both levels!

Fletcher Munson Compensation, essential for consistent sound quality over a large dynamic range. The above illustrates that smart fysiological processing is essential for good sound balance over a large dynamic range. With Fletcher Munson Compensation, the system is much easier to handle, and it's performance virtually independent of the actual sound level. In our D-series products, we implemented the scientific work of Fletcher, Munson, Robinson and Dadson. Once you experienced the advantages, you will have a hard time working with anything less!

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