About RMS
Audio Design

Audio Design White Papers

In the course of the research and development of subwoofer designs, strongly supported by Grimm Audio in the design of their DMF subwoofer, a lot of knowledge is gathered from the audio world. It has originally been the intention to write a book on all discovered matter but for different reasons it was decided to bring this material in the form of so called White Papers, dealing with one subject at a time and free for anyone to download the paper in pdf format.

One of the reasons for not binding them in a book is that each subject is treated in a different way, making it not sufficiently coherent and hence less readable when combined. Each subject stands on its own but there is some logic in the order in which it is shown here. I tried to bring the material in such a form that it does not require pre-reading and I reduced the mathematics as much as possible. For more background knowledge on the mechatronic aspects I refer to my high-tech oriented book "The Design of High Performance Mechatronics".

Several subjects are also covered by many other publications but I added a "mechatronic flavour" by giving more attention to mechanics and dynamics then generally is done. For some of you this may give a different angle to the subject.

An important issue that comes with writing a book is the need to be perfectly clear about sources and references, especially when the writer is to be traced back as a university professor, like I am. When writing my book, mainly from knowledge gathered over many years in my memory, I needed to find applicable sources afterwards, which was extremely time consuming.
This means that the papers do not or hardly contain references and sources. If needed, internet can provide many examples of sources that I used in one or another way, however, like in my book, I wrote most of it based on my own investigations mixed with knowledge gained in contacts with many people over many years, thereby "standing on the shoulders of giants", like Bernard de Chartres stated so many years ago.

I express hereby my sincere gratitude to all those unnamed in the papers who have provided me with this knowledge.

Finally I'd appreciate, when copying these papers, to retain the front page which shows its source. It should also be noted that the documents will most propbably be updated when necessary due to errors or otherwise, so it is always good to check the latest version.

It should be noted that the documents are uncompressed so sownloading via weak internet connections can take a while.

1: Low Frequency Sound Generation by Loudspeaker Drivers

This paper presents the basic physics, mathematics and engineering practice for estimating and modelling the sound generation by a loudspeaker driver.
It focuses on the classical electrodynamic loudspeaker driver with a moving coil actuator, which drives a moving solid diaphragm. It addresses the role of the enclosure, focusing on the closed-box enclosure and also explains the extremely low efficiency of such systems.

2: Distortion Sources in Loudspeaker Drivers

This paper deals with the error sources in classical electrodynamic loudspeakers, which cause the significant levels of distortion at low frequencies.

3: Why Bassreflex is not Suitable for Low-Frequency Musical Sound Reproduction

This paper was originally made with a less confronting title as a real life example with a demonstrator for teaching dynamics of coupled mass-spring systems to university students.
Its secondary purpose was to determine the real value of bass-reflex systems in high quality loudspeakers. Unfortunately it only proves that the bassreflex principle is at best a compromise between size and sound quality but the latter suffers so much that the principle should not be applied for the low frequency sound in transparent high quality loudspeakers.

4: Motional Feedback Theory in a Nutshell

This paper presents the basics of control theory related to active feedback control of a loudspeaker. It uses parts of the chapter on Motion Control from the book "The Design of High Performance Mechatronics". The purpose is to create sufficient awareness of issues regarding stability, error reduction and sensitivity and is required for judging designs that apply feedback.

5: Sensorless Velocity Feedback Subwoofer

This paper is a combined and updated reprint of two articles that were originally written for Linear Audio from Jan Didden. At the time of publishing it was allowed to provide copies to university students and for that reason I can also post it here.

The original purpose of the study was, like with the previous paper, to teach dynamics by demonstration on a real system in the classroom. In fact the used demonstrator is identical to the one described in the paper on the bassreflex principle. In this case the interaction between actuators and amplifiers was the targeted goal. By adapting the output impedance of the amplifier, the effect of the motion EMF is influenced in such a way that a sensorless velocity feedback system is created. It shows some benefits in full control of the system resonance although distortion reduction is not achieved.

6: Acceleration Feedback Design

This paper describes the steps to design a subwoofer with acceleration feedback. It is intended for study and inspiration, rather than a ready to make cookbook for "do-it-yourself". Nevertheless it shows the key issues and possible solutions. Only the real execution remains proprietary.
Previous reading of paper 1,2 and 4 is strongly advised.

7: Impulse Compensated Driver Configuration for MFB

The configuration with two drivers from whitepaper 5: is posing some questions about how to connect them in a real acceleration feedback setup. This paper deals with different options. It is also available in Dutch.

The applied LTSpice model for the configurations can be downloaded here





2021 Rob Munnig Schmidt