The gap between measured and perceived quality, linked to sneaky pitfalls of feedback
There is an unexplained inconsistency between the measured and perceived quality of audio equipment. The paper analyses the causes of this inconsistency and comes to the conclusion that the theory of feedback is a crude approximation of the actual situation and that testing procedures are flawed. The conclusion is that designing audio equipment needs to be changed significantly in order to close the gap between the measured and perceived quality.
The presentation has been used during a tutorial at the Convention of the Audio Engineering Society in Madrid (Spain) in June 2024.
Feedback theory has several sneaky pitfalls
The commonly used theory of feedback has several sneaky pitfalls, which disable the possibility to calculate the response of electronics exactly and enforces the use of approximations and simplifications. As a result, the calculated and realized properties can differ significantly. This can explain why there is so much controversy around this subject, especially in the High-End community and why electronics with similar specifications can sound rather different. The paper also explains why testing with continuous sine waves of fixed amplitude gives ambiguous results, which do not correlate well with perceptual observations.
The required conditions for the applications of Fourier Theory
Fourier Theory is an elegant piece of mathematics, which is commonly used in audio. However, in order to apply the theory correctly, its conditions should also be fulfilled, but neither electronics nor human hearing do so. As a result, incorrect conclusions are drawn. This paper describes these conditions and discusses the incorrect conclusions, which result from neglecting / ignoring these conditions.
The measurement of the impulse response of microphones using white noise
The microphone contributes significantly to the time smear in the whole audio recording and reproduction chain. By measuring the impulse response of microphones, this can be quantified, but manufacturers are reluctant to publish these. However, by using white noise as the exciting signal and a good tweeter, the impulse response of microphones can be determined with relative simple equipment. It also enables correction of the properties of microphones, thus improving their perceptual quality. This paper explains the background and both theoretical and experimental results are presented.
Measuring the impulse response of microphones using noise: a feasibility study
The measurement of the impulse response of microphones is crucial to understand their temporal properties. Little information is disclosed by manufacturers, claiming this is hard to measure due to the requirement of specific hardware and the risk of non-linear response of the microphone. Both objections could be circumvented by using Gaussian distributed white noise as exciting signal and the cross correlation technique. As the use of a loudspeaker is required, a major stumbling block is the requirement of deconvolution of the obtained impulse response. This study shows that the deconvolution is feasible in case of a measured impulse response of a loudspeaker and the impulse response of a modelled (and thus well known) microphone, even if the impulse response of the loudspeaker is significantly wider than the impulse response of the microphone.
The second part of the feasibility study includes a Monte Carlo simulation of the actual measurement procedure, including the exciting Gaussian distributed white noise signal. The exciting signal has been verified to fulfill the requirements for this application. The main conclusion is that the technique is feasible for the determination of the microphone impulse response, but the results show some slight imperfections, which can be reduced by using the same noise signal for the determination of the impulse response of the loudspeaker.
The major conclusion of both parts is that the measurement of the microphone impulse response using noise is feasible without special hardware and that there is no reason for manufacturers to keep this information away from users.
Distortion Analysis
Many perceived differences between amplifiers and other consumer electronics cannot be explained by the common measurements, specifications and analysis.This has given rise to severe discussions on fora, e.g. on the effect of feedback, on which completely opposite opinions can be found. The root causes of the confusion are the use of incorrect equations for systems with feedback, the analysis of non-linear systems using theories, only valid for linear systems and the use of steady-state, continuous signals, whereas music is -fortunately- far from steady state. In this paper, the response of non-linear systems to non steady-state, non continuous signals is calculated and the artefacts, introduced by the electronics are revealed. The technique is used to reveal the validity of several statements, found on fora and to detemine better ways to design amplifiers with high perceived quality. 
Distortion_Analysis_EN.pdf
Phono preamplifier.
We have recently unveiled our phono preamplifier.In this paper, we discuss the concepts and the measured results of this amplifier RIAAmeasurements.pdf. See also PhonoAmplifier-MusicEmotionfinal.pdf in which the review of this amplifier can be found.
Is feedback the miracle cure for high-end audio?
On high-end audio fora, amplifiers with feedback are often rated low on their perceived quality, whereas the theory learns that all kinds of undesirable misery are suppressed by feedback. How come? This paper analyses the problems with feedback and comes to the conclusion that the often applied equations for feedback are fundamentally in error. The gratuitous application of these may introduce audible artefacts. So feedback can improve the perceived quality of equipment, if applied with in-depth knowhow, but it is certainly no miracle cure.
Tone burst response of amplifiers to determine some properties of their dynamic behaviour
An amplifier should have a response which, within its operational range, should not depend on the strength of the output signal. Is that always the case? Read the findings of some measurements in DynamicResponseAmplifiers.pdf.
The current through loudspeakers when excited with tone-burst signals
In order to get a better understanding of the interaction between the amplifier and the loudspeaker, measurements of the current through a bass-reflex loudspeaker have been made and these have been compared with the calculated current. The results can be read in CurrentBassReflex.pdf.
Presentation Hans van Maanen AES Berlin 2017
During the Convention of the Audio Engineering Society in Berlin (May 2017), a workshop has been held on the audibility of the temporal response and temporal resolution of audio systems. The presentation of Hans van Maanen can be found at Workshop3.pdf
Presentation Mike Turner at AES Berlin 2017
During Workshop #3 at the Audio Engineering Society Convention in Berlin (May 2017), Mike Turner presented his ideas and plans to (strongly) improve the low frequency response of loudspeakers. This presentation is now available. It dives deeper into the theory, but to those readers, who think it will be (too) difficult: it includes also a number of very illustrative figures of the temporal response which are certainly worthwhile to study in more detail.
Distortion
Distortion, generated by electronics, is even at low levels audible and annoying for human hearing.
"Temporal Coherence" strives for the lowest possible level and at least as annoying to human hearing
as is practical. To that end, a number of innovative methods have been developed, which are explained
in simple terms in this article.
Amplifier Issues
Although the specifications of amplifiers are usually significantly better than those of loudspeakers, different amplifiers are qualified with large differences. This paper tries to identify a number of causes of this paradox without pretending it is the final answer. It is rather meant to stimulate the discussion and reactions are more than welcome.
Report on the ALMA winter meeting in Las Vegas
At the end of November, Hans received and invation to present the paper, he presented at the conference of the Audio Engineering Society (AES) in Helsinki, Finland, August 2013, again at the winter meeting of the ALMA (Association of Loudspeaker Manufacturers and Acoustics) in Las Vegas (Nevada, U.S.A.). With some shuffling. Hans was able to fit this meeting in his agenda and departed for Las Vegas.
Concepts of the Diamond and Pyramide audio systems
Temporal Coherence has demonstrated its "Pyramide" system at several audio clubs in The Netherlands. To elucidate the backgrounds, this presentation has been given. A number of sheets have been added to compensate the lack of explanation by the speaker.
Report on the Consumer Electronics Show in Las Vegas
Because Hans was in Las Vegas to present at the ALMA winter meeting anyway, he managed his agenda such that he could pay a one day visit to the Consumer Electronics Show (CES) there. The CES is probably the largest show for consumer electronics in the world and there is no way you can tackle it in one day. The show is held in three different (large!) buildings and includes about anything from washing machines to computers and from robots to giga audio systems.
Read the report on the Consumer Electronics Show in Las Vegas
Home Video Test of July 2014
On the contents page of Home Video Test of July 2014 it said "During the listening session at the triumvirate of Temporal Coherence it became more than clear that the loudspeaker design approaches the ideal".
Loudspeakers and Headphones in the High Resolution Audio Era
The high resolution digital formats are an improvement over the CD-format, which has audible deficiencies. But to express the advantages clearly, the loudspeakers, especially the tweeters, and headphones, have to fulfil a number of requirements. In this paper, presented at the AES conference in Helsinki, Finland, in August 2013, these requirements are underpinned and elucidated. It shows that there are still many shortcomings.
Impedance compensation of loudspeakers
Paper for the AES conference in Amsterdam (1994) to obtain an impedance compensation network in order to create a close-to-Ohmic impedance of a electro-dynamic loudspeaker unit in an acoustic box (in English).