©2006 Mark B. Anstendig

The Anstendig Institute is a pioneer in the research and education of both the public and the industry on the practical problems of sound-reproduction, particularly the need for equalization to correct the inevitable distortions created by the playback process

Sound consists of vibrating tones from low to high called frequencies. A sound source, in the space in which it is located, produces tones/frequencies from high to low, and those separate tones/frequencies are not all equal in volume. And it is mainly the specific balance of the volume of all the separate tones to each other, called “timbre”, that gives all sound its main characteristics and distinguishes one sound from another.

In the world of sound reproduction and playback, for purposes of analysis and communication, the range of sounds/tones that we hear is divided into sections, usually into octaves and 1/3 octaves. In all natural sounds, almost no tones have exactly the same loudness, and each section of the frequency spectrum can vary greatly in loudness in comparison to sections.

Equalization is a form of highly flexible tone control that allows the user to finely adjust the volume level of any frequency anywhere in the frequency range without affecting a large range of other frequencies, as would happen with a typical volume control.

With the advent of new technologies, sound reproduction has reached a high level of accuracy in everything except the equalization of the final sound that one hears. And it is the balance (or equalization) of that final sound that is the crucial factor in what the sounds we end up hearing actually sound like.

Listening rooms/spaces are different acoustically from the rooms/spaces, in which the sounds were produced. Microphones have their own characteristic balance of frequencies (EQ) that change the timbre of the sound. Most stages of the sound recording and playback process introduce their own small or large changes in the balance of the tones’ volume levels (EQ). And human hearing has its own ever changing anomalies in how we hear the timbre of sounds. But adequate adjustment of the sound to restore the original timbres of the sound during listening are usually not provided or even taught.  Therefore sound reproduction still seldom actually sounds like the original in timbre. And those distortions usually disturb the listening experience, especially our perception of the all important expression, which is usually the main content of the sound. This problem, can only be corrected by means of specialized equalizers.

The field of sound reproduction has finally admitted the crucial flaws and failings of the CD format and introduced newer, more accurate recording systems, such as SACD (Super Audio CD). Finally recording systems can capture all the important values of sound

But while recording systems are technically approaching the possibility of very high accuracy, the remaining distortion of the balance of frequencies, i.e., of EQ, still disturbs, changes, and often ruins the experience of the sound, especially the experience of the expressive content of the sound. This is true because these distortions usually change the timbre of the sound in the ranges where our hearing is most sensitive, as in certain high-frequency emphases, where our hearing is most sensitive, which can keep the listener from physically relaxing totally into the flow of the music, as is necessary to experience the finer expressive content. These concepts are published at this site, www.anstendig.org, in the papers dealing with sound reproduction and sound equalization, especially the paper entitled “The Massing of Overtones”. They have been available to the general public and circulated within the free of charge for over two and a half decades and are responsible for the current universal use of sound equalization in the recording processes.

But the use of equalization in the recording process cannot correct the problems caused in the playback process. Only the adjustment of the sound by ear, during playback, by means of “program EQ” can correct those problems.

In the 1930s, famous tests at the Bell Telephone Laboratory showed that our ears do not hear all the frequencies equally loudly. But it also, even more importantly,  showed that the relative loudness, with which we actually hear the frequencies, changes when the overall volume level is changed (i.e. whenever the main volume control on the sound system is changed). In other words, with any sound selection that we are hearing, in addition to the imbalances introduced by the recording process and by our playback situation, the entire balance to each other, with which we hear the volume levels of the different frequencies from low to high, changes every time we change the volume level of our sound system. It is the final balances, due to the distortions in the way we actually hear frequencies at differing volume levels, that result in the final timbres that we hear.  And that final timbre that we hear will differ radically from the timbres that the performers played or that other sound sources generated, unless the balance of frequencies during playback is restored to something close to that heard by the performers.

The equalization has, therefore, to be changed (rebalanced) every time the main volume control in the listening room is changed. (It even has to be changed when the recording engineers change their master volume control and also to compensate for frequency imbalances within the Microphones themselves and for acoustical differences between what the performers are hearing and adjusting to and what the microphone is receiving at the position of the microphone). The listener’s hearing also changes because the listener’s body changes over time, even over the course of a single day. Therefore, the final equalization has to be done during listening, in order to compensate for the distortions in timbre that are inevitably introduces during the recording and playback process. Without this “program equalization” adjustment, there can never be natural-sounding sound reproduction. Especially not at louder volume levels, where the human ear is most sensitive to, and irritated by, these differences in the timbre of the sound.

The Anstendig Institute has written all the necessary papers on why we need to equalize. But it has not been able to write the crucial publication on how to equalize the sound during playback. For such a project, sonically neutral recordings would have to be made under controlled conditions with expensive musicians and ensembles. And examples of adequate equalization of those recordings in a neutral room would have to be recorded in graphic form as examples of successful equalization that can be replicated in other neutral rooms. But how to perform program equalization can readily be shown or taught live, in any decent listening room, with adequate equipment.


The Anstendig Institute is a non-profit, tax-exempt, research institute that was founded to investigate the vibrational influences in our lives and to pursue research in the fields of sight and sound; to provide material designed to help the public become aware of and understand vibrational influences; to instruct the public in how to improve the quality of those influences in their lives; and to provide the research and explanations that are necessary for an understanding of how we see and hear.