Hearing, Columns and Comb Filtering


By Roger Russell


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Our brain is able to do many amazing things. We are still learning how it works. One of the characteristics of seeing is how we can look at a bunch of progressive still frames, as in a movie, and end up by seeing them as continuous motion. How this fusion of separate images is accomplished is still not fully understood. Someday, we may know whether information is processed by the eye or the brain, but most likely by both. To the novice, it might seem difficult to understand how this could possibly work in the first place, but it does. One of the conditions is that the number of frames per second is fast enough. At least 16 frames per second are needed and more recent productions are at 72 frames per second to give even better resolution, particularly for motion scenes. Perhaps our ability to do this is linked to a survival trait


An analysis of the operation of a movie projection can easily show that such a device is not feasible because it is only made up of a large number of still frames projected on to a screen with a blank interval to allow for frame change. The analysis is correct but this is not the way the eye/brain works and movies have been enjoyed for many, many years. Even television pictures are nothing more than rapid scans of individual bits of interlaced information and in the same analysis are also rapid changes of information. Today, all of this is taken for granted and no thought is given to why it is so.


The ear and brain also have a unique behavior when it comes to listening. There are many characteristics of hearing that have been learned but when it comes to listening, experience for many is lacking, particularly when it comes to column speakers. Some skeptics are obsessed with the words “comb filtering” and how this could affect the listening performance of a column speaker. Comb filtering can be defined as "the frequency response caused by combining a sound with its delayed duplicate. The frequency response displays a series of peaks and dips caused by phase interference. The peaks and dips look like the teeth in a comb, with very narrow, deep notches where signals are attenuated." An analysis of the different arrival times for all the drivers can easily be calculated and even measured to show the comb shaped pattern of response. This analysis is also correct but again, this is not how the ears and the brain work.


One of the unique features of the IDS-25 column design is that the sound is the same whether you are standing, sitting or at any other listening height. The sound always appears to come from the column at your listening height. You can move your head up and down and the sound follows you up and down. If you were to listen to the column with your head tilted so that one ear is above the other, the same sound still appears to come from the column at your listening height. The same hearing process is at work in either case and there is no need to be concerned with combing.


Listeners, who have been misinformed that columns exhibit combing but are unable hear this “effect” in the IDS-25, must wonder about the truth of this myth. After hearing the IDS-25 it is easy to replace this peripheral belief with more reliable first-hand experience.


Perhaps there is some confusion between driver spacing and what is thought to be combing. If the drivers in the column are a few feet apart and you move your head up and down, you can clearly hear sound from each individual driver, even far away from the system. This does not form a cylindrical source of sound. However, when the drivers are closely spaced, you must be within a foot of the column to hear any difference. Otherwise, further back from the system, there is no audible difference with height. When you are not able to hear any difference, then the sound effectively describes a cylindrical radiator.


The reason for this is the precedence effect, also known as the Hass Effect that describes the ability to correctly identify the direction of a sound source. The first sound to arrive at the ears enables you to determine the direction of the source, whether it is ahead, above or below. In this case, it is the sound from the nearest part of the cylinder.


After hearing an initial signal, the brain will suppress any later signal, such as an echo, for a time up to about 30 or 40 milliseconds. This inhibition is called time or temporal masking. In effect, you do not hear the sound from the higher or lower drivers that would otherwise interfere with locating the nearest source of the sound.


If the arrival time of any echo is longer than this, then two distinct sounds are heard, even if the second arrival is as much as10 dB higher. On the other hand, if there is no arrival time difference or only a very small time difference, the source will be heard to be coming from straight ahead. Even in a room having several reflections, we can reliably localize the real source.


Some of the misconceptions still persist today. Comb filtering is said to interfere with high frequency accuracy and voice articulation, yet typical listening impressions from visitors at the 2006 Rocky Mountain Audio Fest are:


1. “The 3.5" drivers running full range (no crossovers) violate the first rule of comb filtering, but I heard no comb filter problems and these speakers really sounded good.”


2. “The sound was exceedingly clean, articulate without being edgy and tonally consistent at any height position. Its greatest strength was a boxless, transparent character that occasionally reminded me of electrostats at their best.”


3. “…the real surprise was the open and totally transparent vocals. The high end was crystalline and I even stood on a chair to discover that up or down, side to side there was no discernable sweet spot.”


by Roger Russell  All Rights Reserved


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