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ADVANCED SPEAKER RESEARCH FROM INDIA

No Listening Fatigue

Q-Balance > Details

 

Speaker systems have a frequency response curve that due to various electromechanical limitations, cannot be the ideal flat line. They exhibit peaks and dips in their response as shown in Fig1. A conventional speaker system exhibits sharp peaks and dips and this is further worsened at the crossover points. These sharp peaks exhibit a sharp slope or high Q at that point. A sharp dip also exhibits a high Q, but in the negative direction.

Scientific research, listening tests and measurement correlation studies have shown that sharp peaks and dips lead to harshness and sub-consciously perceived distortions that are the main cause of listening fatigue in modern day loudspeakers.

 

Lithos uses Q-balance technology to effectively eliminate these sharp peaks and dips in the frequency response. Q-balance incorporates complex response shaping techniques with specialized crossover designs that smoothen out these sharp Q points and maintain a very low Q throughout the frequency response (fig.2). These results are effectively extended to the speaker impedance curves as well (fig.3 & 4). Furthermore, since all Q- balance circuit elements are not placed in the direct signal path of the speaker, none of the clarity and transient attack of the music is lost.

UNDERSTANDING EAR FATIGUE

 

A lot has been said about ear-fatigue and its relationship with distortion. Common thinking is that the more the sound is different from the original recording or the source, the more distorted it is, and it is this difference that contributes to ear-fatigue.

 

Distortion is the difference in what you hear from your music system as against what is originally in the recording. Let us put things in perspective straight away. At its fundamental level - every system, even the most expensive million dollar super high end system - is in many ways a distortion of the original source or CD! While it is obvious that distortion causes ear fatigue, the attempt to associate all differences between the sound source and the output from the audio system as distortion that leads to ear fatigue is incorrect. All distortions that typify this difference do not cause ear-fatigue,  only some specific distortions do. The following article is an attempt at understanding the mechanisms of these specific distortions that cause ear fatigue.

 

To begin with, most of what our senses perceive as reality (or distortion) is incorrect. To understand this concept, let us take an example from our visual sense and then extend the same to the aural one.

 

Understanding Distortion

 

Consider our view of a railway track that keeps going on over a long distance. We all see that the tracks seem to come closer together as their distance from the eye increases, and eventually, the tracks would meet at a point. This, in reality, couldn't be more wrong. The tracks, as we know, are always parallel. Yet, this distortion doesn't confuse  or fatigue us in any way, even though it is totally different from the truth. In fact if you were to draw 2 parallel lines going into the distance to represent railway tracks, the parallel lines would look weird (distorted) and cause us stress. Draw the lines and see for yourself.

 

There are many more visual distortions that the eye is known to make. Take for example the 'impossible triangle' on the right (courtesy Illusionworks). Studied closely, this is not possible in real life, only in 2D. Yet the eye is very comfortable in distorting the image for us - we see and accept the image as a 3D reality. Thus, we can see that even though the eye distorts the image there is no eye fatigue!

The same is true of the ear. A simple form of aural distortion is that we are incapable of hearing all sounds of the frequency spectrum at the same loudness (even when they are played at the same level). The 1-3kHz (vocal) sounds will sound louder;  the bass will be softer. There are many other forms of distortion that characterize the ear. These are due to various bio-organo-mechanical limitations of the hearing process. This makes it impossible for humans to ever hear what the actual sound of a voice or instrument is really like (even when heard live!!).

So, the difference (distortion?) between source (live concert) and output (music system) is inevitable - no matter what our advances in science and technology, hi-end audio and recording techniques may be. This is because the distortion produced by the chain of electronics, that is used in recording (even if it is ever eliminated), will never be the same as the distortion caused by the chain of physics, psychoacoustics and the organic mechanisms that dictate what we hear.

 

Ear Fatigue

 

Since there is distortion in all our senses, it becomes obvious that these distortions soon become the reality and are accepted without confusion. We can easily add some more distortions, (the difference between source and output, for example) and still accept it as reality, without any problems. For example, the 'thump' that you hear in a discotheque is a distortion in the level and type of bass, that could be quite different from the original recording. Yet we all love it! In the light of this, it is wrong to associate all source and output differences as contributors to ear fatigue. In fact, many 'distortions' - like the thump in a disco - may actually make the sound better, and less fatiguing, to us. Distortion is thus a unique evil. It often makes things more pleasing than they really are. It makes us accept the impossible, accept the unreal and it sometimes even excites our senses.

 

Now that we've realized that the difference in source and output is inevitable and does not cause ear fatigue, what does? We know ear-fatigue exists.

 

Over the years, many institutions have carried out research related studies to understand this mechanism of ear fatigue, and though no scientific-mathematical equation has come up yet, all measurement correlation** studies have pointed to one significant aspect that associates with ear-fatigue.

This one significant aspect is the presence of sharp peaks and dips in any (or all) of the measurement curves that are used to evaluate speaker systems. Sharp peaks and dips have been correlated to ear fatigue over many years of listening tests and research studies carried out all over the audio-scientific world. This includes our own blind testing of various speaker systems across a sampling of audiophiles and listeners. Thus any sharp peak or dip, be it in the frequency response curve, the impedance curve, the phase response curve or the step response, among others, has the potential to cause ear fatigue.

 

Only an attempt to eliminate these 'peaky' distortions can be considered an attempt at solving the problem of ear fatigue. This requires extensive knowledge of speaker and crossover system design and entails highly complex crossover networks. This also requires that you take adequate care in the placement of your speakers in the listening room.

 

To the audiophile/educated layman, the recognition of ear-fatigue may be characterized by sensations like sharpness, harshness, boominess etc. and that of no ear-fatigue as smoothness and the ability to have extended listening sessions. (Often, a more fatiguing system strikes the layman as being a better system in typical showroom environments, because of its in-your-face vocals, big bass etc). Thus the choice of a speaker system entails that one should recognise a fatigue-inducing system.

 

Understanding the details of each of these peaky distortions, (resonance etc) and learning how to recognize systems that will cause ear-fatigue is below.

 

**What's a measurement correlation study? Expert audiophiles are made to listen to different types of systems with music of their choice and their comments are noted down. These comments are then compared with the measurements of the same audio equipment, and then one tries to establish a correlation between what is heard and what is measured. For example 'punchy' bass is correlated to speakers having a slight peak (bass boost) between 60-80Hz, which can be pleasing when listening to certain types of music like pop, techno etc.

 

© Lithos Acoustics, May 7, 2001:

Understanding Ear Fatigue: Part II

 

In part 1 of this article, we identified the sure-fire cause for ear-fatigue; i.e. the specific distortion mechanism that causes any speaker measurement curve to have sharp peaks and dips. These measurement curves could be the frequency response, the impedance curve and the phase curve among others.

 

In this article, we will look at what causes these peaky characteristics and how they can be eliminated. This discussion is restricted to speaker systems only.

 

In the speaker frequency response curves, sharp peaks and dips are caused by…

 

- Incorrect crossover alignment at the crossover points.

- Failure to compensate for mid-band gain in 3 way systems

- Incorrect bass alignments in cabinet design

- Lack of time alignment between drivers (midrange, tweeter)

- Multiple drivers radiating the same frequency bandwidth (2 midranges or 2 tweeters, especially if

  they are placed horizontally, like most center speakers in home theater systems)

- Cone material resonance

- Sharp peaks and dips in the frequency response are also caused due to incorrect speaker place-    ment in the room and incorrect seating arrangement, typically causing comb filter effect and   standing waves, which also add to ear fatigue.

 

In impedance curves, sharp peaks and dips are caused by…

 

- Improper tuning of bass reflex alignments

- Failure to compensate for reactive rise in voice coils at higher frequencies

- Cabinet resonance

 

The points listed above are just a few of the factors that cause ear fatigue.

 

The discern ability level of these sharp peaks and dips is different for all these curves. It is easier getting fatigued with sharp peaks and dips in the frequency response curve; these are the most obvious. The peaks in the impedance curve are less easily discernable as ear fatigue and in the phase response curve, even less. But peaks in all these curves do cause fatigue anyway. It's just a question of how good your existing listening level is.

As you can see, there are many causes of ear fatigue, they take the form of incorrect cabinet construction, improper bass alignments, incorrect choice of cone materials, speaker drivers and most importantly, incorrect crossover design.

 

Did you know that even changing the shape or width of the cabinet would require a change in the crossover design?

 

The crossover is the most important factor that dictates the final sound of your speaker system and its use in eliminating ear fatigue has been neglected by almost every manufacturer in the world. It is so much more fashionable to talk about a Kevlar cone or a dipole surround speaker than to talk about the step response of a 6dB Butterworth crossover.

 

There are many crossover design techniques that are used to eliminate sharp peaks and dips in speaker systems.

Firstly, of course, is the use of correct crossover alignments at the crossover points (easier said than done). The othertechniques involve impedance equalization, resonance damping, sensitivity matching, notch filtering, low frequency diffraction compensation etc. All these constitute to what is known as complex response shaping crossover circuitry.

 

In effect, one has to use any (mostly all) of these techniques and still not induce any undue losses or phase anomalies in the speaker output. Of course, the best way to eliminate cabinet resonance is to improve the structural rigidity of the cabinet, not use a complex crossover filter. But many problems can only be solved by these complex response-shaping techniques. Metal cone resonance is an example.

 

At the end of it all, the elimination of sharp peaks and dips in the measurement response curves of a speaker is the way to eliminate ear fatigue. This is all encompassing, but mostly a job of the crossover, typically involving complex response shaping techniques.

 

© Lithos Acoustics, May 14, 2001:

Note: All material in this article is the sole property of lithos acoustics and cannot be reprinted without prior permission.

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