O que é a estereofonia? - Gradient (em inglês)

A Gradient teve em tempos no seu site este texto intitulado "O que é a estereofonia?":


WHAT THE STEREO IS ALL ABOUT?

From time to time it is useful even for an expert to give a thought to the basics of sound reproduction.
For instance what the stereo is all about?

In stereophonic sound reproduction we have two channels.
What does this mean in practice?
In a recording session we can use two microphones or its multiples, the most important thing being the necessity to mix the results into two channels: the right channel and the left channel.
In principle we have two signals, two wave forms.
Acoustically these are the exact replicas of what happened at the recording location.
These two soundwaves represent the sound pressure level variations produced by the performers as captured by the recording engineer and the producer.
These acoustic pressure wave forms are transformed into electrical wave forms and preserved on magnetic tape, analog or digital.

Now we have two electrical signals on CD.
These signals are not similar and they are changing all the time.
They include all the information we need to reproduce the width, the depth even the height of the performance and its acoustic space.
All information we have and all we need are in these two electric signals.
If we stop these signals for a moment we ́ll notice that they both have single values, voltage i.e. pressure.
At a certain fragment of time we have only two figures.
This is how the digital system works.
What is the problem?
It is how to reproduce these signals without quality loss.
We all know that electronic equipment like amplifiers alter the signal very little.
So the problem is how to carry these two signals to the listener's ears in ordinary rooms without deteriorating the information.




Standardized chain

Carrying the signal unmodified from control room to listener´s livingroom calls for that the equipments used are compatible with each other worldwide.
Program sources (CD, cassette, LP, DVD) and electronics are therefore manufactured following international standards.
That is why an HiFi enthusiast can build the system using different brands.
It is even possible to mix tube and solid state equipments.
Biggest variations exist between power amplifiers and loudspeakers.
Some speakers may have a very low impedance working only with certain amplifiers. But usually all works well.


Listening rooms are varying acoustic loads

Analog to the amplifier loudspeaker interface the listening room works as an acoustic load to the loudspeaker.
In recording studios monitor loudspeakers and control rooms are usually integrated to obtain the best possible sound. However this is not the case in normal living rooms.
When building a house there are no standards for the acoustic load.
That is why rooms vary much from each other and usually are sooner bad than good for sound reproduction.

The loudspeaker room interface being unexpected it is difficult to imagine a loudspeaker suiting to all rooms.
The variations of bass volume alone let the designer give up.
In a room built of stone or concrete you´ll get too much bass but when the speakers are taken into a wooden house the result will be bass shy.
To put it simply there is no such thing like an all purpose loudspeaker.


Why do loudspeakers sound different in different rooms?

This problem has been studied to some content by various loudspeaker designers and other authorities.
For instance the famous Bruel & Kjaer of Denmark (manufacturer of audio measuring equipments) published in 1974 an AES (Audio engineering Society) paper called "Relevant loudspeaker test in studios, demo rooms and in the home using 1/3 octave pink noise".

The B&K team took five different make loudspeakers to three different rooms.
Sound quality was evaluated by five listeners in a blindfold listening test.
The results were surprising.
In advance one could not say which loudspeaker would give the best results in a certain room.
None of the loudspeakers sounded best in all three rooms.
This is easy to understand because every room creates a different acoustic load to the loudspeaker.
When you change the load you get different results.
It´s just like with amplifiers where the performance greatly depends on the load.

What was the advice of the B&K research team to the customer wanting to buy a loudspeaker system?
Take different loudspeakers to your home and measure them using pink noise *.
Finally select the loudspeaker pair giving the smoothest results on the listening spot in the frequency range from 60 Hz to 6000 Hz.
Usually this very loudspeaker also sounds the best.

Before founding Gradient Ltd., Jorma Salmi decided to study the room/loudspeaker interface problem (1980).
Instead of listening and measuring loudspeakers in ordinary rooms he went into an anechoic chamber.
See his findings in "The Absolute Listening Test".

* - This kind of method is still in use. For instance John Atkinson of Stereophile uses a computerized system giving a curve averaged from 60 measurements on a single loudspeaker, totalling 120 measurements for a stereo pair. This kind of a measurement gives good correlation to subjective listening results. See Stereophile March 97 to notice that the in room measurement of the Gradient Revolution is fantastic 32 Hz - 10 kHz +/-1.3 dB! (see fig.5: http://www.stereophile.com/loudspeakerreviews/616/index5.html )


The absolute listening test

At first Salmi arranged a listening test in a big anechoic room.
What is an anechoic room?
To put it simply it is a room without reflections i.e. reverberation, standing waves, flutter echo etc.
In fact it´s a room with no acoustics at all.

He listened to various loudspeakers on the spot where the measured frequency responses were as flat as possible (usually you can find this spot on the so called design axis).
When listening to high quality recordings with high quality speakers he was surprised to learn how good they sounded.
Here are his main findings:

• the sound was very clean and pure
• imaging was excellent
• acoustics of the recording venue were reproduced excellently
• spatial information was reproduced much better than in ordinary rooms
• sound quality was much better than in ordinary rooms

After listening to loudspeakers in an anechoic room Salmi arranged the so called Absolute Listening Test (ALT).
It was conducted as follows:

• the loudspeaker under test was situated inside an anechoic chamber
• a high quality measuring microphone (B&K 4133) was placed on the design axis of the speaker
• music program was fed to the speaker from a source (record player, microphone or tape recorder)
• monitoring took place outside the anechoic chamber
• by switching it was possible to detect the difference between the speaker/microphone combination and the source (a straight wire)
• both headphones and loudspeakers were used in monitoring

This test produced the following rather surprising results:

• when the frequency response curve of the test speaker measured by the microphone was flat, the listeners could not make a distinction between sounds arriving directly and those reproduced via the speaker under test
• non-linear and phase distortion, delayed resonances and similar phenomena did not appear to have an audible effect upon the sound quality of high quality speakers
• flat frequency response was the most important factor

Listening to good quality recordings in an anechoic room sounded excellent.
The difference between the direct sound compared to that passing through the loudspeaker and the microphone was minimal.
At that stage Salmi decided to investigate what happens in an ordinary room.
Or how your listening room kills the sound quality.


The room-loudspeaker interface or how your listening room kills the sound quality

The fundamental question was: Why did loudspeakers having similar frequency response curves sound different from each other in ordinary rooms?

Gradient's R&D team began to research the effect the room has on sound.

In an ordinary reverberant space like a living room only 20% of the dominant sound energy is transmitted directly to the listener, whilst 80% of the sound energy reaching the listener consists of reflections.
The reflected sound is made up of components coming from the floor, the walls and the ceiling.


Reverberation

Since there are no acoustic reflections in an anechoic chamber, the solution must lie in the reflected sounds of the listening room.
This proved to be the case.
The heart of the problem was how did the reflected sound deteriorate the frequency response?
Research done into the human sense of hearing shed some light on the matter.
The solution proved to be in the first reflected sounds received by the ear, since these sounds travel a longer distance than sounds arriving directly to the ear (3...70cm longer, or 0.1...2.0 ms in time).
The human ear and brain perceive these early reflections as contiguous to the sounds received directly.
A "comb filter" effect occurs, and the frequency response curve dips and rises alternately.
An apparently good speaker's response becomes worse.
The extent of this deteriorating depends on the directivity of the speaker and its distance from reflective surfaces, especially the floor.

What are the subjective effects of early reflections on the sound?

• imaging suffers
• lack of spatial information
• three dimensionality suffers
• the sound is coming from loudspeakers, not "out of nowhere" (loudspeakers do not disappear)

P.S.: In addition to early reflections there are also other factors deteriorating the sound quality: standing waves, uneven reverberation time vs. frequency and flutter echo.