Stereo Perception
Although some reflections in the small listening room are supportive to understanding spoken words, too many reflections, for too long a period of time will confuse. Such is the sound we hear when discussing furniture arrangements inside an empty house or apartment, it is an echo problem. In hi end audio, the echo can be a problem but in typical carpeted, furnished rooms the problem is usually not an echo. Most small rooms have too many early reflections within the sound fusion period which degrades musical clarity and sound stage imaging. First, we look into the process of acoustical imaging in audio.
To begin, let’s return to the desert and listen to just one speaker. We relax; stare ahead and begin to “see” the sound coming from the speaker as we expect to occur. Next, we turn up the sound from the second speaker. By the time they are equally loud, the acoustic image floats between the two speakers. This is exactly what stereo is really all about, not two sounds from two separate speakers but the appearance of one sound in the space between two speakers. The stereo experience is intended to be an acoustic hologram, a mirage of sound in the desert.
In the next experiment we again use one speaker and add a wall located just to the side of the speaker. Now we receive the direct signal from the speaker and very quickly following is a reflection off the wall. The image of the speaker shifts again away from the speaker and floats somewhere between the speaker and the wall. The wall reflection produces a sound not exactly like a second speaker yet similar effects are observed. The wall reflection is weaker and slightly time delayed compared to the signal from the speaker. This is quite different from the previous stereo speaker experiment in which the two signals arrived at the same time and with the same intensity.
This entire imaging process has been greatly studied and is called the “Haas effect”, after a scientist of that name. Signals as low as 10 dB below the direct signal and as time delayed as much as 10 milliseconds will produce image position changes. Clearly, with sound traveling 1,130 ft/second (334 m/sec), the wall reflections in the typical sized listening room fall well within the Haas effect time window.
When we get the same signal from the two separate locations and within a very short period of time, our brain is tricked into thinking that sound came from a spot between those two locations, that can even be in front of or behind them. These signals create a “virtual” sound source location. If we get 3 such signals then the image is localized amongst the 3 directions. But if we get many signals of different strengths and delays and from different directions – we lose track of where the sound seems to come from. We think the sound comes from everywhere which is the same as nowhere. Too many early reflections cause confusion in stereo imaging. The sound stage is no longer crystal clear, it becomes fogged over, detail is lost, image is blurred and sound depth becomes flat.
We recognize this problem to exist whenever the speakers do not “disappear”. In a good room, the speakers actually seem to be silent most of the time, all while and throughout the front of the room a vivid audio stage show is taking place.
Musical Quality
Small room acoustic reflections appear in two, and very different forms. Not only form but their effects are also different. Imagine the piano keyboard and middle C – 256 Hz. All notes to the left of center are in the BASS clef and those to the right are TREBLE. Room reflections occur for all of the frequency range. Those reflections that cause problems with imaging belong for the most part to the treble clef, above middle C. Reflection in the bass range creates problems not so much for imaging but effect the clarity, musical honesty and realism. Unlike treble, reflections in the bass range do not bounce around the room like balls. Their wavelengths are so long that the reflected waves fold back one onto another. The resulting compositions are generally known as room resonances or room modes.
Room resonances will cause one note to sound loud and the next to sound quiet. They also cause attack transients to pick up a coloration in tone. Speakers project the treble forward, directly into the room. If you step behind the speaker the treble is no longer heard coming from the speaker. Not so with the bass range. The wavelengths are larger than the speaker and the sound expands evenly in all directions. You hear bass as loudly behind the speaker as in front of the speaker. The treble is beamed forward but the bass cannot be beamed. We actually have to put 10 times more bass power into the room than treble to get the same direct sound level to the listener. This is partially due to acoustic efficiency of speakers in the bass range but primarily due to the direct bass wave having to be delivered to the whole room. The treble is more efficiently produced and directed primarily towards the listener.
There are two types of resonance reflection problems. The sound that the speakers make first expands outward and contacts the walls, floor and ceiling in the front of the room. These very early reflections fold back upon one another to create “head end ringing”. The quivering of the air in the front of the room occurs very quickly within 1/10 second and so it colors the attack transients. It is only during the attack transients of music can we identify phase and time alignment. This most important quality in modern hi end audio equipment is heavily degraded by head end ringing in the mid bass range.
The second type of resonance takes much longer to develop, typically 1/4 second. It engages the full length of the room and tends to belong to the lower notes, the deep bass. These are the full 3 dimensional room modes that are usually talked about. They cause some notes to be excessively loud and others too quiet. Room resonances cause the otherwise smooth frequency response curve of even the very best audio equipment to become very irregular at the listening position.
Room Acoustic Upgrade
Reflections do need control in the audio listening room. Treble reflections are the most commonly understood. Carpet, drapery and furniture work reasonably well with treble. The typical products such as acoustical foam and fiberglass wall panels are only for upper treble acoustics. It takes about 4 inches or 10 cm of such material to effect the full treble range. One must be very careful about over damping the brightness of the listening room by excessive misuse of low cost treble absorptive materials. Some of the more recent audio quality, treble range acoustic products are not only full treble bandwidth but have built-in sound scattering panels to keep the listening area controlled yet still sounding bright.
The lower half of the musical scale, the bass range, needs BASS TRAPS. For many years bass traps were only found built into the walls of recording studios. But over the last decade acoustic products available in audio have been expanded to include small, efficient, free-standing bass traps. Sound absorption depends on two factors, surface area and its absorption coefficient or efficiency. Modern bass traps are cylinder-shaped in order to provide a large surface area as well as high efficiency in a small package.
One very fortunate aspect of bass in small rooms is that the corners of the room act like sound collectors, megaphones in reverse. Because bass goes backwards as well as forwards, it should be no surprise that bass traps are regularly found both behind the speakers and behind the listener. Bass is always extra loud in the corners of a room. That is why it is such a good idea to locate bass traps in the corners of the room. You can always recognize a properly set up a listening room by the presence of corner-loaded bass traps in every corner of the room.
And so, now we should know a little more why the ROOM ACOUSTIC is called the last link in the audio chain and even more, what kinds of modern acoustic upgrades are to be expected in this last and all too often forgotten link of the high end audio chain. |
