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Listening on Speakers

Page history last edited by Randy Coppinger 12 years, 7 months ago

If anything here is confusing, inspiring or absolutely incorrect your comments would be much appreciated.  This is a work in progress and your help improving the information is requested.  Thanks!

 

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Let’s take this opportunity to get some jargon out of the way.  When people talk/write about stereo, they often use words like soundstage and image.  These refer to the “illusion that between and beyond the pair of speakers there exists another acoustic environment within which the musicians are located and performing.” [Lipshitz] If actors and stage seem to transport us to another time and place because we are so engaged by the play, people and theatre have been transformed.  If paint and canvas seem to disappear because we are so engaged by the painting, we have transformed pigment and cloth into an image.  If we present sound at speakers that seems to stage an aural play or paint a sonic image, we transform sound pressure into a presentation that seems to have breadth, depth and perhaps motion.  Hopefully we are so engaged that we forget about the speakers and the sound pressure, but instead involve ourselves in the presentation.

 

Now back to the physical world.  What happens when we present stereo on two speakers?  Sound from the left speaker reaches the left ear and sound from the right speaker reaches the right ear.  Simple, agreed?

 

But sound from the left speaker also reaches the right ear, albeit later in time and lower in volume, varying by frequency.  And sound from the right speaker also reaches the left ear, also later in time and lower in volume, varying by frequency.  This is sometimes called Crosstalk.

 

And then the sound from those speakers bounces off nearby boundaries, some of which reflects back to our ears.  We may even hear reflections of those reflections.  Hmmm... not so simple anymore.

 

The good news is, stereo works despite all this complexity.  If you make a singing vocal the same volume in both speakers (Phantom Center), then slowly make it louder in the left while softer in the right, it will move left.  Bruce Bartlett figures that when the left signal is about 7.5 dB louder than the right, it will seem to be halfway between Phantom Center and the left speaker.  At about 15 to 20 dB it will seem to be completely located at the left speaker, even though some sound is still coming from the right speaker.

Changing the volume to position a sound is like locating a subject for a photograph or painting, with the entire background (the distance between the speakers) behind it.  Working inside such a Panorama, early audio pioneers developed an electronic device to help locate sounds between the speakers by raising volume in one speaker while lowering it in the other in a useful ratio.  This Panoramic Potentiometer (or Pan Pot, or simply Pan) makes positioning sounds with volume quite easy.

 

To some degree you can also position sounds with time delays, with 0.5 ms in the right speaker moving the vocal left halfway between Phantom Center and the left speaker.  A 1.2 to 1.5 ms delay in the right puts the vocal all the way left.


And Bartlett says you can mix and match.  Use some volume change with some time delay and they will accumulate, sort of adding to each other.  And he goes on to suggest that convincing stereo is better achieved using both, rather than just one or the other.

 

Yet it is uncommon to see a Panoramic Delay knob.  Why is that?  Well first of all a delayed signal summed back onto the original signal can Comb Filter, which sounds artificial and weird.  But let’s save our Comb Filter discussion for later.  Another reason is that we tend to locate more accurately by level difference than by time difference.  In other words, level differences are more effective and there aren’t any serious side-effects.  That doesn’t mean we can’t use delays for position, just that it is a little more challenging to accomplish using speakers.

 

It is important to know that Bartlett used broadband signals for his experiments, such as human voice.  If you are using sine waves or other signals with a very limited frequency range, the changes to volume or timing may not position sources as well.  Also note that his results are averages from expert listeners.  In other words, these figures can be used as guidelines; they are not pinpoint accurate.

 

Using EQ to create stereo images on speakers isn’t very useful.  The complex interaction between both ears, both speakers and the acoustics of the room pretty well render those subtleties of human hearing ineffective.  Certainly high frequency content is easier to locate on speakers for the same reasons that it is in real spaces.  But applying EQ differences to one or both channels to help differentiate them and thus create a stereo signal is probably not worth our time when we listen on speakers, especially compared to overall volume differences and time delays.

 

We also know that we can locate sounds fore and aft by changing their relative volume and by changing the ratio of direct sound to Ambience.  This can get tricky on speakers, because we may also use volume and/or time delays to locate sounds left and right.  They key is the relationship between the volume of one sound, which we want to seem close, compared to the volume of another sound, which we want to seem further.  Also consider that mics closer to sources sound closer because the ratio of direct sound to Ambience is very high.  Mics that are farther away from sources sound farther because the ratio of direct sound to Ambience is lower.  We could also control depth perception on speakers by adding sound from room mics or effects devices to manipulate that ratio.

Now let’s think again about the reflections of the listening space where you use your speakers.  If the room where you have your speakers is really live, that is, if the Early Reflections and Reverb are significantly loud relative to the speaker volume, it may cause you some problems.  Timing differences from listening room reflections can confuse timing differences used to place sounds left/right or fore/aft.  Managing the acoustics of a room is worth entire books of discussion, and certainly beyond the scope of this series.  Suffice it to say that some good sounding speakers also need to be carefully placed in a room with some acoustical considerations if they are going to provide convincing stereo and help you monitor your audio work in general.

 

We continue to clarify how differences between the two stereo channels – volume and to some degree delay – can be used to position a signal in the soundscape from left to right speaker.  Pans are easy and effective.  Delays are less effective for localization, and have side-effects, but are useful for creating a sense of space.  EQ seems less effective still, at least on speakers.  Volume ratios between each sound, and the ratio of direct sound to Ambience, can be used to create a sense of depth.  Placement of speakers and their interaction with room acoustics play a significant role in how well you can hear what’s supposed to be happening in your stereo presentation.

 

Listening on Headphones >

 

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