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Stereo: Two is a Magic Number


Stereo. We’re all familiar with the word but what does it mean, what’s so good about stereo and how do you produce it?


Sit on a chair. Close your eyes. Ask a friend to stand a couple of metres or so in front of you. Have them click the fingers on one hand straight in front of you, then to the left of you, then to the right and anywhere in between, randomly, a few seconds apart. Before even trying this out you pretty much know that even with your eyes closed you’ll be able to point fairly precisely to where your friend is clicking. Do it anyway, just to convince yourself.


With both eyes still closed, try again but this time with one ear ‘closed’ too. Perhaps the simplest way to achieve this rather than sticking anything in your ear is to press a finger on the flap of skin on your earlobe in front of the ear canal (the Tragus) so that it temporarily blocks your ear. Have your friend repeat the randomly positioned finger clicking while you use the hand which isn’t closing your ear to point to where you think the clicks are coming from. After a few moments, what you are likely to hear is your friend’s laughter as you fail big-time to be able to point to where the clicks are coming from! (You can then swap over to give yourself the opportunity to laugh at your friend too).


All great fun but what’s all of this got to do with stereo?


The close-your-eyes-and-point game is a fun way of proving how very useful it is to have two ears rather than one when it comes to being able to tell where sounds are coming from. The difference in the time of arrival of a sound to your two ears, combined with the difference in the loudness of a sound between your ears allows you to work out where the source of that sound is. For example, when your friend clicks their fingers straight in front of you the sound reaches both ears at the same time and at the same loudness. However, when they click their fingers to the right of you the sound arrives at your right ear fractionally before it reaches your left ear. It will also be a little louder in your right ear than your left, mainly on account of your head being in the way of your left ear to a sound coming from the right and the fact that the sound has that little bit further to travel. This is pretty much all the brain needs to calculate the likely position of the sound. Stereo also uses level difference and time of arrival differences to simulate positional information and spaciousness in a recording, but more on that a bit later.


Our ability to locate objects by hearing alone is very useful, not least when doing day to day, yet potentially hazardous tasks such as crossing the road. In a more cultural setting it means when we sit in a concert hall in front of a symphony orchestra we hear the violins off to the left, the cellos and basses off to the right and so on.


Imagine, however, listening to a recording of an orchestra through a single loudspeaker. With just the one speaker, no matter how many different instruments there are in the ensemble, or where the players sat in the space where the recording was made, all of the sound appears to come from one place; the loudspeaker. Very one dimensional. Monophonic.


A grand and beautiful gramophone horn but all of the sound from one place in 'mono'

You might then imagine an experiment where the sound of each instrument is recorded on a separate track and then each replayed through a separate speaker, the speakers being set out in your room in the same configuration the players were sitting in during the recording. That would certainly add spaciousness and positional detail to the sound of the recording. It’s not very practical though; for an orchestral recording you’d need space (and the budget) for 60 or so loudspeakers, which doesn’t leave a lot of space for your sofa and coffee table!


Fortunately, thanks to the work of many pioneers, not least that of Alan Blumlein, an English electronics engineer born in 1903, a system was developed which gave the listener an accurate impression of space and position using only two loudspeakers. Beautiful in its simplicity and originally known as ‘binaural’, it soon became what we now know as ‘stereo’ sound.


It’s important to note at this stage that simply having two loudspeakers replaying a recording does not make it stereo. The recording itself must be produced in a way which results in there being differences between the left and right tracks of the recording and hence in the sound from the two speakers. It’s those differences which result in the stereo effect for the listener.


Back to our earlier game where a friend was clicking fingers. When they made the sound directly in front of us it arrived at both ears at the same time and at the same loudness. Even with our eyes closed, we knew where it was . If we sit midway between two loudspeakers and exactly the same sound comes from both speakers at the same volume, what happens? Perhaps surprisingly we don’t hear it as the same sound coming from two places - speaker A and speaker B. Just as in the clicking game, because the identical sound from both speakers reaches our ears at the same time and the same loudness our good old brain determines that it must be coming from right in front of us and so we perceive it as a single sound coming from the midway between the two speakers. Imagine now that the sound is that of a singer. In our stereo sound stage they are now front and centre, just where we want them.


Let’s arrange for the sound of the piano to come out of just the left hand speaker. That sound would reach our left ear slightly before the right, and be slightly louder in our left ear than the right. You guessed it; the sound of the piano appears to come from the left. Next, we arrange for the sound of the guitar to come out of just the right hand speaker. We’ve now made a stereo image with the piano on the far left, the singer in the middle and the guitar on the far right. Ta da!


With a little more sophistication in the system we can add even finer detail. Two backing singers join the band. We make the sound of the first backing singer come mostly from the left speaker but also a little bit from the right speaker. This is where the stereo illusion gets really clever. As before we don’t perceive the sound of the backing singer as coming a lot from one speaker and a little from the other, but rather as one single sound; coming from the mid left position. If we do the opposite with the sound of the other backing singer; mostly out of the right speaker but a little out of the left, they then appear as a single sound in the mid right position.


In contrast to the effect of a mono recording of the band where all of the sounds come from a single point, now, in glorious stereo, using only one additional speaker and a bit of control we hear the band spread across the stage between the speakers from left to right – piano, backing singer, lead singer, backing singer, guitar, each occupying their own space and the band sounding much more ‘real’ than in mono. Listen to any stereo recording with headphones on or sitting midway between the speakers to hear how music recording engineers use the stereo space to position instruments and voices, and in the case of radio-drama and film, the sound effects too.


In the second part of Two is a Magic Number we’ll look at the mixing desk control recording engineers use to create a stereo image, and the techniques used by Classical engineers to create stereo using only microphones.


© 2023 Ken Blair and BMP The Sound Recording Company Ltd


Stereo logo photo by MarkusSpiske on Unsplash

Gramophone photo by Sudhith Xavier on Unsplash




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