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Sound Advice

Q:
Hey Garry, I’m an installer at a high end shop here in Dallas, Texas. I enjoy the magazine every month along with your no-bull attitude. Can you do a write up sometime on stereo imaging? I think I understand what it is, but I don’t know how to tell if my systems are good at it or not. I know speaker placement is important, but does the specific car have much to do with it? And what is staging

Dazed and Confused (via e-mail)

A: Hey there Dazed. Thanks for having the cojones to ask the question so many people want to, but are afraid to admit they don’t know.

A stereo image is a listener’s perception of the location of the various sounds across an ‘imagined’ soundstage. Think of it this way. Let’s pretend you have front row center tickets to a rock concert with a five-piece band. On the far left side of the stage is the keyboard player. Moving closer toward center is the lead guitar player. The lead singer is dead center. Behind him and slightly to the right is where the drummer sits. On the far right is the bass player.

When you hear the band live, you hear these different elements of the music from their various points on stage. Stereo recordings were invented to try to replicate that experience, right down to where each player’s sound comes from during that performance. Back in the early days of audio, recordings were made with a single microphone. The result was a recording that has equal phase and amplitude on both channels and it’s also known as mono. When stereoscopic sound was invented by Western Electric in the late 1920s, they simply mixed two mono tracks together, and changed the amplitude (volume) of the left and right tracks separately.

The way stereo sound works is actually very simple, because most of the hard decoding work is done inside your brain. Try this fun and simple experiment sometime. Take a pair of speakers and place them four or five feet apart, roughly at ear level. Then sit four or five feet in front, but directly between them to form an equilateral triangle between you and each speaker. You now have created the optimum environment to reproduce a proper stereo image. 

When a sound with equal phase and amplitude is played through both speakers, it will seem to come from a magical point in space exactly between the two speakers – this is called a “phantom image.” To make a sound come more from one side of the “stage” or the other, the recording engineer changes the amplitude on that one specific channel only. This will make the image for that sound shift to whichever speaker is louder. 

The amount the image moves is controlled by the difference in output between the two speakers. Obviously if you shut the sound completely off on one side, the entire image will go fully to the other edge of the stage. You can use your system balance control to play with what happens to the phantom image when the amplitude changes between the two speakers.

Another way to shift the image is to delay one channel slightly. The image will then be pulled to the un-delayed, ‘closer’ speaker.

Try this at home. Play a musical track with a good, strong center image. If you don’t have one, simply rip a song with good vocals to a mono track. Set up the system as above, and note the strong and focused phantom (or “center”) image. Everything should be coming from a point between the speakers. Now, sit two feet further left or right. The image will be pulled to the speaker nearest you. Now, use the balance control to reduce the output of the near speaker, and the center image will be shifted back to the middle, but it won’t be as focused or sharp as it was. 

Now, play the same track but in regular stereo. Sit dead center, and you should be able to tell where the band members are across the stage. If you have very good speakers, you may also perceive a sense of depth and height to the stage. It’s possible that the actual stereo image seems even wider than your speakers are apart. That’s a stereo image, or what some people call a proper soundstage. The best way to judge stereo imaging is to set up system that’s good at it, then listen to a lot of music, paying attention where the sounds seem like they are coming from. Memorize a few good tracks, and use those to judge you’re the system in your car. One of my favorite tracks for this is Marc Cohn’s Perfect Love.

By now it should be pretty evident that getting a really accurate stereo image in a car is very difficult, if not impossible. We don’t sit in the center of our cars, and we generally don’t have the speakers at ear level. Yes, we can compensate for some of this through the use of DSP trickery, but it will always only correct for one listening position – at the expense of everyone else. For the best stereo imaging, you’d like the left and right speakers at similar distances from your ears and as high up as possible. That’s a pretty tall order for most vehicles. 

In my opinion, to make a great-sounding car system I concentrate on clarity, tonal accuracy, and timbre (pronounced TAM-bur). The stereo image will never be as good as your home system, where you can sit equidistant from speakers that aren’t playing into your pant leg. And because different car interiors have completely different acoustical properties, an identical system in two different cars will always sound very different. As a pro installer, it’s your job to figure out what works for each car, then educate the shop sales team.

Thanks for the question, and good luck!

Garry