The third article in our series on understanding high-end car audio systems delves into the fascinating world of imaging. In essence, imaging is the ability of an audio system to recreate the precise location of a performer or instrument on a soundstage. While not all recordings excel in this aspect, some are genuinely awe-inspiring. Let’s explore this captivating facet of music reproduction.
What Is Imaging in a Car Audio System?
As mentioned, a truly high-end audio system, be it in your vehicle or at home, should be able to accurately present each performer’s position on the soundstage. This sounds similar to our previous discussion about the source of your music. That is the soundstage. The soundstage defines where the music is coming from. Imaging describes the positional accuracy of the sounds on that soundstage.
When two audio signals are the same in amplitude and phase in a recording, they should be reproduced from the exact center of the soundstage.
For example, let’s say we have a recording of a four-piece band. The drums are centered on the stage and are located at the back. The bassist is on the right, a few feet back from the front edge of the stage. The guitarist is on the left side of the stage at a similar distance. Finally, the lead singer is center-stage, right at the front edge.
Whether or not a recording has imaging cues depends on how the performance is captured. For this initial discussion, let’s assume the microphones on a drum kit are mixed into a single mono channel. Each performer has a microphone for vocals, and the guitar and bass also have a single audio channel. If all of these are brought into a mixer, summed together at appropriate levels to produce good tonal balance, and then recorded, all the music would seem to come from the center of an audio system’s soundstage. Why? There is no left or right information captured in the mix. All the music should seem to come from that single blue dot in the center of the soundstage.
A Stereo Microphone Mix
What if the drum kit is mic’ed with 10 microphones, and the floor tom and the ride cymbal on the left of the stage are panned left, and the hi-hats and crash cymbal are panned to the right? The snare might be panned a moderate amount to the right, with the high tom panned slightly right and the mid tom panned slightly to the left. The kick drum mic is likely to be in the center. Further, a pair of overhead microphones placed 5 feet above the drum kit might be dedicated exclusively to left and right channels. The map might look something like this.
The exact amount of panning depends on the drum technician and the recording engineer. If they want the drums to stay relatively focused in the center of the soundstage, perhaps they should be panned no more than 25% to the right or left. They’ll fine-tune this in the studio.
Next, the guitarist and bassist might have their mics panned to the left and right to separate them from the lead singer.
The image below might represent what you’d hear with some panning added to the microphones on the drums, the bassist, the guitarist and the lead singer.
Would this sound the same as what you’d hear if you stood in the studio with the band? Absolutely not. Would it sound better if all the microphones were mixed into a mono signal? Most definitely.
For a genuinely realistic stereo recording that captures more of a sense of the room, the engineer might use more audio information from the overhead microphones. Some performances, like a choir or an orchestra, might be recorded with fewer microphones. When done accurately, capturing the reverberations in the room can add a fantastic sense of realism to the listening experience. The art of creating a recording is equal in skill and talent to that of the performers themselves.
What It Takes to Create Excellent Car Audio System Imaging
For this discussion, let’s consider a stereo sound system. The alternative would be something with an upmixer that adds a center and possible side and surround channels. For a stereo system to provide pinpoint imaging, the signals from the left and right channels must arrive at the listening position at the same amplitude and at the same time. This concept can be expanded by considering that all frequencies must arrive at the same time as those from the opposite channel. Likewise, all frequencies must be at the same amplitude.
Connect a set of headphones to your smartphone and play some music. You’ll likely find that the lead performer’s voice appears to come from a spot in the center of your head, as if they were singing from the middle of your brain. Now, go into the phone’s settings and adjust the balance about 75% toward the left. All the music will seem to come from just inside your left ear. This is a perfect example of how signal level affects imaging.
Now, if one frequency is louder from one side of the car compared with the other, the source of the music that contains that frequency will seem to move its position on the soundstage. We call this phenomenon frequency steering. This is undesirable because the sound source shouldn’t move based on frequency, only amplitude and phase.
Imagine if a stage is recorded with a high-quality stereo microphone, or better yet, a binaural mic placed exactly in the middle of the stage. If the performer is directly in front of the mic, audio signals will simultaneously arrive at the left and right recording elements. Now, if the performer walks to the right side of the stage, the audio signal will arrive at the right microphone element just before the left one and be slightly louder on the right. If the playback system is truly symmetrical in its ability to recreate the recording, we’ll hear the performer move to the right.
How Human Hearing Detects Sound Sources
Our ears work the same way as a pair of microphones to detect the source of a sound. We can triangulate the arrival time and slight differences in amplitude between one ear and the other to locate a sound source. We are accustomed to the changes in frequency response that occur as sound wraps around our heads. This is how we know whether something is in front of or behind us. We can also detect reflections off adjacent surfaces to correlate height. A bird in a tree sounds different from one standing on the ground.
Companies like Harman International have invested a great deal of time in measuring how we perceive sounds from different locations. They’ve used that data to create stereo headphones like the JBL Quantum One that can recreate surround-sound effects from movies and video games. Using only two speakers, you can hear if a bad guy is walking up behind you in Halo or Call of Duty. Yes, it sounds like the person is just behind you. How do they do it? They adjust the frequency response of the sound to mimic what the helix, antihelix, antihelical fold and antitragus do to sounds. This takes some serious math, but it works surprisingly well. The headphones also include head tracking, so if you turn your head to the side, the source of sounds changes. It works so well; it’s almost creepy.
Tips for Excellent Car Stereo System Imaging
To create a car audio system with excellent imaging and a soundstage that is well out in front of the listener, there are a few items to consider. The first is speaker placement. If you want the stage to be on the dash, you’ll need the tweeters to be in line with the dash. This might require a mounting position on the dash or in the A-pillars. It’s better if you can get a small midrange driver onto the dash; this will help solidify the soundstage position at that depth. Many new cars and trucks have midrange speaker positions at the base of the windshield. This works great for depth. Combining that with tweeters in the sail panels or pillars can increase the system’s perception of width.
Next, you’ll want the tweeters installed within 15 to 20 degrees of being on-axis with the listening position. If you use a three-way speaker set with midrange and midbass drivers, angling these speakers only changes the reflections or equalizes the signal.
Next, you need a way to control the output amplitude of each speaker in the audio system. This means that you’ll likely want a fully active system where each speaker has a dedicated amplifier channel.
Next, your system must have some sort of stereo equalizer that’s precise enough to ensure that the signals from the left and right speakers are the same in amplitude across a wide range of frequencies and with enough bands to make sure nothing goes unchecked.
Finally, from a system design and hardware perspective, you’ll need a way to delay the signals going to the speakers closest to the listening position. Your system will need a digital signal processor or a source unit with signal delay features.
Finding a technician who is well-versed in calibrating digital signal processors in vehicles is the most critical aspect of creating a sound system with exceptional imaging. There’s a big difference in a vocal that sounds like it’s the size of an umbrella in the center of the car compared to a softball or even a golf ball. Once the system is calibrated precisely, you’ll hear additional information being revealed. An accurate sense of room ambiance is, for example, often only audible when the system’s imaging is excellent. You’ll start to hear reflections from the ceiling and back of the recording venue, assuming they’re in the recording at all. If the system is calibrated precisely, this information becomes confusing and unrealistic.
Center Image Position
There is one last point of discussion before we wrap up. There are two common options for where the center image should be on the soundstage. Most professionals like the center to be located equidistant from the left and right boundaries of the soundstage itself. In most cases, this would put the center in the middle of the windshield under the rearview mirror.
With that said some people like the center position to be in front of the driver’s seat. Based on our experience, this is a bit more common for factory-installed systems. We won’t say that your preference is wrong, if you enjoy this “right in front of you” calibration. However, we find that it compresses the size of all the instruments on the left side of the soundstage. If the lead vocals are in front of you, the system might have 12 inches of width to the left boundary to place all the performers on the left side of the stage. Conversely, you might have three feet on the right side. It’s your choice, and you should discuss this with the person configuring and calibrating your car audio system before work begins.
Single-Seat System Calibrations
So far, we have discussed audio systems designed, configured and calibrated to provide an exceptional listening experience from the driver’s seat. However, whoever sits in the passenger seat will likely hear all the music coming from the right side of the dash or the door. Why? The speaker will be louder and sounds from the opposite side of the car are delayed. Sadly, when it comes to listening to the music, it’s not very enjoyable to be a passenger in a vehicle with an audio system set up for a single-seat calibration.
Currently, very few digital signal processors on the market can properly extract a center channel signal from the left/right channels and feed it to a discreet output. This feature, called an upmixer, is the only option for creating a detailed soundstage for the vehicle’s driver and passenger. Your installer and the calibration technician might be able to create something that works satisfactorily using all-pass filters. Still, it might lack the precision of a truly amazing single-seat system. Make sure the signal processor you choose has several preset options. You can have an amazing single-seat calibration for those times when you are alone in the vehicle, then a two-seat tune when someone is in the car or truck with you.
Upgrade Your Car Audio System Today for Excellent Imaging
If you want to improve the realism and detail of your car audio system, visit a specialty mobile enhancement retailer with extensive experience calibrating digital signal processors. Audition several vehicles they configured and calibrated to ensure that they can deliver an experience that matches your expectations.
This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.
