We recently talked about the difference between good and great speakers. In a nutshell, when you choose great speakers, you get more power handling, the potential for more output and dramatically improved clarity, thanks to a reduction in distortion. In this issue of Bang for Your Buck, we are going to look at two car audio speaker technologies that reduce distortion. This article will be a true behind-the-scenes look at how speakers work.

Current Flow and Magnetic Fields

According to Lenz’s law, when current flows through a conductor, a magnetic field is created around the conductor. At the same time, when we move a conductor through a magnetic field, a current is created in the conductor.

The diagram above shows a conductor (in grey) with a current flowing through it. The green lines show the direction of the magnetic field around the conductor.

In a speaker, the voice coil is a coil of wire wrapped around a bobbin or former. Current from our amplifier flows through this conductor and sets up a magnetic field around the voice coil. We use this property to move the speaker in and out of the basket. When the polarity of the magnetic field is the same as that of the fixed magnet, the similar magnetic fields repel each other, and the speaker moves forward. When the polarity of the magnetic field reverses, the speaker is attracted to the magnetic field and the speaker moves rearwards.

Unfortunately, when it comes to dealing with alternating current, the behavior of magnetic fields can work against you. When the polarity of the current reverses, it has to fight against the magnetic field it created. You can think of it as momentum. If marbles are rolling across the floor, it takes energy to make them change direction. This opposition to the change in the flow of current is called inductance. The electrical momentum resists the desire to set up a new magnetic field of the opposite polarity.

Managing Voice Coil Inductance

The amount of inductance in a speaker voice coil is determined by several factors. The size of the voice coil conductor, the geometry of the conductor, the number of layers in the voice coil and the proximity of the voice coil to the top plate and the pole piece – just to name a few.

So, how does inductance cause distortion in a speaker? As you can see in the diagram above when the voice coil winding (in red) is at rest, it is centered on the top plate (in green). As the cone moves down, more of the voice coil is beside the magnet (in blue) and the pole piece (in pink). Conversely, as the come moves outward, less of the cone is near the pole. In a conventional speaker design, the changes in proximity to the steel pole piece cause changes in inductance. As the inductance decreases, there is less opposition to the flow of high-frequency current and an increase in high-frequency performance. Changes in performance based on the position of the speaker cone result in distortion.

The image above shows the inductance of a voice coil relative to its position in the speaker. The red curve is the inductance graph of a conventional speaker. The blue curve is the inductance graph of a speaker that includes an aluminum shorting ring at the bottom of the T-yoke. As you can see in the image above, without the shorting ring, the speaker has dramatically different inductive characteristics depending on the position of the cone.The graph above shows the frequency response of a speaker without a shorting ring (in red) and that of a very similar speaker with a shorting ring (in blue). As is clearly evident, the inclusion of a shorting ring dramatically improves the high-frequency performance of a speaker. Further improvements in linearity can be achieved by including a copper cap on top of the T-yoke.

When you are shopping for great speakers, look for inductance-reducing caps on all speakers (subwoofers, midrange drivers and tweeters), and in larger speakers (subwoofers and midrange drivers) where the room is available in the motor assembly, look for the presence of a shorting ring.

Comparing Inductance Numbers

If we can’t determine whether a speaker has a design that mitigates changes in inductive characteristics, can we simply look at the specifications? They most certainly do provide a hint. A 6.5-inch woofer without a cap or shorting ring may have an inductance of 0.7 to 1.1 mH (millihenries), whereas a speaker with these technologies will be closer to 0.1 or 0.2mH. In terms of how they sound, all other design criteria being equal, the driver with the lower inductance will have better high-frequency performance and produce less distortion.

Speaker Suspension Nonlinearities

The purpose of the spider (also known as a damper) is to keep the voice coil laterally centered in the air gap between the top plate and the pole piece and to help return the cone to the resting position when the audio signal is removed.

Picking the perfect damper stiffness (compliance) for a given cone mass and desired resonant frequency is one of the biggest balancing acts involved in designing a speaker. If the spider is too stiff, the resonant frequency of the speaker may be too high for the desired application and its efficiency may suffer.

A variety of materials are available, as well as different sizes and different geometries. A spider is a spring. Some spiders are designed for linear compliance and some are progressive. More importantly, because of variances in voice coil winding height and basket design, some spiders include an offset mounting lip. This is called a cupped spider. This cup or spacer allows the spider to attach to the voice coil former above the winding, then connect to the chassis while keeping the voice coil vertically centered in the magnetic gap.

The above graph shows the suspension compliance of two different 6.5-inch diameter speakers based on the position of the cone. The graph in red shows the compliance of a speaker that uses a cupped spider. You can see that at 6.5mm of inward travel, the suspension is 25 percent stiffer than at 6.5mm of outward travel. The blue graph shows a similarly sized speaker with a flat spider. Though the overall compliance is different, the behavior in the forward and rearward directions is nearly identical.

What To Look For in a Spider

Ultimately, we want the spider to exert the same amount of force on the cone and voice coil as it moves forward or backward from the rest position. The amount of force should not change based on the direction of cone travel. Imagine the distortion created by a speaker playing a sine wave where the cone doesn’t move as far rearward for a given amount of current as it moves forward. As such, try to avoid speakers that use cupped spiders.

Hear Different Car Audio Speaker Technologies In Person

The next time you head to your local mobile enhancement retailer to listen to new speakers, choose two wildly different price points and listen to the same portion of a song on each speaker. Listen to them at a reasonably loud volume level, and position yourself across the room. Switch back and forth until you determine the differences.

Then, add a third speaker option, priced and featured somewhere in the middle. Make the same comparison with this new speaker and the expensive speakers. Once you have listened to the speakers, ask to look at a sample of each and see if you can correlate some of the design characteristics we have discussed with their performance. It’s not only a great way to audition products but to learn about what makes one car audio speaker technology or design better than another.

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.

If you have ever been to a movie theatre, you have probably experienced what is known as surround sound. There are at least five discreet audio tracks encoded into the film you are watching, and most also include a Low-Frequency Effects (LFE) channel. The purpose of these audio channels is to allow the source of each sound to be directed to a particular location in the theater so everyone can enjoy a realistic experience, no matter where they are sitting.

This is what has brought about the introduction of surround sound processing to the automotive world. It is relatively easy for an audio system designer to set up a realistic listening experience from a single listening position using equalization, precise output level adjustment and signal delay. Even when you sit much closer to one speaker, careful tuning can make the music appear to come from an imaginary speaker array all across the soundstage.

The Front Stage

For mobile audio enthusiasts, much of our efforts in system design and tuning go into the creation of the front stage. This term describes the speakers that are installed in front of us and their locations. We can use signal processing to adjust the output of each speaker so the music sounds as if it is coming from the center of the dash, even though you are sitting on the left side of the vehicle. The problem with these adjustments is that they only work for one seating location. The passenger often suffers with less-than-ideal frequency response and most of the music seems to emanate from the right side of the dash. For realism, this is less than ideal.

Sound-steering Technology

Most movie theaters have dedicated left, center and right speakers behind the main screen. Including the center channel allows the audio producer to fix the position of voices and effects in a specific location relative to the screen. If someone is being interviewed and is centered in the image, their voice will most likely be reproduced by the center speaker only. If they move across the picture, the level of their voice decreases in the center and increases in one of the left or right speakers relative to their position. Even if you are sitting on the far right side of the theatre, voices and sounds are steered for incredible realism.

Surround Sound in Cars and Trucks

Think about how well the ability to steer sound works in a theater when you are in a less-than-ideal seating position. Now think about your vehicle. You are sitting much closer to the left-side speakers. You will hear sound from them before anything from the right speakers, and those sounds will be louder. Your music will seem to come from the door or left side of the dash.

By including a center channel in the middle of the dash, the system designer can route audio signals that are equal in amplitude in both left and right channels to the center speaker. To improve the accuracy of the system a step further, those signals are also removed from the left and right speakers. The result is a system that reproduces music evenly across the dash from both the left and right seat. Music sounds great from the back seats as well.

Rear and Side Speakers

In our movie theater, the rear or side speakers are primarily used for special effects. A helicopter or plane flying into the scene from behind us is a good example. In modern movie production, that information is hardcoded into the surround channels.

Because we are only listening to two-channel audio (left and right) in our vehicles, there is no opportunity for the music producers to direct specific sounds to rear channels. However, complex signal processors from companies like Bose and Harman do an excellent job of extracting audio information that is suitable for the rear speakers. Sophisticated algorithms can also produce an increased sense of soundstage width in systems with speakers in the rear doors. Rear deck speakers add a sense of ambiance and room size to the performance. How they analyze the relative phase and amplitude of signals within the stereo recording is simply amazing. So, too, are the results of their efforts. Your music can appear to come from spaces beyond the boundaries of your vehicle.

Mix it Up

OK, we admit that the title of this article is click bait. We are talking about upmixers here. An upmixer, by definition, takes a number of audio channels (in our case, two) and turns them into a greater number of channels (usually six). If we feed an upmixer left and right signals, it can generate left, center, right, left rear, right rear and subwoofer output signals. Some upmixers provide eight channels: left, center, right, left side, right side, left rear, right rear and subwoofer. Many OEM systems use these 7.1 systems with great success!

You sold me! Gimme, Gimme, Gimme!

When it comes to the aftermarket, the options for upmixers are limited. Some people have mucked around with using computers or tablets with external sound cards, but the interface is clunky and frustrating. On top of that, you don’t get a radio tuner, there is no backup camera input and you can’t add SiriusXM. Tablets look cool, but they are not ideal for use in vehicles.

The Alpine PXA-H800 processor (and companion RUX-C800 controller) includes Dolby Digital, Dolby Pro Logic II and DTS Digital Surround decoding. This processor will generate a genuine center channel signal with audio information that is removed from the left and right channels.

Audiofrog hasn’t been shy about the fact that they are working on a new signal processor that is specifically designed to create a realistic center channel and surround channels. The solution is called Multiseat Stereo. Unlike most systems that use versions of DTS or Dolby Digital, Audiofrog has partnered with PerfectSurround to create a system that is free from phase artifacts.

Confused? Don’t be. In a nutshell, PerfectSurround does a better job of steering sound than other processing systems. The results are more realistic and provide better bass focus in the front of the vehicle. There is no firm release date as yet, due to patent applications and all that business stuff, but keep your eyes peeled.

The Future of Great Mobile Audio for Everyone

If you and your co-pilot both love great car audio, then a mobile sound system with a correctly processed center channel is an easy way for both of you to enjoy a realistic performance from any position in the vehicle. If your car or truck came with a surround sound system, visit your local mobile enhancement retailer to find out how they can upgrade it with better speakers, more power and tuning that matches your listening preferences.

If you are designing a new system, then keep an eye out for new processors to arrive on the market. The future of mobile entertainment that sounds realistic for everyone in the vehicle will be based on upmixers.

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.