HD Radio is a broadcast technology that adds digitally encoded information to an analog AM or FM station. Where once you were limited to listening to, say, 101.1 in New York City, the broadcaster has added a news and talk channel as well as a sports channel to the same 101.1 MHz frequency. The best part is that HD Radio is completely free. If the tuner in your vehicle has this feature, you can make use of it without any activation or subscription costs.

How Does FM Radio Work?

Without getting too technical, each FM radio station in North America is assigned a specific range of frequencies it can use to broadcast its content. The limits for analog FM radio station frequencies are 88.1 to 107.9 MHz, and each station has 200 kHz of bandwidth to transmit both a mono and a stereo signal. The Federal Communications Commission (FCC) in the US and Industry Canada (IC) control the standards and handle licensing in their respective countries. As such, you can’t just build a radio transmitter and start broadcasting a station from your basement.

Technically, with 20 MHz of bandwidth, there is room for 100 stations within 88 and 108 MHz to simultaneously broadcast within the same area. In reality, few cities have that many radio stations. The FCC and IC see to it that radio station broadcast frequencies are spaced far enough apart so they don’t overlap. Likewise, stations that broadcast on the same carrier frequency need a transmitter that is far enough away from a similar station so that the signals won’t overlap within the target market.

Adding HD Radio Digital Information

When a broadcaster decides that it wants to transmit additional information using HD Radio, it is permitted to increase the bandwidth of its transmitted signal and send digital information. This technology is called an in-band on-channel (IBOC) digital radio.

Because this signal is digital, it’s much more resistant to noise. As such, stations rebroadcast their analog signal on HD1 – the first of the four available HD Radio stations available on a single carrier frequency. While the system does use a lossy compression algorithm to reduce bandwidth, most users report that the HD1 channel sounds better than the analog signal.

Broadcasters can divide their available digital bandwidth however they choose. If they want the clearest, most detailed digital option for their main station, they may not broadcast anything other than HD1. In one example, HD1 is the main FM radio station and HD2 is a feed from an AM station owned by the same broadcaster. Talk radio and news-only broadcasts that require less overall bandwidth are also common uses for HD2 and HD3.

Compatible tuners are designed in a way that a failure to decode any of the digital radio stations results in the tuner dropping back to playing the analog audio signal. The technology also allows broadcasters to transmit station information, album and artist details and, in some instances, album or radio station artwork for display on a compatible receiving radio.

Currently, HD Radio signals are broadcast in the US, Canada and Mexico. XPERI Corp., the company that owns HD Radio, reports that India will be the next market to add HD Radio broadcasts.

Upgrade Your Vehicle with an HD Radio-Equipped Receiver

If your car or truck didn’t come with an HD Radio-equipped receiver, drop by your local specialty mobile enhancement retailer to find out how you can upgrade your radio. Sony, Kenwood, Pioneer and Alpine offer solutions to provide a whole new world of entertainment to make your commute more enjoyable.

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.

Though it seems contrary to what I’ve preached, sometimes it’s better to avoid purchasing car audio speaker upgrades based on specifications alone. The information most manufacturers provide does very little to describe the actual quality of the product. A prime example of this are speaker Xmax specification numbers. Does this linear measurement tell us how loudly a speaker plays? Does it describe linearity? Does it inform us about distortion?

Don’t Get Hung Up on the Wrong Numbers

The Xmax specification is a tricky number that gets thrown around a lot when we’re talking about subwoofers and midbass drivers. Historically, this specification has described how far the cone of a speaker can move in one direction without having the voice coil move outside of the top plate. When the coil moves too far, the magnetic field strength is reduced dramatically and distortion will occur.

Many enthusiasts attribute more Xmax to an ability for the speaker to play more loudly. While this seems reasonable in theory, cone excursion is determined by the compliance of suspension components and the strength of the motor assembly.

What if components such as the spider or surround don’t provide the same compliance in both directions? What if the woofer cone might be able to move rearward more easily than it can move forward? Given a specific input signal, this characteristic would deliver what could be described as unbalanced performance. In reality, it results in the addition of distortion to the output of the speaker.

Looking at the graph above, we can see that the suspension offers a resistance of 1.6 Newtons per mm in the forward direction and 2.1 Newtons per mm in the rearward direction.

Another characteristic that might affect speaker performance and our usable Xmax specification is the effectiveness of the motor (voice coil and magnet structure) in delivering linear force in both directions. If the motor is more efficient in one direction, the speaker could potentially move farther (or not far enough) based on a given input signal.

In the graph above, we can see that the motor assembly in this speaker produces a magnetic flux of 4.1 N/A in the forward direction at an excursion of 4 mm, and 4.9 N/A in the rearward direction at the same excursion level.

What Does Distortion Look Like?

In order to explain how nonlinearities in speaker operation affect what we hear, we created a 20 Hz sine wave in Adobe Audition. We manually manipulated the three positive cycles of the waveform on the left channel and boosted their amplitude by 6 dB.

The frequency response plot below shows that the 20 Hz signal has been increased in amplitude by 1.1 dB. Harmonics have been added at 40, 80, 100, 120, 140, 180, 200, 220, 240, 280 and 300 Hz. All of this unwanted information is directly due to the nonlinear behavior of the waveform.

This addition of undesirable harmonic content is called harmonic distortion. Looked at another way, it’s the addition of sounds to the output of a speaker (or any other audio device) that weren’t in the original recording.

Can You Measure Speaker Sound Quality?

There are no characteristics of a speaker’s performance that can’t be measured. We can quantify the frequency response. We can test power handling. We can measure compression. There are tests for inductance versus position and resonant frequency versus excursion. The list goes on and on. What’s difficult is determining when a design has a measurement that is a glaring issue, or just a characteristic. Is some amount of nonlinearity allowable? If so, how much? Does the answer depend on how the speaker is going to be used?

Different measurements have relevance in different applications. If you are looking at detailed specifications of an 8-inch subwoofer, the information you need will be very different than if you were looking at an 8-inch midrange intended for use in a public address speaker in a theatre or concert venue. If you are going to be playing the driver from 300 Hz up, the excursion isn’t much of an issue.

Knowledgeable speaker engineers understand the application of the speakers they are designing and manipulate the component (cone, spider, surround and motor) design to optimize the entire system for specific performance characteristics.

Sadly, for those of us who want the best speakers possible for our car audio systems, manufacturers rarely share this information with consumers. Worse, some don’t have the equipment to make these measurements and optimize their drivers. Nevertheless, your best bet is to visit your local specialty mobile enhancement retailer and audition as many speakers as you can. Quickly you’ll learn to pick out the solutions that are the clearest and most detailed. These are typically the speakers that add the least distortion to their output. Those are the ones you want to use to upgrade the mobile audio system in your vehicle.

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.

Most of us have heard of many types of car audio subwoofer enclosures. Many enthusiasts have debated the benefits and drawbacks of acoustic suspension (sealed), bass-reflex (vented), bandpass and infinite baffle designs in hopes of choosing the best solution for their application and expectations. In all cases, the enclosure has two specific purposes that are crucial to ensuring that your subwoofer sounds excellent.

Rear Wave Cancellation

If you were to take a subwoofer out of its carton, connect it to an amplifier and play music, it wouldn’t make any bass. As the cone moves forward, the pressurized air in front of the cone is canceled by the rarefied air behind the cone, and vice versa. We call this back-wave cancellation.

In the graph below, you can see two sine waves, one in yellow and another with opposite polarity in blue. When they are added together, they cancel each other out, and you get the green line.

Understanding that the sound coming off the back of the subwoofer (or speaker) cone cancels the sound coming from the front is the first step in comprehending why a subwoofer needs an enclosure. We need to separate those two sound sources from each other for us to hear bass information.

The simplest solution is to create a wall or baffle that keeps these sound sources separate. That’s exactly what happens in an infinite baffle subwoofer installation. Your installer will either cut a hole in the rear deck of your sedan or mount a subwoofer on a board behind the rear seat so that you hear only the music coming from the front of the speaker. The sound coming from the rear is trapped in the trunk. This type of design doesn’t work with SUVs or hatchbacks since they don’t have a trunk to capture the sound from the back of the subwoofer cone. In reality, this is very similar to a sealed enclosure, except that the enclosure is extremely large and doesn’t affect the frequency response of the speaker.

Benefits of an Infinite Baffle Car Audio Subwoofer System

Infinite baffle installations are very popular in factory-installed subwoofers systems in sedans. The subwoofer can be mounted in the rear deck of the vehicle, and the trunk becomes the enclosure. These systems are also relatively easy to construct, as long as the trunk is sealed well. The drawback of this type of installation is that there is nothing to control the motion of the subwoofer cone other than its suspension.

Subwoofer Physical Power Handling

For a midrange driver playing above 300 Hz, cone excursion isn’t a concern because it doesn’t have to move very far at these frequencies. As audio frequencies decrease, cone excursion increases dramatically. As an example, to produce, say, 90 dB of output at 100 Hz, a subwoofer cone might have to move 1 mm forward and rearward. To produce the same output at 50 Hz, the cone needs to move 4 mm, and at 25 Hz, the cone will have to move back and forth a total of 16 mm. Stated scientifically, cone excursion at a given output level increases with the inverse square of the frequency. So, for each drop of one octave, excursion quadruples.

Every speaker has a limit as to how far the cone can move forward or rearward. While there are different methods of determining this limit based on acceptable distortion performance, we’ll use the Xmax standard of the voice coil needing to not move outside of the top plate.

In the case of this Sony XS-GSW101 subwoofer, Xmax is specified as being 6.4 mm in each direction. This definition means the cone can move through a range of 12.8 mm without the strength of the magnetic field imposed on the voice coil by the top plate changing. What happens when the voice coil starts to move out of the top plate? In short, the amount of distortion added to the output skyrockets because the cone movement isn’t linear relative to the drive signal. You don’t want that.

How a Subwoofer Enclosure Improves Power Handling

When we put a subwoofer into an enclosure, the air inside the enclosure acts to limit how far the cone can move. In a tiny enclosure, the total air volume is small, so the cone can’t move much. In a large enclosure, the volume of air is large and is easier to compress and rarify, so the cone can move farther.

The graph above shows how the compliance (resistance to pressurization and rarefaction) of the air inside a sealed enclosure affects the output of the system. The red curve shows the response of the subwoofer in an infinite baffle application. As you can see, there is lots of output down in the 20-40 Hz range. The white curve shows the sub in a 0.6-cubic-foot enclosure. It’s not as loud at lower frequencies. Finally, the blue curve shows the response of the subwoofer in a very small 0.2-cubic-foot enclosure. This is much smaller than you’d ever want to use but serves to show how the cabinet acts to limit cone travel and, consequently, bass output.

In the graph above, the three traces show us how much the subwoofer cone moves relative to the size of the enclosure at a drive level of 350 watts. The red curve is our infinite baffle simulation, the white is 0.6 cubic feet, and the blue is an enclosure with a net internal volume of 0.2 cubic feet. Since we don’t want the subwoofer sound distorted at high output levels, our installer needs to choose an enclosure design that balances bass output with maximum cone excursion capability. In this case, the blue enclosure is the only application where we can provide the full rated 350 watts to the subwoofer without the cone moving too far.

We did some extra simulations and confirmed that an enclosure volume of 0.29 cubic feet allows the full 350 watts of power to be applied without excursion issues.

I Want More Bass Than That!

If we look at the frequency response graph above, it shows us that our subwoofer has a -3 dB output frequency of about 92 Hz. While this doesn’t seem like much fun in terms of making bass, it actually works out OK because our vehicles boost low frequencies. The result would be relatively flat frequency response. What do we do if we want our subwoofer to play louder? The answer is to have the retailer you are working with build a bass reflex (vented) enclosure.

In the image above, you can see how much more bass the same subwoofer produces when installed in a vented enclosure. At 40 Hz, the vented enclosure is 9 dB louder than the sealed enclosure. At 30 Hz, it’s 8.2 dB louder. It seems like a win, doesn’t it? When you combine this increased output with the cabin gain, we find all vehicles benefit; suddenly a single 10-inch subwoofer can really rock!

Is there a drawback to using a bass reflex enclosure? The answer is yes. Below the tuning frequency, the enclosure doesn’t control cone motion well.

The violet curve in the graph above represents the typical cone excursion response from a bass reflect enclosure. The cone moves a little more around 55 Hz, but a lot less at 35 Hz, which is the tuning frequency of the vent. This is because most of the sound produced by the enclosure is coming from that vent. Below the tuning frequency, cone excursion skyrockets. Below 30 Hz, we can run into excursion issues. To prevent this from damaging the subwoofer, it’s best to implement a high-pass filter (often called an infrasonic filter, or incorrectly called a subsonic filter) to limit output at these frequencies.

Choose Your Subwoofer Enclosure Solution Wisely

Your car audio subwoofer needs an enclosure to prevent the sound coming from the rear of the cone from canceling with the sound coming from the front. Likewise, it needs an enclosure to control cone motion to prevent distortion at high drive levels at low frequencies.

When you visit your local specialty mobile enhancement retailer to discuss the prospect of upgrading your vehicle with a subwoofer, it’s important to talk about the style of enclosure that will be used. A design that helps to optimize the efficiency of the overall system can make your car stereo sound great while reducing the power required from the amplifier. When executed properly, even a single 10-inch subwoofer can deliver impressive bass response that will make your music sound amazing.

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.