If you are a mobile electronics enthusiast like we are, then it’s quite possible that you enjoy reading product specifications. You can learn a lot about the design and application of a product from the specs. Amplifier power ratings are probably the most popular specs, but there are a lot more. One product specification that tends to confuse people more than help them is speaker efficiency. This article explains what the numbers mean and how you should analyze the appropriateness of subwoofer specifications, one subwoofer to another.

Speaker Efficiency

In a nutshell, the efficiency specification attempts to quantify how much sound a speaker will produce when provided with a given amount of power. Manufacturers provide the specification in two common methods. The most common are decibels of output when fed with 1 watt of power and measured at 1 meter from the speaker, written as 1 W/1 m. The other specification method involves replacing the 1 watt power measurement with 2.83 volts. For a 4 ohm car audio speaker, 2.83 volts works out to 2 watts of power or 4 watts into a 2 ohm speaker. Make sure to take the impedance of the speaker into consideration with the latter format.

For a subwoofer, we derive this efficiency number with a formula that includes the driver’s resonant frequency, equivalent suspension compliance and driver’s electrical Q. In reality, the resulting number is purely theoretical and applies most directly to output in the frequency range above where most subwoofers play. There is the heart of the matter: The efficiency specification doesn’t describe how loud the subwoofer is at low frequencies. The only way to predict and compare performance is to model the behavior of the driver in simulation software.

Frequency Response Simulation

Let’s compare two 10-inch speakers. For sample A, we will use a very high-quality car audio subwoofer. Since we want to make this example somewhat extreme, we will use a 10-inch pro audio woofer as Sample B in our comparison. We will use BassBox Pro 6 to make the comparisons.

Sample A has a calculated efficiency of 93.21 dB when driven with 2.83 V. Sample B has an efficiency of 95.07 dB. Without modeling the low-frequency behavior of the driver, you’d happily think that Sample B was the louder of the two by 1.86 dB.

We modeled each driver in a sealed enclosure with a volume that provides a total system Q (Qtc) of 0.707. The car audio subwoofer is in 0.694 cubic feet, and Sample B is in 0.378 cubic feet net.

As you can see from the graph, the output of the two woofers varies dramatically. Sample A is louder at 40 Hz by an impressive 4.95 dB. That is contradictory to the efficiency specification, isn’t it? It is, however, not wrong.

Subwoofer Specifications

When an engineer designs a speaker, the first thing to decide is the application. Will this be a high-SPL car audio speaker, or a pro-sound speaker? The differences make a great deal of difference.

Our sample subwoofers also have dramatically different excursion capabilities. Sample A has an Xmax of 17.6 mm and Sample B has an Xmax of only 4 mm. It is worth noting and reminding everyone that cone excursion quadruples for every halving of frequency. If these speakers were given enough power to play 100 dB at 80 Hz with an excursion of 1 mm, then they would need to move 4 mm at 40 Hz. At 20 Hz, they would need to move 16 mm. An excursion requirement of 16 mm is no problem for Sample A, but will likely rip Sample B to shreds.

Comparing the output of two drivers requires that we ensure the driver can handle the excursion requirements necessary to meet our needs.

In the case of our subwoofer simulations, due to excursion limits, Sample B can produce a maximum output of only 94.9 dB at 40 Hz. Sample A can produce 108.3 dB at the same frequency. That is a difference of 13.4 dB. This difference is significant. Sample B simply cannot produce 100 dB of output at 40 Hz in this enclosure. Thus, Sample B not a suitable choice for a subwoofer, which makes sense, since it was designed to be a bass guitar speaker.

Choosing the Right Subwoofer

Unless you own speaker-modeling software and know how to use it, it can be tricky to determine the performance of one subwoofer versus another. Even more difficult is attempting to predict how one sounds when compared to another. Frequency response is just one of the many criteria that differentiate one subwoofer from another. Excursion capabilities, enclosure requirements, distortion characteristics and – of course – cost are all factors to be considered.

Many people think that just because it is harder to hear distortion at low frequencies, the design of a subwoofer matters less than that of a midrange speaker. You would be stunned at how a good subwoofer can bring out details in your music that you may have never heard before.

When it is time to go subwoofer shopping, visit your local mobile electronics specialist retailer. Discuss your needs with them and work with them to find a subwoofer solution that fits your application. You will be happy that you did.

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.

Enclosure, box or cabinet: Whatever you want to call them, where you install your speaker or subwoofer is critically important to their resulting performance. In this article, we focus on the simplest and most forgiving of enclosures to design and construct – the acoustic suspension or sealed enclosure.

The Laws of Physics

There are a few characteristics to keep in mind about every speaker. The first is that as frequency decreases, cone excursion increases. In fact, to produce the same acoustic output, a speaker must move four times as far for every halving of frequency. As an example, if your subwoofer were moving 1 mm at 80 Hz, it would have to move 4 mm to produce the same output at 40 Hz. To produce the same output at 20 Hz, it would have to move 16 mm.

A speaker includes an element called a spider. The spider stores energy when the voice coil of a speaker moves the cone forward or rearward from its resting position. When the cone reaches the end of its travel and comes to a stop, the stored potential energy in the spider wants to be released. This stored energy pulls the cone in the opposite direction. Each transfer of energy includes some losses, and eventually, the cone comes to rest.

Think of the cone motion like a swing at the park. You exert a force on the swing to get it started, and it continues to swing back and forth with a decreasing amplitude until it comes to a stop. Thankfully, a speaker stops moving a lot faster than the swing at the park.

In a speaker, this transfer of energy from the cone to the spider and back is most efficient at a specific frequency. We call this the resonant frequency of the speaker. At the resonant frequency, there is a dramatic increase in impedance because the spider stores a great deal of energy. This energy storage causes the cone to want to continue to move. The movement of the voice coil moving through the magnetic field generates a voltage. This voltage generates a flow of current in the opposite direction to the current flowing from the amp. We represent this opposition to current flow as an increase in impedance.

We also have to consider that every speaker is limited in how far the cone can move. Once we exceed the excursion limitations of the speaker, bad things happen. The voice coil former can hit the back plate. The suspension components may be compromised and start to fail. As a by-product of the cone, dust cap, surround, spider and motor geometry, harmonic distortion also increases as excursion increases.

Our goal in designing any audio system should be to keep distortion as low as possible. Most of the distortion at low frequencies is resonance. These resonances decrease as we move above the resonant frequency of the speaker. The spider and the changing motor force, as the coil moves past the edge of the gap, are the biggest contributors to distortion.

Why Do We Need an Enclosure?

Let’s consider a few additional characteristics. The low-frequency roll-off of a speaker is a high-pass filter. The spider in the speaker is like a capacitor—a spring stores energy and so does a capacitor. The air inside the box is also a spring, and it is in parallel with the spider. The air spring and the spider work together at the same time to do the same thing. The combination of the air spring and the spider increases the high-pass filter frequency. Yes: Contrary to our efforts to produce as much low-frequency information as possible, an enclosure limits low-frequency reproduction.

If that is the case, why do we want to limit cone motion? Consider what we’ve said about how much excursion is required to reproduce low frequencies and about distortion. Limiting low-frequency output from our speaker is not an ideal goal, but limiting some of the really low frequencies to get the right amount of bass at higher frequencies is worthwhile.

There is a benefit to increasing the resonant frequency of the speaker and enclosure system. Let us say we have a subwoofer with a Q of 0.5 and it is our goal to have a total system Q of 0.707. We choose an enclosure air volume that increases the Q, which then increases the system output at the new resonant frequency. Yes, we sacrifice output at lower frequencies, but we gain output around the new system resonant frequency.

I Want More Bass!

Modern speaker designs continue to reduce distortion through computer simulation and modeling of material behavior. Qualified and properly equipped speaker designers can simulate spider, cone and surround behavior to analyze individual resonance and distortion behaviors. They also can model the interaction between the voice coil and the motor structure to predict changes in magnetic field strength and inductance that can further affect how a speaker will sound at moderate to high excursion levels.

These advancements have resulted in speakers that produce less distortion at higher excursion levels. This improvement in performance allows enclosure designers to build speaker systems that will play lower and louder.

Some basic principles govern low-frequency sound reproduction. Cone area is critical. An old article published by the Audio Engineering Society called “The Problem with Low-Frequency Reproduction,” by Saul J. White, included a graph that compared cone excursion vs. frequency vs. system output for a 12- and 15-inch loudspeaker. In the chart, it shows that a 15-inch driver cone only has to move half as much as a 12-inch driver to produce the same output.

To produce sound, we need to displace air. Displacement is calculated by the product of speaker cone area times the distance the cone can travel. In other words, bore times stroke. For the same displacement, more bore requires less stroke.

What is the punch line? If you want it louder, buy more speakers or subwoofers.

Driver Behavior in an Enclosure

The increase in the system Q caused by the addition of air stiffness in the enclosure can cause distortion if the Q is increased excessively. This increase in Q works against our desire for a low-distortion system. Making the enclosure too small increases the Q too much, and we wind up with a system that produces a great deal of output in a narrow frequency range. These undersized enclosures are often referred to as a “one-note-wonders.”

What causes this behavior? The one-note quality is a result of the increased energy storage and transference in the resonant system. The bass just keeps going and going – like our swing at the park.

Power Handling

In an acoustic suspension enclosure, cone excursion increases as frequency decreases. This increase in excursion continues down to the frequency at which the force of the spider and the box exceeds the force of the motor. At that point, the excursion level is limited, and we will not see the increase in excursion . The result: We protect the speaker from physical damage due to cone excursion beyond the design characteristics of the speaker.

Predicting the limits of cone excursion relative to frequency and power is relatively simple for a sealed enclosure. The volume of the enclosure is inversely proportional to the amount of power the speaker can handle when perceived from the standpoint of excursion. A small enclosure limits cone excursion a great deal at very low frequencies, but the system does not produce a lot of deep bass. A large enclosure allows the speaker to move further and produce more low-frequency output, but we cannot drive the speaker with as much power for fear of damaging it.

As we increase the volume of the subwoofer enclosure, the air inside has less “spring effect” on the subwoofer’s motion. This graph shows the increase in driver excursion as air volume increases in four different enclosures.

Acoustic Suspension Overview

An acoustic suspension speaker enclosure reduces bass output at a rate of -12 dB per octave below the resonant frequency. When you combine this roll-off with the cabin gain associated with most vehicles, you can get excellent and linear low-frequency extension well into the infrasonic region. Acoustic suspension enclosures are easy to calculate and to construct. They are very forgiving of minor errors in volume calculation.

Finally, it is worth remembering that acoustic suspension enclosures are not the lowest-distortion enclosure designs available.

When it comes time to design a subwoofer enclosure for your car or truck, visit your local mobile electronics retailer and discuss your requirements. They can help you choose a subwoofer and enclosure design that will give you a solid foundation on which to build your audio system.

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.

When an audio enthusiast goes shopping for an amplifier, the question of “how much power do I need?” comes up almost every time. There are a few factors to take into consideration when answering this question. This article looks at those factors and provides some technical background to help support your decision.

Why Do We Need Power?

Quite simply, when you send more power to a speaker, it moves farther and produces more output. Two limiting factors within the speaker itself control how much power it can handle. At higher frequencies, the limit is heat. Speakers are notoriously inefficient. The best convert about 2% of the energy sent to them into sound and the rest is converted to heat. When you send 60 watts of power to a speaker, most of that energy heats up the voice coil and the components around it. Eventually, those components will reach a temperature where they will fail. The speaker will usually stop working at this point, or shortly after.

The second limiting factor is how far the speaker can move. Inexpensive midrange speakers may be able to move back and forth about half an inch without creating massive distortion. Higher-end speakers have as much as twice as much cone excursion capability. (Speakers don’t sound the same at high volumes as they do at low. Audition your speakers at the volume you will be using them.)

Power vs. Output

Power works like this: When you double the power going to a speaker, the output increases by 3 dB. That is not a large amount. In fact, it is the smallest change in amplitude that is perceivable across the audible frequency range. (1 dB is the smallest perceivable change in amplitude where our hearing is most sensitive – 1 to 2 kHz).

Perceived volume is a different beast. An increase of 10 times the power sent to a speaker produces a doubling of perceived volume.

Speaker Efficiency

Another consideration in choosing an amplifier is the efficiency of your speakers. An inexpensive conventional midrange speaker may produce an average output of 91 dB when measured 1 meter away from the speaker cone and when driven with 1 watt of power. A high-quality speaker will likely be less efficient, but capable of playing over a wide range of frequencies. A measurement of 85 dB efficiency at the same distance and power level is not uncommon.

How Loud Do We Need it?

A typical RMS sound pressure level for an orchestra, when you’re seated three or four rows back from the musicians, is around 100 dB. If we use our analogy of the 85 dB efficient speaker, we need 31.6 watts to get that speaker to play 100 dB. The problem is that this is the average power, not the peak power. Perhaps the performance crests at 110 dB? In that case, we need a peak power level of 316 watts. Just keep in mind that the speaker components are likely to melt if you keep this effort up for any significant amount of time.

We don’t suggest buying any speaker based on its efficiency. Criteria like linearity, lack of distortion, application limitations and frequency range are far more important. If you need it loud, buy more speakers, or larger speakers.

Distortion Happens

What happens if we run out of power in an amplifier? We get distortion. This distortion creates all sorts of high-frequency harmonic content. That increased high-frequency energy is what causes tweeters to fail. We need to choose an amplifier that will allow our speakers to play loudly enough without running out of power.

You are better off buying a 100 watt per channel amplifier and only using 50 watts than you are buying a 50 watt amplifier and occasionally causing it to distort. Remember, those 50 extra watts only result in an increase in output of 3 dB – assuming the speaker can handle it.

It Takes Power to Make Power

A consideration that many people overlook is the ability to supply an amplifier with the power it needs to produce the power you want. Modern vehicles have electrical systems with reduced power production capabilities. Smaller alternators, smaller batteries and smaller wiring save weight. Reduced weight transforms into better fuel economy for the vehicle.

As a general rule of thumb for power consumption calculations, you can assume that every 100 watts of power from an amplifier will require about 10 amps of current from your electrical system. Yes, some amplifiers are more efficient than others, but this serves as a good, quick guideline.

If you want to purchase a 650 watt amplifier to power your subwoofer, then your electrical system (battery and alternator) has to be able to provide it with about 65 amps of current. This power requirement is on top of what is required to run the vehicle. The computers, lights, ignition system, radio and heater all consume power as well. On a modern compact car, it would be no surprise if you only had 30 to 40 amps of power left over for an amplifier.

You can get away with a big amplifier – but you can’t play it indefinitely, even with the vehicle running. Once you have exceeded the power delivery capabilities of the amplifier, the battery will start to supply current. You can kill a car battery, even with the vehicle running. Once you shut the car off, you may not have enough energy in the battery to restart it.

Blowing up Amplifiers

Amplifiers do not like to be starved for power. When you run out of power to drive your amplifier, in most cases, the amplifier rail voltage starts to drop. Power starvation causes the maximum undistorted power production of the amplifier to decrease. We are back to the same scenario: Distortion causes harmonics, and harmonics can damage fragile speakers.

If you have had an amplifier fail, and the failure was because the power supply section of the amp self-destructed, chances are you were not able to feed the amp properly.

How Much Amplifier Power Do You Need?

The solution: Buy as much as power as you can afford. Buy the biggest that will physically fit in your application. Get the highest-performance amplifier you can. Make sure your installer uses properly sized wiring to install the amplifier. Upgrade your car battery to a high-performance, high-capacity unit if you need more reserve power.

For more information, visit your local mobile electronics specialist retailer. Be honest about your needs and expectations for your audio system. They will be able to suggest a solution that sounds fantastic and will offer years of reliable performance.

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.

You cannot have talked about audio and computers any time in the last 15 years and not have heard of an MP3 file. MP3 audio files and websites, like the original Napster, started a shift in where, how and when people acquired music. If you are on the older end of the spectrum, like many of us in the mobile electronics industry, then you bought your CDs, cassettes and maybe even your vinyl at a record store. Computers and the Internet changed that. You could go online after dinner and download an illegal copy of a song in a few minutes. It was wrong, but people acquired tens of millions of songs this way.

In the 1990s and early 2000s, accessing the Internet was slow. We started connecting to the Internet using phone lines and modems. Each byte of information took time to transfer to your computer, so anything that would speed up the process was a treat. Downloading (stealing) music using the Internet is where the popularity of the MP3 audio file met its calling.

A Primer on Digital Audio

We could write 10 articles about digital audio – and we just might. For now, we are going to look at the basics and use the compact disc (CD) as our reference. CDs store digital audio sampled at 44.1 kHz with a resolution of 16 bits. These numbers mean each sample can have an amplitude that is a single value within a range of 65,536 different levels (2 to the power of 16). The information is sampled 44,100 times a second. Sampling at what is known as 44.1/16 allows capturing the audible range of audio (considered 20 Hz to 20 kHz) with good detail and accuracy.

To store 1 second of audio at this resolution, we need to store 1,411,200 bits of information. Anyone who has played with audio transcoding software may recognize 1,411 kbps as a standard data rate. This number is calculated by multiplying the number bits per sample (16) times the number of samples per second (44,100) times 2. The times-2 factor is because we record in stereo – which is two channels. So, a 3-minute long song is 254,016,000 bits or 31,752,000 bytes.

Let’s round it off to 31 megabytes of information. Can you imagine how long it takes to download that with a dial-up modem running at 14,400 baud? The answer is at least 3.5 minutes – without error checking, line noise and other factors that slow the real download time to about 5.5 minutes.

Data Compression

What if someone found a way to shrink the size of the audio file to speed up download time and reduce bandwidth usage? The caveat is that the audio still sounds essentially the same on most basic audio systems, such as a TV, computer speakers or a 1990s factory car radio. In 1991, a group of companies, including the Fraunhofer Institute, France Telecom, Philips, TDF and IRT, started working on a way to reduce file size while maintaining relevant information. That is the key to how file size is reduced using MP3 compression.

The MP3 file format is a “lossy compression” algorithm. Lossy compression means that information is thrown away to reduce file size. The development team worked on a compression method called perceptual encoding to decide what information to remove. Perceptual encoding is based on how we hear sounds relative to other information, and the limits of our hearing.

What MP3 Files Throw Out

We are going to analyze the information that MP3 files remove to reduce file size. One of the easiest ways to cut back on information storage is to reduce the highest frequency that will be reproduced. If we analyze a 128 kbps MP3 file, we see that the highest reproduced frequency is just below 16 kHz. If that were the only information that was removed, our new bitrate with 16-bit samples in stereo would be about 1,004,800 kbps instead of 1,411,200 kbps for 20.05 kHz.

The next part of the compression process analyzes content that is common to both channels. It is common for some parts of a recording to be virtually in mono. The encoding process removes duplicated information from the file and adds code to copy the opposite channel. If the audio track were purely mono, the file size would be divided in two. Few tracks are completely mono, but we can see more space saving from this process.

Subsequent processing looks at low-level information during high-amplitude passages. Let’s use the example of a song with a lot of bass in it and some very quiet harmonic midrange information. Perceptual encoding processes like MP3 will remove this low-level information from the audio track. This process is called audio masking. There is enough audio information at other frequencies to distract you from hearing what is removed.

Can You Hear the Difference?

Dozens – nay, hundreds – of tests have compared MP3 files to full CD-quality audio tracks. Are there differences? There most certainly are. One thing became apparent during our research: How an MP3 file is created is crucial to its subjective sound quality. Different encoders work in different ways with different results.

Perhaps the best way to describe the difference between a CD-quality recording and an MP3 file is to look at the difference between the two. I wish we could share some samples here for you to listen to, but that would break copyright laws. What we can do is visually show you the difference.

We took a 3-second sample from Daft Punk’s “Give Life Back to Music.” We chose this track because of Daft Punk’s clear and conscious effort to make a high-resolution version of the album commercially available. We want to thank them for that! The sample is from 31.5 seconds to 34.5 seconds into the song.

This Spectrogram shows the frequency content of the sample. The horizontal scale is time. The vertical scale is frequency. Finally, the color intensity shows the amplitude.

You can see that there is frequency content up to 30 kHz, clearly demonstrating the high-resolution nature of this track. Each vertical color band represents a drum machine beat – more or less.

128 kbs MP3 File Analyzation

It is clear that audio information above 16 kHz has been removed. Infrasonic frequency content is clearly different as well. There is more information in the MP3 file below 30 Hz compared to the original. This increase in information will, however, present itself as less-dynamic range.

MP3 Vs Original File

We inverted the MP3 file and added it to the original sample to make the image you see here. The net result is the difference between the two tracks. You can see the high-frequency content that was removed above 16 kHz. In fact, information was removed at all frequencies, and that information follows the intensity pattern of the audio file.

The original file has a peak amplitude of -0.1 dB for both channels and an average amplitude of about -14.2 dB. The removed information has a peak level of -10.9 dB and an average amplitude of -37.01. The removed information is buried deep below the peak amplitude information.

What does the removed audio sound like? We would describe the clip as the sound of a distant marching band. The audio is mostly high-frequency information. The track has a decidedly warbled texture to it as well: The drum machine beats are clear and present, but they sound like distorted cymbal hits.

Even with a high-end headphone preamp and studio grade headphones, the difference is hard to perceive when switching between the original track and the MP3 file. In a listening environment with a larger soundstage, it may be more apparent.

Conclusions about MP3 Files

Purists will tell you that you should have the highest-quality recordings available. There is no fault to this logic. Why skimp when you can have it all? High bitrate MP3 files, like those at 320 kbps, for example, are excellent in quality. Repeated testing has shown that when created with quality compression algorithms, the sound difference between a CD-quality recording and a 320 kbps MP3 file is almost impossible to detect. Lower bitrate MP3 files start to dispose of more information, and the differences become bigger.

The latest source units on the market are capable of playing WAV and FLAC audio files of great resolution and bit depth. Shortly, we will see units that will play MQA files over digital connections. Almost every source will handle MP3 and WMA files.

Drop into your local mobile electronics specialist retailer today, and bring along some music to enjoy. We think you will be impressed – no matter what format you choose.

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.