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When people are looking at purchasing a car audio amplifier, the specification they check most often is how much power it can produce. Power is rated in watts – a universal unit of measurement of power. In this article, we explain what a watt is, and how it is measured – both the correct and incorrect way.

Dictionary Time!

Let’s get the formal definition of a watt out of the way first. A watt is an SI (Systéme International) unit of the measurement of power. The power does not have to be electrical. In fact, the unit watt was named after James Watt and created to quantify the work a steam engine could do. In that kinetic application, a watt was the work done when the velocity of an object was moving steadily at 1 meter per second with a force of 1 newton opposing it. When referring to an electrical motor, 1 horsepower equals 746 watts.

As much fun as talking about horsepower is, we are car audio enthusiasts, so let’s get back on track with an explanation of the electrical watt.

In electrical terms, a watt is a transfer of 1 joule of energy over a period of 1 second. The next logical question is what is a joule? A joule is yet another SI unit of measurement, and it defines the amount of work required to move a charge of 1 coulomb through an electrical potential of 1 volt. Yes, the question now moves to the coulomb – what in the world is that? A coulomb is a unit of electrical charge – and is equal to -6.242 x 10^18 electrons.

Lost yet? Don’t fret; we are just appeasing the math and measurement nerds among us. Let’s break this down to what matters.

When we want to use electricity to do work, we have to flow electrons through a device like a filament, motor or voice coil. The result will be, in the case of a speaker, that the magnetic field created by the flow of electrons will cause the voice coil to be attracted to or repelled from the fixed magnet in our speaker. When we flow more electrons, more work is done, and the speaker moves farther toward or away from the magnet.

Power Math

Here is where we start to talk about power equations. There are three common methods of calculating the power in a circuit – but we need to know the values of other variables such as voltage, resistance or amperage. Any two of these variables can be used to calculate the power done in a circuit. Here are the equations:
If we have a circuit with a resistance of 4 ohms and we apply a voltage to it with a potential of 10 volts, then we have 25 watts of power. Increasing that voltage to 20 volts means the power available is now 100 watts. We can substitute and rearrange the variables in the equations above to figure out any other variable – it’s simple algebra.

Measuring Power

When a technician has an amplifier on a test bench and wants to measure power, the technician typically connects the amp to a bank of high-power load resistors, then measures the output of the amplifier when the signal has reached a distortion level of 1%. The measurement taken is voltage. Most often, we assume the load is not variable. Let’s say we measure 44 Volts RMS out of an amplifier and we have the amp connected to a 2 ohm load. That works out to 968 watts. It’s very simple and very repeatable – but it doesn’t work in the real world. Let’s look at why.

Resistance versus Reactance

This is going to get a bit technical. Audio signals are alternating current (AC) signals. AC signals are required to make the speaker cone move back and forth from its rest position, but they make power measurement much more complicated. The way conductors and loads react to AC signals is different from direct current (DC) signals.

Because AC signals change direction, the polarity of the magnetic fields they create also changes direction. Trying to change the polarity of magnetic fields wreaks havoc with the behavior of current flow. Once current gets flowing and sets up a magnetic field, it doesn’t like to stop. Imagine a DC voltage – all the electrons are moving in the same direction all the time. They are happy and have no complaints. When it comes to AC signals, though, that flow of electrons has to change directions. With a 20 k Hz signal, the change of directions happens 20,000 times a second. Electrons are lazy – they like to keep doing what they were doing. Because of this, they oppose a change of direction.

An inductor is truly nothing more than a coil of wire. We see inductors in passive crossover networks and the filter stages of Class D amplifiers. When electrons are flowing through an inductor, they set up a strong magnetic field. When you take away the voltage source, the electrons try to keep flowing. In fact, if you have seen a relay with a diode connected to it, that diode is there to give that flow of electrons somewhere to go, other than back into the circuit that was controlling the function of the relay.

When we apply an AC signal to an inductor, the higher the frequency, the harder it is to change the direction of the flow of electrons. The resistance to the flow of alternating current is called inductive reactance. Think of it as resistance, but only applicable to AC signals. Inductors oppose a change in current flow. If we disconnect our alternating current source and measure the DC resistance of an inductor with a multimeter, the number we see on the screen is the resistance. To measure the reactance of an inductor, we need a device that can apply an AC signal and measure the effective voltage drop across the inductor.

The formula to calculate inductive reactance is Xl = 2 x pi x F x L, where F is the frequency of the applied AC signal, L is the inductance value of the inductor measured in henries and Xl is the inductive reactance in ohms. You can see that inductance increases with frequency, as we mentioned earlier.

The voice coil of a speaker is and acts as an inductor.

Current and Voltage

We have more bad news for you. Because an inductor opposes the change in current flow, a timing error arises. Timing of what, you ask? The relative time between the AC voltage across the inductor and the AC value of the current flowing in the inductor. In a perfect inductor (one with no DC resistance), the current through the inductor lags the voltage across the inductor by 90 degrees or ¼ of the frequency of the signal being passed through.

Let that sink in for a second, then think back to our equations for power. Power is voltage times current. But what if the current peak isn’t happening at the same time as the voltage peak? We can’t simply multiply the two numbers together to get the power in the circuit. Worse, the amount of time that the current lags voltage depends on the DC resistance of the inductor and the inductive reactance – for most car audio speakers, the DC resistance is usually somewhere between 2 and 8 ohms. The inductance is in between 0.04 mH for a high-quality tweeter to more than 5 mH for a big subwoofer.

There’s one more challenge: The inductance changes depending on the drive level of the speaker and the position of the speaker cone.

We’re sure you agree – It’s all very complicated, but don’t give up just yet.

How do we measure the real power in an AC circuit? There are a couple of ways. We can measure instantaneous current and voltage at a very high sampling rate and multiply them together. The sampling rate would have to be 20 or 30 times the frequency we measure to be reasonably accurate. We can also use conventional meters to measure the amount of current and voltage in the circuit, then use a Phase Angle Meter to find the relative relationship between the two. Pretty much none of us have a standalone phase angle meter in our toolboxes. What we can’t do is just multiply voltage and current times each other.

Those SPL Guys And Watts

If you are reading this, then you likely roam the Internet with some frequency. You have undoubtedly seen SPL enthusiasts attempt to measure the power produced by their amplifiers by “clamping”’ it. They connect a current clamp to one of the speaker wires coming out of the amp and put a voltmeter across the terminals of the amplifier.

This creates three problems:

1. They should connect the voltmeter to the speaker terminals. Because of the high current flow, the resistance in speaker wire can waste a measurable amount of power.
2. With a voltmeter and current clamp, we don’t know the phase relationship between the current flowing through the voice coils and the voltage across the voice coil.
3. They typically perform these tests at extremely high power levels. The massive amounts of power heat up the voice coils quickly. This heat also increases their resistance quickly. This increase in resistance will cause the current flowing through the speaker to decrease. If the connected current clamp is in “peak hold” mode, it will store a peak reading of the initial current flowing through the voice coil. The reduction in current flow eases the load on the amplifier power supply and allows it to produce more voltage. As current decreases, the voltage out of the amplifier may increase, giving a false reading to the voltmeter in peak hold mode. This heating and resistance increase can happen in a matter of seconds.

If you thought our definition of the watt was complicated, then explaining how to calculate power in a reactive load would push you over the edge, so we won’t explain it all. That’s a topic saved for college or university courses on AC power. What we will do is provide a solution for making complicated power measurements.

The reality is when it comes to measuring power out of an amplifier while connected to a speaker, getting accurate results is very difficult. A few companies produce car audio power meters. The most popular unit is the D’Amore Engineering AMM-1. The AMM-1 is a handheld meter that simultaneously measures current and voltage, and calculates the phase angle between them to provide an accurate power measurement. The AMM-1 will show you how much real-world power your amplifier is making. (Please don’t cry if it’s less than you thought.)

The AMM-1 can also show volt-amps. Volt-amps are calculated by multiplying current times the voltage. You can also see the phase angle of the load on yet another screen. If you are serious about measuring power when an amplifier is driving a reactive load like a speaker, then this is the tool you need.

What You Need to Know

When you are shopping for an amplifier, the numbers you usually see quoted are measured into resistive loads. Most amplifiers have no problem with driving reactive loads, so you can trust the published numbers, as long as the distortion specification is clearly defined.

The CEA-2006A (now called CTA-2006A) specification for power measurement defines the maximum signal distortion during measurement as being 1%, and no more than 14.4 volts can be supplying the amp. Comparing power specs using this standard has leveled the playing field in the car audio industry.

We will look at some other very important amplifier specifications in another article. These other specifications may, in fact, be more important to choosing the right amp for your system than how much power the amp makes. Until then, drop into your local car audio specialist retailer to find out about the latest amplifiers available for your system. There are some amazing new amps on the market with a lot of cool features.

Happy listening!

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 purchased a set of premium car audio speakers from a respected mobile electronics retailer in the past few years, then you should be familiar with the concept of sound deadening. If you aren’t familiar with this, or want to know more, then read on! We think you will find sound deadening is an often-overlooked upgrade that has more benefits than most people are aware of.

What Is Sound Deadening?

Automobile manufacturers apply small sheets of dense asphalt or butyl-based material to the floor, firewall or door panels of their vehicles. This damping material adds mass to the panel, making it more difficult for sound and vibration to move the panel and transfer sound into the interior of the vehicle. Automakers walk a fine line between adding weight to a vehicle to reduce noise versus losing fuel economy and handling characteristics due to this added mass. For this reason, most don’t go overboard with sound deadening. They are missing out on a great opportunity.

In spite of what they say in their marketing materials, manufacturers don’t really put that much emphasis on their audio systems. Even when vehicles include multichannel systems with well-recognised namebrands like Bose, Lexicon or JBL, little effort is put into maximizing the performance of the speakers. Proper application of sound deadening can have a dramatic effect on the performance of an audio system.

Aftermarket Deadening Materials

One of the first companies to actively promote sound deadening was Dynamat. Dozens have since followed suit with different approaches to controlling noise inside the vehicle. All of them work on the same principle of absorbing sound energy in one fashion or another and preventing it from being transferred to the interior of the vehicle. Sound deadening has two main benefits when it comes to car audio – exterior noise blocking and audio system performance improvement by preventing backwave cancellation.

When you look at the inside metal skin of a car or truck door, you can see that there are openings to allow access to power window motors, door handles and other components in the door cavity. These openings are typically covered with a thin sheet of plastic. The purpose of the plastic is to keep water away from the interior door panel. That’s important, of course, but these openings work against your efforts to get good sound from your new speakers. There is just as much sound energy being produced from the rear of the speaker as there is from the front. If this rearward-facing sound is allowed to mix with the sound coming from the front, they cancel each other. The result is poor bass and midbass response. Sealing up these openings with a layer of sound deadening means the energy being produced by the rear of the speaker cannot mix with the frontal energy.

Just how dramatic can this cancellation affect be? We have seen instrumented measurements of a factory 6×9” speaker where the difference between having sound deadening or not produced an increase in output of up to 8 dB at several frequencies between 100 and 500 Hz. If you think about how much additional amplifier power it would take to produce the same increase in output, that’s more than six times are much. To be clearer, if you put 10 watts of power into the speaker and measured the response, you would need 63 watts of power into the same speaker to get the same output without the sound deadening. As you can see, that’s a significant difference, and the benefit is not just in efficiency, but in improved low frequency output. The speaker doesn’t have to work as hard, and that alone will improve the overall sound of your system.

It is well worth noting that an upgrade in speaker quality will not produce the same improvement in performance. With a properly sealed and damped door, an inexpensive speaker can easily outperform speakers costing five to 10 times as much money. Sound deadening is critical to the performance of an audio system.

Signal To Noise

The second benefit of sound deadening is in keeping the interior of the vehicle quiet. When you make the interior quieter, the benefit is two-fold. Driving is more comfortable, since you hear less road, wind and tire noise. This reduction in noise also makes it easier to hear your audio system. You don’t have to turn it up quite as loud to drown out the remaining noise. You can hear the quiet parts of your music more easily. Your Bluetooth hands-free system will also sound better. In the same way that controlling backwave cancellation reduces the need for a speaker to work hard, having a quieter interior does the same.

Kinds Of Deadening

There are many different kinds of sound deadening. The most popular are butyl sheets bonded to a thin aluminum layer. The combination works well to span large openings, but is thin and flexible enough to adhere to complex shapes. Other materials are made of vinyl and asphalt-based.

There are three key considerations when looking at different sound deadening products: How flexible is it? How thick is it? How well does it stay adhered once installed? On the engineering and development side, testing the damping characteristics at different temperatures can show quite varied results. Some materials don’t work as well in high or low temperatures. We have seen many people attempt to use materials not specifically designed for automotive applications. When the material melts and ends up as a gooey, black mess at the bottom of your door or leaks onto your carpet, the cost to repair the damage can be significant.

There are also several products on the market that add a layer of foam to the top of the aluminum layer. This foam is great when used between the inside door skin and the metal door because it eliminates buzzes and rattles.

See Your Specialist Car Audio Retailer To Learn More

The next time you are driving by a specialist car audio retailer, drop in and ask about sound deadening. Many people have chosen to apply sound deadening to otherwise stock vehicles. We guarantee the difference in performance from the audio system, combined with the increased comfort while driving, will be well worth the investment.

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.

For decades, there has been discussion after discussion about which of the different subwoofer enclosures are “the best” and why. Let’s take a look at why we need a subwoofer enclosure at all, and how the three popular styles – sealed, vented and bandpass – differ in their design and performance.

Back-Wave Management

If you were to hook any speaker up to an amplifier, hold it in your hand and play music into it, you would find that you don’t hear any bass. That is because the sound coming from the front of the speaker cancels out the sound coming from the back. We need a way to keep the sound coming from the back of the speaker cone from interfering with the sound coming from the front. If you were to cut a hole in the middle of a large, flat piece of wood and mount the speaker in it, you would hear a lot more bass. In fact, until the half-wavelength of the bass frequencies becomes longer than the dimensions of the piece of wood, you will get really good, solid bass. If we put a speaker in an airtight enclosure, none of the sound coming from the back interferes with the sound coming from the front.

Power Handling

The ability of a speaker to use the power produced by an amplifier is limited by two criteria – how far the speaker cone can move and how much heat the voice coil of the speaker can handle. Thermal power-handling limitations are based primarily on the design of a speaker – the size of the voice coil, how airflow is managed around the voice coil and the proximity of the stationary components of the motor assembly to the voice coil are the key contributing factors. The excursion-limited constraints are also part of the speaker’s design – how long the voice coil winding is, how tall the top plate is and how much suspension travel is available are the key factors.

Excursion

When it comes to reproducing bass, a speaker has to move four times as far each time the input frequency is halved. For example, a speaker moving 0.125 inches at 100 Hz has to move 0.5 inches to reproduce the same output level at 50 Hz and 2 inches at 20 Hz. You can see that, for the lowest of frequencies, cone excursion limitations are significant – very few speakers can move 2 inches without significant distortion.

When we put a speaker in an enclosure, the combination of the enclosure and the speaker create a high-pass filter. We are effectively decreasing the low-frequency output of the speaker. Why would we want to do this? The benefit of an enclosure is that we can control the motion of the speaker cone. Looking at a simple acoustic suspension (also known as a sealed) enclosure will be the simplest illustration of this explanation.

Compliance

Each and every speaker – from the biggest of subwoofers to the smallest of tweeters – has a springiness to the cone. We call this the compliance. We measure compliance by comparing it to a volume of air with the equivalent springiness. We call this characteristic of the speaker Vas. In general terms, a speaker with a very small Vas specification has a tight suspension, and a speaker with a large Vas has a softer suspension. There is a lot more to it than that, but for the discussion of enclosure features and benefits, that’s all we need to get into for now.

When we put a speaker in an enclosure, we stiffen the suspension. When you push in on the speaker cone, you are pushing against the speaker’s suspension (which wants to center the cone) and you are trying to pressurize the air in the enclosure. When the cone tries to move outward from rest, you are putting the air in the into a vacuum state – it wants to pull the cone back to its resting position. We do sacrifice low-frequency output, but we gain significant power handling and control over the motion of the speaker cone. For the latter, the combination of the air in the enclosure and the speaker suspension helps to stop the speaker cone from moving once an electrical signal starts it in motion.

Think of it like a shock absorber on a vehicle. You can see that having an enclosure is critical.

Acoustic Suspension Subwoofer Enclosures

The simplest of enclosures is called an acoustic suspension or sealed enclosure. In these enclosures, we are putting the speaker into an airtight box. When we put a speaker in an enclosure, the system resonates at a specific frequency that – we call this Fc. Below that frequency, the output is reduced at a rate of -12 dB per octave. If the system has a resonant frequency of 50 Hz, the output will be 12 dB quieter at 25 Hz.

Acoustic suspension enclosures are amongst the smallest of the different enclosures we will discuss. They are also the easiest to construct, and most forgiving regarding calculation error. If you combine the roll-off of the enclosure and speaker system with the increase in efficiency you get from the relatively small air volume of the vehicle interior (often called transfer function or cabin gain), you can get a very flat in-car response with good infrasonic output. Bass from an acoustic suspension enclosure is very tight and controlled, thanks to excellent transient response.

There is a down side. If you are looking for loud bass, then you need a driver that has a lot of excursion capability, and you need a reasonable amount of power to move the speaker cone back and forth to get the level of output you want. There is another drawback that isn’t talked about as much, and that is distortion. As a speaker increases in excursion, the amount of distortion it creates increases. Likewise, distortion increases near the resonant frequency of the speaker. So, what can you do?

Bass Reflex Subwoofer Enclosures

A bass reflex (also known as ported or vented) enclosure uses a vent to increase low-frequency output by making use of the speakers back-wave energy. The vent, often a round tube or sometimes a rectangular slot, has an area and a length. The specific area and length of the vent and their relationship to the total volume of the enclosure cause the column of air in the vent to resonate at a specific frequency when excited by the speaker. We typically tune bass reflex enclosures quite low to emphasize the bottom octave or so of the audible frequency range. They can be tuned higher to increase efficiency for high-SPL applications. There is always a sacrifice, though – when we tune the enclosure higher, we sacrifice low-frequency performance.

Bass reflex enclosures are typically larger than sealed enclosures. There is no hard-and-fast rule to associate with the size relationship, but 25–50% large is common. The trade-off for that extra volume is two-fold – more efficiency in the tuning frequency and more power handling, at some frequencies.

When the subwoofer used in a bass-reflex subwoofer enclosure produces frequencies around the resonant frequency of the vent/enclosure combination, the driver excursion is reduced to almost nothing and all the “work” is done by the vent. Put more succinctly, around the tuning frequency, most of the music is being produced by the vent. The benefit to this is that power-handling problems caused by cone excursion limitations are dramatically increased. Since the cone is barely moving, very high sound pressure levels can be achieved. Around the tuning frequency, power handling is limited by the thermal capabilities of the subwoofer.

As we mentioned earlier, one factor that contributes to loudspeaker distortion is cone excursion. With a bass reflex enclosure, the driver moves significantly less than with an acoustic suspension enclosure design. As long as the vent itself has enough area and a smooth transition at both openings, the distortion produced by a properly designed bass reflex enclosure can be impressively small.

Nothing is free, is it? A factor in deciding to use a bass reflex design is how fast the output decreases below the tuning frequency. Where an acoustic suspension enclosure rolls off at -12 dB per octave, a bass reflex enclosure rolls off at 24 dB per octave. Below the tuning frequency, the vent acts more and more like a hole in the enclosure, offering increasingly less back pressure as frequency decreases. Designing for, and managing, driver excursion is a fundamental part of bass reflex enclosure design.

Bandpass Subwoofer Enclosures

We will quickly touch on bandpass enclosures to wrap up this article. There are several different designs for bandpass enclosures. Some use a sealed enclosure, and some a vented one. Independent of whether the rear chamber is sealed or vented, the output of the subwoofer plays into a vented enclosure. This enclosure acts as a low-pass filter. Why would we want to design a bandpass enclosure?

First and foremost, all of the output of the enclosure is produced by the vent or vents. This allows a creative designer to build an enclosure in the trunk of a vehicle and have the vent opening play through the rear parcel shelf. There have been some amazing bandpass enclosures build in the front storage area of mid- or rear-engine vehicles. The vent allows the bass to enter the interior of the vehicle. Bandpass enclosures can also offer impressive gains in efficiency over acoustic suspension and bass reflex enclosures, but they do so at the sacrifice of bandwidth and enclosure volume.

A bandpass enclosure has two resonant frequencies – one for each of the enclosures. The resultant management of cone excursion can allow a great deal of bass to be produced from limited excursion drivers. While the speaker cone itself does not move a great deal, the amount of work done by the motor assembly is still significant. You are still putting power into the speaker, and work is being done. Because the front chamber of the enclosure acts as a filter, it can also be very difficult to hear when the speaker is distorting.

Regarding the complexity of design, and forgiveness of construction error, bandpass enclosures are the most complicated to execute perfectly. Unlike an acoustic suspension or bass-reflex design, bandpass enclosure designs must be tailored exactly to the speaker they are being used with. Never trust the concept of a “generic” bandpass enclosure.

Lastly, because a bandpass enclosure includes an acoustic low-pass filter, it has to be used with good-quality, appropriately sized midbass drivers. If not, the bass can sound lost or disconnected relative to the rest of the music.

For More Details On Subwoofer Enclosures, Visit Your Local Specialist

As you can see, there are many ways to install a subwoofer – or any speaker, for that matter. Navigating the available space in the vehicle, as well as different speaker sizes and designs, can be tricky. The design and construction of an enclosure can be complex, especially when complex shapes are involved. Visit your local car audio specialist retailer to explore different enclosure options for 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.