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What the FAQ? – Your Subwoofer Questions Answered

What the FAQ? – Your Subwoofer Questions Answered

Posted by Glenn Rowswell on 8th August 2017

Why do I need a subwoofer in my system? What’s better for my sub, a bridged amp or a mono amp? What’s the best size sub to have? Find out the answers to all these questions and more in our subwoofer Q&A…

Here at FC we’re often asked technical stuff via the mediums of email, Facebook and being shouted at in the local Maccie Ds. Now obviously we are pretty amazing but, contra to popular belief, we don’t know absolutely everything about everything.

Luckily though we happen to have a team of industry experts on hand to deal with the really tricky stuff. This month we catch up with top American fellas and JL Audio’s director of training Steve Turissi, to answer all your questions about subwoofers. Boom!

Our hearing can detect sounds between 20 and 20,000 Hertz and music can contain frequencies from across the entire spectrum. While the lower notes may only represent a small number of frequencies, they also require a significant amount of energy to reproduce. This energy, simply put, comes from both an amplifier and the speaker’s ability to move. Standard speakers may be able to play these notes at very low levels, but when you turn the volume up these simply cannot reproduce the sound properly.

A subwoofer is specifically engineered to play these lower frequencies. When set up properly, it can transform the sound of even a very basic audio system, by not only adding authority to those lower notes, but also clarity to the remaining sounds – that’s because the other speakers no longer need to struggle to reproduce everything.

car sub facts info guide

Why do I need an amplifier to have a sub?
An amplifier takes a small amount of electrical signal and makes it ‘bigger’. In the case of music, the signal is read off of a disc or from a file and the small signal is sent to an amplifier to make it more powerful so that it can move larger speakers.

The smaller signal may be strong enough to power earphones or very small speakers, but larger speakers will all benefit from having more power available. Subwoofers in particular will need quite a bit of power. In mobile audio all speakers need some sort of amp, be that the very basic built-in headunit item or an external unit.

Can I run a sub straight off a headunit?
Scientifically speaking you can hook up a subwoofer to the wires from a headunit, but you will not get the performance from the subwoofer that it’ll be capable of delivering. So, technically, the question can be answered: yes, but you shouldn’t.

What’s better for my sub, a bridged amp or a mono amp?
I guess I’m making dangerous assumptions here, but they are effectively the same thing. A bridged amplifier is simply the combination of two individual channels into one more powerful combined channel. Mono amplifiers are just a single channel to begin with.

The reason some multi-channel amps can be bridged is purely down to flexibility. Someone may want to use a two-channel for a pair of speakers, some may want to run two subwoofers and some may want to run a single more powerful subwoofer.

car sub facts info guide

What’s impedance?
In electrical terms, impedance is like resistance. Resistance is the term we use in DC (direct current) circuits to describe the resistance to the flow of electricity. Music is AC (alternating current) and as a result, the electrical resistance will change depending upon the frequency you play and how you play it. The 1-ohm or 4-ohm value you see rated is the nominal or average value.

As the speaker moves, the value changes, but we may still call it a 4-ohm speaker even though the value may be anywhere between, as an example, 3.6-ohms and 60-ohms.

All amplifiers need some amount of resistance connected to them in order to function. Generally the impedance of the speaker is of no consequence to how well it may perform. The amplifier you use, however, will most likely change based upon the impedance of the subwoofer you use. Always check to make sure that the nominal impedance value of the subwoofer(s) you plan to use are within the range of values that the amplifier is rated to work with.

What does a voice coil do and why do some subs have more than one?
When you run electricity through a piece of wire, it creates a magnetic field around that wire. If you coil it up and put it near another magnet, you can make it move. Strap a cone and a frame around it all and you have a very basic speaker. The vast majority of all speakers use this moving coil design.

The vast majority of subs, if not all, use only a single voice coil, despite any names that imply otherwise. When you see ‘dual voice coil’, this is an example of lazy speech. It should actually be ‘dual voice coil connections’, meaning that this single coil has two connection points. It may seem like two coils, but it’s more about configuration. These will both be in the same area of the speaker and should be wired to work together simultaneously – to work together as one coil.

Is a bigger, heavier magnet better?
A wise man once said, the right amount of magnet is the right amount of magnet. In a well-designed speaker, you should use the right amount of magnet for the design, no more and certainly no less. The weight of the magnet is not a vital specification when determining performance capability. It is about as important as the colour.

In a well-engineered speaker, the magnet is part of a delicate balancing act. Too much magnetic material will often lead to a speaker that is ‘over-damped’, meaning you need more power to overcome the magnetic energy of the magnet. Not having enough magnetic material can lead to an ‘under-damped’ speaker, meaning, again in general terms, that the speaker lacks control.

Finally the reason many speakers have very heavy magnets is to do with the quality of the magnetic material. In some cases very low quality material is used and it doesn’t have the capacity needed to hold a strong magnetic charge. To make up for this, some companies will use more of the low capacity material.

car sub facts info guide

What’s the most important part of the construction?
The two most important aspects of the speaker are the designer(s) and the people that build it. That is not a joke. Since every single part of the speaker will have some significant impact on performance, it’s almost impossible to identify one part as the most important.

You might say the voice coil, but I might point out that without the magnet or the cone, the coil does nothing. Similar cases can be made for just about every singe functioning part of the speaker. It is the careful selection of the best parts for the deign that matters most, as well as how it is all put together.

What’s the best size sub to have?
The best size depends upon a few things. High on the list is how much space is available. If you are very pressed for space, a smaller subwoofer is a better option because, in general, it will need a smaller enclosure to perform well. What you don’t want to do is force a larger speaker into a less than ideal enclosure. That will compromise performance and lead to less than optimum results. It’s better to have a smaller subwoofer done right, than a larger one done wrong.

car sub facts info guide

Why do subs need an enclosure?
In the most simple form, at the frequencies that a subwoofer is asked to play, it’s important to isolate the back part of the speaker from the front. Since the speaker moves in and out to reproduce the wave forms, any inward motion would effectively cancel out the outward motion if the two wave forms could meet. It’s also about supporting the woofer cone.

Many speakers are designed expecting a specific volume of air to push against. Air has a springy-ness to it that can be used as an additional suspension for the speaker. By controlling that springy-ness you can alter the performance of a speaker. If you get it wrong, the speaker can either move too far or not move far enough. When a speaker moves too far, it can fail when the surround and spider get stretched to the point that they can tear.

When a speaker can not move far enough it will struggle to play the lowest frequencies accurately as well as some other reliability issues. In short, if the manufacturer of a product specifies a certain application, there is most likely a good reason for it.

car sub facts info guide

What’s the difference between sealed and ported enclosures?
Sealed boxes are simple – build a box with a certain amount of air in it and drop the sub in. If it’s right, you get good, tight response and good power handling. If you get it wrong, you get sloppy/muddy bass or peaky/resonant bass and some reliability concerns. Pros include small size when compared to a ported enclosure. Cons include lower efficiency than other designs.

Ported boxes are a little more complicated. In addition to having to get the air volume right, you also have to get the port diameter and length right. There are a lot of variables to mess with and with such flexibility comes the chance for errors. As a result, most ported boxes are done wrong in some way. When you get them right, you are rewarded with deep, powerful and potentially louder bass than a sealed unit.

What about the JL MicroSubs – what are they about?
The genius of the JL MicroSub is proof that there is a real value in quality design. Many people think, understandably, that these are tuned in such a way as to get the performance of a larger subwoofer. While this is a great compliment, it is a bit misplaced.

In truth, the MicroSubs are designed to get the most out of the speakers used in the enclosure. The fact that they outperform larger speakers has more to do with a sub-optimum design for the larger speaker when compared to the MicroSub. This is not a shot at other manufacturers in any way. In fact our CP108LG-W3v3 (the single 8-inch MicroSub) will hold its own against some of our own sealed 10-inch solutions.

The real magic is that our engineers (specifically Lucio) designed the speakers used in the MicroSub enclosures at the same time they were developing the enclosures. Any changes they had to make to the speaker, they altered the enclosure to match. The result is an incredibly well- engineered system that sounds great and is shockingly powerful for the size. Not by accident. But by very careful design.

How do I know if the power ratings on a subwoofer and amplifier are true?
You’ll usually match your subwoofer to an amplifier with a similar power rating, but in real world terms these figures can be the most misleading and misunderstood specification you’ll find. Trying to decipher what is real and what is printed to make a product look good is one of the most common problems in audio. Even the accepted term RMS, which stands for root mean square, can be misleading.

When it comes to the true power figure for an amplifier, the equation you need to know is Power (Watts) is equal to voltage (Volts) times Current (Amps). If you’re working with a car system (12 Volts) and your amp is rated at 2,000watts RMS it will need around 160amps of current. A lot more than most cars can deliver.

The easiest way to debunk power ratings is to check the fuses. If the fuse in your 1,000watt amp is say, 40amps, then you know it’s not really a 1,000watt amplifier – 480watts (12volts x 40amps) is more like the true figure.

With speakers, including subwoofers, the ratings almost seem arbitrary and in some cases just plain made up. It’s tricky to specify power for a subwoofer though, since music is dynamic, and different enclosures can impact the performance of a speaker. The best any manufacturer can really do is give a guideline as to what amount of power might be the best for most applications. Smaller subwoofers may need a bit less power, while some of the larger monsters may need more.