Speed & Sound: Plugged In – A First Look at Subwoofers
Text by Dave MacKinnon (www.osmlabs.com), Photos by the Manufacturers
Last year, we took a look at how speakers worked, then at what separated the also-rans from the truly great products. In this installment of Plugged In, we are going to delve into the topic of subwoofers.
If you ask the average guy or girl on the street what a subwoofer is, they’ll tell you one of two things – it’s either a smart answer something like “Oh, that’s the bass part in my home theatre” or something less flattering like “those darn kids boomin’ down my street at all hours have those.” While both are right, they are far from a technical answer – which, of course, is where I come in.
A subwoofer is a speaker, typically fairly large in size, that is designed to reproduce the bottom two octaves of the audible spectrum. If we consider the audible range of sound to be 20Hz to 20kHz, then a subwoofer covers 20Hz to 80Hz. In a later issue of Plugged In, we’ll look at high-end subwoofers and explain how some, combined with their use in a car, can play down another octave.
So, then, is a subwoofer important to accurately reproducing music? It most certainly is – high fidelity means that we want to reproduce music as accurately as possible with respect to the original recording. I did a quick dig on the Internet to get some backup. For those of you with an orchestral background, then there are a few instruments that would be lost in a performance without a quality subwoofer. The contrabassoon, for example, plays down to an impressive 29.1Hz, and a Harp (which I didn’t know) plays down to 30.9Hz. The lowest A note on a piano is 27.5Hz, and a Bosendorfer Imperial Grand Piano and most quality church pipe organs will play to a bone-rattling 16.35Hz. Mmmm, I get shivers of delight just thinking about it!
So, what differs in the design of a subwoofer as compared to a regular speaker? Other than their physical size, the key change is the distance the speaker cone must move to do its job. For every octave lower a subwoofer must play, cone excursion quadruples. So, if a subwoofer must move 1mm back and forth to play 80Hz at a specific volume level, it has to move 4mm to play 40Hz at the same level, and a whopping 16mm to play 20Hz. Which is why some won’t play 20Hz very well – they can’t move that far.
A key and critical factor with respect to subwoofer performance is the enclosure in which it is installed. Because the cone moves quite far, the air inside the enclosure is pressurized and then put under vacuum. So the amount of air inside the enclosure has a dramatic effect on the subwoofer cone’s ability to move. We won’t get into the math behind that right now, but know that the design of the enclosure you choose and the quality of its construction is as critical to its performance in your car (or home) as the design of the subwoofer itself.
Since we know it takes lots of cone excursion to move a subwoofer, and we know that it takes lots of power to make a speaker cone move, then we can quickly surmise that subwoofers need to handle lots of power. Foolishly, people continue to rate speakers based on their power handling capabilities. It would be like rating a car’s handling based on its top speed capabilities – fine for a Veyron, but the plan falls apart when John Force deep stages his Mustang in front of your house but plans on making a U-turn at the end of the street and coming back.
The problem with putting lots of power into a speaker is that, sadly, speakers are really inefficient. Most subwoofers convert about one percent of the energy you put into them into sound. This means, a sub getting 100W of power is making 1W of musical energy and is heating the car with 99W of thermal energy. When things get hot, they fail. So, the parts used to build a subwoofer need to be able to withstand a great deal of heat. Most subwoofers are designed to allow air to flow in and around the voice coil to help move heat away from it, so it won’t overheat the varnish that keeps it neatly wound together.
What separates one subwoofer from another? Its design. Most are made of good materials and most can move back and forth well, to some extent. Where one subwoofer differentiates itself from another is in the speed at which it can respond to an input signal, how fast it stops moving once the signal is gone, how accurately it tracks the input signal (linearity), how far the speaker cone can move and the frequency response (output level) of the subwoofer in a given enclosure.
We’ll save subwoofer enclosure designs and construction for a later installment, but let’s look at a few numbers that you should understand when you go subwoofer shopping. The first one to know (to make you sound smart) is Xmax. Xmax is the distance the cone of the subwoofer can move forward or backward from its rest position before the voice coil comes out of the magnetic gap. It can get more complicated, but really, it tells you how much air the cone can move, and given the correct enclosure application, how loud the sub can play. More is better – to a point.
Efficiency is the next specification to talk about, but this one gets tricky, since the number specified is typically in the 85 to 90dB range. This refers to how much sound the subwoofer will produce when powered (typically) with 1 Watt. It’s often written as 89dB 1W/1M – which reads 89 Decibels with 1 Watt measured at 1 Metre. The problem with this specification is that it doesn’t tell you at what frequency it produces this output, and it’s often derived mathematically.
Getting a little more technical, we should discuss ‘low frequency cut-off.’ This specification is 100 percent dependent on the enclosure you use and describes how low (or deep) a subwoofer will play. For example, a ported subwoofer enclosure for most cars with the speakers I am going to highlight here, should play down to 35Hz or so. Sadly, as an enclosure allows a subwoofer to play lower, you typically lose more efficiency. There are dozens more, but we’ll get into those next time – sorry for the tease.
So what about all the fancy materials and designs you see out there? Flat-cone subwoofers? Square, hexagonal and triangular subwoofers? These are just variations on a theme. Sure, some let the designers cram more cone area into a smaller space (both in terms of cone surface area and driver depth), but if done properly, that’s the extent of the benefit. No fear though, that can be worth the ‘price of admission’ and they are well worth checking out for specific applications.
I’ll wrap things up by saying that the choice of one subwoofer over another should be done by listening to them. Ideally, if you can try the subwoofer you want in your car, that would be best – each and every subwoofer from the hundreds of companies out there sounds a little different than another. Some play really low, some are very tight and controlled, some are boomy and loud. Let your local car audio shop know what your needs are in terms of a subwoofer and they should be able to offer you something that really brings some emotion and realism to your music – be it the 1812 Overture, Money for Nothing, Down in It or The Real Slim Shady!
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