Let's Checking Out Your Subwoofer Specs !

Like many other components, subwoofers come with a list of specs that help you compare different subs when shopping. Because subwoofers also require enclosures, there’s a whole slew of specs that relate to how a sub will behave in a box. Known as Thiele-Small parameters, these specs are difficult even for the most learned enthusiast to interpret and comprehend.

Checking Out Your Subwoofer Specs

In the next few sections, I cover the most general and important subwoofer specs you should pay attention to.

Frequency response

Frequency response is a measure of the frequency range that a subwoofer can accurately reproduce. For a 10-inch subwoofer, a frequency response spec will look like this:

30 Hz to 700 Hz

This means that the subwoofer reproduces sounds ranging from 30 Hz to 700 Hz, although you may only use the subwoofer to reproduce sounds from, say, 30 to 100 Hz. 

Power handling

This is a measure of the amount of power a subwoofer can handle, in watts, and lets you know how much amplifier power is required to effectively drive the speaker. For a 10-inch subwoofer, the power-handling spec may look like this:

Peak power 900 watts, continuous power 300 watts

This means that 900 watts is the maximum amount of power you’ll want to supply to the sub. More power could cause the sub to distort or become damaged or blown. The continuous rating is the average amount of power the sub can handle and indicates how much power you’ll need to efficiently drive the sub.

A subwoofer is as likely to fail from getting too little power as it is from getting too much power. A subwoofer is under-driven when an amplifier runs out of juice (if, for example, the amplifier is mismatched to the sub) and begins to clip or distort the audio signal. It’s this distortion that is most likely to damage a subwoofer, so make sure you drive a subwoofer with sufficient power.

Car audio manufacturers sometimes massage specs to make them compare more favorably with competitive speakers. So consumers can compare subwoofer specs, the Mobile Electronics division of the Consumer Electronics Association, the trade group representing the consumer-electronics industry, came up with the CEA-2031 standard. To participate, manufacturers must obtain subwoofer power-handling specs using a uniform method defined by the standard, so that consumers can compare apples to apples. CEA-2031 is a voluntary standard and only a handful of manufacturers have signed on to the program at this point. Still, look for the CEA-2031 logo, shown in Figure 16-9, if you want to be absolutely sure that a subwoofer’s power-handling spec is legit. 

Sensitivity

Sensitivity or efficiency indicates how much power a subwoofer needs to achieve a relative volume level and tells you how loud the sub will play when provided a certain amount of power. Subs with a higher sensitivity rating need less power to play loud, and those with lower sensitivity need more.

The sensitivity spec is obtained by supplying a subwoofer with 1 watt of power and measuring the output in decibels (dB), with a microphone 1 meter away. A sensitivity spec usually looks like this:

85 dB (1 watt/one meter)

This means that the subwoofer puts out 85 dB of sound-pressure level (SPL), measured 1 meter away, when fed a 1-watt signal.

Sensitivity is the best indication of how much amplifier power you’ll need to drive a subwoofer. Low-power amps work better with subwoofers with a higher sensitivity rating, whereas high-power amps are required with less- sensitive subwoofers.

Physical specs

A subwoofer’s physical specs are also important because they tell you the size of the sub and other information. Physical specs include

Mounting depth or displacement: How deep the sub’s frame or basket extends into an enclosure

Mounting diameter: How large a hole needs to be cut into a baffle to hold the sub

Enclosure volume or size: The volume or air space required in an enclosure

Impedance

For most car speakers, impedance isn’t usually a concern because almost all of them have a 4-ohm impedance. But with subwoofers, impedance plays a very crucial role. Impedance is a measure of how much resistance a sub¬woofer presents to the power flowing from the amplifier, and is often referred to as the load a sub places on an amp. Another way to think of impedance is how much a speaker restricts flow of power from an amplifier. The higher the impedance, the more the power from the amplifier is restricted, and the lower the impedance, the less the power from the amplifier is restricted. 

Impedance is an important subwoofer spec because, depending on a subwoofer’s impedance and the way the subwoofer is wired to an amplifier, it causes the amp to increase or decrease its power output. In general, most car audio amps produce more power with every halving of impedance — up to a point, that is.

For example, if a two-channel or stereo amplifier produces 60 watts x 2 into a 4-ohm load, it may produce around 75 watts into a 2-ohm load. When the two channels are bridged to form a single channel, meaning the two separate stereo channels are combined into one mono channel, the power more than doubles to about 150 watts.

But the amplifier has to be stable at that load, meaning it has been designed to operate at that impedance. Otherwise, the amp is being asked to perform beyond its design capabilities and either shuts down (if it has protection circuitry) or it fries (if it does not). Some amps, called high current amps, can drive loads as low as half an ohm (although these are usually specialized for sound-off competitors who are trying to squeeze every last drop of power out of their systems, and they often require an upgrade of a car’s charging system).

The various ways in which subs can be wired to an amplifier are the deter¬mining factors of the final impedance at the amplifier’s outputs, and therefore the ultimate power output of the amp. That’s why you see amplifiers rated at different impedances. For example, the same 120-watt stereo amplifier mentioned earlier may have power ratings that look like this:

60 watts x 2 into 4 ohms

75 watts x 2 into 2 ohms

150 watts x 1 into 4 ohms

Almost all car audio amplifiers are designed to produce their optimum power output into a specific impedance. If the impedance goes up past that opti¬mum point, the amp performs less powerfully and you’re not getting all the power you paid for. If the amp is driven into a lower impedance than it’s capable of handling, it could either shut down, if built-in protection circuitry kicks in, or become damaged, if it does not.

Another important thing to know about amplifiers in relation to subwoofers is that subs are almost always driven in mono as opposed to stereo. That’s because stereo separation cannot be detected at low frequencies. That’s why it’s common to bridge two channels of a stereo amp or a mutichannel amp to form a single mono channel to power a sub, and why mono block subwoofer amps are often used to power subwoofers. (Mono block is just a fancy way of describing a single-channel amplifier, and the terminology came from the high- end home audio world, in which mono block amps are preferred for powering 

all speakers.) As mentioned earlier, a single bridged channel of an amp is usually about twice as powerful as two separate stereo channels if all are driving the same load, which is a good thing because subwoofers require roughly twice the power of other speakers.