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UNDERSTANING THE SPECS

When you begin to choose a loudspeaker component, you will soon begin to feel a little overwhelmed by the confusing and somewhat contradictory numbers and words thrown at you. For a quick overview of the specifications that matter the most for subwoofer selection, we will begin with the ones that are the most broadly applicable. When we get a little further along with discussing particular subwoofer box configurations, we will more thoroughly examine the parameters that affect those individual designs.

POWER HANDLING

The industry seems to have almost convinced us that power handling capacity is the most important number to consider when choosing speakers. High power handling is certainly a factor to consider, depending on your application, but be sure that you are looking at the right numbers. The RMS rating is based on a mathematical formula (RMS= .707 X peak measured power) for determining average power that the speaker is subjected to, and it is usually a fairly conservative number compared to the continuous program power or peak power ratings that we will discuss in a moment. RMS ratings are only as good as the "qualifiers" that accompany them, for example, rating a hypothetical subwoofer at 600 watts/RMS doesn't mean much if the frequency chosen for measurement (1000 Hz) is outside the range that you intend for the subwoofer to be used (25 to 125 Hz). In the case of this example, the power handling number would give us some idea of what the subwoofer could withstand in terms of thermal power handling capacity, but it would not reveal much about the mechanical limits at the lower frequencies where the subwoofer will be used. Lower frequencies require greater cone travel, and we will be expecting that subwoofer cone to be moving quite a bit! For the purpose of selecting a speaker it is useful to know the range of frequencies tested, or the single frequency if that is what was used. Since these qualifiers are sometimes a little hard to nail down, your best bet is to stick with manufacturers that demonstrate the greatest credibility, and suppliers that take performance specifications seriously.

Continuous program power is generally understood to be double a given RMS rating, and tends to represent the kind of stress that a speaker is subjected to when that amount of music program is played through it. Although music can place dynamic stresses on a speaker, it also gives the components a chance to "rest" at intervals, even if those intervals are only a few milliseconds in duration. If you use a subwoofer rated at 200 watts RMS or 400 watts continuous program power, with an amp rated at 250 watts RMS, you are actually getting a pretty good match as long as the amp is not driven so hard that it exceeds it's "clean" power rating of 250 watts RMS. When overdriven, the amp can exceed it's RMS power rating and produce even more power, and that extra power will be loaded with a lot of harmful distortion.

Peak power is probably the least helpful rating for speakers. Any speaker can handle extremely high peaks of undistorted signal for milliseconds or microseconds, but these impressive numbers are rarely qualified by the disclosure of the test conditions that were used to determine them. In effect, peak power ratings are an example of a "numbers game" that marketing pressures have forced the manufacturers to compete in because higher numbers impress some buyers. The key to power handling ratings is reviewing and understanding the specifications that actually apply to the job at hand.

IMPEDANCE

Help Guides
Speaker Connection In
Series And Parallel

The diagram above shows the basic electrical difference between connecting 4O speakers in parallel and in series. The connection point where the speakers are attached to the amp is shown at the right side of each frame with the resulting impedance that each situation would provide to the amplifier's output.

Speaker impedance can be defined as the combined total of all resistance, inductance, and capacitance that the speaker presents to an amplifier. It is also referred to as "nominal impedance" because the actual number measured in ohms can vary quite a lot over the frequency range of the speaker. The actual nominal impedance value of 2, 4, 6, 8, or 16 ohms has no connection whatsoever with subwoofer performance or quality, but it is important for the proper match of an amplifier to the intended load. The lower the impedance value, the more power will flow through the speaker. If a 4 ohm and an 8 ohm speaker are connected to the same amplifier, the 4 ohm speaker will receive twice the power of the 8 ohm speaker, so matching values within the same frequency range can be pretty important. Multiple driver designs will require an awareness of the impedance of the intended drivers as various wiring configurations can increase or decrease the total value that the amp will see. It is also important to correctly identify subwoofer impedance as it is also a factor in establishing the values used to determine passive crossover components.





XMAX, EFFICIENCY, AND SENSITIVITY


Xmax is the term used to describe the linear excursion capability of a loudspeaker. It is basically determined by the length of the voice coil relative to the height of the magnetic gap of the speaker "motor". The subwoofer can only provide clear, undistorted output if the magnet is able to maintain control of the voice coil, and lower frequencies demand greater travel or excursion. The voice coil tends to have greater mass due to the increased amount of wire required for the longer surface area covered; this usually results in a reduction in higher frequency response, and lower overall driver efficiency.

Efficiency and sensitivity are very closely related. Efficiency is usually shown as a percentage that describes the ability of the speaker to convert an electrical input into an acoustical output. You will hear a lot of advertisements for speakers that declare the product to be "high efficiency", but in fact that is a relative term.

Sensitivity ratings for loudspeaker products are representations of relative loudness, using a scale of decibels (dB), and with an input power of 1 watt (w). The resulting Sound Pressure Level (SPL) generated is typically measured at a distance of 1 meter (m) from the loudspeaker, so it would appear that we have everything we need to compare one speaker to another, right? Actually, it is just a start.

It is a surprising fact that loudspeakers in general and subwoofers in particular are very inefficient devices, electrically speaking. An "efficient" pro sound 12" midrange speaker with a sensitivity rating of 103 dB at one watt/one meter might have a reference efficiency of around 5 or 6%. This means that for all of the amplifier power going into the speaker, as much as 94% is wasted as heat and mechanical losses. Wait, it gets worse! Your state-of-the-art subwoofer loudspeaker has a sensitivity of only 87 dB 1w/1m, or a reference efficiency of only 0.27%, so what good is that? Before you get mad at your speakers for being very expensive round heating elements, just relax and enjoy the benefits of modern technology! High performance amplifier power has gotten relatively cheap and reliable, and improvements with adhesives and high tech design have pushed speaker power handling to limits that could only have been dreamed of a generation ago. It was not that long ago that you were the king of the hill if your (probably tube) amp was cranking out a blazing 25 watts per channel, and your woofers (probably not subwoofers) were rated at 50 watts each. The high efficiency designs of that era were actually able to provide a very dynamic hi fi experience, but there are some alternatives for the contemporary user.

The most popular current thinking regarding efficiency relative to subwoofer output is to select a driver with a fairly high mass stiff cone, good linear excursion capability, and power handling that is high enough to withstand the power required to move that massive cone and long voice coil. This driver will have a fairly low sensitivity rating because of these design characteristics. In a properly designed enclosure, and driven by the appropriate amplifier, our example subwoofer will be able to deliver deep, clean, high impact bass.

We have established that power handling ratings are only relevant within a driver's actual operating range; sensitivity ratings for a subwoofer only count in the sub-bass region below approximately 125 Hz. A subwoofer could actually have a response that is rising at frequencies that are outside the range that you intend for the sub to be used. The sound pressure of these higher frequencies will affect a sensitivity rating if the manufacturer has averaged the boosted response of the higher frequencies with the reduced output that we would expect at lower frequencies.

FREQUENCY RESPONSE

The frequency response of a subwoofer is important but becomes far more meaningful when we are able to determine the actual response in the enclosure of our choice. As we have seen with other key specifications, frequency response numbers mean little without some kind of qualification. To simply state that a driver has a response "from 19 Hz to 1000 Hz" is leaving out critical information. If the relative output (remember SPL?) versus frequency is not disclosed, we could be looking at a response at 19 Hz that is 3, 6, or even 16 dB below the average response for the speaker. Similarly, there could be a 10 dB peak at 90 Hz. Frequency response graphs can be far more helpful, because the associated graph will display the variations in sound pressure level at any given frequency. Be sure to try to determine if the woofer shown on the graph was tested in an enclosure or free air, because free air plots will make the bass response look very puny indeed. Computer modeling programs like BassBox 6 Pro can fill in a lot of information as far as what to expect for bass response from a given enclosure design. Keep in mind that since we are discussing subwoofers, the response of the driver above 125 or 150 Hz is not a matter of great importance to us.


 
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