Subwoofer Primer

Reading this file will not guarantee bass ecstasy but may assist in maximum enjoyment of your tunes and flicks. Think of this discussion as pragmatic and practical rather than as a precise engineering dissertation.

I authored this FAQ file in 1994 for upload to CompuServe's Home Audio forum. I am proud to say that for years this was by far the "most-read" speaker file in that library. When time permits I make updates and revisions to this version. Regards, Mike Dzurko.

Reasons to consider adding a subwoofer(s) to your system:

• Improve the quality of bass and lower midrange
• Reach deeper in the bass than your current speakers
• Get louder bass than your current speakers
• Be like all your audiophile friends
• Increase the dynamic range of your system, (Peel Plaster!)

Contents: We suggest reading these in order

1. Passive and Powered Subwoofers
2. Powered subwoofer design features
3. Choosing the best subwoofer (s) for your needs
4. Single or multiple subwoofers
5. Subwoofer setup tips
6. Terms to know

1) Passive and powered subwoofers

-PASSIVE SUBWOOFERS: Passive subs may be mono or stereo and contain no amplification. A mono passive subwoofer contains two woofers or a single woofer with a dual voice coil. A passive low pass filter feeds the woofer(s). Some passive subs contain a passive high-pass for output to the main speakers.

Another type of passive sub is one in which no filters are contained. In this case, the user generally needs to supply an electronic crossover and a power amplifier.

Considering a passive subwoofer? They can cost less and can be worthwhile if they are designed to be used with the speakers you already have. But it is difficult to adjust a passive sub's output or crossover without sonic problems, (unless using a separate, electronic crossover). They will blend properly only with speakers they were designed to work with. Passive subs draw power from your system and bass requires a lot of power. Unless your amp/receiver has plenty of reserve power you might well run out of power when adding a passive sub. Consider passive subs only if;

A) They are offered as an option by the manufacturer of your main speakers.

B) You really know your stuff (or have competent help) and know that your Playgo Model 1901 speakers are similar enough to the Zoomtite Model Y that the Zoomtite Model SUB is designed to go with.

C) You just need to add some inexpensive bass punch to a pair of small speakers and you're not very picky. Powered subwoofers cost more, but offer far better value.

D) You have an extra amplifier and the ability to bi-amplify to the passive subs.

-POWERED SUBWOOFERS: Also referred to as active subwoofers. Powered subs have their own amplification and an active low-pass crossover to feed the woofer(s). Powered subs can also utilize various circuits for equalization, self-protection and distortion reduction. They may also have active or passive high-pass outputs. Powered subs can be adjusted to match your system. On a powered sub you can adjust the output level and crossover frequency to match a wide range of speakers. Powered subwoofers have their own amplification so instead of drawing power from your system they increase your total system power. By designing the amplifier, enclosure and driver (woofer) together, the manufacturer can optimize the performance of all three. As a result, powered subwoofers usually offer higher performance, smaller size, better value and greater reliability than passive subs.

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2) Powered subwoofer design features

If space and money aren't limited it's easy to get flat response below 20Hz at high SPL with low distortion. The problem is getting good bass response from reasonably sized enclosures for a reasonable amount of money. There is a three-way relationship between enclosure size, low bass output and required amplifier power. The powered subwoofer designer has a lot of tools at his or her disposal to get acceptable low bass out of cabinets that will actually fit in your room.

* Variable gain control. Virtually every powered subwoofer has some sort of volume knob which controls the amount of output. This allows the powered subwoofer to be level matched to virtually any main speaker no matter how high or low its sensitivity.

* Variable crossover frequency. Dial adjust the crossover frequency up or down until you get the best match to the main speakers. The vast majority of subwoofers on the market utilize 2nd or 4th order lowpass filters. A few, like Vandersteen, use 1st order lowpass. ACI subwoofers use either dual 12db/oct networks that are usually used in tandem to create 4th order filters, or a single 24db/oct. filter as used in the Maestro XL. Higher order filters have the advantage of cutting the sub off at a more rapid rate. This makes it easier to "hide" the sub sonically. Why? Because frequencies above the subwoofers cutoff are what often makes is easy to localize the sub. It is difficult to localize 80Hz. However, if the sub is still putting out a lot of energy one or two octaves up, those frequencies are easy to localize.  Some powered subs even have switchable crossover slopes.

* High pass filters. Some powered subs have line-level and or speaker level high-pass filters built in. Using built in filters requires running extra cabling. Speaker level filters are rarely very effective because of the varying load of the main speakers. Another option is to use a plug-in high-pass filter such as the ACI High-Pass filter at the input of the main amplifier. Because the amplifier is a constant load, an inexpensive plug-in filter can be quite effective.

* Switchable or adjustable phase. A handy feature that allows the user to determine the correct system phase by flicking a switch or turning a dial.  This is most easily accomplished using test tones and an inexpensive SPL meter.

* Driver sizes from 6" to 18", single or multiple drivers. Yes, it is theoretically possible to make a 6" woofer with enough excursion to equal a 12", but, it's not very practical. Remember, no two drivers are the same. It is very possible for a high quality 10"woofer, for example, to go lower with less distortion than a lesser quality 12" woofer.

* Subwoofer enclosures are commonly one of three types. Sealed boxes (acoustic suspension, some employ active equalization), Vented (4th order or greater) or Bandpass. You will find many convincing arguments for why each is superior. Each has its own advantages and disadvantages. All three can sound great or terrible depending on how well they are designed.

* Amplifier type and power. The designer of a powered subwoofer has the advantage of using an amplifier specifically engineered for that application. Unlike a separate amplifier which may be used with any of thousands of different speakers, the powered sub amp can be designed for optimum performance with a specified speaker. The designer can use just the right power supply and output section to best match the rest of the subwoofer without needing to over-design the amp for speakers it will never have to drive. High power can be used to get high output and low bass from relatively small boxes. Most subs use relatively use plate amps with  "amps on a chip" while the best powered subs usually use discrete output devices. The newest type of high quality amplification is the ICE type unit used in many high-end separate amps and our own Maestro XL. So far we haven't seen any triode tube amps being built into subwoofers but give it time :)!

* Limiters and compression can be designed into the electronics that will allow the amp to push the woofer to maximum output levels but no farther. If the system is properly designed the result equals high output levels, low bass, low distortion levels and a nearly bullet proof system. Poorly designed and the compression and limiting may add audible artifacts to the sound.

* Feedback, either motional or current sensing, can be used in the design. Feedback can be used to reduce distortion and or reduce box size. With abundant power, feedback can be used to get extended, flat response out of a box that would otherwise be too small.

* Active low frequency cut-off filters can be used to block frequencies that are too low for the woofer to handle and would only cause distortion or damage.

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3) Choosing the best subwoofer(s)

Ask yourself a lot of questions. You should have some ideas of what you want to help narrow down your decision. Do you want to really extend your bass response or just add some extra muscle, perhaps for videos. What is your budget? How large of box can you fit in your room? Is the appearance of the subwoofer important? (Personally I'm not thrilled with big black boxes, I'd rather have a speaker be useful as furniture. In designing ACI subwoofers, my wife insisted on true furniture quality. Mike D.)

Generalizations:

• The bass that knocks your socks off when enjoying your favorite blockbuster might well seem bloated and muddy when listening to a jazz quartet.
• Accurate bass does not always sound spectacular, it sounds real. Listen to bass guitar or plucked string bass. The notes should be clearly defined with no particular note(s)  jumping out at you (lumpy bass can often be the fault of room placement or poor setup).
• For video based applications, quantity of bass may well be as important as quality. If you have the money, space (and time to fine tune) you can have quality and quantity, but quantity is easier to get.
• You should demo any speaker you are considering (including subs) in your home system. Take some time to experiment with it to see if it will be satisfying to you for the long term. Most store demos will only prove that the sub can move some air.
• Consider your evolving tastes. Initially, you might get a real kick out of excessive bass. After awhile you might get a headache. People often start off with the sub(s) cranked and watch the latest SciFi or Battle movie over and over. After awhile they start enjoying a wider range of films and music and begin to notice that there is too much bass and not much definition. People often find that the sub(s) degrades their enjoyment of music because it calls too much attention to itself. Make sure the sub(s) you keep have enough quality to keep you happy for years. . . not months.
• Seamless integration is often overlooked by first-time subwoofer buyers. A really good sub will integrate so seamlessly into your system that it will not draw attention to itself. Yes, you'll have the added foundation of the bass, yes, you should experience added ambience in your soundstage. You should not hear the subwoofer as a separate entity, and the bass should sound tuneful and integrated with the rest of the music. This seamless integration is one of the hardest things to design into a sub.

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4) Single or multiple subwoofers

A single sub provides a major and cost-effective improvement for most people in most rooms with most music. However, there are a number of reasons to consider more than one sub.

-Stereo subwoofers, are more important if the main speakers need a rather high crossover frequency. Some very careful listeners may want stereo subwoofers with crossover as low as 60Hz while many others might be more than happy with a single subwoofer crossed over above 100Hz. Many of the popular three-piece satellite-subwoofer systems in the $500-$1000 range have crossovers above 160Hz! Some listeners hear the problems immediately while others never even notice them.

-Multiple subs can increase output levels and lower distortion. People with large rooms and those who need higher than average output levels might want to use more than one sub. Two smaller medium cabinets might fit your space better than one large box. For large spaces, and or very high volume requirements, any number of subs can be added to a system.

-Multiple subs can yield smoother in-room response than a single subwoofer. Loading the bass into the room from two or more points can give a better balanced bass throughout the room. However: if the crossover frequency is low, (usually below 80Hz or even lower), the best bass may be with the multiple subs placed next to each other.

-A third sub may be used for the rear surround channels. Many films now have significant low bass in the rear channels. Some processing modes require extra subs for the ultimate experience. Some film lovers feel that having a subwoofer for the rear channel improves impact and realism with such films.

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5) System setup

Getting the optimum performance from your sub(s) is going to require some careful listening. Keep in mind the sound you want. Using test instruments, test CDS, etc., can be helpful but also be fairly confusing. Low frequency room response testing is a complicated subject. Suffice it to say that inexpensive test equipment used without a lot of experience and understanding may give very misleading results. We suggest making your final judgment based on how music/film sounds to you.

Read your subwoofer owner's manual. It may give you all the information you need to optimize your system. The tips below may or may not be in your owner's manual.

-Placing the sub (s) near your main speakers is the easiest way to get a good match. The ideal placement is often somewhere between the main speakers. Placements outside of the main speakers can also work well. If you are crossing over below 80-100Hz, try a corner placement behind your main speakers. A corner placement is usually the optimum placement for maximum, undistorted output with the most accuracy.

-Instead of moving a heavy subwoofer around the room to find the best response try this simple trick. Place the sub where you normally sit to listen. While playing either test tones or music with a steady bass, walk around the room and listen for the deepest, tightest and smoothest bass response. Move the sub to the place where you heard the best bass.

-Placing the sub(s) close to a wall will increase bass output but may also make the bass less even. Moving the sub(s) into a corner will increase bass output even further. Use the placement to fine tune the bass your system produces.

-Placement of the sub(s) will often be determined by your room's configuration and furniture layout. Experienced audiophiles know that "fine-tuning" room placement can yield important gains in sound quality. Most listeners will be more than happy with the results they get from the most "convenient" placement. Very serious listeners will want to experiment with various placements to really optimize the match to the main speakers.

-Correct phase of the subwoofer is dependent on the frequency response of the main speakers, the crossover filters and the placement of the subwoofer relative to the main speakers. If you have a phase or polarity switch on your sub try both positions. The correct position yields the greatest amount of bass. If you have infinitely adjustable phase, dial it in until you have the greatest bass level. If you don't have adjustable phase, start with all speaker connections positive-to-positive and negative-to-negative. If there seems to be a gap in the response between the sub(s) and main speakers, try reversing the leads to either the subwoofer(s) or the main speakers. You will then probably have to re-adjust the crossover frequency and output level. The correct phase is the one with the most bass.

-Use (line-level) connections if you have line-outputs in your system. If you don't you will need to use the speaker-level inputs or adapters. The high input impedance of most powered sub(s) will not use any power from your receiver or amp.

-The low-frequency response of your main speakers must be considered. Crossover frequency should be set as low as possible. Lower crossovers will allow the subwoofer to audibly "disappear" and placement is less critical. The deeper your main speakers go, the easier it will be to get a seamless match. If your main speakers are a ported (vented) design, you will often get the best results with the vent plugged. A cloth or towel will work fine. Just stuff it so that you no longer get air pressure coming from the vent when the speaker is playing.

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6) Terms to know

-FREQUENCY: The range of human hearing is commonly given as 20-20,000Hz (20Hz-20kHz). One hertz (Hz) represents one cycle per second, 20Hz represents 20 cycles per second and so on. Lower numbers are lower frequencies. Subwoofers typically reproduce frequencies from 50-150Hz on down. Music has a lot of energy in the 40-80Hz octave but little fundamental energy below 40Hz. Large pipe organs and synthesizers, however, reach below 20Hz and "hall sound" contains energy below 40Hz. Special effects in the movies; crashes, explosions, etc., have tons of energy in the 30s. Recently,  more movies have been mixed with strong content below 30Hz.

-SOUND WAVES: Sound waves can be thought of like the waves in water. Frequency determines the length of the waves; amplitude or volume determines the height of the waves. At 20Hz, the wavelength is 56 feet long! These long waves give bass its penetrating ability, (why you can hear car boomers blocks away).

-SOUND PRESSURE LEVEL (SPL): Given in decibels (DB) is an expression of loudness or volume. A 10db increase in SPL represents a doubling in volume. Live orchestral music reaches brief peaks in the 105db range and live rock easily goes over 120db. Caution, excessive volume can and will cause permanent hearing damage. Don't overdo it!

-HEARING SENSITIVITY: The human ear is less sensitive at low frequencies than in the midrange. Turn your volume knob down and notice how the bass seems to"disappear". To hear low bass requires an adequate SPL level. To hear 25Hz requires a much higher SPL level than to hear 250Hz. In the REAL world, low frequency sounds are reproduced by large objects; bass drums, string bass, concert grand pianos, etc. Listen to the exhaust rumble of a 454 cubic inch V8 engine vs. the whine of the little four banger. The growl of a lion vs. the meow of your favorite kitty. As frequency decreases we perceive more by feel than actual hearing and we gradually lose our ability to hear exact pitch.

-OCTAVE: An octave is a doubling or halving of frequency. 20Hz-40Hz is often considered the bottom octave. Some electronically synthesized music as well as move soundtracks actually contain significant energy below 20Hz. The largest pipe organs in the world will hit 16Hz.  Each octave you add on the bottom requires that your speakers move four times as much air!

-LOW FREQUENCY EXTENSION: Manufacturers, writers and salespeople toss around all kinds of numbers and terminology that can be very confusing and misleading. "This $300 shoebox sized sub is flat to 20Hz". Right, in your dreams... How is that tiny box and driver going to reproduce a 56 foot wavelength with enough power to be heard? It will not do it. Good bass reproduction requires moving a lot of air and playback at realistic volumes. Remember the rule of needing to move four times the air to go down one octave. Example: You have a pair of good quality tower speakers with 10" woofers that produce good bass down to around 40Hz. The salesman is telling you that his little subwoofer with a single 10" woofer will extend your system down to 20Hz. If you've been paying attention you know that his woofer will have to move eight times as much air as each of your 10" woofers, not likely. Adding that subwoofer to your system might give you more apparent bass energy, and in fact may help a little with movie special effects, but it is unlikely to extend bass response significantly. Note, there are now subwoofers that are quite small, (less than a cubic foot), that will move significant air as low as 20Hz. These subwoofers feature drivers with extremely high excursion powered by amplifiers that are usually over 1000 watts. They do not sell for $300:)

-LINE LEVEL: CD players, VCRs, DVDs, Laserdisc Players etc., are connected in a system at line level, usually with shielded RCA type interconnects. Line level is before power amplification. In a system with separate pre-amp and power-amp the pre-amp output is line level. Many surround sound decoders and receivers have line level outputs as well.

-SPEAKER LEVEL: Taken from the speaker terminals. This signal has already been amplified.

-ACTIVE CROSSOVER: A frequency divider at line level. Can be based on integrated circuits (ICs), discreet transistors or tubes. An active crossover is impedance buffered and gives a consistent and accurate transition regardless of load (see impedance below).

-PASSIVE CROSSOVER: A frequency divider that relies on capacitors, coils and resistors. Usually used at speaker level, (but our ACI high-pass line level filters are at line-level). Unlike an active crossover, a passive crossover is load dependent (the transition may not be very smooth or accurate if a different speaker is substituted for the one the crossover was designed for).

-HIGH-PASS FILTER: A circuit that allows high frequencies to pass but rolls off the low frequencies. When adding a subwoofer it is often desirable to roll-off the low frequencies to the main amplifiers and speakers. This will allow the main speakers to play louder with less distortion. High-pass filters used at speaker level are usually not very effective unless properly designed for a specific main speaker (see impedance below). ACI High-Pass filters are line-level devices.

-LOW-PASS FILTER: A circuit that allows low frequencies to pass but rolls off the high frequencies. Most subwoofers have low-pass filters built in and many surround sound decoders have subwoofer outputs that have been low-pass filtered.

-CROSSOVER SLOPE: High and low pass filters used for speakers do not cut-off frequencies like brick walls. The rolloff occurs over a number of octaves. Common filter slopes for speakers are 1st through 4th order corresponding to 6db/oct to 24db/oct. For example, a 1st. order, 6db/oct high pass filter at 100hz will pass 6db less energy at 50Hz and 12db less energy at 25Hz. Within the common 1st through 4th filters there is an endless variety of types including Butterworth, Linkwitz-Riley, Bessel, Chebychev, etc. Salesmen and product literature will sometimes make claims of clear superiority for the filter they have chosen. Since the subject fills books, suffice it to say there is no one best filter, it depends on application and intended outcome.

-IMPEDANCE: Impedance is a measure of electrical resistance specified in ohms. Speakers are commonly listed as 4 or 8 ohms but speakers are reactive devices and a nominal 8 ohm speaker might measure from below 4 ohms to 60 or more ohms over its frequency range. This varying impedance curve is different for each speaker model and makes it impossible to design a really effective "generic" speaker level high-pass filter. Active devices like amplifiers typically have an input impedance between about 10,000-100,000 ohms and the impedance is the same regardless of frequency.

-PHASE: For this discussion we'll consider only absolute phase; also referred to as polarity. With a pair of stereo speakers it is very important that the + and - be connected correctly on both speakers. Correct phase means both speakers will move out when fed a positive signal. With subwoofers the situation gets more complicated because we are adding the variables of crossover slopes and distance both of which can change phase. Example; with the sub the same distance from your listening position as your main speakers, the two may be correctly phased. Move the subwoofer a few feet back and it may now be out of phase with your main speakers. The result will be a hole or dip in the response around the transition between the two. (We'll discuss fixes later.)

We hope this information is useful to you. Please keep it in mind the next time someone asks your advice about subwoofers. We welcome your comments. Email: sales@audioc.com 

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ACI, Sound that Satisfies...Since 1977