Updated 07 Nov 2015

Disclaimer

The information presented below is just that, information. Use it at your own risk.. You (the reader) are responsible for anything foolish that you might do as a result of reading this article. You assume complete and total responsibility for your actions! The information presented below is believed to be technically correct (however mistakes occasionally are made, this being the "real world"). In simple terms, if you do something foolish (as a result of reading this article), don't blame me!

 

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SPEAKER FAQs and ABC's...

This page will provide information about speakers and answers to FAQs. It is not particularly well organized at this time. The information presented refers to professional type speakers that DJ's typically use (although much of the info applies to home and car speakers as well). Hopefully it will be useful and will help clear up some mysteries and misconceptions...

General

Speakers are highly complicated electromechanical devices. The primary purpose of a speaker is to convert the electrical energy from an amplifier to sound (acoustic) energy. Speakers currently represent the weakest technological link in an audio system. Although many speakers of today are very good or even outstanding, there remains considerable room for improvement (compared to other audio equipment).

Wattage Ratings

Often, consumers are attracted to and impressed by speakers that can handle the largest amount of wattage. However, wattage is just one factor to consider among many. Just because a speaker can handle more wattage does not necessarily mean that it will play louder or sound better.

To find out the real story, one needs to consider the sensitivity specification together with the wattage rating. The sensitivity specification basically states how much sound comes out for a given input (analogous to a miles-per-gallon rating for a car). The sensitivity spec is determined by the manufacturer, and often takes the following form: "x" dB / 1 watt / 1 meter. "x" is the sound level in dB, and may be anywhere from about 85 to 105 dB, depending upon the speaker model (a spec in the low to mid 90s is common). The spec is determined (at the vendor's factory) by driving the speaker with 1 watt of electrical energy and measuring the acoustic output at a distance of one meter (using a calibrated microphone). There are a number of variations in measurement procedures due to the variety of speaker models, too many details to cover at this time.

So, what does this all mean? Consider two speakers, Model A and Model B. Model A has a wattage rating of 200 watts and a sensitivity rating of 101dB/W/M. Model B has a wattage rating of 600 watts and has a sensitivity rating of 97 dB/W/M. The maximum output from Model A (with 200 W input) is 124 dB; the maximum output of Model B (with 600 W input) is 124.8 dB (most people would not be able to hear a 0.8 dB difference in sound level). So, as can be seen from the numbers, Model A and Model B have nearly identical maximum output, even though Model B can handle three times as much power! (I haven't gone into mathematical details of how I came up with the maximum numbers... a topic for another day). It is interesting to note however... for top end speakers (of the same type), the maximum output of them are all pretty similar. In other words, speakers that handle a lot of power often have lower sensitivity numbers. Speakers with the highest sensitivity numbers typically can't handle as much power. You are not likely to find a speaker than can both handle a ton of power and also have a high sensitivity rating. Amplifier power is relatively costly, so speakers that are more efficient (generate more sound with a smaller input) are generally desired so long as you are satisfied with the sound characteristic.

CONCLUSION: DON"T SELECT A SPEAKER BASED ON WATTAGE RATING ALONE! As we saw in the above example, two speakers that (to the unknowing) might seem much different actually put out nearly the same amount of sound. Model B requires a much more powerful amplifier to get the same maximum output; powerful amplifiers are certainly available but weigh more and cost more!

Why Speakers "blow out"

Most people have blown a speaker at one time or another. A typical scenario is a large party where the guests are drunk and someone says “crank it up!”  The result is that the volume control is turned up as far as it can physically go (despite the offensive sound quality), and the next morning the house owner realizes that one or more speakers are no longer functioning properly. What happened?

There are two major ways speakers can fail: mechanical failure (most often limited to woofers, resulting from too much cone excursion) and thermal failure (the voice coil overheated and melted or burned).   The most common reason for blown speakers: driving them with too much electrical power!  Quite often the person operating the equipment does not fully realize how much power is being sent to the speaker.  Below are some photos of a medium duty woofer (disassembled) to show some of the parts we are talking about:

Description: wooferWoofer with cone removed (click on the image to bring up a large image in a new window) This is a medium duty woofer with its cone removed. The frame is sometimes called the "basket". The tan item at the center is the spider, this holds the cone centered. If a woofer is overdriven with too much cone travel, the voice coil can rip free from the spider.

Description: wooferWoofer with cone removed (click on the image to bring up a large image in a new window) This is the cone from a medium duty woofer. Note the voice coil at the center, this is a 1.5" coil rated for about 50 watts. Had this woofer been blown due to thermal overload the coil may be charred, melted or the windings might be coming apart (this coil is in good shape).

Description: wooferClose up of Woofer Voice Coil (click on the image to bring up a large image in a new window) Here is a close up of the coil. This woofer had been suffering from severe foam rot (see discussion below). This caused the coil to rub against the magnet structure. This is evident from the lines and scratches on the inside of the coil former.

 

Mechanical failure of a speaker (most often a woofer) occurs because the speaker's cone is being asked to move farther than it was designed to safely do. When this happens, parts of the speaker become stressed due to stretching and excessive vibration, and in severe cases the cone and/or parts of the voice coil structure can collide with the frame of the speaker. If operation of the speaker in this fashion is not ceased, sooner or later something will rip, break, rupture or come loose. If the woofer's spider (a component made of a mesh like material that holds the voice coil in alignment) comes loose, the voice coil will basically be allowed to flail wildly inside the voice coil gap, and the insulation on the coil will be damaged as it rubs against the metal parts of the magnetic structure. When this occurs, the sound quality will take on a very gritty tone, it should be very obvious (to a sober person anyway!); if left to continue in this fashion, the coil will eventually break (resulting in no sound at all). Mechanical failure of midrange and tweeter components can and does occur, but it is much less common than woofer failures. Almost all of the time, failure of a midrange or tweeter speaker is due to thermal failure.

Thermal failure of a speaker occurs when the speaker is fed more power than it is designed to safely handle. This basically causes the voice coil to get too hot, and two main things can happen. First, the adhesives used to hold the voice coil together can soften, resulting in the coil coming apart (this is a partial mechanical failure). If the coil gets too hot, it will simply melt or burn the wires in the coil, most often resulting in an open circuit (in this case there will be no sound at all).

In the majority of cases, a speaker blowing out is usually due to an accident or carelessness. An example of an accident is someone making cable connections to components (a CD player) at the back of the amp while unknowingly having the volume at full blast while the amp is on (the very loud hum that is generated could blow up a speaker). More often, carelessness is the cause (the drunken party example above). It should also be mentioned that it can sometimes be easier to blow a speaker with an amp that puts out less power than the speaker is designed to handle! The reason for this is that an amp that is being overdriven can generate a lot of distortion and the RMS level of the amplifier output will be a lot higher than what the operator thinks it might be, and the speaker can blow (clarification: distortion itself does not blow speakers, but amplifiers driven into distortion cause more power to go to the speaker than one realizes). You may sometimes hear the statement "speakers can take a lot of power if it is "clean" power". This is true to a point. However, if you try to drive a speaker rated at 100 W with an amp that puts out 1000W, chances are you could wreck the speaker even though the power was totally clean (no clipping or distortion)! So remember, most speaker failures are caused by driving them with too much power.  This is most often caused by amplifiers being driven into distortion, however the distortion itself is not the culprit.  Driving an amplifier into distortion causes signal compression which results in a significantly higher average power level being delivered to the speaker.  If your speakers or subwoofers blow out and you do not want to fix them, you may want to consider looking at the best powered subwoofer deals at OneCall.

 

Are 4 Ohm Speakers better than 8 Ohm Speakers?

NO!!! The Ohm rating of a speaker has nothing to do with the quality of the speaker!

What is an "ohm" rating anyway?

All speakers have a characteristic known as impedance which is measured in units called ohms. The most common values for speakers are 8 ohms and 4 ohms. Many older speakers have ohm ratings of 16 and even 32 ohms (this is because in the old days amplifiers used vacuum tubes, and higher impedance speakers were more compatible with the output impedance of vacuum tube amplifiers). A speaker with a lower ohm rating represents a more demanding load for an amplifier to drive. As stated before, the impedance of a speaker has no relation to the quality of the speaker.

Why do speakers come in 8 ohm and 4 ohm versions?

Many manufacturers of speakers (especially raw drivers) offer a choice of impedance... 8 ohms or 4 ohms. Often, the remaining specifications are very similar (except that the lower impedance versions of the speaker usually have a smaller sensitivity value). The reason for offering two versions is for special applications. For example, in a speaker system with dual woofers, two 4 ohm woofers can be wired up to form an 8 ohm system.

Some musicians prefer the "sound" of a 4 ohm speaker as compared to an 8 ohm version. I believe that this "sound" is not so much a result of the speaker itself, but a result of the heavier loading that a 4 ohm speaker places on an amplifier.

In the car stereo market, virtually all speakers are 4 ohms. The reason is due to voltage limitations available in cars (namely the 12 volt battery). More power can be driven into a 4 ohm speaker as compared to an 8 ohm speaker (assuming the same driving signal!).

What should I know about using 4 ohm speakers?

4 ohm speakers place a significantly heavier demand on power amplifiers as compared to 8 ohm speakers. Because of their lower impedance, twice as much current will flow through a 4 ohm speaker (as compared to an 8 ohm speaker) for a given volume control setting (assuming the amp can keep up). This translates to amplifiers getting significantly hotter (and heat is among the top enemies of electronic devices!).

If you use 4 ohm speakers, your speaker wiring will have to be (or should be) larger. This is because the resistance of the speaker wires becomes more significant with respect to that of the speaker. The result is that more power is "wasted" (in the form of heat) in the wires leading to the speakers! Amplifier power is relatively costly, so it does not make sense to waste that power in the lines leading to the speakers.

If you run two sets of 4 ohm speakers from a single amplifier (and the amplifier puts them in parallel), you are asking for trouble. The reason is that the combined impedance will be down around 2 ohms (and could actually be lower still... see below). A 2 ohm load is a load that only the most robust amplifiers will tolerate. There are amplifiers which are rated to be able to handle 2 ohm loads with no problems; however in my opinion operating a system in this configuration should generally be avoided. Despite the fact that the amp may be designed to handle the load, operating in this fashion places more stress on circuits and can lead to less reliability.  You CAN drive a very low impedance load with most amplifiers so long as you keep the volume low.  However, for practical reasons this is not often done.

Incidentally, you CANNOT make a 4 ohm speaker into an 8 ohm speaker by wiring a 4 ohm resistor in series with it (I once saw someone trying to do just this at Radio Shack). This will make the amplifier happier, since as far as it is concerned it has an 8 ohm load (which is easier to drive). However, resistors dissipate energy, they DO NOT generate sound. Wiring a 4 ohm resistor in series with a 4 ohm speaker will halve the amount of power which reaches the speaker (it will also degrade the damping factor, which won't hurt anything but it can degrade the sound quality!). Since amplifier power is relatively expensive, it would be extremely foolish to "throw away" half of it by wiring a resistor into the speaker's electrical path! Again, if you have 4 ohm speakers, you cannot change them into 8 ohm speakers... get an amplifier that is capable of driving 4 ohm systems!

Is an 8 ohm speaker really 8 ohms?

Not really! Due to the highly complicated nature of a loudspeaker, its impedance is NOT a simple number such as 8 (or 4) ohms. When a speaker is said to be 8 or 4 ohms, this is understood to be its nominal impedance.

A speaker system is an extremely complicated electromechanical device, and its behavior is correspondingly difficult to quantify. The impedance of a speaker system in actuality is said to be reactive; that is, in addition to resistance it can take on inductive and capacitive characteristics. An inductor is a device which stores energy in a magnetic field, and a capacitor is a device which store energy in an electric field. These characteristics are not "designed into" a speaker system; they are characteristics that are "inherent" (based on the physical laws which govern the way things work in our world).

Why do speakers take on reactive properties? As stated, speakers are very complicated devices, so this answer will be highly simplified. When a signal is first applied to a speaker, there is a time delay which occurs before the speaker cone starts to move. This is because the speaker cone has a non zero mass. Basically, it is like a person trying to start a heavy cart in motion… the cart will “fight” and tend to not want to move at first.  Then, once the speaker’s cone is moving, and the input signal suddenly says "go the other way", the cone will resist the change in motion (again because it has mass and is moving in a particular direction). Basically, it takes effort to start a mass moving, and once in motion, it takes effort to stop it and make it go the other way! An amplifier must deliver more current in order to start a speaker cone moving, and when the amplifier signal tells the speaker "now go in the other direction", the speaker responds by generating an electrical signal which "fights" the amplifier! Again, this explanation is highly simplified. The best way to handle a difficult speaker load is to use an amplifier which has the ability to deliver tremendous current (and has zero output impedance). This will better keep a wild speaker "in line".

So, why do I need to know or care about this? It is important to realize that even though a speaker may have a rating of 8 ohms, the actual value can vary greatly. The impedance of a speaker varies as a function of input signal frequency. As stated above, speaker impedance is in general reactive. This means that the impedance consists of a resistive part and either an inductive or capacitive portion (inductive and capacitive impedances cannot exist at the same time). The actual impedance (for an 8 ohm system) might vary from around 5 or 6 ohms up to as high as 50 or 60 ohms! The high numbers (50 or 60) are not so much the issue; it's the low numbers that cause trouble for amplifiers. Most amplifiers can easily handle the load of a "typical" 8 ohm speaker; the trouble comes in when one tries to drive two systems (in parallel) from the same amplifier. If you have an amplifier that is not too good with low impedance loads AND you have two "difficult" 8 ohms systems connected to it, you are inviting trouble. In general, amplifiers do not like to drive reactive loads (which all speakers are of course). The best way to combat this is to use an amplifier that can dish out substantial current (i.e. one that claims it is good with 2 ohm loads). Lesser amplifier designs will be the first to "quit" whenever difficult speaker loads are connected to it.

Advanced topics... As stated, the impedance of a speaker varies with frequency. If one looks at a complex impedance plot (generated with a variable frequency sine wave input), it can be seen that the impedance makes several "loops" on the plot. There will be several frequencies where the impedance is "real" (purely resistive). The first occurrence (starting from the low frequency end of the plot) of a purely real impedance occurs at the resonance frequency of the system (technically DC is the first "real" impedance, but is of little interest because normally DC is not applied to speakers). For most systems, this value will be somewhere between 40 and 80 Hz. The impedance at resonance is high, perhaps the highest value for the system (it might be anywhere from 20 - 70 ohms). A speaker system is very efficient (relatively speaking) at resonance. For bass-reflex speaker systems (which most DJ speakers are), the vibration of the woofer cone at resonance will be very small (however the amount of air coming out of the vent will be tremendous). The speaker system presents an "easy" load to the amplifier at resonance because (a) the impedance is purely resistive and (b) the impedance is relatively high.

Speaker Efficiency

Efficiency is typically defined as the ratio of useful power output of a system to the power input. Most devices are less than 100% efficient; that is at least a portion of the input energy is wasted, usually in the form of heat. As an example, consider an incandescent light bulb. A light bulb ideally generates only visible light, however anyone who has ever touched a 100 watt bulb (which has been on for a while) knows that it gets quite hot. This heat represents wasted energy and is a result of the bulb being less than 100% efficient at generating visible light. Anyway, on to the topic at hand...

Speakers are in general notoriously inefficient. A speaker takes energy from an amplifier (electrical watts) and converts it to sound energy (acoustical watts); however, most of the power is wasted (in the form of heat). A typical speaker might be about 5% efficient; this means that if 100 watts of amplifier power are being sent to the speaker, only 5 watts of acoustic sound comes out! If speakers were 100% efficient, the average "boom box" would be able to fill a gymnasium with sound!

Why are speakers so inefficient? They are not designed to be inefficient, it just "works out that way" due to the laws of physics which govern how things behave in our world. The biggest problem is that a speaker cone forms a very poor impedance match with the air it moves to create the sound. Whenever a poor impedance match occurs, energy transfer (in this case from the speaker cone to the air) is also poor. Basically, air is "too thin" to work well with a speaker. A speaker placed underwater would be much more efficient (but probably wouldn't work for very long!) since water is much denser than air.

Can you explain "impedance match" in familiar terms? (I'll try): Consider a person with a 10 speed bike. The person is riding on flat terrain with a slight headwind, and has the bike in first (the "easiest") gear. The bike is very easy to pedal, however, the distance covered by the bike is minimal. If the rider pedals very fast, more distance is covered, but soon the rider is out of breath from pedaling at a high speed. Now, the rider shifts to tenth gear (the "hardest" to pedal). The bike goes much farther with a single revolution of the pedals, but it is much harder to pedal. Even though the rider only has to pedal at a slow rate, the extreme force required to move the pedals eventually causes fatigue. What is the solution? Pick a gear "somewhere in the middle" that provides a good balance of speed and pedal effort. That is, select a gear that "feels right". One of the middle gears will provide a good match for the riding conditions. By selecting the "comfortable" gear, the rider has essentially "matched" the impedance of his/her legs to the "load" provided by the bike pedals. Note here that we are talking about impedance match between the speaker and the air, not the electrical impedance relationship between the speaker and the amplifier.

Some speaker types are much more efficient than others: most notably, horn type systems are the most efficient. Some narrowband horn systems have efficiencies approaching 50% (this may still seem like a low number but for a speaker it is extremely high). The reason horn systems are so efficient is that the horn acts like an impedance matching device (to the air); essentially an acoustical transformer. Whenever you cup your hands around your mouth to increase the volume when shouting at someone, you are forming a crude (but effective) horn. Many high frequency and midrange speakers are of the horn variety. Due to physics, low frequency horn speakers are very large (if not huge); hence the reason for "horn woofers" being scarce. A number of low frequency horn systems are available, but they are usually of the folded horn design. Because a low frequency horn would be excessively large (a big consideration for portable sound systems), engineers came up with the folded horn design. Folded horns result in some losses (as compared to a "straight" horn), but the result is still a very efficient (again, relatively speaking) system.

Big Magnets mean a Great Speaker, right?

NOT NECESSARILY!!! Many people are impressed by speakers with gigantic magnets, thinking that they make a superior speaker. The purpose of a magnet in any speaker is to generate a magnetic field in the voice coil gap of the speaker. When a speaker driver is designed, calculations are made as to how much magnetic flux must be present in the voice coil gap for desired operation. A quality speaker has a magnet size based on these flux requirements. A larger magnet, although "looking impressive", is a waste: it costs more, weighs more, and may even degrade performance!

Woofers in general have larger magnets than mid and high frequency drivers primarily due to the necessarily larger width of the voice coil gap. Woofers have larger voice coil gaps than high frequency drivers since the woofer's cone (and voice coil) is required to move much larger distances (up to an inch or so forward and backward). In order to maintain clearances between the voice coil and the magnetic gap under such conditions, the voice coil gap must be wider. The magnet of a speaker is just one part of what is called the "magnetic circuit". There are other steel parts that direct where the "magnetic flux" flows. Magnetic flux "likes to stay" in highly permeable materials like steel, it does not travel well in air (and the voice coil gap is full of air). Thus, as a voice coil gap gets wider, the strength of the magnet required to maintain the proper value of magnetic flux gets larger still. This is why woofer magnets tend to be quite large compared to midrange and tweeter speakers. For the latter, the voice coil does not move nearly as far, so tighter tolerances can be maintained (and smaller magnets are needed). It is not uncommon (despite their smaller magnets) for tweeters to have more magnetic flux in the voice coil gap than giant woofers!

The "giant magnet" hype is primarily confined to lower quality car stereo speakers. It is largely a marketing ploy. An example of such a speaker is a ten inch woofer with an 80 ounce magnet (selling for $9.95 at a "tent sale"). Such speakers are likely to do more for one's ego than for their ears.

Can a speaker reproduce a square wave?

No… at least not very well.  A perfect square wave (which does not occur in normal music) changes from one voltage level to another instantaneously.  A speaker could not reproduce this sound as there is no way to physically move a speaker cone from one position to the other instantaneously.  It will try to do so if driven with a square wave, but the cone will take time to get from where it was to where the signal wants it to be.  Also, there will be overshoot as the cone cannot stop instantaneously once it is moving, and the result will be some form of “ringing” in the sound.

 Can a square wave damage a speaker?

Yes and no.  If the square wave driving the speaker is within the speaker’s continuous power rating it will be OK so long as the frequency being driven is not causing excessive cone motion.  If the square wave driving the speaker is more powerful than the speaker’s continuous rating it could blow, and if the frequency of the square wave is low enough (even though the power of the square wave might be well below the speaker’s rating) you could damage the speaker.  Low frequencies require a lot more cone movement as compared to high frequencies, so you must be careful when driving low frequencies at high power.  Slamming a 100 watt woofer with a 50 watt signal at 20 Hz could cause mechanical failure due to the excessive motion the speaker would be subject to.

Can distortion damage a speaker?

The answer is very similar to that for a square wave.  So long as power being applied to the speaker is within its power rating (and the signal is not causing the cone to move beyond design limits) you can play any signal you like, distorted or otherwise.  The main reason people equate distortion with blown speakers is that speakers tend to blow when people crank up the sound to the point where the sound is badly distorted.  What is really happening here is that the signal is being compressed and a lot more power ends up going to the speaker than one realizes.  So the answer is no, distortion itself does not blow speakers.  Overdriving them does!

Notes on raw midrange and tweeter speakers

Note:  If you build your own speakers and get raw tweeter and/or midrange drivers, it is absolutely essential to use them with a crossover network.  A crossover network divides the incoming signal into different bands (low/high for two way systems and low/mid/high for 3 ways systems).  Most midrange drivers and virtually all tweeter drivers (with the exception of piezo electric drivers) will quickly blow out if you do not use a crossover that blocks the low frequency signals.  Crossover design is an entire field within itself so I cannot go into all the nuances of it right now.  At very minimum a midrange and/or tweeter must have an appropriate value capacitor in series with it to block the low end signals (this represents the simplest possible crossover).

How many OHMS are you running?

While setting up a sound system for a school dance, someone came up and asked me how much power one of my amplifiers had. I said "it's 465 wpc at 8 ohms, and 625 wpc at 4 ohms". He then said, "Run your speakers at 4 ohms!" I sensed immediate confusion on the part of the person asking the question. IMPORTANT: If you have 8 ohm speakers, you cannot "run" them at 4 ohms; they are what they are. You cannot (easily) change the impedance of your speakers. A given set of speakers with a given amplifier will have a particular maximum output; you cannot get more by "running" your speakers at 4 ohms. If you have 8 ohm speakers and your amp puts out 200 watts at 8 ohms, that's what you'll get. That same amp might put out 300 watts at 4 ohms, but to "get that extra power" you'll have to use a 4 ohm set of speakers.

There is a way to "adapt" the impedance of a speaker to a different value by using a transformer. However, this practice is restricted to office PA systems and other applications where audio quality is less important. Transformers that maintain high audio quality at high power are extremely expensive, very large and very heavy (in other words, totally impractical for DJ use!). You will find them (transformers) in high end audiophile vacuum tube amplifiers, but the price tag of some of these amps is not that different from a luxury sedan!

Again, you cannot "run" your speakers at 4 ohms in an attempt to get more power out of your amp! (You can connect two sets of 8 ohm speakers (in parallel) to your amp to get more power out of the amp. However, under such conditions the power to any one speaker in general will be less than the amount it would receive if powered by itself at 8 ohms!

Rotate your Woofers????

This is not a joke... in some cases it is wise to "rotate your woofers"! Like rotating tires on a car (to extend their life), it can be important to rotate your woofers to extend their life. If you have large woofers (15" or more) that have heavy cones, rotating them periodically can be critical to the life of the woofer. Why? Woofers with large heavy cones tend to "sag" over time, and it is possible for this to cause the voice coil to become misarranged in the voice coil gap. If this occurs, the coil can rub the metal parts of the magnet structure when it is playing, possibly leading to a failure of the coil. The way to combat this issue is to rotate the woofer periodically. Basically, remove the screws that hold the woofer in place, and then rotate it 180 degrees from its current position and then replace the screws (be sure to check that there is enough slack in the wires connected to the woofer before rotating it!). Rotating the woofer 180 degrees will cause gravity to make the cone sag in the opposite direction, basically counteracting the problem. This problem will not occur with all speakers, but I have seen it occur with 18" woofers in a number of cases. Rotating them should not be needed more than once per year. NOTE: if your woofers are mounted such that the cone aims straight up, this section does not apply (rotating the woofers in this case will do no good).

Low Frequency Speaker Systems

The dominating factor in a low frequency speaker system's performance is the enclosure design. A woofer sitting in open air (no cabinet) has very poor low frequency response. Most "raw" speakers are designed with a particular type of cabinet design in mind (for DJ speakers, the two main types are bass reflex and folded horn). "Acoustic suspension" or "sealed box" enclosures are not often used for DJ speakers because they tend to be too inefficient.

Bass Reflex (sometimes called "vented" enclosures) are designed to take advantage of the acoustical energy inside the enclosure. The design essentially uses the sound inside the enclosure to reinforce the "direct" energy from the speaker. The result is more sound (as compared to a sealed box system) for the same wattage input. However, this phenomenon only works at low frequencies (from resonance to about 150 Hz or so). The mid band output of a speaker is much less dependent upon enclosure design.

Bass reflex enclosures are not easy to design. Often, much "tweaking" and "tuning" is necessary, requiring expensive equipment and a lot of brainpower. Unless you love building and tinkering with speakers, you are probably better off purchasing a system from one of the many excellent manufacturers that offer them!

One disadvantage of bass reflex type speakers is that the low end bass response falls off very rapidly as compared to sealed box systems. In technical terms, the bass reflex is a "second order" system, meaning that the low end response (below resonance) falls off at approximately 24 dB per octave (as compared to 12 dB per octave for sealed box systems). A number of bass reflex systems have an advertised low end (-3 dB point) that is lower than that of sealed box systems (possibly implying better low end bass). However, because the low frequency response of a bass reflex system drops so rapidly below resonance, such a system may actually seem to have less low end bass than a sealed box system!

Folded horn low frequency systems are even more complicated to design, and probably "out of scope" for all except those with an intense interest in speaker design (and lots of time and money)! However, a well designed horn system is tough to match in terms of the amount of acoustical energy they can generate. Horns are not noted extremely deep bass response (mainly because no one builds one large enough), but the "punch" they can generate is unmistakable.

 

Woofer Foam Rot

The surrounds of many woofers are made from a foam material that eventually fails and needs replacement.  This condition is often called “foam rot”.  If the woofer is a high end unit it is usually worth having the unit reconed, or you can buy kits and do the re-foaming yourself (I have done this and it worked very well on the woofers I had).  If the woofer was an entry level unit, it may be cheaper just to get new woofers.  Not all surrounds are subject to this condition.  Many of the woofers made in the 50s and 60s used cloth surrounds (doped with an elastic material), these do not rot out (I have speakers from the 60s that are as good today as they were then).  Woofers with butyl rubber surrounds do not seem to fail due to rot either.  Below is a photo of a woofer with severe foam rot condition.  Note: if you have high end woofers and note foam rot (and you plan to do the replacement yourself), do not let the foam rot out to the point where the voice coil starts rubbing.  This can damage the coil and you will then have to have the woofer reconed.

Description: wooferWoofer with severe Foam Rot (click on the image to bring up a large image in a new window) Here's an example of a woofer with severe foam rot. Operating a speaker with this condition will lead to more damage as the cone has only the spider to keep the cone aligned, and this alone is not enough to prevent voice coil rubbing. The sound from a woofer in this condition will be very bad!

Speaker Placement

Where you place your speakers in a room can have a large effect on the sound, especially the low frequencies. For DJ applications, it is often best to keep low frequencies on (or near) the floor, and to keep the mid and high frequencies above the tallest person in the room (6 feet or so). This presents a dilemma for people using single speakers (as opposed to those who run separate subwoofers and mid/high units).

In my experience, an excellent speaker configuration (for most rooms anyway) is to use two subwoofers and two smaller mid/high frequency systems. The two subwoofers are placed together (like one big unit); the two small speakers are placed on stands (about 7 feet up). This configuration offers a number of benefits:

Note that since low frequencies are very non-directional, the subwoofers can be placed in an out-of -the-way place. If you are creative you can hide them behind other items in the room. Your guests will wonder how "those two little speakers" (the ones they actually see) can put out so much bass!

A Sampling of Woofers

This page has photos of four different woofers ranging from vintage 8” to professional 18” woofers: A Sampling of Woofers  


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