Arpeggio Music & Sound

    
 

Shopping Basket Home Text Menu Glossary Contact

Introduction for Beginners

This section is intended as an introduction to speaker building for raw beginners. It is purposefully oversimplified and omits details that are important to advanced speaker builders. If you are already familiar with speaker building and would like to proceed immediately to learning to use this site to manage your speaker projects, click on the tutorial link.

The Basics

Speaker designing and building is an enjoyable activity that incorporates woodworking, analog electronics, advanced mathematics, and acoustical physics. It is as much an art as it is a science, and it can be as simple or as complex as one wishes to make it. If you have the tools and skills to cut a square board, glue a box together, follow simple circuit diagrams, and solder together electrical components, you can probably make speakers that far surpass most store-brands. But a good "rule of thumb" to remember is the common saying: Speed, Quality, Economy -- pick any two. This applies to making speakers as much as it does to anything else. If made quickly or cheaply, it will be a matter of sheer luck if they end up sounding good, and to make very good speakers can take a great deal of time, skill and money.

A speaker is an electro-mechanical device that converts electronic signals to audio signals. It is part of a larger system that includes a source signal, a receiver and an amplifier. All speakers consist of the following:

  1. The transducer, usually called a 'driver'
  2. Fixture to which the driver is mounted, usually called an 'enclosure' or a 'baffle'
  3. Electrical leads or circuitry that connect the driver to the amplifier

Receiver/Amplifier

There are many sources for audio signals. Radio broadcast towers, CD and tape players, musical instruments, cable stations, all emit a variety of electromagnitic signals at very low strength. A device called a receiver decodes these signals and converts it to a 'line level' signal (some items, musical instruments in particular, emit close-to line level signals directly). This is the actual, desired audio signal, but at a very low voltage. It would be barely audible if connected directly to most speakers, and so this signal is sent to an amplifier.

The amplifier does just what it is called: It amplifies an electrical signal. This amplified signal is then sent to the speakers via cables. In many cases the receiver and amplifier -- and sometimes even speakers -- are combined into a single unit.

Transducer or Driver

There are many different sizes, kinds and costs for drivers. They can cost as little as about $5, up to thousands, and anywhere in-between. Of course, the highest price drivers will only be found in very expensive speaker systems, but this does not mean that lower priced drivers are not very good. While the lowest price drivers are priced thus for good reason, many drivers even $30 and up are excellent and can be found in highly-regarded speaker systems costing many thousands of dollars.

The most common type of driver available today is the 'cone' style, in which an electrical signal creates a magnetic field that causes the cone to vibrate. The vibrating cone creates pressure waves in the air that can be detected by the ear. The process by which this is done is effective, but relatively inefficient. Only a small percentage of the amplifier's power actually gets transformed into sound. But it works, and the quality of the reproduced sound can be quite faithful to the original.

People can hear (or feel) sound below 20Hz and some people all the way up to almost 20,000Hz. This is about ten octaves, or doublings of the frequency. Starting at 20Hz, the next octaves are 40, 80, 160, 320, 640, 1280, 2560, 5120, 10240 and 20480Hz. Typical conversation rarely varies outside of one octave. A good singer might be able to cover three. Musical instruments typically cover three to seven, not counting harmonics/overtones. It is an unusual (and expensive) driver that can cover this range evenly, if at all. 20Hz is close to the lowest notes on a full organ. Some people cannot hear frequencies at the ends of these ranges, though they can sometimes phsyically sense them if the power is high enough. The ear is especially sensitive to frequencies between about 300 and 3000Hz. Frequencies above 3000Hz are usually 'harmonics' of other tones (also known as overtones). The combination of different harmonics allow the mind to recognize what is being heard. For example, almost all conventional music instruments (and people) can produce a 440Hz 'A' note. But the 440Hz signal is not the only signal they are generating. They are also generating many, many other higher 'harmonic' frequencies above the 440 Hz root tone. The unique combination of these overtones, and the speed with which they attack and fade, allows us to tell the difference between an A played on a piano from an A played on a clarinet. Were it not for harmonics, everything (and everyone) would sound roughly the same; there would be no recognizing people by their voices, etc.

Ideally, a driver will completely reproduce the electrical signal it receives; nothing more and nothing less. But due to their size, weight and construction, specific drivers can usually only cover a portion of the audible frequency spectrum (they also color -- add to and take away from -- the signal in various ways and to differing degrees):

Subwoofers are the largest drivers, usually 10" or more in diameter. They are typically used to cover frequencies from 20Hz through about 100Hz. They can go higher, but are rarely employed to do so.

Woofers are slightly smaller, ranging from about 8" to 4", covering the frequencies from as low as about 60Hz to 7000Hz.

Midrange drivers are less common, usually 5" to 3" in diameter, and focus on the frequencies between 200Hz to 10kHz.

Tweeters are many, the smallest of the drivers, and are used to handle frequencies above 2500Hz all the way to 20kHz. There are even 'super' tweeters that focus only on the highest portion of the frequencies.

Enclosure, Cabinet or just 'Box'

The speaker enclosure serves several practical purposes beyond supporting a cool drink from time to time. It holds and protects the driver and any circuitry to which it is connected. For lower frequency sound, the enclosure can prevent the desired sound waves from being canceled (a topic beyond the scope of the present discussion). The air in the box has a key resonant frequency -- a frequency at which it tends to vibrate -- that can affect the sound. This resonant frequency is usually used by the enclosure designer to help boost the volume of lower frequencies. Air is compressible, a little like a spring; the air inside the enclosure also acts as a kind of spring-ballast for the driver, and subwoofer, woofer and midrange drivers. These are almost all designed to work best when supported by a specific amount of air. For tweeters, the size, shape and porting of the enclosure usually doesn't matter.

Speaker boxes are generally quite rigid and strong, with excess bracing inside in an attempt to prevent the box, itself, from vibrating and affecting the sound.

There are many different enclosure designs and shapes. The simplest and most common are 'sealed' and 'ported' boxes. A sealed enclosure is just that, entirely sealed. A ported enclosure includes an opening to an internal tube of specific length and diameter to help control the box's resonant frequency. Ported enclosures tend to be larger than sealed enclosures. The driver's parameters dictate whether it will work best in a sealed or ported enclosure; driver manufacturers usually recommend one kind or another for a specific driver.

There are many additional, complex enclosure designs of interest to advanced speaker builders.

Beyond problems just making a box of a particular size -- the tools, skills, materials and supplies to do so -- adding bracing, sealing or ports, etc., there is one problem common to almost all speaker boxes. It goes by a few different names; baffle step, diffraction, and others. It is a result of the physical reality that larger, lower sound waves tend to bend around objects, even the box from which they are generated. What this means is that higher frequencies are more likely to project fairly straight forward and away from the box, while lower frequency sounds will spread, sometimes significantly. Let us suppose that your most excellent speaker just emit one watt of sound (which is approximately how much is actually emitted from most speakers connected to a 100W amp with the volume turned all the way up -- yes, really). That one watt spreads out as the sound waves proceed from the speaker. Let's imagine that you are standing 10 feet from the speaker and, that far away, the sound has spread to cover a circle with a diameter of ten feet. Assuming that your ear collects the sound energy from about two square inches of that 78 square foot circle, you will "receive" not one percent of that watt, but roughly one 100th of a percent of the emitted watt. Let's just say "a very small amount."

Now, in addition to the higher frequency tones of which your ear is receiving "a very small amount," there are many lower tones that are spreading much differently. In fact, they are wrapping right around the speaker and spreading in all directions to cover a spherical surface area of ~312 square feet -- or four times the surface covered by the lower frequency. All things being equal, this means that only 1/4th of "a very small amount" of lower frequency energy will reach your ear, even if it is at the same volume as everything else. In other words, without special compensation, the acoustical physics is going to make your music sound weak in the low end. This is a big change and, yes, you will notice it. There are four ways of fixing this problem:

  1. Make an incredibly wide speaker box so that the sound waves won't wrap around it (or just place it in a wall or something). This is called an "infinite baffle."
  2. Place the speaker very near or against a wall to reduce (but not entirely eliminate) this effect.
  3. Use an equalizer to boost the lower end frequencies before they get to the speaker -- a great option if you do not know where the speaker will be placed or just want future flexibility.
  4. Add circuitry to the speaker that reduces the higher frequency power by a similar amount, thereby "evening the playing field" for all frequencies -- but this requires making some assumption about where, in relation to nearby walls, a speaker will be placed.

This last option is called baffle step compensation, and requires the addition of some components to your...

Circuitry

If we take a single woofer, for example, place it in an appropriate enclosure and connect it to our fine stereo/amplifier, the results will be unpleasant, bland. Woofers reproduce only half of the audible frequency spectrum (or less), and so a lot of the character -- the harmonics -- in the music would be left out. To address the missing sound information, we add a tweeter to the box, too. Rather than just wiring the drivers in parallel and then to the amplifier, circuitry is added -- usually called a crossover circuit or a filter -- that divides the incoming signal between the drivers so that the right frequencies go to the driver that can handle them best. Failing to properly direct the frequencies can not only result in poor audio output, but can even damage some drivers.

Crossover circuits consist of capacitors, inductors (also known as coils or chokes), and resistors, all connected by wire and solder. Some are placed on printed circuit boards (PCBs). Some just on plywood. Capacitors tend to allow higher frequencies to pass and they block lower ones, and are used mostly with tweeters. Inductors tend to allow lower frequencies to pass and block higher ones, and so usually go with woofers. Resistors 'resist' the flow of electrical current, regardless of its frequency.

The quality of these parts is important; not just any old part will do. They should be designed specifically for audio applications, and can cost much, much more than their generic cousins. For example, a common resistor can cost a penny. An quality audio circuit resistor will be three to five dollars each. Some cost almost fifty. It is even worse with coils, and if you are already choking, don't even look at quality audio capacitor prices. They can be almost $100 each (though the $3 - $10 ones are more than sufficient for many speakers).

Crossovers are sometimes categorized by 'type,' which affects their behavior in the regions where they are actively dividing signals between drivers (this is unimportant to beginners, so don't worry about it now). A crossover's complexity and expense is proportional to the number of drivers it controls, the quality of the components and the 'order' of the crossover. 'Order' refers to the strength with which they suppress the unwanted frequencies. For example, consider a speaker enclosure that has two drivers, a woofer and a tweeter. You wish for the frequencies below 2500Hz to go to the woofer, and frequencies above 2500Hz to go to the tweeter. The crossover is really two filter circuits: A 'low-pass' filter circuit that allows frequencies below 2500Hz to pass, but attenuates or diminishes frequencies aboe 2500Hz, and a 'high-pass' circuit that allows frequencies above 2500Hz to pass, but attenuates those below 2500Hz.

The low-pass circuitry goes between the incoming signal from the amplifier and the woofer. The high-pass circuitry goes between the incoming signal and the tweeter. Unfortunately, crossover circuits aren't exactly gates; they can't stop unwanted frequencies completely. The simplest, first-order filter, reduces the signal strength by 6 decibels (dB) for each octave from the pivot point (2500Hz). A decibel is a measure of power relative to some other figure. A 3dB change is a doubling or halving of the sound strength; +3dB is a doubling, -3dB is a halving. The mathematical formula is n = 10 x log10(P2/P1). So if P2 = 100W and P1 = 50W, n = 10 x log10(100/50) = 10 x log10(2) = 10 x 0.301 = 3.

Most people can detect volume changes as small as 1dB. So -6dB reduces the signal to roughly 1/4 of its original strength every octave. If 2500Hz is our key point, the next octave above is 5000Hz, and the octave below is 1250Hz. So, on our low-pass filter, a first order filter would reduce the sound strength by about 3/4 at ~5000Hz. At the same time, the high-pass filter should be allowing the 5000Hz signal to pass to the tweeter with full power, so most of the sound will be coming from the tweeter, which is designed to handle those frequencies.

Having two or more drivers in a single box solves one problem -- how to cover the audible frequency range -- but creates several others. These are related to the spacing between the speakers, their individual geometries, and the way in which they interact with their supporting circuitry.

Higher order filters accelerate the rate at which unwanted signals are attenuated, but require more components. A simple, first-order crossover requires only two components, an inductor and a capacitor, and so can be relatively-inexpensive. Maybe $15 if one is using mid-quality parts, or as little as $4 for cheap ones. A third-order crossover would require at least six components, and with quality parts could easily approach $50. This may not seem like much, but that is just for a single enclosure. A home theater system would require at least five such crossovers, plus drivers, etc. It can add up quickly. 2nd, 3rd and higher order filters are found in higher quality, more expensive systems.

In addition to the crossover frequency.

Multiple Enclosures

Three applications are common in home use; stereo music, computer accessory or home theater. The first two are similar in design, consisting usually of left and right stereo channels, and an optional bass/subwoofer channel. Home theater systems include these, plus a center channel and at least two surround speakers (the words 'channel', 'enclosure', and 'speaker' can often be used interchangeably when referring to a sound system).

Almost all music, some computer audio, video game consoles, many movies, and many television broadcasts have stereo audio -- left and right channels. The purpose of stereo sound is to attempt to simulate, using two speakers, the location of sound sources from left to right. Surround sound has a similar goal, but in a full 360 degree sense; not as if the event is taking place before you, but all around you.

Because music is usually recorded and played in stereo, the greatest effort at 'fidelity' (faithfulness to the original) is usually found in the stereo left and right front speakers. Less effort and expense is placed in the other surround speakers, though there is little reason, apart from budget considerations, to make them with any less quality than the stereo left/right pair.

Why Build My Own Speakers?

Take your pick:

Where Next?

There are many books and sites available that can help teach more about speaker building. Peruse the tutorial section -- a step-by-step description of a real speaker project -- to see how this site can assist with your own speaker projects. When you are ready, this site can help perform the mathematics and book-keeping for anything from the very simple to complex speaker systems.

If you'd rather not start from scratch, but would like a high quality kit, see our Speakers and Kits menu. It has several basic designs available that can be personalized and ordered complete or in kit form.




Copyright 2005 Simple Way Limited Partnership DBA Arpeggio Music and Sound. All Rights Reserved. http://speaker.rosaryshop.com/index.php Terms of Use

Arpeggio Music and Sound, 805 NW Alder St, McMinnville, OR, USA. 97128
Telephone 503-472-0688; FAX 503-434-5803; Hours 10 AM - 5 PM Pacific Time, Monday through Friday
Visits by appointment only.