From Sound to Electrical Current and Back Again
It’s pretty incredible when you think about it, magnets attached to a tweeter or woofer, the basic components of a speaker, can reproduce any sound simply by moving back and forth in a precise fashion. When the tweeter or woofer, also known as a driver, moves back and forth it creates vibrations in the air, otherwise known as sound waves. How does the driver move in just the right way so as to faithfully reproduce the recording that is being played? The recording contains an encoding of the sound wave in some method, either by transforming the sound waves into grooves on a record, pits on a CD or just binary data in digital file. That encoded translation of the sound wave is then converted into an electrical signal which goes from the playback device and is then amplified either at an external amp, receiver or at an amp inside the speakers. It’s these electrical signals, also known as current, that change the polarity of a magnet inside the speaker called a voice coil. As the polarity of the voice coil is switched back and forth by the current, it moves closer or further away to another magnet next to it. Since the voice coil is attached to the driver, they both move together and the current is transformed back into sound waves.
One of the more interesting parts of the process is that once the sound reaches our ears, our nervous system once again translates the vibrations from sound back into an electrical current which is then processed by our brain where the truly mysterious part begins as we interpret that sound through perception. So basically the whole process of recording, sound reproduction and even our hearing is merely the conversion of vibration to electrical current and vice versa. The one exception is the storage of the information on a disc, piece of vinyl, etc until it can be played back, or converted back to current, at a later time.
Let’s take a more in depth look at the components that make up just the driver and how each functions:
The Anatomy of a Speaker Driver
- Cone: The cone is connected to the voice coil and moves air to create sound waves. Most modern tweeters move air with a dome rather than a cone.
- Voice coil: The electromagnet that drives the cone and is alternately charged postively and negatively
- Magnet: The non-changing magnetic field that allows the voice coil’s alternating magnetic force to be attracted or repelled.
- Top plate, back plate and pole piece: The magnetically conductive parts that efficiently concentrate the magnet’s energy around the voice coil.
- Spider: A springy cloth disc that keeps the voice coil and bottom of the cone from moving off to the side and focuses the coils motion in a forward and backward motion.
- Surround: A flexible ring that keeps the cone from moving side to side while allowing it to push forward and backwards. Together with the spider, a suspension system is formed for the parts that move, the moving parts being the cone and voice coil.
- Flex wires and wire terminals: These components move the electrical current from the amplifier to the voice coil.
- Dust cap: Covers the middle section of the cone and keeps debris from getting into the gap between the magnet and the pole piece where the voice coil resides.
- Frame (or basket): Holds the entire speaker assembly together and attaches it to the cabinet.
In addition to the driver there are a couple other parts that we need to make a complete speaker. First the cabinet which is just the box into which the drivers are installed. Why do we need a cabinet? The main purpose is to trap the sound waves that come off the backside of the driver and to ensure that they do not cancel out the sound coming from the front of the driver. The cabinet also ensures that the drivers are positioned properly with respect to one another and allows them to work efficiently.
Another feature you will see on many speakers is a port, which is merely an opening in the speaker which allows the long wavelengths of low frequencies to escape the cabinet and reinforces the speaker’s bass response. By including a port, the speaker will be able to reproduce bass at higher volumes than without it. Another method to increase a speaker’s bass response is to include a passive radiator, which has all the parts of a regular driver, except for a voice coil and magnet ans is not wired to an amplifier. The passive radiator moves back and forth with bass sound waves created by the other drivers and allows for more bass output from the speaker. A passive radiator can be preferable to a port in some cases because it doesn’t have the same tendency to turbulence or port noise. It also allows the speaker cabinet to remain small, which is an engineering method that we at Aperion use for our small footprint subs and center channels.
Finally speakers with more than one driver, that is nearly all loudspeakers, use crossover networks of circuitry to ensure that the different drivers play the frequencies for which they are designed. For instance in a two way speaker, which is a speaker with a tweeter and one or more woofers that play the same frequency range, the crossover will filter out low frequencies before the signal reaches the tweeter and then filter out high frequencies before they reach the woofer(s). This ensures that the drivers do not waste energy attempting to reproduce frequencies that are inaudible to our ears when produced by that driver. Commonly, capacitors are used to filter out lower frequencies and a coil or inductor is used to filter high frequencies. The crossover point is the frequency when one driver’s response falls off in decibels (dB) and another driver’s frequency response increases. You can think of the crossover point as the “hand off” of the sound from one driver to another. Using components to create an ideal crossover point for each driver is critical to ensure that the different drivers in a speaker blend together seamlessly while faithfully reproducing the full audio spectrum.
Audio Specs Explained
- What is Nominal Impedance? Impedance is a measure of resistance. All electrical devices resist the flow of electricity to some degree. Because they resist at some frequencies more than at others, engineers thought they would confuse the layperson. Thus they chose the word impedance instead of resistance. But the concept is the same. The nominal impedance (resistance) means “the lowest the impedance (resistance) will be at any frequency is not much lower than the spec listed.” This spec is unrelated to the quality of speaker performance but it can effect how much power your speaker draws from your receiver. The lower the impedance, the more power the speaker will draw from your receiver with the “volume” knob at a given position. Therefore if your speaker has a low nominal impedance spec, typically 4 ohms or lower, you will need a higher powered amplifier to drive it.
- What is Efficiency (also known as SPL)? The efficiency of a speaker is how loud it will play, in dB, when fed one watt of power and measured from one meter away. If a speaker is 10dB higher in efficiency than another, it will play twice as loud with the same amount of power. An increase of 3 dB means it will play just as loud with half the power. Don’t mistake efficiency for quality. In fact, many good speakers, like high performance cars, are low in efficiency.
- What is Sensitivity? This takes efficiency and adds the effect of impedance. If two speakers are the same in efficiency but one as half the impedance (resistance), it will play 3dB louder since it’s drawing twice the power from your amp (and that’s without touching the volume knob).
- What is Frequency Response? This is usually the most reliable indicator of a speaker’s sound quality. Unfortunately, it’s also the easiest spec to manipulate given that it depends on microphone placement, room placement, how the graph of the response is scaled and smoothed, etc. If your speaker system really is +/- 3dB, that is only 3dB of deviation, from 300 Hz to 18,000 Hz in a lab condition and +/- 5 dB from 30 Hz to 500 Hz in your listening position it’s likely to be a fine sounding system indeed. If you choose to look at response graphs, keep in mind, rough peaks that are narrow in frequency range will not be as audible as graphs that have a wide frequency range that is low or elevated.
- What is Power Handling? Continuous power handling is only limited by how hot the voice coil can get before the glue that holds the wire begins to melt. Peak power handling refers to how big of a momentary burst (at the most troublesome bass frequency) a speaker can take. This is a somewhat tricky spec, because if a speaker is rated for 100 watts, we don’t know how long it can really sustain that power level. Also, just because a speaker can handle a large amount of power does not mean that it is better than a speaker with a lower power handling spec. Finally the power handling spec is only really important if you plan on playing your speakers at a loud volume for a long period of time.
- What is Recommended (amplifier) Power? This defines the manufacturer’s idea of a sensible range of amplifier (or receiver) power for their speaker. This specification is a helpful way of subjectively combining power handling and efficiency into one useful rating.
- What is Max SPL? This is a measure of how loud a speaker can play. It’s hard to make sense of this spec. This isn’t the most helpful specification because there isn’t a real standard in terms of the conditions that are used to test the maximum SPL of a speaker, so you may not be comparing apples to apples when looking at different speakers.
Hopefully this article helped demystify how speakers work and also will help you make an informed buying decision the next time you are shopping for speakers. Just remember, the most important thing is to trust your ears and as always, happy listening!