Loudspeaker anatomy:
In this document, we use the terms ‘loudspeaker’ or ’speaker system’ to denote a unit consisting of one or more drivers in an acoustic enclosure perhaps along with a frequency selective crossover, tone controls and switches, fuses or circuit breakers. Connections to the amplifier or receiver are via terminals on the rear.
The front is covered with an (optically) opaque or semitransparent grille which provides protection and improves the appearance (depending on your point of view).
A ‘driver’ is the actual unit that converts electrical energy into sound energy. Most drivers use voice coil technology: a very low mass coil wound on a light rigid tube is suspended within a powerful magnetic field and attached to a paper, plastic, or composite cone. The audio signal causes the coil to move back and forth and this motion causes the cone to move which causes the air to move which we perceive as sound.
The typical driver consists of several parts:
* Frame – a rigid steel or composite structure on which the driver is constructed. The frame holds the magnet and core, cone suspension, and connection terminals.
* Magnet – this includes a powerful (usually ceramic, AlNiCo, or rare earth) magnet including a core structure provide a very narrow cylindrical air gap. This accounts for most of the mass of a driver.
* Voice coil – a one or two layer coil of fine wire wound on a light rigid cardboard, plastic, or composite tube suspended within the air gap of the magnet and connected via flexible wires to the electrical terminals.
* Cone – a roughly cone shaped very light and rigid structure that does the actual work of moving air molecules. The cone in a woofer may be 12 or more inches across. The cone in a tweeter may only be an inch in diameter. This is the part of the driver you actually see from the front of the speaker system with the grille removed. The center is usually protected with a small plastic dome.
* Suspension – a corrugated flexible mounting for the voice coil called a ’spider’ and outer ring of very soft plastic or foam. Together, these allow the voice coil/cone combination to move readily in and out as a unit without tilting or or rubbing. For most designs, there is a certain amount of springiness to this suspension. Acoustic suspension loudspeaker, however, use the trapped air in a totally sealed speaker enclosure to provide the restoring force.
Inexpensive ‘LoFi’ devices like portable and clock radios, many TVs, intercoms, and so forth use a single, cheap driver. Some have a coaxial pair of cones but this does little to improve the frequency response.
HiFi speakers systems will divide the audio frequency spectrum into several bands and use drivers optimized for each. The reason is that it is not possible to design a single driver that has a uniform response for the entire audio frequency spectrum. A ‘woofer’ is large and massive and handles the low base notes.
A ‘tweeter’ has a very low mass structure and is used for the high frequencies. A ‘mid-range’ handles the mid frequencies. There may also be ’sub-woofers’ for the very very low notes that we feel more than hear. Some systems may include ’super-tweeters’ for the very highest frequencies (which few people can hear. This may make for some impressive specifications but perhaps little else.)
A ‘crossover’ network – a set of inductors and capacitors – implements a set of filters to direct the electrical signal (mostly) to the proper drivers.
Various controls or switches may be provided to allow for the adjustment of low, mid, and high frequency response to match the room acoustics more faithfully or to taste. Fuses or circuit breakers may be included to protect the speaker system from intentional (high volume levels) or accidental (amplifier output stage blows) abuse.
Loudspeaker problems:
If you have a high quality and expensive set of loudspeaker, then the cost of professional repair may be justified. However, if the problem is with speaker systems you might not write home about, then read on. Playing your music system at very high volume levels, especially CDs which may have peaks that way exceed the ratings of your loudspeakers is asking for trouble – but you knew that! CDs can be deceiving because the noise floor is so low that you are tempted to turn up the volume. A peak comes along and your speaker cones are clear across the county (remember the movie ‘Back to the Future’?). Loudspeaker systems are generally pretty robust but continuous abuse can take its toll.
Problems with loudspeakers:
1. An entire speaker system is dead.
Verify that the connections both at the speaker system and at the source are secure. Check circuit breakers or fuses in the speaker system. Reset or replace as needed. Make sure it is not the amplifier or other source that is defective by swapping channels if that is possible. Alternatively, test for output using a speaker from another system or even a set of headphones (but keep the volume turned way down). Assuming that these tests confirm that the speaker system is indeed not responding, you will need to get inside.
It would take quite a blast of power to kill an entire speaker system. Therefore, it is likely that there is a simple bad connection inside, perhaps right at the terminal block. You should be able to easily trace the circuitry – this is not a missile guidance system after all – to locate the bad connection. If nothing is found, then proceed to test the individual drivers as outlined below.
2. One or more drivers (the name for the individual speakers in a loudspeaker enclosure) is dead – no sound at all even when you place you ear right up to it. The cause may be a bad driver, a bad component or bad connection in the crossover network. Test these components as outlined below.
3. One or more drivers produces distorted or weak sound. Distorted may mean fuzzy, buzzing, or scratchy a various volume levels. Most likely this is due to a bad driver but it could also be a defective component in the crossover – a capacitor for example or even a marginal connection.
Getting inside a speaker system usually means removing the decorative grille if it snaps off or unscrewing the backpanel and/or terminal block. Use your judgement. With the grille removed, you will be able to unscrew the individual drivers one at a time. With the back off, you will have access to all the internal components. If sealing putty is used, don’t lose it or expect to obtain some replacement putty (non-hardening window caulking like Mortite is suitable).
Test the components in the crossover network with a multimeter. These are simple parts like capacitors, inductors, and potentiometers or reostats. Confirm that any circuit breakers or fuse holders have continuity.
Test the drivers on the low ohms scale of your multimeter. Disconnect one wire so that the crossover components will not influence the reading. Woofers and midrange drivers should measure a few ohms. If their impedance is marked, the reading you get will probably be somewhat lower but not 0. If possible compare your readings with the same driver in the good speaker system (if this is a stereo setup). Some tweeters (very small high frequency drivers) may have a series capacitor built in which will result in an infinite ohms measurement. Other than these, a high reading indicates an open voice coil which means a bad driver. In a comparison with an identical unit, a very low reading would mean a partially or totally shorted voice coil, again meaning a bad driver. Except for expensive systems with removable voice coil assemblies, either of these usually mean that a replacement will be required for the entire driver. Sometimes an open voice coil can be repaired if the break can be found.
To confirm these tests, use an audio source to power just the suspect driver. Your stereo system, a small amplifier attached to an audio source, or even a pocket radio (use its speaker output if the headphone output does not have enough power) will suffice. The resulting sound will not be of high quality because you do not have the enclosure sealed and it is only one of the drivers in the system, but it should give you some idea of its condition. Again, comparing with an identical unit would be another confirmation.
Repairing loudspeaker drivers:
As noted above, if you are dealing with a high quality system, leave these repairs to professionals or obtain an entire replacement as some reduction in audio quality may result from the abuse you are about to inflict on the poor defenseless driver.
Minor damage to the cone can be repaired using a flexible adhesive like weatherstrip cement and a piece of thick paper to reinforce the seam or hole if necessary. Since this will not totally perfect match with the original paper cone, there could be audible distortion at certain frequencies particularly at higher volume levels. However, such a repair will be better than nothing. Cut the paper in a shape and size to just overlap both sides of the torn area or completely cover the puncture. Use just the smallest amount of adhesive to fasten your ’splint’ to the cone. The less material you add, the more likely that the audio effects will be minimal.
An open driver can sometimes be rescued by tracing the input wires through the cone and under the center protective dome. The most likely places for these wires to break are right at the place where they pass through the cone and just after they pass under the dome. Note: some drivers have replaceable voice coil units. If this is the case, you should probably just replace the entire unit.
First, scrape away the insulating varnish on the front of the cone where the wires emerge and head toward the center. Use your ohmmeter to test for continuity here. If you find that you now are measuring a reasonable resistance – a few ohms, then trace back to determine which of the two wires is broken or has had the solder connection come loose. If it is still infinite, you will have to go under the dome.
Use an Xacto knife to carefully remove the dome. Use a shallow angle and cut as near the edge as you can. Take care not to puncture the paper cone which may continue under the dome as the voice coil may be of a smaller diameter than the dome. The shallow cut will also provide a base to reattach the dome if you are successful. Carefully scrape off a bit of the enamel insulation as near to the voice coil as possible and test with your ohmmeter once again. If the resistance is still infinite, there is nothing more you can do but salvage the magnet for fun experiments or erasing floppy disks. There is essentially no way to replace just the voice coil unless your driver has a removable voice coil unit (in which case you would not be reading this).
If the resistance now measures normal – a few ohms, trace back to determine which wire is broken and use some fine (e.g., #30 gauge) wire to bridge the break. You will have to scrape off the enamel insulation to permit the solder to adhere. Make sure it is secure mechanically first – a speaker cone is a rather violent environment for soldered connections. Finally, use some flexible adhesive to protect and reinforce the solder connections, to glue down your new wire along its entire length, to protect and reinforce the place where the wire passes through the cone, and finally, to reattach the central dome. Let the adhesive dry thoroughly before playing the 1812 Overture.
Loudspeakers – repair or replace?:
Assuming that the cabinet is in reasonable condition, the question arises: is it worth replacing broken, damaged, or worn out drivers or faulty crossover components that are not repairable rather than just dumping the speaker systems?
It is very straightforward to swap drivers as long as you get ones with similar characteristics. It all depends on what you want out of a loudspeaker. If you are basically happy with them, then it will be a lot cheaper than replacing the entire speaker system(s). However, speaker system quality has improved considerably in the last 15 years so now may be the time to upgrade.
As far as crossover components are concerned, these are basically common electronic parts and replacement is probably worthwhile.
However, if one driver has a deteriorated suspension, it is likely that its mate does as well and that other drivers may not far behind. Replacing **all** the internal components of a loudspeaker may not be worth it
No comments:
Post a Comment