Speakers, Impedance, Radios

 


----- Speakers, Impedance, Radios -----

NOTE: The examples are based on manufacturer's specifications for representative equipment. While the information is accurate, always check the manufacturer's specification for your equipment and adjust any calculations as necessary.

1) Speaker Placement. Besides replacing the existing speakers with new ones, you may want to add additional speakers (see below) at other locations. Just a couple of minor things to keep in mind. Low frequencies are non-directional. That is they radiate from the speaker pretty much in all directions. Mid and high frequencies ARE directional so it's easy to tell that a sound is coming from one channel or the other. Where you place the woofers doesn't matter a whole lot ~ it has little effect on the "stereo image". (This explains why you find a lot of "dual-coil" woofers that you hook both left and right channels to.) But where you place the mids or tweeters WILL effect the image. The bottom line is: you can mount the larger woofers anywhere they'll fit and the smaller mids and tweeters in locations best suited for stereo. ~ see also Radios and amplifiers, below

Speaker terminals are usually marked with a plus (or a dot - usually red) and minus. This designation is to help you to get your speakers all hooked up "in phase". This is, so they are all moving the in same direction at the same time. Failure to observe this will result in some of the sound waves cancelling each other out with a noticable loss of bass response.

2) Speaker resistance is more properly called impedance. Resistance is only a direct current (DC) measurement. Impedance is the effective resistance to a changing signal like alternating current (AC) or an audio signal. It cannot be measured with a simple ohmmeter. A speaker's impedance, in ohms, is plotted across it's frequency operating range. Somewhere in the lower frequencies, usually around 100 Hz, there is a peak in the impedance where the natural resonant frequency of the speaker is reached. Following this resonance, the impedance will dip to a low value before slowly rising again. This "dip" just after the resonant peak is the speakers nominal rating. So "8" ohms is only an approximate minimum impedance ~ a guideline.

For the record: there is little difference between a 8 and a 10 ohm speaker (there are 4, 6, 8, 10, 16 and a few oddballs). An "8 ohm" speaker, as noted above, will vary its impedance through it's operating range. The general tolerance for common electronic components is plus or minus 20%. So 8ohms and 10ohms are just within that 20%

It is ALWAYS safe to go to a higher impedance speaker to replace a lower one. This is, you can replace a 6ohm with an 8, a 10ohm with a 16. It is NOT safe to use a lower one in place of a higher one unless it is within the 20% tolerance noted above. Therefore, it is OK to replace a 10ohm with an 8 but NOT OK to replace an 8ohm with a 4 (unless the manufacturer says so).

3) Speaker systems fall into two broad catagories: Those that use crossover networks and those that do not. Crossover network speaker systems use a type of filter that separates frequencies into two or more bands each of which is routed to a speaker specifically designed to handle that band. These are also referred to as "2-way" or "3-way" systems. Classically the speakers themselves are called the woofer (bass), the mid-range, and the tweeter(highs). If you use a system that comes from a manufacturer with multiple speakers (such as coaxial speakers) and it is rated by the manufacturer at 8ohms, you can consider it to be one speaker with that impedance. Single speakers that do not use a crossover are called "full" or "wide" range speakers. They are just what the name implies: a single speaker that covers the entire frequency range. The full range speaker is less expensive with poorer high and low frequency response; the 2 or 3-ways give better response but at a premium. (also see Piezo Speakers, below)

Multiple speakers (including 2 and 3-ways) can be hooked up in parallel, in series, or a combination of both.

Hook up speakers in PARALLEL will DECREASE impedance. That is, two 8ohm speakers can be hooked together to create a 4ohm impedance. Remember than decreasing impedance (increasing load) will usually cause an amplifier to put out more power.

The math used to calculate the impedance (represented by "Z" - don't ask why) of parallel speakers gets a little tough but here goes:

                       1
       ---------------------------------
Zt  =     1        1        1
        ----  +  ----  +  ----  +  etc...
         Z1       Z2       Z3
      

So an 8ohm and a 4ohm would be:

             1
       -------------            1               1
Zt  =    1       1    =  ---------------  =  -------  =  2.67 ohms
        ---  +  ---       0.125 + 0.250       0.375
         8       4
      

The good news is when hooking speaker of the SAME impedance in parallel, simply divide the impedance by the number of speakers. For example: hooking three 8ohm speakers in parallel gives 8 / 3 = 2.67ohms. MAKE SURE THE AMP CAN HANDLE IMPEDANCES LESS THAN 4ohms! And make sure you hook the positives (or dots) and negatives as shown:

       |----------------------------[+]AMP
       |
       |         |------------------[-]AMP
       |         |
      (+)SPEAKER(-)
       |         |
       |         |
      (+)SPEAKER(-)
       |         |
       |         |
      (+)SPEAKER(-)
      

Hook up speakers in SERIES increases the impedance. That is, two 4-ohms can be hooked together to create an 8-ohm impedance. Remember than increasing impedance (decreasing load) will usually cause an amplifier to put out less power. The math is simple, simply add the impedances: an 8ohm and a 4ohm equals 12ohms. Make sure you hook positives (or dots) and negatives as shown:

       |----------------------------[+]AMP
       |
      (+)SPEAKER(-)
                 |
                 |
       |---------|
       |
       |
      (+)SPEAKER(-)
                 |
                 |------------------[-]AMP
      

4) Piezo Speakers: Most of the speakers we have been discussing and most of the speakers you're likely to run across are called "dynamic" speakers. They have the typical cone, voice coil, and a magnet on the back. But there is another type around called a "piezo" speaker. These use a crystal element to generate sound rather than a magnet/coil/cone. These are almost always found as tweeters. Piezo tweeters are small (so you can stick them anywhere), require no crossover, usually have a fairly high power rating, and have a very high impedance ~ usually about 100ohms. The nice thing about this is you can hook several up without overloading the amp. Hooking two 100ohm piezo tweeters in parallel to an existing 8ohm speaker would give a total impedance of about 7ohms ... a safe bet. (A 2" piezo tweeter from Radio Shack is only 5 bucks, cheap enough for an experiment.)

If your considering adding additional tweeters, use a "main" speaker that has a little too much bass or a woofer ~ which will be balanced out by the added tweeters.

5) Radios and amplifiers are rated by their manufacturers with the maximum continuous power (RMS) output and a minimum speaker impedance in ohms. The maximum RMS power may often be rated at differing loads such as 25 Watts RMS per channel at 8ohms and 33 Watts RMS per channel at 4ohms or with only a single rating. Since amplifiers will produce more power at higher loads (lower impedances), a single rating usually reflects the maximum power at the lowest impedance the amplifier can tolerate. If the specification states 33 Watts RMS at 4ohms, we can be pretty sure that the amp will deliver a maximum of 33 watts RMS into a minimum impedance of 4ohms. Unless the manufacturer specificly states that the amp can tolerate an impedance lower than 4ohms, DO NOT ASSUME YOU CAN DO IT.

Please note the amplifier ratings discussed are presented only as an example. Actual specifications vary from manufacturer to manufacturer and from model to model. Consult your owners handbook or user's guide for the actual specifications of your equipment.

If, as discussed above, you hook multiple speakers or speaker systems up, the question becomes which speakers get what power. If the amp discussed put out 25 Watts RMS at 8ohms and you paralleled a single 8ohm speaker with a second 8ohm speakers, the amp, as specified by the manufacturer, will now put out 33 Watts RMS at the new 4ohm impedance. With two 8ohm speakers, the answer is simple ~ each speaker will get half the maximum power. This would hold true even if they were hooked in series (though the amp would put out less power at 16ohms).

If, however, you hooked up an 8ohm and a 4ohm speaker in parallel (and assuming the manufacturer says it's OK), the picture becomes a little different. Lets' assume the amp will now put out 36 Watts RMS at our new 2.67ohms impedance. The power will now be split between the two speakers INVERSELY proportional to their impedances. That is, the 4ohm speaker would get 24 watts and the 8ohm speaker would get 12 watts. Therefore the 4ohm, getting twice the power, will be louder than the 8ohm ~ something you might want to remember when considering where to mount them.

If you hooked up an 8ohm and a 4ohm speaker in series, the power will now be split between the two speakers DIRECTLY proportional to their impedances. If the amp now put out 18 Watts RMS at our new impedance of 12ohms, the 4ohm speaker would get 6 watts and the 8ohm speaker would get 12 watts. The 8ohm would now get twice the power of the 4ohm and will be the louder speaker.

6) Adding new life to old radios (especially if the output transistors keep quitting ~ see below) without sacrificing the OEM radio (or the points) is too stash a power booster out of sight. Even a cheapie like a Spark-o-matic (what-a-name) probably has better specs than the original radio. Note that the input impedance (what you'd hook you radio's outputs to) of the booster is usually pretty high, 100 ohms or more, so they present no loading problem to an old radio. You can hide them anywhere if you're worried about appearance and they usually have the added benefit of being able to drive larger loads such 4ohms or less.

For mono radios, simply hook the speaker output from the radio to BOTH left and right inputs on the booster. Just make sure you hook them in the same direction, don't cross the wires. This won't give you stereo but it will sound a lot better. You can get a (sort of) stereo-like effect by placing woofers in one end of the car, like the rear deck, and the mids/tweeters towards the front. It won't be stereo but it will have the effect of spreading the sound around a bit and give it a bigger "feel" .

A word about old radios that keep quiting: If you are taking your radio/amp back to the shop evey couple of months or so to have the ouput transistors replaced, here's a tip: The amplification stage just before the output stage is called the output driver. When the output transistors short, they usually put a pretty good whomping on the driver transistors. When it goes to the shop, the tech checks the outputs, finds them shorted, replaces them, fires it up, it works, hands it back, and collects his money. BUT... the drivers have been permanetly twisted by the shorted outputs and will operate slightly out of spec. This causes the outputs to run a little harder than they should so they fail a few months (rather than many years) later. The next time, tell the technician to humor you and change the transistors and any diodes in the driver stage of the offending outputs. Before I started doing this, I had WAY too many systems come back to the shop. Once I started changing the drivers along with the outputs, they almost never came back. ~ The Voice of Experience ~

 

BTW, I don't recommend any particular brands. Too much depends on the listener. If it sounds good to you it IS good even if your buddy hates it ~ and even if it was made by RonCo.

 

Submitted by: Mike Frederick

 

1/13/97