Last Modified: June 26, 2015
Contents: Basics; Using Vehicle Stereo Systems; Mounting Considerations; Non-DSP Powered Speakers; Plain Old Speakers;
Passive audio filters have been around for many years. The Skytec CW-1, made during the 70s, is a good example. It utilized a ported 2 inch speaker mounted in a tuned cavity made from thin wall PVC pipe. At the mouth was a little sleeve you could adjust to set the center of the bandpass. If your receiver didn't have a CW filter, this little jewel worked fairly well. It sold for about $20 at the time.
Discrete-component passive filters for SSB have also been produced over the years by just about every manufacturer of amateur equipment. When these units were first introduced, few radios had built in bandpass or IF shift features, to say nothing of the DSP systems we have nowadays.
Unlike the aforementioned CW filter, all of these designs used a combination of inductors and capacitors typically built into a station speaker. The schematic at left is that of the Icom SP-20 (click on it for a larger view). Note the switches to select the desired mode; lowpass, highpass, or both. Also note the passive elements are in series with the speaker for good reason; you have to be careful using directly shunted elements with single-ended audio amplifiers, as doing so can cause them to fail.
It should be noted that passive filters have insertion loss (3 to 5 dB depending on the design), so if your DSP-equipped mobile transceiver is already lacking in the audio out department, a DSP and/or powered speaker is better choice. The Hear It® unit shown at right is marketed by GAP. It sports a wide range of DSP settings, and sells for about $180. It comes with a headphone jack, a fused power cord.
West Mountain Radio now sells a new model of their popular ClearSpeech, shown at left. It is larger than its competitors mainly because of its larger speaker. As a result, it does have more bass response than the others. Whether this is an attribute remains with the listener.
All of these devices use an audio-interfaced DSP (Digital Signal Processor) to remove band noise, static peaks, and other bothersome background hash. In some cases, they'll out perform built in audio-based DSP units, however, none of them are as good as a properly designed IF DSP. The main reason is, most built in IF units are placed before the AGC loop, so nearby large signals don't overload them.
If you need a bit more audio power, BHI in England makes the DSPKR with 10 watts of power which should go a long ways in noisy environments. Incidentally, BHI makes most of the private-labeled DSP speakers sold in the US.
There is no argument about their usefulness, as long as you don't mind the additional level of complexity. They do require DC power which must be switched on, and they must be positioned to allow operation of their controls. And using the accessory jack to power external DSP speakers isn't a good idea. The limiting factor isn't necessarily the current rating (typically one amp). Rather, it is the voltage drop through the radio—as much as 2 volts in some cases. This can cause some powered speakers to operate erratically.
Using Vehicle Stereo Systems
Vehicle stereo systems are designed specifically for each model in an effort to maximize linear frequency response. The higher end systems even have built in equalizers which allow owners to further tailor the frequency response to suit their desires. But EQs typically do not have the range to effectively cut off frequencies above 2.7 kHz. As a result, using one in place of a decent mobile speaker is a stopgap solution you'll soon change.
All too often, operators mount their speakers too close to their heads. One popular mounting location utilizes the headrest support posts. This isn't a very sound idea (pun intended!), and here's the reasons why. First, modern vehicles have SRS devices (airbags) mounted in every nook and cranny. Besides the dashboard area, they're often mounted in the sides of the seats, the headliner, and even inside the headrests! It should be obvious why these areas are poor mounting locations for speakers, no matter their size.
Secondly, sound pressure levels can be deceiving especially in a noisy environment like the interior of some vehicles (trucks mainly). And, contrary to popular belief, it isn't just the low frequencies which can be damaging to one's hearing. The high frequency hash we all put up with when listening to weak SSB signals is just as annoying, and deaf-producing! Here's a little experiment you can carry out on your own. No matter where your speaker is mounted, after about 10 minutes of listening, just turn off the radio. If you suddenly feel like you're in an insulated sound proof booth, chances are your speaker volume setting is too high!
Here (and hear) is a suggestion to minimize the hash. Place the speaker under the front driver's seat, pointing upward. This tends to increase the bass response while muffling the high-frequency hash. Up under the dashboard pointing into the foot well works almost as good. Either way, you'll end up using less audio gain, which is a good thing in more ways than one!
Non-DSP Powered Speakers
The audio output in modern transceivers seldom tops 3 watts, which is barely adequate especially in a noisy mobile environment. Fortunately, there are several companies making amplified speakers these days, including the MFJ-382 shown at right. It sells for under $40. Midland®, Cobra®, and Shakespeare® also offer amplified speakers for about the same price.
Occasionally, you'll find old Motorola HSN1000B amplified speakers at hamfests for as little as $10. New ones cost about $75. It's amazing how good they are as long as the speaker cone hasn't deteriorated—caveat emptor.
Plain Old Speakers
There is nothing wrong with plain old speakers. However, there are a few things to consider when selecting one.
First, price means nothing. Some really cheap speakers will out perform ones costing 10 times as much. One of the reasons for this is. Expensive speakers tend to have a much wider bandwidth (high frequencies especially) than cheaper ones, and they're typically larger in size. While they might sound a little fuller at home, when you're mobile you need all of the definition you can get. Conversely, you don't want one too small or you won't have enough bass response, and the highs will be accented which reduces readability.
Because each person's hearing is slightly different, you should attempt to try before you buy (always a good idea!). Just don't buy one smaller than 3 x 5 inches. Those tiny 2 inch jobs don't sound very good unless you mount them very close to your ear—something you shouldn't do!