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Semi-precious Jewels

Nuggets of Knowledge
from Dennis, K1YPP

Acoustiwhat?
Most amateurs can understand what happens to a radio signal from the time it starts in the oscillator, gets amplified and multiplied, transferred to the outer world via the antenna, is received at the distant station, detected and amplified and sent on to the loudspeaker. It’s at this point that most amateurs feel their work is done and they can now go have a coffee. What about that electro-mechanical transducer, the loudspeaker, and in many cases the microphone as well? The sound has to get from the electrical state to the acoustic state and back to the electrical state in the brain.

To understand how this can happen some knowledge of acoustics is needed. Acoustics can get complicated, so lets keep it simple.

Each evening some weather reports on the evening news give the barometric pressure in millibars; this is nothing more than a means of measuring air pressure. As far as acoustics is concerned, millibars is a rather large pressure, so it is more common to work with microbars. (This sounds like using Decibels instead of Bels, doesn’t it?)

What is a microbar? Well, a microbar is considered to be the minimum audible level of sound that the average person can hear. To be more exact its 0 dBa. I’ve taken the liberty here of adding the “a” to indicate we’re discussing audio, or decibels audio. 0 dBa is equal to a pressure of .000204 dynes, which is another unit of measurement.

You may wonder how much is a dyne, well, it takes 27805.5 dynes to give an equivalent of 1 ounce of pressure per square centimeter! This means of course that the typical human ear can hear sounds as low as 7.32 x 10^9 ounces per square centimeter!

At the other end of the scale we start to feel pain in our ears at 105 dBa, which is a sound pressure level (SPL) of 36.3 dynes or 0.0023 ounce. If some of you old timers would clean your ears once in a while, it would be like raising that beam another 200 feet (61 meters). The ears are very delicate receivers indeed! It was once stated that if you took all the audio power in the air at a full football stadium in one afternoon, it would be enough energy to warm a cup of coffee!

So, how does all of this relate to amateur radio equipment? If you have ever looked at a microphone specification sheet you have no doubt seen something like -66 dB REF 1V/dyne. What the manufacturer is saying is this: If you expose this microphone to an audio signal of 1 dyne (which is 73.81 dBa, approximately normal conversation levels of speech) it will produce a voltage into a matched impedance that is -66 dB down from one volt. Well, how much is that?

Dragging our minds through the coals, it’s:

   Voltage
-66 dB =20 Log---------
   Ratio

Divide both sides by 20:

   Voltage
-3.3 dB=Log---------
   Ratio

Take the anti-log of both sides:

Power = 0.3 x 103 = 0.0005

In other words, the microphone will provide 0.5 millivolts output at normal levels. In the case of an electret or condensor microphone a typical impedance would be 1 - 5 KW. This is a good signal because electrical noise is fairly low with these devices. Now all you have to do is amplify it to the desired level. Most electrets have an FET built right into them to build up the signal before it travels very far and picks up additional noise. Actually the FET is there to isolate the element capacitance, but the gain helps.

Microphones have fairly good response ranges, even the cheap ones, but this is not so with loudspeakers. Loudspeakers, especially in most amateur radio applications, are terrible things. Their response curves look more like West Virginia road maps!

Only in very recent times have there been any improvements by the amateur fraternity. There are several folks on the QRP list on the Internet doing serious experiments and several years ago there was a product manufactured by Skytec in California. The Skytec product was designed with an end result resembling that of a good active filter, excellent for CW reception. This is really a Band-Aid for that CW receiver that is spilling out garbage from 30 Hz to 10 KHz. It cannot fix a sick front-end, but it certainly can filter out much of the noise. Unfortunately, most amateur radio equipment designers worry more about the aesthetic beauty of the speaker cabinet than its function in life.

It does not, repeat, does not, take much power to make lots of audio noise. A telephone dial tone is approaching 0.5 milliwatt, and you can hear that across a quiet room. Hi-Fi buffs have amplifiers that now rival anything amateurs have for finals on 20M. What does one do with 500 watts of audio? Your ears can’t take that sort of punishment for any length of time, how about “worked all states” on audio. There are folks today that have these super high- powered audio systems in their cars, pumping lots of rap drumbeats into their ears. The audio stores get these guys coming back to complain that the system isn’t as loud as it used to be, they have to run the volume all the way up. The truth is, their equipment is fine, but their ears are gone!

An amateur receiver only NEEDS 1 or 2 watts of audio unless it is to be used in a particularly noisy environment. A good stereo system should work well with 50 watts of power (this is real power, not all the hocus-pocus power measurement schemes that the marketing guys use). Anything more than this, in most cases, is wasted power.

It may not be easy to create audio enclosures that will operate over a specific band of audio frequencies, but the benefits are great, and more work must be done in this area.

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