Acoustic Devices

Different kinds of transducers can be used for generating, detec ting, or measuring acoustic waves in various media. Some of these, which we have already touched on, are piezoelectric, inductive, or capacitive devices. The constructional details of these devices have been outlined in previous sections. Here, we will be concerned with applications of acoustic transducers, especially for short-distance communications purposes.

THE DYNAMIC TRANSDUCER

The least expensive, most rugged, and perhaps most versatile acoustic transducer is an ordinary dynamic speaker or microphone. The dynamic device operates by means of electric currents generated by moving magnetic fields (speaker) or by means of mechanical forces caused by varying currents in the presence of a magnetic field (microphone). These principles are illustrated in Figs. 23-1A and 23-1B.

ill. 23-1. Principle of operation of dynamic speaker (A) and microphone (B). These renditions are greatly simplified.

Actually, a speaker can operate as a microphone. You may have tried this by connecting a small dynamic speaker to the input of a tape recorder or public-address system. The use of a speaker as a microphone is somewhat analogous to the operation of a generator as a motor. The converse of this is also often observed—a microphone can be used as a tiny speaker—but this practice is not recommended because a microphone is not normally expected to handle very much current, and damage is likely if a microphone is connected to the output terminals of an audio amplifier!

Dynamic transducers can operate over a wide range of frequencies, from below the human hearing range (less than 16 Hz) to tens or even hundreds of kilohertz. The design of a dynamic transducer is dictated by the range of frequencies over which it's to be used. In general, for a given power requirement the size of a transducer is proportional to the acoustic wavelength over which it's operated. A low-frequency device is therefore larger than a high-frequency device. The hi-fi enthusiast will recognize this by comparing the sizes of a woofer (bass speaker), midrange speaker, and tweeter (treble or high-frequency speaker).

ELECTROSTATIC TRANSDUCERS

In recent years another form of acoustic transducer has become common. This is the electrostatic device, which is actually a form of dynamic transducer that operates via electric rather than magnetic fields. As with the conventional dynamic transducer, the electrostatic device can be used either as a microphone (acoustic energy to electrical energy) or as a speaker (electrical energy to acoustic energy). The principle of operation of an electrostatic transducer is as follows. In the case of a microphone, impinging sound waves cause a flexible metal plate to move in the electric field created by two charged, fixed plates (ill. 23-2). This causes a fluctuation in the charge on the moving plate as it comes nearer to the positive fixed plate and then the negative fixed plate in accordance with the movement of the medium. In the case of a speaker, a varying charge on the movable plate causes it to be alternately attracted and repelled by the fixed plates.

ill. 23-2. Principle of electrostatic speaker or microphone.

Electrostatic transducers are somewhat more expensive than their magnetic counterparts, but they offer certain advantages. The impedance of the electrostatic transducer can be made very high (in fact, practically infinite), so that the sensitivity and power requirements are miniscule. Also, small electrostatic transducers exhibit excellent high-frequency response and can be used with good results at ultrasonic wavelengths. Another advantage of electrostatic transducers is their relatively light weight and small physical depth.

Both the magnetic and electrostatic types of dynamic acoustic transducer are physically rugged.

PEIZOELECTRIC TRANSDUCERS

The piezoelectric effect can be used to advantage in acoustic transducers, particularly those devices that convert sound energy into electrical impulses. The principle of the piezoelectric transducer is discussed in Section 6.

The main advantage of the piezoelectric transducer is its exceptional high-frequency response. Crystals can in fact respond to acoustic disturbances at megahertz frequencies. This is the principle of operation of a selective filter, e.g., used in a radio receiver. The natural (resonant) frequency of a quartz crystal may be as high as 10 MHz or more.

Piezoelectric transducers are light in weight, small in physical size, and fairly sensitive. Their main drawback is that they are somewhat fragile, both mechanically and electrically. They can not tolerate excessive input currents or voltages. Thus, they have limited usefulness as “speaker-mode” devices, except at low power levels.

AN ULTRASOUND RECEIVER

With a sensitive acoustic transducer, an audio amplifier, and a signal converter, it's possible to literally listen to ultrasound. The normal upper limit of human hearing is approximately 16 kHz to 20 kHz for a young person and decreases with age until it might be as low as 6 kHz to 8 kHz for an elderly person. With the device described here, it's possible to “hear” sounds up to several tens of kilohertz—frequencies higher than even dogs can hear.

A block diagram of the circuit is shown in ill. 23-3. The transducer may be a tweeter from a hi-fi speaker system, or it may be an electrostatic or piezoelectric device (commonly available in electronics stores). The audio amplifier can be an ordinary module obtainable in an electronics store, but a better design is to use an operational amplifier circuit. The input impedance of the operational amplifier should be high if a piezoelectric or electrostatic transducer is used (ill. 23-4A), low if a magnetic (tweeter) device is used (ill. 23-4B). With piezoelectric or electrostatic input transducers, a field-effect-transistor preamplifier stage can be put ahead of the op amp for increased sensitivity.

ill. 23-3. Block diagram of an ultrasound receiver.

ill. 23-4. Operational amplifier circuits for high input impedance (A) and low input impedance (B).

The converter can be designed in either of two ways. One method is to build a small amplitude-modulated (AM) transmitter consisting of a modulated local oscillator operating at a frequency of a few megahertz. The output of the oscillator can be fed directly to the antenna terminals of a communications receiver. The receiver must have a product detector, so that it can receive continuous- wave signals. The ultrasound noises will appear as sidebands above and below the carrier frequency of the modulated oscillator. If the oscillator operates at 3.500 MHz, e.g., then a 30-kHz ultrasonic note will appear as two signals—one at 3.470 MHz, and the other at 3.530 MHz. The receiver dial can be tuned from zero (in this case 3.500 MHz) to several tens of kilohertz (it makes the most sense to go upward in frequency on the receiver dial—3.510, 3.520, 3.530, and so on—so that the signals appear “right side up”).

The other method of detecting the ultrasound is to construct a variable-frequency oscillator, tunable from perhaps 10 kHz up ward to several tens of kHz, and mix the output of this oscillator with the output of the audio amplifier. This scheme is shown in block form in ill. 23-5. Another audio amplifier at the output, incorporating a selective filter resonant at about 700 Hz, allows some discrimination between various ultrasonic noises. The main problem with this scheme is that it results in double-signal reception, whereas modern communications receivers allow single-signal reception and also offer variable selectivity.

There is another, quite interesting, way to monitor the ultrasonic output of the circuit of ill. 23-4, if you happen to have access to a spectrum analyzer. (You might want to borrow the one in the lab where you work—borrow the use of it, that is.) Simply connect the amplifier output to the spectrum analyzer through the appropriate attenuator and set the analyzer for zero-left and about 5 kHz per graticule division. The resolution should be well under 1 kHz, perhaps even less than 100 Hz, although the optimum set ting will depend on the ultrasonic noise. With this arrangement you will be able to literally see the spectrum of sound from zero up to several tens of kilohertz. It may not even be necessary to use the preamplifier at all. It will be imperative that all leads be carefully shielded, especially between the transducer and amplifier and between the amplifier and the spectrum analyzer. The amplifier enclosure, too, will have to be shielded. Otherwise the interconnecting wires will pick up radio signals in the very-low and low- frequency ranges, and these signals will appear as false displays on the analyzer screen.

EXPERIMENTS

What can you expect to “hear” with a device such as this? Dog whistles? Actually, there is quite a lot of ultrasonic noise around. Most of it's in the form of “pink” noise, a broadband form of noise that gets less and less intense with increasing frequency. However there are ultrasonic components in your own voice; various small animals and insects may emit ultrasonic noises (I don’t know for sure; it might be interesting to find out). An especially interesting application of this device would be for monitoring of sound and ultrasound under water. Some piezoelectric transducers are submersible and could be used for this purpose.

In conjunction with the ultrasonic transmitter about to be de scribed, short-range sound communication can be carried out through the air. Under water, longer distances can be spanned because sound travels much faster through water than through air.

ill. 23-5. An ultrasound receiver employing a communications receiver in conjunction with an amplifier and a converter.

ill. 23-6. A simple oscillator for producing ultrasound.

AN ULTRASOUND TRANSMITTER

The ultrasound transmitter is actually simpler to build than the receiver. An oscillator, preferably of variable frequency, is constructed using a tuned circuit as shown in ill. 23-6. A common 88-mN toroid can be used as the inductor; a 365-pF variable capacitor available in most electronics stores, can be used at C. The output of the oscillator can be fed to the input of a hi-fi amplifier for use at frequencies up to perhaps 30 to 40 kHz. Alternatively, a broadband power amplifier circuit can be built using a power transistor.

The transducer must be capable of handling the power output from the amplifier. The logical choice is a hi-fi tweeter with a maximum power rating of at least 50 percent more than the amplifier output. Tweeter devices are available that can handle quite large amounts of power. The cost, of course, depends on the power- handling capacity. it's important that a high-pass network be in stalled in the audio line to the tweeter; a simple means of doing this is to place a 0.1-pF ceramic capacitor in series with the line. This will also prevent the tweeter from being subject to direct currents.

Ultrasound, like ordinary sound, is reflected and diffracted. For this reason a direct line of sight is not necessary for communications. However, diffraction will not take place to as great an ex tent as with ordinary sound, and most objects don't reflect 100 percent of the sound energy that strikes them. Consequently the best results are obtained with a line-of-sight path. Path loss is affected by rain, fog, or dust in the air.

Using a tweeter with 10 W of audio power and a direct line-of- sight path, signals can be transmitted over fairly long distances (1 or 2 km) under ideal conditions. The range can be increased by concentrating the ultrasound into a narrow “beam” using a reflecting dish (ill. 23-7). Because the wavelength of ultrasonic energy is quite short—only 13 mm at 26 kHz, e.g.,—the dish need not be especially large. A diameter of 2 or 3 feet is adequate.

When working with ultrasound, it's important to remember that it can be quite irritating at high levels. Some people develop headaches from exposure to ultrasound even though they can’t hear it. and dogs are liable to go crazy as you conduct your experiments. It might not be a good idea, therefore, to employ power levels of more than a few watts!

ill. 23-7. A dish reflector can be used to concentrate the ultrasound emanating from a transducer. The same technique can be used for reception, enhancing the range and reducing interference from stray sources.

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