engineering

Subaudible tone

Subaudible tone is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Subaudible tone rather than just read about it. In short: A subaudible tone is a tone that is used to trigger an automated event at a radio station. A subaudible tone is within the audible frequency range; however, it is usually at a low level that is not noticeable to the average listener at normal volumes.

Key takeaways

Subaudible tone belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Subaudible tone to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Subaudible tone from memory before moving on to harder problems.

Reference excerpt

A subaudible tone is a tone that is used to trigger an automated event at a radio station. A subaudible tone is within the audible frequency range; however, it is usually at a low level that is not noticeable to the average listener at normal volumes. It is a form of in-band signaling.

Overview These tones are included in the audible main portion of audio in the case of a satellite; on tape, these often are filtered. Typically, subaudible tones are at one of the frequencies: 25, 35, 50, 75 hertz (Hz), or combinations of those frequencies. Until computerized radio automation became inexpensive and common, 25 and 35 Hz were used either in the audio stream or, in the case of tape cartridges used in radio broadcasting (better known as "carts"), on a special track on the tape to indicate to a radio station's automation system that it was time to trigger another event. With the advent of computers and digital satellites, these tones are relegated to triggering commercial announcements and legal IDs on a dwindling number of radio networks. External data channels have supplanted tones in the audio sent independent of audio on digital satellite feeds for radio. These trigger relay closure terminals on the satellite receiver (Starguide is a prominent system).

Use for filmstrips Later, filmstrip projectors used subaudible tones to advance to the next frame in a filmstrip presentation. Previously, the phonographic record or audio cassette accompanying a filmstrip to provide its soundtrack would have an audible tone to signal the person operating the projector to advance the film to the next frame. However, automatic filmstrip projectors were introduced in the 1970s (that had an integrated phonograph or cassette player) that would read a subaudible tone of 50 Hz recorded on the soundtrack to trigger the projector to advance to the next frame automatically. Most cassettes accompanying filmstrips from the 1970s and 80s have one side of the media with audible tones for use with manual projectors and the other side with the same program audio but with 50 Hz subaudible tones instead for automatic projectors. Some filmstrip releases combine audible and subaudible tones, making the filmstrip and its companion cassette or record compatible with any filmstrip projector.

External links Examples of subaudible tone units

Worked examples

Example 1 — a first encounter with Subaudible tone

Start with the simplest possible case. Write down what Subaudible tone claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Subaudible tone before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Subaudible tone ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Subaudible tone

In research: Subaudible tone appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Subaudible tone in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Subaudible tone is common in secondary-school and first-year university syllabi. It links to neighbouring topics Broadcast engineering, Broadcasting stubs, Radio technology, so understanding it makes those chapters shorter.
In everyday life: Look for Subaudible tone outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Subaudible tone in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Subaudible tone means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Subaudible tone out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Subaudible tone in simple terms?

Why does Subaudible tone matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Subaudible tone?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Subaudible tone.