engineering

Gain before feedback

Gain before feedback 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 Gain before feedback rather than just read about it. In short: In live sound mixing, gain before feedback (GBF) is a practical measure of how much a microphone can be amplified in a sound reinforcement system before causing audio feedback. In audiology, GBF is a measure of hearing aid performance.

Key takeaways

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

Reference excerpt

In live sound mixing, gain before feedback (GBF) is a practical measure of how much a microphone can be amplified in a sound reinforcement system before causing audio feedback. In audiology, GBF is a measure of hearing aid performance. In both fields the amount of gain is measured in decibels at or just below the point at which the sound from the speaker driver re-enters the microphone and the system begins to ring or feed back. Potential acoustic gain (PAG) is a calculated figure representing gain that a system can support without feeding back.

Live sound In live sound mixing, GBF is dependent on a wide variety of conditions: the pickup pattern (polar pattern) of the microphone, the frequency response of the microphone and of the rest of the sound system, the number of active microphones and loudspeakers, the acoustic conditions of the environment including reverberation and echo, and the relative positions of the microphones, the loudspeakers, the sound sources and the audience. Each doubling of the number of open microphones (NOM) reduces the PAG by 3 dB. Directional microphones are used in live sound to maximize GBF. Directional microphones with cardioid and hypercardioid pickup patterns are designed with reduced sensitivity to the rear (cardioid) or to an angle between the side and the rear (hypercardioid). Such microphones are aimed such that their pickup pattern is weakest in the direction of the loudspeakers. This is especially useful in the presence of foldback monitors (stage wedges). Directional loudspeaker systems may also be used to increase GBF. The distance from the sound source to the microphone is a critical element of GBF. Greater GBF is obtained with the performer closer to the microphone; an instance of the inverse-square law. If the performer reduces the distance to the microphone by half, the PAG is increased by 6 dB while the environmental sounds remain relatively the same. The sound system operator can use equalization to change the frequency response of a microphone or loudspeakers system to increase GBF. The frequency which first begins ringing or feeding back is identified by the operator, and a notch or parametric filter is engaged to reduce the overall level of that frequency. This process is repeated several times to identify and reduce the level of further feedback frequencies. A graphic equalizer can be used for the same purpose but with somewhat less precision. Automatic feedback suppressors automate and speed the process of identifying and reducing feedback frequencies. A small amount of pitch shift applied to the signal can increase GBF, as can the addition of a few milliseconds of straight delay. The latter will increase the number of feedback frequencies while reducing the frequency range within which they occur, but it will slow the rate at which feedback grows. In practice, adding straight delay to a signal improves GBF.

Hearing aid A hearing aid incorporates a miniature microphone and a very small speaker driver, and various conditions may increase or decrease the amount of gain that can be applied to the microphone signal before feedback. A well-fitted hearing aid has more GBF than one that is loose. The shape of the earmold is a factor, with larger and heavier designs capable of delivering greater GBF. Hearing aid designs deliver gain in increasing steps depending on the severity of the patient's hearing loss; the range spans from 10 to 65 dB of gain. To prevent feedback with the highest amounts of gain, such designs require the tightest-fitting earmolds with no venting, and the deepest penetration of the ear canal to place the speaker driver as close as practical to the eardrum.

Worked examples

Example 1 — a first encounter with Gain before feedback

Start with the simplest possible case. Write down what Gain before feedback 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 Gain before feedback 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 Gain before feedback 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 Gain before feedback

In research: Gain before feedback 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 Gain before feedback 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: Gain before feedback is common in secondary-school and first-year university syllabi. It links to neighbouring topics Audio engineering, Sound production technology, Tone, EQ and filter, so understanding it makes those chapters shorter.
In everyday life: Look for Gain before feedback 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 Gain before feedback in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Gain before feedback 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 Gain before feedback out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Gain before feedback in simple terms?

Why does Gain before feedback 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 Gain before feedback?

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 Gain before feedback.