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I have seen people claim these two scenarios as being possible acoustic amplifications. My question is, are they TRULY amplification or are there any other types of acoustic amplification in existence.

  1. An acoustic megaphone (is it amplification or just a focus of the sound?)

  2. Singing in a chapel (is the sound amplified or is it just reverb and delay)

So, is amplification actually even possible in acoustics or does it have to be turned into an electrical signal first?

Note: I understand the definition of acoustic is not having electrical amplification. I’m not sure how to word it other than “amplification of air”, and I'm not sure if that is correct either. If someone can edit it for me or tell me how to fix the terminology please let me know. Thanks.

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13 Answers 13

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Previous answers have already elaborated how mechanical means can improve the efficiency of sound transfer from source to ears (or transfer from vibration to sound). From deflectors, to horns to sound boards to tin-can-telephone-toys. What they have in common, is that the perceived sound is louder than without, but they do not add energy into the systems. That puts practical limits on the amount of amplification.

Electric solutions (mic/pickup + amp + speaker) can also make sound louder, but they insert extra energy (the amp draws power). There are fewer practical limits here: You can blast a full Wembley stadion with the sound of a single needle drop.

There are, however, also non-electric means to amplify sound by injecting extra energy. In the early 1900’s they had gramophone players with pneumatic amplifiers. The sound is amplified through valves, powered by compressed air. I’m pretty sure that the same principle could be used for a compressed-air-powered megaphone

See also http://www.vias.org/crowhurstba/crowhurst_basic_audio_vol1_028.html

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An example of actual amplification (rather than just impedance transformation like most acoustic string instruments do, i.e. leading to much reduced sustain times of concert acoustic guitars compared to electric guitars) would be the amplification of free and forced reed vibrations (harmonica, accordion, reeded organ pipes, reed and brass instruments), namely the amplification of a small primary oscillator by letting it control the passage of a compressed air stream.

The energy of the amplified sound comes from the compressed air, not the primary oscillator (that actually gets its energy from the same source).

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  • In addition to playing, I've spent VERY much time thinking about harmonicas and would claim to know how they work. I know that it's the airstream that's making the sound, and that the frequency is determined by how fast the reed opens and closes the airway. Further, I'm a certified radio amateur, so I know a little about electric oscillators and amplifiers. Yet, I never thought about harmonicas as amplifiers before now. This is great!
    – EdvinW
    Dec 20, 2023 at 16:25
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According to the law of conservation of energy you can't amplify sound without an energy input. What you can do is concentrate the sound locally in one direction using a megaphone at the cost of reducing the sound output in other directions.

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    So it is amplification just that it isn’t anywhere close to as powerful as it can be once turned to electricity? And are brass or like woodwind instruments technically amplification as well? Basically, are there any instances of very loud amplification in air or is that as loud we can get.
    – Lecifer
    Dec 7, 2023 at 22:46
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    @Lecifer No, your examples are not amplification, neither are wind instruments.
    – PiedPiper
    Dec 7, 2023 at 23:00
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    Even though sound boards and resonant bodies used in chordophones do not add total energy, we still call them "amplifiers". The acoustic energy put out by a vibrating guitar string in free space is much lower than the acoustic energy of the same string vibrating with one end connected to a guitar body. Kinetic energy is changed into acoustic energy by the guitar top and body. The acoustic gain of the top and body is greater than 1, so it is an acoustic amplifier. Dec 7, 2023 at 23:37
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    @ToddWilcox we call it amplifier by convention, but the word is misleading. In terms of physics, it's actually wrong. Strings have little effect on sound and acoustic energy, because they have a very small surface affecting the air, and a lot of their kinetic energy may be dissipated by the parts that keep the string in place (eventually transforming it in small amounts of heat). When at least one of those parts allows transfer of kinetic energy, then we have a better acoustic energy performance because the dissipation has a longer run: the bridge, then the sound board, and finally the air. Dec 8, 2023 at 1:14
  • Comments have been moved to chat; please continue this interesting discussion there. Dec 9, 2023 at 1:01
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Adding on PiedPiper's answer, energy must be conserved. Acoustic "amplification" is, as already stated, focusing the energy in one direction but also (the point of this answer) to do acoustic impedance adaptation between the medium creating the sound (tube, string) and the open air. That's for instance the purpose of parabolic shapes of horns but also what is done with the sound tables of string instruments.

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    +1 for mentioning impedance matching. Increasing coupling efficiency of energy already there rather than adding more energy.
    – DKNguyen
    Dec 8, 2023 at 15:46
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Does natural amplification TRULY exist...?

If we replace the word "natural" (since it's not clear what that means in this context) with "non-electrical", then the answer is definitely:

Yes

Discussions of conservation of energy and/or requirement of the input of additional (i.e., non-signal) energy are irrelevant, since the word "amplifier" is used in engineering contexts to refer to devices that do not use an additional source of energy to increase the strength of the input signal.

The category of non-electrical amplifiers relevant to sound and music are called mechanical amplifiers. A very simple non-resonant mechanical amplifier is a lever. In a lever, input force and displacement are coupled to output force and displacement such as either the force is amplified at the cost of displacement or vice-versa. Despite the fact that no additional energy is added to the system beyond the "input" energy, a lever is considered a mechanical amplifier.

Another type of mechanical amplifier that is clearly relevant to acoustics is the resonant amplifier. Soundboards and bodies of string instruments combine resonant and non-resonant mechanical amplification.

While some may disagree that the definition of "amplifier" should include passive devices (i.e., no non-signal energy is added), it is clear that such devices are called "amplifiers" in many contexts. One particular example is the following quote from the Wikipedia page on mechanical amplifiers:

No new energy is created. However, through mechanical amplification, more of the available power spectrum can be utilised at a more optimal efficiency rather than dissipated.

https://en.wikipedia.org/wiki/Mechanical_amplifier

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  • I concur with your explanation, but, at the same time, we could call the same amplifier device as an attenuator, if we consider the other involved component (without mentioning other energy transfers, like heat). For instance, we can cut paper closer to the fulcrum of a scissor either to get more power, or to get less and more precise movement. And your last quote is straight to the point: efficiency. An acoustic "sound amplifier" doesn't really amplify the sound (as opposed to a real amplifier), it transmits it more efficiently, through mechanical amplification, preventing dissipation. Dec 8, 2023 at 21:28
  • Throw in constructive interferance too. Not sure why this whole Q&A lost its mind, but the comments sections are in dire need of moderation.
    – Yorik
    Dec 8, 2023 at 21:28
  • @Yorik I understand resonance is the result of constructive interference. Dec 9, 2023 at 2:00
  • @ToddWilcox However, not all constructive interference is resonance...I think...
    – DKNguyen
    Dec 9, 2023 at 20:18
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As the other stated, you can't make energy out of nothing.

In addition to the two mechanisms you listed (focusing the sound towards the listener and adding reverberation), I would add one more: increasing the efficiency of transferring the vibration to the air. Many of the common sound sources don't do it well naturally.

For example, a moving string creates a slight overpressure in front of it, and a slight underpressure behind it. As the string has a very small diameter, these two regions are very close to each other, and the two resulting sound waves in opposite phases almost entirely cancel each other. This is why an electric guitar without amplification is so quiet. In a classical or acoustic guitar the string vibrations are transferred to the body of the instrument, which is large and much more efficient in transferring the vibration to the air. But this still cannot be called an amplification, as this efficiency is never greater than 100%, i.e. the amount of the sound energy cannot exceed the work performed by the musician to pluck the string.

So, is amplification actually even possible in acoustics or does it have to be turned into an electrical signal first?

What you certainly do need is an additional source of energy. In an electric amplifier you draw this energy from the electrical outlet. I don't know if non-electrical amplifiers were ever made, but in principal no laws of physics forbid it.

Consider pipe organ. A musician presses the key, they allow the air to flow through a pipe. The sound energy can be much larger than the work the musician did to press the key. This is because the energy comes from an air pump, which is powered by a separate motor, or a human assistant, rather than the fingers of the musician. This is an example of purely mechanical amplification of energy. This is not really sound amplification, as the sound is produced only in the "amplifier", not before it, but I hope it can be an inspiring example of amplification that doesn't require electricity.

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On the basic assumption that 'amplification' means making a sound louder, then, no, a sound doesn't need to be turned into an electrical signal first.

What's needed is an 'acoustic amplifier'. Consider a tuning fork: in open air, very quiet, placed on a table, for example, much louder - amplified. A solid electric guitar, with a slab for the body; against a round hole acoustic guitar. A trumpet mouthpiece which then gets attached to the rest of the pipework, particularly the bell. A drum head compared a complete drum. A megaphone compared to just a voice.

Al those devices are amplifiers without using an electrical signal. Even the design of a church or cathedral will bear in mind these factors, and try to amplify the sound source at the pulpit, for example.

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The term "amplification" in acoustic context is ambiguous and often misleading. While it is widely used and accepted, it's not correct: you can use it, but with awareness.

You are having correct doubts about aspects related to electric amplification, and, interestingly enough, you are involuntarily getting to the point: it doesn't really matter if we're talking about sound or electricity.

Sound is still energy.

To extend what correctly pointed out in other answers, since energy is always conserved (in some form), it cannot be created from nothing, but must be transferred ("added") in some way, so that the original vibrations can be amplified.

In order to amplify something, you always need to add energy from somewhere else.

Electric (real) amplification

Let's take the example of voice amplification (instead of stringed instruments with sound boards, I'll explain why later).

A microphone is an electrical device that transforms kinetic energy of a membrane (that moves following the vibrations in the air) into changes in an electric current: when there's no air pressure, there is no passage of current, when the pressure increases, more current passes through it. Since sound is based on variations of pressure (including negative pressure) over small amounts of time, you get lots of changes in the current.
This is an oversimplification, but try to follow me anyway.

Then that current comes and returns through a powered circuit that is able to multiply (amplify) it. For instance, if it gets current 1, it becomes 4, if it gets 2 it becomes 8, if it gets 0, it remains 0.

Finally, that current goes to a speaker, that works like an inverted microphone, transforming electric signals into kinetic energy: more current, more pressure. And the resulting sound is much louder than the original.

Acoustic attenuation

Before going on on the concept of "natural amplification", let's understand what actually happens whenever we hear a sound.

As we all know, physical sound is an abstract concept that, for us humans, is the result of variations of air pressure in time. We need to have frequent changes in air pressure in order to perceive sound, and we need something that alters those pressure changes.

When some object is in contact to the air and that object starts to "vibrate", changes in its physical displacement create pressure waves that may eventually arrive to our ears.

Pressure changes expand radially in waves from their origin point (the classic pebble in the pond example), and eventually dissipate after some time, possibly transforming the kinetic energy into thermal energy in the meantime.

If the waves encounter a different pressure (like a harder surface), they may be reflected back in the opposite direction, meaning that a certain point may receive back a wave similar to one they already got.
But if the waves encounter a surface that is able to absorb ("stop") that kinetic energy, those waves are easily dissipated. That energy doesn't disappear, it is normally transformed in small amounts of heat.

An electric guitar has a fixed body that absorbs most of the vibrations of the strings, meaning that most of the potential energy that could eventually be transmitted from those strings is practically nullified. The pick-up is able to transform a small part of the energy of the vibration (causing changes in the magnetic field) into an faint but sufficient electric signal, usually between 100 and 300 mV.

Acoustic ("natural") amplification

Simply put: it doesn't exist. That's because, in normal conditions, no energy is being added. If you want "more sound", it means that you want "more energy", but, as said above, you cannot create energy from nothing.

So, how is it possible that, with the same sound source, we can get "louder" sounds?

The reason is that "acoustic amplifiers" are, in reality, energy transformers and better energy "conductors". They are more efficient in transforming and propagating the original vibrations (including those of physical objects like strings).

When you get a louder sound, you're not getting more energy than originally produced, but more of the originally produced energy, which is otherwise dissipated in other ways as explained above (heat).

The acoustic megaphone

An acoustic megaphone just redirects the acoustic energy in a focused direction.

Imagine somebody speaking in a direction in an open, flat field, and you standing a few meters behind them.

  • when they speak, you will probably hear them somehow clearly;
  • when speaking into the megaphone, keeping it close to their face, you will hear them much less than before; if there is a vertical obstacle large enough (like a building) a few meters in front of you, you'll hear the reflected sound much more clearly;

That's because when the megaphone is put right in front of their mouth, the sound is forced to a specific direction.

It would be almost the same as they were speaking with their mouth next to a hole in a wall: if you're standing behind them, you'll hear almost nothing, but if you're on the other side of that hole, you'll hear them very clearly.

Sound in a chapel or church

Again, not amplification.

Chapels and churches are large structures with big and very reflective surfaces (concrete walls/domes/arches, glass windows, marble floors, etc) that are both flat and curved (concave): a flat surface reflects sounds in the opposite direction, while a concave curve focuses it.

The sum of that combination results in focused sounds easily reflected by flat surfaces, and since some of those reflections arrive at very close intervals, you get a "louder" sound because of the sum of those vibrations arriving almost at the same time.

One common effect of similar structures is that known as "whispering-gallery waves": when one whispers in a very specific point of the structure, that whisper can be clearly heard at another very specific point which is sometimes quite far away, even if, in normal condition, that distance wouldn't allow the listener to hear the original sound as clearly.

Concert halls

The concept is similar to that of churches, but with less reverberation: the sound waves are better and uniformly propagated in the whole space, allowing audiences to clearly hear any sound coming from the stage even at 30-40 meters away. Specific architecture elements are used to provide better sound propagation, including special panels put on walls or hanging from the ceiling, shaped in ways that improve sound reflections.

Sound boards

Most of the sound of instruments having sound boards doesn't come directly from the strings, but from the vibrations of the sound board itself.

The vibration of the strings passes through elements that are able to resonate (vibrate at similar frequencies): the bridge, the sound/soul post, and the sound board itself.

The hollow space, type of materials and their thinness used in sound boards all contribute in a better efficiency of sound propagation, due to their matching the acoustic impedence. Simply put: they are able to get a better performance of the kinetic energy coming from the vibration of the strings, transforming most of it in air vibrations instead of dissipating that energy.

Pneumatic amplification

There are other ways of amplifying sound, not involving electricity. As it's now quite clear, we still need power coming from another source, though.

One example is the pneumatic amplifier.
It used a simple mechanism: a diaphragm put at the end of a "microphone" cone would transmit the vibration through a valve, put within a channel that had pressurized air coming from a pump: the increased air pressure, finally exiting through an output cone, made the sound much louder than its original source. It obviously wasn't very accurate, but the concept was quite ingenuous.

Here is a diagram of its mechanism.

diagram of the pneumatic amplifier

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    "In order to amplify something, you always need to add energy from somewhere else." - I guess this is the same assertion you've made in a comment. Do you have a source for this? You mention acoustic/mechanical impedance matching devices that do not add energy and therefor you do not call them amplifiers, but there are passive electronic devices that do impedance matching and have a voltage gain greater than unity - so they are voltage amplifiers. Are you asserting that such devices are not amplifiers even though they have a gain greater than 1? Dec 8, 2023 at 19:37
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    @ToddWilcox Voltage amplifiers increase voltage but also reduce current. A mechanical amplifier (like a lever) can only increase one component, while losing the other: it's either force or displacement. The amplitude of the vibrations (the kinetic energy) is not increased, it's transmitted more efficiently instead of being absorbed by the dampening (small amounts of thermal energy) or by vibrations that have insufficient areas or mechanical properties to cause any resonance, possibly decreasing the frequency in some way or, again, transforming it into thermal energy. Dec 8, 2023 at 20:09
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    @ToddWilcox As pointed in other comments, we could have different levels of the concept of "amplifier". I admit, I should rephrase the last part and be more specific: in terms of physics (thus, considering all forces in a closed system), it's not correct. A "voltage amplifier" is a known and accepted concept, but it is also based on the awareness that electricity has two fundamental components: current and power. In reality it's not a real amplifier, it's a transformer: its very definition assumes that the output impedance is lower than that of the input. Dec 8, 2023 at 21:09
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    @ToddWilcox I agree that the definition of "amplifier" might not be very strict, but note that the passive component used to increase the voltage is commonly called a transformer, not amplifier. Dec 9, 2023 at 12:29
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    The problem here seems to be hinging on definitions. A lever can amplify one of displacement or force, at the expense of the other. What is not amplified is work, which is the product of the two. (Nor power, which is the time derivative of work.) The same is true of electrical analogs, like voltage and current. For sounds, however, amplification usually requires additional energy to be put into the system, usually electrical. One can argue that a mechanical phonograph can be amplified by using a stronger motor and a heavier needle, adding more mechanical energy into the system...
    – JimC
    Dec 9, 2023 at 21:38
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Most string instruments include a natural amplifier. As one example, the sound-board of a piano amplifies the sound of the string, which would be quite soft otherwise.

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  • The resonance of the sound-board creates the sound, it doesn't amplify it.
    – PiedPiper
    Dec 7, 2023 at 22:53
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    @PiedPiper The vibration of the strings creates the sound. The bridge transmits the vibration to the sound-board, which amplifies it.
    – Aaron
    Dec 7, 2023 at 23:04
  • @Aaron While we normally consider it amplification, from the physics point of view that's not the correct term. Vibrating strings alone have a defined amount of energy which cannot be "amplified" (due to the law of conservation mentioned by PiedPiper). If there's no sound board, the strings are the only mean of sound diffusion (the capacity of moving a certain volume of air) due to their little area and most of their energy is dissipated by "fixed" (non resonating) elements that keep strings in place. The sound board becomes that element, vibrating along with the strings, and since it has a » Dec 8, 2023 at 0:42
  • @Aaron » greater area exposed to the air, it's able to move much more air around it, causing a louder sound. Also, the strings do not "create the sound": they create the initial vibration that affects the overall sound, but most of it comes from the soundboard (and other elements, like the bridge). While the choice of strings certainly is important, they have limited effects on the final sound: good strings can only make a slightly better sound on a bad instrument, but it will still be bad; bad strings may not get 100% from a good instrument, but it will still be good. Dec 8, 2023 at 0:51
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    @musicamante "from the physics point of view that's not the correct term" - can you please cite a source for this assertion? I've been looking for one on my own and haven't found it yet. Dec 8, 2023 at 19:36
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I think word missing is resonator. In old churches, you can see a large, thick, octagonal or circular board over the pulpet. It resonates the speaker's words from the pulpit and the acoustics of the sanctuary conduct the sound down the length of the building. If you've ever spoken or sang in an old church, you will know that you don't necessarily need electric microphones, your voice is resonated from the front of the church to the back seats.

Similarly, the sound board in some instruments resonates the music, it doesn't amplify it. Just like an echo can be resonated against the walls of a tunnel or narrow hall. Some structures make sound "bounce".

This is how "sound amplification before electricity" was explained to me. I hope this helps.

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    Consider an Edison cylinder (or even platter) recorder. The entire sound is driven by the ridges on the plastic cylinder. It's transferred from a diamond-tipped steel needle to a metal or plastic horn. If the horn isn't attached, there is no sound. I think the vibrations of the needle do not disturb the air very much, The needle can transfer its energy to the horn and thence to the air. The coupling constants from cylinder to needle, needle to horn, and horn to air are large. The needle and cylinder do not couple the air. Much of this may be frequency-dependent.
    – ttw
    Dec 9, 2023 at 2:34
  • If you take a (once-common) electric phonograph and disconnect the speaker, you can still hear faint sounds coming from the needle. A horn-less acoustic phonograph doesn't have no sound. But neither is usable this way.
    – JimC
    Dec 9, 2023 at 21:29
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A flame can provide non-electronic amplification of sound. Typically the gas flow in the flame must be close to a transition from laminar to turbulent regimes. https://massless.info/images/Popular%20Electronics%20May%201968%2031401532-Flame-Amplification.pdf

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As well as mechanical amplification, there is Chemical amplification (where sound pressure amplifies chemical reaction, which amplifies sound pressure) and Ultrasonic amplification (modulating high energy ultrasonic pressure).

Both are inherently non-linear processes, and although they can be linearized, to my knowledge, neither has been demonstrated with good fidelity.

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To add to the other examples of non-electric amplification, you have humans [*]!

Imagine a single speaker who says a few words, and then a crowd that repeats the words said by the speaker. Your signal has been amplified.

The signal has changed in the process - but the content is the same. It is not uncommon for amplification to distort signals (e.g. guitar amps)

It doesn't have to be words - it works with melodies and rythm too, for example Freddy Mercury making a crowd at Wembley sing simple phrases.

[*] Unless you consider humans to be electric...

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