Understand loudspeakers – start from zero

What is sound?

A loudspeaker sits on a table. You hear it across the room, even though nothing visible flies between it and your ear. What actually reaches you? Before looking at loudspeaker technology, we will follow that journey.

A movement is passed along

Rest two fingers lightly on the outside of your throat and hum quietly. You can feel a vibration. When you speak or sing, your vocal folds set the air into motion. A loudspeaker uses a moving surface called a diaphragm. It moves backwards and forwards.

When the diaphragm moves forwards, it pushes nearby air particles slightly closer together. Air pressure briefly rises there. When it moves back, a region of lower pressure forms nearby. These pressure changes travel through the air. On reaching your ear, they can make your eardrum vibrate.

The air does not travel across the room

Imagine a row of people passing a small nudge from one neighbour to the next. The disturbance reaches the far end even though nobody walks along the whole row. This comparison illustrates transmission; real air particles are neither people nor arranged in a neat line.

In a sound wave, air particles move around their resting positions while the pressure disturbance travels onwards. Particle movement and wave propagation are different things. A loudspeaker is therefore not a fan that continuously transports fresh air from its diaphragm all the way to your ear.

Reading the picture

The dots below represent small regions of air. Where they are closer together, pressure is higher in this model. The burgundy dot lets you follow one location. Movement is greatly slowed down and enlarged; real air particles are not visible like this.

Now explain it yourself

Two final questions

Next: High or low? Loud or quiet?.

High or low? Loud or quiet?

A deep voice can whisper. A high-pitched beep can be very quiet. “High” and “loud” therefore describe different things. We need to separate them before introducing numbers.

How quickly does the movement repeat?

Hum a comfortable note, then a slightly higher one. You change how quickly the vibration repeats. For a simple tone, faster repetition is heard as a higher pitch and slower repetition as a lower pitch.

A complete oscillation ends only when the movement returns to the same state: the same position and direction of movement. Going from front to back alone is not a complete cycle. Think of a swing going forwards, backwards and returning to its initial state.

How strongly does pressure vary?

Now imagine the same note played quietly and then more strongly. The repetition rate can remain the same while the pressure variation grows. At the same frequency and under comparable conditions, we normally hear a stronger pressure variation as louder. Our hearing is not equally sensitive to every pitch, however.

What the curve means

The horizontal direction in this picture shows a fixed time interval. The vertical direction shows changing sound pressure relative to the centre line. It is not the path that air particles follow across a room. More cycles within the same interval mean a faster repetition rate. Larger excursions mean stronger pressure changes.

The controls deliberately do not use hertz or decibels yet. First recognise the property you are changing. This graphic produces no audio; “higher” and “louder” are interpretations of a simple visual model.

Now explain it yourself

Two final questions

Next: How does a vibration become music?.

How does a vibration become music?

One steady tone does not yet sound like a guitar or a voice. Music changes continuously: notes start and stop, instruments play together, and even one note has its own characteristic sound.

A simple tone as a building block

The smooth repeating wave from the previous lesson is called a sine curve. It describes a particularly simple tone. An instrument usually produces additional vibrations alongside a fundamental vibration. Many repeat faster. In harmonic sounds, their frequencies are whole-number multiples of the fundamental.

You do not need to calculate those terms yet. The important point is that several contributions can exist at the same time. Their relative strengths and the way a sound starts and fades help shape its character. A piano and a guitar can therefore play the same note and still sound different.

One diaphragm, one combined movement

A single speaker driver does not have a separate diaphragm for each instrument. Electrical signal components are reproduced together. At every instant they result in one combined movement. Their contributions to air pressure also superimpose.

The graphic shows two simple contributions and their sum. A black curve that is less round is therefore not automatically an error. It may be the correct shape of a combined signal. Distortion means that a system changes the input signal in an unwanted way; a complicated waveform alone does not establish that.

A deliberately simple model

Here we add only one contribution, repeating twice as fast as the fundamental. Real music contains many more components and changes over time. This drawing is not a measurement of a particular driver. It explains why a speaker must follow more than one isolated sine vibration.

Now explain it yourself

Two final questions

Next: What is inside a loudspeaker?.

What is inside a loudspeaker?

We now know the job: an electrical music signal must produce controlled movement, which changes air pressure. Let us look at the parts that do this in a common moving-coil loudspeaker.

From signal to force

A voice coil is a winding of wire. It sits in a strong magnetic field and is attached to the diaphragm. Current flowing through the coil produces a force on it. Reversing the current reverses the force. The signal can therefore move the coil and diaphragm backwards and forwards.

The magnet and magnetic circuit provide the magnetic field. They do not produce music on their own. The changing electrical drive comes from the amplifier. This lesson concerns a moving-coil loudspeaker; other operating principles also exist.

What guides the movement?

The surround flexibly connects the outer edge of the diaphragm to the frame. The spider guides the voice coil and contributes restoring force. Along with other system properties, these parts influence how the diaphragm can move. They permit the intended movement while helping to keep the coil centred in its narrow gap.

The frame supports the assembly. The diaphragm moves air. On many drivers the central dust cap is a cover, not automatically a separate tweeter. Do not press a real diaphragm or dust cap to explore it: the schematic below is sufficient for this lesson.

Driver or finished speaker?

The individual assembly containing diaphragm, motor and frame is called a driver. A finished loudspeaker additionally includes an enclosure, connections and, depending on its design, more drivers and a crossover. A woofer is typically designed for larger air movements at low frequencies, while a tweeter is designed for high frequencies. Suitability depends on the whole construction, not just diameter.

Now explain it yourself

Two final questions

Next: Why does the enclosure matter?.

Why does the enclosure matter?

If a driver already moves air, why build a box around it? One major reason is on the back of the diaphragm. Its rear side does not disappear acoustically just because we cannot see it from the front.

Two sides of one movement

When the diaphragm moves forwards, it compresses air in front. The opposite pressure change occurs behind it at the same time. If the contributions can meet around the diaphragm edge, they can partly cancel at a listening position. Low tones are particularly affected with a small, freely standing driver.

Why low tones? Their pressure variation changes more slowly. The extra journey from the rear around the edge is often short compared with the spatial extent of that oscillation. The opposing contributions can then arrive with similar strength and nearly opposite phase. This is often called an acoustic short circuit. It is not an electrical short circuit.

Handle the rear radiation deliberately

A sealed enclosure separates the rear radiation from the direct forward path. The trapped air also behaves like an additional spring. The cabinet is more than packaging: its volume changes the driver's behaviour.

A bass-reflex enclosure also uses a tuned port. A passive-radiator system uses an unpowered diaphragm for a corresponding resonance role. Both need suitable design. An arbitrary hole or an arbitrary box size is not a dependable improvement.

What the comparison can demonstrate

We compare just one idealised low tone at the same listening position. Without separation, we assume equal contributions that are exactly opposite: their sum is zero. With the rear separated, we show only the direct front contribution. This does not predict the bass gain of a real cabinet. Path lengths, levels, the enclosure air spring and the room are omitted.

A sufficiently large baffle or a deliberately designed open system can also work. “Every loudspeaker must have a sealed box” would therefore be the wrong conclusion.

Now explain it yourself

Two final questions

Next: Now give the ideas numbers.

Now give the ideas numbers

You can now explain what vibrates, what travels and why a loudspeaker is a system. Only now will we give two familiar observations their technical names. To begin with, you only need counting and division.

Hertz: repetitions per second

The number of complete oscillations per second is called frequency. Its unit is the hertz, abbreviated Hz. One oscillation per second is one hertz. One hundred complete oscillations per second are one hundred hertz. Under otherwise comparable conditions, a simple 200 Hz tone is higher in pitch than a 100 Hz tone.

For practice, we use slow movements you could count. Such low repetition rates would not be ordinary audible tones. Likewise, do not read the earlier slow-motion drawings as actual playback frequencies.

The first calculation starts with a question

You observe twelve complete oscillations in three seconds. The question is: how many occur, on average, in one second? Divide the count by the observation time. Twelve divided by three is four. Only now do we write a formula for that idea:

The symbol f means frequency, n the number counted and t the time in seconds. These letters abbreviate a question you already understand. They do not add a mysterious new concept.

Decibels: establish the reference first

For sound levels you will encounter the decibel, abbreviated dB. This is a logarithmic scale. Here it describes a ratio to a defined reference, not a count of oscillations. At the same frequency and under comparable conditions, a higher sound pressure level normally means a louder tone.

This does not mean “60 dB is twice as loud as 30 dB”. Nor does 0 dB SPL mean “no sound”: it denotes the reference of the sound-pressure-level scale. Perceived loudness, frequency, duration and measurement conditions need to be distinguished. We will leave decibel calculations for further study instead of introducing them without preparation.

Your first project brief

Finish by describing a use case: where will the speaker stand, how far away will you listen, how much space is available and can a subwoofer help? You do not need to choose a driver yet. The aim is a clear task that gives reasons for later technical choices.

Now explain it yourself

Two final questions