Audio 101: What Sound Is

Sound begins with movement

A talker’s vocal folds vibrate. A loudspeaker’s diaphragm moves. Each creates small pressure changes that travel through the surrounding air. Areas of slightly higher pressure alternate with areas of slightly lower pressure. Reaching an ear, these variations can produce the experience of hearing.

Air does not stream across a room from talker to listener. Air particles move back and forth locally while the disturbance travels through them, rather like people passing a message down a line.

Sound needs a medium, such as air, water, or a solid. Commercial AV usually focuses on airborne sound, but structure-borne vibration matters too. A bumped table can carry vibration into a microphone mount, producing a thump.

A microphone changes sound into an electrical signal. A loudspeaker performs the opposite conversion. Equipment between them may change level, frequency balance, timing, or routing.

Understand the acoustic event before assuming an equipment setting is responsible. [1.1]

Reading a waveform

A waveform plots a changing quantity against time. For airborne sound, the vertical axis can represent pressure variation above and below the surrounding air pressure. The horizontal axis represents time. The center line represents the reference around which sound-pressure changes occur—not an absence of air pressure. Positive and negative parts show the direction of variation relative to that reference.

A sine wave is a smooth, repeating shape associated with one frequency. Speech rarely looks so simple, but this model makes the vocabulary clear.

Frequency, period, and wavelength

Frequency is the number of complete cycles occurring each second. Its unit is the hertz, abbreviated Hz. A 100 Hz tone completes 100 cycles each second; a 1,000 Hz tone completes 1,000. For simple tones, higher frequency generally means higher perceived pitch.

Period is the time taken for one complete cycle. Frequency and period describe the same repetition from opposite directions. More cycles per second means less time per cycle. A 100 Hz tone has a period of one-hundredth of a second. A 1,000 Hz tone has a period of one-thousandth of a second.

Wavelength is the distance traveled during one cycle. In the same medium under the same conditions, a higher-frequency sound has a shorter wavelength. A lower-frequency sound has a longer wavelength. At about 20°C, sound travels through air at roughly 343 meters per second, giving a 1,000 Hz tone a wavelength of about 34 centimeters. A doorway, tabletop, or room therefore interacts differently with different frequencies.

Frequency is a rate, period is a duration, and wavelength is a distance. These distinctions will matter when studying rooms, arrays, and timing. [1.1]

Amplitude, pressure, and loudness

Amplitude describes the size of a variation. On our pressure waveform, a larger amplitude means a greater pressure change. When comparing measurements, also notice whether amplitude means peak value or an effective measure called RMS (root mean square). Different measures should not be treated as interchangeable.

Sound pressure is a physical quantity measured in pascals. Loudness is a listener’s perception. They are related, but they are not the same thing. Frequency content, duration, background noise, and the listener’s hearing all influence perceived loudness. Two sounds with similar measured levels need not seem equally loud or equally clear. [1.2]

Audio levels are often expressed in decibels, written dB. A decibel describes a logarithmic ratio rather than an ordinary count. The reference matters: dB SPL describes sound-pressure level relative to an acoustic reference; a digital level uses a different reference. A number followed only by “dB” may be incomplete information.

For now, remember the practical lesson: ask what is being measured and what the reference is. Do not assume that a digital meter reading directly tells you the sound-pressure level at a listener’s seat.

Doubling pressure amplitude is about +6 dB; doubling power is about +3 dB. Neither automatically means “twice as loud.” We will develop the math later. [1.4]

Why voices do not sound like test tones

A sustained voiced sound can contain a fundamental frequency plus harmonics at whole-number multiples of that fundamental. The relative strengths of those components help shape its character. The vocal tract further shapes the spectrum, emphasizing some regions and reducing others.

Speech also contains noise-like components, such as the airflow in “s” and “f.” These are not neatly described as a single repeating tone. A complete voice is a changing combination of periodic and nonperiodic sounds.

“More level” is not necessarily “more intelligibility.” Turning up a muffled voice does not restore missing speech detail. [1.3]

In a conference room

A remote participant says, “I can hear the meeting, but I cannot understand the words.” Before increasing the entire system level, separate the possibilities. Is the speech too quiet? Is background noise masking it? Are important speech components being altered? Is the microphone capturing too much room sound?

These possibilities call for different investigations. Describe the symptom before choosing a control.

Try it

At a comfortable speaking level, hum a low note and then a higher note. Do not strain or try to reach an extreme pitch. Notice that changing pitch does not require deliberately making the sound louder.

Next, say the same short sentence softly and then at your normal conversational level. Keep the wording and position unchanged. You have mainly changed level, although real voices also change tone when effort changes. This is an observation exercise, not a calibrated measurement. Stop if speaking or humming feels uncomfortable.

Troubleshooting habit

Replace “the audio is bad” with a specific observation: “speech is quiet,” “consonants are unclear,” or “there is a low rumble.” Clear descriptions narrow the next test.

Recap and check yourself

1. If frequency increases, what happens to period?
2. In unchanged air conditions, which has the longer wavelength: a low tone or a high tone?
3. Is sound pressure identical to loudness?

Answers: 1. Period decreases. 2. The low tone. 3. No. Sound pressure is physical; loudness is perceptual.

Further reading

• 1.1 OpenStax: Speed of Sound, Frequency, and Wavelength
• 1.2 OpenStax: Sound Intensity and Sound Level
• 1.3 UNSW Physics: Spectrum, harmonics, and timbre
• 1.4 Biamp: Decibels