Amplitude of Sound: Meaning, Facts and More

What is Amplitude?

Amplitude refers to the maximum displacement of a wave from its resting position. In sound waves, it is directly related to the loudness or volume of the sound we perceive. A high amplitude sound wave will be louder, while a low amplitude sound wave will be quieter.

The amplitude of a sound wave is measured in decibels (dB). Decibels are logarithmic units that compare the sound intensity to a reference level. 0 dB is considered the threshold of human hearing. As the amplitude and loudness increases, the decibel level goes up. For example, normal conversation is about 60 dB, while a rock concert can be 120 dB or higher.

In summary, amplitude is the physical quality of a sound wave that determines how loud or quiet it is to our ears. It is directly correlated with the subjective perception of volume and intensity. Measuring amplitude in decibels gives us a standardized way to quantify and compare the loudness of different sounds (1).

Measuring Amplitude

The amplitude of sound is commonly measured using the decibel scale, which is a logarithmic unit used to express the ratio between two values of power or intensity. The decibel scale allows a wide range of amplitudes to be compared.[1]

Sound pressure level (SPL) is commonly used to measure amplitude and is expressed in decibels. SPL represents the pressure variations a sound wave generates above and below atmospheric pressure. Standard reference pressures have been established to compare different SPLs on a consistent basis.[1]

Audiometry testing is a technique that measures an individual’s hearing sensitivity across different frequencies. This can help determine hearing thresholds and the faintest sound someone can detect at those frequencies. The results are plotted on an audiogram to visualize hearing abilities.[2]

[1] https://studylib.net/doc/6869530/2011-mjc-ph-h2-p1-prelim-soln

[2] https://www.asha.org/public/hearing/audiogram/

Hearing Thresholds

The minimum sound pressure level that an average human ear can detect is called the absolute threshold of hearing or minimum audibility. This threshold varies significantly with frequency, as the human ear is not equally sensitive to all frequencies of sound.

According to research, the lowest threshold exists around 2-5 kHz, where the average minimum amplitude for hearing is around 0 dB SPL (sound pressure level). At lower frequencies around 100 Hz, the threshold rises to around 30-40 dB SPL. At higher frequencies of 10-20 kHz, the threshold also increases to around 60-100 dB SPL [1].

Overall, the normal hearing range for humans spans frequencies from 20 Hz to 20,000 Hz with minimum thresholds ranging from 0 dB SPL in the mid-range to over 100 dB SPL at the extremes. This frequency-dependent sensitivity is important to understand, as it affects how we perceive different sounds.

Safe Listening Levels

Exposure to loud sounds, even for brief periods, can lead to hearing loss over time. To protect hearing, guidelines on safe listening levels have been established by organizations like the Occupational Safety and Health Administration (OSHA) and World Health Organization (WHO).

OSHA sets legal limits on noise exposure in the workplace. The permissible exposure limit is 90 dBA for 8 hours per day. Exposure to sounds above 85 dBA over extended periods can result in gradual hearing damage. Sounds above 140 dBA can cause immediate harm.

For the general public, the WHO recommends keeping volume levels below 85 dBA for no more than 8 hours to avoid hearing loss. They advise using hearing protection when exposed to sounds above 100 dBA. Examples of loud sounds above 120 dBA that can be painful include fireworks at close range, turbofan aircrafts at takeoff, and sirens.

The risks of excessive noise exposure include hearing damage or disorders like tinnitus, hyperacusis, and diplacusis. Hearing loss from noise can be temporary or permanent based on factors like volume, duration, and frequency. Gradual loss typically begins at higher frequencies. With continued exposure, lower frequencies become affected.

To prevent hearing damage, use protective equipment like earmuffs or earplugs when exposed to loud equipment and environments. Turn down volume levels when possible and limit time spent in noisy areas. Get annual hearing check-ups to monitor for early signs of hearing loss.

Dynamic Range

The dynamic range of human hearing refers to the range between the quietest and loudest sounds we can perceive. According to Wikipedia, the dynamic range spans around 140 dB, but this can vary depending on frequency. The threshold of human hearing is around -9 dB SPL at 3 kHz, while the threshold of pain is around 130 dB.

In analog audio, dynamic range is the difference between the noise floor and the maximum undistorted signal level. Digital audio has a wider dynamic range, as it is not limited by noise and distortion in the same way. The dynamic range of CD audio is around 96 dB, while 24-bit digital audio has a dynamic range of around 144 dB. This allows digital audio to capture a wider range of volumes without clipping or distorting.

Having a wide dynamic range allows us to hear both very soft and very loud sounds. It also enables greater expressiveness in music and more realistic sound in audio recordings. However, the large disparity between loud and soft sounds can also present challenges for audio engineers in terms of compression and normalization.

Compression

Audio compression is used to reduce the dynamic range and file size of audio recordings. This is especially important for broadcast applications, where bandwidth is limited. According to Stautner (1992) from High Quality Audio Compression for Broadcast and Computer Applications, advanced compression techniques can provide high quality audio while reducing the bitrate. Broadcast standards like Dolby AC-3 use perceptual audio coding to compress the dynamic range. This reduces loudness variability while maintaining audio quality and listener satisfaction.

Compression allows broadcasters to transmit more audio channels within limited bandwidth. The reduced file sizes also decrease storage needs. However, excessive compression can degrade quality, so an appropriate balance is necessary. Audio engineers utilize specialized codecs and tools to compress dynamic range for broadcast, while retaining clarity and loudness consistency.

Psychoacoustics

Psychoacoustics is the study of how humans perceive sound. An important concept in psychoacoustics is the perception of loudness. Despite having the same amplitude, humans perceive some frequencies as louder than others. According to one study, perception of loudness depends on the frequency, intensity, and duration of a sound.

Equal-loudness contours demonstrate this phenomenon. These contours show the sound pressure levels necessary for tones at different frequencies to be perceived as equally loud. The contours reveal that humans are most sensitive to frequencies around 2-5 kHz, so less intensity is needed at those frequencies to be perceived as loud. Lower and higher frequencies require greater sound intensity for humans to judge them as equally loud. Understanding psychoacoustics and equal-loudness contours helps explain why certain sounds seem louder or softer than others at the same amplitude.

Acoustics

The acoustics of a space refer to how sound behaves in that environment. Two key concepts in room acoustics are sound absorption and reverberation.

Sound absorption refers to the ability of materials to absorb sound energy rather than reflect it. Soft, porous materials like carpet, curtains, and upholstery are good sound absorbers that can help reduce noise and echo. The more sound absorption in a space, the less reverberant it will be. According to the Master Handbook of Acoustics, controlling sound absorption is an important aspect of room acoustics.

Reverberation refers to the persistence of sound in an enclosed space after the original sound source has stopped. It is caused by repeated reflections of the sound off the surfaces in the room. The reverberation time quantifies the amount of reverberation and is related to the size, shape, and absorption characteristics of the space. Longer reverberation times can make speech less intelligible but can be desirable for music in concert halls. Adjusting the reverberation time by controlling the amount of sound absorption is an important goal in room acoustics.

Musical Expression

Amplitude changes, known as dynamics in music, are an important expressive aspect of music performance. Dynamics help create emotions, drama, and nuance in music (https://pages.uoregon.edu/emi/5.php). Musicians utilize a range of amplitude variations to convey certain moods or ideas. Playing loudly can communicate intensity, power, or triumph. Soft playing can suggest delicacy, subtlety, or pensiveness. Sudden shifts from loud to quiet or vice versa add contrast and surprise. Gradual crescendos build tension, while decrescendos release it.

The ability to precisely control amplitude is crucial for musical expression. It allows performers to shape the music and create emotion. Dynamics engage listeners, highlighting musical ideas and adding meaning. From the dramatic thunder of a full symphony orchestra to the gentle whisper of a single violin, amplitude variations are an essential part of impactful and moving musical performances.

Applications

Amplitude of sound has important real-world applications in many fields.

In sonar systems, amplitude data provides information about the size and material composition of underwater objects. This is critical for naval vessels detecting submarines or sea mines. Medical ultrasound imaging also relies on measuring sound wave amplitudes to generate images of internal body structures.

Other industries like aerospace engineering and automotive design utilize amplitude readings to identify vibration issues or sources of unwanted noise. This knowledge enables them to fine-tune components and minimize disruptive sounds.

From an audio perspective, recording engineers use compressors and limiters during mixing to control amplitude variations. This helps balance loud and quiet passages, allowing all parts of a song to be heard clearly.

Acousticians and noise control engineers measure sound levels in homes, offices, and public spaces. They then recommend modifications to reduce excessive amplitude that can potentially cause hearing damage or disturbances.

Understanding amplitude is crucial for developing effective audio reproduction and noise mitigation strategies across many sectors of industry and research.