Lena Torres
The directivity of sound is influenced by the size of the sound source relative to the wavelengths it generates. Generally, low-frequency sounds are omnidirectional, while high-frequency sounds are more directional. This is because high-frequency sound waves have shorter wavelengths, making them more easily absorbed and localized compared to low-frequency sound waves, which are often perceived as spherical. The larger the speaker array, the more directional the sound, and certain techniques, such as using phased arrays or ultrasound technology, can enhance directionality. The environment, such as the size of the room and the presence of reflective surfaces, also plays a role in how directional sound is perceived.
| Characteristics | Values |
|---|---|
| Directional sound | The notion of using various devices to create fields of sound that spread less than most traditional loudspeakers |
| Directivity | The larger the speaker array, the more directional; the smaller the speaker array, the less directional |
| Beaming | A function of the effective driver size; the dispersion narrows as the wavelength (frequency) becomes smaller than the size of the drive unit |
| High-frequency sound | High-frequency sound waves repeat many times in one second; a 10 KHz wave repeats 10,000 times in a second |
| Low-frequency sound | Low-frequency waves are omnidirectional and spherical |
| High-frequency sound | High-frequency waves are directional and planar |
| Low-frequency sound | Low-frequency sound waves are comparable in size to a room's walls and the source of the sound |
| High-frequency sound | High-frequency sound waves are smaller in scale than the room and the source |
| Determining direction | Above 1600 Hz, the dimensions of the head are greater than the length of the sound waves, making it difficult to determine the input direction |
| Determining direction | Below 80 Hz, it becomes very difficult or impossible to determine directionality |
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What You'll Learn
Speaker size and frequency
The directivity of sound is influenced by the size of the speaker and the frequency of the sound waves it produces. The fundamental principle is that the larger the speaker compared to the wavelength of the sound waves it generates, the more directional the sound will be. This is because the sound field becomes narrower as the source size increases relative to the wavelength.
In the context of speaker size and frequency, it is important to understand the concept of "beaming." Beaming occurs when the wavelength (frequency) of the sound becomes smaller than the size of the speaker driver. The dispersion of sound narrows as the wavelength decreases relative to the driver size. This means that at certain frequencies, the speaker will be able to move air at full force, which is known as the tuning of the speaker. For example, a 4" midrange driver with an effective diameter of 3" will experience beaming at approximately 2kHz.
The size of the speaker does not directly determine the frequency response, but it does impact the ability to move air. Larger speakers can move more air, which is necessary for producing lower frequencies. For instance, a 12" subwoofer will outperform a 12" top speaker at the same SPL due to the increased air movement. However, as cone size increases, it becomes more challenging to prevent excessive cone flexing, which can affect high-frequency response.
Additionally, the directivity of sound is also influenced by the shape of the speaker array. Speaker arrays can be arranged in various shapes and sizes, but reducing the physical dimensions relative to the wavelength will result in decreased directivity. To achieve strong directivity with small speakers, very short wavelengths, such as those of high-frequency ultrasound, can be used. This technology creates an invisible beam of ultrasound that serves as the sound source, resulting in extremely directional audio.
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Human perception of sound direction
Human beings can identify the origin of a sound in terms of both direction and distance. This is known as sound localization. The human auditory system uses several cues for sound source localization, including time difference and level difference (or intensity difference) between the ears, and spectral information.
The ability to localize the direction of sound is dependent primarily on differences in the phase and intensity of the sound received by the two ears. This is known as the interaural time difference (ITD). The human auditory system can discern interaural time differences of 10 microseconds or less. For frequencies below 800 Hz, the dimensions of the head are smaller than the half-wavelength of the sound waves, so the auditory system can determine phase delays between both ears without confusion. Interaural level differences are very low in this frequency range, especially below 200 Hz, so a precise evaluation of the input direction is nearly impossible on the basis of level differences alone.
As the frequency drops below 80 Hz, it becomes difficult or impossible to use either time difference or level difference to determine a sound's lateral source, because the phase difference between the ears becomes too small for a directional evaluation. Directional sounds become less localizable/directional at 1,500 Hz and above.
Other factors also come into play when determining the direction of a sound source. For example, the Haas effect states that a difference in timing between the original sound and reflected sound increases the spaciousness, allowing the brain to discern the true location of the original sound. This is especially important in reverberant environments.
Additionally, the movement of the listener can help determine the location of a sound source. Similar to the visual system, there is the phenomenon of motion parallax in acoustical perception. For a moving listener, nearby sound sources pass by faster than distant sound sources.
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Sound localisation
For frequencies below 800 Hz, the dimensions of the head are smaller than the half-wavelength of the sound waves, so the auditory system can determine phase delays between both ears. As the frequency drops below 80 Hz, it becomes difficult to determine a sound's lateral source, because the phase difference between the ears becomes too small. For frequencies above 1600 Hz, the dimensions of the head are greater than the length of the sound waves, so an unambiguous determination of the input direction based on interaural phase alone is not possible. Directional sounds become less localizable at 1,500 Hz and above.
The acuity of sound localisation differs widely in birds, with barn owls having an acuity of 3° to 5° and two hawk species ranging from 2° to 8°. Canaries, on the other hand, have an acuity of about 25°. In general, small animals that hear low-frequency sounds have poorer sound localisation acuity than larger animals hearing higher-frequency sounds.
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High-frequency waves and low-frequency waves
The directivity of sound is determined by the size of the sound source relative to the wavelengths it generates. Larger sound sources, such as large speaker arrays, produce more directional sound compared to smaller speakers. This is due to the larger source size creating a narrower sound field.
High-frequency sound waves have shorter wavelengths and are characterised by a higher pitch. They are more easily absorbed by materials and tend to reflect off surfaces, causing echoes. High-frequency sounds, such as those produced by birds chirping or sirens, add presence and clarity to noise. Human ears can typically perceive sounds in the high-frequency range of 2,000 Hz and above, although this may vary depending on the individual.
Low-frequency sound waves, on the other hand, have longer wavelengths and are perceived as having a lower pitch. These sound waves are highly resilient, capable of travelling long distances, and can more easily permeate solid surfaces. Low-frequency noise is often felt as vibrations rather than heard, especially at frequencies closer to 20 Hz. Examples of low-frequency sounds include the bass from a speaker system or the rumble of an engine.
The transition between high and low frequencies is marked by middle-frequency sounds, which form the basis of the sounds we perceive. They provide essential information for our ears to interpret and discern different noises. Adjusting the bass and treble settings on a stereo system allows us to control the balance between low- and high-frequency sound waves.
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Sound field and sound dome technology
The directivity of sound is dependent on the size of the sound source relative to the wavelengths it generates. Sound waves with longer wavelengths are less directional than those with shorter wavelengths. High-frequency sounds have shorter wavelengths and are therefore more directional than low-frequency sounds.
To improve the sound in a dome, a uniform sound field should be created by increasing absorption through acoustic dampening and improving the diffusion of sound with respect to the reflection on the internal concave surfaces. This can be achieved through various methods that adjust for low-, mid-, and high-frequency sounds.
Sound dome technology has been developed to address the challenge of creating a large sound source that produces narrow beams of sound. Sound domes are acoustically similar to large speaker arrays, but they tend to be much lighter in weight. The AudioDome, for example, is an 11-foot (3.4 meters) speaker system that employs advanced audio-rendering techniques to create rich virtual sound fields that accurately simulate the precise locations of recorded sounds. This technology can be used for research into how the human brain processes sound and for creating immersive experiences for listeners.
Another example of sound dome technology is the patented sound dome by Brown Innovations, which features a hemispheric design that delivers precise audio isolation and rich stereo sound. This design outperforms parabolic domes, which struggle to focus sound waves and produce true stereo.
In summary, sound field and sound dome technology aim to create highly directional sound sources by manipulating the acoustic properties of speaker arrays and dome structures. These technologies have applications in both scientific research and audio entertainment, offering new possibilities for immersive and controlled sound experiences.
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Frequently asked questions
Directional sound refers to the use of various devices to create sound fields that spread less than those of traditional loudspeakers. The directivity of a sound source depends on the size of the source compared to the wavelengths it generates.
High-frequency sound waves are directional because they have a shorter wavelength and a higher repetition rate, making them appear flat or planar when viewed from a human scale. Low-frequency sound waves, on the other hand, have longer wavelengths that are comparable to the size of a room, giving them an omnidirectional or spherical pattern.
Larger speaker arrays tend to be more directional, while smaller speaker arrays are less directional. This is due to the relationship between the size of the speaker and the wavelength of the sound it produces.
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