Part 2
The Sound
When soundwave are produced, it create a pressure wave that are “heard” by the ears. In audio reproduction, this wave is reproduced by the ‘speaker(s) or ‘drivers’. There are 3 basic sound elements that are important to human interpretation of sound.
Frequencies - measured in Hz.
Levels - measured in decibels.
Localization - measure in time and space - In this case, degree and milliseconds
All 3 aspects intertwine - meaning each element must also be in consideration when dealing with another. One might have more influence on another but they are not isolated onto themselves. There are subsets of each aspect and I will limit it as much as possible to facilitate easier reading and understanding.
Frequency:
Frequencies are ripples or cycles which causes air pressure/vibrations at a given time. For example, when something is vibrating at 20 cycles per second, it translate to 20 Hertz (Hz). These cycles are measurable in term of size or often referred to as acoustic wavelength. Imagine an acoustic wavelength as generated by a pebble hitting on a calm pond. The amount of ripples from one to the next indicate its’ frequency if one can measure the time it takes from one cycle to the next. Not only is the wavelength size discernible but also the cycle. The lower the frequency, the bigger the wavelength. For example, a 20 Hz wavelength is more than 58 feet long. This phenomenon plays to huge advantage in car audio which are addressed later.
The human ears are capable of “hearing” frequencies between 20Hz to 20 KHz. Some believe to have the ability to hear beyond 20khz but generally, the accepted norm is between the noted frequencies. ‘The lower the Hertz the lower the sound‘. Anything below 20Hz are ‘felt rather than heard’. Keep this frequency band in mind when viewing specification data. When apply to our ears, we are most sensitive to the frequency between 500hz and 2Khz in terms of volume/level or localization. Typically, anything below 85Hz is very difficult to locate. As stated above, the interaction between these 3 basic elements from one to the next cannot be avoided.
Level:
In general, the larger the atmospheric pressure that are generated, the higher the perceived loudness levels or Sound Pressure Level (spl). The spl level as measured in decibels (db) also to some degree, give clues to our brains to interpret its’ direction due to localization impression via psychoacoustic. We tend to perceive something is closer if it is louder than something less so. A note about levels, the louder the sound, the less sensitive our ears becomes at the extremes of our frequency detection. An odd phenomenon also applies when the levels are low. Manufacturers often uses “loudness” equalization to address this phenomenon. What this means is our sensitivity to higher and lower frequencies are diminished as pressure (loudness) rises or lower. Part of this is due to our ears inability to transfer the pressure waves effectively at the extreme. This phenomenon should be considered when tuning a system for various loudness levels. Also note that at certain extreme levels, our ears will introduce distortion as well.
Keep in mind that the amount of air moved in terms of volumes is different from the size of the wavelength. What we are referring to here is the volume (amount) of the waves at any given time. Not the size in duration as it pertain to frequency. To use the same pond analogy as above, imagine that the distance of the wavelength is the same but the height from top to bottom of each ripple is larger. This will equate to a perceived level given the same frequency.
A subset of Level is something that is crucial in quality music reproduction: Dynamic. Dynamic or Dynamic Response or Headroom, also measurement in decibels, is the db difference between silence and full volume in a timed event. For instance a gunshot is heard in the middle of the a field. The db difference between silence and the bang in that short duration is what is considered to be the dynamic response. This translate into the system’s ability to reproduce sounds that are faithful, true and natural. If a system has poor dynamic, it would sound “compressed or monotone”. Keep in mind every recording is compressed to some degree. The majority of music recordings are purposely compressed to prevent overloading the process or system which when occurs would create distortion which we would find much more objectionable than compression.
Localization:
Probably one of the most important and least appreciated aspect of great sound is how our ears interpret localization. In other words, where the sound is coming from. Localization is subtle but very important to great sound. We perceive localization using our stereophonic ability to judge distance, depth and above all it’s location within our environment. Our brains mainly uses time delay to interpret direction with levels being second. Frequency plays a part but timing, measured typically in milliseconds (ms) is what matters in location. Delays, levels and frequency also provide what our brain interpret as depth which translate to the size and type of our environment. Localization is greatly diminished the lower the frequency or level due to the brain’s inability to decipher sound masking and our sensitivity to very narrow bands (between 500hz and 2Khz)
To illustrate, if one were to hear a sound with a frequency that has no bearing between the distance of our ears was generated from our left side, the left ear will pick up the sound several milliseconds faster than the right thus our brain will interpret it to have come from the left. This lateral detection provide us with a sense of horizontal tracking. We are less sensitive vertically since 3D detection rely on the third factor; reflection. Until we develop a third ear above our head or below our chin, we are limited to only two receptors along with reflected sound to determine vertical localization. If one were to be in an Anechoic chamber which all reflected sound are absorbed by the room, vertical detection is virtually impossible. Fortunately, we don’t live in an Anechoic chamber so our environment will assist us with vertical detection. That being said, low frequency or frequency with wavelengths larger than the distance between our ears are much more difficult to detect if the initial sound are generated within the immediate range. That means that sound 20hz wave having some 57 ft in length would be impossible to locate if one were within the 57th range of the source. Beyond that range, our localization rely more heavily on levels.
Why is this important? Localization give us depth and proper soundstage and illustrate why stereo sounds ‘better’ than mono.
Bare in mind the above 3 major elements assume that the source producing the sound is from a source that is capable of producing the required frequency and levels. This get complicated when we encounter real world limitations thus the importance of the above and will be redress below.
Continue to Part 3
Last edited by zakimak; 12-04-2013 at 04:21 PM.
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