Wiring circuit

Sound?
Websters dictionary defines sound as:

"The perceived object occasioned by the impulse or vibration of a material substance affecting the ear; a sensation or perception of the mind received through the ear, and produced by the impulse or vibration of the air or other medium with which the ear is in contact; the effect of an impression made on the organs of hearing by an impulse or vibration of the air caused by a collision of bodies, or by other means; noise; report; as, the sound of a drum; the sound of the human voice; a horrid sound; a charming sound; a sharp, high, or shrill sound."

Mmm, fancy. We all know what a sound is. Its a noise, something we sense with our ears. But what is sound? Its the vibration of the air by a moving body. For our purposes in car audio that moving body is our speakers. The movement of the speaker forward and backward causes a vibration in the air that our ear receives and we hear music. Heres what sound looks like as a wave:



 
A sound wave goes through a cycle of 360 degrees. As the speaker moves forward it goes from rest (0 degrees) through one quarter of its cycle to 90 degrees. As it starts to move back to rest it travels another 90 degrees to the 180 degree mark before moving completely rearward to 270 degrees. As it moves back to the rest position it travels the final 90 degrees to the 360 degree mark. The cycle then repeats itself and we hear the result as music. To summarize, as the speaker moves up and down it travels one cycle which equals 360 degrees. These cycles are measured in Hertz (Hz) with one Hertz being equal to one cycle. The lower the number of Hz the lower the sound we hear. A human can hear, on average, the sounds between 20 Hz and 20,000 Hz (also known as 20 kHz).

We do not hear all of the sounds equally though. Our ears are less sensitive to sounds at the far ends of the scale. This was discovered through research done by Fletcher and Munson and they have given us the "Fletcher-Munson" curve which illustrates this point.



Notice how the curve goes up at both ends of the scale. Also notice how the curve is greater at lower volume levels (sound pressure levels). This is what the loudness circuit on a head unit tries to do. Increase the level of the low and high frequencies at lower volume levels. As the volume goes up the level of boost at these frequencies is reduced. However, some loudness circuits do nothing more than boost the low and high frequencies by a constant amount. If youve seen a vehicle with an in-dash equalizer the chances are very good that the vertical sliders were arranged in a similar fashion to the Fletcher-Munson curve. This is sometimes called "making your EQ smile".