If you would like to know more about sound and the principles of hi-fi sound reproduction, there are many good books on the subject.
So what is sound?
Sound is caused by the movement of air. If a large wooden panel is vibrating, the air next to it is pushed away.
Pressure Waves
If we know the frequency (the number of waves per second), we can calculate the distance between corresponding points on successive waves – in other words, we can measure the length of a wave, or the ‘wavelength’. A sound that has a frequency of 1100Hz has a wavelength of one foot. A sound of 2200Hz will have a wavelength of six inches, and a sound with a frequency of 550Hz will have a wavelength of two feet.
The notes of the musical scale simply represent sounds at certain frequencies. It may produce very low frequencies, but only at very low output levels. This means that if you could somehow freeze the sound and see the wavelengths emanating from the two speakers, both wavelengths would be at the same point in their cycle.
When sound is recorded for stereo reproduction, it is assumed that the listener will be positioned equidistant between the two loudspeakers.
If we are sitting in the front of the car, close to the left-side speaker, then we will hear the sound from that speaker very slightly earlier than the sound from the speaker on the right, and from those speakers behind us. If you use both hands to start two waves at exactly the same time, the peaks of the waves will occur at exactly the same point and the waves could be said to be ‘in phase’. Now imagine speakers at the front and rear of a vehicle, each producing sound pressure waves. These mix and create some very complex alterations to the sound. In the latter case, if the two waves are identical in terms of frequency and pressure and are exactly half a wavelength out of phase, they will cancel themselves out completely, leaving silence.
The lower the frequencies being produced and the more loudspeakers there are reproducing that range of frequencies, the more likely it is that phase cancellation will occur. If speakers are wired with the + and – terminals reversed on one speaker, the speakers are placed 180 degrees (half a wavelength) ‘out of phase’.
By installing many speakers inside a vehicle, we create a very complex mix of pressure waves which can cause problems with the overall sound. We may want to split the frequency range into small portions – sub-bass, bass, mid-bass, midrange, upper midrange, high frequencies and ultra-high frequencies – so that each range has a pair of speakers dedicated to it. Of course space in a vehicle is limited and so we tend to install fewer speakers, usually covering the sub-bass, mid-bass, midrange and upper frequencies.
We should generally try to avoid reproducing the same frequencies from speakers positioned at different distances from the listeners. For example, if we have a pair of 6-inch speakers producing mid-bass at the front of the vehicle, we should avoid having another pair of speakers reproducing exactly the same range of frequencies from the rear shelf. The footwell positions often work well for midrange speakers and sometimes tweeters too.
Larger bass speakers can often be installed in the doors without too much affect on imaging, if you make sure that the crossover point is set so that their output does not overlap the frequency range of the midrange speakers too much. If you need to install speakers here to provide ‘rear ambience’ or a ‘double front stage’ because you often carry passengers in the rear seats, adjust the front/rear fader control on the CD player so that the front speakers are considerably louder than the rear speakers when listening from the driver’s seat.
It may sound crazy but sometimes works well. A speaker’s output characteristics alter when it is listened to ‘off axis’. Its frequency response is affected, and phase changes may also occur. In the case of sound deadening sheet, this bonds an acoustically ‘dead’ material to the more easily excited panel, such as the vehicle’s metal inner side panel, door panel or inner boot (trunk). The sheet helps to absorb panel resonance because its own resonant frequency is very low, so the resonance occurs at a much lower frequency and becomes less noticeable.
An Introduction to Car Audio System Design, Stereo Imaging and Staging
If there is an incident involving you, another person if they were driving your vehicle or the registered owner of the vehicle, third party car insurance will give you cover for any claim that may be made against your insurance.