Car speaker basics
Some people think a 200 watt speaker will be louder than a 100 watt speaker, the greater the amplifier’s power the louder will be the sound. This isn’t necessarily so.
Speakers are the single most important part of a vehicle’s sound system and their installation should only be done by experienced installers. To get good sound you must have a good speaker system. But what makes a good speaker system? To answer this we need to look first at what sound is.
Sound theory
Sound consists of vibrations which travel through air. These vibrations are transmitted by compression and refraction waves, and sounds are heard when these waves reach the ear. Whether deep or high-pitched noises are heard depends on the frequency of the vibrations. The very lowest sound frequency which the human ear can detect is about 20 Hertz (or in other words, 20 vibration waves per second). At the other end of the scale a young person can hear frequencies of up to 20,000 Hertz (20 kilohertz). Twenty Hz is the deepest rumbling sound of an earthquake, while 20,000 Hz is the highest of screams which shatter glass.
Unlike other sound reproducers like cassettes, a CD player (and its amplifier) can reproduce sound all the way from 20 Hz to 20,000 Hz. Speakers produce the pressure variations in the air by vibrating their cones back and forth. A speaker cone vibrating 1000 times per second produces a sound frequency of 1000 Hz. One vibrating twice as fast produces a higher pitched sound. When reproducing music the speaker’s cone changes very rapidly in its vibrating frequency as it produces all the different sounds. The loudness of the sound produced depends not on how quickly the speaker’s cone vibrates but how far the cone moves. So a loud, deep bass note requires the speaker to vibrate only relatively slowly (only say 50 times per second) but to have a fairly great cone excursion.
Speaker frequency range
A single wide-range 20-cm speaker might be able to reproduce all the frequencies from 100-10,000 Hz. This would be fine in a public address system, where it would only be called on to reproduce speech and canned-music. But none of the low frequency instruments like bass drums and bass guitars would be heard and high-pitched instruments like cymbals would also be inaudible.
Many poor quality speakers reproduce a frequency range of even less than this – although the manufacturers often claim a better range of 50 Hz to 18,000 Hz. This is because they don’t say in the specs how loud the low and high frequencies will actually be when compared with the mid-range. If the bass drum is so soft in volume that it is inaudible then it’s irrelevant – even though the speaker is reproducing it.
The relative loudness of the different sounds produced by a speaker is measured in decibel (dB) units. Three dB is the smallest difference in sound level which can be detected by the human ear and a quoted frequency response of a speaker must always have a dB figure attached. A speaker with a frequency response of 120-17,000 Hz (+3 dB) means that at either end of the frequency range the loudness of the speaker will be only just down in level. Frequency responses without a (+3 dB) attachment aren’t reliable.
But running a pair of speakers with a frequency range of 120-17,000 Hz (+3 dB) will still mean you miss out on the highest of highs and (more importantly) on all the bass. A single speaker capable of reproducing a deep bass note of 40 Hz is incapable of also being light enough to vibrate easily at 20,000 Hz. This means the speaker function is specialized with specific speakers designed to reproduce just the lows (woofers), the middle frequencies (mid-ranges) and the highs (tweeters). Two-way speaker systems use a woofer and a tweeter in the same package, while three-ways use a woofer, mid-range and a tweeter.
Sensitivity
A speaker’s sensitivity is a measurement of how loud the speaker will be for a given input of electrical energy. This is a critical spec for those who like their music loud, or want to save money. Why is this so important? Because it takes a doubling of amplifier power to get the same sound level output from a speaker which is only 3 dB less sensitive! So, asking the bloke how many watts he’s got in amp power is as relevant as asking the sensitivity of his speaker system. If you want to break it down, each single dB of reduced speaker sensitivity requires 25 percent more amp power to get the same sound pressure level.
Note the speaker’s sensitivity is measured in free space and (like resonant frequency) will change when mounted in the 4WD. It’s still a vital parameter to look for.