The lowest frequencies in audio call for massive amounts of air to be moved by the woofer drive units. In other words air displacement. The area of the driver times the linear excursion. A woofer with a 15” diameter and a throw of 0.25 inches (one way) is going to move a total of 44 cubic inches of air. A 10” diameter woofer with a throw of 0.6 inches (one way) is going to move about 47 cubic inches of air. The area times the excursion will give us the displacement figure. In other words, the diameter of the drive unit only tells half the story.
The marketing boys are keen to quote the overall outside diameter of the woofer frame. The actual diameter measurement is taken from the middle of the surround to the opposite side.
Here are a few quick examples of random drive units taken off the shelf.
FRAME 4.93 inches CONE DIAMETER 3.25 inches
FRAME 5.93 inches CONE DIAMETER 4.30 inches
FRAME 7.10 inches CONE DIAMETER 5.13 inches
FRAME 10.00 inches CONE DIAMETER 8.25 inches
As can be seen from the above examples the actual cone diameter can be 83% to as low as 66% of the frame diameter on the smaller drive units.
How do we get long excursion?
The cheapest way to get a long excursion is to use a long voice coil with a short magnetic gap. The trade off is that of efficiency The part of the coil hanging outside the gap is doing nothing. There is also the added length of the coil which adds to the inductance. Not a happy circumstance. The better way to achieve a long throw is to use a short coil - minimum inductance - immersed in a long magnetic gap. The trade off is price. A powerful magnet structure costs a lot more than a few turns of wire. Caution! The claims for woofer excursion get a bit fuzzy. Downright creative. For example, if we have a 1 inch tall voice coil sitting in a 0.25 inch tall gap then the linear throw will be 0.375 inch one way. If we fudge the numbers we can call the total throw as being 0.5 inch one way. At this point the voice coil is at the hairy edge of leaving the gap completely and the distortion numbers will skyrocket accordingly.
What kind of cabinet do we use?
The cheapest way to get low frequencies is to use a ported cabinet. In other words, a box with a hole in it. The drive unit parameters will determine the size of the box and the diameter and length of the port. The bad news is that the parameters change with power input level as well as over time. Like aiming at a moving target. There is also the problem of having an extra resonance caused by the port which muddies the signal. The low frequency rolloff in a ported box is steeper than with a sealed one. For truly accurate low frequency reproduction a sealed cabinet is the answer. The tradeoff is that of lower efficiency but watts are cheap to-day unlike 50 years ago. A sealed cabinet must be braced and any unwanted vibrations must be damped. This bears repeating. You can have a solid cabinet or a well damped cabinet. The goal is to have it both SOLID and WELL DAMPED. There are manufacturers of subwoofers who leave the box empty. They claim that since the wavelengths are so long the stuffing makes no difference. These are the same chaps who build the dreaded home theatre thud, thud “bottom of a barrel” sort of cabinets. For accurate low frequency reproduction the stuffing of the cabinet must be carefully installed and the overall measurements must be taken until the optimum impedance level is achieved. In other words the cabinet is stuffed to the point where the impedance hump is reduced to a minimum value while lowering the resonance of the driver/box combination. When the addition of more stuffing causes the resonance to sneak back up the material is then removed to achieve optimum damping.
What sort of cone materials are required?
We are dealing with extremely long wavelengths here. At 20 hZ the wavelength is over 50 feet long. We are not concerned with cone breakup. We are concerned with the surround and spider having the correct parameters to allow long excursions and the voice coil and former having the ability to dissipate heat. The cone material is of little consequence. It can be made of sawdust or chicken livers.
The power handling figures are rather useless. The lads in marketing will always outdo each other with silly meaningless numbers. Several areas must be taken into consideration. Does maximum power mean when the voice coils melts? Does it mean when the surround rips? Does it mean when the spider rips? Does it mean when the voice coil jumps out of the gap? Does it mean when the distortion rises? How much distortion? 10%? 50%? How many watts for how long? For 5 seconds? For an hour? For a fortnight? At what frequency or combination of frequencies? It becomes a numbers game. The family sedan has 7 thousand horsepower. Nice.
Perception of low frequencies
Popular music contains rather limited low frequencies. Not much below 40 hZ. A chest thumping 80 hZ is usually enough to convince the listener that he is listening to low frequencies. Pipe organ recordings on the other hand will often call for tremendous reach into the 20ʼs. Another area of interest for the music lover, almost never talked about is life like reproduction. Recordings done with 2 microphones in a large space. A church for example. The movement of air in the room, if captured on the recording can be reproduced and although very subtle adds to the realism of the performance. This calls for very solid and deep reproduction by the subwoofer. Not cheap.
How many subs are required?
Home theatre is an artificial means of entertainment. The audio portion is totally spectacular. And totally fake. The signal is manipulated and massaged to achieve a pleasing effect. The low frequencies are there for the crashes and rumblings. You can get away with using just one subwoofer. It need only play loud. Not necessarily low. Buy more beer and popcorn with the savings. On the other hand if you wish to reproduce accurate stereo, two subwoofers are required. A correct recording using only 2 microphones done in a large space as in a church will yield lots of very subtle low frequency information. A pair of good subwoofers will add to the realism.
What about efficiency?
One of my pet peeves is dates. No, not the ones grown on trees. Take a look at any invoice. If the date at the top says 01 - 07 -18 is that January 7th or July 1st? “Oh we always write month first then day.” How the hell does anyone else know? The Gotham Grommet Company may very well chose Day / Month. To curtail this unnecessary confusion why not just write Mar. 4th or Sept 6th? Where was I? Oh yes, efficiency of speakers. Some measurements are taken at 2 volts input. Some at 2.83 volts. Some readings are taken with the measurement mic at 1 meter. Some at 2 meters. Some at half a meter. So far we have 6 possible variations. Add to this the sloppiness of measurements and finally the outright lies of the creative marketing departments and youʼve got chaos. You can bet your bottom dollar that these numbers will be fudged to impress the innocent.
What the hell is a fast woofer?
A misnomer if there ever was one. A steep pulse applied to a speaker, any speaker will result in a pretty good looking vertical line. So far so good. Now if we remove the signal the “recovery” of the speaker should be displayed by zero output. Now thereʼs trouble in River City. Most speakers wonʼt shut up when the signal is removed. This can be caused by several factors. A lack of force over area is the Achilles heel of planar type speakers. Ported speakers will have a poor recovery of steep pulses since they are not critically damped. A long voice coil in a short magnetic gap is also a poor performer when it comes to signal recovery. The extra length of coil winding simply adds weight and extra inductance. Speaking of weight, an extra light cone assembly is sometimes thought of producing “fast” bass. Not true. One of the planar type of speakers with a diaphragm weighing next to nothing exhibits some of the worst distortion measurements in the industry. Would you believe 25% distortion at 90 dB at 1 meter? Poor damping and excessive inductance either in the voice coil or the coil in a passive crossover plays havoc with the recovery of the pulse. An active crossover meaning the power amp is tied directly to the drive units, is far superior in terms of accurate recovery. To sum up, it ainʼt the “speed.” Itʼs the recovery that determines accurate low frequencies.
We donʼt guess. We Measure.