Identifying Legitimately High Fidelity Loudspeakers: Vented vs. Non-Vented Woofers
Turbulence Reduction, Cooling and more…
A vented pole is basically a hole bored out at the back of the motor structure as seen in the figure below. Its primary job is to reduce the highly non-linear spring effect of the air trapped behind the dustcap/dome, and reducing distortion at high excursion. There is debate on the effects of cooling with vented pole pieces as it actually can be detrimental if the vent is too large. Not only that, but the magnetic permeability of the pole piece can be reduced given the smaller amount of metal in the pole piece which can affect motor efficiency. These issues were much more of a concern years ago. However, with advent of driver modeling and design programs such as FINEMotor, a well designed driver with a vented pole piece can be very effective at balancing the needed effect of reducing pressure under the dust cap for improved linearity and still providing cooling for the voice coil. Let’s discuss this in further detail.
Sectional View of Driver with Vented Backplate (courtesy of JAES Vol 52, #1/2, 2004 Jan/Feb)
Reducing Air Turbulence Under the Dust Cap
A solid dust cap does not allow air to pass through its surface, and creates a small acoustic chamber that will generate air pressures as the cone moves back and forth over the pole piece. This compression and rarefaction can have detrimental effects on speaker operation which can sometimes be heard as a “squeaky wizzer” sound when the driver is really moving back and forth close to its excursion limits. With tweeters, this can produce what has been referred to as a “cupped hand” resonance. Since the area between the voice coil and pole piece is too small to effectively relieve this pressure caused by the motion of the dust cap, manufacturers use two practical solutions to the problem.
SVS PB13 Ultra Woofer (left pic) ; SVS NSD Woofer (right pic)
One (as previously mentioned) is to vent the pole piece, which requires a small hole to be drilled through the pole piece so that air can pass out an opening in the backplate. Another way of venting a driver is to punch holes into the voice coil former where it attaches to the cone. This will allow air to flow out of the small chambered area and relieve the pressure between the pole piece and the dust cap. Both options have their own merits and can really enhance the performance of a midrange or woofer as opposed to a similar design with no venting at all.
The above left picture of the SVS PB13 Ultra woofer shows a flared vented pole piece while the above right picture of the SVS NSD woofer shows the punched vent holes into the voice coil former and cone.
The Role of the Dust Cap
We haven’t spoken much about the dust cap of the loudspeaker diaphragm. Some argue that it plays little to no role on overall performance of the driver, but those who really understand loudspeaker mechanics know that this generalization is untrue. Years ago when drivers employed soft lossy cloth dust caps, they did little to nothing to add rigidity to the diaphragm. Their primary role than was literally to block the dust from getting into the gap and also offer some cooling for the voice coil by allowing air flow. Loudspeaker driver technology is a lot more advanced these days and as a result, superior drivers are being produced and employed by companies with a genuine design goal of producing better speakers.
The dust cap does more than just protect the motor assembly from dust. A hard dust cap for a well-engineered driver acts as a stiffing membrane for the diaphragm. Hence it helps the driver act more pistonic as we discussed earlier. Inverted dust caps can further improve cone rigidity and reduce frequency response anomalies better than protruding dust caps, all other things being equal. The shape of the cone must also be factored into this equation but that goes beyond the scope of this article.
Beryllium Phase Plug Mid Woofer (courtesy of Status Acoustics)
Phase Plug Drivers
Phase plug drivers are a unique situation. They don’t have a rear vent for cooling. When a driver employs a true phase plug, meaning the actual plug is affixed to the motor pole piece of the driver and not the cone, it does several things:
- It can reduce the moving mass of the driver, thus allowing it to extend its usable frequency response
- It acts as a heat sink to cool the voice coil and increase power handling
- It slightly reduces on-axis beaming by dispersing the high frequencies the driver is producing
- It acts as a vent to allow airflow through the voice coil much like a vented pole piece
All things being equal, phase plug drivers have an advantage when used for upper bass and midrange applications, but also create a hole at low frequencies, thus reducing total cone surface area and the driver’s ability to effectively produce low frequencies. If not properly engineered, they can also increase mechanical noise when excessive air spills from the gap. You will typically find these used as dedicated midrange drivers more so than dedicated bass drivers for these very reasons. The Definitive Technology patented BDSS design addresses these concerns and allows phase plug drivers to be used as bass-mid drivers more effectively than many conventional designs.
Editorial Note about Phase Plugs:
Some loudspeaker manufacturers use a quasi phase plug which looks like a phase plug for marketing purposes, but does not physically separate from the cone of the driver. These "so called" phase plugs are really nothing more than fancy dust caps. They can help improve frequency response but in some cases, they can do more harm than good as they can provide an inconsistent surface area and also increase mass of the cone yielding poorer frequency response and linearity.
Editorial Note about Driver Beaming:
Because the diaphragm of the loudspeaker becomes increasingly directional at high frequencies and phase differences of high frequency signals owing to cone material, properties and geometry, it ceases to behave in a pistonic manner displaying high Q resonances. At low frequencies the diaphragm placed in a small enclosure, will radiate equally in all directions. However as frequency increases and wavelengths decrease, the driver will start to beam at approximately the frequency where the cone diameter equals the wavelength it is producing. This results in the speaker producing sound predominately in the forward direction. As frequencies increase, the beaming effect becomes narrower and narrower and more intense until finally all of the sound will radiate straight forward from the driver forming a beam having the same diameter as the driver diaphragm itself. This is one of the many reasons why bandwidth limiting the driver and handing off the higher frequencies to a smaller drive unit like a tweeter can resolve this problem.
Heat management and dealing with air pressure build up under the dust cap in a loudspeaker are important parts of the overall design. As mentioned earlier, the effectiveness of using a vented pole piece for heat management is still hotly (no pun intended) debated among driver designers, but many engineers designing pro drivers for high output and excursions are quite adamant of the positive effects it can have if properly executed.
We need to use steel to complete the magnetic circuit and make the speaker work! Unfortunately, steel makes a poor heatsink. The way that better speakers are made is often by using top plates that are considerably thicker than normal speakers. All the power being sent to a loudspeaker has to go through the voice coil at some point. Typically, 97 to 99 percent of the power is lost as heat. One additional way to manage the heating of the loudspeaker motor is by venting. If you have examined loudspeakers closely, you will notice a great many of those woofers, especially the woofers which use larger diameter voice coils, have a hole down the center of the pole-piece (hence the term “vented pole piece”. A vented pole piece is essentially a cylinder which goes through the center of a ring magnet to bring the north and south poles close enough to concentrate the magnetic force in the small air gap in which the voice coil is immersed. The reason for this is simple. As the voice coil gets hot (they can stabilize typically in seconds when hit with high power), the heat has to escape, or else the voice coil will simply burn up—not good. So the metal close to the voice coil (top plate on the outside, and pole-piece on the inside) needs to stay cool. If they don't, the heat won't escape the voice coil, and eventually it will simply fail.
If the speaker in question is a woofer, then under high power, it will undergo high excursions. This allows us to use the cone and dust cap as a pump, moving the air across the voice coil. Now moving the same air continually without any exchange won't help much. We want that air to be refreshed and exchanged with the air in the surrounding vicinity so that we can keep that voice coil (VC) from burning up! This is the primary reason why we have gone to the trouble and expense to cut a hole in the pole-piece. Its presence allows the exchange of hot air (unlike the halls of Congress, where hot air may pass without any real work getting done).
Another viable option of reducing heat build up in a voice coil is to directly couple an aluminum voice coil former and diaphragm. This effectively allows the actual loudspeaker cone to act as the heatsink. This type of design can be very successful at reducing heat. But, it doesn’t address one primary concern that a vented pole piece does, reducing air turbulence under the dust cap.
About 20 years ago, a well-known mid-line speaker company decided to enter the high-end market with a family of aluminum-cone speakers. The voice-coil former was aluminum, connecting directly to the aluminum cone, forming a continuous aluminum heat-transfer path. They were long-throw 1 ½-inch voice coils on smallish 6 ½-inch woofers, so the entire structure was pretty robust. During power testing one day late in the development cycle, they wanted to really “see what these babies could do,” as the saying goes. The gain was turned up louder and louder, and the speakers were really cranking, but not complaining. Someone touched the woofer’s cone, and it was hot! Then someone flicked a little water at the cone with their finger and it sizzled and evaporated like water hitting a hot frying pan.
Does an aluminum cone provide any heatsinking? Yes it does, especially when the voice coil former is aluminum also and is bonded directly to the cone. That was twenty years ago and the experience is still crystal clear. Of course, it needed to have such high power handling, because the greater mass of the aluminum cones made for a low system sensitivity of only 84dB 1w/1m.
Many tweeters these days employ ferrofluid cooling. Its purpose is three-fold: to conduct heat away from the voice coil, provide damping, and center the voice coil. Ferrofluid is an inexpensive and effective way of increasing short term power handling operated under normal conditions, and not when the tweeter is already pushed to its limits. The downside can be consistency in performance of such drivers when under high output load conditions. Also, over time, the fluid can dry out making the cooling it provides ineffective. Some designers engineer the loudspeaker crossover at low power levels, not realizing how the frequency response and damping can vary depending on how hard the loudspeaker system is driven.
It is possible to use ferrofluids in mid-range drivers and woofers. However, since tweeters tend to have the smallest peak power handling by nature, they stand to gain the most from the short term peak power handling increases provided by ferrofluid. Also the gap and coil in woofers is much larger often making it cost prohibitive and impractical to use ferrofluid cooling. There are many different formulations of ferrofluid available, allowing the designer to tailor the characteristics to achieve his/her desired design goals with less compromise.
Bottom Line on Driver Venting
Venting the pole piece of a woofer has significant advantages in reducing highly non-linear spring effect of the air trapped behind the dustcap and reducing distortion under high excursions. A secondary benefit of venting the pole piece can be heat dissipation, thus increasing the driver’s power handling and reducing its susceptibility to thermal compression. You will typically find the woofers and midrange drivers of the very best high output loudspeakers employing either a vented pole piece or hole punches in the voice coil former and/or cone to achieve the best possible performance. Phase plugs can also be a useful viable alternative to venting a pole piece or hole punching the cone/former.
Ceramic (Ferrite) vs. Neodymium Magnets
Since we know that acoustic interference is present from multiple sources (be they multiple tweeters, or the direct and reflected sound in a room) causing the same signal (read frequency) to arrive at the listener’s location at slightly different times, we can consider an ideal source (eliminating this multiple time arrival issue) that is infinitely small. We can stack up a large number of these very close together, they can sum without cancellation up to a very high frequency. Unfortunately, as we shrink the size of our loudspeaker, we also limit its ability to radiate acoustic power, which is based, in part, on the resistance of the air load to which it is coupled.
So, we have our dilemma. Everything is a compromise. Suppose we had a magnetic material which would allow us to make a small (say 25 mm or 19 mm) tweeter, and still have a sufficient amount of magnetic force to not starve the product of motor force (BL, where B =Tesla and L=Meters of wire immersed in B). Well, we have had exotic magnetic materials for quite some time, the most popular of which today is neodymium. The advantage of this product is that it contains about 10 times more magnetic energy per unit volume than does the much more commonly used ceramic magnet. This allows us to make the diameter of the tweeters very small. The smaller the faceplate, the closer we can mount the tweeters (or any speakers with a small diaphragm size) together. The closer we can mount them together, the higher the frequency to which they will act as if they were ONE speaker, and the less acoustical interference we will have to endure from the array. This advantage is not to be seen in 18” woofers. That said, if you are hauling about 50 boxes of them on tour with the band, the difference in weight alone can be offset by the decrease in your cost of fuel.
Ferrite Magnet Tweeter (left pic) ; Neodymium Tweeter (right pic)
The above left pic is of a ferrite magnet tweeter courtesy of Peerless. The above right pic is of a neodymium magnet tweeter courtesy of Vifa. Notice the heatsink on the Vifa tweeter to help keep the small magnet structure cool.
Neodymium has different technical challenges in its employment than do ceramic magnets. Most notably, as ceramics get hotter, they become more stable and harder to demagnetize (until they reach a point beyond where the entire speaker has gone up in smoke, called the Curie temperature.) While consumers need not concern themselves with this point, those in the professional business must and do. It is not uncommon, even in consumer parts today, to see a heatsink attached to a neodymium motor structure.
The reason is so it does not become demagnetized from the excess heat caused by great amounts of input power. The fact that the steel surrounding the neodymium is far smaller and therefore has less capacity to hold heat than the larger heavier structures required by ceramic magnet motors is another exacerbating factor when trying to use neodymium in higher powered loudspeakers. Some of the more clever manufacturers design their cast baskets with heatsink ribs that encapsulate the small neo magnet structure, thus giving the entire driver/basket/magnet assembly the same thermal heatsinking area of a traditional ceramic magnet driver. The Boston VR-M90 3 1/2” midrange from 1999 or 2000 was one such driver. It will be interesting to see the short term direction of the marketplace in response to recent tripling of the price of neodymium in China. Sadly in the last few years, small and light just got a lot more expensive, making neodymium a less practical choice when trying to shrink the size of the motor structure of a driver.
Bottom Line on Driver Magnets
Neodymium has the advantage of shrinking the weight and size of the driver motor structure but care must be taken in design execution to ensure the magnet is properly vented to avoid excessive thermal compression issues a ferrite alternative will typically not suffer from.
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