Speaker Design By RJB Driver SelectionBefore selecting drivers you will have to determine what type of speaker design you want to build, a subwoofer, a fullrange, a two-way, a three-way or some other combination. You also have to consider what type of enclosure you want to use for the lower frequency drivers. Then you will have to determine what sensitivity you are looking for and what minimum impedance you can tolerate as well as how deep you want the speaker to play. You also have to consider what type of crossover you want to use including crossover frequencies and slopes. Different drivers have different applications. Some drivers can play very low in a smaller enclosure but suffer from low sensitivity (the Adire Audio Extremis 6.8 and Dayton RS125 are good examples of this type of driver). Some drivers have higher efficiency but usually suffer from poor bass extension. Some drivers require steep crossover slopes to mitigate the effects of cone resonances while others can be used with lower order crossovers because these resonances are damped within the cone.I am not going to get really technical with the discussion of how a driver works and different electromechanical interactions within the driver but intend to focus on driver parameters that are important in determining whether a driver can suit your needs. There is information on driver construction in many of the basic speaker design books and there is more advanced information available on the net. Some of the topics I'm not going to cover are motor design, magnet types, surround types, spider types, voice coil alignment, and other similar areas. Instead I will focus on driver size/type and cone material. My area of expertise is crossovers and I will focus most of my attention in that area and try to cover as much material as possible there. Driver MaterialWith regard to driver types I will only be discussing drivers with cone/dome materials that I've used in my projects. So this list includes polypropylene, paper and aluminum cone woofers and fabric and aluminum dome tweeters. Below is a list of the drivers I've used in each category.
Polypropylene ConeI have the least experience with poly cone drivers out of the drivers I've used but I can still discuss their general properties. Poly cones have a tendency to flex more than other cones and this allows it to dampen any of the resonances that usually show up at the top end of its response. This means they often have a very smooth response at the top end which makes it easy to use them with low order crossovers. Some would argue that even though the resonances are damped you are still better off crossing lower and using higher order slopes. I haven't experimented with this concept but have used poly cones in two different designs with a lowe crossover point and 4th order slopes. In general I would have to say that poly cones tend to have a warmer, more resonant sound to them and don't have as much detail as the other two cone materials. Of the three I think poly is my least favorite overall.Paper ConeI've used a few different paper cone drivers in the past with different crossover topologies for each one. Paper cones still flex a little bit but not as much as poly cones and the cone resonances are still damped but not as well as with the poly cone. Even though the cone is a bit stiffer, paper cones can still usually be used with lower order crossovers. I've used mine with 1st order series, 2nd order Linkwitz-Riley and 4th order Linkwitz-Riley networks with great success. Paper cones are more detailed than poly cones and a bit less warm although they tend to be warmer than most aluminum cones. Paper cones can be very smooth while still retaining a decent level of detail. I think that paper cones have the best aspects of both poly and aluminum cone drivers in that they have the detail of the aluminum cone but the smoothness and warmth of a poly cone and can still be used with lower order crossovers. My favorite cone material is paper especially for midrange applications (2 way or 3 way).Aluminum ConeI probably have the most experience with aluminum cones having built two three way speakers using them. Aluminum cones are extremely rigid and cone resonances are not damped very well. As a result, they have large resonant peaks at the top end of their response. The larger the driver, the lower in response these peaks will occur. Due to these peaks you usually want to go with 4th order or higher crossovers unless used as a bass driver low enough in frequency in which case you can use 2nd order slopes. For midrange/midbass drivers I only use 4th order filters and like to get the resonance peaks down by at least 30 dB, 25 dB at the absolute minimum. Aluminum drivers have the most detail compared to the other two types. However, this detail often comes with a price because they can sound a bit harsh with poorly recorded source material because they are so detailed. My analogy for these drivers is they have similar properties as solid state amplifiers with lots of detail but less smooth. I feel that paper drivers have similar properties as paper drivers because they are a bit smoother in the midrange and I also feel that they have a more spacious sound for some reason. I thought some of these differences could be to differences in distortion. Paper cones sometimes have higher 2nd order distortion and lower high order distortion while aluminum cones can sometimes have lower 2nd order distortion and higher high order distortion and this analogy is similar to the differences between tube and solid state amps. I prefer paper over aluminum for midrange/midbass drivers but for bass drivers I like aluminum quite a bit. I think that the high order distortion that may be an issue with aluminum cones doesn't hurt the sound when used as a bass driver because the harmonics aren't high enough to be irritating and this can actual give the bass more (artificial) detail. I have yet to use a paper cone as a bass driver in a three way so I can't comment on how the bass compares.Fabric DomeI've used quite a few good fabric domes in my projects. Fabric domes are somewhat similar to paper cones in that they flex a bit and dampen high frequency resonances. With a fabric dome you can get a nice flat extended response from a tweeter unlike aluminum domes that usually have a dip an then resonant peak in the 25 kHz area. I've found that fabric dome tweeters usually have a bit more air and sparkle at the top end than aluminum domes. My favorite all time tweeter, the Hiquphon OW1, uses a fabric dome. I generally prefer fabric dome tweeters over aluminum dome tweeters.Aluminum DomeI've only used two aluminum dome tweeters so far. Aluminum domes are very stiff with very little damping and tend to "ring" above 20 kHz. This ringing is beyond most people's hearing range so it shouldn't be an issue. However, there is often a bit of high frequency roll off above 15 kHz for a lot of the 1" models while a comparable fabric dome will be flat in this region. Metal domes can be very detailed but I've found that they sound a bit harder than fabric domes. They also don't seem to have as much air and sparkle at the top end. The RS28a is the best metal dome I've heard but it still isn't as nice as the OW1.Driver MeasurementsThere are a few different driver measurements that are important in determining how well the driver will perform. It is essential to know the impedance and frequency responses of the driver in order to determine if it will work well in your application and these are also required to model the crossover. A driver with a flat frequency response is the easiest to work with. Any peaks or dips must be dealt with using the crossover tuning. It is easier to tame peaks than it is to elevate dips. The impedance response can also tell you something about the driver's performance. If there are any irregularities in the response like small dips or peaks then this is an indication of a driver issue and often shows up in the frequency response in the region of the irregularity. You also want to pick a driver with an impedance that will work in your system. Most amps can handle 8 ohm loads easily but might have problems with a 4 ohm speaker. So if you plan to use two drivers in parallel and stick with the typical 8 ohm driver you will end up with a 4 ohm speaker. However if you used two 16 ohm speakers in parallel or used two 4 ohm speakers in series you have an 8 ohm design. For subwoofers I think that 4 ohms is preferred because most subwoofer amplifiers are designed to drive a 4 ohm load and you'll get more power out of it as well (as opposed to using an 8 ohm driver). It is not necessary to match the impedances of the different drivers in the design and the overall impedance is based on each driver's operation range so putting a 4 ohm woofer with an 8 ohm midrange is most likely going to give you an impedance near 4 ohms in the woofer's range of operation and near 8 ohms in the midrange's range of operation. The best way to determine if the impedance of the final design is good is to look at the impedance response of the crossover simulation.Distortion measurements can tell you a bit more about the sound of the drivers and it is a good way to compare the performance of similar drivers. Mark K has some great write-ups that discuss the properties of linear and nonlinear distortion at the following links. Nonlinear distortion testing Linear distortion testing You can go to the link above for some good technical information regarding distortion but I will try to quantify types of harmonic distortion by how they sound. Harmonic distortion occurs when higher frequency tones at harmonic intervals accompany the base tone. Harmonic distortion can be characterized by the order of the distortion with second order distortion being the distortion component that occurs at twice the fundamental frequency and you can probably figure out what third, fourth, fifth and higher order distortion tones are. From what I've read and heard even order distortion is easier on your ears and has a more pleasing sound. Odd order distortion can sound harsh but can also give a driver a greater sense of detail. Lower order distortion is harder to detect. It takes a much lower level of higher order distortion to detect it compared to lower order distortion. High order distortion in a tweeter can give it a more detailed sound. Based on my experience I tend to prefer drivers with low high order distortion even if I have to deal with higher 2nd order distortion because I find that the low order distortion is tolerable while the high order distortion can sound harsh even though it may provide a more detailed sound. I think that you can compare drivers to amplifiers regarding distortion. Tube amps have a lot more 2nd order distortion than solid state amps and they tend to have a smoother, less detailed sound overall. I think the same analogy works for drivers with similar distortion characteristics as the amps described. Driver ChoiceIn this section I'll cover how to choose a set of drivers for a particular design configuration. I'm going to break it down by frequency range and cover the tweeter, midrange(3 way)/midbass(2way), woofer and subwoofer.TweeterWhen choosing a tweeter you first have to determine what type of tweeter you want to use including fabric dome, metal dome, planar, ribbon or some other type. In order to choose a tweeter you will need to know how low you need the crossover frequency to be and how sensitive it should be. A larger tweeter will tend to be able to cross lower and you can usually determine the lowest crossover frequency by looking at the resonant frequency of the tweeter and you usually want to cross at least 1 octave higher than the fs of the tweeter (with high order slopes). A larger tweeter with a low resonance frequency is a better match for an aluminum cone driver because you can cross low with steep slopes and attenuate the resonance modes of the aluminum driver well. Most ribbon tweeters are better suited to higher crossover frequencies with steeper slopes but longer ribbons can be used lower (however the vertical dispersion suffers significantly).The dispersion of a tweeter is directly proportional to the size of the tweeter. For a dome tweeter the dispersion is symmetric about all radial angles. For a ribbon tweeter the horizontal dispersion is determined by the width of the ribbon and the vertical dispersion is determined by the height of the ribbon. The smaller the ribbon or dome, the higher in frequency the response will remain flat as you move off axis. This is due to the fact that at higher frequencies with wavelengths smaller than the width of the ribbon or dome will tend to cancel off axis because the sound radiating from one side of the diaphragm will cancel with the sound coming from the other side of the dome as the difference in distance is great enough to put these waves out of phase. For this reason I tend to prefer smaller dome tweeters. Ribbon tweeters usually have good horizontal dispersion but the vertical dispersion suffers due to the longer ribbon length. My favorite tweeter, the OW1, has excellent dispersion because it is a 3/4" dome and an added bonus is that it can cross lower than most domes its size. Sensitivity usually isn't much of an issue with tweeters because most of the time it is greater than that of the other drivers especially when baffle step losses are taken into consideration. The limiting factor to the overall sensitivity of a design is the bass driver whether its the woofer in a three way design or the midbass in a two way. Then you must consider that you'll lose 6 dB in the bass response due to baffle step losses (I discuss baffle step later on). Assuming that the amount of baffle step compensation is usually in the 3-6 dB range the overall design sensitivity should end up 3 to 6 dB less than the sensitivity of the woofer. Conservatively assuming that you will only use 3 dB of BSC (baffle step compensation) you'll need a tweeter that is about 3 dB less sensitive than that woofer. Midrange/MidbassI think this is the most critical driver because it covers most of the range in which are hearing is most sensitive. There are a few main decisions to make. First of all, what type of cone material do you want to go with. I've found that paper is my favorite but if you want a more detailed sound and listen to a lot of Jazz and Classical music aluminum might be a better choice but it may limit your crossover possibilities. Determining what size driver you'll need is the next choice. For a two way design you usually need a larger driver in order to get good bass extension. However, a larger driver will often require a larger enclosure and will require a lower tweeter crossover frequency because it will begin to roll off and exhibit cone resonances lower in frequency. The other issue with a larger driver is it becomes directional lower in frequency (similar to the dispersion phenomenon with tweeters) and you'll want to cross low enough that the off axis response doesn't have severe dips. For larger two way designs I think 8" is the largest size driver that should be used and even then you should cross in the 1kHz to 2kHz range for the best performance. I think that 6.5"-7" drivers are actually ideal for this application and that is probably why there are so many 6.5"-7" two way designs in existence. 5"-6" drivers also work well but you usually sacrifice bass extension and power handling.Driver parameters can indicate what size/type of enclosure is best for a driver and there is usually a tradeoff between bass extension and efficiency. The drivers that I've found that work well in small enclosures and have great bass extension always end up being the least efficient because you can't cheat physics. In order to get a higher sensitivity speaker to play deep you'll need one or both of two things... large cone area or a large enclosure. So based on what size cabinet you want, what f3 you desire and what sensitivity you need you can hopefully find a driver that suits these needs. For a dedicated midrange you can usually go with a smaller driver because the woofer in the three way design can usually cover most of the bottom end. I like to use 4.5" to 6" midrange drivers because they usually have enough excursion and decent bass distortion properties above 200 Hz. 7" drivers can also be used as a midrange and can provide a bit more sensitivity and power handling but may require a lower crossover point with the tweeter. WooferI'm going to discuss this driver as being used as the low frequency driver in a three way design with a crossover frequency below 400 Hz. You have a lot of the same tradeoffs as mentioned before regarding enclosure size, bass extension and sensitivity. The woofer in a three way design will determine the overall sensitivity of the design. I've found that the sensitivity of most of my three way designs ends up being about 3 dB less than the sensitivity of the woofer in the design. Three way designs usually use bass drivers anywhere from 6.5" to 15" and the larger the driver, the higher the low pass filter will have to be. Some subwoofer drivers can be used as the bass driver in a three way design but you'll have to set the crossover frequency lower to get the best response out of the subwoofer driver. My favorite drivers for a three way design are 8" and 10" sizes because they don't require a real huge enclosure and offer decent bass performance at a reasonable sensitivity.Driver TradeoffsWhen choosing a driver for use as a midbass or bass driver there are a few tradeoffs between design parameters. The three main parameters of concern are the size of the enclosure required, the sensitivity of the driver and the f3 of the design. Dick Morgan suggested to arrange these parameters in a triangle so I created a visual aid in this fashion but I don't know if this idea originated elsewhere. The driver tradeoff triangle is shown below. I have some examples of drivers that I've used that fall on different parts of this triangle. The first example is the Adire Audio Extremis 6.8. I used this driver in a 24 liter enclosure with an f3 of around 30 Hz but the overall design sensitivity is only 80 dB after baffle step compensation is factored in. So this driver has a low f3 and a relatively small enclosure but suffers from poor sensitivity. So this driver would be located near the middle of the left side of the triangle. Another example is the Vifa XT18WH driver that I used in my Asterion project. This driver has a pretty low f3 of 40 Hz (maybe lower according to measurements) and has decent senstivity after BSC of about 85 dB but requires a larger enclosure of 28 liters. So this driver has decent sensitivity, decent f3 but requires a larger enclosure to get this. I would say that this driver should fall on the right side of the triangle but maybe not right on the side but closer to the center because the box size requirements aren't that great. This driver probably offers a very good compromise between the different parameters. The third driver in the comparison is the Seas CB17RCY/P used in my Galatea design. This driver has the highest sensitivity (85 dB after BSC) of the bunch and only requires a 16 liter cabinet but only has an f3 in the low 50's. So this driver has the best sensitivity and works well in a small enclosure but suffers from bass extension. This driver would be located on the bottom side of the triangle probably centered between the small box and high sensitivity points. So based on these tradeoffs you can determine what requirements you need and choose the driver that can best meet those requirements. Driver choice is mainly based on the application and resources. For example if you want a small subwoofer that can play very deep you can use a low sensitivity driver that works well in a small cabinet with a low f3 but it will require more power. If you want a high efficiency two way design that plays very deep you will usually need to either consider a driver that works well in a larger enclosure or choose a driver that works well in a smaller enclosure but suffer some loss in bass extension. These tradeoffs should be considered when picking any bass/midbass driver for a design and should help you determine the physical limitations of different design concepts.  |