Understanding and Using Basic Loudspeaker Parameters

By Richard Ramsdell, Ramsdell Pro Audio

By knowing how to implement loudspeaker parameters into your designs and purchasing decisions you will be able to make the right choice the first time.

Raw driver manufacturers have gone to great lengths to develop their own unique product lines. Each loudspeaker has been designed with an intended specific use. You can build an enclosure or upgrade your specific box to the performance levels you want without the disappointment of purchasing the wrong speaker. We have so many customers whose only specs they look at are power handling, sensitivity and impedance. They purchase the speaker and can’t understand why it’s just not doing what they expected it to do. Being familiar with loudspeaker parameters, you will know how to look through speakers’ specifications and choose the correct subwoofer, full range driver or mid bass/midrange unit..

The following is a list and explanations of the most basic parameters. We have done our best to explain them in the most non engineering terms as possible.

Qts: The Qts could be considered the overall quality factor of the complete interaction of all its working parts. This involves all the compromises the manufacturer had to make to build this speaker. The lower Qts number is, the smaller the box you will be able to load your speaker into and achieve the results you want. The higher the Qts goes the bigger the box will have to put your speaker into. The Qts is a number that is derived from two other parameters, the Qes and the Qms. The Qes is directly related to how strong the speaker voice coil and magnetic circuit are working together. The lower the Qes number, the more push and pull your voice coil has available to move the cone. Not only does this mean that it can start and stop the cone better but it also allows the speaker to work in a smaller box. The higher the Qes the weaker the force that the voice coil can deliver. The Qms refers to the mechanical parts of your speaker. This would include the cone, the surround and the spider. The lower the Qms number the less mechanical losses you will have. A detailed study on Qms is beyond the scope of this article. It is sufficient to understand that the lower the figure the more compliant the spider and surround combination is.

Fs: This is the free air resonance of the speaker. If you hold your speaker away from any nearby objects and tap on the cone with your finger it will vibrate or resonate at a particular frequency until it slowly comes to a stop. This will be the speakers free air resonance frequency or Fs. If this speaker had a heavier cone it would resonate at a lower frequency. If the same speaker had a lighter cone it would resonate at a higher frequency.

Vas: This figure represents the volume of air usually given in cubic liters or cubic feet that has the same acoustic springiness as the cone did when you tapped on it. For example, Say the speaker you are looking at had a Vas of 10 cubic feet. Imagine having a box built that has volume of 10 cubic feet. If you were able to stretch an invisible membrane over a hole the same diameter as the speaker cone and tap on it , it would have the same springiness as tapping on the speaker that had a Vas of 10 cubic feet. If you made that same box bigger, it would resonate at a lower frequency. Conversely if you made the box smaller it would resonate at a higher frequency. The Qts, Fs and the Vas are going to be the three figures you will need to calculate your optimum vented box size.

Re: The Re is the actual DC measurement of your speakers’ resistance. If you were to take a good quality volt ohm meter and take a reading across the terminals the Re would represent the reading you would get. An 8 ohm speaker usually has an Re of 5 to 7 ohms/..Do not confuse this with the 8 ohm rating of the speaker. Your speaker can vary from 5 to 200 ohms depending on the way you plan on using it. The 8 ohm rating is supposed to represent the overall average impedance of your speaker.

Impedance: We have found over the years that a lot of customers do not understand Impedance verses Resistance. Resistance is a DC measurement and Impedance is an AC measurement. Your speaker only has one DC measurement, this would be the Re. But due to the fact that your speaker cone vibrates in a different manner at each individual frequency it can exhibit a different measurement in ohms at each of these frequencies. For example it may be 60 ohms at 50 hz, 15 ohms at 100 hz and 8 ohms at 500 hz. This will change for each different cabinet that you would put the same speaker into. It will end up reacting differently to each box it may be put into also.

Impedance of passive crossovers: Here is a very important warning. If you are not knowledgeable in passive crossover design it is best to purchase a crossover made by the same manufacturer of the speakers you would like to combine together. By guessing at building your own crossover you can end up with dangerously low impedance areas in your design. This will cause your amplifier to prematurely over heat causing damage or thermal shut down. This problem is even compounded further by putting two ore more of these projects in parallel on the same channel of an amplifier.

X max: This number represents the linear travel in millimeters of your speakers voice coil. For comparison purposes there are 24.5 millimeters in an inch. For your speaker to work properly as it moves in and out the magnetic gap the coil rides in must be completely filled up with the turns of wire that make up the voice coil itself. For maximum linearity and usefulness there must be extra voice coil that is available to keep the magnetic gap filled as signal is applied to the speaker. This voice coil extends above and below the magnetic gap in most designs.

The Xmax is the measurement in millimeters of how much of this coil sticks up out of the magnetic gap. The Xmax is the amount of overhang on one end of the coil. Once you begin to overdrive your speaker and run out of Xmax the gap is no longer completely filled with the voice coil. The output of your speaker begins to drop off quickly. The speaker begins to loose it's ability to control the moving mass of the cone. The movement becomes erratic at best and the possibility of mechanical damage increases quickly. At this point if you don’t turn it down everything begins to go downhill at the same time. The ability of your speaker to dissipate excess heat is reduced also.

In addition, it affects the overall impedance curve and the amplifier has a more difficult time controlling it at high power. This is where it becomes extremely important to have your speaker in a properly designed enclosure. At the conclusion of these parameter definitions we are going to show you how to easily take a few of the more important parameters and plug them into a box design formula. You will be able to design the optimum box for high power use for the speaker you have chosen to use.

No. or Eff: This figure represents what is called the half space reference efficiency. This number is the product of a formula involving several other parameters. You will notice that as you study these numbers that loudspeakers are relatively inefficient devices. Even the electric motor in your fan is probably 70 percent efficient.. The average high output subwoofer is 3 to 5 percent efficient at best. The rest of the 100 percent is turned into heating up your voice coil which has to be effectively dissipated as possible. This figure is best used to compare similar sized speakers to each other. It is difficult to compare this to the sensitivity number. It gives you an idea of how well this particular speaker will work as a mid bass or low bass reproducer.

A lot of times smaller very efficient 10 inch and 8 inch midrange type speakers will not exhibit a higher half space reference number. I tend to use this figure in part of my decision process of choosing a certain speaker if other parameter measurements are similar. For example I would never dream of using a 15 or 18 inch woofer with an efficiency of under 3 percent. I also like to see 12 or 15 inch speakers I want to put into as mid bass horn or other mid bass applications in the 5 to 6 percent range. As you get more familiar in using parameters for evaluating woofers, you notice that the higher half space reference efficiency woofers usually like larger boxes. The higher efficiency speakers also tend to be more accurate, tighter and have better punch.

Additional information on the use of No. or Eff. More efficient speakers are more linear in smaller boxes. Due to a more powerful motor design they can still compress the air in their box the same way as you reduce the box volume. Here is an effect we discovered designing a powerful home theater subwoofer years ago. Even a car audio subwoofer with a .75 percent efficiency sounded great when we were able to hit it with a 1000 watts during home theater product testing. How ever it failed to follow the soundtrack faithfully at 2 or 3 watt levels during the more quiet passages. It simply disappeared.

When we installed a more efficient low frequency driver the passages were audible and faithfully reproduced at all levels. We discovered that somewhere in the 2 percent range and higher they did a more accurate job of following all passages at all levels. There has been a design trend lately to build inefficient super long Xmax subwoofer drivers for sound reinforcement applications. They are no where near as accurate sounding when played at the exact same levels as higher efficiency drivers.

One advantage of using lower efficiency drivers is that they will go into a smaller box. They get their inefficiency from a higher moving mass and a stiffer suspension that is needed to control it. The result of this type of design is smaller box size because of a smaller Vas .Sometimes it is not possible to use a larger box to get your low frequency requirements. This type of design is very popular in automotive sound and home theater applications.

MMS and MMD: These symbols represent the moving mass of your speaker. The moving mass would include the cone, The voice coil and one half of the surround and spider. Mmd would be the moving mass of the cone, voice coil and one half of the spider and the surround. The Mms is the same thing but it includes what is called the “air load”. When the speaker cone is moving in and out there is a “can” of air on both sides of the cone that tends to ride back and forth with cone at the same time. This air also has a few grams of mass that needs to be added to the over all moving mass. Naturally, The larger the speaker cone the larger the can of air would be. Your speaker does not move air along like a fan. It compresses and expands the air near it and this produces the sound that you hear.

SPL: Spl stands for sound pressure level in decibels. When published with in a list of loudspeaker parameters this value is usually computer generated number from a parameter formula. It usually represents an overall average level. If you were to measure individual frequencies they would probably be higher. Here is the most important part you need to know. Try to find out what the Spl is in the frequency range you intend on using it. A lot of manufactures publish their values from measurements made in the 100 to 1000hz area. They do this because this is where the speaker will give them the highest most impressive number for their specs. This not going help you if you intend to use it from 80hz and down. Reputable manufactures usually publish how they made all their measurements.

BL: Sometimes this is written as the Bl product. This is because it is the result of multiplying these two values together. B is simply the strength of the speakers magnetic gap that the voice coil is rides up and down inside of. L stands for the length of wire that is wound on the voice coil that is actually inside of the magnetic field. By multiplying these two numbers together you end up with the Bl product. The higher the Bl product the more force the coil can exert on the cone. There are many different ways of trying to get as much wire as possible in the gap. Some manufactures use edge winding, some use multiple layers of round wire windings and still others use triangular or hexagonal wire winding. They all work slightly different depending on what the manufacture is trying to accomplish. The Bl product is one of the parameters I look at after I have determined that the speaker is close to the correct value I have chosen for my box size and low frequency requirements. I consider the Bl and the Mms relationship to be very important. The heavier the cone the more powerful of a magnet assembly you are going to need to control it. We are going to go more into depth on this relationship in the enclosure design section.

PE: The Pe should represent the continuous overall long term power handling or the speaker. An entire book could be written on this subject. It is difficult at best to convert ever changing audio signals with tremendous peaks and dips into a long term published safe operating value in watts. What the manufacture has done is to try to combine thermal and mechanical limitations together in one figure. The problem is that any one of these areas can over power the speaker without exceeding the other values.

For example, certain types of high impact music and exceed the mechanical limits without overheating the voice coil. Doing this for too long can tear cones or surrounds or even physically damage the voice coil. The opposite can also occur. You can put too much continuous power to the coil and burn it up without exceeding the mechanical limits. The manufacturer has gone to great lengths to come with real world value. The Pe should be the watt rating you should consider.

It is the opinion of the writer that you should match the Pe to the Rms output rating on your amplifier. If you are using an amplifier that has an Rms rating in watts that exceeds 10 to 20 percent of your speakers Pe it becomes more important. We have had over 30 years of experience in live sound applications. Let me tell you what can happen. Almost all amplifiers made today come with some sort of a soft clip circuit. If you are considering a purchase of an amplifier definitely make sure this is a feature that is available and installed on the amplifier. This will protect you from overdriving your amp and producing a “clipped” or distorted signal. When this occurs there is all sorts of high Frequency garbage that is produced. Your speaker can not respond fast enough to convert this distortion into sound. The voice coil effectively just sits there and this distortion turns into heat. This extra heat is added to the normally generated heat and will eventually overheat and burn the voice coil up.

When used properly your amplifiers soft clip circuit will protect against damaging distortion. If this circuit stays on too long other problems can occur. Your speaker likes to see variations in your level so it can cool itself naturally. The harder you drive your amp the anti clip circuit begins to change your program material more and more into what the speaker sees as a continuous or Rms signal. If your amp can put out a greater Rms signal then your speakers Pe rating you will eventually experience speaker failure.

Your speaker naturally sounds better with a huge amp over a smaller one but you need to be careful. Your speaker can handle tremendous power for short bursts. However if you greatly exceed your Xmax rating for an extended time your speaker will die a premature death from mechanical fatigue. Your spider and surround simply loose their strength from being over stretched. It is our recommendation that if you just starting out in this high power arena that you heed this advice. Instead of driving one speaker four times as hard add a second one and achieve your long term output requirements that way. Believe it or not, in the long run it will cost you less. You will end up with a system that will last years instead of months.

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Reprinted with permission of www.RamsdellProAudio.com © 2005 Ramsdell Pro Audio. All Rights Reserved.

 

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