So Low iT Hz
Member
So I have noticed some misconceptions about speakers, particularly subwoofers, that have become way too commonplace. I have always tried to learn from the people that the industry itself looks up to, not the local know it all. So to debunk some myths I've that have become rampant I have located a good article as a reference point to list some basics that everyone should understand.
The following is not my write-up, no copyright infringement intended.
There are so many myths about subwoofers and their behavior that we decided to make a formal list to clarify what really goes on. With 15 years of practical design and engineering experience we have literally built everything in the high end audio book, so here are the facts from the fictions
---------- Post added at 02:13 PM ---------- Previous post was at 02:13 PM ----------
# 1 Subwoofers have an RMS rating
Speakers actually have a very complex thermal compression relationship and certainly can not be quantified by just one or two numbers typically called RMS and Program or Peak. Because voice coils in traditional drivers are inherently resistors, any amount of voltage generates some amount of heat which then adversely changes the resistance and properties of the speaker. This is the principle of thermal compression: As the voice coil heats up, the resistance changes and the efficiency and performance of the driver decrease until the point of maximum thermal compression. There are some unique types of materials that have a close to zero temperature coefficient and of course there is also superconducting metals that operate at subzero temperatures with no indications of any sort of resistance. In theory, only these types of materials would have no thermal compression, but they are not employed or very practical yet. Copper and Aluminum are still the two most widely used materials for voice coils. Both copper and aluminum heat up considerably and the resistance changes as a function temperature, and there lies the problem, therefore a discrete RMS scalar value is entirely inappropriate.
Under heavy use, the TSP parameters can shift as much as 35% and in a generally un-favored direction. (higher Qts, lower sensitivity). The common ultra high RMS ratings we see of large and expensive subwoofers are at best marketing ploys to make the driver seem far more worthy than it is, or in fact they are really intended to give the customer an idea of what type of amplifier to buy. The fact is, even the highest “RMS” rated subwoofers in the world in excess of 5 digit figures will begin to compress with far less power than you would ever image, try only a few hundred watts! (no joke!). Now this doesn’t mean you still don’t need lots of power to reach the maximum potential of the driver. As a rule of thumb, the amplifier should be much more capable than what the driver needs on average. For example, quick short bursts will produce huge SPL’s and the voice coil will not have time to heat up as much, but longer term high power use will result in considerable performance regression if not failure from glues giving way due to heat or differences in the thermal expansion of materials around the glues. Under heavy use thermal compression limits begin to play a large part in SPL but most people are oblivious to this concept. It is true that woofers can be used well into their thermal compression state, and typically that is what occurs. As the power increases linearity, the SPL does not increase linearly. This is some form of compression, usually thermally related unless the woofer is beyond or close to xmax. In an ideal non-compression circumstance of either power, BL or otherwise, you can expect a 3dB increase every time the power is doubled. Rarely does this ever occur, in extremely compressed and dangerous states it can be less than 1dB!
As a woofers reaches its very limits, unless failure occurs there will become a point where the resistance of the voice coil is rising faster than the power going into the subwoofer. When the resistance doubles as the power doubles then absolute thermal compression has set in. In practice you can’t actually increase the power from the amplifier because most amplifiers start to produce less power as the resistance increases because almost every car, home and pro audio amplifier is a constant voltage source rather than a constant current source. So in a way this phenomenon is a self limited occurrence that accidentally works to protect the driver. However, running the driver at or near the maximum thermal compression limit will likely result in rapid failure. Ultimately, thermal compression is a very large but unavoidable shortcoming of mass controlled transducers. Likely, compliance controlled transducers, or rather subsonic transducers are not limited by their thermal properties as much, but rather their compliance or linear limits (xmax). It is believed by a few experts in the field that thermal compression plays a much greater role in linearity and distortion than we know of, but it’s rarely discussed.
# 2 More xmax means more SPL
Subwoofer drivers really can be broken down in two categories: “Mass” controlled drivers and “compliance” controlled drivers. Mass controlled drivers tend to have low xmax and high sensitivity. These tend to be punchy and very loud and mostly used in live concerts for sound reinforcement or even car SPL competitions. Compliance controlled subwoofers which tend to be the majority of car audio subwoofers have high xmax, more weight, lower sensitivity, but more SPL in the lower frequency spectrum. Then there are of course hybrid drivers which are basically mixes of the two. Any driver in these categories can sound good or bad, but more important is being able to use the woofer where it performs the best. Using a low xmax woofer for subsonic content is probably not wise, likewise using a high xmax low sensitivity driver for sound reinforcement is not going to be very effective. In truth, there is no best driver and most drivers can overlap these zones with good results. We are not really used to the idea of a two way subwoofer, but as we demand more and more SPL and deeper bass, we may some day find that two different types of subwoofers used together are required to get the full reference SPL effect we all hunger for!
So yes, more specified xmax does mean more SPL but only for lower frequencies. Generally speaking, during lower frequencies, the driver tends to run out of usable throw (beyond xmax) before high thermal compression states occur and mechanical failure is a greater risk. 0-40Hz is primarily mechanical, 40-60 is in between) 60 and up is going to be more thermally limited. 0-20Hz is the subsonic content and in fact there are more efficient methods of producing bass in this spectrum rather than a regular piston based transducer. Surprisingly, even the largest drivers with high xmax and big voice coils can be bottomed out or run past a safe mechanical state with only a few hundred watts if the frequencies are low enough. Without a high pass (subsonic) filter, or in a low tuned system, bottoming out or breaking a driver could be a very real possibility without careful modeling and testing. The difference in displacement from 40Hz to 20Hz or rather half the frequency, or one octave, is quadruple! In the simple large sealed box example, that means if your woofer displacement is 1” peak to peak at 40Hz, you’ll bottom out just about anything in existence by the time you dip below 20Hz without protection.
Often times when people want more SPL, they really need higher sensitivity in the form of higher BL product or less moving mass, rather than more xmax because 50-60Hz is really what they are after. This is a very sensual frequency range for humans and much of the bass in music content exists in that frequency domain.
The following is not my write-up, no copyright infringement intended.
There are so many myths about subwoofers and their behavior that we decided to make a formal list to clarify what really goes on. With 15 years of practical design and engineering experience we have literally built everything in the high end audio book, so here are the facts from the fictions
---------- Post added at 02:13 PM ---------- Previous post was at 02:13 PM ----------
# 1 Subwoofers have an RMS rating
Speakers actually have a very complex thermal compression relationship and certainly can not be quantified by just one or two numbers typically called RMS and Program or Peak. Because voice coils in traditional drivers are inherently resistors, any amount of voltage generates some amount of heat which then adversely changes the resistance and properties of the speaker. This is the principle of thermal compression: As the voice coil heats up, the resistance changes and the efficiency and performance of the driver decrease until the point of maximum thermal compression. There are some unique types of materials that have a close to zero temperature coefficient and of course there is also superconducting metals that operate at subzero temperatures with no indications of any sort of resistance. In theory, only these types of materials would have no thermal compression, but they are not employed or very practical yet. Copper and Aluminum are still the two most widely used materials for voice coils. Both copper and aluminum heat up considerably and the resistance changes as a function temperature, and there lies the problem, therefore a discrete RMS scalar value is entirely inappropriate.
Under heavy use, the TSP parameters can shift as much as 35% and in a generally un-favored direction. (higher Qts, lower sensitivity). The common ultra high RMS ratings we see of large and expensive subwoofers are at best marketing ploys to make the driver seem far more worthy than it is, or in fact they are really intended to give the customer an idea of what type of amplifier to buy. The fact is, even the highest “RMS” rated subwoofers in the world in excess of 5 digit figures will begin to compress with far less power than you would ever image, try only a few hundred watts! (no joke!). Now this doesn’t mean you still don’t need lots of power to reach the maximum potential of the driver. As a rule of thumb, the amplifier should be much more capable than what the driver needs on average. For example, quick short bursts will produce huge SPL’s and the voice coil will not have time to heat up as much, but longer term high power use will result in considerable performance regression if not failure from glues giving way due to heat or differences in the thermal expansion of materials around the glues. Under heavy use thermal compression limits begin to play a large part in SPL but most people are oblivious to this concept. It is true that woofers can be used well into their thermal compression state, and typically that is what occurs. As the power increases linearity, the SPL does not increase linearly. This is some form of compression, usually thermally related unless the woofer is beyond or close to xmax. In an ideal non-compression circumstance of either power, BL or otherwise, you can expect a 3dB increase every time the power is doubled. Rarely does this ever occur, in extremely compressed and dangerous states it can be less than 1dB!
As a woofers reaches its very limits, unless failure occurs there will become a point where the resistance of the voice coil is rising faster than the power going into the subwoofer. When the resistance doubles as the power doubles then absolute thermal compression has set in. In practice you can’t actually increase the power from the amplifier because most amplifiers start to produce less power as the resistance increases because almost every car, home and pro audio amplifier is a constant voltage source rather than a constant current source. So in a way this phenomenon is a self limited occurrence that accidentally works to protect the driver. However, running the driver at or near the maximum thermal compression limit will likely result in rapid failure. Ultimately, thermal compression is a very large but unavoidable shortcoming of mass controlled transducers. Likely, compliance controlled transducers, or rather subsonic transducers are not limited by their thermal properties as much, but rather their compliance or linear limits (xmax). It is believed by a few experts in the field that thermal compression plays a much greater role in linearity and distortion than we know of, but it’s rarely discussed.
# 2 More xmax means more SPL
Subwoofer drivers really can be broken down in two categories: “Mass” controlled drivers and “compliance” controlled drivers. Mass controlled drivers tend to have low xmax and high sensitivity. These tend to be punchy and very loud and mostly used in live concerts for sound reinforcement or even car SPL competitions. Compliance controlled subwoofers which tend to be the majority of car audio subwoofers have high xmax, more weight, lower sensitivity, but more SPL in the lower frequency spectrum. Then there are of course hybrid drivers which are basically mixes of the two. Any driver in these categories can sound good or bad, but more important is being able to use the woofer where it performs the best. Using a low xmax woofer for subsonic content is probably not wise, likewise using a high xmax low sensitivity driver for sound reinforcement is not going to be very effective. In truth, there is no best driver and most drivers can overlap these zones with good results. We are not really used to the idea of a two way subwoofer, but as we demand more and more SPL and deeper bass, we may some day find that two different types of subwoofers used together are required to get the full reference SPL effect we all hunger for!
So yes, more specified xmax does mean more SPL but only for lower frequencies. Generally speaking, during lower frequencies, the driver tends to run out of usable throw (beyond xmax) before high thermal compression states occur and mechanical failure is a greater risk. 0-40Hz is primarily mechanical, 40-60 is in between) 60 and up is going to be more thermally limited. 0-20Hz is the subsonic content and in fact there are more efficient methods of producing bass in this spectrum rather than a regular piston based transducer. Surprisingly, even the largest drivers with high xmax and big voice coils can be bottomed out or run past a safe mechanical state with only a few hundred watts if the frequencies are low enough. Without a high pass (subsonic) filter, or in a low tuned system, bottoming out or breaking a driver could be a very real possibility without careful modeling and testing. The difference in displacement from 40Hz to 20Hz or rather half the frequency, or one octave, is quadruple! In the simple large sealed box example, that means if your woofer displacement is 1” peak to peak at 40Hz, you’ll bottom out just about anything in existence by the time you dip below 20Hz without protection.
Often times when people want more SPL, they really need higher sensitivity in the form of higher BL product or less moving mass, rather than more xmax because 50-60Hz is really what they are after. This is a very sensual frequency range for humans and much of the bass in music content exists in that frequency domain.