Frequently Asked Questions
How do I choose from all the different systems?
The Performance Comparison Table highlights some of the differences among the various options.
It is important to keep in mind that there are frequently many different solutions that will result in the same level of user enjoyment and different people can have different opinions of any given system.
For folks with very limited budget and/or who may just want to get some bass to compliment a soundbar or bookshelf speakers, a pair of MBM-18 cabs, PA-460 drivers, and a Yung 500 watt plate amp (6dB boost option; one amp can run two subs) will provide a surprisingly amount of bass (approximately 115dB) into the mid-20's Hz. Such a system would blow away the 6.5" - 8.0" woofers-in-a-box that are sometimes marketed as subwoofers.
For folks who are looking for movie bass with extension below 20Hz at the best value, MartySubs may be your best bet. Two MartySubs and an NX Series amplifier will provide Reference Level bass output in most rooms. We recommend Mini-Marties as providing the best combination of output and extension for most folks. For larger rooms or for folks who like to run their bass "hot", four subs has proven a very popular choice. If Mini-Marties are too large, go for the Cubes. For the last couple Hz of extension (and if size permits), go for Full Marties.
For folks who feels that they need a little more mid-bass punch in their systems, MBM-18's with PA460 drivers can be added to any of the MartySub-based systems.
For music-only subs, a pair of MBM-18's with TBW-100 or SAF184.03 drivers is an excellent choice that will provide very clean bass. For a little more output, the MBM-21 with 21DS115, 21SW152, or SAN214.50 drivers can be the way to go.
For folks who want it all, a quad set of 21-inch HT subs loaded with 21DS115, 21SW152 or SAN214.50 drivers and either a couple NX6000D amps or a SpeakerPower SP8000 leaves little to be desired. Very few subs in the commercial world compare with systems at this level. Those that do tend to have price tags in-line with premium automobiles.
There are always some folks who want to be at the ultimate edge of performance. If this is you, then Devastators are probably what you are looking for. :-)
After reading the performance table, I'm still unsure. What is the difference between the PA460 and the UM18?
The PA460 is better in most every way EXCEPT very low end output. The UM18 has about three times the excursion capability of the PA460, which in many cases is what limits the very low end output. So for the same amount of low end as one UM18, 2.5-3.0 PA460's are required depending on which enclosure is being compared.
For example, in order to get the same amount of low end as a pair of UM18's in MiniMarty enclosures, FIVE subs would be required if using PA460's. Of course, you'd get a whole lot more mid-bass with the 5 sub solution, but for the guy whose application is mostly movies and wants the most very low end, UM18's are the way to go.
The 21-inch drivers are somewhat more expensive, but provide somewhat cleaner sound and good midbass in many ways similar to a PA460, but also have the low end output capability of the UM18. So as to the question of which driver is the "best", it sort of depends on the application, the size of the space, number of enclosures, and the budget as to which is the best way to go.
How much performance can I really expect from MartySubs?
Compared to a typical home-theater-in-a-box (HTIB) or even a typical 12” subwoofer powered by a 250 watt amplifier (as you might find in many home theaters today), there is simply no comparison to a MartySub.
We could cite performance specifications (such as 120dB+ output from 20Hz up) as being typical of what a MartySub will do in a typical room, and “over Full Reference” output from a pair, but that would be meaningless to many folks.
Let’s just put it this way, a pair of MartySubs and the recommended amplification will meet and in many cases exceed (by going lower and playing louder) the movie bass experience that you have come to expect from the "main theater" at your local cineplex.
Will a MartySub by GSGTM work with my system?
If your receiver has a “subwoofer” RCA output, you are good to go.
Despite using professional grade amplifiers, MartySubs by GSGTM are connected to your system in the traditional fashion—a simple “RCA” cable goes from your receiver (or other subwoofer output) to the amplifier’s input. The input on the amplifier uses a professional grade connection called an “XLR”. Therefore to connect your RCA output to the XLR input, you will need an RCA-to-XLR cable. Such cables only cost a few dollars and a link to a cable is provided on the Checklist page.
If you are unsure how to connect a MartySub by GSGTM to your system, please contact us at email@example.com and we will try to help you figure it out.
Do MartySubs by GSGTM come with instructions?
Yes! We have put together step-by-step Assembly Instructions that include pictures of how the panels go together as well as several build tips. No complicated tools are required for a MartySub build.Builder Guides and Assembly Instructions
I was hoping to use a Speakon connector instead of the terminal cup. What should I do?
For many folks, the standard terminal cup with the traditional banana plugs is the easiest to work with. Other folks prefer the Speakon connector. In order to accomodate both groups, we have made the basic cutout for the terminal cup and have included an adapter for folks who want to use an NL4MPR Speakon connector. Just glue the adapter into the terminal cup hole and then attach the Speakon connector to the adapter. Designing it was a bit of an effort, but using it is a piece of cake! The Speakon is even recessed (flush with the back of the cabinet) for a professional look. If you want to use the Speakon adapter, we recommend attaching it before assembling the rest of your cabinet.
Which Speakon connector and Speakon cable do I need?
Step 1: The female connector on the subwoofer enclosure will be the NL4MPR for ALL SUBWOOFER ENCLOSURES.
The driver will be wired to 1+ (positive/red lead) and 1- (negative/black lead) on the Speakon connector for everyone EXCEPT folks using a 3000 or 3000D amp in BRIDGED mode.
ONLY for folks using using a 3000 or 3000D amp in BRIDGED mode, the driver will be wired to 1+ (positive/red lead) and 2+ (negative/black lead) on the Speakon.
Step 2: The cable depends on which amp and setting you are using.
If using the NX6000 or NX6000D amp (which is not a bridgeable amp), choose one of the following cables:
Standard cable with straight connectors (3, 10, 25, 50, 100 foot)
Optional: Right angle connector cable (allows for closer positioning of subwoofer to the wall)
Right angle (15 foot 2-pack, 50 foot 1-pack, 50 foot 2-pack)
Right angle (25 foot 1-pack)
If using the NX3000 or NX3000D amp (which IS a bridgeable amp), choose one of the following cables:
The choice of cable depends on whether or not you will be running the NX3000 or NX3000D amp in STEREO mode or BRIDGED mode.
If running the amp STEREO mode (one sub on each channel), then the choice of cable is the same as for the 6000 amp above. See the options above and choose your cable.
IF running the NX3000 or NX3000D amp in BRIDGED mode (one sub on the amp), choose the following cable:
Speakon cable for bridged mode amps (25 foot)
Speakon cable for bridged mode amps (50 foot)
Optional: If you are running BRIDGED mode NX3000 or NX3000D amp and want a 90 degree connector, you can switch the straight connector on the cable to the right angle connector listed below (NL4FRX). Just be sure to get each wire (1+, 1-, 2+, 2-) attached appropriately in the new connector. By unscrewing the round plastic strain relief on the connector on the cable, you can see how the wires are attached and what is involved in making the switch.
How do I set up the digital signal processing ("DSP") on my amplifier?
IMPORTANT: These settings MUST be applied BEFORE running your subwoofer or it could be damaged.
The protective high pass filter should be set at or just below the tuning frequency of the subwoofer or MBM.
If you are unsure of the tuning frequency of your subwoofer or MBM, check the product page.
NOTE: While the settings can be entered using the DSP controls on the front panel of the amplifier, it is much easier to update the settings using the PC software. The software is available at the Behringer website on the amplifer page by clicking on "downloads" in the upper right corner of the page.
Without getting too technical, the motion of the cone will go "wild" below the tuning frequency in all ported subwoofers. This can damage the driver. In order to protect the driver, frequencies below the tuning point of the subwoofer need to be filtered out of the signal. This is accomplished with a "high pass" filter (which allows frequencies above the setting to "pass", while mostly filtering out frequencies below the setting).
Note: the Dayton SA1000 amplifier comes with a protective high pass filter already in place. No additional configuration is necessary.
In order to set up your high pass filter, click on the "Filter/Crossover" tab and then enter the parameters shown in the red boxes below. NX3000D (3000DSP) only requires setting the high pass on Channel A, as the high pass on Channel B won't be used.
Depending on which amplifier you are running, NX3000D (3000DSP) or the NX6000D (6000DSP), there are different settings in the software on the "Configuration" tab.
NX3000D (3000DSP) Specific (Set amp to Bridge mode):For the NX3000D (3000DSP), you will want to set your amp to "bridged mode". Bridged mode directs power from both channels to a single subwoofer providing significantly more output.
NX6000D (6000DSP) Specific (Set amp to Bi-Amp 1 mode):For the NX6000D (6000DSP), there are two options:
1. (Most common) If you will be running a single subwoofer output from your AV receiver, then choose Bi-amp1 mode (this will take a single input signal and send it to both channels on the amp). Only one cable is required and goes from the Subwoofer Output on the AVR to the Input on Channel A on your amp.
2. (Less common) If you will be running dual subwoofer outputs from your AV receiver, then choose Stereo mode (this will send a unique signal from your AVR to each subwoofer). With Stereo mode, two cables are required, one goes from the Subwoofer Output 1 on the AVR to the Input on Channel A on your amp, the other goes from Subwoofer Output 2 on the AVR to the Input on Channel B on your amp.
With your protective high pass filter in place and your Configuration set up, you are now ready to plug your amp into your receiver and begin enjoying your MartySub!
The design of the subwoofer and the settings work together. In a large room, typical response should be strong down to just under 20Hz. In a medium or smallish room, typical response should be strong into the mid-teens or lower.
NX3000D (3000DSP) AND NX6000D (6000DSP) Amplifiers (Set the Limiter):
As a final step to setting up your amplifier, set the limiter to 94 volts peak, which is 1104 watts at 4 ohms. This will help protect the driver from overheating. Even with the limiter engaged, use good judgment with content (full power sine waves and similar "destructive content" are not recommended).
Do I need stuffing or lining in my subwoofer?
No. The wavelengths in the bass are very long relative to the size of the enclosure, so there will be no resonances in the range where the subwoofer is providing output. For a more detailed analysis, Josh provides a good study here: General Effects of Stuffing and Lining on Subwoofer Performance
Who is "Marty"?
The original Marty subwoofer was designed and named by our lead designer, John (he’s the LTD02 guy), in 2013 on the AVS audio forum for a forum member named Martycool007. Since then, there have been have been hundreds of Marty sub builds and countless variations on the theme.
How do MartySubs compare to sealed subwoofers that use the same driver and amp?
MartySubs are large compared to most sealed subwoofers that use the same driver. The additional size provides a significant increase in low end output. In the image below, a Full Marty is compared with a sealed subwoofer (4 cubic feet net internal volume) using the same driver (UM18) and the same amount of amp power (1,500 watts). As can be seen in the image, above 50Hz, both systems produce essentially the same amount of output. Below 50Hz, the port on the Full Marty begins to provide additional output (in much the same way as the air in a pipe organ resonates increasing its output).
At the tuning point on the Full Marty (16Hz), the port is providing approximately 10dB of additional output relative to the sealed sub. Doubling subwoofers, drivers, and amps provides +6dB of gain, so +10dB is more than twice the output of the sealed sub. So in many ways, a large ported sub is like getting two sealed subs on the low end.*
Of course, not everybody has the room for a MartySub, or two, or more :-), so in those cases a sealed sub or two may be the only option. Sealed subs can also have a benefit of providing some tactile response into the single digits, but their primary benefit is their more compact form factor. Where space is available, large ported subs such as the MartySubs provide the most output per dollar aka 'bang-for-the-buck'. The same rule applies for choosing a MartySub. Get the largest one that will fit in your space. Bigger models come with slightly lower tuning points and slightly more low end output.
Ported subs and sealed subs can both provide an enjoyable bass experience. Discussions around "which is best" should generally be re-framed to "which is best for you", since they both have a role to play depending on the application and the goals of the user.
* Technically, +10dB relative output is just over 3.16 times the output of a sealed sub.
Where does the Dolby Atmos Reference Level of 125dB at 31.5Hz that you mention come from?
Decibels (dB) are measured on a logarithmic scale, so the addition of multiple sources can be confusing, but here goes...
Output from each channel is specified as 85dB + 20dB headroom, or 105dB max from each channel.
For a 7.1 system with bass from all channels redirected to the subwoofer channel:
7 channels of redirected bass at 105dB = 121.86dB
1 channel of LFE at 115dB = 115dB
121.86dB + 115dB = 125.11dB
The 31.5Hz frequency response specification is given on page 3 of Dolby Atmos Specifications, Issue 3
What is with the trademark? I thought “Marty Subs” came from the DIY community?
During an R&D process that spanned several months, extensive modifications were made to the original Marty sub concept in order to make it as easy to build and as good looking as possible in the process. We simply differentiate our line of Marty subs from all other Marty subs by designating them MartySubs by GSGTM .
Folks are still free (of course) to build Marty subs as originally conceived in the DIY community and we fully support folks doing so.
What is the GSG Classified Ads site?
GSG Classified Ads is our way of bringing together everybody who has a Marty sub for sale with everybody who wants to purchase a Marty sub. Examples of things you can do on the GSG Classified Ads site:
- If you want to gauge interest for a mini-group buy in your area
- If you are moving and looking to sell some Marty subs
- If you are selling Marty sub flat packs as a side business out of your home
- If you are selling Marty sub flat packs as part of your retail business
What is an MBM?
MBM is short for "mid-bass module" though we like to think of it more as "music bass module" because GSG MBM's are tuned to cover most all of the bass that is commonly found in music, even elecronic music such as EDM and dubstep. GSG MBMs are tuned to 27Hz and provide extension into the low 20's in most rooms (as compared with the Marty Sub home theater subs which are tuned from 16-20Hz and provide extension into the teens).
Do I need an MBM?
Many folks find that traditional, high-excursion, subwoofers lack some of the "punch" in the mid to upper bass.
MBM's are designed to use pro-audio drivers which are characterized by very light moving mass (cone, voice coil, etc.). That in turns gives them very high efficiency in the mid- to upper-bass. As a result, very little power is required to achieve high SPL, which in turn minimizes compression (and other effects) and gives a perception of good "kick in the chest" bass.
As an additional benefit, the bass from an MBM may sound subjectively cleaner or more articulate than that of a high-excursion driver.
The downside of MBMs is that to achieve their very high efficiency, MBMs employ short voice coils which limits their output on the low end. For example, the PA460 pro-audio driver provides more mid-bass 'kick' than the UM18, but it would take THREE of them to provide the same output on the very low end (16-30Hz) as ONE UM18.
Some folks have constructed subwoofer systems comprised of 6-8 high efficiency drivers in low-tuned cabinets, but that ends up being a lot of enclosures! Others have found that a combination of 2 high excursion drivers (such as UM18s) in low-tuned cabs (such as Marty Subs) combined with a 2 pro-audio drivers (such as PA460s in GSG MBM-18 cabs) achieves the best of both worlds (big strong low end as well as incredible mid-bass kick).
For folks who are looking for subwoofers that will principally be used for music and occassionally for movies, MBMs by GSG can make a great choice.
Because of their extreme high efficiency, MBMs don't require a huge amp to hit hard. 500 watts is all that is required for a PA460 driver. MBM's, like all ported cabs, do need a protective high pass filter. 25Hz is recommended in the case of the GSG MBM-18 cab.
A BRIEF TECHNICAL EXAMPLE OF THE MBM
The following charts highlight the differences between a MBM loaded with a pro-audio driver (in this case a PA-460) and a Full Marty loaded with a big excursion driver (in this case a UM-18).
The first chart shows the frequency response (with no EQ) of both systems with an arbitrary 500 watts of power. What can clearly be seen is for the same amount of "nominal power", the MBM produces significantly more output in the upper bass. 70Hz is roughly the frequency where folks report "kick-in-the-chest" bass.
Frequency response at 500 watts, PA-460 (red line), UM-18 (black line):
Additionally, each of these two systems has a different electrical impedance. The electrical impedance is caused by the natural resonant frequency created by the driver and the enclosure. The peaks respresent the frequencies where the system is highly resonant, i.e. if you thump the cone, this is likely the frequency that you will hear.
Impedance, PA-460 (red line), UM-18 (black line):
The importance of the electrical impedance can be seen in how much power the amplifier is required to output at each frequency for the above example. Amplifier power requirements are inversely proportional to the electrical impedance, so the higher the impedance, the less power is required from the amplifier to produce the output at a given frequency.
Even though the PA-460 loaded MBM is producing much higher output than the UM-18 loaded Full Marty (122dB vs. 115dB at 70Hz), it is doing so by using far less power (125 watts vs. 375 watts). This results in less heating in the voice coil and less power compression in the amplifier, with the final result being a much harder hitting mid-bass.*
Amplifier Power, PA-460 (red line), UM-18 (black line):
*Instantaneous heating in the voice coil is a cause of distortion, so the output will tend to sound subjectively cleaner as well.
After all these effects are considered, the PA-460 loaded MBM is approximately 14 times more efficient at producing mid-bass than a UM-18 loaded Full Marty. That translates into a huge, real-world, difference that can be heard as well as felt.
What about UM15 (15 inch) drivers instead of the 18's, will they work in MartySubs?
The UM15 models very similar to the UM18. It simply has a little less max output given its slightly smaller size.
With lower displaced volume, the port velocity will be slightly lower than the 18, so nothing to worry about there.
We can modify the cutout for the UM15 on any of the Marty Subs without much difficulty. Send us an email with what you would like (no extra charges).
The Mini-Marty is typically the best choice if the size fits in your space because it provides a couple dB more output on the very low end and has a couple Hz of additional extension. Otherwise, the Marty Cube will still provide 4-5dB more output relative to sealed in the range from just under 20Hz up to around 30Hz and a little less increase on either side of that range.
What is a high pass filter and what does it do?
A high pass filter is simply that: it "filters" out frequencies below a certain point (called the crossover point) and allows frequencies higher than that point to "pass" without being filtered out.
An example illustrates how it works to protect your driver.
Here is the modeled frequency response of the MBM-21 with 2,000 watts of power:
This image shows that below the tuning frequency (in this case 28Hz), the driver is no longer "loaded" by the air in the cabinet. As a result, the excursion of cone increases very rapidly. When the cone motion is too large, the driver can be damaged. This driver has very clean output when excursion is kept below about 15mm (xmax) and the damage point doesn't occur until about 30mm (which is very high for a pro-audio driver).
In order to protect the driver from over-excursion a "protective" high pass filter can be applied. This is the frequency response with a very common protective high pass filter (2nd order Butterworth 12dB/Oct) applied at 25Hz.
As can be seen in the following image, below the tuning point, the cone motion is controlled, so there is no risk of over excursion.
The bottom line is all ported subwoofers behave in this fashion and so all ported subwoofers should have a protective high pass filter just below their tuning point. The result is that it will be far less likely that the driver is damaged from over excursion and because the driver excursion is controlled, the output will be much cleaner too!
There are different many different kinds of filters. The two most common types are the 2nd order a.k.a. 12dB per octave Butterworth and the 4th order a.k.a. 24dB per octave Linkwitz Riley.
The higher the "order" the steeper the slope and the greater the rolloff. 2nd order a.k.a. 12dB per octave Butterworth is all that is needed for GSG ported subs, while 4th order a.k.a. 24dB per octave Linkwitz Riley is more common in speaker crossovers and/or in the crossover from the main speakers to the subwoofers. Some of our specialized products may require other types of high pass filters, but we will always let you know when this is the case!
My system has a 60Hz hum or ground loop.
What causes it and how can I eliminate it?
A book could be written on this topic (and has!), so we’ll just address the most common culprits and identify some solutions here.
Interference on the power line from external sources
A common cause of noise is the 60Hz ballast in a lighting fixture and/or a dimmer in a lamp, light switch, or light fixture. Switch off all the lights (in the whole house). If the hum is caused by a dimmer switch, higher quality dimmers may be a solution. Some have filters to smooth the line current and reduce hum while others use a different technology altogether for dimming (auto-transformers). If the problem is not a ballast or dimmer, then it is most likely a ground loop.
Solution A: Isolation transformer on the CATV coaxial line.
60Hz is the frequency of electrical power in the USA. Any time that electrical gear has a “connection to ground” through two or more devices, the possibility exists for a ground loop. When this occurs, some residual 60Hz noise will be transferred onto the signal part of the connection, which in-turn is then amplified by the audio system.
The most common circuit for a ground loop is from a piece of grounded audio equipment to the coaxial cable television line (which is also grounded).
In order to check if the cable or satellite line is the source of the ground loop, simply disconnect the coaxial cable leading into the box. If the hum goes away, then the problem can be solved with a device called an isolation transformer. An isolation transformer is a little piece of electrical gear that allows an electrical signal to pass, but breaks the ground connection.
If the cable company cannot provide an isolation transformer, then one may need to be purchased.
While an isolation transformer can break the ground loop, it must have a sufficiently high frequency spec to all the signal to pass. For digital cable, the upper limit tends to be somewhere in the 850MHz range (analog cable is much less). For analog and digital cable systems, the Jensen Iso-Max VRD-1FF 75 Ohm Inline Digital CATV Isolator Hum Eliminator can work. The Viewsonics VSIS-EU Cable TV Ground Loop Isolator is also proven, but appears to no longer be available. Other, low-cost options may be available and may work. Just be sure to check the specs.
We are unaware of isolation transformers that works with satellite TV, which can have frequencies up to 2,300MHz or so for DISH Network and up to 40,000MHz (40GHz) for some DirectTV and Wildblue systems. In these cases, other solutions will be required.
Solution B: Isolation transformer on the line-level connection to the subwoofer amp.
If the hum only occurs through the subwoofer when the subwoofer amp is connected and/or turned on, then the ground loop is passing through it. This most commonly occurs when the amp is connected to a different receptacle than the rest of the gear.
Connecting the subwoofer amplifier to the same receptacle or power strip as the rest of the equipment frequently eliminates the hum, so try that first. If that does not eliminate the hum, then an isolation transformer can be placed in between the AVR and the subwoofer amp. This will break the ground loop between the subwoofer amplifier and the rest of the system.
Low-cost isolation transformers have difficulty passing low bass signals. If you are running a sealed system and want to preserve frequencies into the single digits, then we would recommend one of the Jensen transformers such as the PI-XX.
while the top-of-the-line Jensen will work for folks running ported cabs, it may be overkill for folks running ported cabs.
The ART Pro DualZDirect Dual Professional Passive Direct Box and the Pro ZDirect Professional Passive Direct Box include audio isolation transformers that can break ground loops as well. They begin rolling off around 10Hz, which is fine for use with all ported cabs tuned above 10Hz and are available for a much lower cost than the Jensen device. If you need two channels of isolation, get the DualZDirect.
The Behringer Direct Box rolls off the bass under 40Hz, so we don’t recommend it. This is the same for many low cost isolation transformers, so be sure to check the specs! If no specs are provided, ask the manufacturer.
How "low" will a driver go?
From time to time we get asked about drivers and cabinets. Frequently, folks see a particular driver specification (such as the driver free-air resonance (Fs), a claim by the manufacturer that the driver is good for a particular range, such as 20-150Hz, or that a driver has a particular -6dB point such as 28Hz) and are led to believe that these numbers determine how low a subwoofer can play in a particular enclosure. This is NOT THE CASE!
When a driver is placed in a cabinet, the two form a *system*. As such, not knowing anything about the cabinet that a driver will be placed in tells us nothing about how the driver is capable of performing.
For the first example, we will use a Dayton UM-18 driver, which is a long-throw, low efficiency (88dB/1w1m), subwoofer with a ton of excursion capability (usable excursion of over 30 mm). It has a free-air resonance (Fs) of 21 Hz. For the second example, we will use a LaVoce SAF184.03 driver, which is a hybrid driver that has medium-high efficiency (95 dB/1w1m) and medium-high excursion capability (usable excursion of approximately 19 mm). It has a free-air resonance (Fs) of 29 Hz.
So which driver “will play lower”, the 21 Hz driver or the 29 Hz driver?
It turns out that when these drivers are placed in a Mini-Marty subwoofer cabinet (which is approximately 7.5 cubic feet of internal volume and tuned to 18Hz), both drivers will have the same low end extension! The picture shows output at a nominal 1,100 watts of power, just for comparison. (Note that frequencies below approximately 16 Hz will be filtered out by the protective high pass.)
At 1,100 watts of power, neither driver exceeds its excursion capability. The LaVoce, however, provides quite a bit more output in the mid-bass region (60-120 Hz). When EQ’d to have the same response as the Dayton, the LaVoce will require quite a bit less power to produce the same amount of mid-bass output. This in turn gives it a subjectively harder-hitting mid-bass.
In summation, be careful when reading about driver parameters, manufacturers’ claims, or folks on the internet stating “how low a driver will go”. In order to know for sure, one has to model it up using the Thiele/Small parameters and for the most part, the enclosure itself is the dominating factor in factor in determining how low a driver will play. Of course, how much usable excursion a driver has is important when selecting an enclosure and an amplifier, as that will ultimately dictate output limits in many cases.
Why don’t you tune your subs lower?
When choosing the tuning of a ported subwoofer, there are several performance aspects that must be considered. With respect to selection of the tuning frequency, one of the most significant is the tradeoff between maximum output (high SPL) and extension (low Hz).
For most subs, as the tuning frequency on a ported sub is lowered, the efficiency and the maximum output are both reduced. This is because at frequencies just above the tuning frequency the driver is doing most of the work, not the port, and because lower frequencies are harder to produce.
These effects can be seen in a comparison between several subs using the same driver. In this case, a UM18 is used for comparison. It has roughly 1100 watts RMS power handling, and roughly 28mm excursion before distortion begins setting in.Full Marty-size cab, UM18 driver 20Hz tuning (Green line)
Full Marty-size cab, UM18 driver 16Hz tuning (Blue line)
Full Marty-size cab, UM18 driver 10Hz tuning (Red line)
Sealed cab, 4 cubic feet (Black line)
* This simulated response does not include room gain. Many rooms will have 3-6dB of room gain at around 20Hz. Typically, the smaller the room, the greater that amount of room gain. Large rooms may have little or usable room gain at 20Hz.
As can be seen in the SPL graphic, the green line (20Hz tune) actually produces the highest SPL in the 20-30Hz region, but it also starts to roll off at around 20HZ.
The blue line (16Hz tune) has a couple of dB lower output in the 20-30Hz ballpark than the green line, but with its lower tune it has more extension.
The red line (10Hz tune) has quite a bit lower efficiency than the green and the blue, but it also has the greatest amount of extension.
The next graphic shows the driver excursion for each of the subs.
Starting on the right side of the graphic at around 100 Hz, excursion for all the subs is the same and the amount of excursion is very low. The driver must only move a few millimeters in order to produce the 118dB of output shown in the previous chart at 100Hz.
Going down in frequency from there, driver excursion increases for all the subs down to about 30Hz. Starting around 30Hz, the port on the green line begins to take over producing output. As a result, output remains pretty flat, but cone excursion starts to decline. For the green line the driver excursion drops rapidly until it hits a minimum at 20HZ, which is the tuning frequency of the sub. Below the tuning frequency, cone excursion increases rapidly and could damage the driver (which is why protective high-pass filters are necessary on ported subs).
The same pattern as the green line (20Hz tune) shows up in the blue line (16Hz tune) and the red line (10Hz tune) with the excursion minimum values corresponding to their tuning frequencies. The excursion for the sealed sub is different since it does not have a port (all the output is produced by the driver only).
None of these designs exceed the peak excursion of the driver (28mm) with 1100 watts of input power.
We have analyzed the spectral content of movies (what sounds are present at what frequencies) as well as how different tunings translate into real-world perceived performance.
At Reference Level, many folks have a difficult time hearing frequencies below about 18Hz and, in many cases, folks have a difficult time even detecting their presence unless there is a way to impart the vibration to the listener, such as if the theater/home construction resonates at low frequencies or the subwoofer is placed right up against the back of the listener’s seat (nearfield subs). However, even in the latter case, there may be better ways to impart vibration effects to the audience (see Crowson transducers, for example).
Different drivers are limited in different ways in different cabs, so there is more to designing a subwoofer than this single dimension. Nonetheless, optimizing the tradeoff between output and extension is one of the more important choices in subwoofer design.
While there may be situations where having a very low tune can be beneficial, we believe that the ‘sweet spot’ for subwoofer tuning that best balances output and extension in most high performance theaters is somewhere in the 16-20Hz region. So, perhaps unsurprisingly, that is where our subs are tuned.
Is the Devastator a horn?
There have been some questions around the internet as to whether or not the Devastator is a horn. The issue relates to the fact that the horn path in front of the driver does not have a flare, i.e. it is straight. To some, this means the Devastator is a horn with no flare, to others it is a 'pipe-horn', while to others it isn't a horn at all.
In the design of any horn, various horn parameters (cross-sectional area of horn path, horn path length, horn flare rate, etc.) are adjusted. Changes in these parameters result in changes to the frequency response, the efficiency, and other measures that indicate how a horn will perform. After modelling countless combinations of parameters, it turned out that performance of the Devastator was optimized as the flare rate of the horn approached zero (i.e. no flare rate). Of course careful balancing of all the horn parameters was required to get the Devastator 'just right'. The end result is a horn with maximum efficiency from just under 60Hz to just over 100Hz, precisely where we wanted it to be for maximum mid-bass punch. The port then adds its own efficiency to the bottom end. So while the Devastator is technically a 5th order bandpass subwoofer, it looks and performs like a horn with a ported rear section, so for simplicity we call it a "ported horn". Irrespective of what it is called, it won't affect performance; we promise.
4-ohm vs. 8-ohm drivers, which is best?
Sometimes we get questions about our recommendation for the LaVoce 8-ohm driver. The story typically goes that the 4-ohm driver will “pull” twice the power from the amp and since “more power is always better”, the 4-ohm driver is clearly better, right?
As a quick primer on amp power, watts are equal to volts squared divided by resistance:
W = V^2 ÷ R
W = watts
V = volts
R = resistance (or impedance)
An example can illustrate how this works. A Behringer NX6000D will be used for the amp and the drivers will be the B&C 21DS115-4 (nominal 4 ohm) and B&C 21DS115-8 (nominal 8-ohm). The actual resistance of each driver is
B&C 21DS115-4: 2.2 ohms
B&C 21DS115-8: 5.1 ohms
Each channel on the amp is capable of 100 volts unloaded (roughly); and each channel on the amp is capable of sustaining 1200 watts of power continuous (roughly). Any more than that and the amp will “power cycle” (i.e. it will shut down so that it doesn’t destroy itself).
And each channel of the NX6000D amp is limited by either:
Max voltage: 100 volts
Max sustained power: 1,200 watts (roughly based on actual measured results)
The GSG 21” Full Marty Roundover cab will be used for the enclosure.
This is the response of the two drivers at precisely 1 watt each in the cab:
B&C 21DS115-4 = black line
B&C 21DS115-8 = red line
The two drivers have essentially the same efficiency, perhaps the 8-ohm has a hair more, but for all practical purposes, the efficiency across the whole bass region is the same.
So let’s turn up the gain and see how many watts will each driver attempt to “pull” from the amp?
Using the equation from above W = V^2 ÷ R
B&C 21DS115-4: W = 100^2 ÷ 2.2 = 10,000 ÷ 2.2 = 4,545 watts.
B&C 21DS115-8: W = 100^2 ÷ 5.1 = 10,000 ÷ 2.2 = 1,961 watts.
Assuming that the amp could survive under those conditions, this is the response that would result:
In the model, the 4 ohm driver is pulling a little more than twice the power than the 8-ohm driver, so it would be expected to have a little more than 3dB advantage across the board.
However, we need to look further into what is going on.
Recall that subwoofers typically have a large change in resistance (impedance) based on frequency). This is how it looks for these subs (the value on the Y axis is ohms). What can be seen here is that where impedance is lowest (around the tuning frequency 17 Hz or so), the amp will be producing the greatest power and where the impedance is highest (around 44 Hz), it will be producing the least power.
[Side note, guys in the car audio world who “burp” their cabs in SPL competitions will typically do so right on the impedance peak frequency in their cabs because it makes the loudest noise while putting the least demands on the amp.
The chart below shows how many watts each driver is pulling as a function of frequency.
At 20Hz, the 4-ohm driver (black line) runs up to about 2,900 watts.
At 20Hz, the 8-ohm driver (red line) runs up to about 1,300 watts.
Remember, the amp can only sustain about 1,200 watts before shutting down.
This means is that there are many frequencies where if you attempt to play a full power output into the 4-ohm driver, the amp is going to attempt to output more than its rated power. This causes rapid heating in the amplifier components and if it doesn’t shut down, it may be destroyed. Some very high end amps (such as certain SpeakerPower amps) have current detection and current limiting capability, the Behringer, unfortunately, does not.
Notice that from about 20Hz up to 100Hz, the 8-ohm driver is comfortably below the red-dashed line for almost the entire frequency range. This means that it is unlikely to cause a shutdown, even at full output.
If a limiter is placed on the 4-ohm driver so that its power does not cause a shutdown on the amp (in this case, limited to 68 volts RMS), power might look like this:
By limiting the power, the amp will be no more likely to shutdown with the 4-ohm driver than the 8-ohm driver.
However, by limiting the power, the output is also reduced:
Another way that can limit out is driver excursion. Once a driver has reached its useful limit, hitting it with more power will create only a hair more output, but distortion, compression, and driver heating will result. These drivers have a usable excursion of about 18mm. The blue dashed line is shown at a little past the 10% distortion point.
At 100 volts into the two drivers, this is the how the cone excursion looks.
As before, a limiter can be applied so the cone excursion is kept in check.
However, as before, this just results in the same output again.
So which driver wins?
Well, it depends.
While there are some situations using complex dynamic equalization where one could get a few more dB out the 4-ohm driver, for all practical purposes, the two have the same output (8-ohm run wide open or 4-ohm with a 68 volt RMS limiter, 96 volts peak).
The 8-ohm driver has the advantage that you don’t have to set any limiters on the NX6000D and the NX6000D has enough power to push the driver to its useable excursion (roughly) in the 21” Full Marty cab. The 4-ohm driver could cause amplifier stress, shutdown, and possibly damage if not properly limited.
In this case, the 8-ohm driver, the 21” Full Marty cab, and the NX6000D (or a bridged 3000D) create a very balanced system. This same phenomenon holds true for the LaVoce drivers in many of our cabs including the Devastator.
So just because you may see certain drivers only available in 8-ohms, it does not necessarily mean that you are sacrificing performance. In fact, depending on the cab (size and tuning) and amp (voltage and current capabilities), you may actually be better off choosing the 8-ohm driver.
Do you provide help with modelling subwoofers (WinISD for example)?
Owing to time restaints, we can no longer provide assistance with respect to modeling GSG subwoofers or DIY subwoofer projects or other specifications (such as enclosure volume, port size/length, non-GSG driver and amplifier recommendations, etc.).
I have a question not answered here.
How can I get some help?
We recommend the DIY section of the AVSForum.com for general purpose and setup questions. There are many MartySub builders there sharing helpful tips and helping each other with their builds.
If you have a question that you can't otherwise find an answer to, just drop us a note at firstname.lastname@example.org and we'll do our best to get back to you. As you might imagine, we get a lot of questions, so sometimes it will take a little longer than we would like to get back to you. We appreciate your patience and understanding.