Articles › November 2011 › Sealed vs Bass Reflex vs Tapped Horn
The debate over sealed vs bass reflex vs tapped horn often comes up with people comparing alignments or championing their favorite over another. This got us wondering... what would happen if you took one driver, put it into a representative of a couple of the major types of alignments and measured the results? How would the enclosures leverage the driver's output and how would it effect performance? With that same driver or driver compliment, what could be accomplished if you choose a bass reflex alignment verses a tapped horn? Even further along this path of thinking, what would happen if you EQ them to have the same response so that their output levels are the same? Which one would have better distortion, less compression, etc?
We sought to find answers to these questions and share the results with you, our Data-Bass readers.
The Driver - The driver used for these tests is a TC Sounds LMSR-12 which is a high power, high displacement 12” driver.
Sealed Enclosure- The sealed enclosure the driver was placed in for our tests measured slightly over 1 cu ft after driver and bracing displacements. It had a moderate amount of poly stuffing inside of it as well.
Tapped Horn (TH)- We used the Danley Sound Labs DTS-10 cabinet for our tapped horn enclosure in order to avoid building yet another cabinet for this test. The DTS-10 cabinet provides a very deep loading of the driver at about 14.5Hz and has effective output from almost 10Hz up until just past 100hz.
Bass Reflex - We used a passive radiator (PR) cabinet to represent a bass reflex design. It is about 2.3 cu ft after displacements and bracing. The passive radiator cabinet was tuned as low as possible using a pair of TC Sounds 12” VMP (variable mass passive) radiators with full weight loaded onto them. The resultant tuning was around 12.5-13Hz for the passive radiator system.
Comparison Range - A comparison range of 10-103Hz was chosen with the intent to compare the systems over as wide of a bass bandwidth as possible. 103Hz is the effective upper end limit of the tapped horn before its response abruptly crashes into a huge null, ending any hope of using it higher in frequency. 10Hz is slightly below the effective loadings of the tapped horn and the passive radiator system, and since none of the systems had appreciable output below that point, it was chosen as the cut-off. Still the comparison range covers a full decade of bandwidth.
Using the same driver, amplifier and equipment for each test ensures that what differences show up will be attributable to the enclosure's effect on the driver performance.
Previous testing has shown the sealed system to have the lowest headroom and smoothest, easiest response to match to over this entire range, so it was chosen as the base for the other two systems to have their response EQ’d to match. A Symetrix 551E and a Behringer DCX2496 were employed to EQ the response curves for the PR and TH systems. After the systems had their response curves matched as closely as feasibly possible, all of the systems were run through the same series of tests so that the results could be compared to each other.
Using the same driver, amplifier and equipment for each test ensures that what differences show up will be attributable to the enclosure's effect on the driver performance. Having the responses each EQ’d into the same basic shape ensures that each system is asked for the same output levels during each test and allows the results to be more directly compared. Basically, each system is considered and being asked to do the same job as the others. Any large differences could be a possible cause of the system sounding different from the others despite having the same basic response.
The next couple of graphs show the basic response of the PR and TH systems before and after EQ and compared to the baseline sealed system response. The responses were matched as closely as feasible but obviously there are still differences and the match isn’t perfect.
The TH was particularly troublesome. The goal was to be within 2dB at any frequency 10-100Hz, but this was not quite realized. Matching the responses below 20Hz was the most difficult due to EQing limitations and the corner for the TH and PR systems being in that range. Below 12Hz it was a question of close enough and realizing that another hour of EQ work was not worth the trouble. Note also that background noise at those very low frequencies makes the accuracy questionable at low volumes as well.
Note About These Tests
The DTS-10 cabinet is huge compared to the passive radiator and sealed systems and contains two drivers. To account for this, the DTS-10 was driven 6dB harder for all tests. All of the: sensitivity, max output, and SPL measurements have been scaled up 6dB for the sealed and passive radiator systems on the assumption of doubling the systems and power in order to keep it a driver in a cab versus driver in a cab scenario, or in this case two drivers in a cab versus two drivers in a cab.
The first thing we will look at is the response decay of each system. Bass-reflex systems usually have some delayed energy storage near tuning and tapped horn systems have shown to have some issues with large resonances in the pass band. Sealed systems are generally held to do very well here.
Sealed and Passive Radiator Decay
The PR system seems to do just as well as the sealed system here, largely in part due to the uber low tuning and also to the EQ’d response shape which lowers the low end output.
Sealed and Tapped Horn Decay
The TH system does very well over the majority of the range but at 55Hz and 100Hz there is a distinct and very noticeable ringing of energy seen.
Next, let's take a look at how well each system held the basic response shape and responded to level increases without compressing. Interestingly, at the highest sweep level used, the PR system seems to offer less output compression across the board over the sealed system. The maximum output level used was determined by the sealed system which was running out of driver excursion at the lowest frequencies. The PR system dramatically lowers excursion from the active driver for the same output level in the deep bass frequencies.
Note here that a typical vented system would likely have faired far worse than the PR system, as vents typically compress a lot at high output levels; however, the VMP’s used here did not seem to compress much at all, despite huge excursion levels.
Looking at the magnitude of the TH system's compression, it is a little bit of a mixed bag. Above 40Hz it has the least compression. From 25-35Hz it has the most and also right at its effective tuning. Actually, all of the systems fared pretty well at the output levels used and were below 3dB of compression effectively.
There are some interesting results here at the maximum drive level used which was a 113dB nominal sweep for the PR and sealed system and 119dB nominal for the TH (since it uses two drivers).
The sealed system looks fairly typical, exhibiting low distortion down to the system impedance maximum where it has a slight notch of even lower distortion. After this, the excursion rises rapidly as the frequency descends, so the distortion rises accordingly. The sharp rise is indicative that the 12” cone was being driven very hard at the lowest frequencies at this point.
The passive radiator system acted as normal for a bass reflex, exhibiting a low point in distortion at the system impedance maximum, followed by a rise tracking the cone excursion and then another low point on distortion at the system tuning where driver excursion is minimal. Below tune, the Total Harmonic Distortion (THD) skyrocketed as usual. The PR system has slightly higher distortion above 35Hz but is still well below 10% THD. However, it has significantly lower distortion from 35Hz until at least 11Hz, where it too, breaks 30%THD and climbs off the graph.
Once again, the tapped horn behaved entirely differently from the other two systems due to the complex loading on the drivers. From about 52Hz up until 100Hz, it has significantly less THD than the other two systems. At 50hz, there is spike in distortion but it still remains below 10%. However at 42Hz, there is a large precipitous spike in the THD where it tops 17%. Below 35Hz, the THD is very low and is the best behaved all of the way down to 10Hz. Essentially, the tapped horns THD is well below 10% between 12Hz and 38Hz, which is a major improvement over the other two systems.
The tapped horn maintains better overall distortion levels from 12-33Hz and 65-100Hz even when being driven 8dB harder.
In the second graph, the output level of the sealed and PR systems is dropped 8dB lower to the 105dB nominal sweep level and is compared with the TH distortion at its 119dB nominal sweep level. (Remember that the DTS-10 cab uses two drivers here which gives it a 6dB advantage.) The tapped horn maintains better overall distortion levels from 12-33Hz and 65-100Hz even when being driven 8dB harder. This is an interesting comparison because all of the systems are being asked for equivalent SPL levels and frequency response everywhere across the 10-103Hz range. You have to wonder what audible differences there would be between these systems? Is the extra-clean, deep bass of the TH obvious? What about the spike at 42Hz and the jaggedness at 50 and 63Hz?
System Sensitivity Comparison
Here the equivalent 1w/1m and 100w/10m sensitivities of each system are compared. The sealed and PR system have been raised by 3dB to account for a doubling of each system since the DTS-10 uses two drivers. Unsurprisingly the extra large tapped horn enclosure is much more sensitive. The PR system is only slightly more sensitive than the sealed system.
We present here the maximum output levels achievable by each system with each in their native response. The passive radiator and tapped horn systems both had a 12Hz HPF employed. You can see that the much larger TH system has much greater headroom overall and the PR system has a little more overall output than the sealed system. Both the passive radiator and sealed systems' SPL have been increased by 6dB to account for a doubling of systems and power (since the DTS-10 uses two drivers). In this case, it appears that the much larger TH system does allow the same driver compliment to compare favorably to sealed or reflex systems utilizing two or even three times the drivers and power in overall maximum headroom available.
This was an interesting comparison. Obviously, the tapped horn is many, many times larger and heavier than the other two systems and even the PR system is almost twice the size of the sealed system. Most would not be looking at or comparing systems of such a disparate size but that was not the point of this exercise. The question was, how does each alignment take the same basic driver parameters and fundamental performance and leverage it? Our testing shows some of the very wide possibilities of what can be done with an individual driver or two.
Tapped Horn - If you are a financially strapped individual with a decent bass driver, it might make sense for you to attempt something like a large horn because it really can force multiply a driver's abilities. However there are some tradeoffs, most notably of which seems to be limited bandwidth and issues with ringing.
Bass Reflex - It is easy to see why bass-reflex designs are so prevalent as they leverage more out of a single driver over a large bandwidth. They can be made reasonable in size and don’t have major issues, other than excessive group delay near tuning and issues with port resonances and compression.
Sealed Systems - Sealed systems are very simple, very small, and are more idiot-proof from damage. They also have no major acoustic issues related to the alignment itself to contend with, other than the fact that they do not offer the same amount of headroom and sensitivity over the intended bandwidth, driver for driver. However with enough drivers and power, the headroom can be increased as much as needed if the funds are there.
Nothing too ground breaking here but interesting results nonetheless. Note that we aren't championing any of these enclosure types. Each has strengths and weaknesses and each may be a better fit depending on the situation or scenario.