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NHT Absolute Tower Design Overview

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NHT Absolute Tower Cabinet Loudspeaker Cabinet

As is the case with much of NHT’s lineup, the build quality is truly first rate.  The cabinet is made of MDF and has substantial bracing.  Due to the small enclosure size, bracing and thick MDF, the cabinet should not contribute much extraneous sound due to resonance.  This MDF is methodically finished with several layers of primer, paint and clear coat yielding a reasonably durable and beautiful piano black finish.

The top of the cabinet is slightly curved adding to the aesthetics.  The left and right sections of the baffle around the tweeter are substantially chamfered.  NHT claims this is done to randomize diffraction effects.  While this will divert reflected tweeter energy off-axis, it doesn’t look so random in the polar response as this energy is not destroyed and causes off-axis response anomalies discussed in the measurement section.

The speaker is a 3-way acoustic-suspension system with a separate internal enclosure volume for the woofers.  Separating the midrange from the woofers is essential as it prevents the woofer’s backwave from re-radiating through the midrange cone.  A separate enclosure also allows the designer to optimize the enclosure volume of the midrange.  Without this, I would suspect that the measurements would not have been nearly as good.

The cabinet base is solid and attaches with hearty machined Phillips head screws. Since the cabinets are relatively short at 37.9” and are not very top heavy, they feel relatively stable on carpet or hard surfaces.  They are not toddler proof, but should work well in the majority of homes.  The NHT Absolute Tower ships with floor spikes for carpet and cups to place the spikes in for hard floors.

NHT Absolute Tower Crossover

The loudspeaker crossover component count and quality is on par with what is found in this price classification.  There is a mixture of iron core and air core inductors.  The majority of the capacitors are electrolytic but there is a single polypropylene capacitor likely placed in series with the tweeter.  The component count is relatively low because the crossover is listed as 3rd order low pass and 2nd order high pass.  A shallow slope 2nd order crossover at 2.2kHz to a 1” aluminum dome tweeter is not something that typically works well unless the volume is kept low or the tweeter is beefy and fluid-cooled.

 Crossover

Absolute Tower Crossover


A 2nd order crossover reduces phase error, but as all things in engineering, there are tradeoffs.  Specifically, a 2nd order crossover has a shallower roll-off of low frequencies for a tweeter than higher order crossovers.  This means that a 2nd order crossover at the same crossover frequency as a higher order crossover will subject a tweeter to more output at frequencies below the crossover frequency.  If the crossover point is too low then this can significantly reduce power handling and increase distortion.  Every inductor and resistor in the crossover adds 90 degrees of delay to the signal heading for the speaker.  This means that for a signal going through two crossover components, the signal is 180 degrees out of phase compared to the original signal.  If the sound from two drivers radiates from the same plane and one goes through two crossover components and the other is directly connected then one driver will go in and the other will go out when the signal starts.  This will of course cause cancellations.  Luckily with a simple second order crossover, the driver polarity can be reversed and the phase response is corrected.  In the case of a crossover between a midrange and a tweeter, the acoustic center of the driver is not in the same range because the midrange radiates from the center of the cone which is typically an inch or more deeper than the acoustic center of the tweeter.  In an effort to create a linear phase speaker, the designer can either physically align the acoustic center of the drivers or use a ladder delay network to align the phase difference electrically.

In the case of the Absolute Tower, the crossover between the midrange and tweeter is 2nd order electrically, but close to 4th order acoustically.  The difference is due to the fact that NHT specifically engineered the drivers to reduce output below the crossover frequency for the tweeter and above the crossover frequency for the midrange.  The woofer to midrange crossover is 3rd order electrically but also close to 4th order acoustically.  The 4th order acoustic crossover allows the NHT absolute towers to play at high power and still maintain relatively low distortion.

NHT Absolute Tower Speaker Drivers

This three way acoustic-suspension design uses two 5.25” woofers in a separate air volume, a 5.25” midrange and a 1” fluid-cooled aluminum dome tweeter.  All drivers are magnetically shielded with good-sized motors, materials and workmanship.  Since CRT TV’s and VHS tapes are not popular anymore, I’m not sure if the shielding provides much of a real world benefit.

The tweeter uses a neodymium magnet and appears to be well made as evidenced by the measurements.  Most metal dome tweeters have resonances in the 1-2kHz range and then again at higher frequencies.  The measurements for this tweeter do not reveal any resonance issues within the operating range up to 20kHz!

The woofers and midrange are all 5.25” drivers with matching polypropylene cones and full magnetic shielding for the rare application requiring magnetic shielding.  The motor structure looks pretty beefy and the spider and frame are also high quality.  Using two 5.25” woofers in a pretty small sealed enclosure does not provide a great deal of low frequency extension.  The response of the woofers is tuned pretty flat and rolls off at 12dB per octave.  This speaker can be used with a subwoofer with a 12dB per octave low pass at 80Hz without a high-pass filter on the loudspeakers.  For the two channel snobs, this means that the speaker will integrate with a subwoofer without any sound degrading high pass filter.

Absolute Tower Wofer

Absolute Tower Woofer

 

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Recent Forum Posts:

Steve81 posts on August 13, 2014 12:40
konajoe, post: 1045846
But, in this particular case, the author went out of his way to report that this speaker was different in this respect.

Hi Joe,

The issue in question isn't uncommon:
Between the crossover point of 2.2kHz and 5kHz, the tweeter is transitioning across the angled chamfer, increasing the output off-axis. This seems like a good thing because the response is close to linear when sitting up to 30 degrees off axis. However, this has consequences in rooms without acoustic treatments.

This is a simple matter of a directivity mismatch, which is to say a 1“ tweeter will have a very wide dispersion pattern at the bottom of its passband (i.e. where the wavelengths are large with respect to driver diameter), while a 5.25” midrange driver's dispersion will be narrowing at the top of its passband (i.e. where the wavelengths are getting smaller with respect to driver diameter).

There are a couple common ways around this:

1. As seen in NHT's Classic 4 tower and Classic 3 bookshelf, you can use a smaller midrange driver; in the case of the NHTs, they use a 2" dome midrange down to 800Hz, which ensures a wide dispersion pattern throughout the midrange.

2. Use a waveguide or horn to constrain the tweeter's dispersion, which you can see on a lot of Harman speakers (JBL, Infinity, Revel), KEF, etc.

XTZ also looks to have an interesting solution with their Cinema series, i.e. use an array of tightly spaced tweeters down to a lower XO point of 1.2kHz, with only one of them handling the band above 3kHz.
alphaiii posts on August 13, 2014 12:19
I think the reviewer is making the point that, because of the chamfered baffle, some excess off-axis energy in the 3-5kHz range is projected into the room… In a reflective room, this excess may be perceived as brightness, or unforgiving mid-treble. As the reviewer mentions, some placement adjustments can help counter that.

I found the Absolute Towers to sound bit forward in my room, and liked the Classic Three better (which did not sound forward in my room)… but I can't say it was because of that 3-5kHz region…
AcuDefTechGuy posts on August 13, 2014 12:07
konajoe, post: 1045846
But, in this particular case, the author went out of his way to report that this speaker was different in this respect. He doesn't say “unforgiving in reflective rooms” about all speakers. He goes out of his way to explain, in depth, why this would be so. I'm not sure he would go out of his way to note this as a ‘con’ if it were true for all speakers.

As a total newb, I hate to appear that I am disagreeing with knowledgeable folks who have been very helpful.

My younger brother still has the NHT SuperZero bookshelf speakers + SW2P sub I gave him. The NHT system now resides in his large family room. This room is reflective as hell - I mean you can hear the echo when you clap your hands.

Amazingly, the NHT still manages to sound good.

I think the take-home message here is that in general, great speakers tend to sound good in most rooms. They sound best in the better rooms, but they still sound pretty good in lesser rooms. These NHT speakers are just one such example.
zieglj01 posts on August 13, 2014 11:36
konajoe, post: 1045846
But, in this particular case, the author went out of his way to report that this speaker was different in this respect. He doesn't say “unforgiving in reflective rooms” about all speakers. He goes out of his way to explain, in depth, why this would be so. I'm not sure he would go out of his way to note this as a ‘con’ if it were true for all speakers.

As a total newb, I hate to appear that I am disagreeing with knowledgeable folks who have been very helpful.

NHT does not make sharp/edgy and aggressive speakers - their speakers do measure nice.

I have owned about 85 sets of speakers, including some NHT ones - and the room/rooms, are important.
None of these speakers would sound good with hard and reflective surfaces all around. Now it is really cool,
what reducing reflections can do - with something like carpet/rugs, curtains, some soft furniture - and with
some pictures and/or other type things on walls that do help. Do not need to get real fancy - however, it is
cool and nice, to even tame the first reflections.

Your option choice and call - do what you wish - may (hope) you enjoy the adventure.
konajoe posts on August 13, 2014 06:34
But, in this particular case, the author went out of his way to report that this speaker was different in this respect. He doesn't say “unforgiving in reflective rooms” about all speakers. He goes out of his way to explain, in depth, why this would be so. I'm not sure he would go out of his way to note this as a ‘con’ if it were true for all speakers.

As a total newb, I hate to appear that I am disagreeing with knowledgeable folks who have been very helpful.
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