October 3, 2011

Bass horns in the works

My point source horn project has revealed the magic of horn loading. There is something spectacular about very high efficiency when it is achieved by horn loading. All else being equal, higher efficiency leads to higher fidelity. Why? From the moment more power is applied and diaphragm excursion is increased, we see the increasing effects of thermal and mechanical problems. Power compression and thermal dynamic issues come into play. Mechanical memory effects and nonlinearities increase. Everything gets worse as excursion goes up.

Horn loaded treble

The typical solution for treble is a 1" dome with about 89 dB 1w1m. At levels of about 90 dB in the listening position the sound has degraded considerably, and we are stuck with the very wide dispersion that comes from a small piston and we can't run it low enough that we might cross over where the mid also has wide dispersion - at least not without a complex crossover and many drivers. A 1" exit compression driver will have a 44mm diaphragm and 3x the piston area. We end up with around 109 dB sensitivity and greater thermal power handling due to a larger voice coil. We can also run the crossover down to 1k, at least an octave lower than the dome tweeter. The 20 dB increase in sensitivity means 1% of the power input for a given output level. The sound quality of the compression driver now becomes mostly related to two main things:

1. The way the driver behaves regarding breakup. Titanium units seem to have some issues there while drivers with polyimide and polyester seem to be the most preferred.

2. The design of the horn or waveguide.

The advantages are compelling:
  1. Virtually unlimited output in a domestic system.
  2. An effortless and very dynamic sound that allows music to be enjoyed at much greater levels when desired.
  3. The ability to match the dispersion of the midrange for a smooth polar response transition.
  4. Flexibility with crossover points - 1k or below is possible
To be fair, it is much easier to get a good result with a dome tweeter. They tend to be "easier on the ears" and some prefer the way they illuminate the room with wide dispersion. 

Horn loaded midrange

If you are starting with a compression driver at the top end, then it is easier to get high sensitivity speaker in the midrange without horn loading. You can get 100 dB sensitivity with a pair of 15" pro midwoofers. In fact, even an 8" high sensitivity mid can do very well. However, there is a certain magic in horn loaded midrange as well. In this type of horn in particular, people talk about the ability to reveal low level detail. Sounds not noticed before, buried in the mix start to emerge. I can only guess where this comes from, but I suspect that it relates to a few things. One of them might be the bandpass aspect that acoustically filters out harmonic distortion, leading to greater clarity and less masking of detail. Another could be the ability to use small low mass drivers where cone breakup is well outside the passband. The horn is doing the work so we are relying much less on the motor itself.

Horn loaded bass

This is the area that I now intend to investigate. In the bass range, cone area and excursion as well as motor issues are all very important. I have two projects in the works:

1. Midbass horn 50 - 250 Hz with my current 18" Eminence Magnum driver
2. Sub bass horn with dual opposed Rythmik 12" drivers 15 - 50 Hz.

As direct radiators, the 18" woofers can reach THX levels with some headroom, but it's not so much about output as it is arriving in style. 95 dB becomes more like 105 dB and 50 Hz extension seems possible with a corner design. A further advantage is the bandpass aspect where the horn also has an acoustic filtering function. The main problem is not taking up too much bass trap real estate.

The tapped horn subs increases the efficiency at 20 Hz by 15 dB, since the drivers start rolling off around 40 Hz without EQ. At their top end, the tapped horn adds about 6 dB. As direct radiators, the subs can't keep up with everything else. The problem is not their sound, because they sound fantastic. The amps can overheat when trying to keep up because a lot of power is needed to overcome the low efficiency. So there are two solutions - one is to simply add more of them. The other is to horn load them with a tapped horn and limit their bandwidth. I've chosen the latter - the horn loading increases the excursion limited LF output and the narrower bandwidth also means a less continuous power demand.

I should mention that there is no problem with the Rythmik subs. It's simply asking too much of a pair of 12" subs.

1 comment:

  1. Hi Paul

    I have to say that I completely agree on the subject of horn loading.

    Working in home theater industry and doing installations / calibrations and selling equipment I just *cannot* find a 'traditional' cone/dome speaker that delivers the visceral impact, dynamics and effortlessness of a compression driver / horn combination. The result of this is massively reduced engagement with the whatever you are watching, be it sport, film or HDTV.

    Certainly there is no way a traditional speaker is going to be capable of delivering 'reference' level (105dB) at normal listening distances. So for me I've reached the conclusion that high efficiency is the only way to go.

    For me the jury is still out on applications in traditional two channel audio. The compression drivers have got *a lot* better and I can only see this tech improving further and distortion being reduced more.

    Good work on the blog BTW!


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