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Some notes on High Sensitivity Speakers

Posted by Thorsten on December 16, 1999 at 12:29:03

Hi all,

As I have been around this a few times here some pretty general notes:

1) The Driver
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The Maximum SPL for a given Driver at a given Diameter and a given Frequency is pretty much the same, regardless of driver specifics, assuming there is no horn to reinforce the Low Frequency SPL....

As a result a 50Hz output for one Electrical Watt for an 8" Driver tends to be around 90db. For a 10" there are usually 93db possible and for a 12" 96db.... These are "ballpark" Values and depending upon box Design and Driver Design give about +/-3db....

One can increase the sensitivity of a given Driver by making the Magnet Larger/Stronger. This raises the Midrange Sensitivity, but not the sensitivity at low Frequencies. It does however reduce the Q and as a result reduces the amount of "reinforcement" of the Driver by the Box.

Another Item is the so called Baffle-Loss, which results is a 3db loss at about 250Hz for a 12" Wide Baffle, theoretically reaching -6db and then leveling out. All these issues are relevant for ANY application of Drivers in non-hornloaded (and even hornloaded) boxes.

The result is that I can take (for example) a Driver with a Qt of about 0.5 to 0.7 (say an 8" Audax Fullrange Driver) and mount it into a nice Box of about 50L Volume with a suitable bass reflex or sealed. This Driver will actually under anaechonic conditions peak up at about 100Hz (and by doing so fill in at least partially the baffle loss) and roll off quite quickely below that.

In room and with the speaker free-standing we will have a fairly evenly balanced Frequency Response (up to a few kHz at least where cone breakup and resonances between whizzer and main cone make things go scanbark) and about 93 to 94 db of genuine sensitivity down to about 45Hz.

Now let's take a Fostex FE208 with a Qt of about 0.27 and place the same Driver in a similar Box. Not only is there no peaking, the Driver already shows a rolloff by itself in the Box from the 96db in the midrange going down to about 91db at 100Hz where the SPL becomes "shelved" untill it hits about 50Hz and looses it completely. This is the low Q reducing the possible reinforcement from the box and at the same time lifting the midrange. BTW, the Driver in our example could also be a Lowther.

In the end we get a speaker which in room will play at 96 to 98 db/W/m in the midrange, will dip below 90db in the lower midrange and will recover to about 92db and carry down to about 50Hz....

It is very clear that the first example (based on a really cheap and nasty Driver) will end up sounding a lot more balanced than the second example. Of course you could equialise the second example flat, but you'd be off worse with respect to SPL when comapred to example one.

If we now use a Lowther Acousta (Medallion) style rear Horn (with about 150 to 200 liter net internal volume) something interresting happens. The cheap driver sounds horribly overblown in the bass but just not very loud. It's already notable resonance peak get's "amplified" by the horn. Now let's try the Fostex. Wow, bass, well balanced sound and high sensitivity all the way to the bank....

What has happened? Well below about 300Hz our horn kicks in and gives us about 6 to 9db accoustic gain. This neatly offsets our loss from the Low Q of the driver and the Baffle loss giving us 97 to 100db/1W/1m for 100Hz. So now we have a Speaker which above the cutoff of the Horn and up to where cone breakup make things go majloco manages 100db (or thereabouts) /1W/1m sensitive and actually has a slight DOWNWARD tilt in the response from 100hz towards 1khz (a good thing indeed)....

Note that most wideband drivers tend to have a rising response above about 1kHz (much depends on driver specifics) and that for example the FE208 will reach about 104db in the 2 - 4kHz range. This is a seperate issue to be dealt with. Some notes on that compiled by James from seperate post of myself and others are on the "single driver" webpages.

It is higly instructive to compare the kind of parameters found in Drivers traditionally used in sealed or vented Cabinets (Altec 755, Diatone 610A and similar) with those traditionally used in Horns (Fostex Sigma, Lowther, Pro Audio Drivers explicitly designed for Horns).

Ergo, Drivers with a Qt above 0.5 go into sealed or reflex Boxes, Drivers with a Qt below about 0.3 go into horns. Anything inbetween is not really happy in either....

2) Voight Pipes aka TQWT
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The TQWT or Voight Pipe is a mixture of a conical Horn, a transmission line and a Bass Reflex Box. If done well it has some postential. Generally Drivers somewhat between the Extremes (around 0.4 Qt) seem to be ideal for these. Such drivers require some reinforcement in the lower Mid/Upper bass to counteract the Baffle Loss and hence benefit from the partial and slight Hornloading provided.

In addition the TQWT is a Transmission Line which offers a nice acoustic Load for the Driver over most of it's operating Range and in addition the Bass-Reflex reinforcement can be tuned pretty low so that a pretty deep and tight bass can result.

However, the Voight Pipe (folded or not) has a big Problem. It has a tendency to suffer from severe comb filter effects resulting in a very uneven and lumpy Bass Response (also visible on the Impedance Plot). One way around this it to use a Dual TQWT (per Speaker) with two individual Drivers (one per TQWT) where one TQWT is tuned 1/2 Ocatve above the other. This way the Combs "interlock" and produce a deep and tight Bass with a pretty smooth response.

Also seen is the use of multiple Drivers placed at different positions along the length of the TQWT (Super-Whammodyne, Holli's Transpheres).

One could use two of the Triangle near Full-range Drivers Zalytron sells for dual TQWT. I know of (by now) two commercial Speaker manufacturers who use such a principle. One is the british Castle Accoustics who have added a neat twist to the equation. They fire one Driver forward and the other upward. If one reads Dick Olser on the Samadi Speakers they attempt to gain more spaciousness in the sound with upwards firing woofers and a fairly low crossed over tweeter. I'd guess this could work well with Wideband Drivers and some Lowther based Speakers operate like this (Veritas, Academy...).

The other implementation is Tommy Hoernings new line of speakers. Tommy Hoerning operates a front firing Lowther (whizzer removed) operated without Crossover and a rear firing 12" or 15" Pro Audio Driver. Each operates in a seperate TQWT which eventually end up in one reflex opening. The Big-assed Bass-Driver comes in at 200Hz and a special cone tweeter takes over above 6kHz. These Speakers sounded good enough to me to ask for review samples....

3) Horns
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When it comes to horns we know two distinct types. One type loads the front of the Driver and is generally straight, though PA Systems do employ folded variants. The other loads the rear of the Driver and is very often folded. I have seen few reasonably "unconvoluted" rear Horns and non that are domestically acceptable (look at Be Yamamura and Jim Carfrae for examples).

Now I would challange the view that the "domestic horn" (eg. Acousta) can be seen as true Horns, especially at very low frequencies. A much more complex system is at work here, making such designs very unpredicatble. Here is how I see it.

First of all we have a direct radiating Wideband Driver operating in a relatively small chamber. Behind the chamber we find another and another and another, operating a chain of relatively weak Helmholtz resonators (these can be seen in an Impedance Plot). The behaviour of all these chambers is not easily covered mathematically but they will cause resonance peaks and dips in the Frequency response. Really smart use of these chanbers could help to deal with another problem discussed furter down.

At any extent, these Chambers will also interact with the driver in a number of ways and this will be different for drivers with different Thiele Small Parameters.

These Helmholtz Resonators are combined with a structure that is tapering larger outwardly producing a certain degree of Hornloading. However, ALL domestic Horns I have seen have by far small too small mouth opening to make any reasonable attempt to reproduce frequencies below about 100Hz (often higher).

A classic explanation is that the horn is continued by the rooms walls. This is not entierly incorrect but misleading. Below the Hornmouth determined cutoff the Box no longer operates as horn. It now becomes a Transmission line which is usually loaded into a corner.... As the corner loading will produce 9db gain when compared to free space conditions and as we are very close to the corner (usually) this little trick tends to work and will result in Bass dow to the cutoff of the Line.

Below that it's "sandmaennchen".... there is not much live below the Transmissionline 1/4 Wavelength TL cutoff.... And as the Transmission line is not stuffed (it's also a horn) we have problems with the harmonics of the linefrequency causing suckouts in the Bass.... The beforementioned Helmholtz Chambers if used smartly give use some handle at eliminating these Dips....

All this leads to the problem of constructing a really well working variant of the "domestic horn". More modern Lowther Factory enclousers adress some of these issues by using additional helmholtz resonantors (the "Bicor Principle") which seem designed on trial & error as again a lot of math is needed to make sense of all the various components and their interactions.

Jon Risch should be able to give some input on both the Horn Side of things and the TL side of things and maybe even the Resonantors. I looked at the math neccesary and thought that this is definitly NO FUN. At any extent, to make something that will go significantly lower than about 70Hz the Speaker cannot be small. Or it cannot be high sensitivity.

Now some of the schemes presented here in the asylum by various people seem to rely on a most commendable experimental spirit which seems to go:

Oh, these Drivers look interresting. These Enclosures look interresting. I wonder if these would make a nice speaker together.....

The answer is - more often than not - no. If the Drivers are cheap, well so what. if they cost some real money experimenting here can be costly. And one thing is already clear. You cannot have small size, good (low) bass and high efficiency. It's the usual Triangle where two sides exclude by definition the third....

The VHSDCT Speaker
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Here is my take on a VHSDCT (Very High Sensitivity Dirt Cheap Trick) that will work quite well:

Take the Eminence Catalogue and find the Eminence Beta 12LT.

http://www.eminence-speaker.com/Pages/Speakers/Beta/BetaF.html

This Driver will operate in a 80 to 100 Liter Box tuned to 35 Hz in such a way that at least 40Hz in room are achievable. No - the tuning is not any optimal alignment, it's extended bass shelf and will rely in the speakers being fairly close to rear and sidewalls to make up some senstivity in the lower octaves. I would suggest something like 75cm X 48cm X 30cm outer size. Use 22mm Plywood and MDF and have a port tuned to 35 hz or so.

I personally would suggest to mount the driver in a Fulcrum by the magnet, reducing the sonic problems from the flimsy basket. It is probably a good idea to dampen the basket with Lead and Bitumen too. Wire this driver direct to the amplifier.

It will sound reasonably decent, but not great. Taking a leaf from our japanese friends and from Marc Wauters but also from Warfdale we tweak the driver a little more..

Paint the cone with Dammer paint. Use a lot of thinner on the first coats and apply the stuff VERY VERY thin.... Apply at least four of five VERY THIN coats to both Cone and Whizzer. It makes the cone stiffer but also makes for more natural sound (don't ask me why - just do it).

Now take some foam which is intended for air conditioner filters and adhere it to the rear side of the Wizzer cone covering most of the Whizzer (Marc suggest a different not permanent approach for Lowthers - but here we are just messing around with cheap PA Drivers, so we might as well glue the stuff down). You want a layer of foam stuck to the rear of the Whizzer. Use very little glue and make sure the glue does not "eat" the foam. The Foam damps the resonance between whizzer and cone, alleviating some of the midrange nasties usually found with Drivers employing whizzers.

Removing the Dustacp is often also a good Idea.

Now you have something that sounds half decent, if rolled off in the treble. Radioshack does a cheap and nasty "add on" Horn-Tweeter in it's own pod. It looks very high tekky, jananesey in a cheasy, no very cheasy kind of a a way.

The Tweeter is not half as bad as it looks and if you replace the internal crossover with a decent quality 1uF Cap, ditch the crappy connectors and simply seat the thing on top of the speakers you'll have a pretty decent Speaker covering 40Hz - 15khz with > 97db/W/m and an easy impedance. Use some noise from FM radio between stations and have someone else move the Tweter forward and backward with you in the listening position until the noise "locks in" (you will know what I mean when it happens).

The FFRC Cable described in TNT-Audio lends itself for bi-wiring such a speaker, so make one Set of these while you are at it. That Cat 5 solid core cable (best plenum) is also good for the internal wire of the speakers. On the www.tweakaudio.com you can find a good tip on how to bypass the Binding Posts on speakers, use it here....

Drive from an Bottlead SEX Amp nad you have a pretty surpisingly good system for pretty little money.... No, "True HiFi" it ain't, not even close. But it sure is "Ultra-Fi" if at entry level.

I'd guess I'll be taking the privilidge to NAIM that speaker and with Afterglow and all the other sexy NAIM's taken by Doc B, I have only one choice. This speaker MUST be called the "AFTERBURNER".... ZZ-Top fan's, play it at MACH 2 on Spinal Tap using this Setup....

The HiFi Versions of such Systems as the one described where made by Goodmans (Axiom Drivers) or the Electrovoice (SP Series).... Such Drivers tend to fetch handsome amounts of money in Japan, though they have occasionally been sighted on E-Bay....

I myself now use a Goodmans Axiom 201 12" Full Range and a very nice German supertweeter per side. And yes, the cone is laquered, not with Dammer but with C37 laquer. The system sounds at least as good as the Lowters it replaced in the Midrange but the bass is tons better and some of the remaining Lowther Problems (yes even with Beauhorn Virtuosos there are remaining problems) are simply no longer there.

Now this approach may not sound very kinky, but systems like the AFTERBURNER have been around since the 50's and actually stand a chance (based on physics) to work.

It's straightforward, like the SEX Amp... And it satisfies....

Nothing kinky please, we are british.

Later Thorsten

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