LF sensitivity, response and power handling

These are the critical triumvirate when it comes to producing bottom on the gig: low frequency sensitivity, response and power handling.

The shorter and simpler version:

Our cabs have the volume displacement (cone area x cone excursion) to produce huge lows for their size - each of our 12" woofers can do what it takes three run of the mill 12" woofers to achieve - and the porting, sensitivity and thermal power handling to use that displacement. If there's one thing Barefaced cabs are really really great at it's producing big fat deep loud lows, that is our raison d'être.


The longer and more complicated version:

The first thing we need to make clear is that although a bass cab does not sound like a bass cab if it doesn't do some actual BASS (i.e. ~150Hz and down), the real tone of a bass guitar happens higher up. Without sufficient support lower down then that tone can end up sounding more like a guitar than a bass guitar but without that tone higher up then it doesn't sound like anything, it just adds a rumble which lacks tonality, timing and harmony. So you need tone and you need bottom. You also need to be loud enough otherwise no-one will hear your tone or your lows - and if no-one can hear either, are they there? Effectively not!

Hoffman's will not be denied!

So if we're going to make the ultimate bass cabs (and why would we do anything else?) we need to achieve the best possible low frequency sensitivity, response curve and power handling. Low frequency sensitivity and the related response curve are completely tied together - the response curve is simply a plot of sensitivity against frequency with pistonic sensitivity referenced to a nominal value of 0dB. Hoffman's Iron Law states that any loudspeaker can only achieve two out of three factors of loud, low and small, so as size is an issue (we need to be able to transport our cab to the venue and get it through the doorway) then we have to choose the right compromise of loud and low.

Finding the best balance of compromises for the LF roll-off

When it comes to the actual response curve, over the past few years we've collected numerous customers' experiences, done much testing, much research and much reading of technical papers from the greats in loudspeaker design and related fields. This has led us to a natural response curve which we believe to be the best possible for the amplification of live sound and especially bass guitar with a full-range loudspeaker (not a subwoofer). In particular we've been very careful to consider the time domain and the behaviour in varied acoustic conditions. With matching EQ this curve does not sound as ridiculously deep as that of our previous Big Series cabs but it exhibits usefully better transient response and holds up better in difficult conditions. With some EQ it matches that ridiculous depth whilst still exhibiting slightly better transient response and maintaining much better sensitivity where it counts (where the duty cycle is most demanding).

The four factors affecting power handling low down

Low frequency power handling is a tale of four parts - the woofer's thermal limit, the woofer's excursion limit, the port tuning and the port flow rate. The thermal limit is significant because when you exceed full power continuously for sufficient time then the voice coil will overheat and the woofer die - as in complete gig-ruining expensive death. However, it's more relevant significance is its effect upon amplifier power output - driving 200W continuously into a 400W rated speaker will effectively double its impedance, thus halving the power output of your amp. This loss of power doesn't really manifest itself in a loss of loudness but in a loss of headroom, and that loss of headroom can be heard and felt as a diminishing of punch, slam, thump and bottom - basically the dynamic factors that make you sound BIG.

Excursion is the big one - but excursion costs money!

The woofer's excursion limit is the main factor that holds back bass cabs, especially smaller ones - the more air a woofer can move, the more bottom it will put out before growling and then farting. The reason most successful designs, especially from further back in time, had lots of cone area is to make up for a lack of excursion. In other words, lots of speaker area moving a little was better than not a lot of speaker area moving a little. Our advanced drivers can move a huge amount, so we don't need that compensatory cone area - one 12XN550 will move as much air as FOUR typical 10" drivers or two really good 12" drivers. Reducing the cone area means we can make the cab a lot smaller without killing our bottom - the woofers need space behind them (not depth front to back but total air space) to move efficiently at low frequencies.

Juggling the porting

Regarding the port tuning and how it affects power handling, tune too high and you end with a cab that struggles when you head down the E-string and completely dies on the low B (and below). Tune too low and curiously a similar thing happens, but this time it's the second harmonic (first overtone), rather than the first harmonic (fundamental) causing the problem - and then as you head higher (A-string and up) the fundamental starts to cause problems too. Tuning low sounds impressive on paper (and can be great at low levels) but rarely works well in the unpredictable and very dynamic environment of live music, unless you're happy to carry a much much bigger rig.

It still all comes back to tone

Returning to the low frequency response curve, the sharper eyed amongst you may have twigged that this directly affects the sound of your bass. Those of you who know enough about frequencies to know that your low E has a fundamental frequency of 41Hz and your low B a fundamental at 31Hz may panic that if the response curve has dropped too much by this point that it'll kill your tone on the low notes - a classic case of a little knowledge being a dangerous thing (and a trap I fell into back in the '90s).

The fact of the matter is:

  1. When you play an open E you do not hear just 41Hz. You hear 82, 123, 164, 205, 246, 287, 328, 369Hz, all the way up into the thousands of Hz. There's also some non-harmonic information in terms of the percussives of you striking the string and things like fret noise and rattle, all of which add character to your sound. In fact a huge part of the tone variation from instrument to instrument (not comparing bass guitars but comparing pipe organs to cellos to tubas to pianos to bass guitars etc) is in the complex combination of harmonic and percussive sound during the attack phase of the note. Remove that attack and it becomes much more difficult to identify the instrument.
  2. When you play a note on a stringed instrument (not bowed, plucked or picked or slapped), as that note sustains the higher harmonics diminish in relative amplitude. However, when you're playing notes which are low in fundamental frequency compared to the size of the instrument, the lowest harmonics also diminish in relative amplitude over time. A good way to understand this is to play the 82Hz E at the 17th fret of the B-string, the 12th fret of the E-string, the 7th fret of the A-string and the 2nd fret of the D-string. Each of those notes will have a good thump of bottom when you pluck it and in fact the notes played on the thicker (and fretted higher and thus effectively shorter) strings will be sound bassier because you're plucking closer to the middle of the string and therefore energising the lower harmonics more and because the thicker string is stiffer and thus less keen to vibrate at higher frequencies. But let the notes sustain and you'll notice that the deepest harmonic, the 82Hz fundamental stays stronger for longest on the 2nd fret D-string note. Now consider that when you play a 41Hz open E that the string is barely any longer but the note is twice as low. How long does that fundamental frequency (first harmonic) stay strong for? The answer is not very long!
  3. We listen to music through hi-fi speakers which often tail off by 60Hz and music still sounds great thought them. The most popular bass cab of all time, the Ampeg 8x10", rolls off steadily from about 100Hz and is a fair way down at 60Hz let alone 31Hz low B fundamental. But bass guitar still sounds great through it. How can these be? Well with hi-fi speakers the room tends to give a significant boost in the low frequencies, through a combination of boundary reinforcement and room gain. On the other hand with bass cabs you tend to get quite a lot of boundary reinforcement pumping up the lows and if you need any more bottom then you have EQ to play with.
  4. When you play bass, you don't just sit on the lowest note, thumping away. Even the most meat and potatoes player moves around the neck to a degree, even if it's merely playing the lowest root note that's available at any given moment.

It's our duty to consider the cycle

And thus we come back to the two key words that no-one else seems to have ever considered in this context: duty cycle. Assuming we get our cab to sound great (because that's obviously the most important thing) then our cab will play loudest if it is most sensitive where the duty cycle is most demanding and exhibits lower sensitivity where the duty cycle is easiest. This will mean that the amplifier has to deliver the least power to reach a given SPL and if it's delivering the least power then the speaker voice coils will be staying the coolest possible. If the voice coils are staying as cool as possible then the speaker impedance will be staying as close to the optimum as possible, rather than increasing substantially. If the voice coils were to get hot, then the effective impedance could of the cab could double or more, which would halve the power your amp can put out - and it's that power that allows you to feel and hear the thump and punch and zing of transients when you're playing loud.

Focus your efficiency where it's needed

To clarify further, the duty cycle looks at how the continuous average power when playing bass guitar is distributed across the bandwidth of the instrument. So by maximising the sensitivity wherever the duty cycle is most demanding we get the highest output in the real world of gigging as opposed to the overly simplistic lab situation - so comparing the old 2nd generation Big Series cabs to the new 3rd generation cabs, we have doubled the sensitivity in the core low frequencies (from 60Hz upwards).

Handle the BIG peaks

Now if the duty cycle is telling us where we need to maximise our sensitivity, we also have another factor to consider. Whilst the duty cycle tells us the continuous average power at a certain frequency, it does not tell us the peak power. The peak power handling of a loudspeaker is limited by the thermal power handling and the excursion limited power handling. The peak thermal power handling of our speakers is absolutely huge - the big voice coils take a comparatively long time to heat up because of both their thermal mass and the fast rate at which they can dissipate heat, so it's in the many thousands of watts and thus a non-issue. So the excursion limited power handling becomes the key.

We have to MOVE AIR!

The excursion limited power handling quadruples when you double the excursion limits of the driver - and our 12"s have three times the excursion of typical 12"s and twice the excursion of good 12"s - so somewhere between four and nine times the power handling. To make use of this very high excursion limited power handling (or volume displacement) you also need suitably imposing ports with plenty of radiating area (they're basically like virtual subwoofers where the diaphragm is a plug of air rather than a paper membrane) and smooth flow geometry.

And finally - crest factor.

So the final part of the puzzle is the crest factor - this is the peak to average ratio. Our duty cycle is giving us the average continuous power distribution across the frequency bandwidth and then the crest factor is stacked on top of that showing the transient peaks. And what stands out like a sore thumb is that the average power in the deep lows is not very high at all but the crest factor is particularly high down there so the peak power is high. What does this mean? We don't need high sensitivity at deep low frequencies because the continuous power will not be high enough to cause significant voice coil heating but we do need high peak power handling (which is excursion limited at low frequencies) to ensure those big low frequency transients can can slam out with attitude. And that's exactly what the new Big Series cabs have.





Website design by bmd and Barefaced

Website hosting by 24/7 Hosting

© Copyright 2015 Barefaced Ltd. All rights reserved. | copyright notice | disclaimer | privacy | sitemap