Two WaySystem Using Bass Guitar 12 incher

 

The use of a bass guitar speaker used as a high fidelity woofer has come into mind for quite some time.  In the seventies, I designed and built a pair of Klipsch type corner horns that dwarf the original .  These behemoths unloade3d ay 34hz compared to the original which unloaded at 47hz.  They are described briefly HERE.  So, why make mention of these?  Well, the bass horns are powered by the JBL 2205, a bass guitar speaker.  Initially, I was going to use the LE15 but a JBL engineer recommended the 2205 for use on a horn.  Later, as I "got into" horn theory, I discovered that he was right, not that I doubted him in the first place because after his recommendation, I immediately purchased a pair of 2205's.

So, here we have a pair of Eminence BASSLITE S2012 units in vented cabinets of 2.6 ft^3.  In 2012, I designed and built an enclosure for a guitarist friend using a pair of these speakers.  It was then that this idea came to mind but it took a few years to come to fruition.  They sounded awesome, even outdoors, when that friend fired up his Eden amp and started plucking away.  Still, that sound was attributed to the fact that the sound was direct from the guitar with no messing around with equalizers and such as is done in a recording studio. To make matters worse, the monitors used in studios hardly resemble the guitarists speaker and at times, the guitar is plugged directly into the mixer, removing the effect of the guitarists speaker.  the guitarist may be listening to his/her guitar through a set of ear phones.  So much for the original sound so many "think" is on the recording.  That being said, the main reason for using these was an attempt to tighten the bass.  It was realized that low frequency extension would be compromised but that was acceptable since these speakers were NOT intended for classical or organ music.  They were designed with Rock-a-billy and Country Rock in mind and before I go into the boring tech details, it worked.

I also designed and built a system for that friend's guitar using the Eminence BASSLITE C2515, a 15" version.  Both have neodymium magnets that apparently, a lot of musicians don't seem to like.  The reason may be that they're used to the sound of the weaker ceramic magnets.  the stronger neodymium magnets will, theoretically, tighten (dampen) bass.  Even audio enthusiasts (aka purists) have become used to the so-called muddy bass which, I too like. It creates the illusion of lower bass.  Besides, only the players know how the original sounded and also those who have attended a live performance.  If anyone who has heard a kick drum in a night club and heard the same song at home from a recording and claimed that they sound identical either has a good speaker system or a tin ear.  A kick drum sounds tight, BOOM but usually from a home audio system it will sound like BOOOoooM.

Does all that really matter?  Only to audio fanatics or the curious.  In the end, it's all subjective.  Arguing about such things is as ludicrous as arguing about which flavour of ice cream is the best.

 

PHOTO 1

This is a photo of my living room.  The pair of guitar speaker cabinets are the smaller ones in the center, slightly canted outward due to their spacial proximity.  They are flanked by a previous design using a floor firing 15" woofer.  They are described HERE.

The so-called sweet spot is a blue armchair beyond the lower left corner of the photo and in line with the center of the opposing systems. 

Being single and living alone, there is no WAF (Wife Acceptance Factor) with which to contend; there is only the MAF (My Acceptance Factor).

By the way, the octagonal systems are designed to be disassembled as even MAF has been pushed to a limit.

Also, FYI, the octagonal cabinets are 5.3 ft^3 and the smaller ones are 2.9 ft^3.

 

 

 

 

Figure 1

Two theoretical response curves.

RED = 12" BassLIte

ORANGE = 15" BassLite

Of the systems built for that friend in 2012, the pair of 12's requires a 5 ft^3 cabinet and the 15" required 6.5 ft^3.  These curves are representative of those speakers in those volumes.

Both enclosures are vented.

 

 

 

The Boring Technical Stuff

PHOTO 2

The crossover is two-way, second order, obviously.  This crossover was originally designed for use with a system comprising a Peerless 831857, a 12 inch unit coupled with the Satori MR16P-8 with a papyrus cone. The crossover frequency is 200hz.  As luck would have it, the nominal impedance of the Peerless and the BassLite at 200hz are the same, so another crossover wasn't needed.  The resistors were used to attenuate the more efficient Satori when used with the Peertless.  They were alligator clipped into the circuit to form a fixed L-pad and are not used here.  As can be seen in the photo, I use a lot of wires with alligator clips to temporarily set up a system and I will admit, it does have its drawbacks.

 

 

 

 

 

Figure 2

The light yellow trace is with the two speakers, woofer and squawker (mid-range), wired in phase. Note the dip around the crossover point.

The blue trace is with the mid-range wired out of phase.  The red trace is the much the same as the blue one with 124g of pure wool in mid-range enclosure.  The audible effect is more pronounced than the visual here since the sweep process is 4th order filtered to remove harmonics. Note the removal of the dip.

The upper orange trace is measured at 1W1M.

The 10dB rise between 70hz and 160hz was more of a problem than noted here and is discussed in detail later.

 

 

 

 

 

The following set of traces are measured in my living room with both systems in use.  It's all fine and dandy to get a smooth response when measuring one speaker system but the days of one speaker system (mono) are gone.  Even today, a pair is used even when playing old mono recordings.  The following will show what happens when two systems are fired up in lieu of one.

Hopefully, the following will prove interesting, but not applicable to a different room.

Notes on the equalization mentioned aside figure 2 below.

The parametric EQ at 100 hz was felt to be the better choice for 3 reasons.

1. A peak at 100hz sounds boomy and quite obnoxious.

2. It's very difficult to boost a null caused by room cancellation.

3. Since the 100hz peak in the green trace is about 5dB in amplitude above the 50hz peak, the 100hz peak is more noticeable to the ear.

 

All measurements were made with the microphone in the position shown in photo 1.

 The sound is much the same just about anywhere in the room at ear level.  In the blue chair, the sweet spot it noticeably different.  The bass is thinner and the treble slightly brighter but quite acceptable.  All who come here to give a listen are standing and for good reason; there's but one place to sit facing the speakers.

The orange/red trace is left & right in phase. Note the wider bump; totally unacceptable.

The first and obvious attempt was to use the parametrics to attenuate the bump between 60hz and 120hz.  After failing in many attempts over an hour or so, that process was abandoned in favor of reversing the polarity of one system with respect to the other.  That surely killed the bump but had other effects, a severe bass loss.  The proximity of those two 12 inch systems may have something to do with the irregular bass response.

The green trace is left & right, now out of phase by reversing the wires to one amplifier channel.  The hump is gone but replaced by two smaller ones.  The smaller one is around 50hz to 65hz and the larger one centered at 100hz.

After some tweaking on a parametric equalizers along with a pre-amplifier bass boost of several dB (3 o'clock), the violet/black trace was obtained.  The bump centered at 100hz is considerably reduced.  There is another bump between about 45hz and 70hz that wasn't further tweaked because after several listening tests, it sounded ok, so well enough was left alone for now.

The end result is that the kick drums sound tighter on these BassLites than it does on the 15's next to them and that's having to spend about 30 seconds to reach the amplifier and swap the connections.  A switch box is on the drawing board to do this instantly.

Figure 3

 

 

 

 

Figure 4

The impedance curves of the 12" in air (GRN) and in the 3ft^3 vented cabinet, (RED)

From this, it can be seen that the vent resonance, left red is 30hz; the tuned frequency of about 50hz, the low point between the peaks in the red trace; the driver resonant frequency in the box, about 78hz, the taller red trace at the right.

The free air resonance, Fr is at 50hz.

 

 

 

 

Figure 5

The RED trace is the impedance of the crossover with resistive loads close to the drivers' impedances at the crossover frequency, 200hz.  The peak of the red trace is right at 200hz.

The green trace is the impedance of the crossover with reactive loads, namely the Eminence BassLite and the Satori.

The two left large peaks are those of the red traces in fig.5.  They look much larger than they are due to the left vertical scale.

The dip around 100hz is most likely due to the phase shift of the driver and it's series low pass filter in conjunction with the filter components of the Satori, a series high pass filter and a parallel low pass filter.

 

 

 

 

Impulse Response

The following 3 impulse response traces were performed using the signal generator in the CLIO.  This little fella has the ability to generate a fractional waveform, for example, half a cycle.  So, what was done was to select a frequency of 50hz, 20mS followed with Number of Cycles and the repetition time.  I use half a cycle as it seems to be similar to a plucked string.  Once set free, the string will oscillate until it stops, a damped oscillation.  That method I use has been criticized by a couple of engineers but my argument about the string held to which was added the plucking of a wine glass to make it ring. The leaded crystal will ring far longer that borosilicate glass. For a reason or reasons unbeknownst to me, the CEA* standard defaults to 6.5 cycles.  Perhaps that method by it's name is a standard method used to quantify the oscillation.  The method I use will only qualify it, i.e. use it to compare one speaker against another.

The CEA standard is described in AudioMatica's application note AN-007 in great detail.  It has been printed and will be studied in depth.

*  Consumer Electronics Association

 

Figure 6

TwoWay-BG12/Impulse-12 normal polarity-s.jpg

 

 

 

Figure 7

TwoWay-BG12/Impulse-15 normal polarity-s.jpg

 

 

 

 

 

 

Figure 8

This was actually the first test on the 12".  Nothing unusual was noted until the test on the 15" was compared and that's when the peaks were observed to be reversed.  Well, after some poking around, it was decided to run the 12" again and the fig.6 was obtained.  It was then that it donned on me that the speaker connection tat the amplifier was reversed,  black to red and red to black.  The test was run again with the leads reversed and Fig.8 was the result again.

The speaker responds to the polarity of the signal applied.  The cone motion can't be seen at musical frequencies but it can be seen if a 3hz tone is applied.  With both speakers beside each other and wired opposite polarity, the cones will move in  opposite directions.

It was noted here because it was considered to be a humourous experience.

 

 

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