DIY Bass Guitar Speaker

Design Requirements

Bass SpeakerIn audio, little is straightforward and compromises are rife. This design is no exception. It is an attempt to find a way through the conflicts of portability, frequency response and power handling for a bass guitar speaker. I hope the results have been worth the effort. If nothing else, I'm pleased with the aesthetics of the box.

The requirements were:-

  1. That I should be able to transport the cabinet in my modest family saloon car (Ford Focus) and be able to carry it into the gigging hall even if it involves a few steps.
  2. It should work with a 4-string bass guitar.
  3. It should be capable of competing with a moderately loud drummer.
  4. It should be reasonably economical.
Doubling up of speakers is one way to increase volume. Doubling the speakers results in a doubling of efficiency. So, if a constant voltage drive is applied to two identical speakers, the acoustic power output will be four times of that which would be provided by a single speaker. Four times the power is equivalent to about 6 dB in loudness terms. It follows that if the power to both speakers is halved, the acoustic power remains double that of a single speaker.

I recognise that these are entirely qualitative notions. One man's moderately loud drummer is another man's Ginger Baker as one man's cheap is another's bar of gold..The success (or otherwise) of the design would only become apparent when measured by my ears when deployed in a live gig of my own group - a very limited definition of success but the only measure I can give.

I have a couple of amplifiers, one is a a Fender Rumble 200 watt class D affair which is rated at 125 watts into 8 ohms (200 W into 4 ohms) and a Peavey transistor amp that gives 250 watts into 8 ohms and 700 watts into 2 ohms. It would be good if the cab could handle each of these at 8 ohms and give the possibility of doubling up on the cab to give additional acoustic power.

Speakers vary in efficiency by around 10 dB. That is ten times the output power for a given input power ( the difference between a 100W amp and a 1000W amp) However, there is much more to a speaker than its efficiency. It is instructive to look at the VOX AC30 to understand  the significance of speaker choice. These amps made an awful lot of noise, especially considering their modest 30 Watt rating. Their speakers were a significant factor in that their efficiency was so high (104dB for 1 Watt) However, they could only handle 15 watts each. Compare these to modern 12 inch speakers. 200 watt handling is commonplace but efficiency is often in the region of 98 dB/w, 6 dB below the Alnico Blue speakers of the AC30. So to produce the same acoustic output, as the AC30 on a pair of such speakers 120 watts is required. Or from a single speaker, 240 Watts does it. 
 
 

The choice of speaker is significant to the achieved final result, Whereas a  commercial maker can dictate its specification the amateur must choose "off-the-shelf" models. The range is nevertheless quite wide, as is the price. The speaker I settled on is a Celestion Pulse 15. It is rated at 400 watts and giving 96dB  for 1 watt. It is a mid range speaker costing around £100.

Now, you cannot connect a 400w speaker to a 400w bass amplifier and expect it to survive and that is because the 400w is a thermal rating which is derived from pink-noise tests and not a bass guitar signal. For bass applications, the maximum excursion and the cone area are highly significant.. i.e. the amount of air the speaker can shift. For a particular speaker, the cone area is fixed but its excursion is hugely dependent upon the cabinet. Exceeding the maximum excursion will, over time, wreck the speaker.

This table shows the power distribution of the first four harmonics for each open string, normalised for 100 watts.

String

Fundamental

(watts)

2nd Harmonic

(watts)

3rd Harmonic (watts) 4th Harmonic (watts)
E (41 hz) 26 59 4.7 9.4
A (55 hz) 59 10 14 16
D (73 hz) 67 3.6 24 5.3
G (98 hz) 25 58 12.4 3.7
The Goodmans 18P is a fabulous speaker. It cost me around £50 in 1970, I don't know how that translates to today's prices, must be £500. Its sensitivity is given as 96 dB for 0.6 watts i.e. about 98 dB for 1 watt. Its power handling is not huge at 100 watts. The pamphlet that came with the speaker recommends a maximum amplifier power of 65 watts when used with bass guitar. The acoustic output if fed at 65 watts would be almost identical to the Celestion Pulse 400 at 120 watts. Why am I bothering? Portability is the answer.
 

It would be useful if we knew what signals the bass speaker had to handle. The lowest note of a 4-string bass is 41 hz so we may feel it appropriate to design for a 41 hz sine wave at the full power rating of the amplifier. However, this low bass note is the sum of a number of harmonics and the lowest is not neccessarliy the largest. I recorded the output from my Jazz bass and displayed the frequency spectrum for the four open strings. I am surprised how the distribution varies across the strings. However, it does appear that the fundamentals contribute only a fraction of the total power for any string. This is highly significant to the design of the speaker. It is the lowest frequency that presents the greatest design difficulty and although it cannot be guaranteed that the 41 hz component of the open E-string will account for 26/100 of the total power, it can be assumed to be considerably less than the 100 watts total. For my design, I have decided to go with the assumption that the 41 hz component will be half of the total power. This leaves a decent margin compared to the measurements but ignores the effects of any tone control settings. This is not the full story of cone excursion, its peak value will be the vector addition of all of the displacements caused by all of the frequencies within the driving signal, each component modified by the speakers response to that particular frequency. The actual signal that may arise is unknowable so the only thing to do is to allow a decent margin of safety. I note that the D-sting may give particular difficulty as its fundamental displays a greater percentage of the total, compared to the other strings

The last aspect to be considered is the required frequency response of the final result. There is little guidance on the Internet and makers of commercial bass guitar equipment give only qualitative descriptions such as " Deep Bass". Now, most bass speaker chassis units have open-air resonances in the order of 35 to 65 hz, In any useable form of cabinet, a speaker cut-off frequency has be higher than its open-air resonance and usually a good deal higher, often double. I own a Goodmans 18P 18 inch speaker (from the 1960's) which has an open-air resonance of 45 hz. It is in a 120 litre sealed cabinet and it sounds pretty good to me, with no apparent shortcoming on the open E-string. On this basis,  I'm going to estimate that a 55 to 60 hz cut off is adequate.

I now have my basic requirements, a cut-off frequency of 55 hz and a power handling of 125 watts at 41 hz, this being half the power of my larger amplifier. The next issue is what type of cabinet?

Pin It