Below is the range of loudspeaker drivers Celestion has designed for use with bass guitar. This is by no means an exhaustive list of suitable drivers in the whole of their product line, and other models from within the Celestion catalogue will be researched.
There are two options equipped with ferrite magnets the new Pulse 15, and the BL300x. I will be comparing these to the specifications of the Tf1530, a driver aimed more at the sound reinforcement market but non the less usable for bass guitar. My reason for including the TF1530 is due to it being the preferred driver for 15″ enclosures at Matamp who will be using this project to form the nucleus of a new line of bass guitar speaker cabinets.
The parameters that are of most interest to this project pertain to the loudness (sensitivity), qts (an indicator of the drivers suitability for sealed and ported boxes), fs and qes (divide the former by the latter to calculate ebp. This adds a second set of data to assist in deciding whether a driver works best in sealed/ported boxes). Xmax (how far the cone and voice coil can move forward and rearward) are major considerations as is the bl (the strength of the drivers motor strength) rating and power handling. See the table of results at the bottom.
Firstly I calculate the ebp by dividing fs by qes. As you can see (working below), this identifies the driver as best for a sealed enclosure. However, with a qts of 0.751 it is at the upper end of the range for a sealed box and would also be suitable for free air and infinite baffle applications. There is a caveat in that in that this rule is not true in all cases. Given my inexperience in this field and the variety of choice in this design is will go with what is known to work.
This driver has a respectable power handling of 300 watts and a sensitivity of 96dB. As these enclosures are to be used with the Matamp GT200 which produces 200 watts of clean power and approaching 300w when fully driven, 300w may prove to be underpowered for using with a single driver enclosure.
The Pulse 15 is a new model which appears to have replaced the BL15-400X. It has a very similar frequency response to the 300X but with a stronger motor (bl) and the highest Xmax of the three drivers. An inspection of qts and ebp indicates a sealed enclosure would be optimal in terms of performance. There is also the added 100w of power handling which would make it a superior choice to the former, making the pairing of the common 4xkt88 power section with a single driver enclosure a safer enterprise.
The TF1530 is not offered as a specific bass guitar driver by Celestion. As previously mentioned this speaker is the standard 15″ offering at Matamp and has been proved to perform in both sealed and ported enclosures. Again we see a 400w power handling but a greatly reduced Xmax. Where this driver shines is its sensitivity. At 99 dB it provides twice the perceived volume of the previous two drivers and at 400w will happily take the power of the GT200 or similar. The bl rating is the highest of the three drivers. Interestingly, the information furnished by having a qts of 0.38 indicates it is on the cusp of the divide of sealed/ported suitability and an ebp of 107 placing it in the spectrum for a ported box.
That the TF1530 has been used with great success in sealed design previously makes it an even more appealing choice as while the initial designs resulting from this project will be sealed it would be prudent to add ported versions utilising the same drivers at a later date.
With the design brief of a portable, sealed enclosure in both single and dual ferrite driver variants able to handle 200-300 watts of power and a minimum of 99dB and 103dB (single and dual driver models respectively) it is plain to see that the TF1530 is the best candidate. With its ability to deliver in both sealed and vented designs already being established, and it having the highest bl (research this parameter further to see if there is any correlation between bl and sensitivity) it’s status as prime choice is marred only by its relatively low Xmax.
The additional specs for the BN models feature neodymium magnets and are for information purposes only.
My next job is to procure samples of the three drivers and calculate the optimal dimensions for test enclosures. I hope to aquire the samples for free via Matamp. Once the enclosures are fabricated testing can begin to see how they can be trialled to ascertain which yields the best sound and performance in practice.
Once the final selection is made work can begin on a bracing scheme with the box dimensions expanding to maintain the correct internal air volume.
As a final note in regards to “correct” air volume. When the drivers data is inputted into speaker design software you are not provided with a definitive set of dimensions for an enclosure. You are instead presented with a range of optimal air volumes and you use that to derive the dimensions from via the golden ratio. There is enough scope here to tailor the final dimensions to best fit the amplifier that will reside on top. Ergo, it may be wise to build the initial box in the middle of the range with a further two boxes being made equidistant between the centre point and the two extremes. This will provide a better understanding of the drivers behaviour while making up for my inexperience and allow me to tune the final prototype prior to launch. Thorough groundwork here will speed the progress of subsequent models.