need driver for specialized digital piano loudspeaker

[ChrisA]

Here is yet another what driver? question. But I think this one is different.

I want to build a very specialized speaker system. It will be used only for playing the sounds from a digital piano. (no recorded music)

I have a hollow horizontal beam made plywood 49" long, 12" high and 6" deep. I can't change the dimensions. I want to use this as an enclosure for a bass, to midrange speaker. I need bass to 30Hz. The largest driver that will fit would be an 8" woofer. I can add ports as required.

I could put one driver in the center aimed right at the piano player's knees. and feed it with the sum of the left and right channels. Or I could divide the hollow space in the beam and use two drivers, one for left and other for right. I prefer stereo but not if the smaller volume cabinet can't get me to my 30Hz goal.

Question:

1. Can I fit a stereo pair in the beam?
2. What drivers should I be looking at? I don't need a lot of power handling

I have not yet selected an amp. I will select one that is best matched to the job after the speakers are built.

The speaker system will in addition have a pair of full range drivers above the level of the keyboard. But this will be left for later. I'm building one component at a time. For now I can use small powered studio monitors. The driver(s) in the beam are ment to supliment the smaller speakers and provide the feeling of acoustic power that the low end of a piano can produce. I think to blend well it will need to go up a couple octaves above middle C.


A Bit of background:

The low notes on a piano go down to about 27Hz but I'd be happy if the speaker could reproduce sound down to 30Hz. The upper end is not so high only to about 4K but I don't need even that right now. The current goal is to re-produce the sound that a Grand Piano would radiate downward into the floor from it's sound board. Later I'll worry about the component of the sound that is reflected from the open lid

The piano has two 1/4 inch line-level jacks. One is called "L/L+R" and the other is "R". This means that they are switched such that I can get either both channels summed or two channel stereo.

The digital piano has a stand made of wood. There are two uprights, one on each end and a cross brace (about knee high) the connects them. What I want to do is replace the cross brace with a hollow beam. It will attach the uprights using the same screw holes and will perform the same structural function as the current brace. but being hollow I can place a speaker driver inside of it. The driver would be aimed at the playr's knees. At first this sounds like a poor location until you think about a real acoustic piano. The midrange and bass will come from this beam. On top of the piano there will be some smaller speakers that handle higher frequencies up to about 12Khz and those will be aimed at the wall in back of the piano, again, this is not a hifi music system, I want to emulate the sound of a piano.

[leadbelly]

Do you know if anyone has successfully done this? My first impression is that it will never work, because the acoustic vibration from the speaker system will be picked up by the key transducers.

[PeteMcK]

you've got around 50L to play with, 2x Eminence delta pro 8a in that box tuned to 33Hz will give you an EBS alignment reaching down to 30Hz

[ChrisA]

Originally Posted by leadbelly

Do you know if anyone has successfully done this? My first impression is that it will never work, because the acoustic vibration from the speaker system will be picked up by the key transducers.

Yes this has been done. Kawai sells a digital piano with 9 speakers and 200W of power. (see below)

The transducers are very clever. What they measure is key velocity but indirectly. There are two switches that are contacted in sequence as the piano key moves downward and the sensor measures the time between switch closures to deduce velocity. Piano keys have considerable mass and inertia and this is replicated in the digital piano. So they need not measure force, time is good enough to deduce force


Here is an example of a high end digital piano. I want to do something on a much smaller scale. But the photo does show some round cutouts for drivers. I want the same thing but in the size and shape of a console piano
Kawai CP207 Digital Ensemble Pianos - Concert Performer Digital Pianos by Kawai

My plan is to build some speakers around my Yamaha p155. Here is a photo. You can see the cross brace. I figure the first step is to replace this with a hollow beam. It is held in place with just four 1/4 inch machine screws
P-155

My P155 actully has a very good interal sound engine but the the internal speakers and 6W amp is the weak link. Yamaha's intent was that you connect the line-out jacks to a better sound system with the internal sound uses just for practic volume

[el`Ol]

This tube shaped enclosure is kind of a transmission line, but one could use this fact. Take a driver on one end that has a low Qts and resonance frequency with quarter wavelength around the length of the pipe and a rather short and wide port on the other end. The suggested Eminence might do, also this if it has to be more noble:
8NMB420 - High Output MB Neodymium Driver
Play with the sheets at Quarter Wavelength Loudspeaker Design Will only get you down to 40 Hz however. A matching high sensitivity tweeter (if it has to be noble) could be this: Tweeter TG1 SUPRAVOX
I would invest the money in better drivers instead of stereo.

[ChrisA]

Originally Posted by PeteMcK

you've got around 50L to play with, 2x Eminence delta pro 8a in that box tuned to 33Hz will give you an EBS alignment reaching down to 30Hz

Thanks,

I didn't say anything about power handling. I don't need that much. I'm thinking that 96db at 1M is enough. With a speaker as sensitive as above I only need less then 5W power to get required volume level. I'll likey use a push/pull EL84/6BQ5 based amp that can supply at most about 18W per channel of power.

On a Piano the distance from the listener to the speaker is on the order of one meter. So 200W of power handling ability would go to waste. Being grossly over built does no harm