The pot depends on how you connect the speaker.
Do know that under load the sound will be different.
When you put a speaker in series with this kind of motor, you'll blow the speaker.
Put the 1K pot and the speaker in series, but in parallel with the motor.
Also try to find a capacitor, capacitors in series with the speaker, in parallel with the motor should reduce low frequencies that would damage your speaker
Put a capacitor with pot in parallel with the speaker for high frequency cutoff.
Capacitor 2 is not necessary for the circuit to function, Capacitor 1 is very necessary! Both act to protect the driver,and optimize the sound (like you said, they are the main parts used within a LPF/HPF. The problem in getting the right capacitors or LPF/HPF, is that most of these are meant for line or microphone volumes (1-2V). Not for powerful outputs like these motors (48V).
Without the right capacitors you can only connect a speaker in series with the motor. Because of the large amperes you'll burn the voicecoil in minutes (if not seconds) when connecting it in series with the motor. a series connection might work with smaller engines, and under no load. As soon as the motor demands more power than the speaker,a parallel connection is a must!
- speaker2.GIF (2.95 KiB) Viewed 949 times
M=motor
C1= 1st capacitor, HPF; make it larger for allowing lower frequencies to pass the speaker, smaller for more mid-frequencies to pass
P1= first pot, depending on the current you'll need to open or close it. In load you might want to close it more than running the motor without load.
S1 = Speaker
C2 = 2nd Capacitor, LPF. Depending on the size of the capacitor, you will be able to dial in and out the high frequencies. Usually a 47uF is a good capacitor for line signals or microphone levels, but it probably has to be more for these power applications.
P2 = 2nd pot. This one arranges the effect of high frequency cutoff. Fully opened, and only mid frequencies are heard. Fully closed and sharp frequencies are heard.
How to calculate what kind of speaker you need:
U=I*R
U=48V
R=8Ohms (change to 4ohms for 4 ohms speakers)
I=U/R=48/8= 6A
P=U*I=48*6=288W
For your setup to work without pots, you will need a 288W RMS speaker. But seeing the type of signals you send to the speaker (especially under load and at maximum velocity), you will need at least a 400W driver, if you don't want to destroy your driver. 400W drivers are enormous (starting from 12").
Let's calculate if you had a 4ohms speaker:
I=U/R=48/4=12A
P=U*I=48*12=576watt!
Let's calculate if you had a 16 ohms speaker:
I=U/R=48/16=3A
P=U*I=48*3=144wattRMS
Since the frequencies are odd and damaging the speaker you'll need to have 3/2nd of the above RMS rating, that would be 216watt speaker not to damage your speaker.
As you can see, the higher the resistance, the more maneuverable the speaker becomes.
Suppose you don't want eardeafening sound, you want some lowend to your sound, but not the sharp sound you get from the rotor, you'll have to focus on tweeterless or domeless subwoofers or low frequency drivers. These drivers usually sound ok 'till 4 or 6kHz, still sharp enough!
Regardless you will want to put a priority to budget,so even the cheapest speaker would suit you;You also want to keep the driver small, and not drive around with an 8" speaker, (I look in Amazon, sort by price):
this will limit your choice to $15 40W 5" drivers if you are looking in car door speakers,
http://www.amazon.com/Boss-BRS65-6-5-In ... 682&sr=1-1and 4" 40W speaker in the audio section.
http://www.amazon.com/40w-ShiELded-Woof ... 980&sr=1-5Both of them are 40W, 8ohms, so I'd personally prefer the 4" driver.
Suppose a second scenario,you cut the wire of one of your sattelite speakers of your home theater or desktop computer 2.1 system.
These speakers usually are ~5W 8ohms, and better for a MID frequency sound around 1kHz.
You can also get a speaker from somewhere for free, and use the calculations of the nearest driver in specs to determine the best parts for your circuit.
Now we calculate the P1 resistance needed for both cases:
case 1: 4" 40W 8Ohms speaker:
The total power the driver should get is 2/3rd of the maximum rated RMS rating, as the frequencies are not soft to the driver, but hard.
So P=40*2/3=26.666W, or for ease of calculation 24W.
P=U*I; I=P/U=24/48=2A
R=U/I=48/2=24Ohms
The speaker already is 8Ohms, the pot meter should be in series with the speaker, so it's just 24-8=16ohms.
You can either solder a 16ohms resistor there for maximum loudness, or solder a ~100ohms pot meter in between to be able to adjust the volume of the speaker somewhat (ranging from 16ohms to 2kOhms, a 1kOhms pot is recommended, see here below).
Suppose you don't want the driver to be too loud, and you just want to hear the sound dimly. At 1W the following resistor is needed:
P=1W
P=U*I; I=P/U=1/48=0.02A=20mA
R=U/I=48/0.02=2304Ohms, or 2kohmscase 2: Desktop speaker 5W 8Ohms:
The total power the driver should get is 2/3rd of the maximum rated RMS rating, as the frequencies are not soft to the driver, but hard.
So P=5*2/3=3.333W, or, because these drivers usually can handle somewhat more, and for ease of calculation, let's take 4W.
P=U*I; I=P/U=4/48=0.083A=83mA
R=U/I=48/.083=576Ohms
The speaker already is 8Ohms, the pot meter should be in series with the speaker, so the lowest resistance should be 576-8=568ohms.
You can either solder a 568 (=600)ohms ohms resistor there for maximum loudness, or solder a ~1kohms pot meter in between to be able to adjust the volume of the speaker somewhat (ranging from 568ohms to 2kOhms, a 1kOhms pot is recommended).
Now that you have established the right resistance, for the 2 drivers, now is the challenge to find the right resistors.
In the first case, your resistor must be rated higher than 16 ohms, and to operate in higher than 48V, 2A (or 96W dissipation), environment. If it is rated to handle less, use multiple in parallel. (eg: rated 100V 0.2A = 20W, you will need to solder 5 of them in parallel. It might work with less, but it'll only wear out faster over time)
In the second case the resistor needs to be higher than 568Ohms. 568Ohms does not exist, so the next step up is 600 ohms, and needs to be able to handle 48V.083A or 4W. If the resistor is rated 220V, 0,1A = (p=u*i=) 22W, you will be fine with one. If it's rated 30V 1A, you will need one with higher voltage rating. If it's rated 100V, 10mA =1W you will need 4 in parallel.
The calculations for the capacitors I don't have. It's by trial and error I'd say to figure out which one gives the best sound, and which one can stand the high voltages. make sure they are 48V compatible!
For P2 the calculations are rather simple. P2 needs to go from 0ohms to inf.
At 0ohms C2 will function fully and the sound will be dull.
at inf ohms, C2 will not function and the sound will be sharp.
I presume the 1k pot did not work in above example because of 3 reasons:
1- The pot was broken to begin with
2- The pot was not capable of handling such high amps, and broke
3- The pot was working, however the load was near to zero, and thus it looked like it wasn't working. Once you'll ride under load things might sound a lot louder!
Also note that the desktop computer speaker will not be loud enough.
In order for your bike to be heard, you will need to glue the speaker onto some plastic or chrome pipe (looking like an exhaust pipe) preferably with cone / horn structure (meaning not a flat pipe, but one that gradually becomes wider and acts to amplify the sound of the speaker
Likely some folks there that could help doing it, too.
