N SERIES SPEAKER TINY!! HELP!

N SERIES SPEAKER TINY!! HELP!

Postby vahakn » Mon Feb 09, 2009 12:17 pm

Hello

I and building a project in which I would like to chain up an 8 channel system, using these tiny Nokia N95 speakers:

N SERIES SPEAKER PHOTO
http://www.vahakn.co.uk/x/DSC_1723.JPG


these dont seem to have a data sheet and I would like to find out their power consumption / wattage / rating so I can work out how to drive them.

I have run 1 on the headphone output of my laptop, but signal is pretty distorted.
I need to boost gain way up and i have heard the original mobile phone play pretty loud.

I would like to have 8 chains of 10 speakers.
meaning I need 8 mono amps or 4 stereo amps

they can also be seen here:

here:

I am also trying to work out what would be the best shape horn / sound box for these little drivers

please let me know if you can help

thanks!
vahakn
 
Posts: 1
Joined: Mon Feb 09, 2009 12:15 pm

Re: N SERIES SPEAKER TINY!! HELP!

Postby Klaus Stock » Sun Feb 15, 2009 12:23 pm

Hi,
vahakn wrote:I and building a project in which I would like to chain up an 8 channel system, using these tiny Nokia N95 speakers:
<snip>
these dont seem to have a data sheet and I would like to find out their power consumption / wattage / rating so I can work out how to drive them.

there's no reliable way to find out the power handling capabilities of a driver, without destructive testing I mean.

Note that there is no single power rating which is safe for a driver (asuming that we're talking abour standard dynamic drivers, not ESLs or piezos).

The usual "wattage rating" of a driver for the professional market is the long term thermal power handling capability. It is given in Wrms and means the average power of a signal. Exceeding this power rating can cause the voice coil to overheat and break ("burn"), or that heat from the voice coil melts the glue which keeps the driver together.

Drivers (and speakers) for the consumer market are sometimes labelled with a different power rating, like PMPO (peak music power output). This is a rating which the driver might survive if apllied once for a sufficiently short time - sometimes times as long as one second, often several orders of magnitude less. These rating are made up by marketing, not technical guys, and have no relation or relevance to real life.

The second aspect which is relevant power are the mechanical power handling capabilites. It is usually determined by the length of the voice coil and the magnetic gap. If the driver "moves too far", the voice coil gets outside the strong magnet field (which is located inside the gap). As the strength of the magnetic field weakens, the voice coil generated less force to move the diaphragm. The movement of the driver becomes non-linear, which means distortion. It also means that the voice coil receives less cooling per Watt consumed, so thermal power handling can be reduced by exceeding the meachincal power handling capabilites.

Another mechnical limit are the suspensions of the diaphragm and the diaphragm itself. The suspension is designed for only a certain linear exccursion. If this excursion is exceeded, the suspension may stop the driver from moving further...or it may fail to do so, causing the suspension or the diaphragm to rip apart or deform.

Now, the problem with the mechnical power handling capabilites is that there is no single parameter which can be converted to Watts. The actual excursion which a driver performs depends on the frequency of the signal; it typically gets larger as frequencies get lower. It also depends on the enclosure which the driver sits in.

Some speaker design software can calculate the excuesion for a given driver in a given enclosure at different frequencies.

Speaker design software requires the Thiele-Small parameters (or the physical parameters, which can be converted to T/S params and vice versa) of the driver to be known. These parameters can be taken from the mannufacturer's datasheet, if present. If not, they can be measured, utilizing a computer with a full-duplex capable sound card (today, most are), an inexpensive jig and a software like Speaker Workshop (free, available at http://www.audua.com/). Note that drivers need to be broken in before measurements are performed; the suspension is often too stiff when the drivers come out of the factory. It is said the drivers will settle down to their final parameters after being played at significant power for 24 hours (some even say 48 hours). The break-in period may be distrubuted of several days. Neighbours have a tendency to disapprove of speaker operation at night, especially if done "at significant power".

What means "significant power" anyway. Good point. It means that driver moves enough to excercise the suspension. As the driver may not even sit in an enclosure while it's being broken in, it may yield strong excursion at relatively low power. Care should be taken not to exceed the limits.


Anyway, you asked about how to measure or calculate the power handling capabilites of the driver. There is a way which can help you to get a rough estimate of the thermal power handling capability. You need a multimeter (to measure resistance, voltage and perhaps current) and an adjustable voltage source with enough power to drive the driver to it's (not yet known) limit. For a very small driver, an adjustable DC "wall-wart" may suffice.

First, measure the DC resistance of the driver at room temperature (20°C). You receive a reading which we'll called R20. Assuming that the voice coil is made of copper (most are, nowadays) and the the driver can safely sustain a voice coil temperature of 70°C, we calculate R70 = ((0.0039 * 50) + 1) * R20. Or R70 = R20 * 1.195. For example, a typical home speaker (with "8 Ohm" witten on the back, which is not the DC resistance!) with a DC resistance (R20) of 6 Ohm will have a R70 of 7.17 Ohm.

Now start a measurement series with increasing DC voltages accross the driver. Apply the voltage, measure it (if the DC source isn't calibrated/stabilized), wait for 10 minutes or so (assuming that this time span is suffcient to reach thermal equilibrium) and then detach the voltage source and immediately measure the DC resistance of the voice coil before it cools down again. If the measured resistance is close to the calculated R70 (or exceeds it), stop your measurements. Otherwise, start a new measurement with a slightly higher voltage.

Now, you've found some voltage limit, but you want a power limit. Easy: square the voltage (U70) and divide it by the DC resistance, and you got the thermal power handling in Watt. P = U70 * U70 / R20.

Note that you may not need to use R70 as a target, R40 may suffice as well. This has the advantage of bot getting as close to the arbitraryly chosen temperature limit of 70°C. R40 = ((0.0039 * 20) + 1) * R20, or R40 = R20 * 1.078. The actual power rating (to reach 70°C) will be 2.5 times the power required to reach 40°C.
vahakn wrote:I would like to have 8 chains of 10 speakers.
meaning I need 8 mono amps or 4 stereo amps

Apparently, you want to design a line array. A vertical line array (drivers stacked in a tall column) provides good horizontal coverage and increases efficiency by "beaming" vertically. The vertical beaming means that the center of the driver array should be at ear level.
vahakn wrote:I am also trying to work out what would be the best shape horn / sound box for these little drivers

Sometimes line speakers are built as dipoles (http://en.wikipedia.org/wiki/Dipole_speaker). These have the problem of generating large excursions at lower frequencies (and not producing any sound at low frequencies). If the baffle is 1 meter (~40 inch) wide and extends for 0.5 meters above and below the driver array (or until it reaches floor or ceiling), the low end of the usable frequency range will be 160Hz. The speaker should be operated though a crossover to prevent it from attempting to reproduce lower frequencies (which would probably exceed the mechanical limits, and contribute very little audible sound).

If you know the Thiele-Small parameters of the drivers, you can attempt to design a sealed or vented enclosure. This might yield a visually more pleasing speaker.

Best regards, Klaus
Klaus Stock
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