Speaker parameters

...what does each of them mean...

The first, and often the only parameter which is taken into consideration is input power. However, truth is, input power itself will not tell us how loud will the speaker be. Sensitivity is important, too. Its unit is dB (or dB/1W/1m, or dB/2,83V/1m). It tells us, how loud will the speaker be when powered by 1W (or 2.83V, what is 1W with 8ohm speaker, or 2W with 4ohm speaker). Common sensitivity of HiFi speakers is usually from 86 to 93dB/1W.

For a better visualisation, what do those dB really mean, let's have an example: we have speaker with 200W input power and 87dB sensitivity and another speaker with 90dB sensitivity and 100W input power. We put rated power into each speaker (200W to first and 100W to the second) and we'll find out that both speakers play at the same level of loudness.

From this we can understand that speaker with 3dB higher sensitivity is twice as loud. 6dB is quadruple difference and 10dB is 10-times more (or less). If we want to calculate the difference for another dB values, put 10(dB/10), where dB is the difference in dB.

For a lot of speakers, the sensitivity proclaimed by manufacturer is often quite doubtful, it's very unlikely that the speaker with a heavy polypropylene W diaphragm (cone) will have the 93dB sensitivity, or even more. For speakers with exotic diaphragms (weird shapes, thick plastic), the common sensitivity is 90dB and less. Higher values are typical for classic paper or paper + glass fibre conical diaphragms.

When talking about power, there are two often seen values - rated or sine power (RMS) and music power (or max power)

Sine power will tell us, what power will the speaker handle if it is powered by a sine signal (single frequency tone), for a long time. However, this value isn't exact, if the speaker has 500W rated power, it may be able to handle 600W in some specific cases, it all depends on how well is the voice coil cooled. The better the cooiling, the more the speaker will handle.

Music power is similar, only we replace the sine signal with a music signal, which is less demanding for the voice coil (doesn't produce so much heat).

Speakers with higher rated power have various cooling holes, most often is the hole in the middle of the rear pole piece, less common are the holes leading directly to the voice coil, for example in the basket under the spider, or in rear pole piece directly under the voice coil.

Often we see the PMPO power, what is just a marketing bullshit and is very misleading. It has no meaning for us

Another very important parameter is the resonance frequency. It is related to the weight of the cone and it tells us about the character of the frequency response. If we want from our speaker to play low frequencies, it should have low resonance frequency.

This were the basic parameters, here is the brief explanation of the other parameters we should run into:

fs - resonance frequency
Frequency at which there is a local maximum in the impedance response. On this frequency, the cone of a free-air speaker will have the highest excursion. Together with a Qts parameter, it tells us, what the frequency response will look like. Its unit is Hz

Le - voice coil inductance
It's an electric parameter important for a simulation. Lower inductance means lower distortion. Its unit is mH or uH (milihenry or microhenry)

Re - voice coil resistance
Also an electric parameter, it's the value of the resistance you will measure between the poles of the speaker terminal. Also crucial in simulation. Units - Ohms.

Rms - mechanical resistance
Mechanical property, it expresses the loss created in the suspension when the cone moves. The unit is kg/s or Ns/m.

Mms - moving mass
Moving system mass, including the air in the system. The unit is gramme or kilogramme. In simplicity we can tell the lower the better, the movement of the lighter cone is easier to control. Higher weight decreases the resonance frequency, but also the sensitivity.

Cms - suspension compliance
It tells us, how soft is the cone suspension. The unit is um/N, from which it is easy to understand, that it tells us, how much the cone skews when we apply the force of 1N on it. The lower the value, the harder it is to move the cone. Speakers with lower values usually doesn't need so much baffle volume

Bl - force factor
It's a product of the magnetic induction flow in the air gap and the length of the coil (not the coil winding). The unit is Tm (Tesla meter) or N/A (Newton per Ampere), thus it indicates, what force can the motor of the speaker generate, when there is 1A current throught the voice coil. Speaker with Bl=10N/A with 1A current in the voice coil can hold 1kg on the diaphragm without the cone leaving it's steady position. It is a very important parameter which informs us of the motor strength. Stronger engine will have a better control of a moving cone and thus the cone can follow the signal more precisely.

Xmax - maximum linear excursion
Determined by mechanical construction, specifically by the height of the magnetic gap (usually it's the same as the thickness of front pole piece) and height of the voice coil winding (you will not be able to measure this without disassembling). It tells us what is the maximum excursion in which the gap is still "filled" with the coil.

Xlim, Xmech - maximum mechanical excursion
Maximum possible cone excursion, if we exceed this, we will damage the former and/or the suspension.

Sd - effective piston area
An area which produces the sound, it corresponds with the size of the loudspeaker. Higher area means higher efficiency when producing low frequencies.



Qms - mechanical Q-factor
reflects losses created in mechanical part of the driver. The lower it is, the better.

Qes - electrical Q-factor
reflects losses created in electrical part of the driver. The lower it is, the better.

Qts - total Q-factor
it's the combination of both Qms and Qes. Can be used to determine a suitable enclosure.

Vas - equivalent volume
tells us what volume of air will have the same compliance as the suspension given the effective area. The higher, the bigger the enclosure usually has to be, but it doesn't tell us what specific volume to use.