The Bass Driver, Basics

Dr.-Ing. Peter Strassacker
E-mail: peter@lautsprechershop.de

The cone loudspeaker driver consists of magnet and a (voice) coil. Principle speaker

The magnetic field lines are using the line of least resistance and travel, therefore, focussed through the air gab (green) between the right pole plate and the pole piece (both grey). The voice coil (red) is located in the air gap.

If current flows through the voice coil (vertical to the picture: red dots), then the magnetic field creates a force (green, vertically oriented) that travels vertically to the current and the magnetic field, i.e. to the left or to the right, horizontally in the picture. As a result the cone is moved to the left or to the right (blue arrows).

For the cone to radiate sounds evenly and efficiently to the ambient environment, this cone should have a specific size (similar to a high frequency aerial):the diameter of the cone D should have - considering symmetrical feed-in - roughly half the wavelength λ/2 of the sound wave. The wavelength λ = c / f, where c represents the speed of sound (340 m/s) and f represents the frequency in Hz. To radiate a 20 Hz tone a driver with the diameter of 1/2 * λ = 1/2 * (c/f) = 8,5 m would be required.

For 20 Hz a cone of 8.5 metre diameter is required and for 20 kHz a diameter of 8.5 mm. Consequently, several drivers, each covering a different frequency range, are mounted in loudspeaker cabinet.

Fact is: a cone diameter of 8.5 mm is feasible, of 8 metres certainly not.

Characteristics of the bass driver

For an efficient radiation of low frequencies this cone is definitely too small. This can be corrected in many ways:

The third solution works best: if a smaller magnet is used (or a dropping resistor is placed in the path to the driver), then the force of the driver will be reduced and the volume drops with the same current coming from the amplifier. At low frequencies, close to the resonance frequency, this driver will dominate, the reason why its volume hardly drops. Result: a lot less mid and high frequencies, almost the same amount of low frequencies - meaning the speaker produces more bass. The draw back is that the smaller the drive, the less control over the driver, meaning a lot less oomph.

If a smaller magnet or a dropping resistor is being used, the size of the cabinet usually has to be increased as well, to make sure that the bass resonance is not too pronounced and that the bass doesn't sound loose.

To match drivers some examples with Alcone AC 12..

Tuning by using the cabinet, resistor and capacitor

Theoretical aspects of subwoofer development: How to tune a subwoofer?
- using a closed cabinet
- with or without capacitor in series
- with or without resistor in series
- using a bass reflex cabinet
What are the advantages / disadvantages?

All hobbyists should stop here and first go to our tools pages to calculate the correct size of bass drivers using a choice of cabinets.

Extended theories about drivers

To get into the right mood, here are some formulas on subwoofer drivers (for experts only) and some practical examples about drive power.

The problem of thermal compression

The materials used in a voice coil have a strong positive temperature coefficient. This causes thermal compression producing considerably more slack bass at higher operating temperatures.

The solution:
Use a driver with a strong magnet (Qts smaller than actually necessary) and fit a resistor in series. Don't worry about the damping factor - it doesn't matter here. We'll explain later - or choose the next solution.

Absolute control through a new tuning theory

Summary of results so far:
- Standard drivers sound loose at high inputs (thermal compression)
- This can be compensated for by using a strong magnet or an external resistor
- The voice coil doesn't get hot anymore, is more durable and shows less compression effects
- The resistor has to be fitted, otherwise there is not enough low bass

There is still another tuning method for bass reflex speakers. Basis is again is the idea: the stronger the magnet the better the control over the cone and the smaller the chance of the voice coil getting hot or the bass sounding loose.

In the picture below the red curve represents the frequency response of a bass driver with a strong magnet mounted in a closed cabinet.

If this driver is mounted in a bass reflex cabinet
of the same size, then the bass reflex tube may
be tuned according to the steep, blue curve
resulting in a flatter (blue coloured) overall
frequency response.

If just the bass reflex is just tuned lower (steep,
green curve), then a kinked frequency response
curve is attained.

The low tuning pictured above causes the problem
that the sound pressure level rises with increasing
frequencies (from about 60 Hz). Therefore, this tuning
should only be used for subwoofers where the amplifier's
cut-off frequency of 65 Hz (not pictured) is generating an
overall cut-off frequency of 80 Hz (overall frequency
response: blue curve).

Using this type of subwoofer, the amplifier's cut-off
frequency should be set somewhat lower than actually desired.

If the low tuning needs to be calculated accurately,
then this can be done with commercial simulation software
like e.g. LSP-CAD (picture left).

Pictured is the AC 12 SW4, where
- the black curve represents the overall frequency response
- the blue dotted line represents the frequency response
of the bass driver (not flat due the inner feedback of the
bass reflex tube)
- the red curve represents the frequency response at the bass reflex tube
- the black dotted line represents the phase response

Advantage of this type of tuning

Above tuning leads to:
- A better controlled cone (stronger magnet)
- Lower thermal compression
  - Lower heating up of the voice coil due to strong magnet
  - Shift of operating point at higher voice coil temperatures,
turning a low tuned speaker into a regular tuned one.
    Refer picture to the right: red = low tuned, green = 33%,
blue=66%, resistance increase = regular tuning
    - as a result the sound is always clean
    - volume almost doesn't change at all
- extended life span of the voice coil due to low load

What subwoofers are available featuring this type of tuning

only Subwoofer Sub 10-60.

Special types of subwoofers (URPS, Dipole, RiPole)
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