Special Types of Subwoofers, Basics

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

URPS, Dipole and RiPol

Conventional loudspeakers and subwoofers separate the front from the rear side of the loudspeaker driver. The original idea was to prevent the rear radiating sound cancelling out the front radiating. This unwanted cancelling out of sound waves is clearly noticeable when a bass driver that is not built into a cabinet is connected to an amplifier. This effect is called acoustic short circuit.

The closed cabinet loudspeaker was born where the rear radiating sound could not leave the enclosure any more. At resonance frequency the enclosure has a output level of P, that deviates in relation to higher frequencies as follows: P = 20 dB * log10 (Qtc), where Qtc represents the Total Quality Factor in a closed cabinet.

At a later stage the bass reflex cabinet appeared where the bass reflex tube delays rear radiation so that front and rear radiation are not cancelling out one another. In fact, when this cabinet is tuned correctly both types of radiation are added.

The purpose of the transmission line system was to use the duct to eliminate rear radiation altogether. Later it was found that the phase shift, caused by the long duct, lead to the effect that low frequencies radiating from the rear were added to the ones radiation from the front (considering a correctly tuned cabinet).

URPS (Under Resonance Principle Subwoofer)

The square of the driver's resonance frequency f depends on the Product m * Fr , where:
- m represents the diaphragm mass
- Fr represents the reset force
  determined by diaphragm
  suspension and rear air mass

URPS means to operate a driver at below its resonance frequency. This is achieved in most cases by increasing the resonance frequency, i.e. fitting the driver into a smaller cabinet. The movement of the woofer's diaphragm compresses and expands the air inside the cabinet. Since the cabinet's volume is smaller, the diaphragm's excursion generates a particularly high counter force.

The smaller the cabinet the higher the increase in resonance frequency. Frequenzgang mit LSP CAD lite

The picture to the right shows an example with Alcone AC10 HE. In a closed cabinet of 26 litre the driver has a resonance frequency of 61 Hz, in a closed cabinet of 4.8 litres the resonance frequency is 125 Hz.

The smaller cabinet shows a level peak at around 150 Hz. At lower frequencies, e.g. 40 Hz, the larger cabinet is quite a bit louder with 82.5 dB (1W,1m) than the smaller one with 70.8 dB; the difference of 11.7 dB requires the amplifier's output to rise 15 times to reach the same sound level.

In order to achieve a similar flat frequency response as a bigger cabinet the URPS' response has to be equalised.

Summary: the woofer's diaphragm excursion in a URPS is restricted due to the smaller cabinet size. As a result the resonance frequency goes up and a much higher amplification is needed to achieve the same diaphragm excursion. The higher internal counter force also causes stronger diaphragm bending and, therefore, higher stress on the materials. Tests confirm a relative high distortion factor of 10% and more (for measurement results and tuning please refer to www.weidinger-online.de).

Sound: depending on the tuning, the bass of a URPS should be as deep as the one coming from a conventional subwoofer; the distortion factor (additional harmonics, also called harmonic distortion), however, is considerably higher. This is not necessarily a disadvantage, for some it's even a benefit.

Additional harmonics produce a livelier sound. But is sounds horrible if it's overdone - clearly noticeable when a URPS is cranked up too much.

Dipole

Dipol The main feature of a dipole loudspeaker is that the driver radiates in different directions with inverse phase. For example, every driver that is not built into a cabinet is basically a dipole since front and rear radiation is phase shifted by 180 degrees.

If now both radiating surfaces are separated by no more than one wavelength or the radiated wavelength is smaller than the baffle diameter D, then - with further dropping frequency - the rear sound pressure will diminish by 6 dB per octave.Dipol

Considering a baffle diameter of 1 metre (representing a wavelength of 340 Hz), the sound pressure will fall by 6 dB per octave when the frequency is declining. At 34 Hz the sound pressure is reduced by 20 dB, or put in an other way: to achieve the same sound level as at 340 Hz ten times more amplification is needed. In the picture to the right, the broken line represents the frequency response of an ideal dipole. If the woofer wasn't dipolar it would show a linear frequency response down to 20 Hz (i.e. the actual frequency response would show a steeper decline).

This decline could be compensated with a more powerful amplifier or a larger diaphragm, however, there are specific dipolar loudspeakers designs offering an even more attractive solution that are explained below:

At higher frequencies there will always be - depending on the frequency - either a cancelling out or a maximum gain, exactly then when the phase is either inverse or the same (comb filter effect).

RiPol speakers, Linkwitz H and U Profiles

If a tube is attached to the front and the rear of the driver, the impedance facing the driver changes due to the additional air resistance by the open tubes resulting in a reduced resonance frequency.Resonanzfrequenz

This way the low bass ability is further improved on. Especially H and N as well as W dipoles are taking advantage of this design in a very efficient way. In the picture to the right the blue line shows the impedance of the RiPol R30 woofer in free air, while the red line shows the impedance when the woofer is built into a RiPol cabinet.

The resonance Frequency's shift from 27 Hz to 21 Hz is particularly astonishing, as any additional counter force will increase the resonance frequency.

The cabinet increases the diaphragm's radiation resistance, the air cannot escape that quickly anymore and, therefore, may be compressed much better. Thus the increased radiation resistance reduces the resonance frequency.

Below you'll find diagrams of all profile types known to us.
 
Dipole und RiPole
All profiles have somewhat similar acoustic properties:

      N-Profil nach Ridtahler
N-Profil RiPol
N-Profil RiPol

The diagram to the right shows the frequency response of the R30 driver that is built into a N profile. The extreme low bass is clearly noticeable as well as sound pressure peaks at 200 - 300 Hz that are waning at higher frequencies.

The peaks appearing at higher frequencies are conditioned on physics; they are always found when two transducers are radiating. It is the same comb filter effect as mentioned above with regard to dipoles.

The diagram below left shows the frequency response measured approx. 1 m away from the loudspeaker, after the high frequencies have been removed with a 6 dB coil and after the peaks at 200 - 300 Hz have been softened with an absorber circuit (L in series to C).

This dipole speaker according to Ridtahler produces an extremely clean bass, showing lightness and a presentation without effort; the listener has the impression that less resonance and standing waves were developing and that the bass cannot be pinpointed - an entirely desirable characteristic.

Linkwitz H and U profiles have comparable characteristics. Linkwitz often uses the W profile with two drivers facing one another to iron out pronounced non-linearity between the two drivers partially, resulting in a reduced distortion factor.

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