Created on 24.10.2019 20:33.
Updated on 02/04/2020 20:45.
Author: Pavel Sayk.
Subwoofers are an essential component of multi-way speaker systems. These emitters not only have to confidently work out the bass range itself, but also a significant area of the midrange in two-way designs.
Woofers (or, as they are sometimes called, woofers), as a rule, have a significant diameter and a powerful magnetic system. This is because they need to pump a significant amount of air to create convincing sound pressure in the listening room and increase the immersion effect when watching movies or listening to music.
At low frequencies, the speaker cone should act like a piston.
The woofers are, for the most part, a classic design.
Meanwhile, a conventional conical paper diffuser with an outer diameter of 250 mm has a piston movement only at frequencies of no more than 300–400 Hz. Therefore, the world’s leading companies are literally sophisticated in the use of new materials for diffusers – plastic, cast cones made of durable metallized paper, graphite materials, mica-reinforced paper, etc.
Thus, the Scan-Speak 23W / 4557T00 woofer is equipped with a particularly rigid cone made of silver anodized aluminum, which ensures that the speaker operates in a piston mode in a wide frequency range.
Or the SEAS X2-08 EXOTIC W8 woofer has a paper cone with papyrus fibers to optimize rigidity and damping.
Fisher (USA) makes diffusers by deposition of nickel on a porous base. In its new generation of music centers, Pioneer has applied true piston speakers. The diffuser travel in them reaches ± 20 mm.
Acoustic systems with such speakers are called Power Bass and with a small diameter of the diffuser pleases with powerful and thick bass.
Also, the speakers may differ in the shape of the cone itself.
For example, the AAD C-10 / Q-1000 speaker has a flat cone, which makes it very similar to an engine piston.
Or the unusual Fostex FW 168 HR – Paper speaker cone is made of modern hybrid materials. Its intrinsic damping and strength provide wave propagation speeds close to those of metal diaphragms due to reduced weight and greater resilience.
Many companies pay great attention to increasing the diffuser travel – this is the only way to increase the response at low frequencies while reducing the diameter of the diffuser. Deep corrugation of the woofer cone is widely practiced.
Moreover, the lower the frequency, the larger the effective diameter of the diffuser moving the air mass. Such a diffuser also cannot be squeezed into an accordion – it is, in fact, already similar to it, so that the deformation of the diffuser during shock excitation signals turns out to be quite predictable and not dangerous.
Careful calculation on computers of the profile of the accordion excludes its damage and allows you to get a fairly uniform frequency response. Loudspeakers with such cones are produced by many companies, for example, Fostex, Morel, Peerless, ScanSpeak and many others.
One of the most difficult to solve (and to the end unresolved) problems is the suppression of the resonance of the mechanical system of the speaker. A suspended speaker cone (usually the suspension is done in two places – at the top and at the base of the cone of the cone) behaves like an oscillatory mechanical system with a fairly low damping coefficient.
You can feel this by lightly tapping your finger on the cone, you will hear (and even see if the speaker is powerful) vibrations of the cone decaying rather slowly.
The damping coefficient (and its reciprocal value – the Q-factor of the resonant system Qts) is a measure of the damping of oscillations: the lower the damping coefficient and the higher the Q-factor, the longer the speaker sounds when impulsively applied to the diffuser.
Note that the quality factor can be characterized by the number of periods of damped oscillations with a decrease in their amplitude by a factor of e (e is the base of the natural logarithm, e = 2.718 …). Even with a well-designed woofer, the Q factor often reaches several units, i.e., in response to a shock disturbance, the speaker will respond with several complete cycles of oscillations with a resonant frequency.
A poorly designed speaker will give tens of cycles of damped oscillations before the transient is over.
Subwoofers should have a strong cone with soft suspension.
This is achieved by using special materials for the bellows and centering washer. Thus, the resonant frequency is reduced to 25–50 Hz, sometimes even lower. Below the resonant frequency, the response of the speaker drops sharply and its use becomes ineffective.
The resonance of the speaker leads to two troubles.
When playing percussive sounds (for example, a shot, explosion or hitting a drum), resonant vibrations of the woofer diffuser lead to additional vibrations of the diffuser and, accordingly, to the appearance of additional sounds perceived as strong distortions (“bubbling”, “wheezing”).
In addition, when playing harmonic signals, the response of the speaker sharply increases at frequencies close to the resonance, and at lower frequencies, the drop in the frequency response of the speaker sharply increases. Near the resonant frequency, the amplitude of the oscillations of the diffuser increases sharply (by about a factor of Q) and this can create large nonlinear distortions due to the uneven magnetic field in the gap of the speaker’s magnetic system.
To reduce the effect of resonance, special materials are used for the suspension of the corrugated diaphragm and the centering washer – paper, porous rubber, compressed foam rubber, etc. This reduces the elasticity of the system and lowers the resonance frequency.
The correct choice of materials can significantly reduce the quality factor of the speaker’s mechanical system.
A slight rise in the frequency response due to resonance at the lower frequencies of the audio range can even equalize the resulting frequency response – however, this is possible only with the appropriate acoustic design of the speaker.
A typical means of suppressing resonance is the use of amplifiers with close to zero (and sometimes even negative) output impedance. A speaker with amplifier is an oscillatory electromechanical system for which the impedance (amplifier and speaker coils) acts as a damper.
This electrical damping effectively complements other (purely mechanical) damping methods. German and some other European firms have already made great strides in the development of conventional loudspeakers.
Since the post-war years, Grundig and Philips loudspeakers have been famous for their powerful yet soft and mellow sound. And now these firms produce an extensive range of speakers, although they are experiencing very serious competition from American and Japanese firms.
However, speakers of such high quality are insanely expensive …