Tässä artkkelissa asiaa käsitellään hieman tarkemmin.
Ja ainakin tämä väite puolustaa omaa näkökantaani.
"Note that all of these devices are for noise reduction, rather than intake volumetric efficiency tuning. So if any of the volumes is likely to be causing a restriction to intake airflow (and that's especially the case with a series expansion chamber), you can delete it without too much concern."
The following techniques are used to suppress noise on intakes:
1. Expansion Chambers
By using large volume air cleaners (eg moving from 4 litre to 8 litre designs) or by connecting separate chambers in-line with the aircleaner, a major reduction in intake noise can be gained. Very high volume systems can reduce intake noise by as much as 10dB - a dramatic change. However, the downside of this approach is finding the space to mount the large volumes. The Subaru Liberty RS is an example of a car that used this technique, with an expansion chamber mounted within the guard (fender). Note that in this approach, all of the intake air passes through the volume.
2. Absorption
This approach employs either ducts or volumes lined by sound-absorbing materials. The materials can be added to the existing system, so reducing costs and not requiring the finding of any extra underbonnet space. Noise reductions of 2-5 dB are possible with this technique. Mid-Eighties Nissans used this approach, with perforated sheet steel backed by padding being placed in the upper half of the airbox.
3. Cancellation
This uses resonators tuned to a particular frequency and connected to the intake system. (Ah, so here is a description of those funny things you can see tee'd into the system!) When the intake system produces the frequency of sound that the resonator is tuned to, the air within the resonator is excited. It in turn produces a sound of the same frequency and amplitude, but with exactly the opposite phase. That is, when the intake system's waveform is high, the resonator's waveform is low - and the two cancel each other out.
Two types of resonators are used - Helmholtz and quarter wavelength. Helmholtz resonators are like bottles, with their tuned frequency being the result of their volume, neck area and neck length. Quarter wavelength resonators are characterised by having a constant diameter. Their resonant frequencies are determined by their length - for example, a quarter-wavelength resonator with a centre frequency of 67Hz (the firing frequency of a four cylinder engine at 2000 rpm) is 1250mm long.
Reductions in noise output of 5 - 10+ dB are possible, although usually only over a narrow range of frequencies.
The disadvantage of cancellation volumes is that they again take up space. Additionally, a variety of sizes may be needed if cancellation is to occur over a wide range of frequencies.
Normally, a mixture of techniques is used. For example, as shown here, a quarter-wavelength resonator (yellow), a Helmholtz resonator (green) and an expansion chamber (blue).
Täällä artikkeli kokonaisuudessaan.
http://www.autospeed.com/cms/A_1969/article.html
joten pohdiskelin että mitäpä muuta....
