One of the unheralded changes that has occurred in engines of the last decade is the use of resonant volumes on the intake. You know, those odd blind-ended boxes and tubes that you can see under the bonnet, tee’d into the intake. Sometimes they’re long and thin, other times they’re short and fat. Often they’re in full view but every now and again they’re hidden inside a guard or under a radiator cover plate. So what are these resonant volumes for?

And how do they work?

As the name suggests, they’re part of the ‘tune’ of the intake system. As more commonly understood with exhausts, the opening and closing action of the valves creates a rapid starting and stopping of gasflow in and out of the engine. Each time the inlet valves close, the columns of gas rushing in towards each cylinder are abruptly stopped. This creates a high pressure wave that gets bounced back along the intake runner. When it reaches the beginning of the runner, it’s reflected back towards the intake valves. If the intake runner is of the right length, the reflecting high pressure wave will arrive just as the intake valves are again opening – which will help jam in more air.

In most intake systems, the individual cylinder runners are connected to a common plenum chamber, and it’s the combination of the plenum and the runners which creates the tuned system. In more sophisticated cars, the system tune is able to be altered, perhaps by the intake system swapping from long to short runners, or by plenum chambers being connected together to change volumes. In some very rare cases, the intake system tune is able to be continuously altered to suit the operating conditions of the engine. (A continuously variable intake system from a BMW V8 is shown here.)

But all that’s after the throttle body. So what are these funny volumes doing earlier in the intake?

In short, they’re positioned there to reduce noise. Induction noise is a major concern of new car engineers, who have to meet drive-by noise requirements that sometimes are easier to fail because of induction noise, rather than the more commonly considered exhaust noise. This is especially the case with four-cylinder engines, where the intake noise is dominated by low frequencies, which are harder to quieten.

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).

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.