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Soundproofing, Part 1

Improving the quiet

by Julian Edgar

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A noisy car is tiring to ride in and drive. So what can you do if your car is too noisy? How do you subdue the racket?

Many people fit sound deadening kits without any real understanding of what they are doing - sticking randomly selected stuff here and there. But reducing noise in a car doesn't have to be hit and miss - here's an approach that will cost you less and result in a better outcome.

This issue we look at the basics and then next issue, we start soundproofing.

Types of cabin noise

There are two types of noise that affect the inside of the car's cabin.

The first is air-borne noise. This type of noise is transmitted to the interior of the cabin by air movement. For example, a hole in the engine firewall will allow the entry of airborne noise.

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If you are stuck in traffic in a city and wind down the window, the noise level inside the cabin will increase. The change in noise level is caused by the transmission of airborne noise. Note that the window has to be down only a tiny amount for the noise level to dramatically increase - airborne noise travels through even a small opening.

Another type of airborne noise is that caused by reflections within the cabin. If there are hard surfaces in the cabin, noise will bounce around, being reflected from those hard surfaces. On the other hand, soft surfaces (e.g. a velour headlining) will absorb this noise.

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The second type of noise transmission is that transmitted through the car's metalwork – structure-borne noise. It's like when you put your ear on a railway line.  You can hear the train coming long before you can hear it through the air - the noise is being transmitted along the steel railway line.

In current cars, structure-borne noise is more common than airborne noise.

Strategies

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Airborne noise that is transmitted through holes is best addressed by closing those openings. For example, the openings for hoses and cables that pass through the firewall should be completely sealed.  Openings in the floor of the boot or hatch space (for example where there are missing grommets or rubber blanks) should be re-sealed.  All door and boot seals should be in good condition.

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Airborne noise reflections within the cabin can be reduced by placing absorbent coverings over all acoustically hard surfaces. Note that it is the surface finish that is important - you cannot just put soft materials behind hard surfaces and expect a reduction in noise reflections. Thick flock velour, deep pile carpet, cloth rather than leather seats - all will help.

Fitting an underbonnet noise insulator is another example of absorbing air-borne noise.

Structure-borne noise can be addressed through two strategies.

The first is to reduce panel vibration. To transmit sound, the metalwork must vibrate.  That vibration can be reduced by adding damping material, or by increasing panel weight - or a combination of both approaches.

Damping material reduces the propensity of the panel to ring - when tapped, it will sound a dull thump rather than a 'boing'. A panel that vibrates less will transmit less sound.

Increasing the panel mass changes the resonant frequency of the panel - the sound frequency at which the panel most 'likes' to vibrate.  To put this another way, you want the panel to be less excited by the existing noise sources.

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Both effects can be achieved by the addition of heavy, viscous sheet materials like bituminous compounds, butyl and mass-loaded vinyl. Note that this material must effectively stick to the metal panel across all its area, and must not come loose. If the attachment to the panel is not good enough, the panel will be able to vibrate without flexing the damping material.

The other approach to reducing structure-borne noise is to let the structure vibrate as it did previously, but to separate the cabin space from that panel vibration. In other words, to place an effective noise barrier between the panel and the cabin.

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A good noise barrier comprises a sandwich of mass-filled vinyl between two layers of foam 'rubber'. The heavy vinyl blocks the sound wave transmission but can still vibrate within the foam rubber layers - the barrier is 'decoupled' from the adjoining surfaces by the foam so that vibration is not transmitted.

Mistakes!

The most frequent mistake that people make is to misuse damping layers. 

Remember – these need to be very securely attached to panels that would otherwise vibrate. Therefore, using barrier layers on the inside of door trims is to waste the material – the door trim is unlikely to be transmitting sound through its vibration. 

Damping layers that are simply placed on floors or other panels will also be much less effective – the damping material must be securely attached over all its surface area. This can be achieved by using a contact adhesive (some sheets are self-adhesive) and by using a roller on the sheet so that it conforms to the contours of the panel. 

Summary

Preventing airborne noise transmission to the cabin - close any openings through which the noise could pass.

Preventing airborne noise reflections within the cabin - replace hard reflective surfaces with acoustically soft surfaces.

Preventing airborne noise reflections in the engine bay - fit an underbonnet sound insulator

Preventing structure-borne vibration - add heavy, damping material layers to those panels that vibrate.

Preventing structure-borne noise being communicated with the cabin - place a barrier between these panels and the cabin space.

Sources of noise

When people think of noise sources in cars, most consider engine noise and road noise. But aerodynamic noise can be a considerable contributor.

Drive along at 100 km/h and place the car in neutral. Does all the noise go away? No! In fact, at speed, most vehicle noise is not engine noise but instead noise caused by tyres and aerodynamic air flows.

Aerodynamic noise is not just high-pitched whistles and rustles - it can be loud, low frequency noise of the sort that most people think is coming from the tyres. But does the source matter? - after all, noise is noise. It matters because any large resonant panel, no matter what its location, should be damped and/or weighted. 

For example, the roof panel is exposed to aerodynamic flows over its outer surface and this can excite it. The same applies to the floor panel. The roof and floor should therefore have damping panels attached to them - even though it initially appears that they would not be the source of any low frequency noise.

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