After reading this article you will learn about :- 1. Introduction to Noise 2. Vehicle Noise 3. Methods of Analyzing 4. Sources 5. Isolation and Insulation 6. Boom Frequencies 7. Noise Spectrum in Vehicles 8. Brake Squeal Noise.
- Introduction to Noise
- Vehicle Noise
- Methods of Analyzing Vehicle Noise
- Sources of Vehicle Noise
- Isolation and Insulation of Vehicle Noise
- Boom Frequencies
- Noise Spectrum in Vehicles
- Brake Squeal Noise in Vehicle
1. Introduction to Noise:
Noise has been recognized in recent years as an unjustifiable interference and imposition upon human comfort, health and the quality of life. In contrast to occupational noise (associated with industrial work situations), the disturbance noise is often described as the environmental noise to which people are subjected outside their place of work.
This type of noise is very subjective in its effects. On the other hand, environmental noise is causing increasing concern and disturbance to more and more people, in sharp contrast to the industrial noise which affects a relatively limited number of people.
With increasing awareness of the nuisance and hazards of noise, more and more people are becoming less tolerant to environmental noise. A Market Research International Survey (1972) in the United Kingdom revealed, for example, that 12% of people in a sample population considered noise as the prime environmental hazard.
Within this 12%, there were 72% of people who stated that noise was getting worse, and more than 50% were seriously disturbed by noise.
According to The Association of Public Health Inspectors (UK), complaints about noise nuisance in local neighborhoods were increasing by about 10% per year, with road traffic and aircraft noise being the targets of universal criticism.
According to one reliable estimate, 29 million people in the UK (roughly 50% of the total population) were subjected to unacceptable levels of traffic noise in 1980. According to another estimate by the Department of Environment (UK), 20 million people in Britain in 1976 lived adjacent to roads with unacceptable noise levels.
The situation in India and other developing nations may not be bad as that in England and other advanced countries; but the trends are clear. With further development and increasing industrialisation, more and more people of even developing countries will be exposed to undesirable levels of noise, unless great care is taken to keep the noise within acceptable and safe limits.
2. Vehicle Noise:
In most countries, the control of road vehicle noise as it affects the external observer (i.e., a person outside the vehicle and not its occupant) is a subject of legislation. Although there is not much legislative control of the interior noise in the vehicles, such control is important too, since high noise levels are both objectionable and tiring to the majority of drivers and passengers.
From this point of view, control of interior noise in a vehicle should be an essential feature of its design. Unfortunately, the extent to which this (i.e., reduction of interior noise) is carried out depends, to a large degree, on the “commercial” considerations, i.e., how far a low interior noise level is a selling point for that particular class and design of the vehicle.
This is but natural under the present circumstances, since there are hardly any legal constraints, in most countries, on the interior noise levels in vehicles.
It is not necessarily true, however, that a programme of noise reduction from the “interior” or occupancy point of view will automatically ensure a low level of “exterior” noise.
It is quite possible, by a suitable use of sound-absorbing materials, and isolation and suppression of vibration sources, to design and produce a vehicle which is extremely quiet for its occupants, but still has excessive airborne exhaust noise, engine noise, or even road noise to the exterior listener.
In general, however, one may expect that reduction of exhaust noise to low enough levels (so that it is masked by the engine noise) will meet all legal requirements. In addition, this will also classify the vehicle concerned as “quiet” or “reasonably quiet” from the point of view of the external listener.
It is usually possible to achieve such a degree of silencing without any appreciable reduction in power. The only exception where practical difficulties may arise in this approach is the case of some sports cars with small ground clearance. In this case, accommodation of the necessary silencer volume creates problems.
3. Methods of Analyzing Vehicle Noise:
The number of methods used for noise and vibration analysis of road vehicles is quite large. They range all the way from purely “cut-and-dried” methods to the use of extensive instrumentation and test rigs, etc., to permit detailed study of noise problems. The automobile industry frequently employs special instruments to investigate specific noise sources, e.g., tyre thump meters.
Some of the test and evaluation methods used for the analysis of road vehicle noise are listed in Table 1, but this list is not exhaustive. On the other hand, every automobile manufacturer may not employ such an extensive range of testing and measurement equipment.
For example, specific sources of noise may be investigated by the manufacturers of those components and not the vehicle manufacturers, since the latter are concerned mainly with the overall noise levels. That is why the bulk of tyre noise testing is usually done by tyre manufacturers, while the final evaluation of tyre noise by the manufacturers of road vehicles is normally confined to roll or road tests.
4. Sources of Vehicle Noise:
The engine is the main source of noise in road vehicles. Engine noise consists of intake noise, exhaust noise and the noise produced in the engine itself. Other sources of road vehicle noise and vibrations, along with their cause and treatment are summarized in Table 2. Part of road vehicle noise is produced by resonance in various parts of the vehicle.
Table 3 gives a summary of resonance parameters, most likely causes of excitation and their treatment.
Some of the producers of noise in vehicles can be treated at the source by balancing, damping, silencing, etc. The effect of the remaining noise and vibrations can be reduced, on the other hand, by isolation and damping, as far as is practicable. It may be noted here that isolation is effective for transmitted noise, while damping reduces radiated noise.
Another important point to be kept in mind in this connection is that the solutions of the problem of vehicle noise may not always be straightforward, but depend very much on empirical work. This is due to the fact that a major source of noise may very likely be an unanticipated resonance.
Engine is the main source of noise in road vehicles. The effects of engine noise may be reduced by isolation. This is done by setting the engine unit on resilient mounts. Unfortunately, complete isolation by this method is impossible.
The degree of resilience necessary for complete isolation of the engine unit would result in a mounting which would be much too flexible. This problem is complicated still further by the fact that the support to which the resilient mounts are attached is itself relatively flexible and may itself be excited by road noise, for example.
The support may also be excited by the feedback of its own excitation from the un-sprung mass of the suspension. Thus it becomes necessary for the designer to work out the best possible compromise.
Since no straightforward solutions may be available, such a compromise has to be based initially on previous experience and such empirical data as are available. The design is subsequently adjusted or modified as required in the light of experimental testing and final evaluation under true road conditions.
The purpose of damping engine vibrations to a suitable degree by resilient mounting is to inhibit the transmission of engine noise to other parts of the vehicle, and also to eliminate the effects of suspension “shake” which can be feedback through the engine mounts. It is essential to maintain satisfactory isolation over a wide range of engine speeds, road speeds and operating conditions.
Such a degree of isolation involves adjustment of stiffness in all three planes and consideration of all possible modes of vibration of the engine so that resonance with the frequencies of other disturbances is avoided. It has been found that, in practice, the engine mount resonance usually lies between 5 Hz and 15 Hz.
The former frequency corresponds to the maximum flexibility which can be permitted without excessive static deflection of the engine under torque reaction.
Even when isolation of the engine has been achieved to a good extent, engine and induction noise may still be objectionably high due to bonnet-cavity resonance. This is treated by a sprayed-on sound-deadening lining to the underside of the bonnet. This treatment has become now a commonly adopted standard practice.
It may also be extended down the sides of the bonnet cavity, although such regions are generally stiffer and less likely to resonate at engine and induction noise frequencies.
Now a days, conversion “kits” are also available for reducing the noise produced by bonnet-cavity resonance. These kits comprise of sound-deadening “blankets” or linings, and are based on the same principle of reducing or eliminating bonnet-cavity resonance and acting as a barrier for radiated sound.
5. Isolation and Insulation of Vehicle Noise:
The isolation and insulation of the engine. Isolation may also be extended to various points on the suspension and transmission as shown in table 4. This Table gives the relevant details of isolation treatment for various components (other than the engine) of a road vehicle. It is difficult to estimate the effectiveness of such treatment except experience.
Moreover, the damping effects may not necessarily be cumulative. The solution of this problem depends essentially on the proper design of the vehicle concerned as well as the quality of the road surface over which the vehicle is to run. Thus the problem of isolation is an individual one.
Insulation of damping, on the other hand, is more generally applicable and more consistent in response, as in the case of bonnet-cavity resonance. In the same manner, body-cavity resonance can be treated with linings for the sides, roof and floor. Table 5 gives a summary of insulation treatment of a road vehicle.
Besides eliminating resonance, insulation treatment is especially effective in reducing road noise. In this respect, thick under sealing treatment rates high. In general, the larger the sheet metal area concerned, the greater the noise reduction likely to be achieved by insulation treatment.
This general rule applies particularly to lower-cost cars, since there is a tendency to use thinner gauge metal work for body construction of such cars. Moreover, insulation is more likely to be effective for the reduction of noise produced by resonance than local stiffening, as the latter may merely shift the point of vibration or resonance to another area in a unitary construction.
6. Boom Frequencies:
Even when a more or less complete treatment using isolation and insulation methods has been applied to a road vehicle, certain “boom” frequencies may still remain. These frequencies are usually restricted to a particular engine speed or road speed.
In the case of engine speed, the boom frequencies are excited by the engine noise or, more usually, the exhaust noise, such boom frequencies may arise only because a part of the exhaust system is resonating under those particular conditions.
In some of such cases, the cure may simply be a matter of improving the isolation of the exhaust hangers. In other more complicated cases, however, more elaborate treatment may be necessary.
On the other hand, boom frequencies generated at particular road speeds usually mean that the body cavity resonance requires further damping treatment. Such frequencies may be initiated by suspension shake, resonant flexural vibration of the transmission, tyre noise or other similar causes. Boom frequencies related to particular road speeds require both isolation and damping treatments.
7. Noise Spectrum in Vehicles:
Inside the vehicle, the sound spectrum is a characteristic “print” of the design and construction of the vehicle. It is related also to the isolation and insulation treatments involved. As a general rule, the overall noise level increases with speed almost linearly in the absence of marked resonance effects.
At higher speeds, however, a sharp increase in noise level can be anticipated unless the aerodynamic shape of the vehicle is satisfactory. The typical variation of overall noise level with speed for vehicles with poor and good insulation is shown in Fig. 1.
It should be noted that the difference between the noise levels corresponding to poor and good insulation vehicles can be as high as 5-10 dB between the corresponding curves shown in Fig. 1.
In actual practice, deviation from the almost linear curve may be expected. There may also occur one or more apparent “peaks” of reasonably low order, as shown in Fig. 2. Adequate insulation treatment will only ensure that the overall noise levels are not objectionable.
After this, there may not be much further improvement. Individual sharp peaks, on the other hand, may require a detailed investigation in order to effect a possible cure. For example, these sharp peaks may be due to suspension shake, tyre resonance, structural resonance, body cavity resonance or boom resonance.
The performance of a road vehicle is considered satisfactory in a majority of cases if no such sharp peaks are found in the noise spectrum when the vehicle is running on a normal, smooth road surface. On the other hand, if the vehicle is running over an inferior road surface, a marked increase in noise with sharp peaks at particular speeds may be expected.
Of course, the noise spectrum also varies with the speed of the vehicle and the road surface conditions. The noise spectrum therefore, is specific to a particular vehicle running at a particular speed over a particular road surface. In general, the bulk of the noise content for a road vehicle running at moderate speeds on a reasonably smooth road surface is likely to be concentrated around 50 Hz or even lower frequencies.
With modern suspensions and current sound-insulation treatment, the actual noise levels at these frequencies will rarely be objectionable. These noise levels, in fact, may pass unnoticed. In sharp contrast to this, the road noise generated by more irregular road surfaces may show up as peaks at 200-600 Hz. In this case, the change in noise will immediately become noticeable and, in general, objectionable.
8. Brake Squeal Noise in Vehicle:
Brake squeal is an annoying noise on some vehicles. The cause of excitation of this type of noise usually remains indeterminate and its effects are variable. The brake squeal may develop after some period of use. It may persist on one vehicle but remain completely absent from an identical model.
As regards the frequency spectrum of squeal, the frequencies may range from 2,000 Hz upwards and well into the ultrasonic range; but the most annoying frequencies usually lie between 10 and 15 kHz. If the vehicle has drum brakes, it is commonly believed that the most likely cause of break squeal is the accumulation of dust.
It has been observed, however, that complete cleaning of the accumulated dust will not necessarily clear the squeal. On the other hand, one brake with a given accumulation of dust may not squeal, whilst another does.
The most positive “cut-and-dried” method to cure brake squeal is to change the lining material. This cure, however, is far from infallible. In the case of disc brakes, a cure can often be achieved by using a suitable lubricant which does not result in the loss of braking efficiency. In many cases, brake squeal is self-curing in the sense that it disappears after a while and then it may or may not return.