Why are Stage 1 Isuzu's so noisy ??

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Originally Posted by Offender90

All other things being equal, an exhaust of a turbo engine will be approximately 4.5 dB(A) quieter than the exhaust of its non-turbo equivalent (at the outlet).

In comparison, a well muffled exhaust will be up to 25 dB(A) quieter than its non muffled equivalent.

Problem with Land rovers (especially defenders and counties) is that floor construction has very poor noise transmission loss, so much of the muffler noise breaking out of the exhaust pipe will find its way into the cabin.

This can be minimised by:

1.) fitting the muffler as close to the engine as possible
2.) lagging the exhaust pipe (using a thicker walled exhaust pipe will do the same)
3.) increasing the mass of the floor panels.

Not necessarily in that order.

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In addition, the insulation of the seat box alone in a dfender makes a huge difference:)

JC

Re the 25Db, I find that hard to believe, Isn't sounds measured on a scale where every 3db halves or doubles the noise. (or something like that)

So a 3db reduction would infact halve the noise. So 25 Would reduce it an awful lot!:)

http://www.glenbrook.k12.il.us/GBSSC...nd/u11l2b.html

Reading that link, for exampe. 120 Db is 10x louder that 100

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Originally Posted by Psimpson7

Re the 25Db, I find that hard to believe, Isn't sounds measured on a scale where every 3db halves or doubles the noise. (or something like that)

So a 3db reduction would infact halve the noise. So 25 Would reduce it an awful lot!:)

Intensity and the Decibel Scale

Reading that link, for exampe. 120 Db is 10x louder that 100

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Actually the response of the ear to sound is also logarithmic (which is why the logarithmic decibel scale is used for comparing sounds). In fact, although as you state 3db is a halving of the sound pressure, it is a change that is only barely perceptible to most people. So 3db is barely perceptible, but 25db is a quite significant reduction.

John

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Originally Posted by Psimpson7

Re the 25Db, I find that hard to believe, Isn't sounds measured on a scale where every 3db halves or doubles the noise. (or something like that)

So a 3db reduction would infact halve the noise. So 25 Would reduce it an awful lot!:)

Intensity and the Decibel Scale

Reading that link, for exampe. 120 Db is 10x louder that 100

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Hi Pete,

You're kind of right, but not quite.

3 dB is halving of sound energy, however, in terms of human hearing, it's only noticeable. A change of 10dB is perceptively equivalent to halving (or doubling) of noise...

... 120 dB is actually 100x the sound energy of 100 dB, but would preceptively increase the noise by 4 times.

Quick background on sound quantification:

1.) Sound is a pressure oscillation about a mean pressure (atmospheric pressure).

2.) It's measured in a logarithmic (dB or decibel) scale bacause of the ear's ability to pick up a huge range of pressure oscillation. Sound pressure level Lp( in dB ) = 20*log(P/Pref) where P is the pressure oscillation and Pref is a reference sound pressure (2 x 10^-5 Pa). Pref is the meadian hearing threshold, and was chosen so that the median human hearing threshold is 0 dB(A).

The threshold of very good human hearing is -10dB(A) which in pressure corresponds to 0.000002 Pascals (2 x 10^-6 Pa). The upper range of human hearing on the other hand - say the allowable exposure to impulse noise - is 140 dB(A), which is 200 Pascals (or 2x10^2) which is 100,000,000 (10^8) times more than the minimum.

Its not very intuitive WRT human hearing - to make matters even worse, human hearing has different sensitivities at different frequencies, so 60 dB at 50Hz sounds completely different to 60dB at 1kHz. That's why the A-weighted scale was introduced, as well as other (more obscure) scales (i.e. sons and phons)...

...getting back to the matter at hand, 25 dB(A) reduction would perceptively sound like ~1/6 of the original noise, and yes, it is quite possible with a good muffler system.

Cheers

Bojan

Thanks guys. I will read that again in the morning when I may take it in!:)

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Originally Posted by justinc

This is NOT an engine I would like to use for a conversion.

JC

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and i've got 2 of them:(:)

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Originally Posted by Col.Coleman

For me, my '82 Stage 1,...
There is no acoustic rocker cover,
CC

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"The first forty 3.9D LRs do not have the 'quiet kit' fitted to subsequent production machines. Boy, if you end up owning one of the first forty you'll need to invest in some industrial ear muffs - I've never driven a nosier production vehicle." Doug Hicks, Editor, 4x4 Australia, on a road test of Series III 3.9D Issue No 3 Summer/Autumn 1982 (published March).

It seems like you've got an early example indeed. :cool: The rest of the test generally praises the vehicle, which was done at the press launch at Wonnongatta Station. Wagon price was $15,230.

Rob W

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Originally Posted by isuzubob

"The first forty 3.9D LRs do not have the 'quiet kit' fitted to subsequent production machines. Boy, if you end up owning one of the first forty you'll need to invest in some industrial ear muffs - I've never driven a nosier production vehicle." Doug Hicks, Editor, 4x4 Australia, on a road test of Series III 3.9D Issue No 3 Summer/Autumn 1982 (published March).

It seems like you've got an early example indeed. :cool: The rest of the test generally praises the vehicle, which was done at the press launch at Wonnongatta Station. Wagon price was $15,230.

Rob W

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Build date on the plate is 1/82. I thought the first Stage 1's where 81, but if there where only 40 built without the acoustic cover then it may be one of them. It certainly is noticable. I am reputably the second owner(not counting the car yard that bought and sold it to me) as well.

CC