Noise

[Melv]

While you guys are ripping out silencers and fitting mega db exhaust systems, I've had to go the opposite route to keep on track days!!

Measured 105-107 static at Bedford (964 Cup Car -100 cell race cat, Cup pipe, final silencer) -I've now replaced the Cup pipe with a standard C2 silencer. Result? 100db static at Goodwood and no noticeable difference in power above 4200 rpm -and I also don't have to wear ear plugs anymore!!

Doesn't sound as good though......

Melv

[tony.t]

I think you'd need a dyno to determine the difference in power a cup pipe gives.
At 100db you are still pushing your luck at some days and it's hard to see how it could be made quieter.
In the same boat and have to stick to noisy tracks.

[Melv]

tony.t said:

I think you'd need a dyno to determine the difference in power a cup pipe gives.

As long as I can still keep up with GT3 RS's and GT2's.....

Melv

[neon_fox]

I've been told that a twin output rear box helps, as it spreads the sound out between two sources...

Anyone care to confirm?

Fox
---
964C2

[domster]

Dunno, but I had cat bypass and no secondary silencer on the baron at one stage, and it sounded beautiful. However I was limited to the moon for trackdays. Bugger.

Sadly the secondary silencer will have to go back on.

[Melv]

9M added to a post on this a while ago with respect to db's and different set ups -u'll have to do a search. I also asked if he did any bhp comparisons with these set ups.

Melv

[lightweight]

neon_fox said:

I've been told that a twin output rear box helps, as it spreads the sound out between two sources... Anyone care to confirm? Fox --- 964C2

I failed 105db static at goodwood with 'Quiet'set up that was with twin outlets if it is loud it is loud the noise test guys are prity helpfull but a fail is a fail melv makes a very good point

[peashooter]

Melv said:

While you guys are ripping out silencers and fitting mega db exhaust systems, I've had to go the opposite route to keep on track days!!Melv

Mine was 105dp @ Goodwood and just managed to sqeeze in.......Is 105db about the maximum around the UK tracks then??? or what is the limit for both maximum and minimum????......I've heard of people being turned away at Goodwood because of noise......

[GuyR]

A lot of place run 98dB, as does Goodwood on most days..........

My old 964 with a cup bypass ran 110dB at Goodwood a few years ago, but it was back in the days they ran a 110dB limit, so it was OK.

My current 996 GT2 is far too quiet, like all 996s, but the warranty precludes any changes.........

I've always though the best thing to do would be to have someone fabricate some exhaust tips that could be mounted for trackdays that turned down by 90 degrees ie forcing all the sound into the ground where it would be dispersed 360 degress, rather than aimed straight at the noise-meter as currently..........

Guy

[peashooter]

GuyR said:

I've always though the best thing to do would be to have someone fabricate some exhaust tips that could be mounted for trackdays that turned down by 90 degrees ie forcing all the sound into the ground where it would be dispersed 360 degress, rather than aimed straight at the noise-meter as currently.........Guy

My GT3 has a Porsche supplied and fitted switchable exhaust, so loud and original sound at a flick!........Good idea about directing the flow away, but I think the marshals might suspect it, if they can hear it, but the meter registers a lower db....one marshal said that he’s been doing for so long that he can tell accurately the db for each cars pulling up to get tested before the track run without using a meter.......

[Melv]

Coupla points -I watched the sound marshall move between the exhausts on a twin outlet job (Easter Monday), so you can knock that idea on the head!!

105db seems the max limit, if there is a limit. Goodwood is 98db on a quiet 10 car day, 105db on a noisy 5 car day -that's number of cars out on track per session.

And pointing the exhaust at the ground -IMNVHO a total waste of time as the ground will bounce the noise back and probably make it worse.

The marshall guessing the db -urban myth I reckon!!!

Melv
(Happy at 100db!!)

[neon_fox]

Anyone got a noise test meter? Or know where I can pick one up cheap? How far away from the exhaust and at what angle do they usually measure for the static tests? My previous 968CS passed all noise regs easy, but the 964 with Cup pipe I would guess is marginal at best...

Fox
---

[Melv]

Foxy -RTFM!!

"Measured 105-107 static at Bedford (964 Cup Car -100 cell race cat, Cup pipe, final silencer)"

If u r near a track, most are happy if u pop down for a track day and will test yr car, even if u r not participating.

I can get one (Egham area) for contribution to my wife's drinking, oops, sorry, school fund. Si at Essential Styling also has one (St Albans).

Rgds
Mel

[GuyR]

Melv,

Be careful not to dismiss sometimes simple ideas, a little exerimentation might prove a lot cheaper than a new exhaust...........

As an aside, I would be interested to know how exactly can the ground make the noise louder, since it can neither concentrate it, nor amplify it?

Ever stood to the side of a stereo loudspeaker? It's quieter than if you stand in front of it. What does that prove - answer sound dispersion is not equal......

The sound output from an exhaust emits radially from the tip and is also effected massively by the speed of exhaust gases. You will recall this from statements made by various tracks (notably Bedford) about the sound being carried by the wind to nearby villages. What this means is that a large majority of the sound is carried by the exhaust gases and shape of the tail-pipe to exactly where the SPL (Sound Pressure Level) meter is reading. The shape and mass of the car also affect sound dispersion i.e. it sounds quieter stood in front of the car than behind (seems obvious).

If the pressure waves from the exhaust were modified to disperse sound over a more equal area, then the sound reading will be reduced at the point they read it i.e. behind the exhaust.

One method would be direct to the ground via the 90 degree bend, which would then cause the sound to be dispersed over 360 degrees, including under the car. The other alternative would be to fit an end-cap over the exhaust like a chimney top, that ensures sounds radiated co-axially to the line of emission i.e. a 360 degree circle if viewed from behind (left, right, up and down).

Sound is measured as pressure (i.e. dB is a logarithmically rebased measure of Sound PRESSURE Level in N/m2). Given the basic physics formula of pv=nRT, if you increase the area of dispursement, you decrease the average pressure. In addition pressure dispurses at a rate equal to the inverse-square of the distance from emmission, so adding distance via the drop to ground and back would also reduce the dB reading.

Mind you I could be making up all this Acoustic Engineering mumbo jumbo?

Guy

PS dB Meters can be bought easily on-line for about £130, which is often for the same model the officials use:

www.cybermarket.co.uk/ishop/923/shopscr3093.html

>> Edited by GuyR on Monday 19th April 21:37

[Melv]

My turn to to your superior knowledge Guy.

If you can't dazzle 'em with brilliance, baffle 'em with bullshit -that's what I say! Try telling that to the noise police......

BTW, my C2 silencer cost thirty quid!

Melv