Sound Generator Modification

[acerboo]

must plead ignorance here, what exactly is the sound generator, it sounds more like a silencer to me, is it on all engine sizes, thanks.

[Spud]

It's only on 3 litre ones, though you can buy one and retro fit it to the 2.5 and then take the foam out just the same.

[acerboo]

thanks but what is its purpose?

[Spud]

It generates sound

(to the cockpit)

[sk93]

it is a feed directly from the air intake ducting, that passed a oneway valve (i think that's what it is ) and then leads into the cockpit, behind the glovebox (on RHD versions).

It's soul purpose is to "pipe in" the "induction roar" of the car as it accelerates, giving you a more sporty sound.
However, inside the smallest bit of the piping, are two bits of foam, and removing one, the other, or both of these greatly increases the "roar" you hear within the cockpit.

I personally think that one bit of foam out is the way forward, as you only hear more noise when you ARE giving the beans, but under normal driving, it sounds nearly the same as standard.
it's improved my driving experience of the zed, as I now have the soundtrack to accompany the other sensuary pleasures

[daveg]

it is a feed directly from the air intake ducting, that passed a oneway valve (i think that's what it is ) and then leads into the cockpit, behind the glovebox (on RHD versions).

To me it sounds like it's coming from behind my head which is great as it sounds like a sporty exhaust.

I presume under hard acceleration air is being sucked into the intake from the cabin?

[acerboo]

just sounds an odd thing for a firm like bmw to do, more nissan.

[jwa1]

When I was a university we studied acoustics a little. Sound generation in cars was a topic during a lecture one day. Ok BMW have it to enhance the sound of the engine, but they also do it to get rid of the noise (unwanted sound). I was under the impression that this was the main reason for doing it.

TO REMOVE UNWANTED SOUND
Microphones pick up a sound waveform and then subsequently the inverse of that waveform is outputted through the speakers to cancel out that sound. So to ellaborate, say at a given point in time the amplitude of a sound wave is -1. A value of zero means no sound so therefore the speakers will output +1 to make 0. This analasys is done tens of thousands of times per second because the frequency and amplitude of the waveform changes over time and also because the human ear samples at up to 20,000 times a second (20 kHz).

The system will have been programmed to let certain frequencies through (thrashy engine sounds etc) but not frequencies associated with road noise (lower ones).

By the way - Noise Cancelling Headphone do exactly the same thing!

[sk93]

I presume under hard acceleration air is being sucked into the intake from the cabin?

nope, there's a one-way valve by the looks of things.

[sk93]

When I was a university we studied acoustics a little. Sound generation in cars was a topic during a lecture one day. Ok BMW have it to enhance the sound of the engine, but they also do it to get rid of the noise (unwanted sound). I was under the impression that this was the main reason for doing it.

TO REMOVE UNWANTED SOUND
Microphones pick up a sound waveform and then subsequently the inverse of that waveform is outputted through the speakers to cancel out that sound. So to ellaborate, say at a given point in time the amplitude of a sound wave is -1. A value of zero means no sound so therefore the speakers will output +1 to make 0. This analasys is done tens of thousands of times per second because the frequency and amplitude of the waveform changes over time and also because the human ear samples at up to 20,000 times a second (20 kHz).

The system will have been programmed to let certain frequencies through (thrashy engine sounds etc) but not frequencies associated with road noise (lower ones).

By the way - Noise Cancelling Headphone do exactly the same thing!

this is also acheieved by a resonator isn't it? well.. the same affect I think?
My ZX6R has two of them under the cowel... removal of them makes one hell of a racket!

[jwa1]

Exactly - resonators are a non electrical way of doing things. Basically sound goes in to a cavity / chamber and is reflected out as an inverse wave form that cancels out the noise! Really simple. Another application is in theatres too - they cut out echo and low pitched (talking) frequencies too. (The size and shape of a resonator dictates which sounds come out of it as this dictates the resonators resonant frequency - I won't bother to explain on here but google if you're interested!).

[daveg]

Is there an active/electronic acoustic cancellation system in use in the BMW?

If there is a one way valve that closes and stops air from the cabin being sucked into the air intake why does it get louder when I acccelerate? Surely when the valve closes it would go quiet?

[sk93]

AFAIK, there is no electronic noise cancellation system in the Zed...


To be honest, I've no idea.. nor am I 100% it is a 1-way valve.
It's in the big section that clamps onto the block and without wanting to play around with it much, I could only make a guestimate at what it was by looking at it..

You have, however, got me interested in knowing what it actually is
it's part number is: 13747514863

[sk93]

Wow Guys,
Thanks for the fascinating how to. Now I have a weekend project. Very informative and great recipe style directions I may be able to do it without screwing anything up. But that remains to be seen
great presentation

Hey Johnnybwaiting, Welcome to the forum!
Thank's for the comments and feedback! Let us know how you get on.

[Incompatible]

The sound generator assembly uses a diaphragm to both seal off the intake so as not to have a massive air leak while also magnafying the selected frequencies and tunneling them into the passenger cabin to enhance the "ultimate driving experience." The post facelift (3.0Si) cars have a more complex sound conditioning system then the pre-facelift cars. Both use a diaphragm. The post facelift cars have an intake tube which has two large resonance chambers attached (glued) to the tube body, pre cars have a simpler intake tube without the resonance chambers.