Funny interior sound level numbers?

[Cato]

Love my new BRZ Limited except for the sound generator, so I bought the Dynosty kit and removed it. I measured the cabin sound levels before and after removal, and I was pleasantly surprised by the large difference - 70 mph cruise sound level decreased from 88 dBA to 71 dBA on the same stretch of highway, and the sound level at full throttle in second gear (going from 35 mph to 55 mph) decreased from 97 dBA to 85 dBA.

This decrease is much greater than I have seen referenced in other posts, which generally looked like a 3-4 dB decrease. More on the methodology below, but, as they say, it isn't rocket science.

I have the data sheet from Car and Driver's test results on the 2013 Subaru BRZ Limited, and I noticed that my numbers after removing the noise generator were almost identical to C&D's numbers on the stock car (73 dBA at 70 mph cruise, 85 dBA at full throtle).

QUESTION: Did the 2013 Subaru BRZ Limited not have the noise generator? I don't have the date the C&D testing was done, just the PDF, and it could have been a pre-production version.

The moral of the story is that removing the noise generator will reduce the perceived sound level by over half (the actual acoustic power per unit of area is reduced by over a factor of ten).

METHODOLOGY: I used a Radio Shack Sound Level Meter on the A Weighting. For 70 mph cruise, I took six readings on the 70 dB range (I didn't believe the first three were so high) on the unmodified car and three on the modified car. The omnidirectional mike on the meter was not sensitive to the direction in which it was pointed. It was held by the passenger at 10 inches above the seat, pointed directly forward. These readings used the one-second average.

For the full-throttle, I took three readings for unmodified and modified, both on the 90 dB range, using the maximum value experienced in going from 35 mph to 55 mph.

The values for the modified car are similar to my wife's 2004 Miata, which I used for a sanity check. Miatas are not quiet cars, but they are quieter than the stock BRZ.

[slizoth]

I'm interested to know more about this and perhaps a link to where I can buy the Dynosty kit as well. I've already put sound dampening material in the doors and trunk. Most of the noise comes from the tires though so if anyone can recommend an exceptionally quite set of tires that'd be fantastic!

[PabloN]

Originally Posted by slizoth

I'm interested to know more about this and perhaps a link to where I can buy the Dynosty kit as well. I've already put sound dampening material in the doors and trunk. Most of the noise comes from the tires though so if anyone can recommend an exceptionally quite set of tires that'd be fantastic!

I don't know about quiet tires, but after putting on those underbody panels that are standard on the GT86, I'd swear road noise is reduced (unfortunately I don't have a sound level meter to back it up).

[Rayme]

Really? There's barely any noise coming from the tube going down the highway. I plugged mine and only noticed the quietness at WOT and under harder acceleration. What does that kit do more?

[Cato]

You can buy the Dynosty kit from frsmod.com or dynosty.com. I doubt that it does any more to reduce noise than just plugging the hole in the passenger side firewall, but it is a nice-looking aluminum plug with a beveled outside edge that looks like a factory piece at the air intake. What it does is get rid of three feet of useless tubing and the sound generator itself, making that side of the engine bay much more open.

It is very difficult to guess at relative loudness, but a sound meter is a pretty straightforward piece of gear and says that at 70 mph cruising, the actual acoustic power per unit area will be reduced by more than 90%, which the ear hears as a little more than half. To put this in perspective, eliminating all other sources of noise will not reduce noise in the car at cruise by as much as getting rid of the sound generator.

I stand ready to be corrected by anyone doing the sound meter tests. The difficulty of estimating this by ear alone is complicated the harsh rasp of the sound generator compared to normal road, car and engine sounds. About the only good thing I can say about the sound generator is that it lets those who want an obnoxiously loud car keep more of the goodness inside without having to rely on a loud muffler.

[Poodles]

Does the 2004 Miata have a hardtop and an automatic? Only way I can see it being quieter...

[OICU812]

Do some sound MLV matting like cascade in your trunk area and below back seats and you'll be again impressed by reduced road noise.

[woode]

Personally I didn't buy a sports car for a quiet ride.. actually, I kind of like listening to the engine, exhaust, turbo, road noise, crickets, clutch rattle, etc in a sports car. Part of being more connected and the whole experience..

[PabloN]

^ agreed, except for "road noise" (mainly the white noise from the tires) which is monotonous & fatiguing on long road trips. For me, the stock BRZ/FR-S could use a little more road noise isolation, but not too much--it's useful to hear (as well as feel) when the tires are beginning to lose grip.

[slizoth]

Originally Posted by PabloN

^ agreed, except for "road noise" (mainly the white noise from the tires) which is monotonous & fatiguing on long road trips. For me, the stock BRZ/FR-S could use a little more road noise isolation, but not too much--it's useful to hear (as well as feel) when the tires are beginning to lose grip.

100% with this, but based on the OP, I thought I could cut down on a ton of noise by getting this kit for relatively cheap, easy install, see the difference for myself.

$44 shipped, I'll give it a shot.

[jeffchap]

I plugged the sound tube at the intake with a 1 5/8" marine plug I got at Ace Hardware for $4.29. Fits perfectly and looks professional enough for me.

[Ozzman]

Originally Posted by jeffchap

I plugged the sound tube at the intake with a 1 5/8" marine plug I got at Ace Hardware for $4.29. Fits perfectly and looks professional enough for me.

I hope you are absolutely certain that the bolt in the middle of that thing can't come loose and get sucked into your engine.

[BRZfan]

Ride in a really quiet car and absorb the difference. See what it is like on the "other side".. There is value for some of us with noise reduction.

http://www.worldcarfans.com/10608096...n-luxury-class

Originally Posted by woode

Personally I didn't buy a sports car for a quiet ride.. actually, I kind of like listening to the engine, exhaust, turbo, road noise, crickets, clutch rattle, etc in a sports car. Part of being more connected and the whole experience..

[woode]

Originally Posted by BRZfan

Ride in a really quiet car and absorb the difference. See what it is like on the "other side".. There is value for some of us with noise reduction.

http://www.worldcarfans.com/10608096...n-luxury-class

We have a Lexus IS250. I drive that when I want all the comforts. This isn't a luxury car, lol.


Sounds like some people would prefer more of a sporty luxury sedan/coupe like a BMW 3 series or something than a sports car. They will have a broader power band, be quieter, have more options, and be more comfortable.


But anyways, that's the beauty of the car world.. can morph your car into what you want and everybody wants something a little different.