How LOUD is your exhaust?

[BP1]

I am currently dealing with what I have considered to be a loud exhaust problem. To quantify how loud my exhaust really is I downloaded a free app on my iPhone called "Decibel X". With my car idling (in my case a 3.5L B35 with cam with a 1,200rpm idle) I pointed the phones microphone at the exhaust at pipe level from 2 feet away. I averaged about 100DB! The app said it was a diesel truck!! I am curious as to how loud other member's exhaust are. So if you have time to fool around with this, down load the free app and post your results. Am I the only member driving a diesel truck disguised as a coupe?

 

[Stevehose]

Intrigued. Will do.

 

[sfdon]

This at the opening to the tailpipe at rear of car?

 

[BP1]

Yes.

 

[Cornishman]

Brilliant idea, I am up for this but fear mine will make the most noise in real driving.
How about a test in the car, on the road, at a point near the drivers ear. To make it a fairer comparison, the ear nearest to the centre of the car. Ie dial out those who have the steering wheel on the wrong side of the car.
Could we make a table with
X speed, say 50 mph, constant throttle position
Top gear, but State which revs as some have 5 speed trans, or auto
Windows closed
Fan, radio, wipers off
I appreciate that tyre size and choice, road type, window fit etc will make a huge difference. So an alternative idea is.
Static car
Warm engine at 2000rpm, Ie the minimum revs we use on the road.
Windows closed
Noise at drivers ear.

There must be a noise engineer out there who can devised the “perfect comparison test” for us to do at home.
Now,I will work out how to get that app onto my Samsung.

 

[autokunst]

I am currently dealing with what I have considered to be a loud exhaust problem. To quantify how loud my exhaust really is I downloaded a free app on my iPhone called "Decibel X". With my car idling (in my case a 3.5L B35 with cam with a 1,200rpm idle) I pointed the phones microphone at the exhaust at pipe level from 2 feet away. I averaged about 100DB! The app said it was a diesel truck!! I am curious as to how loud other member's exhaust are. So if you have time to fool around with this, down load the free app and post your results. Am I the only member driving a diesel truck disguised as a coupe?

Oh my. In a past life I was an acoustic engineer, and that is LOUD. My car has open headers right now and I don't think it is that loud. I love this experiment - great idea!!!

I would recommend recording/testing the SPL (sound pressure level) with your app at 1 meter away from the source (in this case, the tail pipes). Or in Cornishman's spreadsheet, I guess the measurements would be taken from inside the car. The one meter distance is an industry standard and will be what most reference material (that diesel truck) would be based upon. Again, very cool!

 

[dang]

Borrow a friend's phone and duct tape it to the back of your car and drive around. No worries...

 

[Belgiumbarry]

surroundings will also do alot …. like we like to floor it in a tunnel :eek:

 

[Arde]

Whaaaat?????

 

[BP1]

Following Autokunst's advise I remeasured from a distance of 1 meter (39") from the exhaust tip and to my surprise the DB level dropped to 92DB. I could not find Sound Pressure Level in the free version of the app, it might be an option in the paid version. Moving the phone by only 15" made a big difference and shows how important a standard of 1 meter would be for consistency in our comparisons. I look forward to all the results. Thanks...BOB

 

[JFENG]

dB SPl will go down approx as the square of the distance. Assume the original was done at 24”, one meter is 1.625 further away. Do the 20log10 conversion to dB pressure and the sound should’ve gone down about 4-5 dB. Perhaps you changed the distance from the ground or you were slightly closer than 2ft the first time? Doubling the distance would reduce dB SPL by 6

 

[BP1]

WOW! This is great! Yes bfeng, I think I was a little bit closer the first time. You can't game the physics of sound waves. Thanks.

 

[JFENG]

these smart phones are reasonably accurate for judging relative differences like this.
Did you try different phone orientations to see if that had a substantial effect on the number?

 

[BP1]

No. The phone was flat...parallel to the ground, pointed at the opening.

 

[Stan]

I would like:
- to know how loud brent's Jaguar is
- to know how loud Arde's spleen is

 

[Arde]

I would like:
- to know how loud brent's Jaguar is
- to know how loud Arde's spleen is

My spleen tops my organ donor card list. Number two is my brain, it has low miles.

 

[autokunst]

dB SPl will go down approx as the square of the distance. Assume the original was done at 24”, one meter is 1.625 further away. Do the 20log10 conversion to dB pressure and the sound should’ve gone down about 4-5 dB. Perhaps you changed the distance from the ground or you were slightly closer than 2ft the first time? Doubling the distance would reduce dB SPL by 6

John, you get a gold star on your physics of acoustics knowledge! I do think that the reflection off the ground will likely affect these results, too - especially if our phones' microphones have different geometry (or how we hold hem, etc). But I still love this experiment. I will try to get the open headers dB tomorrow if I can get the Raven started up quickly.

 

[JFENG]

"low loud Arde's spleen is"
Stan, what?

 

[JFENG]

A simple reflection off the ground is usually not a problem at the distances you tried. And unless there's a wall or ceiling nearby to cause standing waves, you won't have the usual problems of room acoustics which necessitate that you make measurements at multiple points (all equidistant from the sound source) and then average. Hence my assumption you are in a semi-hemispheric free field condition (2pi). That assumption means sound pressure simply scales as the surface area of a hemisphere centered on the noise source. You can test this with your own ears. move slowly away from your tail pipe, without changing your directional orientation. If you feel the sound get louder then you've got non ideal spatial effects and you now need to think about a new test location or you have to average across multiple measurement positions.

 

[autokunst]

You can test this with your own ears. move slowly away from your tail pipe, without changing your directional orientation.

That reminds me of one of my favorite experiments from college (the first official major: acoustic engineering). You can walk down any uniform hallway (any given long office hallway works well), with your eyes shut. Your ears will hear your sounds (foot falls, shuffling clothes, etc), and will not let you deviate from walking right down the center of the hallway. Of course, if your hearing is damaged, there is too much ambient noise, or if one of he walls has dramatically different reflective characteristics, your experience may not be as pure - try with caution.