Recording automotive exhaust sounds?

Special Interest Groups Recording Techniques
1

Hi,
I am new to this forum and very new to sound recording in general. I have a fairly new Lenovo ThinkPad Edge laptop running Windows 7 Pro and a Samson Go Mic. I would like to use this with Audacity to record and edit auto exhaust sounds.

I have tried a few test recordings but it would appear that I have a lot to learn. With the Go Mic set on cardioid placed about 1 meter away from the tail pipe of the vehicle I have tried several recordings. The mic is quite sensitive and will pick up the sound of me walking to the driver’s compartment and starting the engine but it clips the exhaust note. If I turn the recording volume down so there is no clipping, the exhaust note is barely audible.

Can anyone point me in the right direction as to what settings to use or things to try?

Thanks,
Robert

2

Turn down the recording level so that the exhaust note does not clip, then post a short sample - just a couple of seconds in WAV format will be fine. We can then take it from there, See here for how to post audio samples: https://forum.audacityteam.org/t/how-to-attach-files-to-forum-posts/24026/1

3

Hi Steve,
Thanks for your prompt reply.

The learning curve is pretty steep for an old dog like me but I’ll give it a try.

I took one of my test recordings that I had played with the recording levels on. I tried to select a few seconds of the track that had no clipping and pasted it to a second new track. I then exported that clip as a WAV file and attached it to this response. Or at least that’s what I’ve tried to do.

Regards,
Robert

4

Well done - that worked :slight_smile:
A slightly easier way would be to select the part that you want as a file, then use “File > Export Selection”.

The recording is extremely loud !
In fact it is recorded a bit too loud and is clipped (distorted) in each place that it touches the top/bottom of the track. The recording level needs to be reduced so that the recording does not touch the top or bottom of the track.

The recording may sound very quiet on small loudspeakers if they are unable to reproduce the very low frequencies that make up most of the sound. It is almost inaudible on my laptop speakers, but is quite capable of rattling the doors and windows when played on my main sound system.

To make the recording “work” on small speakers; Make an undistorted recording - a peak level about half the track height is fine. Then use the “Amplify” effect to bring the level up to 0dB, then use the “High Pass” filter set to 100 Hz, then use the “Leveller” effect with a fairly “heavy” setting. The Leveller effect will “squash” the dynamic range, making the quieter parts louder, and will also produce some overtones of those bass notes (controlled distortion). Alternatively, get bigger speakers :wink:

5

And now down to actual content for a minute. Are you after real exhaust sound or Hollywood exhaust? You picked one of the “Everybody Knows” effects. “Everybody Knows” that radar has to go 'round and 'round even though radar hasn’t actually done that since 1968. Newsrooms have to sound like model 47 teletype machines (1963), etc.

Car Exhaust has to sound like something from the Dust Bowl era.

Exactly correct. Most exhaust today has been carefully calculated and engineered to be inaudible. It’s still violent movement of air, but some of it comes out tiny pulsing whisping sounds and a majority of it is beyond the bass pedal on pipe organs. That’s why you’re having such a rough time with it. All but the largest speaker systems can’t reproduce it and it shares the Rock Concert problem of base notes destroying the performance.

This brings up other problems as well. Microphones can be engineered not to go there because most of the people using microphones don’t care about 32-foot Diapason organ pipes, so the microphones don’t work very well down there. Vocal microphones aggressively limit bass notes on purpose.

And you thought you were on a regular sound recording job.

Koz

6

If the engine recording is for diagnostic purposes Audacity does have a spectrogram display …
Audacity spectrogram of ''E150''.gif
Other free spectrograph programs are available …
SoX spectrogram of ''E150'' -.png
https://en.wikipedia.org/wiki/SoX

7

And neither display tell you what you need to know. Most exhaust is well below 100Hz, a suggestion of colour at the bottom of those displays. Koz

8

OK, “plot spectrum” in Audacity’s “Analyze” menu, with a log(arithmic) horizontal frequency axis shows the bassy resonant notes of the exhaust (50-150Hz) …
Audacity ''Analyze'', ''plot spectrum'' with log(arithm) frequency axis.png
I did find a (free) real-time frequency analyser (Windows only) … https://forum.audacityteam.org/t/filtering-frequency-during-recording/24899/12

9

Good Morning Gentlemen, and thanks again for your quick responses.

I had considered that the speakers on my laptop might not be able to reproduce the base sounds. This morning I sent the file over to my wife’s computer which has a pretty good sound system, Yup It’s loud alright!

I also was wondering about the fact that the wave form touched the top and bottom of the track, but it didn’t show as being clipped. Now I know better.

I’m an old engine freak, and to me, the sound of a healthy engine, almost any engine, is as good as the finest of symphonies. I don’t expect to get recording studio quality with the equipment I’ve got, but I’m hoping that I can get some realistic reproductions. I’d sure appreciate some advice on how to get the best out of what I’ve got.

To start with, the Go Mic is a pretty basic and inexpensive piece of equipment. No doubt I could do more with a better mic but this is at least a starting point. It is a dual pattern condenser USB using a 16 bit ADC @ 48K sampling rate. A pattern switch allows the user to select cardioid, cardioid with a 10db pad, or Omni directional. The element type is Fixed Charge electret, 10mm dia with a thickness of 4 microns cardioid and 2 microns omni. Sensitivity is -47±2dB/Pa and SPL is 121db. Frequency response is 80Hz-18kHZ Cardioid and 20Hz-20lHz Omni.

From the spec sheet I’m thinking that perhaps I would get better results using Omni because of the better low frequency response. Then again, because exhaust sounds are pulsing perhaps Cardioid with 10db pad so I could turn my recording level up a bit.

Then there is the matter of positioning the mic. At this point I’m just experimenting with a vehicle stationary so I can place it anywhere I want, but eventually I’d like to try recording under actual driving conditions so I’ll be limited to having the mic on a short boom probably within no more than a metre of the tail pipe. I don’t know if the mic should be aimed directly at the pipe or off to one side. Then there is the matter of dual exhaust. I suppose the best starting point would by just a mid point between the two.

I have two engines in my boat and it would be ideal if I could record in stereo, but I understand that this takes more sophisticated equipment than I have. I’m thinking that the best I can do there would be just to have the mic mid point between the two exhaust ports and settle for that.

Once I have the raw tracks recorded so there is no clipping I’m not clear on how I can edit it for best reproduction. I don’t really want to “dumb it down” so it will sound reasonably good on a lap top because I intend to burn the tracks to a CD so it can be played on a reasonably good sound system.

Your thoughts and guidance would be greatly appreciated.

This is fun!

Regards,
Robert

10

Bad news : the real-time frequency analyser I mentioned above , only has a linear frequency scale (not logarithmic) so the low exhaust frequencies (50-150Hz) are bunched-up on the far left of the (green) display …
E150 real-time spectrogram.gif

11

Contact microphones are used to record noise made by engine components, which are attached to the component (in “contact”).
The sound recorded by a contact microphone could localise the sound made by one component, it would sound like you were using a mechanic’s stethoscope, so not how it sounds normally to your unaided ear.

12

…to me, the sound of a healthy engine, almost any engine, is as good as the finest of symphonies.

And a in any live performance, some if it is felt rather than heard. That doesn’t connect until you try to record it.

It is a dual pattern condenser USB

Whose make and model is?

Omni because of the better low frequency response.

Omni or whatever is natural for your microphone is always going to sound better than any of the manufactured patterns. Cardioid patterns have two ratings, what does it sound like straight on and what does it sound like from the back. Everything’s a balancing act. If you’re doing all this outside, then you don’t have to worry so much about echoes. Stay away from large buildings.

Then there is the matter of positioning the mic.

Plug good quality headphones in your computer and listen while you wave the microphones around (Edit > Preferences > Recording > Playthrough). You may have troubles not obvious. Microphones pick up reflections from the ground. Just starting the microphone on the ground and slowing lifting it up can be a revelation. When you see recordings in a studio, the microphone is generally three times closer to the performer than any large flat surface. Drums can mess that up, but those can be tuned so they sound good, not necessarily accurate. This is one place where directional microphones can be valuable.

it would be ideal if I could record in stereo

I wouldn’t do it any other way. If you were on a Mac, there are ways to record two USB microphones, On a PC, you have the option of using two computers and two USB microphones. They don’t have to be identical, but that’s best.

Go out at the beginning and yell “MARKER” and clap your hands. You can use the hand clap to match up the two recordings later, one left and one right.

See what you talked yourself into?

Koz

13

One other item. Vocal Microphones almost always have a low frequency droop and should be avoided in this case. Koz

14

Hi Koz,
The microphone is made by Samson and the model is “Go Mic”.

It does have an output jack on it for head phones so I’ll try that and see if positioning makes much of a difference. I’ll probably have limited options in that regard though.

I made a few more test recordings with the 3 different settings and the recording volume turned right down. The Cardiroid with 10 db pad didn’t cause any clipping but it seemed like the sound quality wasn’t quite as good as the other two. Straight Card was pretty good but it had a few clips, didn’t seem to notice it on a WAV playback though. The Omni had even more clips, but it seemed to sound a bit better and the clips didn’t seem to cause any distortion. It was a tough call between Card and Omni though. It’s interesting that I’m still getting clips with minimum record volume when I punch the throttle. I may be forced to use Card +10 to avoid them.

As far as editing is concerned, would it be best to avoid it completely if I can, to get the best reproduction?