Waveform explained? (problem recording)

Hi there,

I have been recording racing cars for a project, however I have been having mixed success. I used a basic Zoom H1 using an infinite amount of gaffer tape to secure it to the roll cage and some foam padding to stop any vibration.

The first recording was fine, however when we recorded different cars with different engine configurations and different gearboxes, the sound appears to have done bad things to the recording. I am hoping people can point out what has occurred…?

So this one was a Nissan. Recording was fine, no problems.

This one was a Honda, odd crack here and there, 99% ok.

This one is an Alfa-Romeo, sounds pretty bad, but only really on one channel.

This one is a BMW, again, sounds bad. Its a really high pitch recording.

The problem looks like “clipping” distortion.
Clipping occurs when the signal level is too high for some part of the recording system and the top and/or bottom of the waveform is “clipped” off. Notice how flat the bottom of the waveform is in the bad recordings.

I don’t have an H1, but I have an H2. On the H2 there are two ways to control the recording level - there is a high/medium/low (“h/m/l”) switch, and there is a second level control that is set using the buttons on the front. The “h/m/l” switch is a hardware switch that changes the sensitivity of the microphone. The other control operates on the signal “after” it has been converted to digital. The second control just amplifies (scales) the digital signal. If the signal is clipped before it is converted to digital, then lowering the recording level after it has been converted to digital does not help - it just makes the entire sound quieter. The important setting to avoid clipping is the hardware switch “before” it is converted to digital.

In first picture, the recording level is dangerously close to clipping, and is probably just a little clipped in the right channel.
Same for the second picture.
In the last two pictures, the signal is clearly clipped quite badly, but the clipped digital signal is scaled lower so that the clipping occurs below 0 dB.

Ensure that the H1 is set to the least sensitive setting (assuming that it has a switch similar to the H2). If it still clips at the least sensitive, then the sound is just too loud for the H1.

You are overloading the recorder.

I was a party to an ABC Wide World Of Sports the day they sprinkled cameras around the racetrack and tried to cover an automobile race that way. They couldn’t use the cameras right next to the track because it was so loud (how loud was it?) it was so loud it would vibrate the cameras and destroy the video.

There is a limit to how loud sound can get before a recorder fails, and it’s not just a matter of watching the sound meters and turning it down. The h1 uses tiny pieces of metal foil in a cage to pick up the sound. The electronics carefully watch the foil move to tell what the sound is doing. Eventually, the tiny piece of foil runs out of room and that’s when you start getting blue waves with flat tops and bottoms. That’s the foil smashing itself into its little cage.

There isn’t much you can do directly to the H1. I guess you can try wrapping it in towels, but that’s going to sound like you wrapped it in towels. Some News Gathering microphones have provision for very loud sound, or you can use a different type of microphone.

Dynamic (moving coil) microphones like the Shure SM58 rock band microphone don’t overload.

http://www.sweetwater.com/store/detail/SM58S

I suppose they do eventually, but you may not conscious to see it. Follow one of those with a volume pad such as a Shure A15AS attenuator …

http://vintageking.com/shure-a15as-in-line-attenuator?gclid=CI_nuKCXoMoCFUaCfgod_zwM9w

… and you should be good to go. The problem is going to be the H1 doesn’t have XLR connector in the bottom like the H4 does.

So that’s not going to be a quick solution. You’re beating the H1 bloody and somehow or other, you need to stop.

Koz

It could be the H/M/L sensitivity thing, but when you’re in a sound field loud enough to make your jacket move, I don’t think that’s going to help.

Koz

The H/M/L sensitivity thing works up to a point, but I’d not be surprised if close recording of a racing car pushes the sound level beyond that point.

Thanks for the responses. So in effect is it the SPL which is too high in these environments?

If I bought a mic with a much higher SPL would that help this problem?

Thanks so much.

Ben.

Yes, that appears to be the case.

There are two things to consider:

  1. Can the microphone handle the SPL without distorting?
  2. Can the H1 handle the signal level from the mic?

“Dynamic” microphones tend to have better SPL handling capability than “condenser” microphones or “electret” microphones. Some dynamic mics are better than others in this respect. SM58’s are known to have very high SPL handling capabilities. Most “drum mics” also have very good SPL handling ability. This takes care of the first consideration.

The second consideration is the “sensitivity” of the mic - how big is the electrical signal for a given sound pressure level. You could use a mic that can handle terrifically high SPL, but if it also throws out a terrifically high signal level, then it will probably overload the mic input on the H1. You “may” be able to avoid that by setting the H1 to expect a “line level” signal, or by using a low sensitivity mic (cheap drum mics often have low sensitivity). The third option is to use an “attenuator” between the mic and the H1 to reduce the signal level.

Also see trying to mic a rock band.

If I bought a mic with a much higher SPL would that help this problem?

Sorta…

Yes, using a microphone with a higher Sound Pressure Level rating is a very good step, but then you have to keep it from overloading the microphone preamplifier—the next step in the process. That’s what the Shure H15 attenuator does. When there’s tons of electrical sound coming out of the bottom of the microphone, you have to find someplace to put it so it doesn’t cause troubles. The H15 soaks it up (tune the amount with the switch on the side).

And then there’s the matter of being in pro microphone (XLR) territory and no place to plug it in.

As above, once you damage the sound and digitize it, that’s the end of the world. We can’t reverse that.

You are urged to try the High, Medium and Low sensitivity switch on the H1 before you write checks. You may get lucky.

Koz

A cheap option that you could try - earbud headphone can work like microphones. They are not designed to be used as such and have very low sensitivity, but in this case that is exactly what you want. You could try plugging a cheap set of earbud headphones into the mic input of the H1.

Sennheiser has the MKH 410-1 mic for just that kind of problem. Suitable for jet engines, explosions and other very, very loud stuff :astonished:

OK, so I already own this mic http://www.bhphotovideo.com/c/product/744768-REG/Rode_VIDEOMIC_PRO_VideoMic_Pro_Compact_Shotgun.html

If I stripped off the mount and just used that on a clamp bracket or just gaffer taped it to the car, would that mic be ok to use with the H1 in terms of signal?

I guess if I did want to go for a better mic i’d then be looking at a different audio recorder, which I am not ruling out.

Thanks.

I wouldn’t.
A shotgun mic might be OK for recording the cars as they go past, but I wouldn’t use one taped to the car.
I think that cyrano may have changed his mind about the mic he was recommending - I could have sworn that when I read the post first time that it said Sennheiser MKH 70-1 (which is a very expensive shotgun mic, and also unsuitable for the job).

I’d suggest that you try the earbud trick first - I once used a set of earbuds with my H2 to record extreme SPL industrial noise. The earbuds cost about $2 and the result was surprising good.

For a more costly solution, a Shure SM58 might work. The most likely problem using an SM58 is that for extremely high SPL, the signal output will overload the mic input. The SM58 produces around 1volt peak for 150dB SPL at 100 Hz, so it may be OK into a line level input, or using an attenuator as koz suggested.

Kick drum mics typically have very low output (because they are designed for extremely high SPL, but their frequency response is deliberately not at all flat. The output from using earbuds won’t be flat either, but is likely to be low enough for the mic input, and you can use equalization after you have the recording.

If you’re looking for best sound quality and price is not an issue, take a look at the range of DPA omnidirectional miniature microphones. They do some models that can handle well over 150 dB SPL with extremely good sound quality, but they are not cheap.

You have a Special Application. That puts you outside the “cheap as dirt microphones that millions of people bought.”

If someone sent me to a track to do background sounds, I’d bring the Shure SM58 (or other similar type microphone), my attenuator plug-ins, the Zoom H4 and most important, my large Koss Pro4-AA sealed headphones so I had a fighting chance to hear what was going on.

https://www.koss.com/headphones/over-ear-headphones/pro4aa

Also see: Sony MDR-7506 movie studio headphones. Same problem. I need to hear what’s going on.

http://www.sweetwater.com/store/detail/MDR7506

If the requirement was stereo, two microphones and attenuators and then I would need a mic stand.

The headphones would have told you something was wrong back at the track. The monitor point on the H1 would have sounded awful.

Koz

You may have missed an important part of the job spec.

It seems that BenUK wants the recording from the driver’s perspective, not a track-side perspective. A mic stand is not going to be much use “IN” a racing car unless it is welded to it, or at least “securely” clamped, but I’d guess that it’s also a bit tight on available space.

By the way, I checked the H1, there is no microphone sensitivity control, sadly. There are a bunch of controls at the back, but nothing that relates to sensitivity.

I’ve just looked at the user guide - it looks like you’re right.

If you intend to use an external dynamic mic and you are able to solder (or know someone that can), then making an in-line attenuator is pretty straightforward. In its simplest form it just requires 2 resistors (3 for balanced input). There’s an article about how to design and build one here: Uneeda Audio - Build your own attenuator pads

That suggestion is probably only sensible for those of us with existing stocks of resistors and XLR connectors. Google Amazon for “xlr attenuator” and you’ll get lots of hits, most of which are less expensive than the Switchcraft S3FM XLR barrel needed to make the home-brew attenuator look nice.