The Science Of The CrashGuard

• Isolating high frequencies
• Cymbals and high frequency splash
• The shield and internal acoustic absorption

It would be great if we could claim that we 'invented' the CrashGuard. Truth is; devices like the CrashGuard have been custom built by recording engineers for years. Some have tried cardboard and duct tape, others have attempted to cut & glue plastic bottles.

Although these home-made jobbies work to a certain degree, they are usually fragile, hard to position and probably do not afford as much isolation as one would like.

The point here is that necessity is usually the driver for innovation. The CrashGuard is a device whose time has come to break out of obscurity… and this page explains how it works.

Isolating high frequencies

It is well known that wherever air is free to wonder, sound tends to follow. Sound travels through air like waves on water by compressing air molecules. High frequencies and low frequencies both sound different and behave differently. The table below shows some examples.

Given the fact that bass goes everywhere and high frequencies are more directional, highs are easier to predict. And since high frequencies contain less energy than lows, they require less effort to control. This is, in fact, why the CrashGuard is designed to control spill from the cymbals. Highs are easy to mange compared to bass. And controlling low frequencies from the toms is practically impossible.

Reality check: Unless you completely isolate the snare or tom microphone with an air tight heavy mass, sound will get in. And last time we checked, drums are never isolated from the cymbals. Point being, the CrashGuard will not stop the sound of a cymbal from getting into the drum mic, it will attenuate it.

	High Frequencies	Low Frequencies
Wavelength	High frequencies have shorter wavelengths.	Low frequencies have longer wavelengths.
Directivity	High frequencies are uni-directional. They can be aimed using a horn.	Low frequencies are omni-directional. Bass goes everywhere.
Energy	High frequencies (when at the same amplitude as low frequencies) have less energy per cycle.	Low frequencies (when at the same amplitude as high frequencies) have more energy per cycle.

 

Cymbals and high frequency splash

When the hi-hat is played, the high frequency sound from the cymbals will travel outward in a straight line in all directions. You can be certain that some of it will end up right at the nearby snare drum mic. By shielding the snare mic with the CrashGuard, the direct sound from the hi-hat is attenuated. It is important to note that high frequencies easily bounce off of hard surfaces. This can include drum shells, other cymbals and even the drum skin.

Even with the CrashGuard in place, some of the cymbal sound will find its way into the drum mic. The CrashGuard is merely reducing the level to give the engineer more control. This is particularly important when trying to add some attack to a drum. This is normally done using an EQ by adding high frequencies. If the cymbals are left unchecked to spill into the drum mic, their high frequency content will also be accented.

The shield and internal acoustic absorption

In the old days, studios use to line their walls with lead to stop sound. Today, they use multiple layers of gypsum board. To stop sound you need mass. And if you look inside a modern control room, you will find a mixture of both absorbent materials and diffusers. This is because in order to control sound, you have either to absorb it or deflect it away.

The CrashGuard is made from ABS plastic. ABS is a fabulous material… it is rugged yet springy. This means that when you hit it with a drum stick, it will not break. You can find photos of extreme measures where engineers have taped forks and other shields over microphones in effort to protect them from overly aggressive drummers. The CrashGuard offers a good measure of protection and is also heavy enough to actually attenuate some of the high frequency sound. You will also notice that the CrashGuard incorporates angles on both sides. This helps reflect energy away from the microphone inside. In fact, this is exactly the same approach used to build the Stealth bombers: the angular shape of a Stealth bomber is designed to reflect energy away so that it makes it difficult for Radars to pinpoint their location.

The underbelly of the CrashGuard is coated with a layer of open cell acoustic foam. This is designed to help reduce some of the internal reflections that are generated when the drum is hit. The CrashGuard will affect the polar pattern of the microphone which will slightly alter the tone. But by aiming the mic further downward, the effect is minimized.

Who would have thought that such as simple device would actually such in-depth science behind the design… When it really comes down to it, the most important issue here is performance. And the CrashGuard does exactly what it is supposed to do.