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AudioAcoustics

Serving the Acoustics Community Since 1994

Cross-Spectrum Acoustics Inc. offers Sound & Vibration Consulting Services

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Feb 01, 2007

The big news in Boston today is the bomb scare yesterday caused by electronic advertisements placed on or near transportation structures by a ‘guerrilla’ marketing firm. The fallout has included various websites criticizing (or outright mocking) Boston authorities for their “foolish overreaction.”

As with many noise consultants, I often have to leave noise monitoring devices out in the field for days at a time. Often they are chained to telephone poles, trees, fences, etc for security. Through personal experience, as well as the experience of my colleagues, I’ve learned that if you set up an electronic device at or near a public area without informing anyone or providing contact info, there is a 100% chance that the bomb squad will get called in, and about a 95% chance your $5000 sound level meter will get blown up by said bomb squad.

I’m the first one to criticise the paranoid society we have become since September 11, but I can’t find any fault with the Boston authorities here. They didn’t evacuate the city, they didn’t call out the national guard, they didn’t round up all the Arabs - they closed some roads and bridges and investigated some unauthorized devices. The did their job and got down to the truth.

If there was any panic, the faults lies with the marketing firm and Turner Broadcasting who apparently couldn’t be bothered with attaching some identifying info to the devices.


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I have yet another microphone manufacturer that I have added to the client mix: Cascade Microphones of Olympia, WA. They sell inexpensive ribbon microphones, which, in my opinion, are good mics for their price. I recently performed frequency response, polar, noise floor, and output impedance measurements for several of their mics.

They are selling their X-15 stereo mic via their eBay store. The frequency curve they’re showing on the auction is generated using an exponential moving average. As can be seen on the full report for the X-15, I generate three separate data curves for my professional customers: narrow band (FFT), 1/3 octave band, and exponential moving average. I do this because there are no real standards in the prosumer space for microphone frequency response curves. As a result, some manufacturers play games with their specs. It’s not that the specs they give are wrong; they’re just, let’s say, “overly optimistic.”

The point of the three representations is to let you compare apples to apples. Unfortunately that tends to be a little difficult since most manufactures don’t tell you how they measured their mics. There are exceptions of course, Earthworks being most notable. So, if I generate a ragged narrow-band plot (the raggedness is caused by my 20ft by 15ft by 8ft quasi-anechoic measurement environment) and a potential customer tried to compare it with another manufacturer’s moving average or fast-sweep response, one might wrongly conclude that one mic is far superior than the other. In realty, the difference is in the measurement technique.

I just wanted to make people aware that whenever you’re comparing mic specs (even mine!), make sure you comparing the same thing. For example, it would be unfair to compare the X-15’s moving average response with the FFT response of one of the Crowley and Trip mics - remember to compare like with like.

For the record I will mention that when I as a customer am looking for a frequency response data, I tend to favor 1/3 octave band responses. Of course 1/3 octave band smoothing tends to suppress very narrow peaks and dips, but in my experience, the general trends that you’ll see on a 1/3 octave band correspond with what you’ll actually notice in the real world. The FFT measurements is also an indication of what will happen when you use the mic in a real room since most of us don’t record in anechoic spaces. I’m hoping that by giving folks all the facts (good, bad & ugly), customers can make informed decisions.

Please feel free to contact me if you have any questions about the data I generated for microphone distributors.


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