LimitEar – Hearing Dose Pro – Sennheiser HDM Pro 25 Headphones

‘If it’s too loud, you’re too old.’ I think I first saw this rock and roll wisdom attributed to Ted Nugent. And with the passing of the years it now seems Ted was exactly wrong, and in so many ways. In turns out if it’s too quiet when your old, that’s because you listened too loud when your were young. Life is nothing if not ironic. Safe but useful levels it turns out are a particular problem for headphone listening. Now I’m not saying back in Ted’s day that radio DJs wore (and indeed demanded) Pioneer eight ohm headphones driven by 50 watt PA amps. Well, actually I am saying that, that’s exactly what happened. And that was neither safe nor useful, if the cans slipped a millimetre your show was going to howl round magnificently.

One approach is to fit all headphones with limiters, this certainly makes everything safe but it isn’t as useful as it might be. Why? Because it ignores the time dimension of loudness. Safe listening levels integrate loudness over a period of time. Which means you can safely increase your listening volume provided you shorten the exposure. This flexibility isn’t available with the simple catch all level limiter. I’m sure Glensound have done quite a lot of work on this problem in the past, but now another UK company LimitEar have a solution that is available for a range of manufacturer’s products. For professional users this is the HDM Pro.

I tested some Sennheiser HD 25s fitted with the HDM Pro – ‘Hearing Dose Management’ system. The system is the brain child of LimitEar and I guess in theory can be added to any headphones as it is contained within the lead. The clever stuff is packed into a light weight plastic lozenge about the size of a standard usb stick. I took the cans out on a long day’s shoot and was not bothered by the weight and had no real issues with cable flexibility or the device catching on bags, straps and cables. The bane of location life.There’s a multi layer challenge when producing a system like this. Will the product still do the job of professional headphones? How transparent will it be in use? What about charging and usage times and how will fit into your work flow?

To take some of the questions, what is the HDM Pro actually doing? Well in the same way that we have been growing increasingly used to metering loudness levels using time based averaging the LimitEar product is working on average levels calculated from the signal we stuff up our headphone leads. The sums are done on a 24 hour rolling average basis. In action the technology works on three levels. First all peaks are limited to 118dB, sorry Ted, then audio levels above 100 dB but below 118dB are managed down do 100dB and your daily (24 hours) dose is controlled to comply with the 2005 Noise at Work Regulations.

How do they sound? Well that’s a tricky question to answer in any sort of meaningful A/B test. I certainly didn’t notice any obvious artefacts or gain limiting effects. I have a pair of ordinary 60 ohm HD 25s (the HDMs are 70 ohms) and using a multiple output Sound Devices HX-3 headphone amp and a Castle sound level meter did my best to match levels. Feeding the system from the Marenius DAC-S2 and swapping cans over I wasn’t confident I could pick out which was which at ordinary listening levels. The HDM system introduces a 3dB insertion loss and once the system starts working at higher sound levels then level matching becomes a moot point.

As we all know by now you get nothing for nothing and indeed the LimitEar technology uses active electronics that require powering. The system is rechargeable from a standard micro USB charger or PC USB port and a full charge should run the headphones for 7 days and the LEDs warn you when you run down to one day’s charge left. When the battery is exhausted the system applies a 14dB and waits for your to charge it up again.

But how do we know exactly what is happening with our headphones, displays of any sort would presumably eat too many electrons so the LimitEar system makes do with flashing leds, one green and one red. As you might expect, green is good and red is bad, or at least a warning. Working out what the levels are and what the circuitry is doing is a matter of decoding flashes spaced by different amounts and in different ratios. Being a stupid man I found I had to constantly refer to the operating guide. If I was a studio I’d laminate it and stick a copy to the wall. This is probably the weakest point of the system.

The system is clever but it does help to engage your brain. Using the indicators you should be able to set the right levels for the duration of your listening session. And you should take steps to bank your loudness – unplug your cans for twenty minutes while playing the extended 12 inch version of Bohemian Rhapsody or at least turn them down.Why so? Well in this way you will have saved up some audio exposure brownie points. So when you play the 13 inch extended version of Ace of Spades you can turn it up to 11.

Hearing damage is a serious issue and I’m willing to bet virtually everyone of us knows someone who has had their hearing damaged in a business where almost inevitably we are exposed to high audio levels over extended periods. Besides the legal and financial issues of getting sued none of us wants a future with significant hearing loss, of all people we need our ears in good nick. Yes if you buy into the HDM Pro solution you need to manage the charging issue, and yes you need to decode the flashing leds and act accordingly. Small prices to pay against your future hearing.

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