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Sennheiser HDM Pro HD 25

Can Sennheiser’s new collaboration with LimitEar really provide a safer headphone solution while maintaining sound quality? Alistair McGhee finds out.

‘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). It turns out if it’s too quiet when you’re old, that’s because you listened too loud when you 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 50W PA amps. (Well, actually I am saying that). And that was neither safe nor useful. If the cans slipped a millimetre your show was going to howl round magnificently.

CHASING SAFETY One approach is to fit all headphones with limiters. This 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 catchall level limiter. I’m sure Glensound has done quite a lot of work on this problem in the past, but now another UK company, LimitEar, has 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 brainchild of LimitEar and I guess in theory can be added to any of Sennheiser’s headphones as it is contained in the lead. The clever stuff is packed into a

46 April 2014

lightweight 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. 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 it fit into your workflow?


One of the main questions is 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 100dB but below 118dB are managed down to 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 ohm) 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 a Marenius DAC-S2 and swapping cans over I wasn’t confident I could pick out which was which at

“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.” Alistair McGhee

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 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 seven days with the LEDs warning you when you run down to one day’s charge. When the battery is exhausted the system applies a 14dB pad and waits for you to charge it. Displays of any sort would

presumably eat too many electrons so the LimitEar system makes do with flashing LEDs – one green and one red. 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 simple man I found I had to constantly refer to the operating guide and 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 20 minutes while playing the

extended 12in version of Bohemian Rhapsody.Why? So you will have saved up some audio exposure brownie points. So when you play the 13in extended version of Ace of Spades you can turn it up to 11.

CONCLUSION Hearing damage is a serious issue and I’m willing to bet virtually every one 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. 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 but that’s a small price to pay against your future hearing.

THE REVIEWER ALISTAIR McGHEE began audio life in Hi-Fi before joining the BBC as an audio engineer. After 10 years in radio and TV, he moved to production. Most recently, Alistair was assistant editor, BBC Radio Wales and has been helping the UN with broadcast operations in Juba.


• Analogue signal path to ensure high-quality audio • Instantaneous clipping of extreme impulse signals • Rapid management of signals using Rapid Overload Response

• Long-term hearing dose management based on 24-hour assessment

• LED indication of monitoring activity, high dose, and battery level

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