High frequencies key to understanding speech

Do you have trouble understanding what others are saying in noisy places? If so, new research out this week might explain why. Researchers in the US have found that high frequency sounds play a key role in the intelligibility of speech. And if you can’t hear them properly, you struggle in noisy places. But these critical frequencies are usually not routinely checked in hearing tests, and perhaps they should be, as Adam Murphy heard from Cincinnati Children’s Hospital, Ohio’s Lina Motlagh Zadeh and David Moore…

David - So in a normal audiology exam, they generally check out frequencies between about 250 Hertz and over 4000 or 8000 Hertz - where 250 Hertz is low pitched sound and 4-8000 are very high pitched. But actually healthy young people can hear up to twenty thousand hertz (20 kilohertz), but audiologists have tended not to pay attention to that extended high frequency between eight and 20 kilohertz partly because most of the energy of sound is at lower frequencies. We know from everyday experiences of using telephones and so forth we don't actually need to use those extended high frequencies, at least in normal listening conditions.

Adam - Lina now that we've set the groundwork with David, can you explain to me the experiment you've run?

Lina - We recruited young adults, more than 60 percent of them less than 30 years old and we surprisingly found more than half of them have extended high frequency hearing loss, which was related to the self reported difficulty listening to speech in noisy environment. So we first started with routine ideology tests, this involves hearing tests. When you go to visit an audiologist you send some pure tones with different pitches and intensities and you ask them to respond to the sound even if it's soft to obtain the minimum amount of things that they can hear. Then after that we used the 'speech in noise test', we wanted to see if we give access to high frequency hearings, very high frequency hearings, how the speech understanding will be for this participant. So, surprisingly when we put the cutoff of the filter giving access to frequency above 8 kilohertz their score improved significantly. So, it shows that listeners could use these frequencies to understand the speech in noise.

Adam - and why is it that not having these high frequencies means you can't pick noise out of crowds so well?

Lina - Understanding the speech mainly depends on high frequencies, because high frequencies can provide intelligibility or clarity and we need to hear high frequency parts of the speech to follow the conversation in noisy environments. Then you have high frequency hearing loss as a result of age noise or a lot of toxic drugs, this mainly affects high frequency range of hearing - you can't easily follow speech like a person who has normal hearing in these frequencies.

Adam - What do you want people to take away from this research?

David - I think this gives us an opportunity for some sort of personalised medicine, because it's important to realise that some people have this problem while others don't. We can then tailor our intervention, be it protection of hearing, or even some new drugs which are coming online soon to prevent hearing loss to the people who really need them.