How to improve speech clarity in an auditorium

Introduction

In this blog, we will discuss improving speech clarity in an auditorium. We will look into what causes the lack of sound clarity in auditoriums, hear some examples and then discuss ways to improve the situation.

Clients often complain that they have trouble understanding speech programs in many auditoriums, seminar halls, or mangal karyalayas. When Architects and designers consider the aspects behind this issue, they can design the spaces curated to the requirements of their clients. This video will delve deeper into these aspects of auditorium acoustics.

We at consonance acoustics had designed an auditorium with a class A Speech Transmission Index using our patented and other absorbers. You should reach out to us if you want to know about knowing the Speech Transmission Index at your auditorium or hall.

Speech Intelligibility

Let’s begin the chat, hoping that you can hear normally. But even if you hear more than just 75% of the words clearly, you can understand around 90% of the sentences.

Hear this to proceed -

I think you did understand the last sentence even though we deleted two out of eight words which are 25% of the words in that sentence.

You will notice that you were close to getting those sentences' meaning. This happens due to the context and our way of making sense of the language. These things are in-built for us. But imagine, when we are listening to a lecture or a speech, and we can’t understand it correctly, that means not even 75% of the words being said are understood by the audience.

Speech Transmission Index

The most common way to objectively measure speech intelligibility is Speech Transmission Index.

The reason why speech intelligibility is lost in halls and auditoriums is the high reverberation times, or mistakenly called “echo”. Reverberation time is the time taken for the sound impulse, like a clap, to decay by 60 decibels. If the sound is not absorbed a lot by the surfaces in a room, it will keep reflecting and hence take longer to decay. The phenomenon is different for different frequencies of sound.

We learnt in school that the sky is blue because the blue light being high in frequency gets scattered due to the atmosphere. The same happens with the higher frequencies of sound. On the other hand, the lower frequencies are difficult to get absorbed, and their reverberation time is also higher in untreated spaces.

This is of particular importance in the case of speech. While Speech Transmission Index is a good way of getting an idea of speech intelligibility, another critical factor lies in how our language and speech production mechanisms act out.

Have a look at this figure. This curve is our audibility threshold. Notice that it is not flat for all frequencies. In this curve, we have our speech region. It starts from about 120 Hz, for normal male talkers and goes till about 8000 Hz.

In this speech region, we have vowels, which are lower in frequency when compared to consonants. We can take another test, try to guess the following words -

_ _ I _ _ _ e _ o _ e a _ _

Here are the answers -

Think before act

But it could also be -

Blink before art

Stink before ass

Was it difficult? Try the next example -

T _ s t _ p t h _ t r _ _ n p _ l l t h _ c h _ _ n

T h _ s w _ r d c _ _ l d b _ _ n y t h _ n g

Was it easier?

Here are the answers -

To stop the train pull the chain

This word could be anything

The difference is that in the first example, we had no consonants like B or T or F or R etc. and in the next example, we had no vowels like A or E or U etc. You noticed that more of the information of a word is conveyed by the consonants, which are higher in frequency and the shape of the words is conveyed by the vowels which are lower in frequency.

Masking of higher frequencies

Another example that COVID showed us was the use of masks. We find it so difficult to understand when someone is speaking while wearing the mask. You must have found yourself in situations where you have to lower your mask so the person listening to you can hear clearly.

Here is the frequency analysis of my speech when I am wearing a mask. Notice the build-up in the low-frequency zone.

When we remove the mask, you will notice that both the clarity and the high-frequency content go up.

When a space has high reverberation at lower frequencies, they overpower or mask the higher frequencies as their sound levels are higher when all the high frequencies are absorbed. It is, therefore extremely important to look at the absorption coefficients of the sound absorbers across the frequency range instead of just looking at the high NRC number and proclaiming that this is a good sound absorber.

Sound absorption of higher frequencies

Do you know what is the largest source of absorption for high frequencies? It is the audience that sits in the auditorium! We talked a bit about this in another blog that you can check out here.

The audience is one source of absorption we cannot avoid as we cannot ask the audience to sit outside. Since we need a balance in absorbing all frequencies to maintain a constant reverberation time for all frequencies, we need to carefully design the auditorium acoustics to ensure sufficient low-frequency absorption is present.

I hope you now know how to improve speech clarity in an auditorium. It involves careful recommendations of sound absorbers to reduce the reverberation time across the relevant frequency range.

We have not been able to cover the detailed calculations for proper designs in this blog but you can contact us and we would be more than happy to help you in your design process.

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