The following
samples are provided so that you can hear the differences between
several voice input methods, including a handheld microphone, a
competitor's mask and Glottal Enterprises pneumotach and Speech
Tutor masks. Here is the voice clip that you will hear when you
click on the icon next to each mask or microphone in the table
below.
Adult male speaker, spoken at a normal
conversational level:
"Now
is the time for all good Americans to come to the aid of their country."
Speech Distortion Caused by Masks
Speech Distortion Caused by Masks for Measuring Airflow During
Speech
Air Flow from the mouth or nose during speech
or singing is commonly accomplished by means of a mask that funnels
the desired airflow to a flow sensor. In the wire-screen pneumotachograph,
this sensor consists of a fine-mesh wire screen that, by partially
blocking the air path, converts the variations in flow to variations
in air pressure. A sensitive pressure transducer measures
these pressure variations.
The amount of distortion or muffling of the voice caused by the
mask will vary widely, from slight to very strong, depending on
the type of mask used. Presented above are samples of speech taken
using three types of masks that are now sold for speech measurements.
Types of speech distortion caused by
a mask
There are two distinct acoustic factors that combine to result
in the perceived distortion of the radiated speech caused by
a mask: 1. The alteration of the vocal tract acoustics that determines
the identity of the vowel or consonant being spoken (as the general
lowering of the formant frequencies caused by the coupling between
the mask chamber and the vocal tract).
2. The muffling of the speech heard outside the mask caused by
the walls of the mask.
Respiratory masks
The first mask illustrated above is a typical mask used for respiratory
airflow measurements during breathing that is sometimes also
used for speech measurements. The mask funnels the combined oral
and nasal airflow through a chamber containing a fine-mesh wire
screen (in this case a Fleisch pneumotachograph head). The two
nipples on top of this chamber are for the connection of flexible
tubes that lead to a differential pressure transducer (not shown).
This type of mask is used by Kay Elemetrics in the Aerophone™ system
for speech airflow measurements. A sentence spoken with this
mask in place can be heard by clicking on the icon near the mask.
Circumferentially vented (CV) masks
To reduce the speech sound distortion and muffling caused by the
mask, as well as to greatly increase the frequency to which airflow
measurements could be made, the circumferentially vented (CV)
mask was developed by Dr. Martin Rothenberg. The CV mask design
is now marketed by Glottal Enterprises in a number of forms.
Shown in Figures 2 and 3 above, are the Glottal Enterprises
MA-1L and O/N-MA1 CV masks. Sentences spoken with these masks
in place can be heard by clicking on the icon near each mask.
The MA-1 series masks are used by many laboratories worldwide
for low distortion, wideband speech airflow measurements, including
inverse filtering for deriving the glottal airflow waveform.
The O/N series masks were designed for the convenient and accurate
measurement of voice nasalance, but can also be used for wideband
airflow and inverse filtering applications.
The last mask shown above is a version of the O/N-MA1 that has
been modified slightly for further reduction of voice distortion
and muffling by adding a fifth outlet hole and screen ring to the
four already in the oral section of the mask. It can be supplied
by Glottal Enterprises on special order for critical applications
in which the slight speech muffling of the standard O/N mask is
objectionable. A sentence spoken with this mask in place can also
be heard by clicking on the icon near the mask.
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